Melanocytes and Their Diseases

Abstract

Human melanocytes are distributed not only in the epidermis and in hair follicles but also in mucosa, cochlea (ear), iris (eye), and mesencephalon (brain) among other tissues. Melanocytes, which are derived from the neural crest, are unique in that they produce eu-/pheo-melanin pigments in unique membrane-bound organelles termed melanosomes, which can be divided into four stages depending on their degree of maturation. Pigmentation production is determined by three distinct elements: enzymes involved in melanin synthesis, proteins required for melanosome structure, and proteins required for their trafficking and distribution. Many genes are involved in regulating pigmentation at various levels, and mutations in many of them cause pigmentary disorders, which can be classified into three types: hyperpigmentation (including melasma), hypopigmentation (including oculocutaneous albinism [OCA]), and mixed hyper-/hypopigmentation (including dyschromatosis symmetrica hereditaria). We briefly review vitiligo as a representative of an acquired hypopigmentation disorder.

At least 350 genetic loci are involved in pigmentation in mice. Many of these are associated with human hyperpigmentation (e.g., melisma), hypopigmentation (e.g., oculocutaneous albinism), or mixed hyper-/hypopigmentation.

Pigments that determine human skin colors include melanin, hemoglobin (red), hemosiderin (brown), carotene (yellow), and bilin (yellow). Among those, melanins play key roles in determining human skin (and hair) pigmentation. Melanin pigments can be classified into two major types based on their biosynthetic pathways, as updated and reviewed elsewhere: eumelanin (dark brown and black) and pheomelanin (yellow, red, and light brown) (Simon et al. 2009; Hearing 2011; Kondo and Hearing 2011). Eu-/pheo-melanin pigments are produced and deposited in melanosomes, which belong to the LRO (lysosome-related organelle) family in that they contain acid-dependent hydrolases and lysosomal-associated membrane proteins (Raposo and Marks 2007). Melanosomes can be divided into four stages depending on their degree of maturation. Early melanosomes, especially stage I melanosomes, are similar to lysosomes whereas late melanosomes contain a structured matrix and highly dense melanin deposits. Studies of melanosomes are not only performed in medicine but also in archaeology because various morphologies of melanosomes remaining in fossils serve as clues to hypothesize the colors of dinosaurs (Li et al. 2012).

Melanocytes can be defined as cells that possess the unique capacity to synthesize melanins within melanosomes. Factors related to melanin production within melanocytes can be divided into three groups as previously reviewed: structural proteins of melanosomes, enzymes required for melanin synthesis, and proteins required for melanosome transport and distribution (Yamaguchi and Hearing 2009). We briefly update the recent findings regarding pigmentation-related factors.

Disruptions of the functions of many pigmentation-related factors are known to cause pigmentary disorders and a curated list of those are summarized and updated at the homepage of the European Society for Pigment Cell Research (www.espcr.org/micemut). Those disorders include hyperpigmentation, hypopigmentation, and mixed hyper-/hypopigmentation disorders, which are subdivided into congenital or acquired status (Table 1). Their diagnosis depends on the size, location (involved site(s) of the body), and morphology (isolated, multiple, map-like, reticular, or linear) of the lesions. Hypopigmentation disorders are subclassified into those associated with complete or incomplete depigmentation.

Table 1.

