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. Author manuscript; available in PMC: 2021 Dec 28.
Published in final edited form as: Oncogene. 2021 Aug 3;40(39):5781–5787. doi: 10.1038/s41388-021-01979-z

Tumorigenesis in neurofibromatosis type 1: role of the microenvironment

Chunhui Jiang 1, Renee M McKay 1, Lu Q Le 1,2,3,4,✉
PMCID: PMC8713356  NIHMSID: NIHMS1764146  PMID: 34345017

Abstract

Neurofibromatosis Type 1 (NF1) is one of the most common inherited neurological disorders and predisposes patients to develop benign and malignant tumors. Neurofibromas are NF1-associated benign tumors but can cause substantial discomfort and disfigurement. Numerous studies have shown that neurofibromas arise from the Schwann cell lineage but both preclinical mouse models and clinical trials have demonstrated that the neurofibroma tumor microenvironment contributes significantly to tumorigenesis. This offers the opportunity for targeting new therapeutic vulnerabilities to treat neurofibromas. However, a translational gap exists between deciphering the contribution of the neurofibroma tumor microenvironment and clinically applying this knowledge to treat neurofibromas. Here, we discuss the key cellular and molecular components in the neurofibroma tumor microenvironment that can potentially be targeted therapeutically to advance neurofibroma treatment.

NEUROFIBROMATOSIS TYPE 1 (NF1)

Neurofibromatosis Type 1 (NF1), also known as von Recklinghausen’s neurofibromatosis, is one of the most common human genetic diseases, affecting nearly 1 in 3000 individuals with no preference for sex or race. It is caused by mutations in the NF1 gene that are inherited in an autosomal dominant pattern, or by de novo NF1 mutations that occur in up to 50% of NF1 cases, thus projecting a persistent rate of incidence with time in human society [1]. These mutations result in defective function of the NF1 gene product, neurofibromin, which is ubiquitously expressed and plays a role in a variety of fundamental cellular processes, such as cell proliferation and migration, cytoskeleton organization, and neurite outgrowth among others [2]. At the molecular level, neurofibromin primarily acts as a GTPase-activating protein that negatively regulates the small GTPase RAS by accelerating hydrolysis of the GTP-bound RAS active form to the GDP-bound RAS inactive form. In addition to the RAS/MAPK pathway, neurofibromin has been associated with multiple signaling pathways, including ROCK/LIMK/cofilin, AKT/MTOR, and cAMP/PKA pathways [2]. These complexities residing in NF1 cellular and molecular function underlie the phenotypic diversity observed in the range of NF1 clinical manifestations, including neurofibroma pathogenesis.

NF1-ASSOCIATED NEOPLASMS AND NEUROFIBROMAS

NF1 patients have a broad spectrum of clinical presentations that can be classified into three major categories: (1) Non-malignant clinical features, including cutaneous and plexiform neurofibromas (Fig. 1), optic pathway and brainstem gliomas, pigmentary abnormalities, bone deformities, cardiovascular abnormalities, and learning deficits; (2) malignant tumors of the nervous system, including glioblastomas, and malignant peripheral nerve sheath tumors (MPNSTs); (3) non-nervous system malignant tumors, including breast cancer, leukemia, lymphoma, gastrointestinal stromal tumors, pheochromocytoma, and rhabdomyosarcomas [3]. Among these manifestations, neurofibroma is one of the hallmark lesions for NF1 diagnosis.

Fig. 1. Clinical manifestations of neurofibroma.

Fig. 1

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a Multiple cutaneous neurofibromas on the trunk. b Plexiform neurofibroma on the right foot of an individual with Neurofibromatosis Type 1.

Neurofibromas are benign nerve sheath tumors that develop in the peripheral nervous system. Characterized by various clinical appearance and an unpredictable growth pattern, neurofibromas have been divided into two main subtypes: Cutaneous neurofibromas (cNFs) develop along dermal nerve twigs and thus present as numerous soft nodules on the skin in virtually any region of the body, causing severe discomfort and/or disfigurement (Fig. 1a). CNFs develop in the majority of NF1 patients and are one of the most common features of NF1. CNFs typically initiate in early adolescence and expand in size and number with unpredictable occurrence. Due to apparent growth arrest, cNFs do not undergo malignant transformation. Importantly, pregnant women with NF1 will often develop new cNFs, suggesting regulation by sex hormones [4].

In contrast to cNFs, plexiform neurofibromas (pNFs) exhibit distinct clinical features. PNFs develop along the nerve plexus in various regions at birth and continuously enlarge with age (Fig. 1b). While some pNF tumors are small and can only be detected with clinical imaging, others are capable of growing into large tumors weighing in the kilogram range [5]. These pNF tumors can compress surrounding structures, causing substantial pain and neurological dysfunction as well as bone destruction. PNFs also have an 8–13% lifetime risk of transforming into MPNST [6], a malignancy that is among the most challenging sarcoma malignancies to treat, and the leading cause of death in NF1 patients.

It is well accepted that neurofibromas originate from loss of NF1 during Schwann cell development [7], wherein neural crest stem cells differentiate into Schwann cell precursors, followed by further differentiation into immature Schwann cells, which ultimately give rise to mature Schwann cells [8]. Given that cNF and pNF harbor distinct clinical features in terms of spatial distribution (dermal nerve twigs vs. nerve plexus) and pathological onset (embryonic vs. adolescent), it is reasonable to hypothesize that these two tumor subtypes may arise from the Schwann cell lineage at different developmental stages. Indeed, this is supported by studies demonstrating that many NF1 genetically engineered mouse models do not develop cNF and pNF simultaneously [7, 9–13]. The bifurcation of pNF and cNF cells of origin may occur at some point during Schwann cell differentiation as accumulating studies show that dual cells of origin of cNF and pNF reside in Prss56-positive boundary cap cells or the Schwann cell lineage expressing HOXB7 or SOX10 [14–16]. Future studies to further narrow down the cells of origin of cNF and pNF would allow reconstruction of neurofibroma initiation and progression, therefore enabling effective approaches to target the pathological culprits.

