The Connections Between Neural Crest Development and Neuroblastoma

Abstract

Neuroblastoma (NB), the most common extracranial solid tumor in childhood, is an extremely heterogeneous disease both biologically and clinically. Although significant progress has been made in identifying molecular and genetic markers for NB, this disease remains an enigmatic challenge. Since NB is thought to be an embryonal tumor that is derived from precursor cells of the peripheral (sympathetic) nervous system, understanding the development of normal sympathetic nervous system may highlight abnormal events that contribute to NB initiation. Therefore, this review focuses on the development of the peripheral trunk neural crest, the current understanding of how developmental factors may contribute to NB and on recent advances in the identification of important genetic lesions and signaling pathways involved in NB tumorigenesis and metastasis. Finally, we discuss how future advances in identification of molecular alterations in NB may lead to more effective, less toxic therapies, and improve the prognosis for NB patients.

1. Clinical and Biological Characteristics of Neuroblastoma (NB)

NB is the most common extracranial solid tumor in childhood, accounting for approximately 7–10% of pediatric cancers and 15% of all pediatric cancer deaths in patients less than 15 years old (Brodeur, 2003; Maris et al., 2007; Schor, 1999). NB is an extremely heterogeneous disease both biologically and clinically (Brodeur, 2003; Evans et al., 1971; Maris et al., 2007). NB is thought to be an embryonal tumor that is derived from precursor cells of the peripheral (sympathetic) nervous system (Brodeur, 2003; Grimmer and Weiss, 2006; Nakagawara and Ohira, 2004). The tumor can arise anywhere along the sympathetic chain but is most frequently in the adrenal medulla and paraspinal ganglia (Fig. 4.1; Johnsen et al., 2009; Nakagawara and Ohira, 2004).

Figure 4. 1.

Neuroblastoma localization. Neuroblastoma primary tumors derive from precursor cells of the peripheral (sympathetic) nervous system and can arise anywhere along the sympathetic chain, most frequently in the adrenal gland (position 8 as shown). Neuroblastoma may also develop from spinal cord of neck (position 6) and pelvis (position 10). Neuroblastomas mainly metastasize to lymph nodes (position 3), liver (position 4), bone and bone marrow (position 5), and also spread to central nervous system (position 1) and lungs (position 2) in infants.

Historically, NB is clinically classified into five different stages (1–4 and 4S) according to the International Neuroblastoma Staging System (INSS; Brodeur, 2003; Maris et al., 2007; Schor, 1999; van Noesel and Versteeg, 2004). Early stage NB tumors (i.e., stages 1, 2) do not metastasize to bone or bone marrow and are treatable with chemotherapeutic drugs and irradiation. Advanced-stage NB tumors (stages 3 and 4) are highly metastatic and usually respond positively to initial treatment. However, they often become resistant to chemotherapy and irradiation. A fifth stage of NB tumors (stage 4S) undergoes spontaneous regression with minimum treatment or even without medical intervention.

Ninety percent of the children with this disease are diagnosed before the age of 5 years and in those patients older than 1 year, 75% of the patients present with stage 3 or 4 metastatic diseases (Brodeur, 2003; Maris et al., 2007). Metastatic disease remains a major clinical challenge in the treatment of NB since greater than 50% of all NB patients are diagnosed with metastatic diseases (Maris, 2005; Maris et al., 2007). In contrast, infants with this disease tend to be at lower stages (stage 1, 2, and 4s) and to have a better prognosis (Brodeur, 2003; Maris, 2005; Maris et al., 2007; van Noesel and Versteeg, 2004).

In addition to classification by stage, NB tumors are also classified into three risk groups (low, intermediate, and high risk) according to age, MYCN status, and histology (Table 4.1). A new system for tumor staging has recently been implemented by the International Neuroblastoma Risk Group Staging System (INRGSS; Cohn et al., 2009; Monclair et al., 2009). This system, which is based on a combination clinical and imaging data, classifies patients as L1 (localized disease without imaging risk factors), L2 (localized disease with imaging risk factors), M (metastatic tumors), and Ms (metastatic disease with metastasis only in skin, liver, and/or bone marrow; Cohn et al., 2009; Monclair et al., 2009). NB tumors are also graded in terms of histology using the International Pathology Classification System (INPC) which is based on the Shimada histology grading system (Shimada et al., 1999). This system distinguishes good and poor prognosis tumors based on the degree of differentiation, the Schwannian stromal content, the mitotic-karyorrhexis index (MKI), and the age at diagnosis (Shimada et al., 1999). Unfavorable tumors tend to include undifferentiated tumors with high MKI of any age, poorly differentiated tumors or intermediate MKI in patients older than 18 months, and differentiated tumor or low MKI in patients 5 years old or older. In contrast all other cases, including those with gang-lioneuroma or tumor with regions of ganglioneuroma intermixed, have a good prognosis.

Table 4. 1.

Neuroblastoma risk stratification based on age, MYCN status and histology

INSS stage	Age	MYCN status	Histology	Risk group	3-year survival rate
1	0–21 years	Any	Any	Low	>90%
2	0–21 years	Non-Amp	Any	Low	70–90% in this stage
	1–21 years	Amp	Favorable	Low
	1–21 years	Amp	Unfavorable	High
3	<1 year	Non-Amp	Any	Intermediate	30–50% in this stage
	1–21 years	Non-Amp	Favorable	Intermediate
	0–21 years	Amp	Any	High
	0–21 years	Non-Amp	Unfavorable	High
4	<1 year	Non-Amp	Any	Intermediate	<30% in this stage
	0–21 years	Amp	Any	High
4S	<1 year	Non-Amp	Favorable	Low	50–80% in this stage
	<1 year	Non-Amp	Unfavorable	Intermediate
	<1 year	Amp	Any	High

INSS, International Neuroblastoma Staging System; Amp, amplified; Non-Amp, not amplified.

In the remainder of this review, we will discuss the development of the peripheral neural crest with a focus on how developmental factors may contribute to NB tumorigenesis and metastasis, and highlight the current understanding of other genetic changes related to NB and their importance in NB diagnosis and treatment.

2. Neural Development and NB

2.1. Neural crest contribution to sympathetic ganglia and adrenal gland

The majority of NB tumors appear to arise from neural crest-derived cells in the abdomen adjacent to the aorta in the region of the kidney or in the medullary region of the adrenal gland (Brodeur, 2003; Maris et al., 2007). Thus, NB is a sympaticoadrenal lineage neural crest-derived tumor. The neural crest arises from the dorsal region of the closing neural tube beneath the ectoderm (Le Dourin and Kalcheim, 1999). This transient population of cells produces multipotential progenitor cells that give rise to the peripheral nervous system, the enteric nervous system, pigment cells, Schwann cells, adrenal medullary cells, and cells of the craniofacial skeleton (Le Dourin and Kalcheim, 1999). This process is regulated by both extrinsic and intrinsic factors. The Hedgehog and Wnt signaling pathways are especially crucial for proper neural crest development (Dupin et al., 2007; Le Dourin and Kalcheim, 1999; Morales et al., 2005). Lineage studies in the developing embryo have shown that neural crest cells within the trunk region generate multiple neural crest derivatives such as melanocytes, Schwann cells, glia, and neurons of the dorsal root ganglia (Fontaine-Perus et al., 1982; Lallier and Bronner-Fraser, 1988; Teillet et al., 1987; Weston, 1963). A subset of these trunk crest cells, commonly referred to as the sympathoadrenal lineage, contributes to the sympathetic ganglia and medullary region of the adrenal gland (Anderson and Axel, 1986; Anderson et al., 1991). This lineage of cells is thought to be the origin of NB (Brodeur, 2003; Maris et al., 2007). However, given the fact that NB can develop anywhere along the sympathetic axis, it is likely that NB can also arise from earlier crest derivatives, before development of the sympathethoadreanal lineage but after the initial fate specification (Brodeur, 2003; Maris et al., 2007). This could contribute to the heterogeneous histology and pathology of NB.

Neural crest cells develop in response to extracellular signals (Bronner-Fraser, 1993). The signals that induce formation of the crest (BMP/Shh) appear to be similar along the dorsal/ventral axis of the embryo. In contrast, different factors appear to confer cell fate along the anterior/posterior axis of the embryo. Heterotopic crest cell transplantation studies indicate that the positional identity of the cells is based on their location during development, rather than the characteristics of cells in original locations (Bronner-Fraser et al., 1980; Ruffins et al., 1998). Although NB can arise anywhere along the developing sympathetic axis, the majority of cases arise in the abdomen (65%), frequently in the adrenal medulla, where the sympathoadrenal lineage is specified; while others are found in the paraspinal sympathetic ganglia in places such as the neck (5%), chest (20%), and pelvis (5%) (Maris et al., 2007). Therefore, it is reasonable to postulate that the positional identity of the cell along the anterior posterior axis of the embryo, and factors that specify this region, likely contribute to the oncogenic potential of the crest derivatives in this region. As such, developmental signals fating the proper development, migratory pathways, and regulated cell death will be examined here in the context of NB.

2.2. Neural crest migratory pathways

Neural crest-derived cells are highly migratory. Shortly after induction the crest cells undergo an epithelial to mesenchymal transition (EMT). This EMT transition results in acquisition of enhanced migratory abilities and decreased requirements for intercellular contact which allows the neural crest cells to leave the dorsal neural tube (Le Dourin and Kalcheim, 1999). They then migrate either between the dermatome and epidermis in a dorsolateral pathway or delaminate from the neural tube via a ventrolateral pathway (Le Dourin and Kalcheim, 1999). Importantly, a similar EMT transition may also play a role in NB metastasis as described below in Section 6.1.

Neural crest migration pathways are determined by signal from the mesoderm which develops prior to the arrival of the crest cells. Trunk region neural crest cells either migrate ventrolaterally and remain in the sclerotome form the doral root ganglia or continue migrating to a more ventral position to form sympathetic ganglia. N-myc, an oncogene which plays a role in aggressive NB, appears to be required for the migration, survival and/or differentiation of cells that migrate to the dorsal aorta since N-myc deficient mouse embryos have decreased numbers of mature cells in the both the dorsal root ganglia and sympathetic ganglia (Charron et al., 1992; Sawai et al., 1993; Stanton et al., 1992). Hedgehog, Wnt, and additional growth factors all play a role in inducing N-myc expression in the neural crest (Grimmer and Weiss, 2006). More detailed information on the role of N-myc in NB is present below in Sections 3.2.1 and 5.2.3.