Pigmentary disorders and possible responsive genes

Representative disease	Disease brief description	Locus	Mechanism(s) of action
Hyperpigmentation disorder
Congenital
Generalized lentiginosis	Widespread lentigines without associated noncutaneous abnormalities	Chromosome 4q21.1-q22.3
LEOPARD syndrome	Multiple lentigines, congenital cardiac abnormalities, ocular hypertelorism, and retardation of growth	PTPN11	Protein tyrosine phosphatase, nonreceptor type 11
Carney complex	A multiple neoplasia syndrome characterized by spotty skin pigmentation, cardiac and other myxomas, endocrine tumors, and psammomatous melanotic schwannomas	PRKAR1A	Protein kinase A regulatory subunit 1α
Peutz–Jeghers syndrome	Pigments on lips and palmoplantar area	STK11/LKB1	Serine/threonine kinase 11
Others
Nevus cell nevus, Spitz nevus, Nevus spilus, blue nevus, nevus Ohta, dermal melanosis, nevus Ito, Mongolian spot, ephelides, acropigmentio reticularis, Spitzenpigment/acropigmentation, inherited patterned lentiginosis, Laugier–Hunziker–Baran syndrome, Cronkhite–Canada syndrome
Acquired
Melasma/chloasma	Symmetric malar brownish hyperpigmentation	WIF-1 and others	Wnt inhibitory factor-1/Wnt and lipid-metabolism-related genes
Others
Senile lentigines/lentigo, Riehl's melanosis, labial melanotic macule, penile/vulvovaginal melanosis, erythromelanosis follicularis faciei (Kitamura), pigmentation petaloides actinica tanning, postinflammatory pigmentation, chemical/drug-induced pigmentation, pigmentary demarcation lines, foreign material deposition
Hyperpigmentation related with systemic disorders and others
Mastocytosis	Darier's sign	KIT and others	Urticaria pigmentosa
Neurofibromatosis type 1	Neurofibromas and café-au-lait spots/von Recklinghausen's disease	NF1	RAS GTPase-activating protein; Neurofibromin 1
Sotos syndrome	Tall stature, advanced bone age, typical facial abnormalities, and developmental delay	NSD1	Nuclear receptor binding SET domain protein 1
POEMS syndrome	Polyneuropathy, organomegaly, endocrinopathy, M-protein, and skin changes	VEGF and others	Vascular endothelial growth factor and angiogenetic factors
Cantu syndrome	Hypertrichosis, macrosomia, osteochondrodysplasia, and cardiomegaly	ABCC9	ATP-binding cassette, subfamily C, member 9
McCune–Albright syndrome	Clinical triad of fibrous dysplasia of bone, cafe-au-lait skin spots, and precocious puberty	GNAS	Guanine nucleotide-binding protein, α stimulation/stimulatory G protein
Bloom syndrome	Photosensitivity and increased risk of malignancy	BLM	RecQ helicase family
Others
Naegeli syndrome, Watson syndrome, metabolism/enzyme disorders, endocrine disorders, nutritional disorders, collagen diseases, liver dysfunction, kidney dysfunction, infectious diseases
Hypopigmentation disorder
Congenital
Oculocutaneous albinism type 1	Hypopigmentation, nystagmus	TYR GPR143	Melanosomal enzyme G-protein-coupled receptor (GPR143); melanosome biogenesis signal transduction
Oculocutaneous albinism type 2		OCA2	Melanosome biogenesis and size
Oculocutaneous albinism type 3		TYRP1	Melanosomal enzyme; stabilizing factor
Oculocutaneous albinism type 4		SLC45A2	Solute transporter; previously named as membrane-associated transporter protein (MATP)
Hermansky–Pudlak syndrome type 1	Hypopigmentation, bleeding caused by thrombopenia	HPS1	Membrane protein; organelle biogenesis and size
Hermansky–Pudlak syndrome type 2		AP3B1	β3 Subunit of adaptor protein 3 complex; organellar protein routing
Hermansky–Pudlak syndrome type 3		HPS3	Organelle biogenesis
Hermansky–Pudlak syndrome type 4		HPS4	Organelle biogenesis and size
Hermansky–Pudlak syndrome type 5		HPS5	Biogenesis of lysosome-related organelles complex-2
Hermansky–Pudlak syndrome type 6		HPS6	Biogenesis of lysosome-related organelles complex-2
Hermansky–Pudlak syndrome type 7		DTNBP1	Dysbindin, component of the biogenesis of lysosome-related organelles complex-1 (BLOC1)
Hermansky–Pudlak syndrome type 8		BLOC1S3	Component of the BLOC1 protein transport complex
Hermansky–Pudlak syndrome type 9		PLDN	Vesicle-docking and fusion
Chediak–Higashi syndrome	Hypopigmentation, infection caused by immunodeficiency	LYST	Organelle biogenesis and size; membrane protein
Griscelli syndrome type 1	Hypopigmentation, hepatosplenomegaly, pancytopenia, immunologic disorder, and central nervous system abnormalities	MYO5A	Melanosome transport; myosin type Va/dilute mice
Griscelli syndrome type 2		RAB27A	Melanosome transport; RAS-associated protein/ashen mice
Griscelli syndrome type 3		MLPH	Melanosome transport; melanophilin/leaden mice
Phenylketonuria	Phenylalanine hydroxylase deficiency	PAH	Phenylalanine hydroxylase
Piebaldism	White spotting, megacolon, and other neural crest defects	KIT	Receptor for SCF; required for melanoblast survival and homing
Waardenburg syndrome type 1 and 3	White spotting and small or absent eyes	PAX3	Transcription factor; neural tube development
Waardenburg syndrome type 2	White spotting, head blaze, pale hair and skin, neural crest, and other organ defects	MITF SNAI2	Transcription factor; master regulator of melanocyte lineage transcription factor
Waardenburg–Shah syndrome	White spotting, megacolon, and other neural crest defects	EDN3 SOX10	Melanoblast/neuroblast growth and differentiation factor; transcription factor
Hypomelanosis of Ito	Hypopigmentation along Blaschko lines/neural disorders	Duplication of Xp11.3-p11.4 and random X inactivation
Hirschsprung's disease type 2	White spotting, megacolon, and other neural crest defects	EDNRB	Endothelin receptor B
Fraser syndrome	Microphthalmia/anophthalmia, patches of discolored or white fur	FREM2	Extracellular protein related with epithelial–mesenchymal interactions
Charcot–Marie–Tooth disease type 4J	Pale skin, alopecia, clumped melanosomes, and immune effects	FIG4	Phosphatidylinositol-(3,5)-bisphosphatase 5-phosphatase; late endosome-lysosome axis
Menkes disease	Copper transport disorders, kinky hair	ATP7A	ATPase, copper-transporting α polypeptide
Wilson disease	Copper transport disorders, kinky hair	ATP7B	ATPase, copper-transporting β polypeptide
Cystinosis	Multiple organ dysfuctions caused by cystine crystal accumulation	CTNS	Cystinosin, cysteine/H+ symporter, which exports cysteine out of lysosomes
Others
Cross–McKusick–Breen syndrome, nevus depigmentosus
Acquired
Vitiligo	Lesions depigmented completely at initiation phase and pigmented orifices of hair follicles at recovering phase and hyperpigmented ridges surrounded by the lesion at stable/intractable phase	NALP1 MHCI MHCII PTPN22 LPP IL2RA UBASH3A C1QTNF6 RERE GZMB FOXP1 CCR6 and others	NLR family, pyrin domain containing 1/major-histocompatibility-complex class I molecules and class II molecules/protein tyrosine phosphatase, nonreceptor type 22/LIM domain containing preferred translocation partner in lipoma/ubiquitin associated and SH3 domain containing A/C1q and tumor-necrosis-factor-related protein 6 gene/arginine-glutamic acid dipeptide repeats/granzyme B
Vogt–Koyanagi–Harada disease	Bilateral, chronic, diffuse granulomatous uveitis with poliosis, vitiligo, central nervous system, and auditory signs	HLA-D IL17 and others	HLA-D gene locus including HLA-DRB1, DR1, DR4, DQA1, and DQB1/interleukin-17
Others
Sutton nevus/phenomenon, malignancy-induced hypopigmentation, postinflammatory hypopigmentation, pityriasis alba, senile leukodermachemical/drug-induced leukoderma
Hypopigmentation related with systemic disorders and others
Ataxia telangiectasia	Cancer predisposition and neurodegenerative disorders	ATM	Ataxia-telangiectasia mutated, DNA-damage response, signal transduction, and cell-cycle control
Tietz syndrome	Congenital profound deafness and generalized hypopigmentation	MITF	Transcription factor; master regulator of melanocyte lineage
Tuberous sclerosis	Seizures, mental retardation, and cutaneous angiofibromas/development of multiple hamartomas/ash leaf macule	mTOR	Tuberin and hamartin
Werner synderome	Adult onset segmental progeroid syndrome	WRN	Werner syndrome, RecQ helicase-like
Others
Alezzandrini syndrome, Preus syndrome, Tothmund–Thomson syndrome, infectious diseases
Mixed hyper-/hypopigmentation disorder
Congenital
Incontinentia pigmenti	Four stages: vesicles, verrucous lesions, hyperpigmentation, and hypopigmentation	NEMO/IKBKG	Nuclear factor-κB essential modulator/inhibitor of κ light polypeptide gene enhancer in B cells, kinase γ
Dyschromatosis symmetrica hereditaria	Mixture of hyperpigmented and hypopigmented macules distributed on the face and the dorsal aspects of the extremities	ADAR1	Double-stranded RNA-specific adenosine deaminase
Xeroderma pigmentosum type A	UV-induced carcinogenesis	XPA	Xeroderma pigmentosum type A
Xeroderma pigmentosum type B		ERCC3	Excision repair cross-complementation group 3; nucleotide excision repair (NER)
Xeroderma pigmentosum type C		XPC	Xeroderma pigmentosum type C
Xeroderma pigmentosum type D		ERCC2	Excision repair cross-complementation group 2; nucleotide excision repair (NER)
Xeroderma pigmentosum type E		DDB2	Double-stranded DNA-binding protein 2
Xeroderma pigmentosum type F		ERCC4	Excision repair cross-complementation group 4; nucleotide excision repair (NER)
Xeroderma pigmentosum type G		ERCC5	Excision repair cross-complementation group 5; nucleotide excision repair (NER)
Xeroderma pigmentosum type V		POLH	Polymerase (DNA directed), η (RAD 30 related)
Acquired
Photoleukomelanoderma, drug-induced disorders

