Large-Giant Congenital Melanocytic Nevi: Moving Beyond NRAS Mutations
2019; Elsevier BV; Volume: 139; Issue: 4 Linguagem: Inglês
10.1016/j.jid.2018.10.003
ISSN1523-1747
Autores Tópico(s)Melanoma and MAPK Pathways
ResumoLarge-giant congenital melanocytic nevi have been well characterized clinically, yet questions remain about the heterogenous phenotypes observed. Martins da Silva et al. (2018) highlight the genotypic diversity between "classic" and "spilus-like" congenital melanocytic nevi by analyzing multiple biopsy sites and matching satellite nevi. This study provides evidence for alternative modes of development beyond the well-established NRAS mutation paradigm. Large-giant congenital melanocytic nevi have been well characterized clinically, yet questions remain about the heterogenous phenotypes observed. Martins da Silva et al. (2018) highlight the genotypic diversity between "classic" and "spilus-like" congenital melanocytic nevi by analyzing multiple biopsy sites and matching satellite nevi. This study provides evidence for alternative modes of development beyond the well-established NRAS mutation paradigm. Importantly, in five additional biopsy sites, no mutations (e.g., NRAS or BRAF) or fusion genes were detected, which clearly indicates congenital melanocytic nevi develop via alternative mechanisms.Clinical Implications•Somatic mosaicism contributes to congenital melanocytic nevi (CMN) formation and data from Martins da Silva et al., 2018Martins da Silva V. Martinez-Barrios E. Tell-Marti G. Dabad M. Carrera C. Aguilera P. Genetic abnormalities in large to giant congenital nevi: beyond NRAS mutations.J Invest Dermatol. 2019; 139: 900-908Abstract Full Text Full Text PDF PubMed Scopus (47) Google Scholar point to mosaicism occurring within the lesion, which may contribute to lesion heterogeneity.•Large-giant CMN have a higher malignant transformation rate compared to common acquired nevi, which cannot be explained by the presence of mutations in MAPK pathway genes, including NRAS.•The post-zygotic initiating event remains to be discovered in ∼40% of all large-giant CMN.Present at birth, congenital melanocytic nevi (CMN) range in size from small, medium, or large to giant, and are historically known to be associated with a high frequency of NRAS mutations. On the other hand, common acquired melanocytic nevi (AMN) are considered to have mutually exclusive mutations in BRAF (85%) or NRAS (15%) (Tan et al., 2018Tan J.M. Tom L.N. Jagirdar K. Lambie D. Schaider H. Sturm R.A. et al.The BRAF and NRAS mutation prevalence in dermoscopic subtypes of acquired naevi reveals constitutive mitogen-activated protein kinase pathway activation.Br J Dermatol. 2018; 178: 191-197Crossref PubMed Scopus (21) Google Scholar). Despite these frequent somatic mutations, acquired nevi rarely transform into melanoma, whereas large-giant CMN have a relatively high transformation rate (10–15%) (Kinsler et al., 2017Kinsler V.A. O'Hare P. Bulstrode N. Calonje J.E. Chong W.K. Hargrave D. et al.Melanoma in congenital melanocytic naevi.Br J Dermatol. 2017; 176: 1131-1143Crossref PubMed Scopus (101) Google Scholar). Small to medium CMN also have higher transformation rates than AMN. Importantly, people with large-giant CMN are also prone to melanoma in unaffected skin as well as in the central nervous system (Kinsler et al., 2017Kinsler V.A. O'Hare P. Bulstrode N. Calonje J.E. Chong W.K. Hargrave D. et al.Melanoma in congenital melanocytic naevi.Br J Dermatol. 2017; 176: 1131-1143Crossref PubMed Scopus (101) Google Scholar). AMN were recently discovered to be largely induced by UVR, sharing a common UV signature with cutaneous melanoma (Stark et al., 2018Stark M.S. Tan J.M. Tom L. Jagirdar K. Lambie D. Schaider H. et al.Whole-exome sequencing of acquired nevi identifies mechanisms for development and maintenance of benign neoplasms.J Invest Dermatol. 