The most complete studies on neural crest cell migration and development have been carried out in birds. These studies revealed that the avian trunk crest, which is located between somites 6 and the tail, gives rise to sympathetic neurons and that a subset of these (between somites 18 and 24) contribute to the adrenal medulla (Le Dourin and Kalcheim, 1999). The crest cells that contribute to the adrenal medulla, the site where the majority of NB tumors are found, follow the ventrolateral migratory pathway. Upon arrival to the proper target tissue, the cells then undergo final differentiation, and regulated cell death.

2.3. Sympathoadrenal lineage development

The sympathoadrenal lineage is thought to derive from a common progenitor that aggregates at the dorsal aorta after migrating from the crest utilizing the ventral pathway (Anderson, 1993). During the migration to the dorsal aorta, the crest cells encounter signals from the somites, the ventral neural tube, and the notochord. Local signals from the dorsal aorta, such as BMPS, specify the future differentiation of the crest cell as either a catecholaminergic/adrenal chromaffin cell or sympathetic neuron (Ernsberger et al., 2005; Howard et al., 2000; Reissmann et al., 1996; Schneider et al., 1999; Shah et al., 1996). The sympathoadrenal lineage is specified by a tightly regulated set of transcription factors. Trunk crest cells can first be identified by the expression of Sox10 (Betters et al., 2010; Huber et al., 2008). As the crest cells migrate along the ventral pathway, they are exposed to BMP signals (e.g., BMP2, 4, and 7) that induce the expression of Mash1, a helix-loop-helix transcription factor expressed throughout the autonomic progeny (Huber, 2006). Shortly thereafter Phox2b expression occurs in sympathoadrenal lineage cells. Phox2b is required for the maintenance of Mash1 and recent evidence has shown that temporal difference in expression of these two factors may separate the sympathoadrenal lineage into separate sympathetic and adrenal lineages earlier in development than thought (Huber, 2006). Mash 1 induces the expression of Phox2a which is required for the production of the biosynthetic enzymes, dopamine beta-hydroxylase (DBH) and tyrosine hydroxylase (TH) in noradrenergic cells. Phox2b also appears to be important in the development of NB since PHOX2b mutations have been found in a subset of familial NB patients (see Section 3.1.1). Shortly after the migrating cells reach the dorsal aorta, they begin to acquire their respective sympathetic and adrenal cell fates and undergo a secondary migration to the presumptive prevertebral ganglia, the adrenal medulla, and secondary sympathetic ganglia where they complete their differentiation (Fig. 4.2; Huber, 2006; Le Dourin and Kalcheim, 1999; Morales et al., 2005).

Figure 4. 2.

General schema of the development of chromaffin cells and sympathetic ganglia. Cells at the dorsal region of the neural tube undergo EMT (red population), delaminate from the neural tube (orange), and migrate ventrally to the aorta (green) where they are commonly referred to as the sympathoadrenal progenitors (blue and purple). From the aortic region, the cells then migrate to the developing adrenal gland (AP) to become chromaffin cells or differentiate to become sympathetic ganglia (SG). As cells begin to differentiate as sympathetic ganglia they upregulate neural markers while chromaffin cells upregulate proteins found in the adrenal gland. Recent studies suggest that the chromaffin cell and sympathetic ganglia may come from divergent lineages rather than a common sympathoadrenal lineage. The question marks between the migrating crest and the sympathoadrenal progenitors address this possibility. A more detailed discussion on the temporal expression of the transcription factors in the sym-pathoadreanal lineage (including developmental stages) can be found in (Howard et al., 2000) and details about the distinct chromaffin and sympathetic lineages can be found in (Ernsberger et al., 2005). Additional of neurotrophins and their receptors can be found in (Straub et al., 2007). Transcription Factors are shown in bold and factors implicated in neuroblastoma have been underlined. Abbreviations: NT, Neural Tube; NC, notochord; A, Aorta; SG, sympathetic ganglia; AP, adrenal primordial.

Although the complete mechanisms for proper chromaffin cell development are unknown, evidence indicates a highly intrinsic program. In animals that lack an adrenal cortex, chromaffin cells migrate to the suprarenal region, downregulate neuronal markers and contain large chromaffin granules. However, the generation of proper numbers of chromaffin cells and the expression of secretogranin II and PNMT requires intact glucocorticoid signaling. The human adrenal gland is remodeled throughout fetal development, infancy, and into adulthood in a process that is greatly affected by perturbations in levels of IGF II, FGF, and epidermal growth factor (EGF) levels. Importantly, as discussed below several of these growth factors are involved in cellular proliferation and signaling in NB (Section 4.2).

3. Genetic Lesions in NB

3.1. Familial genetic lesions

Hereditary NB is both rare and heterogeneous, accounting for less than 5% of all NBs (Maris et al., 2002). In addition to the known hereditary mutations that are described below, hereditary NB predisposition loci have been mapped to chromosomes 16p12–13 and 4p16 indicating other familiar predisposition mutations may exist, but no genes have been shown to be inactivated or mutated in these regions, to date (Maris et al., 2002; Perri et al., 2002).

3.1.1. Phox2b

Germline mutations in the paired-like homeobox 2B (PHOX2b) gene on chromosome 4p13 are the first predisposition mutations identified in NB (Mosse et al., 2004; Trochet et al., 2004). As mentioned earlier, Phox2b, as well as to MASH1, are expressed early in the developing sympathoadrenal progenitors (Alenina et al., 2006; Nakagawara, 2004; Nakagawara and Ohira, 2004). Shortly after expression of MASH1 and Phox2b in the sympathoadrenal lineage, Hand2, Phox2a, and GATA2/3 appear (Alenina et al., 2006; Nakagawara, 2004; Nakagawara and Ohira, 2004). Phox2b has also been shown to be essential for the expression of the glial family ligand tyrosine kinase coreceptor RET (REarranged during Transfection) and for the specification of noradrenergic fates, particularly the biosynthetic enzymes TH and DBH (Alenina et al., 2006; Nakagawara, 2004; Nakagawara and Ohira, 2004).

NB patients with PHOX2b mutations also have familial disorders of the neural crest such as Hirschsprung’s disease (HSCR) and congenital hypoventilation syndrome (CCHS; Mosse et al., 2004; Trochet et al., 2004). It is unclear that the mutations in PHOX2b found in familiar NB result in gain or loss of function, although many PHOX2b mutations stabilize the Phox2b protein and decrease or eliminate the ability of Phox2b to transactivate the DBH promoter (Raabe et al., 2008; Trochet et al., 2005). The findings that Phox2b is necessary for the differentiation of autonomic neurons and overexpression of Phox2b inhibits proliferation in neuron progenitors and cell lines suggests Phox2b is a tumor suppressor (Raabe et al., 2008; Trochet et al., 2005, 2009). However, the absence of tumors with loss of heterozygosity (LOH) or mutation in second allele suggests gain-of-function, dominant-negative effect, or haploinsufficiency (Benailly et al., 2003; Bourdeaut et al., 2005).

3.1.2. Anaplastic lymphoma kinase (ALK)

ALK is a member of receptor tyrosine kinases (RTK) and was first identified as a part of the fusion gene nucleophosmin (NMP)–ALK in anaplastic large cell lymphoma via chromosome translocation of t(2;5)(p23;q25) (Morris et al., 1994, 1997). ALK is thought to play a role in the normal development of the central and peripheral nervous system since ALK mRNA is expressed throughout the nervous system in mouse and rat, but is not present in normal hematopoietic cells (Degoutin et al., 2009; Hurley et al., 2006; Iwahara et al., 1997; Morris et al., 1997; Vernersson et al., 2006). More detailed studies in chick embryos have shown a similar pattern of ALK expression in the developing central nervous system in which ALK localizes primarily to the spinal motor neuron, the sympathetic ganglia, and the dorsal root ganglia. In mice, expression of ALK in the nervous system decreases after birth but is maintained at low levels in adults. Similar patterns of expression are observed in humans although additional ALK transcripts of differing size, most likely due to alternative splicing, have been observed in colon, prostrate, testis, small intestine, and brain of adults (Iwahara et al., 1997; Palmer et al., 2009).

Full-length ALK protein is comprised of an extracellular region and an intracellular region containing a RTK domain, linked by a transmembrane (TM)-spanning segment, whereas the NMP–ALK fusion protein generated as a result of the t(2;5)(p23;125) translocation contains the N-terminal of NMP and C-terminal kinase domain of ALK. Translocation of the ALK gene is also found in other tumors, such as inflammatory myofibroblastic tumor (IMT), and nonsmall cell lung cancer (NSCLC), but not in NB (Palmer et al., 2009). Overexpression of wild-type ALK has also been observed in thyroid carcinoma, breast cancer, NB, melanoma, small cell lung carcinoma, glioblastoma, astrocytoma, retinoblastoma, Ewing sarcoma, and rhabdomyosarcomas NB (Cheng and Ott, 2010; Mosse et al., 2009; Palmer et al., 2009).

During 2008, at least five papers described ALK point mutations in 8–12% of all NB patient (both hereditary and sporadic) and some NB cell lines as well (Caren et al., 2008; Chen et al., 2008; George et al., 2008; Janoueix-Lerosey et al., 2008; Mosse et al., 2008). With one exception, all the point mutations identified to date occur in the kinase domain and result in the constitutive activation of ALK. Two of these activating ALK mutants were able to transform NIH3T3 fibroblasts and induce tumor formation in nude mice (Chen et al., 2008). In addition, knockdown of ALK or small molecular ALK inhibitors could reduce cell proliferation and induce apoptosis (George et al., 2008; Janoueix-Lerosey et al., 2008). Amplification of the ALK gene and/or overexpression of the ALK protein is seen in as many as 77% of all NB tumors (Passoni et al., 2009) suggesting that overexpression of the ALK protein may also contribute to NB.

