Study of BRAF mutations in melanoma and nevi from patients with germline mutation in the CDKN2A gene

10075 Background: Germline mutations in the CDKN2A tumor suppressor gene is a rare condition associated with a high risk of melanoma. Somatic activating mutations of the BRAF gene are frequently observed in cutaneous melanoma (40–60%) but their occurrence vary according to melanoma subtypes (cutaneous, vs mucosal or uveal) and are less frequent in melanoma located on chronically sun-exposed skin. It is thought that melanomas occurring in patients carrier of a CDKN2A germline mutation are associated a second inactivating genetic event in the tumor, facilitating cellular transformation by loss of function this crucial gatekeeper of the G1-S checkpoint. However, the potential role of the MAP-kinase pathway, and more precisely, the involvement of BRAF activation, in this particular population of patients, are still unknown. Thus, our objective was to evaluate the frequency of BRAF somatic mutations in melanoma and nevi developed by patients carrier of a CDKN2A germline mutation. Methods: DNA was extracted from paraffin-embedded tissues of 36 primary melanomas, 9 metastases and 20 nevi from 31 patients with CDKN2A germline mutation. Ten sporadic melanoma from patients harbouring no CDKN2A mutation were also studied as a control population. BRAF mutations were screened by direct sequencing of exon 11 and 15. Results: BRAF mutations (V599E) were found with a significantly lower frequency of 14% (5/36) in melanomas from patients with proven CDKN2A germline mutation, as compared to a frequency of 55% (5/9) in patients with no genetic predisposition (two-sided Fishers exact test, p=0.02). Frequencies of BRAF mutations in metastases and nevi from patients with CDKN2A germline mutation were 33%(3/9) and 10% (2/20) respectively. Conclusions: BRAF mutations seem to be less frequent in melanoma from patients with familial predisposition harbouring CDKN2A germ line mutation than in sporadic melanoma. These results suggest that CDKN2A loss of function gives rise to tumorigenic pathways where genetic events distinct from BRAF activating mutation can induce cell transformation. Additional biological studies are performed on this rare population of melanoma samples in order to better characterize the specificity of cellular transformation in the context of CDKN2A germ line mutation. No significant financial relationships to disclose.