Telomeres and telomerase in haematopoietic progenitors and bone marrow endothelial cells

Abstract

In normal human somatic cells, the length of telomeres (chromosomal end structures) decreases with each cell division until reaching a critically short length, which halts cell proliferation and induces senescence. The enzyme telomerase, which functions to maintain telomeres at a length that is permissive for cell division, is expressed in approximately 85% of cancer cells and some stem and progenitor cells, including haematopoietic progenitor cells (HPCs), but not most other normal somatic cells. Previous investigations have demonstrated that ectopic expression of telomerase reverse transcriptase (hTERT) reconstitutes telomerase activity, resulting in telomere elongation in some normal human cell types. However, similar experiments performed in HPCs and endothelial cells have demonstrated a dissociation between the expression of telomerase activity and telomere lengthening. This thesis is focussed on further investigating telomerase-mediated telomere length regulation in HPCs and endothelial cells.

Short telomeres in bone marrow and blood leukocytes are associated with the development of disorders linked to bone marrow failure. However, to date a relationship between telomere length and myeloid cell proliferative potential has not been demonstrated. In the current investigations, the telomere length and proliferative potential of 31 cord blood-derived HPCs was determined. Regression analysis revealed a significant correlation between mean telomere length and erythroid cell expansion, but not expansion of other myeloid lineage cells. Another novel finding was that telomerase activity was upregulated in lineage-committed CD34- erythroid cells that were positive for the erythroid-specific lineage marker glycophorin A. It was also functionally demonstrated that telomerase activity facilitates the maximum expansion of erythroid cells.

To address the dissociation between telomerase activity and telomere maintenance in BMECs, a dominant negative mutant of the telomere binding protein TRF1, which functions to regulate telomere accessibility, was over-expressed in hTERT-transduced BMECs. These studies showed that telomere access, as well as oncogene expression and exposure to oxidative stress, contribute to telomere length regulation in BMECs. Overall, the results from these investigations demonstrate for the first time the functional significance of telomere length and telomerase activity in ex vivo expansion of erythroid cells, and provide novel insight to the molecular complexity of telomere length maintenance in endothelial cells.