Telomere Length and Telomerase Complex Mutations Predict Fatal Treatment Toxicity after Stem Cell Transplantation in Patients with Myelodysplastic Syndrome
Authors: Myllymaki Mikko , Redd Robert A. , Cutler Corey S. , Saber Wael , Hu Zhen-Huan , Wang Tao , Spellman Stephen R. , Gibson Christopher J. , Chen Maxine M , Orr Esther , Steensma David P. , Antin Joseph H. , De Vivo Immaculata , Neuberg Donna S , Agarwal Suneet , Lindsley R. Coleman 
Published: October 2019

Abstract
Introduction: Identifying patients at high risk of fatal treatment toxicity is a central challenge in hematopoietic stem cell transplantation (HSCT). Objective metrics that enable more accurate prediction of non-relapse mortality (NRM) could inform clinical decisions about timing and modality of HSCT. Short telomere length, mediated by inherited or acquired factors, impairs cellular response to genotoxic and replicative stress. We therefore evaluated the impact of recipient telomere length on clinical outcomes based on treatment intensity in patients with myelodysplastic syndrome (MDS) receiving HSCT.
Methods: We used qPCR to measure relative telomere lengths in whole blood DNA samples from 1514 patients who received allogeneic HSCT for MDS and were enrolled in the Center for International Blood and Marrow Transplant Research Repository. Within the cohort, patients age 40 and older were grouped into those with short (<25th), intermediate (25-75th) or long (>75th percentile) telomeres. To evaluate germline determinants of telomere length, we sequenced 7 genes involved in telomere maintenance and mutated in dyskeratosis congenita: TERC, TERT, DKC1, TINF2, NHP2, WRAP53 and CTC1. Putative germline variants were classified as "rare" if the allele frequency was <0.001 in all Genome Aggregation Database (gnomAD) populations.
Results: Among patients age 40 and older (n=1267), those with short (HR 1.52, 1.24-1.85, p<0.001) or intermediate (HR 1.35, 1.13-1.61, p<0.001) telomere length had poor overall survival compared with those having long telomeres (Fig 1A). In a competing risks regression model, the adverse effect of shorter telomeres was driven by a significantly higher risk of NRM among patients with short (HR 1.57, 1.20-2.06, p=0.001) and intermediate (HR 1.32, 1.03-1.69, p=0.03) telomere length. The association between telomere length and NRM was evident in patients receiving myeloablative (MAC, p=0.002) but not reduced-intensity conditioning (RIC, p=0.2) regimens (Fig 1B). We observed no association between telomere length and disease relapse in patients receiving MAC or RIC regimens. In a multivariable regression model, the prognostic significance of telomere length was independent of clinical and genetic factors, including age, Karnofsky performance status, hematologic parameters, IPSS-R risk category, donor mismatch, donor age, and TP53 mutation.
We identified 40 patients (2.6% of the cohort) with rare germline TERT variants. Patients with rare TERT variants had significantly shorter telomeres than patients with common (p=0.001) or no (p<0.0001) TERT variants and were diagnosed with MDS at an earlier age than patients with common (52.2 vs. 58.4 years, p=0.01) or no (52.2 vs. 57.9 years, p=0.01) TERT variants. The domain distribution of rare TERT variants mirrored that of validated pathogenic germline TERT mutations, primarily affecting the reverse transcriptase and C-terminal extension domains. Rare variants in TERC (0.4%) and DKC1 (0.2%) were also associated with shorter telomeres (p=0.02 and p=0.04, respectively). In total, we identified germline telomerase complex mutations in 49 of 1514 MDS patients (3.2%), even though only 1 patient had a clinical diagnosis of dyskeratosis congenita. Together, patients with telomerase complex mutations had shorter overall survival than those without mutations (unadjusted p=0.008), attributable to a marked increase in the risk of early NRM among those receiving MAC (1 year cumulative incidence of NRM 48% vs. 26%, p=0.03). The impact of shorter telomeres on NRM was similar in patients with and without identified core telomerase complex mutations, suggesting that additional mechanisms of impaired telomere length maintenance may contribute to MDS pathogenesis and outcome.
Conclusion: Recipient telomere length is independently associated with overall survival after allogeneic HSCT for MDS. Patients age 40 or older with shorter blood cell telomere length have a significantly elevated risk of early NRM with myeloablative conditioning regimens. Clinically unrecognized germline mutations in the telomerase genes TERT, TERC, and DKC1 define a distinct subset of adult patients with sporadic MDS and short telomeres who have poor transplant outcomes. Together, these results indicate that short telomere length in MDS patients mediates fatal treatment toxicity that may be attenuated by lower intensity conditioning approaches.

Disclosures
Antin: Jazz Pharmaceuticals: Membership on an entity's Board of Directors or advisory committees.