SMARCAL1 Resolves Replication Stress at ALT Telomeres

Abstract

SUMMARYCancer cells overcome replicative senescence by exploiting mechanisms of telomere elongation, a process often accomplished by reactivation of the enzyme telomerase. However, a subset of cancer cells lack telomerase activity and rely on the alternative lengthening of telomeres (ALT) pathway, a recombination-based mechanism of telomere elongation. Although the mechanisms regulating ALT are not fully defined, chronic replication stress at telomeres might prime these fragile regions for recombination. Here, we demonstrate that the replication stress response protein SMARCAL1 is a critical regulator of ALT activity. SMARCAL1 associates with ALT telomeres to resolve replication stress and ensure telomere stability. In the absence of SMARCAL1, persistently stalled replication forks at ALT telomeres deteriorate into DNA double-strand breaks promoting the formation of chromosome fusions. Our studies not only define a role for SMARCAL1 in ALT telomere maintenance, but also demonstrate that resolution of replication stress is a crucial step in the ALT mechanism.

Break-induced telomere synthesis underlies alternative telomere maintenance

“…Rad51, together with the Hop2–Mnd1 heterodimer, localize to ALT-associated PML bodies (APBs) and facilitate long-range telomere movement and clustering in ALT cells 7,16,17 . Cells lacking Hop2 from CRISPR–Cas9-mediated excision showed reduced telomere clustering, APB formation, and telomere exchanges in ALT-positive VA13 cells (Extended Data Fig.…”

Precarious maintenance of simple DNA repeats in eukaryotes

“…Telomeric repeats are hard to replicate owing to the formation of abnormal DNA structures and/or stable protein complexes, as is evident form fork stalling that is comparable to or even stronger than at other microsatellite sequences [93,94]. As discussed above, stalled forks can be readily converted into 5′ resected, one-ended DSBs via endonucleases involved in ALT [87,95]. Since telomeres are repetitive in nature, “out of register” strand invasion or multiple template-switching events can lead to lengthening of the repetitive sequence after BIR is complete.…”

Noncanonical views of homology-directed DNA repair

“…Telomere clustering was partially dependent on Rad51 and also required the Hop2-Mnd1 heterodimer, which is crucial for extensive homolog pairing during meiosis (Cho et al 2014). Rad51-dependent telomere clustering was also reported in ALT cells undergoing replication stress after the depletion of annealing helicase SMARCAL1 (Cox et al 2016). Additionally, induction of replication-induced damage by depletion of histone chaperon ASF1 resulted in the induction of ALT activity even in immortalized human cells with longer telomeres (O'Sullivan et al 2014).…”