XPF activates break-induced telomere synthesis

Abstract

Alternative Lengthening of Telomeres (ALT) utilizes a recombination mechanism and break-induced DNA synthesis to maintain telomere length without telomerase, but it is unclear how cells initiate ALT. TERRA, telomeric repeat-containing RNA, forms RNA:DNA hybrids (R-loops) at ALT telomeres. We show that depleting TERRA using an RNA-targeting Cas9 system reduces ALT-associated PML bodies, telomere clustering, and telomere lengthening. TERRA interactome reveals that TERRA interacts with an extensive subset of DNA repair proteins in ALT cells. One of TERRA interacting proteins, the endonuclease XPF, is highly enriched at ALT telomeres and recruited by telomeric R-loops to induce DNA damage response (DDR) independent of CSB and SLX4, and thus triggers break-induced telomere synthesis and lengthening. The attraction of BRCA1 and RAD51 at telomeres requires XPF in FANCM-deficient cells that accumulate telomeric R-loops. Our results suggest that telomeric R-loops activate DDR via XPF to promote homologous recombination and telomere replication to drive ALT.

Flap endonuclease Rad27 cleaves the RNA of R-loop structures to suppress telomere recombination

“…We also analyzed three additional nucleotide excision repair genes, RAD10 , RAD14 and RAD17 , and found that none of them greatly affects type II telomere recombination ( Supplementary Figure S1p–r ). Thus, although mammalian nucleotide excision repair nucleases XPF and XPG have been reported to be involved in processing R-loop structures ( 16 , 17 , 36 ), the yeast nucleotide excision repair enzymes were not essential for telomere recombination. Though mutations in the base-excision repair nucleases (Apn1 and Apn2) and AP lyases (Ntg1 and Ntg2) also did not affect telomere recombination ( Supplementary Figure S1a, k ), the apn1 apn2 double mutant caused a modest delay in type II survivor formation ( Supplementary Figure S1a ), suggesting that Apn1 and Apn2 might function redundantly to modulate telomere recombination.…”

ERCC4: a potential regulatory factor in inflammatory bowel disease and inflammation-associated colorectal cancer

“…Mice overexpressing TRF2 show hypersensitivity to ultraviolet radiation compared to normal mice, resulting in telomere loss in their chromosomes. Conversely, expression of ERCC1-ERCC4 in TRF2-deficient cells leads to increased chromosomal end fusion ( 67 ). Mechanistically, ERCC1-ERCC4 can cleave at the 3’ end of telomeres, causing telomere shortening and premature aging in mice.…”

The Molecular Mechanisms and Therapeutic Prospects of Alternative Lengthening of Telomeres (ALT)

“…Similar to CFS-MiDAS, telomeric MiDAS requires SLX4 and RAD52 although MUS81 is not essential [ 123 ]. Furthermore, the CFS-MiDAS component XPF encourages ALT activity through break-induced synthesis given its activation of DDR pathways that result from the formation of telomeric R-loops [ 167 ]. Targeting SLX4, RAD52, and XPF could thus limit ALT activity through reducing telomeric MiDAS activity.…”

“…Similarly, the PKMYT1 phosphorylates the Tyr15 and Thr14 positions of the CDK1/Cyclin B1 complex [ 171 ]. Targeting the WEE1 protein with inhibitors such as MK-1775 and the PKMYT1 protein with inhibitors such as RP-6306 is particularly promising given the heightened sensitivity of cells with ATRX-deficiencies to WEE1 inhibitors [ 167 , 172 ].…”