Nuclear receptors regulate alternative lengthening of telomeres through a novel noncanonical FANCD2 pathway

Abstract

Alternative lengthening of telomeres (ALT) is known to use homologous recombination (HR) to replicate telomeric DNA in a telomerase-independent manner. However, the detailed process remains largely undefined. It was reported that nuclear receptors COUP-TFII and TR4 are recruited to the enriched GGGTCA variant repeats embedded within ALT telomeres, implicating nuclear receptors in regulating ALT activity. Here, we identified a function of nuclear receptors in ALT telomere maintenance that involves a direct interaction between COUP-TFII/TR4 and FANCD2, the key protein in the Fanconi anemia (FA) DNA repair pathway. The COUP-TFII/TR4-FANCD2 complex actively induces the DNA damage response by recruiting endonuclease MUS81 and promoting the loading of the PCNA-POLD3 replication complex in ALT telomeres. Furthermore, the COUP-TFII/TR4-mediated ALT telomere pathway does not require the FA core complex or the monoubiquitylation of FANCD2, key steps in the canonical FA pathway. Thus, our findings reveal that COUP-TFII/TR4 regulates ALT telomere maintenance through a novel noncanonical FANCD2 pathway.

Alternative lengthening of telomeres: from molecular mechanisms to therapeutic outlooks

“…In addition to DNA repair proteins, several nuclear receptors are also implicated in ALT. Recently, an ALT telomere maintenance pathway mediated by the COUP-TFII/TR4-FANCD2-MUS81-POLD3 axis was reported [70]. The nuclear receptor NR2C/F may recruit the NuRD-ZNF827 complex to telomeres and promote telomere recombination through chromatin remodeling [23,24,71].…”

The absence of (TCAGGG)n repeats in some telomeres, combined with variable responses to NR2F2 depletion, suggest that this nuclear receptor plays an indirect role in the alternative lengthening of telomeres

“…In summary, NR2F2 has a known role in cell cycle regulation that our data corroborate, but its role in the ALT mechanism appears to be more indirect than has been proposed previously. Nevertheless NR2F2 and NR2C2 do bind to some telomeres where they recruit other proteins 31,48 and we speculate that this may serve a different function.…”

“…This is consistent with a lower frequency of NR2F2 and NR2C2 foci at telomeres in the nuclei of four out of five ALT+ cell lines. We propose that telomeres lacking NR2F2 and NR2C2 binding sites are unlikely to engage in direct recruitment of ZNF827, the NuRD complex, or other NR2F2 interacting proteins, such as FANCD2 48 . siRNA mediated NR2F2 down-regulation in three ALT+ cell lines had a modest to severe effect on the cell cycle but the effect on various ALT markers was highly inconsistent between the cell lines.…”

ALT: A Multi-Faceted Phenomenon

“…Telomeres in other ALT cell lines also bind NR2F2/NR2C2 but the density of the (TCAGGG) binding sites varies considerably between cell lines [52] consequently the role of ZNF827-NuRD complex in telomere lengthening is unclear. NR2F2 and NR2C2 have also been shown to recruit FANCD2 to ALT telomeres, a protein from the Fanconi anemia (FA) repair pathway usually involved in DNA interstrand cross-link repair [53]. At ALT telomeres, FANCD2 recruits the endonuclease MUS81 to induce DDR, followed by the loading of the PCNA-POLD3 replication complex.…”