Lysine Demethylase PHF8-Promoted TOPBP1 Demethylation Drives ATR Activation and Contributes to Breast Tumorigenesis

The checkpoint kinase ATR is a master regulator of DNA damage response. Yet, how ATR activity is regulated remains to be investigated. We report here that histone demethylase PHF8 is critically required for ATR activation and cellular response to ATR-activating DNA lesions. With a combined approach of structural and biochemical analysis, we unveiled an extraordinary feature of the association/dissociation of PHF8 with TOPBP1, an essential allosteric activator of ATR. PHF8 removes TOPBP1 K118 mono-methylation to facilitate the association of TOPBP1 with RAD9, thus loading the TOPBP1-RAD9 complex to damaged chromatin and eventually activating ATR signaling and protecting cells against replication stress. We demonstrated that PHF8-promoted TOPBP1 demethylation contributes to breast tumorigenesis, and targeting PHF8 or PHF8-TOPBP1 binding sensitized breast cancer cells to chemotherapeutic drugs. Our study provides molecular insight into non-histone methylation switch in ATR activation and genome integrity and supports pursuing PHF8 as a potential therapeutic target for cancers.