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Abstract

Loss of telomeric repeats has been causally linked to replicative senescence and ageing in human cells. In contrast to normal somatic cells, which are telomerase-negative, hematopoietic stem cells have low levels of telomerase, which can be transiently up-regulated upon cytokine stimulation. Despite telomerase actions, proliferation of progenitor cells is accompanied by telomere shortening possibly leading to a reduction of their replicative potential. To test whether ectopic expression of telomerase can overcome telomere erosion in hematopoietic progenitor cells, we transduced CD34+ cord blood (CB) cells (n=7) with a retroviral vector including hTERT, the catalytic component of telomerase. CD34+ cells were cultured for 4 weeks in serum-free medium in the presence of stem cell factor (SCF). Flt-3 ligand (Flt-3 L), interleukin 3 (IL-3), IL-6, and granulocyte colony-stimulating factor (G-CSF). The hTERT transduced CD34+ CB cells exhibited significantly elevated telomerase activity (10.4-fold) compared to vector-only-transduced CD34+ cells. A peak telomere elongation of 500 bp was found in hTERT transduced CB cells at day 14 of the culture, but telomere shortening was not completely prevented. Surprisingly, the overall expansion potential was found to be significantly reduced in hTERT transduced cells (5.6-fold) relative to control cells which was due to an accelerated loss of CD34 expression and an increased cellular differentiation. These results indicate that telomeres can be elongated in hematopoietic progenitor cells by ectopic expression of hTERT although telomere shortening is not prevented. In addition, our data suggest that hTERT transduction in CD34+ CB cells does not increase their proliferative capacity, but appears to interfere with their differentiation potential.