Translesion synthesis genes POLI and REV1 in breast tumors and response to neoadjuvant chemotherapy

e12642 Background: Translesion synthesis (TLS) of DNA by specialized polymerases (POLs) and REV1, a scaffold protein that recruits other TLS DNA POLs for nascent strand extension, experimentally enable tumor cells to replicate through DNA damage. TLS POLs are hypothesized to contribute to treatment resistance. Methods: We conducted a discovery gene expression study using NanoString technology with a bespoke codeset enriched for DNA damage response genes and RNA from 12 TN, 10 HER2+ and 29 ER+/HER2- breast cancer patients before neoadjuvant chemotherapy (NACT) and when available, in the matching residual tumor. Eleven patients achieved a pathologic complete response (pCR), 12 had < 1 cm residual disease (partial responders) and 28 patients had > 1cm residual disease (non-responders). RNA counts were normalized and log-transformed for differential expression analyses corrected for false discovery. Results: DNA polymerase Iota ( POLI) was the top overexpressed gene in the pretreatment biopsy of non-responders independent of tumor subtype (p corrected = 0.0002). Expression of REV1 was also significantly higher in non-responders and positively correlated with POLI (r = 0.577, p < 0.0001). Despite a small sample size, pretreatment expression levels of POLI and REV1 were significantly higher in ER+ (n = 38) than ER- (n = 13) tumors (p = 0.0006). When restricted to ER+ tumors, POL1 and REV1 expression levels were significantly higher in pretreatment biopsy of non-responders (n = 25) than partial and complete responders (n = 13). For ER+ patients, response by median POL1/REV1 expression levels are shown in Table. Pre/post treatment POL1 and REV1 expression did not change in patients with residual disease. Conclusions: In discovery, two genes involved in TLS of DNA (POLI and REV1) were found overexpressed in pretreatment biopsies from breast cancers poorly responsive to NACT, independent of tumor subtype. Overexpression was more common among ER+ tumors, which exhibit a high inherent resistance to chemotherapy. If confirmed, expressed levels of POLI and REV1 may identify a subgroup of breast cancers resistant to NACT for which targeted inhibition of mutagenic TLS may be beneficial. [Table: see text]