MELANOCYTE DEVELOPMENT

As recently summarized (Kawakami and Fisher 2011; Sommer 2011), melanocytes in the skin are exclusively derived from the neural crest. Melanocytes used to be thought to derive directly from neural crest cells migrating via a dorsolateral path (between the ectoderm and dermamyotome of somites) during embryogenesis, whereas neurons and glial cells were thought to derive from neural crest cells migrating via a ventral path between the neural tube and somites. Adameyko et al. (2009) recently challenged this idea and reported that melanocytes migrate and differentiate from nerve-derived Schwann cell precursors, whose fate is determined by the loss of Hmx1 homeobox gene function in the ventral path. Schwann cell precursors detaching from the nerve differentiate into melanocytes, whereas precursors that stay in contact with nerves eventually differentiate into Schwann cells. Those authors also showed that Schwann cells remain competent to form melanocytes using Krox20 (early growth response 2 or Egr2)-Cre loci crossed to YFP reporter strains. They also showed that Neuregulin-1 (also known as glial growth factor, Heregulin or Neu differentiation factor) regulates the survival and proliferation of Schwann cell precursors and determines the fate of Schwann cells and melanocytes depending on high and low expression levels, respectively, and that secreted signals, including IGF (insulin-like growth factor) and PDGF (platelet-derived growth factor) enhance melanocyte development (Adameyko et al. 2009). Those findings may explain the facts that patients with neurofibromatosis type 1, who develop neurofibromas consisting mainly of Schwann cells, are hyperpigmented, and that segmental vitiligo mostly occurs along with the affected innervation zones or dermatomes.

Taken together, melanocytes in the skin eventually derive from the neural crest and either differentiate directly from neural crest cells via a dorsolateral path or derive from Schwann cell precursors via a ventral path after detaching from the nerve. Various transcription factors, including Hmx1 and Krox20, act as intrinsic factors that regulate the fate of these cell types, which are modulated by extrinsic factors including Neuregulin-1, IGF, and PDGF.

MELANOCYTE HETEROGENEITY

Human melanocytes reside not only in the epidermis and in hair follicles but also in mucosa, cochlea of the ear, iris of the eye, and mesencephalon of the brain as well as other tissues (Plonka et al. 2009). As far as mouse melanocytes are concerned, Aoki et al. reported that noncutaneous (ear, eye, and harderian gland) and dermal melanocytes are different from epidermal melanocytes in that the former do not respond to KIT stimulation but respond well to ET3 (endothelin 3) or HGF (hepatocyte growth factor) signals (Aoki et al. 2009), suggesting the heterogeneity of mouse melanocytes. They also reported that noncutaneous or dermal melanocytes cannot participate in regenerating follicular melanocytes using the hair reconstitution assay, unlike epidermal melanocytes (Aoki et al. 2011). Studies by Tobin’s group also support the hypothesis that follicular and epidermal melanocytes in human skin are different regarding their responses to various biological response modifiers, including αMSH (Tobin 2011). Additionally, Li et al. (2010) also reported that dermal melanocyte stem cells derived from glabrous human foreskin (i.e., with no hair follicles) can differentiate into functional epidermal melanocytes using a three-dimensional skin equivalent model.

