Deletions of BRCA1/2 and p53 R248W gain-of-function mutation suggest impaired homologous recombination repair in fragile histidine triad-negative sebaceous gland carcinomas

Sebaceous gland carcinomas represent rare malignancies of the skin and some 60% of them demonstrate high-grade microsatellite instability on the background of a defective mismatch repair system. However, a significant fraction of periocular sebaceous gland carcinomas exhibits microsatellite stability associated with a frequent loss of the candidate tumour suppressor fragile histidine triad (FHIT). We hypothesized that in those sebaceous gland carcinomas with microsatellite stability and loss of FHIT, effector molecules participating in homologous recombination repair (HRR), such as BRCA1/2, could be somatically inactivated. A pilot series of 10 paraffin-embedded sebaceous gland carcinoma specimens with a defined FHIT status was studied for loss of heterozygosity (LOH) events in the genes BRCA1, BRCA2, FHIT and WWOX. We sequenced the coding exons 5-8 of the p53 gene. Sebaceous gland carcinomas with FHIT negativity displayed LOH and biallelic deletions of the BRCA1 gene in five of 10 (50%) of the sebaceous gland carcinoma specimens analysed. Tumour-specific genomic losses close to BRCA2 were also uncovered. A homozygous p53 R248W gain-of-function mutation as the result of a CGG to TGG transition was identified in one of seven sebaceous gland carcinomas. It has been demonstrated previously that p53 R248W mutants inactivate ATM-directed HRR. This particular sebaceous gland carcinoma presented with concomitant genomic deletions at the BRCA1 and BRCA2 loci, and also at the constitutively fragile sites FRA3B/FHIT and FRA16D/WWOX. Our study demonstrates for the first time that microsatellite-stable FHIT-negative sebaceous gland carcinomas accumulate mutations that target central components of the HRR network. This observation will prompt investigations in synthetic lethality of BRCA-deficient sebaceous gland carcinomas by therapeutic poly(ADP-ribose) polymerase inhibitors.