Telomere Length In Acute Promyelocytic Leukemia

Telomeres are hexameric nucleotide repeats in tandem at the ends of linear chromosomes that function to protect chromosomes and prevent genomic DNA erosion. Telomeric DNA can be elongated by the telomerase complex, which is composed of a reverse transcriptase catalytic subunit (TERT), an RNA template (TERC), and associated proteins. Loss-of-function mutations in genes encoding telomerase complex components are leading causes for telomeres shortening and have been associated with several human pathologies, including aplastic anemia and dyskeratosis congenita, both with a proclivity for progression to acute myeloid leukemia. We have demonstrated that telomere dysfunction and telomerase mutations are genetic risk factors for AML (Calado et al. PNAS 2009; Calado et al. Leukemia 2012). However, the contribution of telomere integrity to acute promyelocytic leukemia (APL) and its clinical significance have not been explored. Here, we retrospectively determined the telomere content of leukemic cells in samples collected from 74 APL patients (age, 15-65y) treated according to combination of all-trans retinoic acid (ATRA) and antracycline (Rego et al. Blood 2013) from Ribeirão Preto,São Paulo, Brazil. The data were analyzed in order to correlate the relationship with clinical and laboratory features, hematologic recovery, relapse, and survival. As controls, peripheral blood cells from 338 healthy subjects (age, 0-87y) were collected. All participants gave written informed consent approved the local Ethics Committee in Human Research (CEP). Telomere content measurement was determined by real-time quantitative PCR (qPCR) using the Rotor-Gene SYBR Green PCR Master Mix (Calado et al. Leukemia 2012). Briefly, this method compares the abundance of telomere content in comparison to the abundance of a invariable standard single gene, providing relative measurement, as determined by a telomere:single gene (T/S) ratio. Telomere content was measured and patients divided into quartiles, according to their telomeres, and dichotomized into short (lower quartile) and long (upper three quartiles) telomeres. Telomeres were significantly and consistently short in APL blasts as compared to peripheral blood leukocytes of age-matched healthy donors (Figure 1; P<0,0001)). There was no relevant difference between APL patients with short (n=21) and long (n=53) telomeres with respect to clinical and laboratory features. Overall, 63 subjects (85%) achieved complete remission (CR). Patients with short telomere APL blasts tended to have poorer CR rate as compared to patients with longer telomere blasts, although the difference was statistically marginally significant (71% vs 90%; P=0.06). Eight patients (11%) experienced early mortality (i.e., death during induction therapy) and telomere length was predictive of early mortality (23% for those with short telomeres in comparison to 5% for those with longer telomeres; P=0.037). With a median follow-up of 33 months (range, 1-72 months), patients with long telomeres also appeared to have better 2-year overall survival (OS) (90%) compared with those with short telomeres (76%) by trend (P = 0.07). Telomere length had no impact on disease-free survival (DFS) rate (P =0.66). Our findings demonstrate that APL blast cells consistently have very short telomeres at presentation, suggesting that telomere dysfunction may contribute to genomic instability in leukemogenesis. Our results suggest that telomere length of APL blasts may predict the achievement of complete remission, early mortality, and overall survival. However, the number of patients enrolled in this study was relatively small and the follow-up was short. These results need to be confirmed in a larger number of patients followed for a longer period of time and validated by an independent cohort. If confirmed, as telomere length measurement is a simple and inexpensive test, it may be applied in the future in the risk assessment of patients with APL.