Telomerase activity and telomere length in CD34+ cells from acute and chronic leukemias, pre- and post-ex vivo expansion
Janus Kinase 2
We studied telomerase regulation and telomere length in hematopoietic progenitor cells from blood and marrow from patients with acute and chronic leuk-mias. CD34+ cells from AML (n=7), CML (nll), CLL (n=4), P.vera (PV, n4) and MDS (n-1) patients were investigated before and after ex vivo expansion culture. Compared to normal hematopoietic progenitors, telomerase activity was 2- to 5ibld increased in chronic phase (CP) -CML, CLL, PV and MDS. In AML, accelerated phase (AP) and blastic phase (BP) -CML, basal telomerase activity was 10- to 50-fold higher. Within 72hrs of ex vivo expansion of CP-CML CD34+ cells in the presence of multiple cytokines (KL, IL-3, IL-6, G-CSF+Epo; Delta assay), telomerase was upregulated, peaked after 1 week of culture and decreased below detection after 2 weeks. In contrast, in AP/BP-CML CD34+ cells, telomerase was downregulated after 1 week of expansion, decreasing even further thereafter. The expansion of cells m culture of CD34+ cells in leukemias was considerably decreased compared to normal hematopoiesis. Average cell expansion was 6.7-, 2.4-. and 0.5-fold in week I, week 2 and week 3+4, respectively, compared to 5- to 15-fold higher expansion in normal hematopoietic progenitors. In serial Delta culture, an average telomeric loss of 0.5-1 kbp within 3 to 4 weeks of expansion was observed. Taken together, we show that telomerase is upregulated in expansion culture in CD34+ cells from CP-CML. CLL, PV and MDS, similar to patterns observed in normal hematopoietic cells in week 1 of Delta with respect to proliferation and cell cycle activation. In AML and AP/BP-CML, however, telomerase was high at baseline and downregulated in expansion culture. Whereas normal primitive hematopoietic cells are mostly quiescent in vivo, leukemic cells, especially in AML and AP/BP-CML, may turnover more rapidly. Increased telomerase expression in leukemic progenitors at baseline may therefore be a feature of their malignant phenotype as well as their cycling and rapid turnover potential. Tetomerase downregulation in Delta culture may be due to their decreased expansion, repression of normal hematopoiesis, or, in AML, due to partial differentiation of AML cells, shown by us previously to be associated with loss of telomerase activity (Cancer Res. 56:1503-1508,1996). Telomere shortening in Delta may be due to levels of telomerase, particularly in chronic leukemic or MDS progenitors insufficient to protect telomere base pair loss upon proliferation and/or the emergence of non-leukemic populations where telomerase levels are insufficient to stabilize telomeres upon extensive prolifération (Blood, in press). Since lelomerase activity displays different patterns in acute leukemic and normal hematopoietic cells, telomerase may be utilized as a molecular marker to distinguish between malignant and normal hematopoietic progenitor cells.