Linear keratinocytic epidermal nevi on trunk skin caused by a somatic FGFR2 p.C382R mutation

Case Report: Sequential postzygotic HRAS mutation and gains of the paternal chromosome 11 carrying the mutated allele in a patient with epidermal nevus and rhabdomyosarcoma: evidence of a multiple-hit mechanism involving HRAS in oncogenic transformation

We report a 7-year-old boy born with epidermal nevi (EN) arranged according to Blaschko's lines involving the face and head, right upper limb, chest, and left lower limb, who developed a left paratesticular embryonal rhabdomyosarcoma at 18 months of age. Parallel sequencing identified a gain-of-function variant (c.37G>C, p.Gly13Arg) of HRAS in both epidermal nevus and tumor but not in leukocytes or buccal mucosal epithelial cells, indicating its postzygotic origin. The variant accounted for 33% and 92% of the total reads in the nevus and tumor DNA specimens, respectively, supporting additional somatic hits in the latter. DNA methylation (DNAm) profiling of the tumor documented a signature consistent with embryonal rhabdomyosarcoma and CNV array analysis inferred from the DNAm arrays and subsequent MLPA analysis demonstrated copy number gains of the entire paternal chromosome 11 carrying the mutated HRAS allele, likely as the result of paternal unidisomy followed by subsequent gain(s) of the paternal chromosome in the tumor. Other structural rearrangements were observed in the tumours, while no additional pathogenic variants affecting genes with role in the RAS-MAPK and PI3K-AKT-MTOR pathways were identified. Our findings provide further evidence of the contribution of "gene dosage" to the multistep process driving cell transformation associated with hyperactive HRAS function.

Cerebriform sebaceous nevus: a subtype of organoid nevus due to specific postzygotic FGFR2 mutations

BACKGROUND

Postzygotic mutations in FGFR2 have been identified in mosaic forms of acne, keratinocytic epidermal nevi, nevoid acanthosis nigricans / rounded and velvety epidermal nevus and in two fetuses with papillomatous pedunculated sebaceous nevus (PPSN).

OBJECTIVES

To determine the clinical and genetic characteristics of children with cerebriform, papillomatous and pedunculated variants of sebaceous nevi.

METHODS

Infants diagnosed with sebaceous nevi characterized by a cerebriform, papillomatous and/or pedunculated morphology over a 10-year period (2010-2019) at three paediatric dermatology centres in Switzerland and France were included in this case series. Clinical and histological characteristics were assessed. Next-generation sequencing was used to assess for FGFR2 mutations.

RESULTS

All nevi were located on the head, with a rounded or linear shape and a typical cerebriform, sometimes papillomatous and pedunculated, surface. No associated extracutaneous anomalies were found. Nevi harboured postzygotic mutations in the transmembrane domain of FGFR2 in 6/8 children (75%), either the known specific p.(Cys382Arg) mutation in 5 cases, or a novel mutation, p.(Val395Asp), in one.

CONCLUSIONS

We found an exquisite genotype-phenotype correlation in these rare nevi, with specific postzygotic mutations in the transmembrane domain of FGFR2. As not all lesions were truly papillomatous and pedunculated, the term cerebriform sebaceous nevus (CSN) appears more suitable than PPSN to describe this entity. The cerebriform pattern of CSN is reminiscent of cutis gyrata, as seen in Beare-Stevenson syndrome, which is caused by closely related germline FGFR2 mutations. While clinically impressive, CSN seem to carry a good prognosis and a low risk for extracutaneous associations.