The downstream effects of ALK in NB need to be defined. Current data suggest that ALK may function through the Shc and MAP kinase pathways (Motegi et al., 2004; Osajima-Hakomori et al., 2005; Souttou et al., 2001). More recent studies also suggest that activation of ALK enhances RAP1 activity via interaction with C3G, a Crk-binding protein and Crk-like protein (CRKL), and that this complex contributes to NB tumor cell growth and neurite outgrowth (Schonherr et al., 2010).

3.2. Chromosome gain and oncogene activation

Many genetic abnormalities have been identified in nonfamilial NB tumors, including amplification of the MYCN proto-oncogene (25–33% of patients) and consistent areas of chromosomal deletion and rearrangement that result in loss of 1p36 (25–35%), 11q23 (35–45%), and 14q23 (16–27%), as well as unbalanced gain of 17q22 (~50%) (Table 4.2; Brodeur, 2003; Maris et al., 2007; Schor, 1999). In contrast, known tumor suppressor genes (TSGs) such as p16^INK4a, pRb, p53, and p14^ARF are not frequently deleted or mutated in NB, although the nuclear localization of the p16^INK4a and p53 proteins has been reported to be altered in some tumor cell lines (Brodeur, 2003; Maris et al., 2007; Schor, 1999; Teitz et al., 2001; van Noesel and Versteeg, 2004). Many of these abnormalities are powerful prognostic markers and are highly related to clinical outcome. For example, amplification of MYCN in NB patients is correlated with chromosome 1p36 LOH. NB tumors which harbor 1p36 LOH and MYCN amplification are usually advanced-stage (stages 3 and 4) aggressive tumors that are frequently metastatic and generally respond poorly to chemotherapy/irradiation (Brodeur, 2003; Maris et al., 2007). In the recent years, clinical trials are increasingly based on the tumor genetic characteristics.

Table 4. 2.

Frequent chromosomal region abnormalities in neuroblastoma

Chromosomal region	Status	Frequency	Relation with MYCN amplification	Involved genes	Clinical group
1p36	Loss	25–35%	Correlation	CHD5, miR-34a, KIF1Bβ	Unfavorable
11q23	Loss	35–45%	Inversed correlation	TSLC1/IGSF4	Unfavorable
14q23	Loss	16–27%	Inversed correlation		All groups
17q22	Gain	~50%	Correlation	Survivin, NM23A	Unfavorable

3.2.1. Amplification of MYCN and the 2p24 locus

In 1983, Schwab et al. found that a novel myc homolog gene was amplified in several NB cell lines and one NB tumor (Schwab et al., 1983). Later, several papers termed this gene as MYCN based on homology to c-myc and expression pattern in the developing nervous system, and identified its location at chromosome 2p24 (Kohl et al., 1983; Schwab et al., 1984). Additional studies have shown that N-myc protein is a nuclear phospho-protein that is a member of the myc family of helix-loop-helix transcription factors (Pelengaris et al., 2002). Amplification of the MYCN gene in patient tumors ranges from 10-fold to more than 500-fold, although the majority of tumors exhibit 50- to 100-fold MYCN gene amplification levels. The amplified DNA typically contains a large region of chromosome 2 ranging from 100 kb to 1 Mb which includes the entire MYCN gene and varying amounts of adjacent DNA. Although other genes may be coamplified with MYCN, MYCN is only consistent amplified gene from this region (Reiter and Brodeur, 1996, 1998).

MYCN amplification is rarely observed on chromosome 2p24 in primary tumors but is found to be at homogeneously staining regions (HSRs) on different chromosomes or, more frequently, as double minutes (DMs; which are small fragments of extrachromosomal DNA; Emanuel et al., 1985; Schwab et al., 1984). During cell culture, the amplification unit frequently integrates into chromosomes to become HSRs. The reason for the differences in the location of the amplicon in primary tumors and cultured cells remains unclear.

Amplification of MYCN is highly associated with aggressive NB tumors and poor outcome. Although the entire role of MYCN in NB is still being uncovered, amplification of the MYCN gene is usually accompanied by overexpression of the N-myc protein. Studies on N-myc regulation suggest that the transcription factor and signaling pathways responsible for the upregulation of N-myc are dependent on cell type (Hurlin, 2005). These factors include IL-7 and Pax-5, NF-κB in pre-B cells, and insulin-like growth factors I and II (IGFI and IGFII) in NB cells (Strieder and Lutz, 2003). In contrast, N-myc transcription is repressed by retinoic acid (RA) in association with E2F binding, nerve growth factor (NGF) binding to TrkA receptor, the iron chelator deferoxamine mesylate, and transforming growth factor-beta 1 (TGF-β1; Strieder and Lutz, 2003; Wada et al., 1992).

Myc proteins form heterodimers with the Max protein. These heterodimers bind to E-box elements (CACGTG) to activate transcription. However, Myc–Max dimers can also associate with other transcription factors such as Miz-1 and Smad and bind to Inr (initiator) element to repress transcription. Max can also form homodimers or heterodimers with Mad to compete or suppress Myc–Max binding (Pelengaris et al., 2002; Thompson, 1998; Fig. 4.3). The targets of Myc–Max are involved in various cellular processes, including cell growth, proliferation, loss of differentiation, and apoptosis (Adhikary and Eilers, 2005; Pelengaris et al., 2002; Thompson, 1998), and include proteins such as MASH1 and important molecules in the normal development of sympathocoadrenal lineage cells, such as the multidrug resistance protein 1 (MRP1), α-prothymosin, telomerase, Id2, MCM7; leukemia inhibitory factor, activin A, Pax-3, and MDM2 (Breit and Schwab, 1989; Haber et al., 1999; Harris et al., 2002; Hatzi et al., 2002; Lasorella et al., 2002; Mac et al., 2000; Pelengaris et al., 2002; Shohet et al., 2002; Slack et al., 2005). Many other putative N-myc targets with E-boxes in or near the promoter have also been identified although studies are still ongoing to determine which E-boxes actually bind Myc. Overexpression of N-myc is also reported to influence the expression of IL-6, NDRG1, MHC class I genes, and integrins (Chambery and Mohseni-Zadeh, 1999; Lutz et al., 1996; Mac et al., 2000), although the mechanisms responsible for these effects are unknown.

Figure 4. 3.

The structure of N-myc protein and transcription regulation by N-myc. (A) The structure of N-myc protein. The N-terminal transactivation domain (TAD) contains two conserved Myc box I and II (MBI and MBII), which are essential for DNA binding. The C-terminal domain (CTD) harbors basic region (BR), helix-loop-helix (HLH) motif, and leucine zipper (LZ) for dimerization with Max. There is a nuclear localization signal (NLS) before CTD. (B) The model for the transcription regulation by N-myc. Myc–Max heterodimer may bind to E-box element (CACGTG) to activate transcription, however, Myc–Max dimer can associate with other transcription factors such as Miz-1, Smad, and bind to Inr (initiator, weak consensus) element to repress transcription. Max can also form homodimers or heterodimers with Mad to compete or suppress Myc–Max binding to E-box.

The transgenic mouse model demonstrates that MYCN overexpression is an initial event in NB tumorigenesis. In this model, overexpression of the human MYCN is driven by the rat TH promoter, which is expressed in migrating cells of the neural crest early in development (Banerjee et al., 1992), causes the formation of NB tumors in transgenic mice (Weiss et al., 1997). These tumors recapitulate most of the histological and pathological aspects of the human disease, including tumor localization, positive staining for neuronal markers, and gains and losses of chromosomes in regions syntenic with those observed in human NB (Weiss et al., 1997). However, other factors are also likely to be involved in the early stages of tumor formation since amplification of the MYCN oncogene occurs in only about one-third of NBs. In addition, the tumors in these transgenic mice rarely exhibit significant metastasis despite the presence of high levels of N-myc protein suggesting that other genetic alterations and/or epigenetic changes are needed for tumor formation and metastasis. These and other studies suggest that N-myc regulates neural progenitor cell proliferation, nuclear size and differentiation (Knoepfler et al., 2002). Importantly, studies using chick/quail chimera reveal that overexpression of N-myc in the early neural crest induces premature ventral migration of neural crest cells and promotes the differentiation of these cells (Wakamatsu et al., 1997). In addition, other studies have shown that high level N-myc mRNA or protein expression in NB cells accelerate cell cycle progression (Lutz et al., 1996), and that overexpression of N-myc in postmitotic sympathetic neurons causes quiescent cells to reenter the cell cycle and enhances the survival of these cells upon NGF withdrawal (Wartiovaara et al., 2002).

The high level expression of N-myc in NB is also consistent with the hypothesis that NB arises during development. N-myc is normally expressed at the beginning of the preimplantation stage of development. As the embryo develops, N-myc expression is observed in the central nervous system and neural crest. By embryonic day 9.5, relatively high levels of N-myc are observed in the fetal brain, kidney, and in the neural crest and early stage migrating neural crest cells (Zimmerman et al., 1986). During later stages of neural crest migration N-myc expression is only observed in cells undergoing neuronal migration. Even in these cells, N-myc expression is gradually downregulated as cells differentiate and become quiescent (Lee et al., 1984; Zimmerman et al., 1986). These data are consistent with studies using N-myc knockout mice that demonstrate that the loss of N-myc results in embryonic death at day 10.5 of gestation due to defects in the nervous system, limb, heart, liver, lung, gut, mesonephros, and genital ridge (Sawai et al., 1991).