These results make us wonder whether human fetal and/or adult melanocytes are heterogeneous. Human adult melanocytes in skin on the palms and soles may be different from melanocytes derived from other sites of the skin based on the facts that melanocyte migration stops at the palms and soles during embryogenesis and that skin on the palms and soles is hypopigmented and contains a fivefold lower density of melanocytes than at other skin areas. Additionally, fibroblasts in the dermis of the palms and soles secrete high levels of DKK1 (dickkopf1), which is an inhibitor of the Wnt signaling pathway and suppresses the proliferation and differentiation of those melanocytes (Yamaguchi et al. 2004). Preliminary results obtained from human fetal melanocytes cultured from four different body sites (scalp, back, abdomen, and sole) indicate that palmoplantar melanocytes express high levels of DKK1, TBX4, WIF1, FGF7, and CHI3L1 (Nakamura et al. 2012). Although the relevance to melanocytes has not been elucidated, a series of studies from Chang’s group showed that the expression patterns of homeotic genes (HOX genes, which are expressed in a nested pattern along developmental axes) determine positional identities within the human body (Chang 2009) and that a long noncoding RNA, which used to be considered to have nonspecific roles, has site-specificity (Rinn et al. 2007) and maintains active chromatin to coordinate HOX gene expression (Wang et al. 2011). Additionally, the expression levels of distal-specific HOXA13 are up-regulated in adult fibroblasts in the skin of paws of mice, thereby inducing the expression of Wnt5A, a morphogen required for distal development, in fibroblasts and of keratin 9, a distal specific marker of epidermis, in keratinocytes (Rinn et al. 2008). These results obtained in studies of mice support the hypothesis that mesenchymal-epithelial interactions play important roles in maintaining the site-specificity of the skin (Yamaguchi et al. 2005).

Taken together, human fetal and adult melanocytes may be heterogeneous/site-specific because those melanocytes are also regulated and maintained by site-specific HOX genes, whose expression patterns are eventually determined by chromatins and long noncoding RNAs. That melanocyte heterogeneity may be affected both by intrinsic factors, including a site-specific transcription factor, HOX, and by extrinsic factors secreted by surrounding resident cell types: fibroblasts and keratinocytes. The fact that acral melanoma is different from other types of melanoma (Curtin et al. 2005), especially in that AMP kinase-related NUAK2 expression levels are high in patients with poor prognosis acral melanoma (Namiki et al. 2011), may also support the idea that melanocytes are heterogeneous.

MELANOCYTE STEM CELLS

As recently summarized by Nishimura (2011), research on melanocyte stem cells (MelSCs) is also in the spotlight. A series of studies by her group showed that MelSCs are immature melanoblasts expressing high levels of dopachrome tautomerase (DCT) and low levels of Kit located in the lower permanent portion of the hair follicle (Nishimura et al. 2002). MelSCs directly adhere to hair follicle stem cells whose high expression levels of collagen XVII (COL17A1, BP180, or BPAG2) play important roles in maintaining MelSCs, which do not express COL17A1 (Tanimura et al. 2011). MelSCs surely serve as a melanocyte reservoir for the pigmentation of both the hair and the skin based on the fact that repigmentation usually occurs at the orifices of hair follicles in the skin of vitiligo patients. Future studies may elucidate the mechanism(s) by which MelSCs are maintained and regulated in skin at the palms and soles, where hair follicles do not physiologically exist.

MELANOCYTE SENESCENCE

The aging process in human skin eventually results not only in brittle/thin/inelastic skin and senile lentigines/lentigo but also hair graying and senile leukoderma. As for hair graying, Nishimura et al. (2005) reported that a Bcl2 deficiency accelerates the selective apoptosis of MelSCs, thereby disturbing the self-maintenance of MelSCs, resulting in hair graying. They also reported that mutations in MITF accelerate ectopic pigmentation or differentiation within the niche, thereby causing the physiological aging of MelSCs (Nishimura et al. 2005). Genotoxic stress also results in the depletion of MelSCs and in the irreversible hair graying caused by their unscheduled differentiation (Inomata et al. 2009). Other factors involved in hair graying include defective TGF-β signaling from hair follicle stem cells and abnormal regulation of the Notch and Wnt signaling pathways (Nishimura 2011). Similar mechanisms may be involved in the pathogenesis of senile leukoderma, although this disease concept is not yet well accepted worldwide.

Ectopic up-regulation of MelSC function caused by aging and/or UV-irradiation may be involved in the formation of senile lentigines/lentigo and other age-related hyperpigmentation disorders. Cosmetic companies usually focus on developing antiaging products, but the senescence process is beneficial in preventing tumorigenesis. Generally speaking, senescent cells are defined as cells with permanent proliferative arrest irrespective of physiological mitogenic stimuli, expressing β-galactosidase and senescence-associated heterochromatic foci. Factors that initiate and maintain the senescence program include BRAF, NRAS, p16INK4a, p21Waf1, p53, and pRb (Haferkamp and Rizos 2010). Benign nevi typically remain growth arrested and contain abundant numbers of senescent cells, although senescent cells are absent in advanced melanomas and in normal melanocytes. The expression of BRAFV600E in melanocytes increases the synthesis and secretion of IGFBP7, which may be required for melanocyte senescence, based on multidisciplinary studies, including the use of shRNA specific for IGFBP7 (Wajapeyee et al. 2010).

Future studies regarding melanocyte senescence will contribute to both tissue regeneration (hair graying and antiaging) and melanoma treatment.

FACTORS THAT REGULATE MELANIN PRODUCTION WITHIN MELANOCYTES

Pigment-specific factors that modulate melanin production within melanocytes are usually located within, on, or close to melanosomes and can be divided into three types (Fig. 1): proteins involved in melanosome structure, proteins that regulate melanin synthesis, and proteins involved in the intracellular trafficking of melanosome components and the transport of melanosomes to the cell’s periphery (Yamaguchi and Hearing 2009). Transcription factors specifically expressed by melanocytes and by melanoma cells (especially MITF) regulate the expression and function of many of those pigment-specific factors. We briefly update several important findings since our review published several years ago (Yamaguchi and Hearing 2009).

