Abstract 1109: MutSα and MutSβ mismatch repair complexes differentially regulate temozolomide efficacy & sensitizing effects of veliparib in glioblastoma cells
Authors: Gupta Shiv K. , Smith Emily J. , Tuma Ann M. , Carlson Brett L. , Schroeder Mark A. , Kitange Gasper J. , Sarkaria Jann N. 
Published: July 2017

Abstract
Mismatch repair (MMR) is essential for temozolomide (TMZ) sensitivity, and MMR deficiency causes TMZ resistance and disease recurrence. However, the role MMR in TMZ response has been linked with Msh2/Msh6 (MutSα), while role of Msh3, a component of MutSβ remains unclear. To delineate role of MutSα and MutSβ in TMZ response, we used siRNA to disrupt MSH6 and MSH3 expression. As expected, silencing of MSH6 led to resistance in a TMZ-sensitive U251 cell line. In contrast, silencing of MSH3 enhanced TMZ sensitivity as compared to the control cells. These results support established role of MutSα in TMZ response via replication stress and DNA double strand breaks (DSB), and reveal that MutSβ may potentially influence TMZ response by modulating DSB repair. Consistently, host cell reactivation (HCR) assays in U251 expressing DRGFP reporter showed decreased homologous recombination (HR) after MSH3 knockdown (KD), while Msh6 KD had no effect on HR efficiency. Supporting the concept of synthetic lethality of HR with PARP inhibition, clinically relevant dose of veliparib (1 µM) showed a modest but consistent increase in TMZ sensitization measured in terms of potentiation factor (PF, ratio of relative growth with TMZ alone/ TMZ and veliparib) in MSH3 KD (PF 1.6±0.2) as compared to control cells (PF 1.1±0.2). Sensitizing effect of 1 µM veliparib in MSH6 KD (PF 1.7±0.5) was comparable to that in MSH3 KD cells. Interestingly, supra-therapeutic dose of veliparib (10 µM) induced more pronounced sensitization in MSH6 KD cells (PF 2.6±0.3) as compared to MSH3 KD (PF 2.4±0.3) or control cells (PF 1.6±0.2). Consistently, veliparib (10 µM) alone had no impact on DNA damage signaling, while TMZ induced prominent increase in phosphorylation of KAP1, Chk1 and Chk2 in control and MSH3 KD cells, but not in MSH6 KD cells; veliparib/TMZ co-treatment led a robust signaling in all 3 types of cells. Increased sensitization by veliparib after MSH3 KD was also examined in a TMZ-resistant (GBM22TMZ) PDX model carrying resistance due to a homozygous mutation in MSH6, and a 4-fold increase in HR efficiency. In clonogenic assays silencing of MSH3 or BRCA1 had no effect on TMZ sensitivity in GBM22TMZ cells. However, veliparib alone (10 µM) decreased colony formation with relative colony formation 0.54±0.03 and 0.32±0.06 in MSH3 and BRCA1 KD cells, respectively as compared to 0.82±0.04 in controls. TMZ/veliparib co-treatment in GBM22TMZ led to further decrease in colony formation in MSH3 KD and BRCA1 KD cells with relative colony formation of 0.30±0.04 and 0.19±0.02, respectively versus 0.46±0.03 in control. Taken together, our results suggest that MutSα and MutSβ differentially regulate cytotoxic effects of TMZ. Decreased HR efficiency and enhanced sensitizing effects of veliparib in MSH3 KD cells partially define synthetic lethality of MMR deficiency with PARP, and may help development of PARP inhibitors as sensitizer of TMZ therapy in GBM.
Citation Format: Shiv K. Gupta, Emily J. Smith, Ann M. Tuma, Brett L. Carlson, Mark A. Schroeder, Gasper J. Kitange, Jann N. Sarkaria. MutSα and MutSβ mismatch repair complexes differentially regulate temozolomide efficacy & sensitizing effects of veliparib in glioblastoma cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 1109. doi:10.1158/1538-7445.AM2017-1109