hTERT, the Catalytic Component of Telomerase, Is Down-Regulated in the Hematopoietic Stem Cells of Patients with Chronic Myeloid Leukemia.

Telomere shortening and increased telomerase activity is associated with disease progression in chronic myeloid leukemia (CML). In order to gain insight into the biology of telomerase and its regulation in CML we have evaluated the expression levels of the components of telomerase, its regulators, and several telomeric associated proteins in the hematopoietic stem cells (HSC)/leukemic blast cells of 21 patients with this disorder, as compared to the HSC population of a group of healthy individuals, using quantitative real time PCR. In this study we have made the surprising observation that hTERT, the catalytic component of telomerase, is down-regulated in the CD34+/blast cells of the majority of CML patients in chronic phase (CP) (7 out of 9, p=0.0339) relative to CD34+ cells from normal controls. Furthermore hTERT was found to be significantly down-regulated in 2 out of 3 patients in accelerated phase (AP) and in 9 out of 9 patients in blast crisis (BC) relative to normal controls (p=0.0003). The expression levels of hTR, the telomerase RNA template, and the telomeric- associated proteins TEP1, TRF1, TRF2 and tankyrase were all up-regulated in the CD34+/blast cells of the majority of the CML patients in CP and AP, relative to normal control CD34+ cells. With the exception of TEP1, the expression levels of all these factors were highest in CP CML and, in the majority of patients, decreased during disease progression. We speculate that the up-regulation of tankyrase (positive regulator of telomere length) and TRF2 (protects critically shortened telomeres) may contribute to telomere maintenance in CML. The expression levels of hTERT and c-myc, a positive regulator of hTERT expression, correlate in all stages of CML (R²=0.623). The expression levels of c-myc decreased with disease progression, falling below the levels observed in normal CD34+ cells in the majority of patients in BC. The expression levels of hTERT were increased in the CD34+ cells of three additional CP CML patients in remission following successful treatment with imatinib, relative to both untreated CP CML patients (p=0.0126) and to normal individuals, reflecting a restoration of haematopoiesis towards normal. In CML the turnover of all types of Ph+ stem and progenitor cells is known to be increased. We suggest that the reduced (and presumably inadequate) expression levels of hTERT in CML CD34+/blast cells, and the consequent telomere shortening with each division, is the direct cause of the shortened telomeres frequently observed in CML patients. These data challenge the widely accepted view that hTERT expression is always increased in tumorigenesis. It is possible that the Bcr-Abl fusion product may play a role in the down-regulation of hTERT observed in the HSC of CML patients in this study.