THE NEUROFIBROMA TUMOR MICROENVIRONMENT

In addition to identifying the cells of origin of neurofibroma, deciphering the role of the tumor microenvironment is also critical for understanding the pathogenesis of neurofibromas [1, 7, 17, 18]. Neurofibromas are very heterogeneous tumors, containing neoplastic Schwann cells, neurons, fibroblasts, immune cells, and endothelial cells, as well as extracellular matrix (ECM) components such as collagen, fibronectin, laminin, and hyaluronic acid [1] (Fig. 2). NF1-null Schwann cells coordinate with these cellular and non-cellular components to promote and support tumor initiation and maintenance. Therapeutically disrupting the interplay between tumor cells and tumor microenvironment will aid in developing interventions that are more effective at treating neurofibroma.

Fig. 2. Components of neurofibroma tumor microenvironment.

Fig. 2

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Schematic diagram showing key components in the tumor microenvironment that impact the development of neurofibroma. Illustration created with BioRender.com.

The contribution of the neurofibroma microenvironment to tumorigenesis has remained a confounding question: On one hand, the miscellaneous composition underlies complex interactions within the tumor microenvironment. (This is discussed in more detail later.) On the other hand, the genetic nature of NF1 indicates that neurofibromas evolve from NF1-homozygous Schwann cell lineage cells and an NF1-heterozygous tumor microenvironment. Mounting evidence has shown that NF1-heterozygosity of the tumor microenvironment is important for neurofibroma tumorigenesis. Zhu et al. reported that mice with Nf1 loss in Krox20-expressing Schwann cells only develop pNF in an Nf1-heterozygous microenvironment and not in a wildtype microenvironment [7]. In addition, the Le lab demonstrated that mice with Nf1 loss in PLP-expressing or Hoxb7-expressing Schwann cells and an Nf1-heterozygous microenvironment develop pNF earlier than mice with a wildtype microenvironment [14, 17, 18]. Of note, in the context of PLP-driven Schwann cell deletion of Nf1, this Nf1-heterozygous microenvironment can also impair the progression of pNF to MPNST [18]. Conversely, Wu et al. reported that an Nf1-heterozygous microenvironment is not required for neurofibromagenesis in a mouse model with deletion of Nf1 in Dhh-expressing Schwann cells [19]. This paradoxical regulation of tumor microenvironment on tumor progression highlights the complicated nature of the interactions between tumor cells and their microenvironment. It seems clear, however, that the tumor microenvironment also likely plays an important role, either to promote or to impair the evolution from pNF to atypical neurofibroma neoplasms of unknown significance (ANNUBP) to MPNST. This is an important area that needs further investigation.

To better understand the complexities of tumor microenvironment contribution to pNF, one can look to the nerve wounding microenvironment for insights: previous research suggests a close link between the two systems wherein mast cells, macrophages, and fibroblasts accumulate in the microenvironment and modify Schwann cell plasticity [20]. Indeed, it has been shown that nerve injury promotes neurofibroma formation specifically at the injury site [21]. However, the mechanisms by which the nerve injury microenvironment transforms into a “soil” capable of seeding neurofibroma growth remain elusive.

THE ROLE OF NERVES IN THE TUMOR MICROENVIRONMENT FOR NEUROFIBROMA DEVELOPMENT

A common clinical manifestation of cNF is chronic itching, which can occur even before the appearance of neurofibroma nodules. Rice et al. discovered increased innervation of skin appendages before neurofibromas appear, suggesting a potential stimulating effect from neurons for Schwann cell hyperplasia and thus neurofibroma development [22]. In normal conditions, Schwann cells are in close contact with nerve tissue. However, loss of this contact occurs frequently in the early stages of neurofibroma development and has been attributed to upregulation of the RAS/RAF/MAPK signaling pathway in NF1-null Schwann cells. Parrinello et al. identified the downregulation of Sema4F as being responsible for the loss of Schwann cell-axonal interaction [23]. Moreover, Liao et al. demonstrated that Nf1-null skin-derived precursor cells (SKPs), a neural crest-derived cell population with stem cell features, can only give rise to neurofibroma when injected into the area surrounding the injured sciatic nerve rather than subcutaneous tissues [24]. Of note, reconstruction with nerve tissues in a 3-dimensional (3D) skin raft culture system can facilitate the development of Nf1-null SKPs into neurofibroma in subcutaneous tissues [24]. Collectively, these results support the important role of nerve tissue in promoting neurofibroma formation.

Regarding the mechanisms and players underlying the interaction between Schwann cells and nerves, a number of interesting candidates identified in the nerve injury model may be informative due to the similarities and connections between these two systems. For example, the neuregulin/ ErbB signaling axis has been reported to mediate the interaction between axons and Schwann cells during nerve injury [25, 26]. Axonal neuregulins bind to the ErbB2/ErbB3 receptor tyrosine kinases in Schwann cells and regulate Schwann cell behavior [25, 26]. Of note, whole-exome sequencing identified ERBB2 mutations in neurofibroma/schwannoma hybrid nerve sheath tumors, and treatment with lapatinib, a small-molecule ERBB inhibitor, provided clinical benefits in patients [27], implicating the neuregulin/ErbB axis in neurofibroma. In addition, as Schwann cells are in close contact with neurons, Ephrin/Eph signaling, which regulates cell-cell adhesion, as well as Notch signaling, may also play key roles in mediating the interaction between neoplastic Schwann cells and nerves in the tumor microenvironment. Multiple lines of evidence have demonstrated that these pathways are involved in axon-glia interactions and can regulate Schwann cell activity [28–30].