2018; 138: 1636-1644Abstract Full Text Full Text PDF PubMed Scopus (30) Google Scholar). The genomic events that transition an AMN to a melanoma have been determined (Shain et al., 2018Shain A.H. Joseph N.M. Yu R. Benhamida J. Liu S. Prow T. et al.Genomic and transcriptomic analysis reveals incremental disruption of key signaling pathways during melanoma evolution.Cancer Cell. 2018; 34: 45-55 e4Abstract Full Text Full Text PDF PubMed Scopus (101) Google Scholar). However, the same cannot be said for CMN, as NRAS mutations are not sufficient to induce malignant transformation. Additionally, what remains to be thoroughly investigated is whether CMN harbor the same somatic alterations across the large CMN and associated satellite nevi. Martins da Silva et al., 2018Martins da Silva V. Martinez-Barrios E. Tell-Marti G. Dabad M. Carrera C. Aguilera P. Genetic abnormalities in large to giant congenital nevi: beyond NRAS mutations.J Invest Dermatol. 2019; 139: 900-908Abstract Full Text Full Text PDF PubMed Scopus (47) Google Scholar aimed to study multiple biopsied regions within large-giant CMN as well as matching satellite nevi. They utilized a focused approach of ultra-deep targeted panel next-generation sequencing to identify the presence or absence of NRAS mutations (plus other genes, such as BRAF, KRAS, KIT, and GNAQ) together with fusion gene analysis (via RNA sequencing) in an attempt to move "beyond NRAS," as their title suggests. Importantly, in five additional biopsy sites, no mutations (e.g., NRAS or BRAF) or fusion genes were detected, which clearly indicates congenital melanocytic nevi develop via alternative mechanisms. •Somatic mosaicism contributes to congenital melanocytic nevi (CMN) formation and data from Martins da Silva et al., 2018Martins da Silva V. Martinez-Barrios E. Tell-Marti G. Dabad M. Carrera C. Aguilera P. Genetic abnormalities in large to giant congenital nevi: beyond NRAS mutations.J Invest Dermatol. 2019; 139: 900-908Abstract Full Text Full Text PDF PubMed Scopus (47) Google Scholar point to mosaicism occurring within the lesion, which may contribute to lesion heterogeneity.•Large-giant CMN have a higher malignant transformation rate compared to common acquired nevi, which cannot be explained by the presence of mutations in MAPK pathway genes, including NRAS.•The post-zygotic initiating event remains to be discovered in ∼40% of all large-giant CMN. If we step back in time, before the advent of next-generation sequencing technologies that enable an unbiased, comprehensive assessment, generally speaking, genomic analysis has been restricted to candidate gene approaches based upon a priori knowledge. In the 1980s, the RAS pathway genes—HRAS, KRAS, and NRAS—were identified as some of the earliest genes involved in neoplastic transformation and, as such, have been the focus of research in a number of malignancies, including melanoma. In sum, NRAS mutations are common in melanoma, whereas HRAS or KRAS mutations are rare, which suggests melanocyte specificity. Evidence for this can be observed in studies involving Ink4a/Arf-deficient mice (CDKN2A locus) with dominant-active human NRAS (N-RasQ61K) targeted to the melanocyte lineage, whereby 90% of mice develop melanoma at 6 months (Ackermann et al., 2005Ackermann J. Frutschi M. Kaloulis K. McKee T. Trumpp A. Beermann F. Metastasizing melanoma formation caused by expression of activated N-RasQ61K on an INK4a-deficient background.Cancer Res. 2005; 65: 4005-4011Crossref PubMed Scopus (224) Google Scholar). In contrast to this, in an earlier study, Chin et al., 1997Chin L. Pomerantz J. Polsky D. Jacobson M. Cohen C. Cordon-Cardo C. et al.Cooperative effects of INK4a and ras in melanoma susceptibility in vivo.Genes Dev. 1997; 11: 2822-2834Crossref PubMed Scopus (350) Google Scholar used Ink4a/Arf-deficient mice with HRAS as the dominant mutation, which resulted in 50% of melanoma development over the same timeframe. To help explain the differences in disease penetrance, Whitwam et al., 2007Whitwam T. Vanbrocklin M.W. Russo M.E. Haak P.T. Bilgili D. Resau J.H. et al.Differential oncogenic potential of activated RAS isoforms in melanocytes.Oncogene. 