Human telomeres are protected by TRF2. Inhibition of this telomeric protein results in partial loss of the telomeric 3′ overhang and chromosome end fusions formed through nonhomologous end-joining (NHEJ). Here we report that ERCC1/XPF-deficient cells retained the telomeric overhang after TRF2 inhibition, identifying this nucleotide excision repair endonuclease as the culprit in overhang removal. Furthermore, these cells did not accumulate telomere fusions, suggesting that overhang processing is a prerequisite for NHEJ of telomeres. ERCC1/XPF was also identified as a component of the telomeric TRF2 complex. ERCC1/XPF-deficient mouse cells had a novel telomere phenotype, characterized by Telomeric DNA-containing Double Minute chromosomes (TDMs). We speculate that TDMs are formed through the recombination of telomeres with interstitial telomere-related sequences and that ERCC1/XPF functions to repress this process. Collectively, these data reveal an unanticipated involvement of the ERCC1/XPF NER endonuclease in the regulation of telomere integrity and establish that TRF2 prevents NHEJ at telomeres through protection of the telomeric overhang from ERCC1/XPF.
Bibliographical noteFunding Information:
We thank N. Heintz and W. Lee for HeLa nuclear extract. K. Hoke, D. Loayza, G. Celli, and R. Wang are thanked for insightful discussion and technical advice. H. Odijk, A.F. Theil, W. Vermeulen, and N.G.J. Jaspers are thanked for technical assistance. X.-D.Z. was supported by a Canadian CIHR postdoctoral fellowship. L.N. was supported by a postdoctoral fellowship number PF-99-142 from the American Cancer Society. K.B. was supported (in part) by a long-term postdoctoral fellowship from the EMBO. The laboratory of M.M. at the University of Southern Denmark is supported by a grant from the Danish National Research Foundation to the Center of Experimental BioInformatics (CEBI).This work was supported by grants from the NIH (AG17242) and the EC to J.H. and by grants from the NIH (AG16642 and GM49046) to T.d.L.