IMMUNE CELLS IN THE NEUROFIBROMA TUMOR MICROENVIRONMENT

Immune cells are important accomplices in facilitating the development of NF1-null Schwann cells into neurofibroma. While rarely found in the normal peripheral nervous system, a substantial number are found within pNFs, especially mast cells and macrophages. CNFs are also replete with infiltrating mast cells and macrophages. As for the roles of these immune cells in the development of neurofibroma, it has been proposed that the neoplastic Schwann cells recruit the immune cells by elevated secretion of cytokines and growth factors. The influx of immune cells then further modifies the microenvironment, for example by stimulating fibroblasts to deposit ECM which drives neurofibroma development [20]. As such, the injury sites where inflammation accumulates are prone to neurofibroma development. Further studies are needed to delineate the network of immune cell regulation on Schwann cells and non-Schwann cells during neurofibroma development.

Mast cells

Mast cell infiltration is regarded as a hallmark feature of neurofibroma due to their response to stem cell factor (SCF; also known as c-Kit ligand or steel factor), a chemoattractant secreted by the neoplastic Schwann cells [31]. Mast cells play multifaceted roles in neurofibroma progression by secreting a variety of factors, including immuno-modulatory factors, such as histamine, to further induce a cascade of immune responses, as well as pro-fibrotic factors, such as TGF-β, to induce fibroblasts to proliferate and deposit collagen [32]. Importantly, NF1-heterozygous mast cells have been shown to increase their pathogenic roles in the context of neurofibroma [31]. Therefore, a reasonable strategy would be to therapeutically target mast cells. In 1987, a clinical trial of ketotifen, an anti-histamine agent and mast cell blocker, showed that ketotifen decreased neurofibroma-associated itching, pain, and tenderness in many of the patients [33]. One patient on long term treatment with ketotifen developed fewer cNFs compared to control patients [34]. Furthermore, genetic or pharmacologic targeting of c-Kit, the receptor for SCF in mast cells, prevents neurofibroma development [35, 36]. To dissect the contribution of mast cells to neurofibroma progression, Liao et al. utilized Plp-CreERT2; Nf1f/f; Scff/f mice to disrupt the recruitment of mast cells. They found that tumor burden was not improved with decreased influx of mast cells in the tumor microenvironment, indicating that mast cells are not required for neurofibroma progression in this mouse model [17]. Taken together, while therapies targeting mast cells provide some efficacy in reducing symptoms of neurofibroma, the extent of mast cell contribution to neurofibroma formation needs further investigation.

Macrophages

In addition to mast cells, macrophages are also highly enriched in neurofibromas. Macrophages are recruited by CSF1, a cytokine secreted by neoplastic Schwann cells [37]. Those neurofibroma-associated macrophages harbor distinct features in terms of secretion of cytokines, chemokines, and growth factors compared to their counterparts in the normal peripheral nerve system [38]. As macrophages are classified into two main categories - a proinflammatory M1 subtype and a protumorigenic M2 subtype-characterizing macrophage identity in neurofibroma has been under investigation. Liao et al. reported that M1 macrophages predominate in the Plp-CreERT2; Nf1f/f neurofibroma model compared to M2 macrophages [17]. However, Choi et al. analyzed the genomic profile of F4/80+;CD11b+ macrophages from Dhh-Cre; Nf1f/f mice and showed a mixture of M1 and M2 subtypes [38]. The complexity of neurofibroma-associated macrophages is also reflected by tumor response to macrophage-targeting therapies. Prada et al. reported that blocking macrophage infiltration in Dhh-Cre; Nf1f/f mice with PLX3397, an inhibitor of the CSF1 receptor on macrophages, resulted in stage-specific effects on neurofibroma formation, with early treatment increasing the tumor burden and late treatment relieving the phenotypes [37]. Overall, further investigation of macrophage subtypes and the mechanisms underlying the stage-specific effects is required to develop macrophage-targeting strategies for neurofibroma.

NEUROFIBROMA-ASSOCIATED FIBROBLASTS (NFAFS)

Developmental origin and biology of NFAFs (Unique signature of NFAFs compared to normal fibroblasts.)

As the name implies, fibrotic components are integral parts of neurofibromas: up to 70% of the dry weight of neurofibroma is collagen and a large percentage of cells in the tumor are fibroblasts [39, 40]. Targeting this fibrotic feature for neurofibroma intervention was attempted using pirfenidone, an effective clinical medication for idiopathic pulmonary fibrosis, however this drug failed in neurofibroma clinical trials [41, 42]. The context-dependent effects suggest a unique signature for neurofibroma-associated fibroblasts (NFAFs), given that fibroblast populations overall are incredibly heterogeneous. For example, NFAFs lack expression of alpha smooth muscle actin (αSMA) [43, 44], a marker for active myofibroblasts in organ fibrosis. Moreover, the gene signature of NFAFs resembles that of normal skin fibroblasts [44], which have been shown to arise from two distinct lineages during skin development: a papillary dermal fibroblast lineage and a hypodermal fibroblast lineage. The papillary dermal fibroblasts (DPP4+Sca1-) contribute exclusively to the formation of upper dermis, including dermal papillae that regulate the growth of hair [45]. Other dermal fibroblasts form the lower dermis, including the Dlk1-expressing reticular fibroblasts responsible for the synthesis of most fibrillar ECM [45]. Further characterization of mouse dermal fibroblasts may shed light on the developmental origin of NFAFs, which is still poorly understood. It also remains to be elucidated whether NFAFs in pNF versus cNF originate from different sources.

Signaling pathways regulating NFAFs

To develop effective fibrosis-targeting therapies for neurofibroma, it is important to understand the molecular mechanisms underlying the pathogenesis of NFAFs. The TGF-β signaling pathway is a master regulator of fibrosis in various pathological contexts, including in NF1. The plasma level of TGF-β is elevated in NF1 patients [46], likely due to secretion from multiple cell types within the neurofibromas, such as neoplastic Schwann cells and NF1-heterozygous mast cells [32, 38]. This TGF-β-induced fibrotic response is dependent upon the activation of non-receptor tyrosine kinase c-abl in NFAF [32] (Fig. 3). Furthermore, as the neoplastic NF1-null Schwann cells secrete excessive basic fibroblast growth factor 2 (FGF-2), platelet-derived growth factor (PDGF) and midkine [47], these secreted factors may regulate NFAF pathological functions (Fig. 3) as they are reported to play important roles in other fibrotic contexts [48–50].

