Telomerase activity and telomere length in mononuclear cells (MNCs) and granulocytes from peripheral blood (PB) and bone marrow (BM) specimens were studied in pediatric acute
leukemia (ALL, n = 15; AML, n = 11) and pediatric solid
tumor (ST) patients (n = 9) at diagnosis, during and after
chemotherapy. In four ST patients,
tumor tissue was also available. For comparative analysis, MNCs from healthy donors (n = 53) were analyzed.
Telomerase was evaluated using a modified telomeric repeat amplification protocol (TRAP) assay, and telomere length by terminal restriction fragment (TRF) analysis. At diagnosis, high
telomerase activity was detected in MNCs from all
leukemia patients, which was similar to the activity from ST biopsy specimens. This exceeded by 10- to 20-fold the activity in PB MNCs from ST patients and healthy donors (P < 0.05). Granulocyte fractions lacked
telomerase activity in all groups. BM MNCs in
leukemia patients revealed a four-fold higher
telomerase activity than PB (P = 0.005). After
induction chemotherapy and response to treatment,
telomerase activity decreased to borderline or undetectable levels in PB MNCs in
leukemia (P < 0.01). Average telomeres in PB MNCs from pediatric patients were significantly longer (n = 25; 10.9 kbp) than telomeres in PB and BM MNCs from adult healthy donors (7.45 kbp) (P < 0.0001). At diagnosis, telomeres were shorter from BM compared to PB specimens in
leukemia (P < 0.05), and two peak TRFs were observed corresponding to the malignant and normal cell clones. With the attainment of remission, the lower TRF peak, reflecting the leukemic population, was lost. In
leukemia patients, mean TRFs increased on average 2.2 kbp after
induction chemotherapy, but decreased thereafter on consolidation and
maintenance chemotherapy (1 kbp). This was comparable to an average telomere loss of 1.2 kbp in PB specimens from ST patients after
chemotherapy. In all patients, telomere loss in granulocytes as compared to MNCs was more pronounced with 1.8 vs 1 kbp, respectively (P = 0.014). Our results demonstrate that at diagnosis,
telomerase was consistently and highly upregulated in BM and PB specimens in
leukemia, decreased after induction
therapy, and correlated with remission. BM specimens in
leukemia had higher
telomerase activity, probably due to the greater leukemic burden than in PB. Telomeres were significantly longer in children than in adults, but shortened as a consequence of
chemotherapy with repeated cycles of hematopoietic regeneration. In acute
leukemia, with the loss of the leukemic burden after
induction chemotherapy, longer mean TRFs were found, a reflection of the repopulation with normal cells. Our findings suggest that
telomerase activity may be useful in the management of childhood
malignancies. The significance of telomere length shortening in pediatric patients undergoing
chemotherapy and possible telomere regeneration after myelosuppressive treatment remain to be determined.