TO THE EDITOR
Grouped patterns of pigmented lesions are infrequent. Here, we analyze a rare case of segmentally distributed, agminated nevi characterized by multiple densely clustered lesions that are confined to a developmental segment. In fact, most segmental nevi are not agminated (Happle, 2002) and arise because of perturbations in the proliferation, migration, and differentiation of embryological precursors, including melanocytes (Misago et al., 1991; Sun and Tsao, 2008). The unusual growth pattern suggests mosaicism, i.e., a condition whereby an organism is composed of two genetically distinct cell populations due to a post-zygotic mutation (Itin and Burger, 2009). Cutaneous mosaicism often manifests as lines of Blaschko, a checkerboard pattern, or a phylloid (leaf-like) pattern (Happle, 1993).
Recently, mutational analyses of melanocytic nevi have revealed that congenital moles, common acquired nevi, Spitz nevi, and blue nevi are associated with significant mutation rates in NRAS (Bauer et al., 2007), BRAF (Pollock et al., 2003), HRAS (Da Forno et al., 2009), and GNAQ (Van Raamsdonk et al., 2009), respectively. These findings suggest that mutational activation of specific signaling molecules drives the formation of these nevi. We thus hypothesized that our patient’s agminated segmental nevi (ASN) resulted from a mosaic event related to one of the known RAS effectors.
The patient is a 58-year-old man who developed numerous grouped nevi on his right leg shortly after birth. There was no other significant personal medical history or family history of melanoma or dysplastic nevus syndrome. On examination of his right thigh and calf, there was an extensive band-like cluster of >100 pigmented macules, plaques, and papules clinically consistent with junctional, compound, or intradermal nevi (Figure 1a). Background skin in the area showed no hypo- or hyperpigmentation. Over multiple visits, 11 clinically atypical nevi within the segment were removed although none had histological features of melanoma. Pathological evaluation of the biopsies largely showed intradermal nevi with congenital patterns (Figure 1b).
This genetic study was approved by the Research Ethics Board at Dalhousie University, and was conducted according to Declaration of Helsinki Principles. The patient also gave his written informed consent. We analyzed the 11 ASN and 1 unrelated mole from the right upper arm, which was excised independently because of its clinical atypia. These 12 formalin-fixed, paraffin-embedded specimens were micro-dissected and screened for mutations in NRAS exons 2 and 3, and BRAF exons 11 and 15. There were no canonical activating mutations in NRAS exons or BRAF exon 11. However, all 11 ASN harbored the identical BRAFT1799A transversion (i.e., the BrafV600E alteration) in exon 15. (Primer set: 5′-AGATCTACTGTTTTCCTTTACTTACTACAC-3′ and 5′-GGCCAAAAATTTAATCAGTGGA-3′). This mutation was notably absent in the non-segmental nevus and in the germline DNA from the patient’s blood. During the course of Sanger sequencing, several samples had minor, but reproducible “A” peaks. To create a cross-validating platform, we devised a liquid bead-based assay to more precisely measure the intratumoral representation of the two BRAF alleles (i.e., wild-type “T” and mutated “A”). (Allele-specific primer for T1799T: 5′-CCACTCCATCGAGATTTCT-3′; and T1799A: 5′-CCACTCCATCGAGATTTCA-3′). The bead assay substantiated the presence of the mutation in several samples (Figure 1c).
There are at least two molecular models that could explain our patient’s pigmented lesions. In a “driver” model, a mutation, such as BrafV600E, occurred as a mosaic event, and therefore triggers the transient proliferation of segmental melanocytes, i.e., mole formation; every ASN should then harbor the same BrafV600E alteration. Our finding of 100% mutational concordance among the 11 lesions is certainly consistent with this “driver” model. However, a second “mutator” model can also be considered if the mosaic event did not occur in the BRAF gene, but rather, at another locus which then predisposes the affected melanocyte to develop activating RAS pathway mutations, including some in BRAF. This model would have more support if the ASN were discordant for BRAF status or harbored distinct changes in BRAF, NRAS or some other gene. If the “mutator” model is in fact correct, 11 independent events would have been necessary to account for the observed concordance of BRAFT1799A mutations. How likely is this latter scenario? Bauer et al. recently found that 0/32 (0%) of true congenital nevi and 20/28 (71.4%) acquired melanocytic nevi “with a congenital pattern” harbored BRAFT1799A mutations. As the mosaic nature of the ASN is by definition present at birth, this man’s segmental nevus may be argued as a variation of a congenital nevus, and therefore, likely devoid of BRAF alterations. However, a more conservative approach would classify these moles as acquired nevi with a congenital pattern. In this case, the probability that independent BRAF mutagenesis occurred in 11/11 lesions examined would be estimated at (0.7111) or 2.3%, a low but certainly not negligible chance. There are two additional factors that must be considered in the calculus. First, the early onset and segmental nature of the ASN argues for a mechanistic link between the populating nevi. In other words, an analysis of 11 randomly distributed and arbitrarily selected adult-onset nevi from a single individual is quite different than an analysis from a single group of ASN. Second, there is a BRAF distinction between the nevi inside and the nevus outside of the segment. In this limited but important specimen, the formation of the non-segmental nevus is clearly triggered by a non-BRAF event. Though our evidence could be strengthened by a large-scale sampling of nevi from different sites, further molecular and statistical pursuits are not clinically tenable given the large number of surgeries that have already transpired.
Recent reports suggest that BRAF mutations, at least in melanomas, occur more frequently on intermittently sunexposed skin and much less commonly in non-sun-exposed sites (i.e., mucosal and acral lentiginous melanomas) (Maldonado et al., 2003). The segmental nature of the ASN definitively eliminates solar participation in the activation of BRAF. Germline mutations in BRAF have also been reported in such conditions as cardio-facio-cutaneous (CFC) syndrome, a developmental disorder with characteristic facial, cardiac, and ectodermal defects (Niihori et al., 2006) However, most BRAF mutations in CFC are non-recurrent and rarely the oncogenic mutations observed in cancers (Rodriguez-Viciana et al., 2006). Perhaps the Braf signaling intensity associated with a germline V600E alteration is incompatible with life, whereas more tolerable mutations producing intermediate Braf activity exist in CFC. If the “driver” model is correct, the activating event may have passed a critical susceptibility period in utero.
In summary, we describe a case of segmentally arranged, agminated nevi that exhibit BrafV600E concordance. Although the evidence strongly supports a “driver” mosaic model for the development of these lesions, the high prevalence of BRAF mutations in general raises the possibility of alternative models.