2007; 26: 4563-4570Crossref PubMed Scopus (54) Google Scholar assessed all RAS genes and revealed that the NRAS mutation in Ink4a/Arf-deficient melanocytes led to increased cellular proliferation. Importantly, however, despite the activation of the RAS pathway, the take-home message is that NRAS mutations in isolation are not sufficient to cause malignant transformation and require loss of function alterations to occur in tumor suppressor genes, such as CDKN2A. The earliest report of the involvement of the NRAS oncogene with CMN was in a study by Carr and Mackie, 1994Carr J. Mackie R.M. Point mutations in the N-ras oncogene in malignant melanoma and congenital naevi.Br J Dermatol. 1994; 131: 72-77Crossref PubMed Scopus (88) Google Scholar, who found NRAS to be mutated in 12 of 43 (28%) specimens. In a follow-up study, Papp et al., 1999Papp T. Pemsel H. Zimmermann R. Bastrop R. Weiss D.G. Schiffmann D. Mutational analysis of the N-ras, p53, p16INK4a, CDK4, and MC1R genes in human congenital melanocytic naevi.J Med Genet. 1999; 36: 610-614PubMed Google Scholar found that 10 of 18 (56%) CMN had an NRAS mutation, and that this mutation was not always present in multiple CMN from the same person. At the time, this study suggested that NRAS mutations were not an essential primary event in CMN formation (Papp et al., 1999Papp T. Pemsel H. Zimmermann R. Bastrop R. Weiss D.G. Schiffmann D. Mutational analysis of the N-ras, p53, p16INK4a, CDK4, and MC1R genes in human congenital melanocytic naevi.J Med Genet. 1999; 36: 610-614PubMed Google Scholar). Since then, in a study of large-giant CMN, Charbel et al., 2014Charbel C. Fontaine R.H. Malouf G.G. Picard A. Kadlub N. El-Murr N. et al.NRAS mutation is the sole recurrent somatic mutation in large congenital melanocytic nevi.J Invest Dermatol. 2014; 134: 1067-1074Abstract Full Text Full Text PDF PubMed Scopus (113) Google Scholar used a variety of sensitive detection methods and concluded that NRAS was the sole recurrent somatic mutation present in 95% of assessed specimens. The authors further assessed five specimens using whole-exome sequencing and found that NRAS was likely to be the initiating event, as no other mutations were found to be at a higher mutation frequency (Charbel et al., 2014Charbel C. Fontaine R.H. Malouf G.G. Picard A. Kadlub N. El-Murr N. et al.NRAS mutation is the sole recurrent somatic mutation in large congenital melanocytic nevi.J Invest Dermatol. 2014; 134: 1067-1074Abstract Full Text Full Text PDF PubMed Scopus (113) Google Scholar). Like AMN, CMN are heterogeneous in many aspects of their clinical appearance (e.g., size, color, and shape). Often congenital nevi are too large to be removed surgically, so for diagnostic and research purposes, a small biopsy is taken for pathologic and molecular analysis. In comparison to AMN, the molecular analyses of CMN have been limited, and each study varies in sample size and heterogeneity of clinical specimens, and they have a diverse range of detection methods. As such, no studies are directly comparable. However, across all studies of CMN, a common theme emerged, with mutations in NRAS being the most commonly known somatic alteration, and it has long been suggested that it is the initiating, post-zygotic mutational event that is the cause of congenital nevus development. Moreover, because NRAS mutations are considered to be present during development, the pattern that the CMN forms on the skin must be the result of mosaicism (Kinsler et al., 2017Kinsler V.A. O'Hare P. Bulstrode N. Calonje J.E. Chong W.K. Hargrave D. et al.Melanoma in congenital melanocytic naevi.Br J Dermatol. 