Sam68 stimulates Theta-Mediated End-Joining by suppressing poison exon inclusion in Polq mRNA

Although the enzymatic mechanism of TMEJ has been extensively studied in recent years, its regulation remains poorly understood. Here, we identify and characterize the RNA-binding protein Sam68 (also known as Khdrbs1) as a key modulator of TMEJ in mammalian cells.

We demonstrate that Sam68 is required for proper expression of Polq, the gene encoding polymerase theta (Polθ), the central factor in TMEJ. Loss of Sam68 results in reduced TMEJ at CRISPR-induced DSBs and increased sensitivity to ionizing radiation. Mechanistically, Sam68 promotes correct splicing of Polq mRNA by suppressing the inclusion of a conserved poison exon that introduces a premature termination codon in the transcript.