3.2.2. Gain of 17q

Gain of chromosome 17q was first identified by G-banded cytogenetic analyses in early 1980s (Gilbert et al., 1984). However, researchers paid little attention to these observations since their interests focused on the genetic abnormalities of MYCN amplification and 1p LOH at that time. In the middle 1990s, NB scientists realized the importance of 17q abnormalities since FISH technology indicated that translocation of this chromosome arm occurs in about half of the NB primary tumors. The translocation results in unbalanced gain of one to three copies of 17q (Brodeur, 2003; Maris et al., 2007; Schor, 1999). Although the breakpoint of 17q varies, the frequent gain of regions from 17q22 suggests that increased dosage of one or more genes from this region may confer a selective survival advantage for NB tumor cells. It is estimated that as much as 20 Mb of the 17q chromosome fragment, which could include more than 200 genes, can be translocated in NB tumors. Therefore, it is difficult to identify the genes responsible for the selective advantages (Brodeur, 2003; Maris et al., 2007; Schor, 1999). Several genes in this area are considered good candidate oncogenes or tumor suppressors based on correlations between expression levels and unbalanced gain of 17q. These include survivin, NM23A, and PPM1D (Godfried et al., 2002; Islam et al., 2000; Saito-Ohara et al., 2003). Notably, survivin is a member of apoptosis inhibiting protein family and is frequently overexpressed in many tumor types, including NB where expression has been correlated with late stage disease and poor prognosis; whereas NM23A is metastasis-related gene and PPM1D a phosphatase that suppresses stress induced apoptosis (Almgren et al., 2004; Bown et al., 1999).

Unbalanced gain of 17q correlates with other chromosomal deletions. The most frequent deletion site is the short arm of chromosome 1, followed by 11q. At least 30 translocation sites on 20 different chromosomes have been identified in various patient samples and cell lines (Bown et al., 1999; Lastowska et al., 1997; Meddeb et al., 1996). Nevertheless, NB tumors harboring unbalanced gain of 17q exhibit a more aggressive phenotype and a poorer prognosis than those without this abnormality.

3.2.3. Amplification and chromosome gain of other loci

In addition to the amplification of MYCN gene, several other regions of gene amplifications have been identified in small groups of NB cases. These include amplification of the MDM2 gene at 12q13, the DDX1 gene at 2p24, the MYCL gene at 1p32, and unidentified DNA from chromosome 2p22 and 2p13 (Corvi et al., 1995a, b; Jinbo et al., 1989; Van et al., 1995). The MDM2 gene was initially found to be amplified in three NB cell lines and one primary tumor (Corvi et al., 1995a). Like the MYCN gene amplification, the MDM2 amplification unit first developed within DMs and then integrates into a different chromosome to form HSRs (Corvi et al., 1995b). The DDX1 gene, which encodes a RNA helicase, was found to be coamplified with MYCN in 4/6 NB cell lines and 6/16 tumors with MYCN amplification; however, DDX1 amplification was not found without MYCN amplification (George et al., 1996). One paper also indicated that MYCL gene is coamplified with MYCN in NB cell lines. MYCL, another member of myc gene family, is frequently overexpressed in small cell lung carcinoma (Jinbo et al., 1989). In addition to gain of 17q, other chromosome gains have been identified on 1q, 4q, 5q, 6p, 7q, 18q using comparative genomic hybridization (CGH) methodology, although their biological and clinical significance remain unclear (Hirai et al., 1999; Lastowska et al., 1997; Meltzer et al., 1996; Takita et al., 2000; Vandesompele et al., 1998).

3.3. Chromosome loss and tumor supressor genes (TSGs)

In addition to mutation, gene amplification and increased chromosome copy number, NB tumors also experience loss of genetic material and deletion of putative TSGs.

3.3.1. LOH of chromosome 1p and CHD5, miR-34, KIF1Bβ

Loss of the short arm of chromosome 1 occurs in about 25–35% NB tumors. 1p LOH is correlated with amplification of MYCN in NB patients. As mentioned above, loss of 1p correlates with and may be a result of unbalanced gain of 17q, however, the exact mechanism that is responsible for these two events is not clear. The importance of 1p LOH is highlighted by studies showing that transferring chromosome 1p material into human NB cells in vitro led to differentiation and suppression of tumorigenicity (Bader et al., 1991). In search of potential TSGs that reside in this region, extensive efforts have been made to identify the smallest region of overlap (SRO) that would include the TSG candidate. These studies delineate the 1p36.1–36.3 as the SRO. Several candidate genes, such as p73, reside in this area. However, further studies failed to demonstrate a correlation between p73 loss and NB development (Ichimiya et al., 1999).

Whereas patients with 1p36 abnormalities without MYCN amplification have been identified, the reverse situation virtually never occurs suggesting either that 1p36 LOH provides a permissive environment for MYCN amplification or that tumors with these two associated genetic defects have a high degree of genomic instability (Brodeur, 2003). NB tumors with 1p36 LOH and MYCN amplification are usually aggressive tumors that are frequently metastatic and generally resistant to chemotherapy/irradiation. Although the chromosomal regions described above are known to be important in NB, the TSGs that reside within these regions have not been definitively identified.

Recent studies have identified three new putative tumor suppressors on chromosome 1p36: the chromodomain helicase DNA-binding domain 5 (CHD5), microRNA-34a (mir-34a), and the kinesin superfamily protein 1B beta (KIF1Bβ; Bagchi et al., 2007; Munirajan et al., 2008; Welch et al., 2007). All three of these proteins have affects on cell growth. For example, Bagchi et al. demonstrated that the effects of CHD5 on cell growth were dependent on p53 and that CDH5 positively regulates p53 via effects on p19^ARF expression. Thus, overexpression of CHD5 results in enhanced apoptosis and senescence, increased p53 and p19^ARF levels, and sequestration of MDM2, the negative regulator of p53, by p19^ARF. Conversely, cells lacking CHD5 exhibit decreased p16 and p19^ARF expression. This decrease in p19^ARF was mirrored by a decrease in p53 levels and enhanced cellular proliferation. Thus, CHD5 appears to function as a tumor suppressor that controls proliferation, apoptosis, and senescence via effects on the p19^ARF/p53 pathway. These effects are most likely due to changes in the accessibility of the p16/p19^ARF gene locus resulting from the chromatin remodeling function of CHD5 (Bagchi et al., 2007).

In addition to CHD5, Chen et al. found that mir-34a was expressed at very low levels in unfavorable primary tumors and NB cell lines. This group further showed that introduction of this microRNA (miRNA) into cell lines resulted in decreased cell proliferation and caspase-dependent apoptosis. They also found that mir-34a directly targeted the E2F3 mRNA and repressed its expression (Chen and Stallings, 2007; Welch et al., 2007). E2F3 is a transcription factor that induces the expression of many genes with roles in cellular proliferation.

Finally, overexpression of KIF1Bβ induced cell death while decreased KIF1Bβ levels correlated with cell proliferation and enhanced tumor formation in nude mice, suggesting that KIF1Bβ is also a potential TSG candidate (Munirajan et al., 2008). Kaelin’s group also found that KIF1Bβ is a downstream target of prolyl hydroxylase EglN3 and induced apoptosis in neuronal progenitor cells or NB cells when NGF is limited. In addition, they identified missense mutations of KIF1Bβ in inherited NBs and pheochromocytomas (Schlisio et al., 2008), supporting the hypothesis that KIF1Bβ is a potential TSG candidate.

3.3.2. Loss of 11q and TSLC1

Loss of the long arm of chromosome 11 has been identified in 35–45% NB primary tumors with a single copy MYCN gene. Two large patient studies analyzed 295 NB primary tumors. These studies found loss of 11q in 44% cases, and common regions of LOH located at 11q23, suggesting there are TSGs residing in this area (Guo et al., 1999; Maris and Matthay, 1999). Loss of 11q correlated with adverse clinical features including late stage disease, older age of disease onset and unfavorable histology, although it is strikingly inversely correlated with MYNC amplification and 1p loss. Therefore, 11q loss is a useful and important marker in determining the clinical prognosis for those advanced-stage tumors without MYCN amplification. Transfer of chromosome 11 induced differentiation in NB cell lines supporting the importance of loss of 11q in tumorigenesis (Bader et al., 1991).

One putative tumor suppressor, the IGSF4 (immunoglobulin superfamily 4) gene, was first localized to the common 11q23 LOH region in 1999 (Gomyo et al., 1999). This gene which is also known as TSLC1/CADM1 (Tumor suppressor in lung cancer 1/cell adhesion molecule 1), is considered as a potential TSG for lung cancers. A recent CGH study which examined 236 primary tumor samples found TSLC1 LOH locus in 35% tumors. Importantly, the level of TSLC1 expression correlated with tumor stage, histological classification, MYCN and TrkA expression levels. Reduced expression of TSLC1 was found in unfavorable tumors. Further, introduction of TSLC1 decreased cell proliferation in NB cell lines (Ando et al., 2008). These results indicated that TSLC1 is a good NB tumor suppressor candidate. Interestingly, a recent study indicates that expression of both KIF1Bβ and TSLC1 is controlled by the polycomb protein Bmi1, whose expression is regulated by N-myc (Ochiai et al., 2010).

3.3.3. LOH of 14q

Loss of the long arm of chromosome 14 is also commonly found in NB primary tumors (~16–27% of the patients). LOH on chromosome 14q was first identified in 1989 using a polymorphic DNA marker which detected allelic deletion at specific 14q23 loci (Suzuki et al., 1989). LOH analysis of 14q in a large number of primary tumors using 11 polymorphic DNA markers found 14q LOH in 83 of 372 tumors (22%) (Thompson et al., 2001). 14q LOH was highly correlated with 11q loss and had an inverse relationship with 1p loss and MYCN amplification (Thompson et al., 2001). However, LOH for 14q was present in tumors from all clinical stages, suggesting this abnormality may be a universal early event during tumor development.

In addition, to the genetic changes described above, there have also been reports of LOH and/or allelic imbalance at chromosome arms 2q, 3p, 4p, 9p, and 19q (Caron, 1996; Ejeskar et al., 1998; Marshall et al., 1997; Mora et al., 2001; Takita et al., 2001), however, the significance of these genetic changes is not clear.