2017; 176: 1131-1143Crossref PubMed Scopus (101) Google Scholar), as NRAS mutations are not detectable in the unaffected skin. While targeted panel sequencing is limited to a set number of genes, it does allow for ultra-high depth of coverage to detect rare mutational events in known genes like NRAS, which reduces the possibility of false negatives. Martins da Silva et al., 2018Martins da Silva V. Martinez-Barrios E. Tell-Marti G. Dabad M. Carrera C. Aguilera P. Genetic abnormalities in large to giant congenital nevi: beyond NRAS mutations.J Invest Dermatol. 2019; 139: 900-908Abstract Full Text Full Text PDF PubMed Scopus (47) Google Scholar analyzed 21 large-giant CMN, which were further classified as classic (57%) or spilus-like (43%), depending on their clinical appearance. There were a total of 54 biopsies panel-sequenced from the 21 study participants and, in most cases, multiple biopsies per participants were available (Supplementary Table S1 online summarizes the data presented in Martins da Silva et al., 2018Martins da Silva V. Martinez-Barrios E. Tell-Marti G. Dabad M. Carrera C. Aguilera P. Genetic abnormalities in large to giant congenital nevi: beyond NRAS mutations.J Invest Dermatol. 2019; 139: 900-908Abstract Full Text Full Text PDF PubMed Scopus (47) Google Scholar). While nevus cellularity was not assessed in this study, one can safely assume that it was the dominant cell type in the biopsied material. Taking into account the mutant allele frequency (MAF), if a given CMN was initiated and driven by a post-zygotic NRAS mutation, the MAF should be relatively high. Classic CMN ranged from 20% to 57% MAF in the main lesion and the spilus-like lesions ranged from 3.6% to 59%. In biopsies with an MAF of <10%, these mutation events are unlikely to be the initiating event. With this in mind, classic lesions had 7 of 12 (58%) NRAS mutations and were likely driven by this mutation. In the classic lesions, when NRAS mutations were not present, novel fusion genes (ZEB2-ALK and SOX5-RAF1 fusion) were found in the main lesions, as well as matching biopsies (samples 3 and 4). Spilus-like CMN were overall more diverse, with 5 of 9 (56%) driven by NRAS/KRAS/BRAF. Samples 14 and 21 were both NRAS-mutant, but had an MAF of 3.6% and 5%, respectively, which are unlikely to be initiating events. Importantly, in combination, six (28%) lesions harbored no known mutation or had low MAF mutations or non-initiating mutations present. The total frequency of NRAS mutations is in line with one of the original studies that postulated that NRAS might not be required for CMN development (Papp et al., 1999Papp T. Pemsel H. Zimmermann R. Bastrop R. Weiss D.G. Schiffmann D. Mutational analysis of the N-ras, p53, p16INK4a, CDK4, and MC1R genes in human congenital melanocytic naevi.J Med Genet. 1999; 36: 610-614PubMed Google Scholar). Martins da Silva et al., 2018Martins da Silva V. Martinez-Barrios E. Tell-Marti G. Dabad M. Carrera C. Aguilera P. Genetic abnormalities in large to giant congenital nevi: beyond NRAS mutations.J Invest Dermatol. 2019; 139: 900-908Abstract Full Text Full Text PDF PubMed Scopus (47) Google Scholar proposed that the differing MAFs observed, or the detection of no mutations, might be the result of low-nevus cell number present within the biopsied tissue; however, this is not very likely. While there are no histopathologic images to confirm the cellularity of each specimen, one would assume that the cellularity would be sufficient, given the size of the overall lesion. Even if the cellularity was as low as 1% nevus cell, the depth of sequencing was more than sufficient (median coverage approximately ×19,000) to detect somatic mutations. Importantly, Martins da Silva et al., 2018Martins da Silva V. Martinez-Barrios E. Tell-Marti G. Dabad M. Carrera C. Aguilera P. Genetic abnormalities in large to giant congenital nevi: beyond NRAS mutations.J Invest Dermatol. 