Diminished Telomeric 3′ Overhangs Are Associated with Telomere Dysfunction in Hoyeraal-Hreidarsson Syndrome

Abstract

BackgroundEukaryotic chromosomes end with telomeres, which in most organisms are composed of tandem DNA repeats associated with telomeric proteins. These DNA repeats are synthesized by the enzyme telomerase, whose activity in most human tissues is tightly regulated, leading to gradual telomere shortening with cell divisions. Shortening beyond a critical length causes telomere uncapping, manifested by the activation of a DNA damage response (DDR) and consequently cell cycle arrest. Thus, telomere length limits the number of cell divisions and provides a tumor-suppressing mechanism. However, not only telomere shortening, but also damaged telomere structure, can cause telomere uncapping. Dyskeratosis Congenita (DC) and its severe form Hoyeraal-Hreidarsson Syndrome (HHS) are genetic disorders mainly characterized by telomerase deficiency, accelerated telomere shortening, impaired cell proliferation, bone marrow failure, and immunodeficiency.Methodology/Principal FindingsWe studied the telomere phenotypes in a family affected with HHS, in which the genes implicated in other cases of DC and HHS have been excluded, and telomerase expression and activity appears to be normal. Telomeres in blood leukocytes derived from the patients were severely short, but in primary fibroblasts they were normal in length. Nevertheless, a significant fraction of telomeres in these fibroblasts activated DDR, an indication of their uncapped state. In addition, the telomeric 3′ overhangs are diminished in blood cells and fibroblasts derived from the patients, consistent with a defect in telomere structure common to both cell types.Conclusions/SignificanceAltogether, these results suggest that the primary defect in these patients lies in the telomere structure, rather than length. We postulate that this defect hinders the access of telomerase to telomeres, thus causing accelerated telomere shortening in blood cells that rely on telomerase to replenish their telomeres. In addition, it activates the DDR and impairs cell proliferation, even in cells with normal telomere length such as fibroblasts. This work demonstrates a telomere length-independent pathway that contributes to a telomere dysfunction disease.

Inherited mutations in the helicase RTEL1 cause telomere dysfunction and Hoyeraal–Hreidarsson syndrome

“…These telomere phenotypes suggested that the cells of the heterozygous carriers of either RTEL1 mutation had a telomere defect, although it was not severe enough to cause a disease. The telomeres of paternal grandfather G1 were shorter than those of G2, suggesting that the genetic defect was transmitted from G1 to P1 and to the affected siblings (9). Sequencing confirmed that G1 and G3 carried the M492I mutation, whereas G2 was WT at this position.…”

“…The heterozygous parents, although healthy, had relatively short telomeres in leukocytes, with broader distribution of lengths compared with the paternal grandmother G2 who does not carry the RTEL1 mutation (9). The shorter telomeres in the younger parents suggest compromised telomere length maintenance as leukocyte telomeres normally shorten with age, and thus telomeres of children are expected to be longer than those of their parents.…”

“…Telomeres in blood cells derived from the patients were severely shortened, and lymphoblastoid cell lines (LCLs) grown in culture showed progressive telomere shortening until reaching senescence, despite the presence of active telomerase. Primary fibroblasts had normal average telomere length but nevertheless displayed telomere dysfunction-induced foci and grew substantially slower than normal fibroblasts (9). Ectopic expression of hTERT, a standard procedure for fibroblast immortalization, failed to stabilize telomere length and prevent senescence of the HHS fibroblasts.…”

“…We have been studying a family in which four of five siblings were diagnosed with HHS; three of them passed away at ages of 3-7, and the fourth died of pulmonary fibrosis 5 y after successful bone marrow transplantation (9) (Fig. 1A).…”

Genetic Variations in Telomere Maintenance, with Implications on Tissue Renewal Capacity and Chronic Disease Pathologies

“…Subsequent molecular analyses confirmed that individuals diagnosed with HHS showed mutations in DKC1 and TERT [181][182][183][184]. Patients were also found to have shortened 3′ telomeric overhangs [185]. Given similarities in clinical presentation, and genetic lesions in the same genes, HHS is now considered to be a severe form of DC [180,182].…”