Abstract
<title>Abstract</title> <p> Background: PARP1 inhibitors (PARPi) are a key therapeutic strategy in BRCA1/2-mutated and increasingly in homologous recombination (HR)-positive breast cancers. However, resistance and relapse remain major challenges. While most studies focus on DNA repair restoration, chromatin-based mechanisms of PARPi response are less understood. PARP1 regulates DNA repair and chromatin organisation through poly(ADP-ribosyl)ation and histone modification. Histone H3 serine 28 phosphorylation (H3S28ph), a stress- and DNA damage-responsive mark catalysed by MSK1/2, is associated with chromatin relaxation and senescence. We hypothesised that H3S28ph promotes a senescence-associated survival programme contributing to PARPi resistance in HR-positive breast cancer. Methods: HR-positive breast cancer cell lines were exposed to UV-induced DNA damage and treated with the PARP inhibitor olaparib. H3S28 phosphorylation dynamics were assessed by immunofluorescence, Western blotting and chromatin immunoprecipitation. Functional effects were evaluated using proliferation, clonogenic growth, spheroid formation and cell cycle analysis. Senescence was quantified by β-galactosidase staining and expression of senescence-associated genes by RT-qPCR. Clinical relevance was assessed using Kaplan–Meier Plotter, GEPIA3, and the GSE173839 cohort. Results: DNA damage induced robust H3S28 phosphorylation, which was enhanced by PARP inhibition. H3S28ph was enriched at promoters of senescence-associated genes, including <italic>CDKN2A</italic> (p14/p16), <italic>CDKN2B</italic> (p15) and FOXA1, leading to their transcriptional activation. Combined UV and PARPi treatment induced G1/S arrest, increased senescence markers and enhanced β-galactosidase activity, consistent with a therapy-induced senescence-like state. Clinically, low expression of MSK1 ( <italic>RPS6KA5</italic> ) and associated senescence genes correlated with poorer overall survival and differential response to DNA-damaging therapies in HR-positive breast cancer cohorts. Conclusions: Our findings indicate that H3S28 phosphorylation acts as a chromatin-based adaptive mechanism linking DNA damage signalling to senescence-associated transcription under PARP inhibition. The MSK1–H3S28ph axis promotes a senescence-like survival state that may contribute to PARPi resistance in HR-positive breast cancer. Targeting this epigenetic pathway may improve therapeutic response and overcome resistance. </p>