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The article focuses on the role of telomerase ribonucleic acid (RNA) levels in telomere length equilibrium. The absence of telomere function could lead chromosomes to break and result in chromosome end-to-end fusion and other rearrangements. It notes that telomerase maintains telomere length by adding telomere repeats on some of the chromosome ends at each cell division. Telomerase is required in maintaining telomere length in cells that undergo various cell divisions.

Discusses research on telomerase activity in cancer stem cells. Result of mutations in telomerase components; Reason for the increase in enzyme activity in later stages of carcinogenesis; Stem cell failure in syndromes of telomere shortening.

Objectives: Systemic sclerosis (SSc) is an autoimmune fibrotic disease affecting multiple tissues including the lung. A subset of patients with SSc with lung disease exhibit short telomeres in circulating lymphocytes, but the mechanisms underlying this observation are unclear., Methods: Sera from the Johns Hopkins and University of California, San Francisco (UCSF) Scleroderma Centers were screened for autoantibodies targeting telomerase and the shelterin proteins using immunoprecipitation and ELISA. We determined the relationship between autoantibodies targeting the shelterin protein TERF1 and telomere length in peripheral leucocytes measured by qPCR and flow cytometry and fluorescent in situ hybridisation (Flow-FISH). We also explored clinical associations of these autoantibodies., Results: In a subset of patients with SSc, we identified autoantibodies targeting telomerase and the shelterin proteins that were rarely present in rheumatoid arthritis, myositis and healthy controls. TERF1 autoantibodies were present in 40/442 (9.0%) patients with SSc and were associated with severe lung disease (OR 2.4, p=0.04, Fisher's exact test) and short lymphocyte telomere length. 6/6 (100%) patients with TERF1 autoantibodies in the Hopkins cohort and 14/18 (78%) patients in the UCSF cohort had a shorter telomere length in lymphocytes or leukocytes, respectively, relative to the expected age-adjusted telomere length. TERF1 autoantibodies were present in 11/152 (7.2%) patients with idiopathic pulmonary fibrosis (IPF), a fibrotic lung disease believed to be mediated by telomere dysfunction., Conclusions: Autoantibodies targeting telomere-associated proteins in a subset of patients with SSc are associated with short lymphocyte telomere length and lung disease. The specificity of these autoantibodies for SSc and IPF suggests that telomere dysfunction may have a distinct role in the pathogenesis of SSc and pulmonary fibrosis., Competing Interests: Competing interests: None declared., (© Author(s) (or their employer(s)) 2021. No commercial re-use. See rights and permissions. Published by BMJ.)

The anti-apoptotic molecules BCL-x(L) and BCL2 delay cell cycle entry from quiescence. We used serum induction and induction of a Myc-estrogen receptor fusion protein (MycER) in quiescent fibroblasts to investigate the mechanisms underlying the cell cycle activity of BCL-x(L) and BCL2. We demonstrate for the first time that BCL-xL and BCL2 delayed serum-induced and Myc-induced, but not E2F-induced, cell cycle entry. The cyclin-dependent kinase inhibitor p27 was elevated during serum deprivation and cell cycle entry in BCL-x(L) or BCL2-expressing NIH3T3 cells and a Rat1MycER cell line. Activation of cyclin-dependent kinase 2 (cdk2) and cyclin-dependent kinase 4 (cdk4) were delayed during progression to S phase, while the induction of cyclin D1 protein, as well as the levels of cyclin E, cdk2, and cdk4 were unaltered by BCL-x(L) or BCL2. Inhibition of cyclin/cdk activities in BCL-x(L) or BCL2 expressing cells was associated with excess p27 in the cyclin/cdk complexes. Neither BCL-x(L) nor BCL2 delayed S phase entry in cells deficient in p27, thus p27 is required for the cell cycle function of BCL-x(L) and BCL2. The cell cycle effects of BCL-x(L) and BCL2 were more profound in Myc-induced than in serum-induced cell cycle entry. Our results suggest that one possible mechanism by which BCL-x(L) and BCL2 delay cell cycle entry may be the inhibition of Myc activity through the elevation of p27.