4. The Role of Neurotrophins and Growth Factors in the Development of the Sympathetic Nervous System and in NB

As the neural crest cells migrate to the aorta but prior to reaching the adrenal medulla they begin to express TH which in turn controls the expression of other enzymes needed for catecholamine biosynthesis as described above in Section 2.3. Since NB appears to arise from cells that are transformed at various times during this migration, the majority of NB tumors secrete catecholamines. Indeed, the presence of high levels of cate-cholamines in patient urine samples is used as one of the diagnostic criteria for the disease (LaBrosse et al., 1976). Based on this data, Sawada began to screen the urine of 6-month-old infants for increased catecholamine metabolites from 1984 and found that the incidence of in situ NB was much higher than the number of sporadic cases that had been observed previously (Sawada, 1992). These data agree with a previous hypothesis of Beckwith and Perrin who postulated that during the development of sympathetic neurons the incidence of in situ NB is higher than the incidence of sporadic cases (Beckwith and Perrin, 1963). Most of these in situ NBs spontaneously regress as the child ages (Brodeur, 2003; Maris, 2005; van Noesel and Versteeg, 2004), suggesting they are resolved using normal developmental programs. Developmental studies and studies from knockout mice suggest the TrkA is crucial for the development of many sympathetic lineage cell types (Brodeur et al., 2009). This is consistent with data indicating the NGF is required for the differentiation and survival of many sympathetic lineage cells (Nakagawara, 2004; Nakagawara and Ohira, 2004). In addition to NGF and TRKs several other growth factors also play roles in the development of the sympathoadrenal lineage cells. These include EGF which is expressed in neural crest cells and is thought to contribute to the formation of neuron and melanocytes at later point during neural crest migration (Erickson and Turley, 1987), vascular endothelial growth factor (VEGF) which is expressed by surrounding cells (McLennan et al., 2010), and IGFI and IGFII which is expressed in the neural crest, the dorsal root, the sympathetic ganglia, and the adrenal medulla (Coppola et al., 2009; D’Ercole et al., 1996).

4.1. Neurotrophin receptors

Although the steps in the transformation of sympathetic neuroblasts to NB cells is not clear, extensive evidence suggests that neurotrophin receptors are involved in NB tumorigenesis and in development of the nervous system and sympaticoadrenal lineage cells. The neurotrophin receptors TrkA, TrkB, and TrkC, are encoded by the NTRK1, 2, 3 genes, respectively. Upon binding to their ligands, NGF, brain-derived neurotrophic factor (BDNF), neurotro-phin-3, respectively, these receptors regulate proliferation, survival, and differentiation in normal neuronal cells (Brodeur et al., 2009; Maris and Matthay, 1999; Maris et al., 2007; Straub et al., 2007). These receptors all associate with p75, a low affinity receptor that may enhance the binding of ligand to the Trk proteins or alter the function of the Trk receptors (Brodeur et al., 2009; Maris and Matthay, 1999; Maris et al., 2007; Straub et al., 2007). High levels of TrkA, in association with very low/no N-myc expression, are detected in favorable NB tumors which often spontaneous regress (Nakagawara, 1993; Nakagawara et al., 1992). These favorable NB tumors cells usually express a small amount of NGF as do some of the surrounding cells (Nakagawara, 1993). Cells that express the most NGF are thought to undergo differentiation, while those that express less NGF undergo apoptosis (Nakagawara, 1993). TrkA expression is dramatically decreased in MYCN amplified NB tumors (Nakagawara et al., 1992). Thus, the TrkA/NGF pathway could play an important role in determining the ability of these favorable NB tumors to differentiate or to regress in response to the microenvironment. While in general TrkA expression is correlated with a good prognosis, a novel TrkA splice variant has been found in advanced-stage tumors with adverse biological features (Tacconelli et al., 2004). This TrkA isoform is constitutively active and promotes cell survival and angiogenesis independently of NGF expression. NGF signaling may also be linked to IGFII expression since a study in which SHSY5Y cells were transfected with TrkA found increased expression of IGFII in response to NGF binding to the transfected receptor (Kim et al., 1999).

In contrast, TrkB is preferentially expressed in clinically unfavorable NB tumors and expression of TrkB strongly correlates with MYCN amplification (Nakagawara et al., 1994). The TrkB ligand, BDNF, is also highly expressed in these tumors. Coexpression of ligand and receptor may form an autocrine loop to enhance survival, metastasis, and drug resistance (Douma et al., 2004; Nakagawara, 1994). Interestingly, a truncated form of TrkB which lacks tyrosine kinase activity is expressed in some favorable NB tumors (Ho et al., 2002).

Finally, TrkC is commonly expressed in favorable NB tumors. These tumors also express very limited amount of the TrkC ligand neurotrophin-3 and coexpress TrkA (Svensson et al., 1997; Yamashiro et al., 1996).

4.2. Other growth factors and growth factor receptors

In addition to NGF and BDNF several other growth factors also play roles in the development of the sympathoadrenal lineage cells and NB tumorigenesis. These include EGF, VEGF, and insulin-like IGFI and IGFII. EGF receptor 1 (EGFR1) expression is found on both primary NB tumors and tumor-derived cell lines (Ho et al., 2005). Binding of EGF to EGFR1 causes receptor autophosphorylation and increases proliferation via effects on the MAPK and PI3K/AKT pathways (Henson and Gibson, 2006). Exogenous VEGF also stimulates the PI3K/AKT pathway and increases expression of survivin, an antiapoptotic gene in NB cells (Beierle et al., 2005). Endocrine-derived VEGF has also been shown to play a role in the proliferation and differentiation of neural crest cells during development and to promote NB cell growth. IGF1 receptors (IGF1Rs) are expressed in the majority of NB primary tumors (Martin et al., 1992). This receptor binds both IGFI and IGFII suggesting that these growth factors are important for NB tumorigenesis. Expression of IGF1R activates the PI3K/AKT and MAPK pathways and enhances cellular proliferation, cell survival, migration, and invasion, and induces chemotherapeutic resistance and reduced response to other apoptotic stimuli (Valentinis and Baserga, 2001). IGFII has been reported to be upregulated upon TRKA activation and downregulated upon TRKA overexpression suggesting potential feedback loops between these proteins (Kim et al., 1999). In addition, IGF1R is transcriptionally activated by N-myc and in turn high IGF1R levels induce N-myc protein and mRNA expression suggesting the presence of an amplification loop that enhances the ability of these proteins to promote tumorigenesis (Chambery and Mohseni-Zadeh, 1999). Inhibition of IGF1R signaling has also been shown to increase N-myc phosphorylation by GSK-3β which inactivates N-myc and enhances N-myc turnover resulting in decreased cell growth both in culture and in mice model systems (Coulter et al., 2009). IGF1R has also been shown to enhance NB metastasis to bone most likely due to its ability to enhance migration and invasion and to the presence of IGF ligand in bone marrow (van Golen et al., 2006). In addition, IGF increases cellular survival under hypoxic conditions via increased expression of hypoxia-inducing factor 1α (HIF1α) and VEGF (Treins et al., 2005).

5. Programmed Cell Death (PCD; Apoptosis) in Development of NB

5.1. The role of cell death in development

Another important process during development of the peripheral nervous system is PCD, also known as apoptosis. This process is used during development to eliminate redundant cells, control cell number, and for remodeling and repair. Cell death also occurs in the developing peripheral nervous system in response to loss of essential growth factors and cytokines (De Zio et al., 2005). Neural crest development therefore is a balance between proliferation, cell death, migration, and differentiation. Errors in any of these processes may leave the cell more prone to transformation and potentially increase tumorigenesis. This is especially true of pediatric cancers, such as NB, since these cancers develop during normal development. Indeed, stage 4S NB spontaneously regresses with little to no intervention (Blaschke et al., 1998; De Zio et al., 2005; Johnsen et al., 2009). Tumors of this stage are present in younger children and the prognosis is good. It is unknown why these tumors suddenly die or cease to grow, although it is thought that apoptosis is likely to be involved in the disappearance of these tumors (Brodeur, 2003; Maris et al., 2007; Schor, 1999). Several hypotheses have been suggested to explain the regression of these tumors: (1) these tumors are dependent on growth factors or other proteins that are present in low levels and that once a tumor reaches a certain size the factors are depleted and the tumor undergoes apoptosis, (2) as the child develops, the tumor is recognized by the immune system and destroyed, and/or (3) that the cells become responsive to the environmental cues and developmental regulatory apoptotic pathways as they mature which triggers apoptosis. Though any of these possibilities may be the case, there is sparse literature investigating apoptotic pathways in the developing trunk crest as well as the peripheral nervous system. Although it is known that crest cells from rhombomeres 3 and 5 in the chicken and mammals undergo PCD (Kulesa et al., 2004; Morales et al., 2005) and that expression of Snail in the early migratory crest cells protects the migratory crest cells form apoptosis (Vega et al., 2004), a more detailed investigation regarding specific cell death programs in crest cells contributing to the developing sympathoadrenal lineage requires more characterization.

5.2. The role of apoptosis genes in NB

As mentioned above, apoptosis or PCD in multicellular organisms is a tightly regulated process required for normal growth, development, and cellular specialization (Danial and Korsmeyer, 2004; Hengartner, 2000). Defective expression of proteins and aberrant function of constituents in the apoptotic cascade have been implicated in oncogenesis, tumor progression, and treatment resistance (Fesik, 2005; Kaufmann and Vaux, 2003; Reed and Tomaselli, 2000). There are two major apoptotic pathways in mammalian cells: the death receptor (or extrinsic) pathway and the mitochondrial (or intrinsic) pathway. Caspases (cysteine proteases) are the central components of the apoptotic machinery. At least 14 distinct caspases have been identified in mammals and are grouped into three categories (nonapoptotic, initiators, and effectors; Danial and Korsmeyer, 2004; Shi, 2004). Importantly for this review, alterations in caspase-8 expression have been observed in NB.

