An siRNA screen identifies the GNAS locus as a driver in 20q amplified breast cancer

Poor prognosis oestrogen receptor positive breast cancer is characterized by the presence of high-level focal amplifications. We utilized a focused siRNA screen in 14 breast cancer cell lines to define genes that were pathogenic in three genomic regions focally amplified in oestrogen receptor positive breast cancer, 8p11-12, 11q13, and 20q. Silencing the GNAS locus, that encodes the G protein alpha stimulatory subunit Gαs, specifically reduced the growth of 20q amplified breast cancer cell lines. Examination of a publically available shRNA data set confirmed GNAS silencing to be selective for 20q amplified cancer cell lines. Cell lines with 20q amplification were found to over-express specifically the extra long Gαs splice variant (XLαs). Over-expression of XLαs induced cAMP levels to a greater extent than Gαs, suggesting that amplification of the GNAS locus, and over-expression of the XLαs variant in particular, enhanced cAMP signalling. GNAS silencing in amplified cell lines reduced ERK1/2 phosphorylation, and conversely over-expression of exogenous XLαs in a non-amplified cell line increased MEK-ERK1/2 phosphorylation, identifying one potential down-stream consequence of enhanced cAMP signalling. Our data indicate that amplification of the GNAS locus may contribute to the pathogenesis of breast cancer, and highlight a previously unrecognized role for the GNAS XLαs variant in cancer.