Figure 1.

Factors that regulate melanin production within melanocytes. Critical factors consist of proteins that affect melanosome structure (Pmel17, MART-1, and GPNMB), proteins that modulate melanin synthesis either directly or indirectly (TYR, TYRP1, DCT, BLOC-1, OA1, P, and SLC45A2), proteins involved in the trafficking of melanosome proteins or the intracellular transport of melanosomes (microtubules, F-actin, kinesin, dynein, Rab27a, melanophilin, myosin Va, RILP, ciliobrevins, and Slp2-a), and melanocyte-specific transcription factors (PAX3, SOX9/10, LEF-1, CREB, DICER, and MITF). Melanosomes mature through distinct stages, noted as I, II, III, and IV in this diagram.

Melanosomal Structural Proteins

Three important structural proteins that form melanosomes include PMEL17/Silv/GP100, MART-1 (melanoma antigen recognized by T cell-1), and GPNMB (glycoprotein non-metastatic melanoma protein b)/DC-HIL/osteoactivin. PMEL17 forms an amyloidogenic fibril depending on the critical acidic pH (5.0 ± 0.5) within melanosomes (Pfefferkorn et al. 2010). Studying PMEL17 mutations can be useful to investigate the conversion between physiological/benign and pathological amyloid proteins (Watt et al. 2011). MART-1, which is abundant in early melanosomes, is required for the maturation of PMEL17 (Hoashi et al. 2005). GPNMB, which is highly homologous to Pmel17 but lacks the RPT domain (imperfect repeats of proline, serine, and threonine-rich motifs), is a melanosome-specific and proteolytically released protein, which is abundant in late melanosomes (Hoashi et al. 2010). GPNMB is critical for the formation of melanosomes in a MITF-independent fashion (Zhang et al. 2012). These structural melanosomal proteins provide scaffold materials to the enzymes required for melanin deposition and on which the melanins are deposited.

Enzymes Involved in Melanin Synthesis

There are three key enzymes that play critical roles in melanin synthesis within melanosomes: TYR (tyrosinase), TYRP1 (tyrosinase related protein-1), and TRP2/DCT (dopachrome tautomerase). Many factors, including BLOC-1, OA1, P, and SLC45A2 (previously known as MATP) influence the trafficking and thus the function of these enzymes. Of note, metal ions including zinc and copper serve as catalysts and/or chelators when melanin is synthesized (Simon et al. 2009) and metal remnants in fossils enable us to predict the colors of dinosaurs (Wogelius et al. 2011), in addition to determining the configurations of melanosomes (Li et al. 2012). Cystinosin, a cysteine/H⁺ symporter that exports cysteine out of lysosomes, is the gene associated with cystinosis, a rare autosomal recessive disorder with multiple organ dysfunctions including hypopigmentation (Chiaverini et al. 2012). Additionally, NAD(P)H:quinone oxidoreductase-1 enhances melanogenesis by increasing the levels of TYR protein (Choi et al. 2010). The regulation of intramelanosomal pH may play important roles for regulating the appropriate enzymatic functions in melanin synthesis as well as the processing and function of melanosomal structural proteins.

Melanosome Trafficking Proteins

Melanin granules are transported from the perinuclear area to the periphery of melanocytes and are eventually transferred to adjacent keratinocytes (Yamaguchi and Hearing 2009). Early melanosomes, produced via the trans-Golgi network and/or endocytosis, originate in the perinuclear area and then mature to late (pigmented) melanosomes as they move toward the periphery of the melanocyte (i.e., the dendrites). In this trafficking pathway, kinesin (prograde) and dynein (retrograde) act like wheels for melanosomal cargo and microtubules function like rails. The clathrin adaptor AP-1 is reported to interact with the kinesin motor KIF13A, suggesting the role of adaptor proteins in melanosome trafficking (Delevoye et al. 2009). The melanosomal cargo is transferred from microtubules to F-actin, which acts as a rail, at the periphery of the melanocyte. Semi-automated analysis of organelle movement and membrane content reveals the involvement of Rab27a and its complex in the regulation of this transfer (Hume et al. 2011). Rab27a, melanophilin, and myosin Va are connected to melanosomes in that order and function like wheels. Finally, Slp2-a may regulate melanosomal cargo exocytosis. Recent findings show that melanoregulin regulates retrograde melanosome transport via the interaction with RILP (Rab-interacting lysosomal protein) and its complex (dynactin subunit 1) (Ohbayashi et al. 2012). Ciliobrevins were discovered as small-molecule inhibitors of the AAA⁺ (ATPases associated with diverse cellular activities) ATPase dynein (Firestone et al. 2012). Studies further elucidating melanosome trafficking might be enhanced in the near future characterizing these factors.

Melanocyte-Specific Transcription Factors

MITF has been investigated intensively among the many transcription factors known to regulate melanocyte function. MITF itself is regulated by many other transcription factors, including PAX3 (a neural-crest-associated transcription factor), SOX9, SOX10, LEF-1/TCF (a downstream regulator of Wnt signaling pathway), and CREB (cAMP responsive-element-binding protein, which is phosphorylated by signals via MC1R, melanocortin-1 receptor) (Yamaguchi and Hearing 2009).

Fisher’s group recently reported that MITF directly up-regulates DICER, an endoribonuclease in the RNase III family that cleaves double-stranded RNA and pre-microRNA into short RNA fragments (20–25 nucleotides long), on melanocyte differentiation (Levy et al. 2010). Enhanced DICER expression plays a crucial role in melanocyte survival via the posttranscriptional processing of the pre-microRNA-17 ∼ 92 cluster, which leads to the down-regulation of BIM, a proapoptotic factor.