Fig. 3. Signaling pathways regulating the functions of NFAFs.

Fig. 3

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Schematic diagram showing key signaling pathways that may contribute to NFAF functions. Dashed arrows indicate that those pathways remain to be further characterized for their role in regulating NFAF functions. Illustration created with BioRender.com. (TGF-β transforming growth factor-β, TGF-βR transforming growth factor-β receptor, ECM extracellular matrix, FGF-2 fibroblast growth factor 2, PDGF platelet-derived growth factor, YAP Yes-associated protein, NF1 neurofibromin).

In addition, as noted above, nerve injury and neurofibroma share common features in their microenvironment, and the molecular regulation of fibrotic responses in nerve injury may provide clues for elucidating NFAF regulation in neurofibroma. Together with the TGF-β pathway, WNT and YAP/TAZ pathways are also integrated into an intricate regulatory network in organ fibrosis [51]. These two pathways have been implicated in regulating fibrotic responses for axonal regrowth. Wehner et al. demonstrated that the Wnt/β-catenin pathway is activated in fibroblast-like cells adjacent to regenerating axons after spinal cord injury in zebrafish, inducing collagen XII expression and deposition as well as modifying the ECM for axonal regeneration [52]. Moleirinho et al. reported that Willin, a regulator of YAP phosphorylation, promotes fibroblast migration towards the injury site for nerve regeneration [53]. In addition, Ephrin-B/EphB2 signaling was shown to mediate the interaction between fibroblasts and Schwann cells for axonal regrowth following sciatic nerve injury [54]. Further elucidation of the potential pathways governing NFAF pathology (Fig. 3) may enable development of novel therapeutic interventions for neurofibroma treatment.

THE EXTRACELLULAR MATRIX (ECM) IN THE NEUROFIBROMA MICROENVIRONMENT

The role of the ECM in neurofibromagenesis has been increasingly appreciated as mounting evidence shows that the ECM is not a static framework simply providing structural support for cell adhesion, but a highly dynamic entity that can impact a broad array of cell activities. For instance, during wound healing the ECM plays a fundamental role in immobilizing growth factors and remodeling the transport routes for cell migration [55]. Mechanistically, ECM can exert effects through biochemical and biophysical cues to guide cellular activities. The biochemical regulation of ECM on cell behaviors is exemplified in the differentiation of mammary gland cells: When laminin-1 is added to mammary epithelial cells in culture, laminin-1 binds to integrin, regulating mammary-specific gene transcription via the prolactin receptor [56]. Meanwhile, biophysical cues from the ECM also play important roles in regulating cellular behaviors via mechanotransduction pathways. Mascharak et al. recently reported that mechanical stress in the wound microenvironment can convert engrailed-1-lineage-negative fibroblasts in the reticular dermis to engrailed-1-expressing fibroblasts contributing to collagen deposition [57]. They show that this mechano-responsive engrailed-1 expression is dependent on YAP protein in the Hippo pathway, a well-established mechanotransduction pathway [57]. Through this biochemical and biophysical feedback between cells and the ECM, ECM components are dynamically modified to fulfill the context-specific demands of the microenvironment.

Another hallmark feature of neurofibromas is the accumulation of ECM, including collagens, glycoproteins, proteoglycans, ECM regulators, ECM affiliated proteins, and secreted proteins among others [44], which are collectively deposited by resident cells in the neurofibroma. Characterization of neurofibroma ECM profiles using advanced techniques, such as proteomics, mass spectrometry, and mass cytometry, will deepen our understanding regarding the contribution of the microenvironment to neurofibroma formation and the response to therapeutic intervention.

Collagen

Collagens are the predominant components in neurofibroma ECM and the histological examination of collagen strand morphology has been regarded as one of the general criteria for neurofibroma diagnosis. The collagen superfamily contains 28 subtypes and each one has unique properties. In order to better understand the neurofibroma microenvironment, Brosseau et al. recently characterized the ECM profiles in human cNF through single-cell RNAseq and found that NFAFs express higher amounts of collagen type I, III, VI, and XV compared to other subtypes [44]. Notably, collagen VI is abundantly expressed in a subset of NFAFs. The protumor effects of collagen VI have been demonstrated in several studies. Park et al. utilized the Col6a1 knock-out mouse model to demonstrate that collagen VI promotes tumor growth through its cleavage product endotrophin by increasing fibrosis, angiogenesis, and inflammation [58]. The pro-fibrotic and pro-inflammation effects were further verified in transgenic mice overexpressing endotrophin [59]. Recently, Wishart et al. reported collagen VI directly binds to the membrane glycoprotein NG2 to drive triple-negative breast cancer cell invasion by cross-talking with the EGFR-MAPK pathway [60], which suggests that NF1 loss can potentially elevate the responses to collagen VI. In addition, Harigai et al. showed that collagen III is important for tumor cell proliferation and contributes to the drug resistance observed in NF1 patient-derived neurofibroma cells [61]. Taken together, those results warrant further investigation on the roles of collagen in neurofibroma development.

Hyaluronic acid

Hyaluronic acid is a large glycosaminoglycan abundantly expressed in the ECM of both cNFs and pNFs, but with significantly more expression in pNF. Importantly, Schwann cells produce more hyaluronic acid than fibroblasts [62]. Hyaluronic acid binds to CD44, a glycoprotein regarded as a prognostic marker for human carcinoma malignancy and resistance to therapy. The interaction between hyaluronic acid and CD44 leads to the formation of a complex of CD44 with other receptors, such as EGFR, thereby boosting the activity of those receptors [63, 64]. As CD44 is expressed in Schwann cells [65], it is plausible that NF1 loss in Schwann cells can further potentiate the response to hyaluronic acid/CD44/EGFR in the context of neurofibroma. Furthermore, hyaluronic acid binds to water, leading to a desmoplastic myxoid environment, which creates barriers to therapeutic drug treatment. Targeting hyaluronic acid for efficient drug delivery has been under intensive investigation [63].

