Deacetylase-independent HDAC1 condensation defines temozolomide response in glioblastoma.

Temozolomide is a standard-of-care therapeutic agent for glioblastoma. However, persons who initially respond well often experience a notable reduction in efficacy over time, with the underlying mechanisms remaining unclear. Here we demonstrate that the reduced response to temozolomide correlates with decreased chromatin accessibility, marked by reduced H3K27ac modification and alterations in chromatin loops. Mechanistically, temozolomide treatment upregulates histone deacetylase 1 (HDAC1) expression. Intriguingly, increased HDAC1 forms condensates independently of its deacetylase function. These condensates arise from multivalent interactions within the intrinsically disordered region and specific interactions with CCCTC-binding factor (CTCF), facilitating resistance to temozolomide by promoting the assembly of DNA repair complexes, even in the absence of direct deacetylase activity of HDAC1. Through phase-separation-based screening, we identified resminostat as an effective disruptor of HDAC1-CTCF condensates, thereby restoring temozolomide sensitivity in patient-derived xenograft models. Our findings introduce deacetylase-independent HDAC1 condensation as a distinct mechanism regulating temozolomide response, providing valuable insights into potential therapeutic strategies.