Bainbridge, MN

Abstract Estrogen receptor positive (ER+) breast cancer is treated with endocrine therapy but intrinsic resistance occurs in ˜1/3 of patients and acquired resistance in ˜1/5 of the remainder. While many resistance mechanisms have been explored, clinical trials testing associated therapeutics have had mixed outcomes. Understanding mechanisms of resistance and finding therapeutic strategies to target them, therefore, remain important challenges. In this study we discover an intriguing link between mismatch repair (MMR) deficiency, specifically of the MutL complex (MLH1/3, PMS1/2), and endocrine therapy resistance in ER+ disease. We find a direct role for MutL loss in inducing endocrine therapy resistance in vitro and in vivo by knocking down multiple MutL genes using CRISPR and stable shRNA approaches validated using standard rescue experiments. We identify the underlying mechanism: MutL deficiency in ER+ breast cancer abrogates Chk2-mediated feedback inhibition of CDK4/6 that appears necessary for endocrine therapy responsiveness. Consequently, pharmacological targeting of CDK4/6 in vitro and in vivo significantly inhibits growth of endocrine therapy resistant MutL-deficient ER+ breast cancer cells. These results are corroborated by data from a neoadjuvant clinical trial demonstrating that cell cycle regulation of MutL-mutant tumors is estrogen-independent but CDK4/6 dependent. The results of this study provide important biological and clinically relevant insights. 1) a novel role for MMR in endocrine therapy resistance 2) A mechanism underlying the effectiveness of CDK4/6 inhibitors in some de novo endocrine therapy resistant tumors. While there are currently no biomarkers to guide the use of CDK4/6 inhibitors for ER+ breast cancer, markers of MMR dysregulation could identify patients in whom CDK4/6 inhibition may, most effectively, prevent disease recurrence. Citation Format: Haricharan S, Punturi N, Singh P, Holloway KR, Anurag M, Schmelz J, Schmidt C, Lei JT, Suman V, Hunt K, Olson Jr JA, Hoog J, Li S, Huang S, Edwards DP, Kavuri SM, Bainbridge MN, Ma CX, Ellis MJ. Loss of mismatch repair predicts resistance to endocrine therapy and sensitivity to CDK4/6 inhibitors in ER+ breast cancer [abstract]. In: Proceedings of the 2017 San Antonio Breast Cancer Symposium; 2017 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2018;78(4 Suppl):Abstract nr P4-04-01.

Abstract Estrogen receptor positive (ER+) breast cancer accounts for the majority of breast cancers diagnosed worldwide but fortunately, outcomes are markedly improved by pharmacological interventions that interrupt ER function. Unfortunately, suppression of relapse risk from early stage disease with endocrine therapy (anti-estrogens or aromatase inhibitors) is only ∼50%, and for advanced disease, pan endocrine therapy resistance is almost inevitable. While many mechanisms for intrinsic and acquired endocrine resistance have been explored, links between defects in DNA repair, the fundamental drivers of cancer pathogenesis, and endocrine therapy resistance have been understudied. Here we link mismatch repair (MMR) deficiency to poor clinical outcomes in ER+ breast cancer using whole exome DNA sequencing data and mRNA expression analysis. We subsequently demonstrate that MMR deficiency bypasses ER dependent cell cycle regulation through disruption of Chk2/p21-mediated feedback inhibition of CDK4 in breast cancer cell lines and tumor samples, as well as through correlations with human clinical data. We also show that pharmacological targeting of CDK4 significantly inhibits growth of MMR-deficient ER+ breast cancer cells in vitro and in vivo. This mechanism provides a new explanation for why endocrine therapy resistant ER+ breast cancers can respond to CDK inhibition and suggests that primary tumors exhibiting MMR deficiency are good candidates for adjuvant CDK4 inhibitor treatment. Citation Format: Haricharan S, Schmelz J, Schmidt C, Singh P, Holloway K, Anurag M, Suman V, Olson JA, Hunt K, Bainbridge MN, Ellis MJ. Mismatch repair deficiency induces endocrine therapy resistance in breast cancer [abstract]. In: Proceedings of the 2016 San Antonio Breast Cancer Symposium; 2016 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2017;77(4 Suppl):Abstract nr PD2-06.