Other components

In addition to collagen and hyaluronic acid, myriad other components have been identified in the ECM of neurofibromas, including fibronectin, laminin, and elastin [66]. Among them, fibronectin is expressed specifically in NFAFs, suggesting its usefulness as a potential marker for NFAF [44]. Fibronectin is a key player regulating ECM composition and stability as well as cell-ECM interaction. Sottile et al. showed that fibronectin polymerization is required for the deposition of collagen I and the formation of collagen fibrillar structure [67]. Dallas et al. showed that fibronectin plays a critical role in incorporating latent transforming growth factor-beta-binding proteins (LTBPs) and TGF-β into the ECM, thereby regulating the TGF-β signaling pathway [68]. The effects of fibronectin on both collagen and TGF-β are highly relevant to neurofibroma development.

INTERACTION BETWEEN SCHWANN CELLS AND FIBROBLASTS IN NEUROFIBROMA

The dynamic interaction between Schwann cells and NFAFs sustains neurofibroma tumorigenesis. Here, we propose possible mechanisms governing the direct and indirect interaction between these two entities (Fig. 4). The direct interaction may occur through Eph/Ephrin pathways, initiating either forward or reverse signaling in Schwann cells and NFAFs. Additional factors, including FGF-2, PDGF, and midkine, may also be involved in the direct interaction. Comparatively, the indirect interaction has been better characterized, and involves immune cells and ECM components. NF1-null Schwann cells secrete chemokines and cytokines, such as SCF and CSF1, to recruit mast cells and macrophages, both of which secrete TGF-β to activate NFAFs for ECM deposition and remodeling. The resulting ECM milieu further regulates the activity of Schwann cells, immune cells, and NFAFs through biochemical and biophysical mechanisms, which likely forms a feed-forward regulation loop.

Fig. 4. Proposed interactions between Schwann cells and fibroblasts in neurofibroma.

Fig. 4

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Schematic diagram showing the direct and indirect interactions between Schwann cells and fibroblasts. Key factors mediating those interactions are listed. Illustration created with BioRender.com. ECM extracellular matrix, TGF-β transforming growth factor-β, FGF-2 fibroblast growth factor 2, PDGF platelet-derived growth factor, MK midkine, SCF stem cell factor, CSF1 colony-stimulating factor 1.

TRANSLATING CURRENT RESEARCH IN THE LABORATORY INTO FUTURE THERAPIES: TARGETING FIBROSIS FOR NEUROFIBROMAS

Fibrosis is inherently involved in the development of neurofibromas. Considering the massive presence of collagen in neurofibromas, targeting fibrosis to decrease collagen deposition per se will likely have tremendous benefits for NF1 patients. Presumably, alleviated fibrosis will in turn affect Schwann cells and immune cells, impairing disease progression. However, the clinical failure of pirfenidone for neurofibroma treatment suggests that a successful strategy for targeting fibrosis requires a more detailed understanding of fibrosis mechanisms in neurofibromas, such as the origin of NFAFs, faithful identification markers, and ECM profiles among others. In addition, targeting fibrosis could also be achieved by disrupting the fibrosis-sensing machinery in neurofibromas, such as integrins and focal adhesion kinase (FAK). In addition, targeting fibrosis can be utilized to boost the efficacy of other neurofibroma therapeutics. To date, the MEK inhibitor selumetinib is the only drug approved by the FDA for the treatment of pNF. During the phase 2 clinical trial of selumetinib, 68% of patients showed at least 20% tumor shrinkage and most of them had clinical benefit [69]. Considering that desmoplasia acts as a barrier for drug delivery in neurofibroma, it will be interesting to determine whether targeting fibrosis in combination with MEK inhibitors can produce a synergistic effect. Taken together, targeting fibrosis offers a potentially promising strategy to combat neurofibromas, and warrants further investigation.

CONCLUSION AND FUTURE PERSPECTIVES

Study of the neurofibroma tumor microenvironment is an exciting area in NF1 research, and likely to identify new targets for therapeutic strategies. Numerous analyses from both preclinical mouse models and human patients indicate that targeting the neurofibroma microenvironment should yield promising outcomes. Indeed, there are clinical trials currently ongoing to this end. For instance, PLX3397, a small molecule drug that targets mast cells and macrophages, is currently under clinical investigation for pNF treatment (NCT02390752). However, expectations should be tempered as previous clinical trials targeting the tumor microenvironment have generated confounding results [70]. A comprehensive understanding of the neurofibroma tumor microenvironment is therefore necessary to successfully translate findings from the bench to the bedside. With advanced technologies for acquiring multiple “omics” datasets, including genomics, epigenomics, transcriptomics, proteomics, and metabolomics, in combination with spatial characterization techniques, such as single-cell RNA sequencing and mass cytometry, one can envision that the cellular and molecular network of the neurofibroma tumor microenvironment will soon be delineated at a much improved resolution. There are also opportunities to fill in this network landscape with data from current clinical trials primarily focusing on neoplastic Schwann cells. The reports of changes in tumor microenvironment components, such as immune cells and fibroblasts, in response to therapies, are important clues to elucidating tumor microenvironment dynamics. In summary, targeting the tumor microenvironment provides an array of opportunities for developing new therapeutic routes to advance the treatment of neurofibroma patients.

ACKNOWLEDGEMENTS

LQL is the Thomas L. Shield, M.D. Endowed Professor in Dermatology. He held a Career Award for Medical Scientists from the Burroughs Wellcome Fund, and is supported by funding from the National Cancer Institute of the NIH (R01 CA166593) and the Developmental and Hyperactive RAS Tumor SPORE (U54 CA196519); the US Department of Defense (W81XWH-21–1-0651); the Neurofibromatosis Therapeutic Acceleration Program; the NF1 Research Consortium Fund; and the Giorgio Foundation.

Footnotes

COMPETING INTERESTS

The authors declare no competing interests.

