TRIM24 directs replicative stress responses to maintain ALT telomeres via chromatin signaling.

An inability to replicate the genome can cause replication stress and genome instability. Here, we develop biotinylation of lac operator (LacO) array replication stress protein network identification (BLOCK-ID) in human cancer cells, a proteomic method to identify and visualize proteins at stressed replication forks. This approach identified mediators of the replication stress response, including the chromatin acetylation reader protein tripartite motif containing 24 (TRIM24). We uncovered a crucial role for TRIM24 in coordinating alternative lengthening of telomeres (ALT), a replication stress-directed telomere extension mechanism. Our data reveal that TRIM24 is recruited to telomeres via a p300/CREB binding protein (CBP)-dependent acetylation chromatin signaling cascade to organize the assembly of ALT-associated promyelocytic leukemia (PML) bodies (APBs) and promote de novo telomere DNA synthesis. Tethering of TRIM24 at telomeres was sufficient to stimulate de novo telomere DNA synthesis in a small ubiquitin-like modifier (SUMO)-dependent but p300/CBP- and PML-independent manner. Collectively, these findings uncover an indispensable epigenetic signaling pathway involving TRIM24 and p300/CBP that mediates ALT-telomere maintenance.