Abstract 3366: APOBEC3 family of cytidine deaminases in sensitizing triple-negative breast cancer cells to cisplatin and carboplatin

Abstract Cisplatin is a DNA damaging chemotherapeutic which forms interstrand crosslinks (ICL) and intrastrand adducts that induce replication fork collapse and lead to apoptosis. These adduct sites can be repaired through several repair pathways, the most important being nucleotide excision repair (NER). Cisplatin and carboplatin ICLs form a unique structure that forces cytosines flanking the adduct site to become extrahelical. Our lab has previously shown that base excision repair (BER) and mismatch repair (MMR) mediate sensitivity to cisplatin and carboplatin as a consequence of nonproductive processing of DNA flanking the ICL structure. The APOBEC3 (A3) protein family of cytidine deaminases have recently been implicated in cancer development and have potential to deaminate the extrahelical cytosines formed by cisplatin ICLs. Polβ knockdown MDA-MB-231 cells are resistant to cisplatin in vivo compared to wild-type cells (p<0.001). Knockdown of APOBEC3 family members in MDA-MB-231 breast cancer cells results in a resistant phenotype to cisplatin and carboplatin treatment compared to the control. Utilizing CRISPR-Cas9 genome editing, we generated knockouts of A3 members. A3D knockdown in the APOBEC3C (A3C) knockout results in resistance to cisplatin compared to the control, suggesting that multiple A3 enzymes can deaminate the ICL induced extrahelical cytosine. APOBEC3A (A3A) knockdown in a cell line with high expression of A3A (SK-BR-3) shows no difference in cisplatin and carboplatin treatment, suggesting that A3A does not deaminate extrahelical cytosines. Overexpression of A3C or A3D in cell lines with low expression of A3 proteins (SK-BR-3) results in sensitivity to cisplatin. These results together suggest that A3C, A3D, and A3G can deaminate the ICL extrahelical cytosines, resulting in the activation of BER. The subsequent nucleotide misincorporation by Polβ followed by MMR protein binding would physically block NER proteins from repairing the ICL, therefore conferring sensitivity to cisplatin and carboplatin. Citation Format: Kayla L. Conner, Asra N. Shaik, Jordan White, Wen Lei, Michele L. Cote, Steve M. Patrick. APOBEC3 family of cytidine deaminases in sensitizing triple-negative breast cancer cells to cisplatin and carboplatin [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 3366.