CONSENT TO PUBLISH

There are no enrolled patients in this review and all authors provided consent for publication.

ADDITIONAL INFORMATION

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REFERENCES

  • 1.Le LQ, Parada LF. Tumor microenvironment and neurofibromatosis type I: connecting the GAPs. Oncogene. 2007;26:4609–16. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 2.Bergoug M, Doudeau M, Godin F, Mosrin C, Vallee B, and Benedetti H, Neurofibromin structure, functions and regulation. Cells. 2020;9:2365. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 3.Hirbe AC, Gutmann DH. Neurofibromatosis type 1: a multidisciplinary approach to care. Lancet Neurol. 2014;13:834–43. [DOI] [PubMed] [Google Scholar]
  • 4.Brosseau JP, Pichard DC, Legius EH, Wolkenstein P, Lavker RM, Blakeley JO, et al. The biology of cutaneous neurofibromas: consensus recommendations for setting research priorities. Neurology. 2018;91:S14–S20. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 5.Dombi E, Solomon J, Gillespie AJ, Fox E, Balis FM, Patronas N, et al. NF1 plexiform neurofibroma growth rate by volumetric MRI: relationship to age and body weight. Neurology. 2007;68:643–7. [DOI] [PubMed] [Google Scholar]
  • 6.Evans DG, Baser ME, McGaughran J, Sharif S, Howard E, Moran A. Malignant peripheral nerve sheath tumours in neurofibromatosis 1. J Med Genet. 2002;39:311–4. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 7.Zhu Y, Ghosh P, Charnay P, Burns DK, Parada LF. Neurofibromas in NF1: Schwann cell origin and role of tumor environment. Science. 2002;296:920–2. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 8.Jessen KR, Mirsky R, Lloyd AC. Schwann cells: development and role in nerve repair. Cold Spring Harb Perspect Biol. 2015;7:a020487. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 9.Zheng H, Chang L, Patel N, Yang J, Lowe L, Burns DK, et al. Induction of abnormal proliferation by nonmyelinating schwann cells triggers neurofibroma formation. Cancer Cell. 2008;13:117–28. [DOI] [PubMed] [Google Scholar]
  • 10.Joseph NM, Mosher JT, Buchstaller J, Snider P, McKeever PE, Lim M, et al. The loss of Nf1 transiently promotes self-renewal but not tumorigenesis by neural crest stem cells. Cancer Cell. 2008;13:129–40. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 11.Le LQ, Liu C, Shipman T, Chen Z, Suter U, Parada LF. Susceptible stages in Schwann cells for NF1-associated plexiform neurofibroma development. Cancer Res. 2011;71:4686–95. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 12.Chen Z, Liu C, Patel AJ, Liao CP, Wang Y, Le LQ. Cells of origin in the embryonic nerve roots for NF1-associated plexiform neurofibroma. Cancer Cell. 2014;26:695–706. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 13.Saito H, Yoshida T, Yamazaki H, Suzuki N. Conditional N-rasG12V expression promotes manifestations of neurofibromatosis in a mouse model. Oncogene. 2007;26:4714–9. [DOI] [PubMed] [Google Scholar]
  • 14.Chen Z, Mo J, Brosseau JP, Shipman T, Wang Y, Liao CP, et al. Spatiotemporal loss of NF1 in Schwann cell lineage leads to different types of cutaneous neurofibroma susceptible to modification by the hippo pathway. Cancer Disco. 2019;9:114–29. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 15.Mo J, Anastasaki C, Chen Z, Shipman T, Papke J, Yin K, et al. , Humanized neurofibroma model from induced pluripotent stem cells delineates tumor pathogenesis and developmental origins. J Clin Invest. 2021;131:e139807. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 16.Radomska KJ, Coulpier F, Gresset A, Schmitt A, Debbiche A, Lemoine S, et al. Cellular origin, tumor progression, and pathogenic mechanisms of cutaneous neurofibromas revealed by mice with Nf1 knockout in boundary cap cells. Cancer Disco. 2019;9:130–47. [DOI] [PubMed] [Google Scholar]
  • 17.Liao CP, Booker RC, Brosseau JP, Chen Z, Mo J, Tchegnon E, et al. Contributions of inflammation and tumor microenvironment to neurofibroma tumorigenesis. J Clin Invest. 2018;128:2848–61. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 18.Brosseau JP, Liao CP, Wang Y, Ramani V, Vandergriff T, Lee M, et al. NF1 heterozygosity fosters de novo tumorigenesis but impairs malignant transformation. Nat Commun. 2018;9:5014. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 19.Wu J, Williams JP, Rizvi TA, Kordich JJ, Witte D, Meijer D, et al. Plexiform and dermal neurofibromas and pigmentation are caused by Nf1 loss in desert hedgehog-expressing cells. Cancer Cell. 2008;13:105–16. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 20.Fletcher JS, Pundavela J, Ratner N. After Nf1 loss in Schwann cells, inflammation drives neurofibroma formation. Neurooncol Adv. 2020;2:i23–i32. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 21.Ribeiro S, Napoli I, White IJ, Parrinello S, Flanagan AM, Suter U, et al. Injury signals cooperate with Nf1 loss to relieve the tumor-suppressive environment of adult peripheral nerve. Cell Rep. 2013;5:126–36. [DOI] [PubMed] [Google Scholar]