PIGMENTARY DISORDERS

As summarized above, the regulation of pigmentation involves many factors required for development, heterogeneity, regeneration, and senescence of melanocytes and their precursors, as well as those involved in determining melanosome structure, melanin synthesis, the trafficking of melanosomal components and the transport and distribution of melanosomes, and melanocyte-specific transcription factors that control the expression and function of all those genes. More than 350 loci are currently known to be directly or indirectly involved with those processes in mice and mutations of many of those genes have been associated with human pigmentary disorders. Those include hyperpigmentation, hypopigmentation, and mixed hyper-/hypopigmentation disorders and can be diagnosed by size (systemic or localized), comorbidities, site of the involvement, and patterns/shapes of the lesions (Table 1). Those are subclassified into congenital and acquired disorders and in addition hypopigmentation disorders can also be subclassified into complete and incomplete depigmentation. Among the many pigmentary disorders summarized in Table 1, we focus on vitiligo because of space limitations.

Congenital Hyperpigmentation Disorders

Congenital hyperpigmentation disorders include those involving epidermal hyperpigmentation (nevus cell nevus, Spitz nevus, and nevus spilus), dermal hyperpigmentation (blue nevus, nevus Ohta, dermal melanosis, nevus Ito, and Mongolian spot), ephelides, acropigmentatio reticularis, Spitzenpigment/acropigmentation, and lentiginosis (generalized lentiginosis, LEOPARD syndrome, inherited patterned lentiginosis, Carney complex, Peutz–Jeghers syndrome, Laugier–Hunziker–Baran syndrome, and Cronkhite–Canada syndrome).

The responsible locus for generalized lentiginosis, characterized by widespread lentigines without systemic involvement, has been localized to chromosome 4q21.1-q22.3 (Xing et al. 2005). LEOPARD syndrome is characterized by multiple lentigines, congenital cardiac abnormalities, ocular hypertelorism, and retardation of growth, and many reports have shown its association with mutations in the PTPN11 (protein tyrosine phosphatase, nonreceptor type 11) gene located at chromosome 12q24 since Legius et al. (2002) first reported the association with Noonan syndrome. Carney complex, a multiple neoplasia syndrome characterized by spotty skin pigmentation, cardiac and other myxomas, endocrine tumors, and psammomatous melanotic schwannomas, has been shown to be caused by mutations in PRKAR1A (protein kinase A regulatory subunit 1α), a tumor-suppressor gene (Kirschner et al. 2000). Peutz-Jeghers syndrome, which predisposes to benign and malignant tumors of many organ systems, has been reported to be associated with mutations in STK11 (serine/threonine protein kinase)/LKB1 (Hemminki et al. 1998).

Acquired Hyperpigmentation Disorders

Acquired hyperpigmentation disorders include senile lentigines/lentigo, melasma/chloasma, Riehl’s melanosis, labial melanotic macule, penile/vulvovaginal melanosis, erythromelanosis follicularis faciei Kitamura, UV-induced pigmentation (tanning and pigmentation petaloides actinica), postinflammatory pigmentation (friction melanosis and ashy dermatosis), chemical/drug-induced pigmentation (polychlorinated biphenyl, arsenic, 5-FU, bleomycin, cyclophosphamide, methotrexate, chlorpromazine, phenytoin, tetracycline, and chloroquine), pigmentary demarcation lines, foreign material deposition (such as carotene, silver, gold, mercury, bismuth, and tattoos). Hyperpigmentation related with systemic disorders includes metabolism/enzyme disorders (hemochromatosis, Wilson’s disease, Gaucher’s disease, Niemann–Pick’s disease, amyloidosis, ochronosis, acanthosis nigricans, and porphyria cutanea tarda), endocrine disorders (Addison’s disease, Cushing syndrome, and hyperthyroidism), nutritional disorders (pellagra, vitamin B12 deficiency, folic acid deficiency, vagabond’s disease, and prurigo pigmentosa), mastocytosis, collagen diseases, liver dysfunction, and kidney dysfunction. Hyperpigmentation can also be related with infectious diseases (measles, syphilis, and Malassezia furfur) and syndromes (von Recklinghausen’s disease, Sotos syndrome, POEMS syndrome, Naegeli syndrome, Cantu syndrome, McCune–Albright syndrome, Watson syndrome, and Bloom syndrome).

Bioinformatics studies have shown that genes responsible for melasma include Wnt and lipid metabolism-related genes in addition to melanogenic markers (Kang et al. 2011) and that reduced expression levels of WIF-1 (Wnt inhibitory factor-1) in keratinocytes and/or fibroblasts may play roles in the development of melasma (Kim et al. 2013). Patients with mastocytosis usually carry the D816 V KIT mutation and a bioinformatics study shows that those patients show the up-regulation of genes involved in innate and inflammatory immune responses, including interferon-induced genes and genes involved in cellular responses to viral antigens, together with complement inhibitory molecules and genes involved in lipid metabolism and protein processing. Aggressive mastocytosis additionally shows deregulation of apoptosis and cell cycle-related genes, whereas patients with indolent mastocytosis display increased expression levels of adhesion-related molecules (Teodosio et al. 2013). Sotos syndrome, characterized by childhood overgrowth with advanced bone age, craniofacial dysmorphic features including macrocephaly and learning difficulties, results from the haploinsufficiency of NSD1 (nuclear receptor binding SET domain protein 1) (Kurotaki et al. 2002). POEMS syndrome (polyneuropathy, organomegaly, endocrinopathy, M-protein, and skin changes) is a multisystem disorder associated with plasma cell dyscrasia, and patients with POEMS syndrome show up-regulated serum levels of angiogenetic factors including VEGF (vascular endothelial growth factor) and HGF (hepatocyte growth factor) (Yamada et al. 2013). Cantu syndrome, characterized by hypertrichosis, macrosomia, osteochondrodysplasia, and cardiomegaly, is reported to be caused by mutations in ABCC9 (ATP-binding cassette, subfamily C, member 9) (van Bon et al. 2012). McCune–Albright syndrome results from somatic mutations of the GNAS gene (G-protein α-subunit) especially mutations in Gsα (stimulatory G protein) (Dumitrescu and Collins 2008). The causative gene for Bloom syndrome, with photosensitivity and increased risk of malignancy, is BLM (Bloom syndrome, RecQ helicase-like), localized at chromosome 15q26.1 (German et al. 1994).

