Mechanism of end protection in stem cells

Division Of Basic Sciences - Nci

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Abstract

We focused on defining how stem cells preserve telomere end protection and how these mechanisms might be co-opted by cancer cells. NMD-TRF1 Isoform Regulation: We performed a genome-wide synthetic lethal CRISPR screen in TRF2-deficient mouse embryonic stem cells (mESCs). The screen identified the nonsense-mediated mRNA decay (NMD) pathway as essential in this setting. Mechanistically, NMD prevents the accumulation of a TRF1 splice isoform lacking exon 8, which functions as a dominant negative. When NMD is impaired, the truncated TRF1 isoform displaces full-length TRF1 from telomeres, leading to loss of end protection and extensive telomere fusions. Importantly, re-expression of full-length TRF1 restores telomere stability. This work uncovers a novel post-transcriptional quality control system that safeguards telomere integrity in stem cells. Dissecting the Separation Between Recruitment and Protection: Using a newly developed single-cell telomerase assay (see also Project 3), we showed that TPP1 is required for telomerase recruitment through TIN2 but is not involved in end protection. Conversely, POT1 is essential for telomere end protection but dispensable for telomerase function. These findings support a model in which recruitment and protection are genetically and functionally separable modules of shelterin. Together, these findings define new mechanisms that preserve genome stability in the pluripotent state and suggest that cancer cells may exploit these safeguards to tolerate telomere dysfunction. Publications Telomere Protection in Stem Cells. (Cold Spring Harb Perspect Biol. 2024. [Journal]) This review discussed the current understanding of telomere protection in stem cells. Manuscript under review: "A post-transcriptional safeguard for telomere end protection in pluripotent cells" Describes the identification of NMD as a safeguard that prevents accumulation of a dominant-negative TRF1 isoform. Establishes a new mechanism through which stem cells maintain end protection under conditions of telomere stress. Manuscript under review: "Modular separation of telomerase recruitment and end protection in stem cells" Demonstrates that telomerase recruitment (via TPP1-TIN2) and end protection (via POT1) are controlled by distinct pathways. Challenges the binary open/closed telomere model and supports a modular framework for shelterin function.