  • 22.Rice FL, Houk G, Wymer JP, Gosline SJC, Guinney J, Wu J, et al. The evolution and multi-molecular properties of NF1 cutaneous neurofibromas originating from C-fiber sensory endings and terminal Schwann cells at normal sites of sensory terminations in the skin. PLoS ONE. 2019;14:e0216527. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 23.Parrinello S, Noon LA, Harrisingh MC, Wingfield Digby P, Rosenberg LH, Cremona CA, et al. NF1 loss disrupts Schwann cell-axonal interactions: a novel role for semaphorin 4F. Genes Dev. 2008;22:3335–48. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 24.Liao CP, Pradhan S, Chen Z, Patel AJ, Booker RC, Le LQ. The role of nerve microenvironment for neurofibroma development. Oncotarget. 2016;7:61500–8. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 25.Fricker FR, Bennett DL. The role of neuregulin-1 in the response to nerve injury. Future Neurol. 2011;6:809–22. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 26.Fricker FR, Lago N, Balarajah S, Tsantoulas C, Tanna S, Zhu N, et al. Axonally derived neuregulin-1 is required for remyelination and regeneration after nerve injury in adulthood. J Neurosci. 2011;31:3225–33. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 27.Ronellenfitsch MW, Harter PN, Kirchner M, Heining C, Hutter B, Gieldon L, et al. Targetable ERBB2 mutations identified in neurofibroma/schwannoma hybrid nerve sheath tumors. J Clin Invest. 2020;130:2488–95. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 28.Harboe M, Torvund-Jensen J, Kjaer-Sorensen K, Laursen LS. Ephrin-A1-EphA4 signaling negatively regulates myelination in the central nervous system. Glia. 2018;66:934–50. [DOI] [PubMed] [Google Scholar]
  • 29.Linneberg C, Harboe M, and Laursen LS, Axo-Glia Interaction preceding cns myelination is regulated by bidirectional Eph-Ephrin signaling. ASN Neuro. 2015;7:1759091415602859. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 30.Woodhoo A, Alonso MB, Droggiti A, Turmaine M, D’Antonio M, Parkinson DB, et al. Notch controls embryonic Schwann cell differentiation, postnatal myelination and adult plasticity. Nat Neurosci. 2009;12:839–47. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 31.Yang FC, Ingram DA, Chen S, Hingtgen CM, Ratner N, Monk KR, et al. Neurofibromin-deficient Schwann cells secrete a potent migratory stimulus for Nf1+/− mast cells. J Clin Invest. 2003;112:1851–61. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 32.Yang FC, Chen S, Clegg T, Li X, Morgan T, Estwick SA, et al. Nf1+/− mast cells induce neurofibroma like phenotypes through secreted TGF-beta signaling. Hum Mol Genet. 2006;15:2421–37. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 33.Riccardi VM. Mast-cell stabilization to decrease neurofibroma growth. Preliminary experience ketotifen. Arch Dermatol. 1987;123:1011–6. [PubMed] [Google Scholar]
  • 34.Riccardi VM. Ketotifen suppression of NF1 neurofibroma growth over 30 years. Am J Med Genet A. 2015;167:1570–7. [DOI] [PubMed] [Google Scholar]
  • 35.Yang FC, Ingram DA, Chen S, Zhu Y, Yuan J, Li X, et al. Nf1-dependent tumors require a microenvironment containing Nf1+/− and c-kit-dependent bone marrow. Cell. 2008;135:437–48. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 36.Robertson KA, Nalepa G, Yang FC, Bowers DC, Ho CY, Hutchins GD, et al. Imatinib mesylate for plexiform neurofibromas in patients with neurofibromatosis type 1: a phase 2 trial. Lancet Oncol. 2012;13:1218–24. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 37.Prada CE, Jousma E, Rizvi TA, Wu J, Dunn RS, Mayes DA, et al. Neurofibroma-associated macrophages play roles in tumor growth and response to pharmacological inhibition. Acta Neuropathol. 2013;125:159–68. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 38.Choi K, Komurov K, Fletcher JS, Jousma E, Cancelas JA, Wu J, et al. An inflammatory gene signature distinguishes neurofibroma Schwann cells and macrophages from cells in the normal peripheral nervous system. Sci Rep. 2017;7:43315. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 39.Peltonen J, Penttinen R, Larjava H, Aho HJ. Collagens in neurofibromas and neurofibroma cell cultures. Ann NY Acad Sci. 1986;486:260–70. [DOI] [PubMed] [Google Scholar]
  • 40.Atit RP, Crowe MJ, Greenhalgh DG, Wenstrup RJ, Ratner N. The Nf1 tumor suppressor regulates mouse skin wound healing, fibroblast proliferation, and collagen deposited by fibroblasts. J Invest Dermatol. 1999;112:835–42. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 41.Widemann BC, Babovic-Vuksanovic D, Dombi E, Wolters PL, Goldman S, Martin S, et al. Phase II trial of pirfenidone in children and young adults with neurofibromatosis type 1 and progressive plexiform neurofibromas. Pediatr Blood Cancer. 2014;61:1598–602. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 42.Babovic-Vuksanovic D, Ballman K, Michels V, McGrann P, Lindor N, King B, et al. Phase II trial of pirfenidone in adults with neurofibromatosis type 1. Neurology. 2006;67:1860–2. [DOI] [PubMed] [Google Scholar]
  • 43.Dundr P, Povysil C, Tvrdik D. Actin expression in neural crest cell-derived tumors including schwannomas, malignant peripheral nerve sheath tumors, neurofibromas and melanocytic tumors. Pathol Int. 2009;59:86–90. [DOI] [PubMed] [Google Scholar]
  • 44.Brosseau JP, Sathe AA, Wang Y, Nguyen T, Glass DA 2nd, Xing C, et al. Human cutaneous neurofibroma matrisome revealed by single-cell RNA sequencing. Acta Neuropathol Commun. 2021;9:11. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 45.Driskell RR, Lichtenberger BM, Hoste E, Kretzschmar K, Simons BD, Charalambous M, et al. Distinct fibroblast lineages determine dermal architecture in skin development and repair. Nature. 2013;504:277–81. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 46.Kadono T, Soma Y, Takehara K, Nakagawa H, Ishibashi Y, Kikuchi K. The growth regulation of neurofibroma cells in neurofibromatosis type-1: increased responses to PDGF-BB and TGF-beta 1. Biochem Biophys Res Commun. 1994;198:827–34. [DOI] [PubMed] [Google Scholar]
  • 47.Mashour GA, Ratner N, Khan GA, Wang HL, Martuza RL, Kurtz A. The angiogenic factor midkine is aberrantly expressed in NF1-deficient Schwann cells and is a mitogen for neurofibroma-derived cells. Oncogene. 2001;20:97–105. [DOI] [PubMed] [Google Scholar]