5.2.1. Caspase-8

Human caspase-8 (also known as FLICE) is encoded by the CASP8 gene which is located on chromosome 2q33–2q34 (Grenet et al., 1999). Caspase-8 exists as a monomer in the cell and requires dimerization or oligomerization for its activation (Boatright and Salvesen, 2003; Salvesen and Abrams, 2004). Subsequent cleavage events, although not essential for activity, further stabilize the activated protein (Boatright and Salvesen, 2003; Boatright et al., 2003; Salvesen and Abrams, 2004). Activated caspase-8 is an important initiator caspase in the death receptor-mediated apoptotic pathway. The death receptor pathway is triggered by members of the death receptor superfamily (FasR, TNFRI, DR5, etc.; Hengartner, 2000). Binding of the ligand (e.g., FasL) to its receptor (FasR) induces the formation of the death-inducing signaling complex (DISC) containing death receptors, adaptor proteins and procaspase-8 (Danial and Korsmeyer, 2004; Hengartner, 2000). Procaspase-8 then dimerizes, resulting in activation and cleavage (Danial and Korsmeyer, 2004; Hengartner, 2000; Shi, 2004). The active enzyme subsequently cleaves downstream effector caspases, resulting in their activation and leading to apoptosis (Danial and Korsmeyer, 2004; Hengartner, 2000; Shi, 2004). Active caspase-8 can also cleave Bid, which is a Bcl2 family member. Cleaved Bid, termed t-Bid, translocates to the mitochondria and promotes cytochrome c release, thereby activating the mitochondrial pathway (Danial and Korsmeyer, 2004; Hengartner, 2000; Shi, 2004).

Our laboratory first found that caspase-8 is deleted or more commonly, silenced in most NB cell lines and patient tumor samples. We further identified a region within the CASP8 gene which is methylated (Teitz et al., 2000). This finding has subsequently been verified by several groups (Fulda and Debatin, 2006; Fulda et al., 2001; Teitz et al., 2000; Yang et al., 2007). Several additional genes with roles in tumorigenesis are also hyper-methylated in NB including the PCDHB gene family, BLU, TSP-1, RASSFIA, TIG1, HIN-1, DcR1, DcR2, DcR4, etc. (Abe et al., 2005; Astuti et al., 2001; van Noesel et al., 2002; Yang et al., 2003, 2007). Among these, CASP8, PCDHB, BLU, DcR2, and HIN-1 are found to be associated with high-risk factors and poor outcome in NB patients (Abe et al., 2007; Yang et al., 2007). Methylation of this region of the CASP8 gene has been correlated with decreased caspase-8 protein expression in many (Fulda et al., 2001; Hopkins-Donaldson et al., 2000; Teitz et al., 2000), but not all studies (Banelli et al., 2002; van Noesel et al., 2002). However, even in studies where methylation has not been correlated with expression, decreased caspase-8 protein levels are observed in 50–70% of NB patient tumors. Since our initial discovery in NB, loss of caspase-8 expression has been found in many other tumors, including peripheral neuroectodermal tumors, medulloblastoma, glioblastoma multiforme, rhabdomyosarcoma, retinoblastoma, small cell lung carcinoma, and Wilms tumors (Gonzalez-Gomez et al., 2004; Harada et al., 2002; Hopkins-Donaldson et al., 2003). Caspase-8 mutations in adult gastric and colorectal tumors have also been reported (Kim et al., 2003; Soung et al., 2005). Finally, loss of caspase-8 expression has been correlated with poor prognosis in medulloblastoma (Pingoud-Meier et al., 2003) and with relapse and aggressive metastatic disease in glioblastoma multiforme (Martinez et al., 2007) A role for caspase-8 in tumorigenesis is also supported by recent studies demonstrating increased transformation of caspase-8 null SV40 immortalized mouse embryo fibroblasts (Krelin et al., 2008). In addition, we have demonstrated that the loss of caspase-8 expression has biological significance in NB cell metastasis (Lahti et al., 2006; Stupack et al., 2006; Teitz et al., 2006). Loss of caspase-8 increases metastasis by blocking integrin-mediated death, a caspase-8-dependent process (Lahti et al., 2006; Stupack et al., 2006; Teitz et al., 2006). The role of caspase-8 in metastasis will be discussed in more detail in the Section 6.2 describing proteins that play a role in NB metastasis.

Multiple groups have shown that caspase-8 deficient NB cells are resistant to death receptor signals and some chemotherapeutic drugs. Importantly, these defects can be corrected by reexpression of caspase-8 via demethylating agents, IFN-γ treatment and caspase-8 expressing retroviral vectors. However, in a few instances secondary defects in death receptor proteins and/or signaling pathway or increased expression of FLIP prevented restoration of the extrinsic cell death pathway (Fulda et al., 2001; Johnsen et al., 2004; Muhlethaler-Mottet et al., 2003; Teitz et al., 2000; Tekautz et al., 2006; Yang et al., 2003).

5.2.2. Bcl-2 family

Another group of proteins that play a role in NB tumorigenesis and metastasis are the Bcl-2 family members. This family of proteins plays a central role in the intrinsic apoptotic pathway. There are more than 20 genes of Bcl-2 family identified in mammals, including antiapoptotic members (Bcl-2, Bcl-XL, Bcl-W, Bcl-G, Bfl1, Mcl-1) and proapoptotic members (Bcl-XS, Bcl-B, Bax, Bak, Bad, Bid, Bik, Bok, Bim, Puma, Noxa, Nix, Nip3, Hrk, Mtd). They usually regulate apoptosis by controlling mitochondrial outer membrane permeabilization (MOMP) to promote or prevent the release of cytochrome c (Green and Kroemer, 2004; Johnsen et al., 2009; Reed, 2006).

The antiapoptotic members, such as Bcl-2, Bcl-XL, Mcl-1, are highly expressed in neural progenitor cells, indicating their protection role in neuron development. Some proapoptotic regulators of the intrinsic pathway, such as Bid and caspase-9, are expressed preferentially in favorable tumors, whereas antiapoptotic regulators such as Mcl-1 were expressed at high levels in unfavorable tumors (Abel et al., 2005), suggesting an imbalance between anti- and proapoptotic factors in NB tumors with favorable or unfavorable biology. Bcl-2 is highly expressed in the majority of NB cell lines and primary tumors and inversely correlates with the degree of cell differentiation (Ramani and Lu, 1994). Although conflicting data exists about the correlation between Bcl-2 levels and MYCN amplification or poor prognosis (Castle et al., 1993; Ikegaki et al., 1995; Mejia et al., 1998), high Bcl-2 level is considered as an important reason for chemoresistance and transfection of Bcl-2 or Bcl-XL into NB cells conferred drug resistance (Dole et al., 1994; Dole et al., 1995).

5.2.3. Regulation of apoptosis-related genes by MYCN

Although the Myc gene was originally identified as an oncogene, it is involved in various cellular processes, including cell growth, proliferation, loss of differentiation, and apoptosis (Adhikary and Eilers, 2005; Pelengaris et al., 2002; Thompson, 1998). N-myc has been found to sensitize NB cells to death receptor induced apoptosis in the absence of cytokines, growth factors or other conditions of stress (Cui et al., 2005; Fulda et al., 1999; Lutz et al., 1998). Cell death in response to MYCN results in caspase-8 cleavage which can be blocked using the caspase inhibitor zVAD-fmk (Cui et al., 2005; Fulda et al., 1999). Upregulation of p53, a direct target of MYCN, is an important mechanism of sensitization of the cell to apoptosis by N-myc (Chen et al., 2010). These data suggest that resistance of NB cells with MYCN amplification to chemotherapy requires additional dysfunction in apoptotic pathways, such as silencing of caspase-8 or inhibiting the p53 pathway. Currently, there are conflicting data on a possible direct relationship between N-myc and caspase-8. Genome wide studies looking for E-box binding sites have identified caspase-8 as a target of both N-myc and c-myc (Fernandez et al., 2003; Perini et al., 2005). Perini and coworkers further demonstrated that the E-box sites in the caspase-8 promoter are methylated in NB preventing binding of N-myc/Max heterodimers. Furthermore, when these NB cells were treated with the demethylating agent 5-azacytidine, N-myc/Max binding was restored and caspase-8 expression was increased (Perini et al., 2005). In contrast to this data which suggests a direct relationship, Fulda and coworkers failed to observe any changes in caspase-8 expression upon overexpression of N-myc or after downregula-tion of N-myc with antisense RNA; however, since these authors did not examine the methylation state of the E-box sequences in caspase-8, this study is somewhat difficult to interpret (Fulda et al., 1999). In addition to data suggesting a possible direct relationship between N-myc and caspase-8, several studies have reported that myc expression promotes activation or priming of the mitochondrial pathway which increases the sensitivity of cells to death receptor signaling. Once the pathway is activated, caspase-8 cleavage of Bid provides a further amplification loop (Klefstrom et al., 2002; Nieminen et al., 2007). Additionally others have shown that the death receptor DR5 contains two E-box sequences that bind N-myc resulting in increased expression of this receptor and further augmentation of the extrinsic pathway (Cui et al., 2005). Some proapoptotic Bcl-2 family members such as Bax and puma are transactivated by MYC, which play an important role in switching p53 downstream effects from G1 arrest to apoptosis (Seoane et al., 2002).

Although p53 is a transcriptional target of N-myc, N-myc also inhibits its function through increased levels of p53 negative regulators, such as MDM2 and TWIST. MDM2, the essential inhibitor of p53, is a direct target of N-myc and inhibition of N-myc results in decreased MDM2, stabilized p53, and apoptosis. TWIST, a transcription factor of bHLH family with antiapoptotic activity, is frequently overexpressed in MYCN amplified NB (Valsesia-Wittmann et al., 2004). Furthermore, TWIST overexpression inhibits ARF/p53 preventing apoptosis in response to N-myc overexpression (Valsesia-Wittmann et al., 2004).

