Histologic and genomic features of breast cancers with alterations affecting the SWI/SNF (SMARC) genes

The SWI/SNF family of proteins is a multisubunit ATPase complex frequently altered in human cancer. Inactivating mutations in SWI/SNF-related matrix-associated actin-dependent regulator of chromatin (SMARCs) underpin a subset of tumors such as the malignant rhabdoid tumor and small cell carcinoma of the ovary, hypercalcemic type. Here, we investigated the genotypic and phenotypic characteristics of breast cancers harboring somatic genetic alterations affecting genes of the SMARC family. We analyzed a series of 6026 primary and metastatic breast cancers subjected to targeted-capture sequencing. SMARC core subunit (SMARCA4, SMARCB1, and SMARCA2) alterations were identified in <1% of all breast cancers, consisting of 27 primary and 30 recurrent/metastatic tumors. The majority of SMARC alterations were monoallelic mutations (47/57, 82%) and thus categorized into two groups: Class 1 alterations consisting of potentially pathogenic mutations and rearrangements and Class 2 alterations consisting of missense mutations and small in-frame deletions of unknown significance. Biallelic events in a SMARC gene were present in a minority of cases (10/57, 18%). Histologic patterns in the form of rhabdoid, composite rhabdoid, sarcomatoid or anaplastic features were observed in a subset of Class 1 primary and metastatic tumors (7/57, 12%). SMARC protein was preserved in nearly all tumors analyzed with immunohistochemistry (26/30, 87%). Four Class 1 tumors demonstrated altered SMARC protein expression in the form of loss (1/30, 3%) or mosaic pattern (3/30, 10%). Complete loss of SMARCA2 (BRM) was observed in a sole tumor with composite rhabdoid morphology, and biallelic hits in the SMARCA2 gene. The genomic landscape of both primary Class 1 and 2 breast cancers did not reveal any characteristic findings. In summary, SMARC alterations likely contribute to the biology of a rare subset of breast cancers in the form of biallelic or pathogenic alterations in SMARC, as evidenced by SMARC-deficient phenotype or altered expression of SMARC protein.