Genomic Stability in Syndromic Basal Cell Carcinoma.

Basal cell cancers (BCCs) are characterized by upregulation of Hedgehog pathway through loss of PTCH1 or activation of SMO, and SMO inhibitors, such as vismodegib, are effective therapies for advanced BCCs. Although most BCCs are sporadic, rare individuals with basal cell nevus syndrome (BCNS) harbor germline defects in PTCH1 and develop up to hundreds of tumors that are histopathologically indistinguishable from sporadic BCCs. Interestingly, BCNS-BCCs are more responsive to SMO inhibitors than sporadic BCCs, with minimal development of resistance. Given differences in clinical course and therapy response, we sought to characterize BCCs in the setting of BCNS. We found that BCNS individuals with low tumor burden demonstrated significantly fewer UV signature somatic mutations and lower overall somatic mutational load compared to BCNS individuals with high burden, supporting a role of UV exposure in driving BCC development in BCNS individuals. However, compared with sporadic BCCs, BCNS-BCCs have a significantly lower mutational load, lower proportion of UV mutagenesis, increased genomic stability, and harbor fewer functionally resistant SMO mutations at baseline, explaining why BCNS-BCCs lack intrinsic resistance to SMO inhibitors. BCNS-BCCs appear to have reduced mutator phenotype compared with sporadic BCCs, which may contribute to their relatively more indolent clinical course and responsiveness to therapy.