6. The Role of Epithelial to Mesenchymal Transition (EMT) in Development and Metastasis

6.1. EMT in development

Although greater than 50% of all NB patients present with metastatic disease, little is known of the process and the mechanism. Microarray mRNA expression analysis studies comparing highly metastatic human NBs (stage 4) to nonmetastatic human tumors (stage 1 and 2) have provided information on some of the proteins that are involved in NB metastasis (Scaruffi et al., 2005). Of note, transcripts encoding proteins related to the developmental program of the epithelial to mesenchymal transition or EMT, are expressed at higher levels in metastatic human NB (Scaruffi et al., 2005). As mentioned above, early in development, as the neural folds join to form the neural tube, the epidermal ectoderm expresses BMP signals and the floorplate of the neural tube produces Shh. These gradients interact and result in the formation of the neural crest at the dorsal aspect of the neural tube, between the epidermal ectoderm and the dorsal neural tube. The epithelial-like cells of the former neural folds undergo the EMT program. EMT is characterized by: (1) lose of epithelial morphology, (2) downregulation of junctional complexes (E-cadherin, cytokeratin, occludins, and claudins), (3) upregulation of intracellular migratory proteins (RhoB), (4) increased expression of matrix modulators (collagenase, matrilysin, urokinase, heparanase, matrix metalloproteinases—MMP), and (5) upregulation of matrix recognition molecules (N-cadherin; Thiery et al., 2009). Metastatic cells exhibit many of these features and evidence is accumulating that several regulators of EMT are misregulated in NB (Shimono et al., 2000; Valsesia-Wittmann et al., 2004; Vitali et al., 2008).

The transcription factors Snail, Twist, and SIP1/ZEB2 are considered to be master regulators of the EMT transition. Snail is induced primarily by TGFB signaling and directly represses E-cadherin by binding of the E-boxes in the E-cadherin promoter (Kang and Massague, 2004). The repression of E-cadherin is a hallmark of the induction of the EMT program, allowing for the motility of the crest cells. Similarly Twist, a basic helix-loop-helix transcription factor developmentally regulated via the NF-κB pathway, represses E-cadherin expression via the E-boxes within the E-cadherin promoter, although this interaction has not been shown to be direct (Sosic and Olson, 2003). Snail also induces the expression of ZEB factors (inhibitors of E-cadherin expression) which are involved in an EMT regulation loop with miRNA-200 family (Bracken et al., 2008). The miRNA-200 family inhibits the expression of the ZEB family that relieves repression of E-cadherin (Bracken et al., 2008).

6.2. Metastasis-related genes

Although greater than 50% of all NB patients present with metastatic disease, little is known of the process and the mechanism. The data from our group and collaborators first indicated that loss of caspase-8 increases metastasis by blocking integrin-mediated death. Loss of caspase-8 facilitates survival in foreign environments both during development and during metastasis. This data may also explain the observation that at least 50% of NB patients present with metastatic disease (Lahti et al., 2006; Stupack et al., 2006; Teitz et al., 2006).

Another protein that is involved in NB metastasis is CD44, a cell surface glycoprotein that plays a role in cell adhesion and metastasis. CD44 is highly expressed in colon tumors where it affects tumor invasion, however, CD44 expression variable in NB tumors (Shtivelman and Bishop, 1991; Tanabe et al., 1993). There are conflicting data about the correlation between CD44 expression and MYCN amplification, however, high expression of CD44 always correlates with favorable tumors and with the presence of more differentiated cells (Combaret et al., 1996; Munchar et al., 2003). The observation that CD44 is only expressed in favorable tumors is consistent with its functions as a metastasis inhibitor (Munchar et al., 2003; Shtivelman and Bishop, 1991).

The Nm23A protein (nucleoside diphosphate kinase A) encoded by NM23-H1 gene mapped to 17q22 locus is highly expressed in unfavorable NB tumors. A point mutation (S120G) was identified in a subset of NB tumors with elevated NM23A expression (Chang et al., 1996). In addition, NM23A promotes NB metastasis in the xenograft NB animal model and S120G mutation reduced cell adhesion and increased cell migration (Almgren et al., 2004). In contrast, NM23A expression is very low in melanoma, breast, colon tumors and NM23A appears to serve as metastasis suppressor in these tumors (Bown et al., 1999). The reason for the opposite expression pattern may be due to the different regulatory mechanisms in specific tissues (Okabe-Kado et al., 2005).

The matrix metalloproteinases (MMPs), such as MMP-2 and MMP-9, are highly expressed in advanced-stage NB tumors. MMP-2 and MMP-9 promote cell invasion and metastasis by degrading extracellular matrix including type IV, V, VII, and X collagens as well as fibronectin. However, there is no correlation between these MMPs and MYCN status (Ribatti et al., 2004; Sugiura et al., 1998).

Twist1, which is originally identified as a key inducer of mesoderm formation in Drosophila, is found to play an essential role during metastasis in many types of tumors. First evidence of Twist1 involvement in metastasis came from human breast cancers. High levels of Twist1 are associated with invasive carcinoma and loss of EMT. Suppression of Twist1 inhibits the ability of cancer cells to metastasize, while overexpression of Twist1 increases cell motility and decreases cell adhesion (Yang et al., 2004). Further study demonstrated Twist1 may promote metastasis through direct induction of microRNA-10b, which inhibits homeobox D10, resulting in upregulation of a well-characterized prometastatic gene, RHOC (Ma et al., 2007). Later studies found that Twist1 also promotes migration and invasion in bladder and prostate cancer, hepatocellular carcinoma and colorectal cancer (Matsuo et al., 2009; McConkey et al., 2009; Valdes-Mora and Gomez Test, 2009). Twist1 is also overexpressed in N-myc amplified NB, where it inhibits N-myc induced apoptosis by inhibiting the p53 pathway in part via downregulation of p19^ARF (Valsesia-Wittmann et al., 2004).

7. The Role of miRNA in Development and NB

miRNAs are endogenous small noncoding RNAs of ~22 nucleotides in length that negatively regulate gene expression by mRNA cleavage or translational repression of the target mRNA (Bartel, 2004). miRNAs play an important role in regulating most cellular processes, and contribute to the process of tumorigenesis and metastasis (Zhang et al., 2010). miRNA expression profiles have been correlated with prognosis, differentiation, and apoptosis in NB tumors (Chen and Stallings, 2007), suggesting that miRNAs could function as TSGs or oncogenes in NB. As mentioned above, miR-34a, which is located at 1p36, is a good TSG candidate (Welch et al., 2007). Mir-34a is a direct target of p53 and knockdown of mir-34a reduced p53-dependent apoptosis (He et al., 2007). In addition, one study found that N-myc is a direct target of mir-34a (Wei et al., 2008), although this data is contrary to a previous study (Welch et al., 2007). The discrepancy between these two studies may reflect the slightly different systems that were used for these two studies.

Recent data showed that seven miRNAs including miR-17 to -92 cluster were induced by N-myc in vitro, and their high expression correlates with MYCN amplification in tumors (Schulte et al., 2008), suggesting this cluster is a potential oncogene in NB. This cluster, which is transcribed as a polycis-tronic transcript containing miR-17, -18, -19, -20, and -92, was first found to be a target of c-myc, a potential oncogene in B-cell lymphoma (He et al., 2005). The miR-17 to -92 cluster is also overexpressed in lung cancers where it has been shown to enhance cell proliferation (Hayashita et al., 2005). miRNAs also mediate tumor metastasis, for example, miR-10b was shown to initiate tumor invasion and metastasis in breast cancer (Ma et al., 2007).

One very recent paper compared miRNA expression patterns between primary and metastatic NB tumors and found significant changes of 54 miRNAs in metastatic samples, among which 35 miRNAs were upregulated and 19 miRNAs were downregulated (Guo et al., 2010). Some miRNAs, such as miR-10b, miR-29a/b, miR-335, which are known to promote metastasis were upregulated in metastatic NB tumors. In contrast, miR-7, miR-338-3p, and the let-7 family were the three of the top 10 downregulated miRNAs in the metastatic group. These miRNAs have been shown to play antimetastatic roles in other tumors (Zhang et al., 2010). The authors also analyzed the predicted miRNA targets of these miRNAs and found that many of these targets are related to metastasis. For instance, both caspase-8 and integrin beta1 are the predicted targets of miR-29a and miR-29b, miRNAs that promote metastasis in breast cancer (Gebeshuber et al., 2009). However, the actual function of these identified miRNAs in metastasis needs to be investigated.

8. Other Important Genes in NB

8.1. Telomerase

Telomerase is a specialized ribonucleoprotein polymerase that synthesizes the TTAGGG telomeric repeats found at the end of chromosomes to maintain the length of the telomere. This enzyme is expressed in germ line cells but not in the majority of somatic cells. Thus, telomeres in somatic cells undergo progressive shortening and eventually lose the ability to protect chromosome ends, resulting in cell senescence and/or death. Increased telomerase expression, which results in unlimited cell replication and repression of cell senescence, is found in many tumors (Bodnar et al., 1998; Hahn et al., 1999). Telomerase activity was detected in most NBs, but not in ganglioneuromas or normal adrenal tissue (Hiyama et al., 1997). In addition, high telomerase activity usually correlated with MYCN amplification and poor outcome, suggesting telomerase could be a prognostic marker for poor survival (Hiyama et al., 1997; Ohali et al., 2006; Reynolds et al., 1997).

8.2. MDR1 and MRP gene family

Most NBs exhibit a strong initial response to chemotherapy followed by the appearance of multidrug resistance (MDR) in more than half of cases with remission/remaining tumor (Keshelava et al., 2001; Tweddle et al., 2003; Xue et al., 2007). Although p53 seems to have an important role, the MDR1 gene (multidrug resistance gene 1) and the MRP (multidrug transporter MDR-associated protein) gene family also contribute to this resistance. MDR1 expression was found to be increased after treatment of NB tumors (Bourhis et al., 1989; Chan et al., 1991). In one report, MDR1 expression was inversely correlated with MYCN amplification and poor outcome (Chan et al., 1991), while other groups did not find this correlation (Dhooge et al., 1997; Kutlik et al., 2002). Likewise, some studies found that expression of MRP gene family members such as MRP1, MRP4, strongly correlated with MYCN expression and chemoresistance in NBs (Haber et al., 2006; Norris et al., 1996, 2005), while other studies failed to find a correlation (De Cremoux et al., 2007; Goto et al., 2000).

