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

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


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.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Cell Line, Tumor
  • DNA Breaks*
  • DNA Polymerase III / genetics
  • DNA Polymerase III / metabolism*
  • DNA Replication / physiology*
  • Humans
  • Rad52 DNA Repair and Recombination Protein / genetics
  • Rad52 DNA Repair and Recombination Protein / metabolism*
  • Telomere Homeostasis / physiology*


  • RAD52 protein, human
  • Rad52 DNA Repair and Recombination Protein
  • DNA polymerase delta p12 subunit, human
  • POLD3 protein, human
  • DNA Polymerase III