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Cell Rep. 2019 Jan 22;26(4):955-968.e3. doi: 10.1016/j.celrep.2018.12.102.

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

Author information

1
Massachusetts General Hospital Cancer Center, Harvard Medical School, Charlestown, MA 02129, USA.
2
Massachusetts General Hospital Cancer Center, Harvard Medical School, Charlestown, MA 02129, USA; Department of Radiation Oncology, Massachusetts General Hospital, Harvard Medical School, Charlestown, MA 02129, USA.
3
Massachusetts General Hospital Cancer Center, Harvard Medical School, Charlestown, MA 02129, USA; Department of Pathology, Massachusetts General Hospital, Harvard Medical School, Boston, MA 02114, USA. Electronic address: zou.lee@mgh.harvard.edu.

Abstract

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.

KEYWORDS:

ALT; ALT DNA synthesis in APBs; ALT-associated PML bodies; APB; ATSA; BIR; BLM; C-circle; POLD3; RAD51; RAD52; Telomere; alternative lengthening of telomeres; break-induced replication

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