8.3. GD2 and Bmi-1

Several other genes are also abnormally expressed in NB tumors such as GD2 and Bmi-1. The ganglioside GD2 is a glycolipid that is most commonly expressed in the majority of NB tumors, thus representing a good diagnostic marker and therapeutic target. Indeed recent studies using an anti-GD2 antibodies in association with GMC-SF and IL2 and RA results in a significant increase in event free survival (2-year estimates of 66% ±5% vs. 46% ±5%), and preliminary data suggest in overall survival (86% ±4% vs. 75% ±5% at 2 years p = 0.0223) (Yu et al., 2009). Bmi-1, a polycomb ring finger gene, was found to be strongly expressed in primary NB tumors in 2006 and very recent data indicated that it is a MYCN target gene that promotes tumorigenesis through inhibition of two potential TSGs (KIF1Bβ and TSCL1) in NB cells (Nowak et al., 2006; Ochiai et al., 2010).

9. Clinical Treatment Overview

Current treatment for NB consists of surgery, chemotherapy, radiation, and biotherapy. The clinical strategy usually depends on a patient’s risk stratification (Table 4.1). For examples, exposure to chemotherapy is generally limited for low risk group patients, whereas, high-risk group patients are treated with multiagent chemotherapy to reduce the overall burden of the disease before the surgical removal of the primary tumor (Haase et al., 1999; Park et al., 2008).

9.1. Low risk group and intermediate risk group

Low risk group encompasses diseases at stages 1, 2, and 4S with favorable characteristics (Brodeur, 2003; Castleberry et al., 1997; Matthay, 1995). Most low risk group patients at stages 1 or 2 are successfully treated with surgery alone and complete resection of the tumor is the goal. The chances of these tumors recurring or progressing to advanced-stage NB are very low and chemotherapy is reserved for those patients with recurrences. NBs at stage 4S without MYCN amplification almost always spontaneously regresses and these patients can be safely observed without any treatment. The survival rate of patients of this group is greater than 95% (Alvarado et al., 2000; Park et al., 2008; Simon et al., 2004).

Intermediate risk group patients include stage 3 patients of any age with favorable features, stage 4 infants with favorable features, and 4S patients without MYCN amplification and with unfavorable histology (Brodeur, 2003; Castleberry et al., 1997; Matthay, 1995). For intermediate risk group patients, surgery, and moderate dose of multiagent chemotherapy are the basic therapeutic strategies. Sometimes radiation is also used to remove the residual tumors. Aggressive chemotherapy is utilized for patients who respond poorly to initial treatment or experience recurrence. Current treatments for intermediate risk group patients have a cure rate about 70–90% (Matthay et al., 1998; Park et al., 2008; Schmidt et al., 2000).

9.2. High-risk group patients

High-risk group consist of patients with unfavorable biological features at stages 2, 3, 4, and those with MYCN amplification at stage 4S (Brodeur, 2003; Castleberry et al., 1997; Matthay, 1995). The standard clinical strategy for this group is comprised of 4 steps: initial induction chemotherapy, local control, consolidation, and biology therapy (Haase et al., 1999; Park et al., 2008). Initial induction chemotherapy consists of combinations of chemo-therapeutic agents such as cisplatin, etoposide, doxorubicin, cyclophosphamide, topectan/ironotecan, and vincristine. Following the completion of induction therapy, local control is achieved by aggressive surgical resection of the primary tumor followed by external beam radiation. After that, consolidation is provided by high-dose chemotherapy and autologous hematopoietic stem cell rescue, using stem cells that are prepared during the induction therapy. The purpose of consolidation therapy is to eliminate any remaining tumors. Consolidation therapy typically employs agents such as carboplatin, etoposide, and melphalan. Sometime focal radiotherapy is also applied to the primary tumor sites. After recovery from the consolidation, patients receive biological therapy to eradicate minimal residual disease because relapse is a frequent occurrence after autologous transplantation in this group of patients. The most common used biological agent is cis-retinoic acid (cis-RA), which is a noncytotoxic differentiation inducer. The use of monoclonal antibodies against tumor-specific antigens, such as GD2, provides an alternative and promising therapy to eliminate minimal residual tumor cells as described above (Matthay et al., 1999; Pearson et al., 2008; Zage et al., 2008).

Even with these complicated and aggressive treatments, the overall cure rate for high-risk patients is only about 30% during the last two decades (Matthay et al., 1999; Pearson et al., 2008; Zage et al., 2008). Therefore, the development of new agents and methods to more effectively treat these patients is underway. These therapeutic approaches include immunetherapy aimed at NB-specific antigens, targeted delivery of radioactive molecules to NB, or new chemotherapy such as retinoids for inducing differentiation, tyrosine kinase inhibitors targeted at ALK, demethylating agents and HDAC inhibitors for inducing reexpression of caspase-8 or other epigenetically silenced genes (Kelleher and McDermott, 2010; Schor, 2009; Wagner and Danks, 2009; Witt et al., 2009).

10. Conclusion

The investigation and identification of genomic abnormalities and gene expression changes has improved the understanding of the molecular basis of biological and clinical characteristics of NBs (summary in Tables 4.2 and 4.3). Although great progress has been made in recent 20 years, much work needs to be done to identify tumor-specific targets for therapy. Also, deeper understanding of the development of normal sympathetic nervous system will help us find the important abnormal events that initiate NB development. In conclusion, more precise identification of molecular alterations should allow more effective and less toxic therapies with improved cure rate.

Table 4. 3.

Summary of genetic changes in neuroblastoma

Gene symbol	Gene name	Chromosomal locus	Gene function	Gene alterations	References
ALK	Anaplastic lymphoma receptor tyrosine kinase	2p23	Receptor tyrosine kinase	Mutation/amplification	Caren et al. (2008), Chen et al. (2008), George et al. (2008), Janoueix-Lerosey et al. (2008), Mosse et al. (2008), Passoni et al. (2009)
Bcl2	B-cell CLL/lymphoma 2	18q21.3	Apoptosis suppression	High expression	Abel et al. (2005), Ramani and Lu (1994), Castle et al. (1993), Ikegaki et al. (1995), Mejia et al. (1998)
Bmi1	BMI1 polycomb ring finger oncogene	10p11.23	Oncogene	Overexpression	Ochiai et al. (2010), Nowak et al. (2006)
Casp8	Caspase 8	2q33–q34	Apoptosis/metastasis	Silence/deletion	Teitz et al. (2000, 2006), Yang et al. (2007), Fulda et al. (2001), Fulda and Debatin (2006), Lahti et al. (2006), Stupack et al. (2006)
CD44	CD44 molecule	11p13	Integrin/antimetastasis	Expression in favorable tumors	Munchar et al. (2003), Combaret et al. (1996)
CHD5	Chromodomain helicase DNA-binding protein 5	1p36.31	Helicase/tumor suppressor	Deletion/low expression	Bagchi et al. (2007)
DDX1	ATP-dependent RNA helicase DDX1	2p24	RNA helicase/Oncogene	Amplification	George et al. (1996)
B4GALNT1	GM2/GD2 synthase	12q13.3	Ganglioside	High expression	Yu et al. (2009)
KIF1b	Kinesin family member 1beta	1p36.2	Kinesin/tumor suppressor	Deletion/low expression	Munirajan et al. (2008), Schlisio et al. (2008)
MDM2	Mdm2 p53-binding protein homolog	12q14.3–q15	Oncogene	Amplification	Corvi et al. (1995a)
ABCB1	Multidrug resistance protein 1 (MRP1)	7q21.12	Multidrug resistance	High expression	Bourhis et al. (1989), Chan et al. (1991), Dhooge et al. (1997), Kutlik et al. (2002)
Mir34a	MicroRNA 34a	1p36.22	Micro RNA/tumor suppressor	Deletion/low expression	Welch et al. (2007), Chen and Stallings (2007)
Mir17–92	MicroRNA cluster 17–92 (mir17, 18, 19, 20, 92)	13q31.3	Micro RNA/Oncogene	High expression	Schulte et al. (2008)
MMP2, MMP	Matrix metallopeptidase 2	16q13–q21	Proteinase/metastasis	High expression	Sugiura et al. (1998), Ribatti et al. (2004)
MMP9	Matrix metallopeptidase 9	20q11.2–q13.1	Proteinase/metastasis	High expression	(Sugiura et al. (1998), Ribatti et al. (2004))
ABCC1	Multidrug resistance-associated protein 1	16p13.1	Multidrug resistance	High expression	Norris et al. (1996, 2005), Haber et al. (2006), Goto et al. (2000), De Cremoux et al. (2007)
MYCL	Myc-related gene from lung cancer	1p34.2	Oncogene	Amplification	Jinbo et al. (1989)
MYCN	Neuroblastoma MYC oncogene	2p24.1	Oncogene	Amplification	Schwab et al. (1983, 1984), Kohl et al. (1983), Pelengaris et al. (2002), Reiter and Brodeur (1996, 1998)
NME1	Metastasis inhibition factor NM23	17q22	Nucleoside kinase/antimetastasis	Overexpression	Godfried et al. (2002), Almgren et al. (2004), Chang et al. (1996)
PHOX2B	Paired-like homeobox 2b	4p13	Neuron development	Mutation	Trochet et al. (2004), Mosse et al. (2004)
TERT	Telomerase reverse transcriptase	5p15.33	Telomere maintenance	High expression	Hiyama et al. (1997), Reynolds et al. (1997), Ohali et al. (2006)
NTRK1	Neurotrophic tyrosine kinase receptor 1 (TrKA)	1q21–q22	Receptor tyrosine kinase	Inverse correlation with Mycn	Nakagawara et al. (1992), Nakagawara (1993), Tacconelli et al. (2004), Kim et al. (1999)
NTRK2	Neurotrophic tyrosine kinase receptor 2 (TrKB)	9q22.1	Receptor tyrosine kinase	Strong correlation with Mycn	Nakagawara et al. (1994), Nakagawara (1994), Douma et al. (2004), Ho et al. (2002)
NTRK3	Neurotrophic tyrosine kinase receptor 3 (TrKC)	15q25	Receptor tyrosine kinase	Coexpression with TrkA	Yamashiro et al. (1996), Svensson et al. (1997)
CADM1	Tumor suppressor in lung cancer 1 (TSLC1)	11q23.2	Cell adhesion/Tumor suppressor	Deletion/low expression	Gomyo et al. (1999), Ando et al. (2008)
TWIST1	Twist homolog 1	7p21.2	Apoptosis/metastasis	High expression	Valsesia-Wittmann et al. (2004)