Telomerase Activity as a Marker For Mobilization Quality

Autologous hematopoietic cell transplantation (HCT) has revolutionized the curative approach to a number of malignancies by providing hematopoietic and immune rescue after high dose cytoreductive therapy. In cases of residual disease, patients are usually treated with chemotherapy to repress the disease, thereafter G-CSF injections are given serially to encourage stem cell proliferation in the bone marrow and mobilization of the stem/progenitor cells to the peripheral blood. Another approach, usually utilizes in myeloma patients is the administration of G-CSF in steady state (without administration of chemotherapy) and consequently collection of the CD34+ cells.

The minimum number of CD34+ cells threshold requires for HCT is currently defined as 2 x 10^6/kg. However, the optimal dose in terms of engraftment may be even higher (>5x10^6/kg), especially when platelet recovery is considered.

In clinical practice, the right timing for collection is decided upon measurement of CD34, a membrane glycoprotein of progenitors and stem-cells. CD34 levels are measured in the expected maximal effect of the G-CSF priming, and accordingly the collection is scheduled. Practically, if the circulating CD34+cell count is ≥20/μL, 90% of collections performed the following day would be expected to yield ≥2.0x106 CD34+cells/kg (Gordon, BMT 1997). However, in cases higher doses of CD34+ cells are required (e.g. for tandem HCT), CD34+ threshold may not be sufficient to determined collection yield.

G-CSF based mobilization regimens have a 5-30% failure rate amongst patients, however in patients with risk factors, up to 60% of the patients are failed to mobilize. Poor mobilization has significant consequences for the patient with potential loss of transplantation as a treatment option. Repeated attempts at mobilization increase resource utilization, morbidity and patient inconvenience. Therefore attempts to identify patients who would mobilize poorly are of clinical significance.

Human telomerase, a unique ribonucleoprotein complex, is inactive in normal somatic cells but present in high levels in more than 90% of all malignancies. The enzyme, synthesize new telomeric repeats at the 3' ends of chromosomes.

As oppose to other normal cells, stem cells are unique in that they carry active telomerase which provides them with longer life span. Previous study from our lab showed that in patients and healthy donors, the administration of GCSF was associated with a 14th fold increase of telomerase activity levels in peripheral blood CD34+ cells.

Working hypothesis:

Telomerase activity in mobilized stem cells is correlated with both the absolute number of the collected cells and with the quality of the future engraftment after high dose therapy and HCT

Specific aims:

1. To investigate the correlation between telomerase activity of CD34+ cells in the day of collection and the total number of collected CD34+ cells in a cohort of 50 patients undergoing stem cells mobilization and collection.
2. To explore the association between telomerase activity of CD34+ cells and the engraftment characteristics (time to platelets>50000/microL, time to platelets>150000/microL, time to neutrophils>500/microL, time to neutrophil>2000/microL).
3. To develop an algorithm based on CD34+ cells levels and CD34+ telomerase activity, both measured on the day of collection, that can serve as predictor for the total number of CD34+ cells to be collected.