Congenital Hypopigmentation Disorders

Congenital hypopigmentation disorders include various types of oculocutaneous albinism (OCA1–4, Hermansky–Pudlak syndrome, Chediak–Higashi syndrome, and Griscelli syndrome), Cross–McKusick–Breen syndrome, phenylketonuria, piebaldism, Waardenburg syndrome, nevus depigmentosus, hypomelanosis of Ito, Hirschprung’s disease, Charcot–Marie–Tooth disease, Menkes disease, and Wilson disease.

Many of the genes responsible for those hypopigmentary disorders have been identified and in general are involved with the intracellular trafficking of proteins to LROs (including melanosomes), in the transport of organelles to the peripheries of the cell and in their transfer to surrounding keratinocytes. Figure 2 outlines the diagnostic decision tree for representative congenital hypopigmentation disorders, and we recommend that interested readers check the many references shown in the ESPCR webpage for further details and updates of genes associated with congenital hypopigmentary diseases. As recently reviewed, Hermansky-Pudlak syndrome, characterized by oculocutaneous albinism, prolonged bleeding times, and pulmonary fibrosis, is triggered by syndromic dysgenesis of specialized LROs including melanosomes, platelet granules, synaptic vesicles, lytic granules, Azurophil granules, and lamellar bodies (Wei and Li 2013). Its responsible dysfunctional proteins include the biogenesis of lysosome-related organelles complex 1 (BLOC-1), BLOC-2, BLOC-3, and AP-3. For example, the responsible gene for Hermansky-Pudlak syndrome type 7 is DTNBP1 (dysbindin), a component of BLOC-1 (Li et al. 2003). Hypomelanosis of Ito may be derived from duplication of Xp11.3-p11.4 and random X-inactivation (Zou and Milunsky 2009).

Figure 2.

Diagnosis of congenital hypopigmentation disorders. Key elements for accurate diagnosis are size (systemic or localized), comorbidities, site of the involvement, and patterns/shapes of the lesions.

Acquired Hypopigmentation Disorders

Acquired hypopigmentation disorders include vitiligo, Sutton nevus, Vogt-Koyanagi-Harada disease, malignancy-induced hypopigmentation (from melanoma and mycosis fungoides), postinflammatory hypopigmentation, chemical/drug-induced leukoderma, senile leukoderma, and pityriasis alba. Most of these acquired hypopigmentation disorders are associated with inflammation and a recent study shows that TNF-α and IL-17 synergistically suppress pigmentation-related signaling and melanin production partly via MC1R (Wang et al. 2013). Hypopigmentation is also related with other syndromes (such as ataxia telangiectasia, Alezzandrini syndrome, Preus syndrome, Tietz syndrome, tuberous sclerosis, Rothmund–Thomson syndrome, and Werner syndrome) and infections (such as HIV, Hansen’s disease, Malassezia furfur, and syphilis). Vitiligo is discussed further below.

Vogt–Koyanagi–Harada disease is a bilateral, chronic, diffuse granulomatous uveitis with poliosis, vitiligo, central nervous system, and auditory signs. Many researchers, especially Yang’s group, have investigated the gene polymorphisms of many factors including IL-17 that are associated with Vogt–Koyanagi–Harada disease in a Chinese Han population (Shu et al. 2010) since Yakura et al. first reported the HLA-D locus linkage (Yakura et al. 1976). The causative gene for ataxia telangiectasia is ATM (ataxia-telangiectasia mutated) localized at 11q22.3–23 (Ambrose and Gatti 2013). Amiel et al. reported that mutations in MITF result in Tietz syndrome (albinism-deafness) similar to Waardenberg syndrome type 2 (Amiel et al. 1998). Tuberous sclerosis results from dysregulation of mTOR signaling caused by mutations in tuberin and/or hamartin and Wataya-Kaneda et al. reported that angiofibromas can be treated with topical rapamycin ointment (Wataya-Kaneda et al. 2011). Although Werner syndrome is generally caused by mutations in the WRN gene, a recent epigenetic study shows that aberrant DNA methylation profiles result in premature aging diseases (Heyn et al. 2013).

Mixed Hyper-/Hypopigmentation Disorders

Congenital mixed hyper-/hypopigmentation disorders include incontinentia pigmenti, dyschromatosis symmetrica hereditaria (DSH), and xeroderma pigmentosum (XP), and acquired hyper-/hypopigmentation disorders include photoleukomelanoderma and drug-induced mixed hyper-/hypopigmentation.