Telomeres provide stability to eukaryotic chromosomes and consist of tandem DNA repeat sequences. Telomeric repeats are synthesized and maintained by a specialized reverse transcriptase, termed telomerase. Tetrahymena thermophila telomerase contains two essential components: Tetrahymena telomerase reverse transcriptase (tTERT), the catalytic protein component, and telomerase RNA that provides the template for telomere repeat synthesis. In addition to these two components, two proteins, p80 and p95, were previously found to copurify with telomerase activity and to interact with tTERT and telomerase RNA. To investigate the role of p80 and p95 in the telomerase enzyme, we tested the interaction of p80, p95, and tTERT in several different recombinant expression systems and in Tetrahymena extracts. Immunoprecipitation of recombinant proteins showed that although p80 and p95 associated with each other, they did not associate with tTERT. In in vitro transcription and translation lysates, tTERT was associated with telomerase activity, but p80 and p95 were not. p80 bound telomerase RNA as well as several other unrelated RNAs, suggesting p80 has a general affinity for RNA. Immunoprecipitations from Tetrahymena extracts also showed no evidence for an interaction between the core tTERT/telomerase RNA complex and the p80 and p95 proteins. These data suggest that p80 and p95 are not associated with the bulk of active telomerase in Tetrahymena.

Loss of telomere function can induce cell cycle arrest and apoptosis. To investigate the processes that trigger cellular responses to telomere dysfunction, we crossed mTR-/- G6 mice that have short telomeres with mice heterozygous for telomerase (mTR+/-) that have long telomeres. The phenotype of the telomerase null offspring was similar to that of the late generation parent, although only half of the chromosomes were short. Strikingly, spectral karyotyping (SKY) analysis revealed that loss of telomere function occurred preferentially on chromosomes with critically short telomeres. Our data indicate that, while average telomere length is measured in most studies, it is not the average but rather the shortest telomeres that constitute telomere dysfunction and limit cellular survival in the absence of telomerase.

The increased tumor incidence in telomerase null mice suggests that telomere dysfunction induces genetic instability. To test this directly, we examined mutation rate in the absence of telomerase in S. cerevisiae. The mutation rate in the CAN1 gene increased 10- to 100-fold in est1Delta strains as telomeres became dysfunctional. This increased mutation rate resulted from an increased frequency of terminal deletions. Chromosome fusions were recovered from est1Delta strains, suggesting that the terminal deletions may occur by a breakage-fusion-bridge type mechanism. At one locus, chromosomes with terminal deletions gained a new telomere through a Rad52p-dependent, Rad51p-independent process consistent with break-induced replication. At a second locus, more complicated rearrangements involving multiple chromosomes were seen. These data suggest that telomerase can inhibit chromosomal instability.

Telomere dysfunction results in fertility defects in a number of organisms. Although data from fission yeast and Caenorhabditis elegans suggests that telomere dysfunction manifests itself primarily as defects in proper meiotic chromosome segregation, it is unclear how mammalian telomere dysfunction results in germ cell death. To investigate the specific effects of telomere dysfunction on mammalian germ cell development, we examined the meiotic progression and germ cell apoptosis in late generation telomerase null mice. Our results indicate that chromosome asynapsis and missegregation are not the cause of infertility in mice with shortened telomeres. Rather, telomere dysfunction is recognized at the onset of meiosis, and cells with telomeric defects are removed from the germ cell precursor pool. This germ cell telomere surveillance may be an important mechanism to protect against the transmission of dysfunctional telomeres and chromosomal abnormalities.

Yeast cells can survive in the absence of telomerase RNA, TLC1, by recombination-mediated telomere elongation. Two types of survivors, type I and type II, can be distinguished by their characteristic telomere patterns. RAD52 is essential for the generation of both types of survivors. Deletion of both RAD50 and RAD51 produces a phenotype similar to that produced by deletion of RAD52. Here we examined the effects of the RAD50 and the RAD51 epistasis groups as well as the RAD52 homologue, RAD59, on the types of survivors generated in the absence of telomerase. rad59 mutations completely abolished the ability to generate type II survivors, while rad50 mutations decreased the growth viability of type II survivors but did not completely eliminate their appearance. Mutations in RAD51, RAD54, and RAD57 had the converse affect: they eliminated the ability of cells to generate type I survivors in a tlc1 strain. The triple mutant, tlc1 rad51 rad59, was not able to generate survivors. Thus either type I or type II recombination pathways can allow cells to survive in the absence of telomerase; however, elimination of both pathways in a telomerase mutant leads to the inability to elongate telomeres and ultimately cell death.