Telomere Length Recovery Strongly Predicts Overall Survival in Acute Promyelocytic Leukemia

Telomeres are the ends of linear chromosomes composed of tandem hexameric nucleotide repeats, maintained by a telomerase enzyme complex, made of reverse transcriptase catalytic subunit (TERT) and an RNA template (TERC). Present in very specific cells, such as embryonic and adult stem cells, the telomerase complex functions to elongate and slow the attrition of telomeric repeats associated with cellular division during replicative cellular age, as well as conferring stability to the ends of the chromosomes. In humans, both telomere length (TL) and loss-of-function mutations in the telomerase complex genes have been linked to malignant transformation and alteration of normal hematopoiesis. Though very short telomeres and germline mutations in TERT and TERC are associated with acute myeloid leukemia (Calado et al. PNAS 2009), the association between TL or telomerase complex integrity and outcome in patients (pts) with acute promyelocytic leukemia (APL) has not been thoroughly analyzed.

We present the largest analysis to date of mean telomere content, as a function of TL, and germline mutations in TERT and TERC in peripheral blood leukemic promyelocytes or total mononuclear cells (remission) from 164 APL pts at diagnosis, complete remission (CR) and relapse, enrolled in the intergroup trials, E2491 and C9710. The goal was to evaluate the correlation of TL in APL blasts at diagnosis and at relapse, in CR in normal total mononuclear cells, as well as delta TL (the difference between TL at diagnosis and CR) with clinical and laboratory parameters, including disease risk status by presenting white blood cell count (WBC ≤ or > 10,000/μl), early mortality, achievement of CR and overall survival (OS). Peripheral blood leukocytes from healthy subjects (ages 0-90 yrs) were collected and analyzed as controls. TL was measured by real-time quantitative PCR, and was dichotomized into short or long by the third quartile. Patient demographics and outcomes were compared using Fisher’s exact test, the Kruskal-Wallis test, and the Wilcoxon signed rank test. To evaluate the effect of TL on clinical outcomes, the Cochran-Mantel-Haenszel statistic was used in the association test on early mortality and CR. OS were compared using stratified log-rank tests. Hazard ratios were computed using stratified univariate and multivariate Cox models.

Median TL at presentation was significantly shorter in the APL than in the control healthy volunteer group (5.1 vs. 7.8 kb, p<0.0001). At CR, the median increase in TL was 2.0 kb (p<0.0001 against 0). There was a statistically significant association between TL at diagnosis and the WBC risk group status (p=0.001). There was no statistically significant difference in pretreatment characteristics (age, sex, ECOG performance status (PS), and risk status) and early mortality (p=0.40) across studies/regimens. However, TL at presentation (p=0.004) and CR (p=0.0003) was shorter in study C9710 than study E2491, though there was no difference in delta TL (p=0.21). TL at diagnosis was not prognostic of achievement of CR (p=0.78) or early mortality (p=0.19). However, delta TL was identified as a powerful predictor for OS (p=0.006), which remained true after adjusting for WBC risk status (p=0.009). In patients who achieved a CR, results from Cox regression with the forward selection criterion demonstrated that delta TL was more potent in predicting OS than the WBC risk status, TL at presentation, age, sex, or ECOG PS. TL at CR (p=0.96) and delta TL (p=0.31) did not predict for likelihood of relapse. Relapsed pts had significantly shorter median OS (p<0.0001) compared to those who did not relapse (2.2 yrs vs. OS not reached). We did not find mutations in the TERT or TERC genes apart from codon 279 and 1062 polymorphisms, which were identified at a frequency similar to that found in normal healthy volunteers.

In the largest study of APL pts analyzed to date, we demonstrate that APL blasts consistently harbor shorter TL at presentation compared to healthy people, and that TL at presentation correlates with higher risk disease. TL significantly recovers by the time of disease remission, and among those pts who achieve CR, delta TL is the most powerful predictor of OS. These findings warrant confirmation in a prospective fashion, and if corroborated, may offer TL measurement as a simple and inexpensive parameter in disease risk stratification and as meaningful predictor of OS in pts with APL.