Telomeres, the protective ends of eukaryotic chromosomes, are elongated by telomerase to compensate for incomplete replication and nuclease degradation. Most human somatic tissues do not express telomerase, and thus telomeres gradually shorten with cell division and age, ultimately leading to cellular senescence. A fatal inherited telomere biology disease termed Hoyeraal-Hreidarsson syndrome (HHS) is associated with accelerated telomere shortening and diverse clinical symptoms. HHS is caused by germline mutations in telomerase subunits or factors essential for its biogenesis and recruitment, and by mutations in the helicase RTEL1. Mouse Rtel1 was found to be genetically associated with telomere length, and thus termed ‘Regulator of Telomere Elongation 1’. Mechanistically, however, it was only shown to protect telomeres from deleterious process such as replication fork stalling and reversal and telomere circles excision, which do not readily explain its proposed role in mouse telomere length regulation and its implication in HHS.
To further understand the telomeric function of RTEL1, we analyzed HHS patient cells carrying various RTEL1 point mutations. Patient fibroblasts expressing telomerase displayed telomere shortening and other aberrations and growth defects in culture. Inducible ectopic expression of a WT RTEL1 splice variant dramatically elongated the telomeres and rescued the cells, while silencing its expression resulted in a gradual telomere shortening. While additional telomeric and non-telomeric damage was also observed, consistent with other proposed functions of RTEL1, this damage was not detrimental. Our observations suggest that HHS-associated RTEL1 mutations cause the disease by compromising telomere elongation by telomerase, similarly to HHS-causing mutations in other genes. Furthermore, based on the HHS-causing mutation we discovered, we generated gene replacement mouse models in Rtel1 that display progressively shortening telomeres, further supporting the critical role of RTEL1 in regulating telomerase action and telomere length.