Abstract 418: BLM helicase unwinds lagging strand substrates to assemble the ALT telomere damage response

Abstract The Bloom Syndrome helicase (BLM) is critical for Alternative Lengthening of Telomeres (ALT), a homology directed repair mediated telomere maintenance mechanism that is prevalent in cancers of mesenchymal origin. The DNA substrates that BLM engages to direct telomere recombination during ALT remain unknown. Here, we determine that BLM helicase acts on lagging strand telomere intermediates that occur specifically in ALT positive cells to assemble a replication stress associated DNA damage response. Loss of ATRX was permissive for BLM localization to ALT telomeres in S and G2 commensurate with the appearance of telomere C-strand specific single-stranded DNA. Moreover, DNA2 nuclease deficiency increased 5’-lagging strand flap formation and ALT in a BLM dependent manner, and BLM promoted ALT in response to telomere C-strand, but not G-strand, single-stranded nicks. These findings define the seminal events in the ALT DNA damage response, linking aberrant lagging strand DNA replication with a BLM directed HDR mechanism that sustains telomere length in a subset of human cancers. Citation Format: Aravind M. Krishnan, Haoyang Jiang, Tianpeng Zhang, Hardeep Kaur, Tao Shi, Youngho Kwon, Patrick Sung, Roger A. Greenberg. BLM helicase unwinds lagging strand substrates to assemble the ALT telomere damage response [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 418.