Dyskerin Is Upregulated During Erythroid Differentiation of Human Hematopoietic Progenitor Cells and Hyperactivates Telomerase in Erythroid Precursor Cells

Abstract 980

Telomerase is a ribonuclear protein complex that functions to maintain the integrity of chromosomal-end structures (telomeres), and thereby enables continued cell replication. The active telomerase holoenzyme includes telomerase reverse transcriptase (hTERT) as its catalytic core, an RNA component (hTR) that functions as a template for synthesis of telomere repeats and an RNA-modifying protein dyskerin, which stabilizes TERC within the telomerase holoenzyme. While telomerase activity is high in acute myeloid leukemia cells, low levels of telomerase enzyme activity are detected in human CD34+ progenitor cells (HPCs). Telomerase activity is increased by cytokine stimulation of HPCs, then downregulated during myelomonocytic cell differentiation. Data accumulated to date suggest that telomerase enzyme levels are principally determined by Myc-mediated transcriptional regulation of the gene encoding hTERT, although the dyskerin gene (DKC1) is also a known Myc target.

Our studies of the regulation and function of telomerase in human myeloid cells utilize cord blood-derived HPCs subject to ex vivo expansion and differentiation along specific myeloid lineages. Our past results revealed for the first time, that in contrast to the repression of telomerase activity observed during granulocytic, monocytic and megakaryocytic differentiation, telomerase activity was upregulated during erythroid differentiation, to reach levels comparable to tumor cell lines in CD34-/Glycophorin A+ erythroid precursor cells (Schuller et al., Leukemia 21: 983–991. 2007). Here we show that the upregulation of telomerase during erythroid differentiation is accompanied by a parallel increase in dyskerin mRNA and protein expression. In contrast to dyskerin expression, but consistent with the pattern of hTERT expression in other myeloid lineages, TERT gene expression was downregulated to a minimally detectable level in erythroid precursor cells. Chromatin-immunoprecipitation revealed that while Myc-binding at the DKC1 promoter decreased, binding of the erythroid-specific transcription factor GATA-1 increased during erythroid differentiation. Overexpression of DKC1 in HPCs confirmed that the upregulation of dyskerin was sufficient to hyperactivate telomerase, while shRNA-mediated depletion of dyskerin from erythroid precursors cells suppressed telomerase activity and decreased expansion of erythroid precursor cells.

These data are the first to demonstrate a cellular context in which the upregulation of dyskerin drives telomerase enzyme activity. Further, the results implicate GATA-1 in the regulation of DKC1 transcription and show that high levels of dyskerin are required to sustain proliferation of Glycophorin A+ erythroid precursors. These results have important implications in relation to the pathogenesis of the inherited syndrome dyskeratosis congenita in which the DKC1 gene is mutated, and illustrate a novel mechanism by which telomerase may be reactivated in myeloid leukemia.