PHRF1 Knockout Cell Line (HEK293)

Plant homeodomain (PHD) fingers are critical effectors of histone post-translational modifications (PTMs), regulating gene expression and genome integrity, and are frequently implicated in human disease. While most PHD fingers recognize unmodified and methylated states of histone H3 lysine 4 (H3K4), the specific functions of many of the over 100 human PHD finger-containing proteins are poorly understood. Here, we present a comprehensive analysis of one such poorly characterized PHD finger-containing protein, PHRF1. Using biochemical, molecular, and cellular approaches, we demonstrate that PHRF1 robustly binds to histone H3, specifically at its N-terminal region. Through integrating RNA-seq and proteomic analyses, we show that PHRF1 regulates transcription and RNA splicing and plays a critical role in DNA damage response (DDR). Crucially, we show that a cancer-associated mutation in the PHRF1 PHD finger (P221L) abolishes its histone interaction and fails to rescue defective DDR in PHRF1 knockout cells. These findings underscore the importance of the PHRF1-H3 interaction in maintaining genome integrity and provide new insight into how PHD fingers contribute to chromatin biology.

The TopBP1-ATR axis is critical for maintaining genomic stability during DNA replication stress, yet the precise regulation of TopBP1 in replication stress responses remains poorly understood. In this study, we identified PHD and Ring Finger Domains 1 (PHRF1) as an important ATR activator through its interaction with TopBP1. Our analysis revealed a correlation between PHRF1 and genomic stability in cancer patients. Mechanistically, PHRF1 is recruited to DNA lesions in a manner dependent on its PHD domain and histone methylation. Subsequently, PHRF1 mono-ubiquitinates TopBP1 at lysine 73, which enhances the TopBP1-ATR interaction and activates ATR. Depletion of PHRF1 disrupts ATR activation and sensitizes cells to replication stress-inducing agents. Furthermore, conditional knockout of Phrf1 in mice leads to early lethality and impaired ATR-Chk1 axis signaling. Collectively, our findings establish PHRF1 as a novel E3 ligase for TopBP1, coordinating the replication stress response by enhancing TopBP1-ATR signaling.