Abstract
A subset of tumors use a recombination-based alternative lengthening of telomere (ALT) pathway to resolve telomeric dysfunction in the absence of TERT. Loss-of-function mutations in the chromatin remodeling factor ATRX are associated with ALT but are insufficient to drive the process. Because many ALT tumors express the mutant isocitrate dehydrogenase IDH1 R132H, including all lower grade astrocytomas and secondary glioblastoma, we examined a hypothesized role for IDH1 R132H in driving the ALT phenotype during gliomagenesis. In p53/pRb-deficient human astrocytes, combined deletion of ATRX and expression of mutant IDH1 were sufficient to create tumorigenic cells with ALT characteristics. The telomere capping complex component RAP1 and the nonhomologous DNA end joining repair factor XRCC1 were each downregulated consistently in these tumorigenic cells, where their coordinate reexpression was sufficient to suppress the ALT phenotype. RAP1 or XRCC1 downregulation cooperated with ATRX loss in driving the ALT phenotype. RAP1 silencing caused telomere dysfunction in ATRX-deficient cells, whereas XRCC1 silencing suppressed lethal fusion of dysfunctional telomeres by allowing IDH1-mutant ATRX-deficient cells to use homologous recombination and ALT to resolve telomeric dysfunction and escape cell death. Overall, our studies show how expression of mutant IDH1 initiates telomeric dysfunction and alters DNA repair pathway preferences at telomeres, cooperating with ATRX loss to defeat a key barrier to gliomagenesis.Significance: Studies show how expression of mutant IDH1 initiates telomeric dysfunction and alters DNA repair pathway preferences at telomeres, cooperating with ATRX loss to defeat a key barrier to gliomagenesis and suggesting new therapeutic options to treat low-grade gliomas. Cancer Res; 78(11); 2966-77. ©2018 AACR.
Highlights
A rate-limiting step in the development of most cancers is the resolution of telomeric dysfunction
The defective p53 and pRb signaling in these astrocytes mimics that noted in lowergrade astrocytoma (LGA) [37], and in these cells (Fig. 1, Cont) neither the CRISPR-based, targeted homozygous deletion of ATRX (Fig. 1A and B, 2 independent clones ATRX KO-1 and KO-2 and Supplementary Fig. S1A) nor stable introduction of mutant IDH1 alone resulted in detectable levels of TERT mRNA or TERT activity via TRAP analysis relative to positive control parental E6E7 cells exogenously expressing hTERT (Fig. 1C)
No control, ATRX-KO, or mutant IDH1–expressing cultures exhibited any signs of the alternative lengthening of telomere (ALT) phenotype (APBs; Fig. 1D; Supplementary Fig. S1B), telomeric sister chromatid exchange (T-SCE) (Fig. 1E; Supplementary Fig. S1C), or C-circles (Fig. 1F; Supplementary Fig. S1D), relative to positive control GM847 fibroblasts known to display the ALT phenotype [38]
Summary
A rate-limiting step in the development of most cancers is the resolution of telomeric dysfunction. Telomeres are DNA–protein complexes that protect chromosomes ends from being erroneously recognized as damaged DNA. Telomeric DNA is elongated by TERT-mediated reverse transcription of a TERC RNA template [1]. TERT expression is silenced during development, and human somatic cells forced to divide in the absence of TERT fail to fully replicate the ends of chromosomes, losing 50 to 200 bp of telomeric DNA per cell division [2]. When the telomeric DNA becomes critically short, the six-protein component of the telomere, known as the shelterin cap, dissociates [3].
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
