Telomeres cap the ends of eukaryotic chromosomes and distinguish them from broken DNA ends to suppress DNA damage response, cell cycle arrest and genomic instability. Telomeres are elongated by telomerase to compensate for incomplete replication and nuclease degradation and to extend the proliferation potential of germ and stem cells and most cancers. However, telomeres in somatic cells gradually shorten with age, ultimately leading to cellular senescence. Hoyeraal-Hreidarsson syndrome (HHS) is characterized by accelerated telomere shortening and diverse symptoms including bone marrow failure, immunodeficiency, and neurodevelopmental defects. HHS is caused by germline mutations in telomerase subunits, factors essential for its biogenesis and recruitment to telomeres, and in the helicase RTEL1. While diverse phenotypes were associated with RTEL1 deficiency, the telomeric role of RTEL1 affected in HHS is yet unknown. Inducible ectopic expression of wild-type RTEL1 in patient fibroblasts rescued the cells, enabled telomerase-dependent telomere elongation and suppressed the abnormal cellular phenotypes, while silencing its expression resulted in gradual telomere shortening. Our observations reveal an essential role of the RTEL1 C-terminus in facilitating telomerase action at the telomeric 3′ overhang. Thus, the common etiology for HHS is the compromised telomerase action, resulting in telomere shortening and reduced lifespan of telomerase positive cells.

SummaryThis study evaluated the association of vitamin D and bone markers with the development osteoporosis in Palestinian postmenopausal women. Even though vitamin D deficiency was very high for the recruited subjects, it was not associated with osteoporosis except for bones of the hip. Age and obesity were the strongest determining factors of the disease.PurposeThe purpose of this study was to investigate the association of bone mineral density (BMD) with serum vitamin D levels, parathyroid hormone (PTH), calcium, obesity, and bone turnover markers in Palestinian postmenopausal women.MethodsThree hundred eighty-two postmenopausal women (≥45 years) were recruited from various women clinics for BMD assessment (131 women had osteoporosis and 251 were normal and served as controls). Blood samples were obtained for serum calcium, PTH, 25(OH)D, bone formation (N-terminal propeptide (PINP)), and bone resorption (serum C-terminal telopeptide of type I collagen (CTX1)) markers.ResultsWomen with osteoporosis had statistically significant lower mean weight, height, body mass index (BMI), and serum calcium (p < 0.05) compared to controls. No significant differences were detected between the mean values of bone turnover markers (CTX and PINP), 25(OH)D, and PTH of the two groups. Women with vitamin D deficiency (severe and insufficiency) represented 85.9% of the study subjects. Multiple and logistic regression showed that age and BMI significantly affected BMD and vitamin D had a significant association with BMD only at the lumbar spine. BMI was positively correlated with BMD and PTH but negatively correlated with vitamin D. Logistic regression showed that the odds ratio (OR) for having osteoporosis decreased with increasing BMI (overweight OR = 0.11, p = 0.053; obese OR = 0.05, p = 0.007).ConclusionsThere was no direct correlation between BMD and PTH, bone turnover markers, and vitamin D except at the lumbar spine. A negative correlation between BMD and age and a positive correlation with BMI were observed. The protective effect of obesity on osteoporosis was complicated by the effect of obesity on vitamin D and PTH.

Inherited bone marrow failure syndromes (IBMFS) represent a group of disorders typified by impaired production of one or several blood cell types. The telomere biology disorders dyskeratosis congenita (DC) and its severe variant Høyeraal-Hreidarsson (HH) syndrome are rare IBMFS characterized by bone marrow failure, developmental defects, and various premature aging complications associated with critically short telomeres. Here we identified biallelic variants in the gene encoding the 5'-to-3' DNA exonuclease Apollo/SNM1B in three unrelated patients presenting with a DC/HH phenotype consisting of early onset hypocellular bone marrow failure, B and NK lymphopenia, developmental anomalies, microcephaly and/or intrauterine growth retardation. All three patients carry a homozygous or compound heterozygous (in combination with a null-allele) missense variant affecting the same residue L142 (L142F or L142S) located in the catalytic domain of Apollo. Apollo-deficient cells from patients exhibited spontaneous chromosome instability and impaired DNA repair that was complemented by CRISPR/Cas9-mediated gene correction. Furthermore, patients' cells showed signs of telomere fragility that were however not associated with global reduction of telomere length. Unlike patients' cells, human Apollo KO HT1080-cell lines showed strong telomere dysfunction accompanied by excessive telomere shortening, suggesting that the L142S and L142F Apollo variants are hypomorphic. Collectively, these findings define human Apollo as a genome caretaker and identify biallelic Apollo variants as a genetic cause of a hitherto unrecognized severe IBMFS combining clinical hallmarks of DC/HH with normal telomere length.

The most distal 2 kb region in the majority of human subtelomeres contains CpG-rich promoters for TERRA, a long non-coding RNA. When the function of the de novo DNA methyltransferase DNMT3B is disrupted, as in ICF1 syndrome, subtelomeres are abnormally hypomethylated, subtelomeric heterochromatin acquires open chromatin characteristics, TERRA is highly expressed, and telomeres shorten rapidly. In this study, we explored whether the regulation of subtelomeric epigenetic characteristics by DNMT3B is conserved between humans and mice. Studying the DNA sequence of the distal 30 kb of the majority of murine q-arm subtelomeres indicated that these regions are relatively CpG-poor and do not contain TERRA promoters similar to those present in humans. Despite the lack of human-like TERRA promoters, we clearly detected TERRA expression originating from at least seven q-arm subtelomeres, and at higher levels in mouse pluripotent stem cells in comparison to mouse embryonic fibroblasts (MEFs). However, these differences in TERRA expression could not be explained by differential methylation of CpG islands present in the TERRA-expressing murine subtelomeres. To determine whether Dnmt3b regulates the expression of TERRA in mice, we characterized subtelomeric methylation and associated telomeric functions in cells derived from ICF1 model mice. Littermate-derived WT and ICF1 MEFs demonstrated no significant differences in subtelomeric DNA methylation, chromatin modifications, TERRA expression levels, telomere sister chromatid exchange or telomere length. We conclude that the epigenetic characteristics of murine subtelomeres differ substantially from their human counterparts and that TERRA transcription in mice is regulated by factors others than Dnmt3b.

Current social trends of delayed reproduction to the fourth and fifth decade of life call for a better understanding of reproductive aging. Demographic studies correlated late reproduction with general health and longevity. Telomeres, the protective ends of eukaryotic chromosomes, were implicated in various aging-associated pathologies and longevity. To examine whether telomeres are also associated with reproductive aging, we measured by Southern analysis the terminal restriction fragments (TRF) in leukocytes of women delivering a healthy infant following a spontaneous pregnancy at 43–48 years of age. We compared them to age-matched previously fertile women who failed to conceive above age 41. The average TRF length in the extended fertility group (9350 bp) was significantly longer than in the normal fertility group (8850 bp; p-value = 0.03). Strikingly, excluding women with nine or more children increased the difference between the groups to over 1000 bp (9920 and 8880 bp; p-value = 0.0009). Nevertheless, we observed no apparent effects of pregnancy, delivery, or parity on telomere length. We propose that longer leukocyte telomere length reflects higher oocyte quality, which can compensate for other limiting physiological and behavioral factors and enable successful reproduction. Leukocyte telomere length should be further explored as a novel biomarker of oocyte quality for assessing reproductive potential and integrating family planning with demanding women’s careers.