Alternative Lengthening of Telomeres: Lessons to Be Learned from Telomeric DNA Double-Strand Break Repair

Abstract

The study of the molecular pathways underlying cancer has given us important insights into how breaks in our DNA are repaired and the dire consequences that can occur when these processes are perturbed. Extensive research over the past 20 years has shown that the key molecular event underpinning a subset of cancers involves the deregulated repair of DNA double-strand breaks (DSBs) at telomeres, which in turn leads to telomere lengthening and the potential for replicative immortality. Here we discuss, in-depth, recent major breakthroughs in our understanding of the mechanisms underpinning this pathway known as the alternative lengthening of telomeres (ALT). We explore how this gives us important insights into how DSB repair at telomeres is regulated, with relevance to the cell-cycle-dependent regulation of repair, repair of stalled replication forks and the spatial regulation of DSB repair.

Keywords: ATRX; alternative lengthening of telomeres; break-induced replication; double-strand break repair; telomeres.

Figure 1

Proposed model for activation of the ALT pathway. Mutation of IDH1 R132H leads to accumulation of 2-hydroxyglutarate, which leads to inhibition of multiple enzymes with functions in both DNA and histone methylation. This has been linked to decreases in expression of XRCC1 and RAP1. Loss of XRCC1 prevents formation of telomeric fusions in ALT cancer cells, whereas loss of RAP1 disrupts Shelterin, potentially leading to the formation of DNA secondary structures on telomeric DNA such as G-loops. The accumulation of reactive oxygen species has also been linked to ALT cancers and the formation of telomeric R-loops. Upon stalling at a replication barrier, replication forks are reversed/regressed. Fork regression is mediated by RAD51 and other DNA translocases, including SMARCAL1, ZRANB3 and HTLF. In the presence of ATRX/DAXX, forks are stabilized and restarted through interaction of ATRX with FANCD2 and CTIP. In the absence of ATRX, reversed forks undergo excessive MRE11-dependent degradation and cleavage by the MUS81 nucleases, leading to the formation of one-ended telomeric DSBs. Damage-induced MMS21-mediated SUMOylation of telomere proteins results in BLM-dependent PML recruitment to telomeres and triggers APB formation through SUMO/SIM-mediated LLPS. PCNA-RFC-Polδ-mediated BIR then occurs between clustered telomeres within the APB in a process that is promoted by BTR complex (BLM, TOP3A, RMI1 and RMI2) and inhibited by SLX4. Strand invasion is out of register, owing to ATRX loss, leading to telomere lengthening through conservative replication.