Incontinentia pigmenti, a rare X-linked genodermatosis, commonly consists of 4 stages: inflammatory vesicular rash, verrucous lesions, linear or reticular hyperpigmentation, and finally atrophic hypopigmented skin. Many investigators have reported that gene mutations responsible for incontinentia pigmenti occur in NEMO (nuclear factor-κB essential modulator)/IKBKG (inhibitor of κ light polypeptide gene enhancer in B cells, kinase γ), located at chromosome Xq28 (Fusco et al. 2012). DSH, also called reticulate acropigmentation of Dohi, is a pigmentary genodermatosis of autosomal dominant inheritance and two groups have reported that the causative gene is ADAR1 (DSRAD; double-stranded RNA-specific adenosine deaminase) (Li et al. 2005; Suzuki et al. 2005). Various types of XP are reviewed elsewhere (DiGiovanna and Kraemer 2012), but mutations of DNA repair related genes after UV-induced damage play key roles in the formation of XP.

Vitiligo as a Representative of an Acquired Hypopigmented Disorder

Simple physical examination (lesions are depigmented completely at the initiation phase, pigmented orifices of hair follicles are seen at the recovering phase, and hyperpigmented ridges surrounded by the lesion are seen at the stable/intractable phase) and/or present history (acquired depigmentation with or without autoimmune comorbidities) should be sensitive and specific enough for the correct diagnosis of vitiligo (Taieb and Picardo 2009), and which can sometimes be confused with other hypopigmented disorders (listed in Table 1 and see above). However, it may be difficult to differentiate vitiligo from nevus depigmentosus at childhood because depigmentation is not usually remarkable at infancy. Vitiligo can be divided into two categories: generalized vitiligo (wide distribution) and segmental vitiligo (confined to the dermatome). Typically, generalized vitiligo starts from the face and/or the dorsal side of the hands, which are cosmetically important part(s) of the body, and occurs mostly during adolescence/puberty. Comorbidities include other autoimmune diseases including thyroid disease and pernicious anemia. The prevalence is approximately 0.5% with different studies estimating a prevalence rate of 0.2%–0.9% (Nordlund 1997) and vitiligo is the 18th commonest disease seen in Japanese dermatology clinics (Furue et al. 2011). The course of the disease appears to be unpredictable; the skin lesions are often stable for more than a year whereas others are slowly/rapidly progressive. The spontaneous healing rate is approximately 10%. The impact of vitiligo on the patients’ quality of life may be higher in more pigmented populations and in young adults based on the contrast of depigmented lesions and their surrounding areas and the social activities, respectively. In general, socially active patients tend to be eager to undergo any possible treatment options aiming for a cure, and socially inactive patients are often untreated or undertreated.

As summarized by Grimes (2005), the presence of CD8⁺ T cells in close apposition to melanocytes suggests that the pathogenesis of vitiligo is T cell mediated. The disease may also be antibody mediated in that vitiligo patients frequently have antibodies to surface and cytoplasmic melanocyte antigens. Increased cytokine levels, including IFN-γ and TNF-α, have been detected in the skin of vitiligo patients. Tacrolimus-induced repigmentation of vitiligo lesions is related with a reduction to normal of the elevated TNF-α in the lesional skin.

Many candidate genes/loci are thought to be involved with the pathogenesis of vitiligo. Among them, Spritz’s group recently reported significant associations between generalized vitiligo (from European-derived white ancestry) and SNPs (single-nucleotide polymorphisms) at many responsible loci using genome-wide association analyses. Most, if not all, of those genes are involved with regulation/function of the immune system. In addition to the NALP1 gene (Jin et al. 2007), those include MHCI, MHCII, PTPN22, LPP, IL2RA, UBASH3A, C1QTNF6, RERE, GZMB (Jin et al. 2010a), FOXP1, CCR6 (Jin et al. 2010b), OCA2-HERC2, MC1R, a region near TYR, IFIH1, CD80, CLNK, BACH2, SLA, CASP7, CD44, IKZF4, SH2B3, and TOB2 (Jin et al. 2012). The same group also reported the involvement of NLRP1, a key regulator of the innate immune system, in the pathogenesis of vitiligo (Levandowski et al. 2013). Although there is currently no single complete cure available to treat vitiligo (Oiso et al. 2013), these series of studies may bring new approaches for therapy to vitiligo patients in the near future after careful clinical trials.

MELANOMA—THE MALIGNANT MELANOCYTE

It is beyond the scope of this review to cover the topic of melanoma, the transformed melanocyte, other than to say that it is the most lethal of all types of skin cancers and its incidence is growing at an alarming rate. A number of gene loci have been characterized that are associated with the process of malignant transformation, growth, and metastasis of melanoma cells, and these have been recently reviewed by Fisher’s group (Tsao et al. 2012). Several melanoma susceptibility genes are related with genes responsible for pigmentation disorders including XP and LEOPARD syndrome. We refer readers to Hawryluk and Tsao (2014) for further information about melanoma. Of note, the gene expression profile of acral melanoma is dramatically different from the profiles of other types of melanoma (Curtin et al. 2005). Because the origin of melanocytes may be diverse depending on the site, UV-induced, non-UV-induced, acral, and mucosal melanomas may need to be treated with different therapeutic regimens. Preliminary data show that acral melanocytes appear at rete ridges during embryogenesis, which may be related with the dermoscopic patterns of the parallel ridge (instead of furrow) in acral melanoma (data not shown).

CONCLUDING REMARKS

Here we summarized recent discoveries of melanocyte biology from the aspects of basic and clinical science although space considerations do not allow us to provide a comprehensive overview of all of the important molecules involved in melanogenesis. We recommend that interested readers check the curated pigment gene database of the ESPCR web page because new pigment-related genes are being identified over time. That web site currently lists more than 350 genes associated with pigmentation and is frequently updated and categorized according to the mechanisms of action of those genes and associated pigmentary diseases. Future studies of melanocytes will further elucidate the mechanisms involved in the regulation of melanocyte development and heterogeneity, by which hair graying and melanocyte senescence occur, and disruptions of which cause most pigmentary disorders. It is our hope that further studies will lead to the development of complete effective therapies for those disorders, especially malignant melanoma.