Histone H3 N-terminal recognition by the PHD finger of PHRF1 is required for proper DNA damage response

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.