Background: 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).
Objectives: 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.
Methods: 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.
Results: 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.
Conclusions: 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.