  • 48.Misa K, Tanino Y, Wang X, Nikaido T, Kikuchi M, Sato Y, et al. , Involvement of midkine in the development of pulmonary fibrosis. Physiol Rep. 2017;5:e13383. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 49.Dolivo DM, Larson SA, Dominko T. Fibroblast growth factor 2 as an antifibrotic: antagonism of myofibroblast differentiation and suppression of pro-fibrotic gene expression. Cytokine Growth Factor Rev. 2017;38:49–58. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 50.Bonner JC. Regulation of PDGF and its receptors in fibrotic diseases. Cytokine Growth Factor Rev. 2004;15:255–73. [DOI] [PubMed] [Google Scholar]
  • 51.Piersma B, Bank RA, Boersema M. Signaling in fibrosis: TGF-beta, WNT, and YAP/TAZ Converge. Front Med (Lausanne). 2015;2:59. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 52.Wehner D, Tsarouchas TM, Michael A, Haase C, Weidinger G, Reimer MM, et al. Wnt signaling controls pro-regenerative collagen XII in functional spinal cord regeneration in zebrafish. Nat Commun. 2017;8:126. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 53.Moleirinho S, Patrick C, Tilston-Lunel AM, Higginson JR, Angus L, Antkowiak M, et al. Willin, an upstream component of the hippo signaling pathway, orchestrates mammalian peripheral nerve fibroblasts. PLoS ONE. 2013;8:e60028. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 54.Parrinello S, Napoli I, Ribeiro S, Wingfield Digby P, Fedorova M, Parkinson DB, et al. EphB signaling directs peripheral nerve regeneration through Sox2-dependent Schwann cell sorting. Cell. 2010;143:145–55. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 55.Henderson NC, Rieder F, Wynn TA. Fibrosis: from mechanisms to medicines. Nature. 2020;587:555–66. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 56.Streuli CH, Schmidhauser C, Bailey N, Yurchenco P, Skubitz AP, Roskelley C, et al. Laminin mediates tissue-specific gene expression in mammary epithelia. J Cell Biol. 1995;129:591–603. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 57.Mascharak S, desJardins-Park HE, Davitt MF, Griffin M, Borrelli MR, Moore AL, et al. , Preventing Engrailed-1 activation in fibroblasts yields wound regeneration without scarring. Science. 2021;372:eaba2374. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 58.Park J, Scherer PE. Adipocyte-derived endotrophin promotes malignant tumor progression. J Clin Invest. 2012;122:4243–56. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 59.Sun K, Park J, Gupta OT, Holland WL, Auerbach P, Zhang N, et al. Endotrophin triggers adipose tissue fibrosis and metabolic dysfunction. Nat Commun. 2014;5:3485. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 60.Wishart AL, Conner SJ, Guarin JR, Fatherree JP, Peng Y, McGinn RA, et al. , Decellularized extracellular matrix scaffolds identify full-length collagen VI as a driver of breast cancer cell invasion in obesity and metastasis. Sci Adv. 2020;6: eabc3175. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 61.Harigai R, Sakai S, Nobusue H, Hirose C, Sampetrean O, Minami N, et al. Tranilast inhibits the expression of genes related to epithelial-mesenchymal transition and angiogenesis in neurofibromin-deficient cells. Sci Rep. 2018;8:6069. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 62.Hagel C, Behrens T, Prehm P, Schnabel C, Glatzel M, Friedrich RE. Hyaluronan in intra-operative edema of NF1-associated neurofibromas. Neuropathology. 2012;32:406–14. [DOI] [PubMed] [Google Scholar]
  • 63.Toole BP. Hyaluronan-CD44 interactions in cancer: paradoxes and possibilities. Clin Cancer Res. 2009;15:7462–8. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 64.Kim Y, Lee YS, Choe J, Lee H, Kim YM, Jeoung D. CD44-epidermal growth factor receptor interaction mediates hyaluronic acid-promoted cell motility by activating protein kinase C signaling involving Akt, Rac1, Phox, reactive oxygen species, focal adhesion kinase, and MMP-2. J Biol Chem. 2008;283:22513–28. [DOI] [PubMed] [Google Scholar]
  • 65.Gorlewicz A, Wlodarczyk J, Wilczek E, Gawlak M, Cabaj A, Majczynski H, et al. CD44 is expressed in non-myelinating Schwann cells of the adult rat, and may play a role in neurodegeneration-induced glial plasticity at the neuromuscular junction. Neurobiol Dis. 2009;34:245–58. [DOI] [PubMed] [Google Scholar]
  • 66.Jaakkola S, Peltonen J, Riccardi V, Chu ML, Uitto J. Type 1 neurofibromatosis: selective expression of extracellular matrix genes by Schwann cells, perineurial cells, and fibroblasts in mixed cultures. J Clin Invest. 1989;84:253–61. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 67.Sottile J, Hocking DC. Fibronectin polymerization regulates the composition and stability of extracellular matrix fibrils and cell-matrix adhesions. Mol Biol Cell. 2002;13:3546–59. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 68.Dallas SL, Sivakumar P, Jones CJ, Chen Q, Peters DM, Mosher DF, et al. Fibronectin regulates latent transforming growth factor-beta (TGF beta) by controlling matrix assembly of latent TGF beta-binding protein-1. J Biol Chem. 2005;280:18871–80. [DOI] [PubMed] [Google Scholar]
  • 69.Gross AM, Wolters PL, Dombi E, Baldwin A, Whitcomb P, Fisher MJ, et al. Selumetinib in children with inoperable plexiform neurofibromas. N Engl J Med. 2020;382:1430–42. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 70.Wang-Gillam A Targeting stroma: a tale of caution. J Clin Oncol. 2019;37:1041–3. [DOI] [PubMed] [Google Scholar]

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