Telomerase activity in dyskeratosis congenita: TERC alone improves cell survival, but is there a trade-off?

In the inherited disorder dyskeratosis congenita (DC), which is linked to deficiencies in telomerase acticity, the principle cause of mortality is bone marrow failure; premature cell death and/or senescence, a consequence of catastrophic telomere shortening, are the likely mechanism(s) underpinning the clinical observations. 

Mounting evidence now indicates that impaired telomerase activity impacts the stem/progenitor compartment, resulting in premature exhaustion of the haemopoietic progenitors, and reduced number of peripheral blood progenitor cells. Mutations in the various subunits of the telomerase complex have been reported, with the RNA subunit, TERC, shown to be the limiting factor for telomerase activity and telomere length in DC patient fibroblast strains transfected with the catalytic subunit, TERT. This has led Kirwan et al. to study growth and telomere dynamics in TERC-transduced primary T-lymphocytes and B-LCLs from DC patients:

Exogenous TERC alone can enhance proliferative potential, telomerase activity and telomere length in lymphocytes from dyskeratosis congenita patients

Dyskeratosis congenita (DC) is an inherited multi-system disorder characterised by muco-cutaneous abnormalities, bone marrow failure and a predisposition to malignancy. Bone marrow failure is the principal cause of mortality and is thought to be the result of premature cell death in the haematopoietic compartment because DC cells age prematurely and tend to have short telomeres. DC is genetically heterogeneous and patients have mutations in genes that encode components of the telomerase complex (DKC1, TERC, TERT, NOP10 and NHP2), and telomere shelterin complex (TINF2), both important in telomere maintenance. Here, we transduced primary T lymphocytes and B lymphocyte lines established from patients with TERC and DKC1 mutations with wild type TERC-bearing lentiviral vectors. We found that transduction with exogenous TERC alone was capable of increasing telomerase activity in mutant T lymphocytes and B lymphocyte lines and improved the survival and thus overall growth of B-lymphocyte lines over a prolonged period, regardless of their disease mutation. Telomeres in TERC-treated lines were longer than in the untreated cultures. This is the first study of its kind in DC lymphocytes and the first to demonstrate that transduction with TERC alone can improve cell survival and telomere length without the need for exogenous TERT.

To assess the impact of TERC-transfection on telomerase activity, the authors established primary T-lymphocyte cultures from four DC patients (two with different TERC mutations; two with different dyskerin mutations) and four heathy subjects. T-cells are difficult to maintain for prolonged periods; therefore, B-lymphoblastoid cell lines (B-LCLs) derived from DC-patients, previously shown to grow poorly when compared to those from healthy volunteers, were utilized for the study of growth dynamics.

In the primary T-cells, telomerase activity in the transfected cells was quite similar between the mutant and wildtype, with all wild-types showing a non-significant increase. Of the DC cells, both TERC mutant samples showed a significant increase, but neither of the DKC1 mutants did (one sample actually saw a decrease). This observation is intuitive, given the respective mutations.

In the B-LCL lines, TERC mRNA was expressed in the transduced cells, although not to a significant level in one each of the wild-type and DKC1 lines. Regardless, telomerase activity was increased in all lines, and to a greater extent than in the T-lymphocytes. (However, this comparison was not completely valid, as activity was assessed at 7 days in the T-cells and 98 days in the B-LCL lines; why not also examine activity at 7 days in the B-LCLs?)

Consistent with the increased telomerase activity, mean telomere length was increased in all but one wild-type line 98 days post-transfection, and there was greater net growth and increased live/dead ratios in the mutant lines overexpressing TERC.

Based on their findings, the authors suggests that TERC alone can rescue the phenotype of DC by increasing telomerase activity, extending telomeres and prolonging cellular lifespan. As TERT, the catalytic unit, was not overexpressed, it would appear that the cell contains spare endogenous capacity for telomerase activity. 

This may be a crucial observation as TERT has recently been shown to have roles other than extending telomeres. During times of cellular stress TERT is reversibly excluded form the nucleus in a dose-dependent manner; this extra-nuclear TERT co-localizes with mitochondria, protecting mtDNA and maintaining mitochondrial function. If over-expressing TERC somehow sequesters this “spare” TERT, what then happens if the cell is “stressed”?  While the maintenance of telomere length is important, mitochondrial damage can lead to increased ROS production thus initiating a detrimental positive feedback loop. In this model system then, there may be a critical trade-off between telomere dynamics and protection of mitochondrial function. Further studies will clearly be required to establish if such a trade-off is worthwhile.