BLM and SLX4 play opposing roles in recombination‐dependent replication at human telomeres

Abstract

Alternative lengthening of telomeres (ALT) is a telomere lengthening pathway that predominates in aggressive tumors of mesenchymal origin; however, the underlying mechanism of telomere synthesis is not fully understood. Here, we show that the BLM-TOP3A-RMI (BTR) dissolvase complex is required for ALT-mediated telomere synthesis. We propose that recombination intermediates formed during strand invasion are processed by the BTR complex, initiating rapid and extensive POLD3-dependent telomere synthesis followed by dissolution, with no overall exchange of telomeric DNA. This process is counteracted by the SLX4-SLX1-ERCC4 complex, which promotes resolution of the recombination intermediate, resulting in telomere exchange in the absence of telomere extension. Our data are consistent with ALT being a conservative DNA replication process, analogous to break-induced replication, which is dependent on BTR and counteracted by SLX4 complex-mediated resolution events.

Bi-allelic MCM10 mutations cause telomere shortening with immune dysfunction and cardiomyopathy

“…Recent studies have outlined distinct differences in telomere replication between telomerase-positive and ALT cancer cells (Dilley et al, 2016, Sobinoff et al, 2017. For example, ALT cells synthesize telomeres in promyelocytic leukemia (PML) bodies and accumulate high levels of t-and C-circle DNA, unlike telomerase-positive cells (Dilley et al, 2016, Sobinoff et al, 2017. In agreement, we detected very low levels of t-or C-circle DNAs in HCT116 ST cells ( Figure 6D,F; Figure S6A,B).…”

“…However, a role for Mcm10 in ALT telomere maintenance has not been investigated. Recent studies have outlined distinct differences in telomere replication between telomerase-positive and ALT cancer cells (Dilley et al, 2016, Sobinoff et al, 2017. For example, ALT cells synthesize telomeres in promyelocytic leukemia (PML) bodies and accumulate high levels of t-and C-circle DNA, unlike telomerase-positive cells (Dilley et al, 2016, Sobinoff et al, 2017.…”

“…Telomere replication not only requires canonical replisome proteins, but is facilitated by components of the Shelterin complex, DNA helicases, homologous recombination and DNA repair factors (Martinez and Blasco, 2015, Pan et al, 2017, Schmutz et al, 2017, Vannier et al, 2012. In contrast, the alternative lengthening of telomeres (ALT) pathway relies heavily on break-induced replication and has defined features that are distinct from telomere maintenance in telomerase-positive cells (Dilley et al, 2016, Sobinoff et al, 2017.…”

Telomere length heterogeneity in ALT cells is maintained by PML-dependent localization of the BTR complex to telomeres

“…Our focus centered on components of the BTR complex, which was seen to be mislocalized from telomeres in PML-null cells (Fig. 1C) and has been implicated in ALT-associated phenotypes (O'Sullivan et al 2014;Sobinoff et al ;Lu et al 2019;Min et al 2019;. In order to test whether loss of the BTR complex would affect ALT activity, we generated U2OS cells deficient in either BLM helicase or RMI1, which is critical for the recruitment of BLM and the rest of the BTR complex to DNA (Xu et al 2008).…”

“…BLM is part of the BTR complex which also includes the topoisomerase TOP3a, and the OB-fold containing structural components RMI1 and RMI2 (Johnson et al 2000;Wu et al 2000;Yin et al 2005;Xu et al 2008). Interestingly, overexpression of BLM or dysregulation of the BTR complex induced by the loss of FANCM in ALT-positive cells has been shown to induce upregulation of ALT-associated phenotypes, suggesting that this factor is limiting for the ALTpathway and led to the proposal that the BTR complex acts in ALT to dissolve recombination intermediates into non-crossover products which results in telomere lengthening (Sobinoff et al 2017;Lu et al 2019;Min et al 2019;Pan et al 2019;Silva et al 2019).…”