Multiple congenital melanocytic nevi and neurocutaneous melanosis are caused by postzygotic mutations in codon 61 of NRAS

Abstract

Congenital melanocytic nevi (CMN) can be associated with neurological abnormalities and an increased risk of melanoma. Mutations in NRAS, BRAF, and Tp53 have been described in individual CMN samples; however, their role in the pathogenesis of multiple CMN within the same subject and development of associated features has not been clear. We hypothesized that a single postzygotic mutation in NRAS could be responsible for multiple CMN in the same individual, as well as for melanocytic and nonmelanocytic central nervous system (CNS) lesions. From 15 patients, 55 samples with multiple CMN were sequenced after site-directed mutagenesis and enzymatic digestion of the wild-type allele. Oncogenic missense mutations in codon 61 of NRAS were found in affected neurological and cutaneous tissues of 12 out of 15 patients, but were absent from unaffected tissues and blood, consistent with NRAS mutation mosaicism. In 10 patients, the mutation was consistently c.181C>A, p.Q61K, and in 2 patients c.182A>G, p.Q61R. All 11 non-melanocytic and melanocytic CNS samples from 5 patients were mutation positive, despite NRAS rarely being reported as mutated in CNS tumors. Loss of heterozygosity was associated with the onset of melanoma in two cases, implying a multistep progression to malignancy. These results suggest that single postzygotic NRAS mutations are responsible for multiple CMN and associated neurological lesions in the majority of cases.

Figure 1

Clinical and radiological images of congenital melanocytic nevus (CMN) syndrome. (From left to right) An example of the cutaneous phenotype of multiple CMNs (written consent for publication was obtained); magnetic resonance (MR) images showing two thoracic spinal tumors (neurocristic hamartomata), diffuse leptomeningeal melanocytosis, and frontal lobe meningioma.

Figure 2

Progression from NRAS Q61K heterozygosity to homozygosity with the onset of malignancy. (Above) Congenital proliferative nodule within a congenital melanocytic nevus (CMN). (From left to right) Clinical appearance, hematoxylin and eosin (H&E)-stained section at × 20 magnification, DNA sequence chromatogram (reverse) showing heterozygous Q61K mutation. (Below) Malignant melanoma arising within the same CMN in the same patient 5 years later. (From left to right) Clinical appearance, H&E-stained section at × 20 magnification, DNA sequence chromatogram showing homozygous Q61K mutation.

Figure 3

Array comparative genomic hybridization (CGH) in melanoma samples. Array CGH findings in both available primary melanoma samples (cases 5 and 12 from Table 2) showing (a) a list of all gains and losses of ⩾30 Mb, and (b) heterozygous partial deletion of chromosome 9p, including the CDKN2A locus.