SAMHD1 restricts the deoxyguanosine triphosphate pool contributing to telomere stability in telomerase‐positive cells

SAMHD1 (Sterile alpha motif and histidine/aspartic acid domain‐containing protein 1) is a dNTP triphosphohydrolase crucial in the maintenance of balanced cellular dNTP pools, which support genome integrity. In SAMHD1 deficient fibroblasts isolated from Aicardi‐Goutières Syndrome (AGS) patients, all four DNA precursors are increased and markedly imbalanced with the largest effect on dGTP, a key player in the modulation of telomerase processivity. Here, we present data showing that SAMHD1, by restricting the dGTP pool, contributes to telomere maintenance in hTERT‐immortalized human fibroblasts from AGS patients as well as in telomerase positive cancer cell lines. Only in cells expressing telomerase, the lack of SAMHD1 causes excessive lengthening of telomeres and telomere fragility, whereas primary fibroblasts lacking both SAMHD1 and telomerase enter normally into senescence. Telomere lengthening observed in SAMHD1 deficient but telomerase proficient cells is a gradual process, in accordance with the intrinsic property of telomerase of adding only a few tens of nucleotides for each cycle. Therefore, only a prolonged exposure to high dGTP content causes telomere over‐elongation. hTERT‐immortalized AGS fibroblasts display also high fragility of chromosome ends, a marker of telomere replication stress. These results not only demonstrate the functional importance of dGTP cellular level but also reveal the critical role played by SAMHD1 in restraining telomerase processivity and safeguarding telomere stability. A Model for SAMHD1 as a gatekeeper of telomere stability in telomerase positive cells. In SAMHD1 proficient cells, the dGTP pool represents less than 10% of the total dNTP pool that preserves telomere length and structure. SAMHD1 deficiency in both immortalized fibroblasts and telomerase positive cancer cells expands the dGTP content, which enhances telomerase processivity leading to telomere elongation (open arrows) and fragility (dark arrows).