Alternative Lengthening of Telomeres through Two Distinct Break-Induced Replication Pathways

• DOI: 10.1016/j.celrep.2018.12.102
• Publication Date: 2019-01-24T02:06:35Z
• Authors: Jia-Min Zhang; Tribhuwan Yadav; Jian Ouyang; Li Lan; Lee Zou
• Keywords: DNA Replication; 0301 basic medicine; 03 medical and health sciences; Cell Line, Tumor; DNA Breaks; Humans; Telomere Homeostasis; Article; DNA Polymerase III; Rad52 DNA Repair and Recombination Protein

Alternative lengthening of telomeres (ALT) is a telomerase-independent but recombination-dependent pathway that maintains telomeres. Here, we describe an assay to visualize ALT-mediated telomeric DNA synthesis in ALT-associated PML bodies (APBs) without DNA-damaging agents or replication inhibitors. Using this assay, we find that ALT occurs through two distinct mechanisms. One of the ALT mechanisms requires RAD52, a protein implicated in break-induced DNA replication (BIR). We demonstrate that RAD52 directly promotes telomeric D-loop formation in vitro and is required for maintaining telomeres in ALT-positive cells. Unexpectedly, however, RAD52 is dispensable for C-circle formation, a hallmark of ALT. In RAD52-knockout ALT cells, C-circle formation and RAD52-independent ALT DNA synthesis gradually increase as telomeres are shortened, and these activities are dependent on BLM and BIR proteins POLD3 and POLD4. These results suggest that ALT occurs through a RAD52-dependent and a RAD52-independent BIR pathway, revealing the bifurcated framework and dynamic nature of this process.