Yuichi Tominaga

A Synthetic Lethality–Based Strategy to Treat Cancers Harboring a Genetic Deficiency in the Chromatin Remodeling Factor BRG1

The occurrence of inactivating mutations in SWI/SNF chromatin-remodeling genes in common cancers has attracted a great deal of interest. However, mechanistic strategies to target tumor cells carrying such mutations are yet to be developed. This study proposes a synthetic-lethality therapy for treating cancers deficient in the SWI/SNF catalytic (ATPase) subunit, BRG1/SMARCA4. The strategy relies upon inhibition of BRM/SMARCA2, another catalytic SWI/SNF subunit with a BRG1-related activity. Immunohistochemical analysis of a cohort of non-small-cell lung carcinomas (NSCLC) indicated that 15.5% (16 of 103) of the cohort, corresponding to preferentially undifferentiated tumors, was deficient in BRG1 expression. All BRG1-deficient cases were negative for alterations in known therapeutic target genes, for example, EGFR and DDR2 gene mutations, ALK gene fusions, or FGFR1 gene amplifications. RNA interference (RNAi)-mediated silencing of BRM suppressed the growth of BRG1-deficient cancer cells relative to BRG1-proficient cancer cells, inducing senescence via activation of p21/CDKN1A. This growth suppression was reversed by transduction of wild-type but not ATPase-deficient BRG1. In support of these in vitro results, a conditional RNAi study conducted in vivo revealed that BRM depletion suppressed the growth of BRG1-deficient tumor xenografts. Our results offer a rationale to develop BRM-ATPase inhibitors as a strategy to treat BRG1/SMARCA4-deficient cancers, including NSCLCs that lack mutations in presently known therapeutic target genes.

Abstract A29: Targeting p300 addiction in CBP-deficient cancers causes synthetic lethality by apoptotic cell death due to abrogation of MYC expression

Loss-of-function mutations in the CBP/CREBBP gene, which encodes a histone acetyltransferase (HAT), are present in a variety of human tumors, including lung, bladder, gastric, and hematopoietic cancers. Consequently, development of a molecular targeting method capable of specifically killing CBP-deficient cancer cells would greatly improve cancer therapy. Functional screening of synthetic-lethal genes in CBP-deficient cancers identified the CBP paralog p300/EP300. Ablation of p300 in CBP-knockout and -deficient cancer cells induced G1/S cell-cycle arrest, followed by apoptosis. Genome-wide gene expression analysis revealed that MYC is a major factor responsible for the synthetic lethality. Indeed, p300 ablation in CBP-deficient cells caused downregulation of MYC expression via reduction of histone acetylation in its promoter, and this lethality was rescued by exogenous MYC expression. The p300-HAT inhibitor C646 specifically suppressed the growth of CBP-deficient lung and hematopoietic cancer cells in vitro and in vivo; thus p300 is a promising therapeutic target for treatment of CBP-deficient cancers. Citation Format: Hideaki Ogiwara, Mariko Sasaki, Takahiro Oike, Saito Higuchi, Yuichi Tominaga, Takashi Kohno. Targeting p300 addiction in CBP-deficient cancers causes synthetic lethality by apoptotic cell death due to abrogation of MYC expression [abstract]. In: Proceedings of the AACR Precision Medicine Series: Opportunities and Challenges of Exploiting Synthetic Lethality in Cancer; Jan 4-7, 2017; San Diego, CA. Philadelphia (PA): AACR; Mol Cancer Ther 2017;16(10 Suppl):Abstract nr A29.