2019; 139: 900-908Abstract Full Text Full Text PDF PubMed Scopus (47) Google Scholar found evidence for non-uniform or mosaic-like development of CMN with assessable mutation frequencies available across the main CMN and/or matching satellite nevi. From the 14 assessable study participants (samples 1, 3–5, 7–10, 13–17, and 20) that had detectable mutations or fusion-genes, three had different mutations present across the biopsies, or were not detected (samples 16, 17, and 20), and seven had the same somatic alteration (samples 1, 5, 9, 15, 16, and 20), but had differing MAFs (see Supplementary Table S1). For example, sample 9, which has a classic CMN phenotype, has the NRAS mutation present in the main lesion at 57% MAF, whereas in two different biopsy sites, the NRAS MAF is only 35%. The matching satellite lesion is 27% for the same NRAS mutation. Using this example and assuming that the nevus cellularity was equal, the biopsy with 57% MAF must have developed first, followed by the other biopsied regions and then the satellite lesion. Another example is sample 16 (spilus-like), which has a BRAF G464E mutation present at 24% MAF and then in another biopsy site, a different BRAF mutation is present (L584F). Importantly, in five additional biopsy sites, no mutations (e.g., NRAS or BRAF) or fusion genes were detected, which clearly indicates CMN develop via alternative mechanisms. Next, Martins da Silva et al., 2018Martins da Silva V. Martinez-Barrios E. Tell-Marti G. Dabad M. Carrera C. Aguilera P. Genetic abnormalities in large to giant congenital nevi: beyond NRAS mutations.J Invest Dermatol. 2019; 139: 900-908Abstract Full Text Full Text PDF PubMed Scopus (47) Google Scholar assessed whether the phenotypic characteristics (e.g., patterns of anatomic distribution, rugosity, hypertrichosis, and number of satellites) of the large-giant CMN were associated with NRAS mutation. The study numbers are relatively small but no associations with the presence of an NRAS mutation were identified. NRAS mutations play an important role in CMN, but there is still much to be learned from CMN. Martins da Silva et al., 2018Martins da Silva V. Martinez-Barrios E. Tell-Marti G. Dabad M. Carrera C. Aguilera P. Genetic abnormalities in large to giant congenital nevi: beyond NRAS mutations.J Invest Dermatol. 2019; 139: 900-908Abstract Full Text Full Text PDF PubMed Scopus (47) Google Scholar performed whole transcriptome sequencing to detect fusion genes. It is anticipated that these data will expand upon our current paradigm to provide novel insights into the signaling pathways involved in CMN development. We presume that the MAPK pathway will be a dominant player, given the proportion of RAS/RAF mutations, but at this juncture a conclusion cannot be drawn. This study has highlighted that classic and spilus-like CMN are both phenotypically and genotypically diverse. Furthermore, it can be concluded that phenotypic characteristics, such as patterns of anatomical distribution, rugosity, hypertrichosis, and number of satellites, are not related to common point mutations in NRAS or BRAF. Perhaps the transcriptome analysis will assist in determining causality? Lastly, because large-giant CMN are prone to melanoma development, the underlying mechanisms remain to be elucidated, which may pave the way for novel therapeutic targets. Mitchell S. Stark: https://orcid.org/0000-0002-4510-2161 The author states no conflict of interest. Download .pdf (.45 MB) Help with pdf files Supplementary Table S1 Genetic Abnormalities in Large to Giant Congenital Nevi: Beyond NRAS MutationsJournal of Investigative DermatologyVol. 139Issue 4PreviewLarge and giant congenital melanocytic nevi (CMN) are rare melanocytic lesions mostly caused by postzygotic NRAS alteration. Molecular characterization is usually focused on NRAS and BRAF genes in a unique biopsy sample of the CMN. However, large/giant CMN may exhibit phenotypic differences among distinct areas, and patients differ in features such as presence of multiple CMN or spilus-like lesions. Herein, we have characterized a series of 21 large/giant CMN including patients with spilus-type nevi (9/21 patients, 42.8%). Full-Text PDF Open Archive
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