Break-induced telomere synthesis underlies alternative telomere maintenance

Abstract

Homology-directed DNA repair is essential for genome maintenance through templated DNA synthesis. Alternative lengthening of telomeres (ALT) necessitates homology-directed DNA repair to maintain telomeres in about 10–15% of human cancers. How DNA damage induces assembly and execution of a DNA replication complex (break-induced replisome) at telomeres or elsewhere in the mammalian genome is poorly understood. Here we define break-induced telomere synthesis and demonstrate that it utilizes a specialized replisome, which underlies ALT telomere maintenance. DNA double-strand breaks enact nascent telomere synthesis by long-tract unidirectional replication. Proliferating cell nuclear antigen (PCNA) loading by replication factor C (RFC) acts as the initial sensor of telomere damage to establish predominance of DNA polymerase δ (Pol δ) through its POLD3 subunit. Break-induced telomere synthesis requires the RFC–PCNA–Pol δ axis, but is independent of other canonical replisome components, ATM and ATR, or the homologous recombination protein Rad51. Thus, the inception of telomere damage recognition by the break-induced replisome orchestrates homology-directed telomere maintenance.

Replication Fork Reversal: Players and Guardians

“…Of note, the same study also elucidates the molecular steps that rescue the degraded forks upon drug withdrawal by showing that the initial resection of the regressed arm generates a reversed fork with a 3′-ssDNA tail that is then cleaved by MUS81 to permit POLD3-dependent rescue of the stalled forks in a BRCA2-deficient background (Figure 2B). Interestingly, a similar mechanism was recently proposed for completion of DNA replication at common fragile sites and telomeric loci (Dilley et al, 2016; Minocherhomji et al, 2015). This mechanism relies on a specialized form of DNA repair originally characterized in yeast and termed break-induced replication (BIR), whereby MUS81 cleavage of stalled replication forks produces a migrating bubble that drives POLD3-dependent DNA synthesis (Sakofsky and Malkova, 2017).…”

Alternative lengthening of telomeres can be maintained by preferential elongation of lagging strands

“…Therefore, these replication problems may also initiate break-induced replication (BIR)-mediated DNA replication processes at the telomere region in ALT cells (55), leading to a preference for lagging strands. We also showed that ATRX depletion partially contributes to preferential elongation in lagging strands (Figure 3).…”

Meiosis-like Functions in Oncogenesis: A New View of Cancer

“…This is dependent upon the inherent ability of these factors to stimulate non-sister chromosome interactions. The ALT pathway operates via a recombination-mediated mechanism in which, in the absence of normal telomerase-mediated elongation, telomeres behave like a broken chromosome end, serving to stimulate RAD51 recombinasemediated strand invasion of an uncapped telomere into a nonsister telomere to enable the invading end to serve as a substrate for DNA replication-dependent de novo telomere elongation (35). This phenomenon can not only help drive tumor formation but also enables tumor cell proliferative activity and is likely to contribute to tumor cell evolutionary potential (36,37), although this latter point requires experimental exploration.…”