Human short-term repopulating cells have enhanced telomerase reverse transcriptase expression

Telomerase activity has been suggested to be critically involved in hematopoietic stem cell (HSC) self-renewal. However, it has been unclear whether human HSCs have telomerase activity and how telomerase activity is regulated within the HSC and progenitor pool. Here, we isolated living cord-blood (CB) CD34(+) cells with up-regulated human telomerase reverse transcriptase (hTERT) expression by using an hTERT-reporting adenoviral vector encoding destabilized green fluorescent protein (dGFP) driven by the hTERT promoter, and functionally characterized them in comparison with control vector-transduced CD34(+) cells expressing GFP. Following a 2-day serum-free transduction protocol, cells were sorted into a dGFP(+) and a GFP(+) fraction. Cell-cycle analysis revealed that the dGFP(+) cells had a greater proportion of cells in S/G(2)/M phase compared with the GFP(+) cells, (56% +/- 1.8% vs 35% +/- 4.3%; P < .001) and fewer cells in G(0) phase (8.1% +/- 3.0% vs 20% +/- 4.7%; P < .01) However, the colony-forming and short-term nonobese diabetic/severe combined immunodeficient (NOD/SCID) B2m(-/-) mice bone marrow-repopulating capacities were similar between the dGFP(+) and the GFP(+) cells. Interestingly, the dGFP(+) cells had a 6-fold lower repopulating capacity in NOD/SCID mice compared with the GFP(+) cells and lacked secondary NOD/SCID B2m(-/-) mice bone marrow-repopulating capacity. Thus, up-regulation of hTERT expression within the CB HSC pool is accompanied by decreased self-renewal capacity.