When first observed, short telomeres of leukocytes in acquired SAA was presumed to be secondary to hematopoietic “stress”.2
The discovery of loss-of-function mutations in genes of the telomerase complex (TERC, TERT
) established a genetic etiology for telomere attrition in marrow failure. Telomerase mutations are etiologic in the constitutional marrow failure syndrome dyskeratosis congenita and are also found in a minority of patients with acquired aplastic anemia. However, in the present series, only one patient later tested positive for a TERT
mutation (codon A202T; his leukocytes’ telomere length was below the tenth percentile). Therefore the current study describes a relationship between variations of telomere length within the normal range
and SAA clinical outcomes in patients who (with a single exception) lacked known genetic explanations for shorter telomeres.
Telomere length was not associated with response to immunosuppression. Why some patients do not respond is unknown: insufficient number of hematopoietic stem cells, inadequate immunosuppression, and a nonimmune etiology for marrow failure each have been suggested.1
That shorter telomeres were not associated with unresponsiveness to immunosuppression indicates that this parameter does not distinguish a nonimmune etiology group. In our cohort, stem cell reserves appeared sufficient for recovery after therapy.
Clonal evolution to myelodysplasia is a major adverse event in SAA; it cannot be routinely predicted and usually signals a poor prognosis. In particular, the finding of monosomy 7 on bone marrow cytogenetics is associated with persistent pancytopenia unresponsive to immunosuppression and progression to myelodysplasia.13
The current work shows that telomere length relates to the development of abnormal marrow clones, in particular monosomy 7, with serious clinical consequences.
Prediction of important disease-related complications is critical to risk stratification and patient management. The major problems of immunosuppressive therapy in SAA are unresponsiveness, relapse, and clonal evolution. Recently, we reported that pre-treatment reticulocyte count was predictive of response to immunosuppression 12
; however, there are no recognized predictors for relapse and clonal evolution. In the current study, we show that pre-treatment telomere length associated with relapse and clonal evolution, two serious late events in SAA. When absolute reticulocyte count and telomere length were combined in our cohort, three groups were observed: 1) a favorable group with high reticulocyte count and longer telomeres; 2) an intermediate group with high reticulocyte count and shorter telomeres or a low reticulocyte count and longer telomere length; 3) and a poor risk group with low reticulocyte count and shorter telomere length (). ATG plus cyclosporine may be adequate for the most favorable group; in contrast, better regimens are needed for the poor risk group. For example, androgen treatment offers a potential for in vivo modulation of telomere length,14
and for those at greater risk for late complications after immunosuppression, higher risk protocols such as stem cell transplantation in older patients and alternative source of stem cells might be considered earlier in younger patients.
Telomere attrition is not simply a biomarker; rather, a plausible mechanism for destabilization of the genome has been inferred from basic telomere biology. Ample in vitro and animal experimentation indicate that critical shortening of telomeres causes chromosome instability, tumor formation, and cancer progression.15–22
Normally, senescence would preclude cells with critically short telomeres from tumorigenesis23
, but cells with malignant potential may escape by failure of mechanisms such as p53 signaling of DNA damage24
or activation of alternative modes of telomere maintenance.25
In telomerase “knockout” mice (Terc−/−
), aneuploidy and end-to-end fusions were observed in fibroblasts after four generations, suggesting an inability to protect chromosome ends when telomerase was deficient.26
Furthermore, on breeding of telomerase-deficient knockout mice with short telomeres with wild-type mice with long telomeres, chromosome fusions and signal-free ends occurred preferentially on chromosomes with critically short telomeres.27
Clinically, telomere length has been associated with human cancer.28
In dyskeratosis congenita, the incidence of cancers was 11-fold higher compared to the general population, with high rates of tongue cancer and leukemia.29
In leukemia patients with no underlying telomerase disorder, hypomorphic mutations in TERT
were three-fold more frequent in patients than in controls.30
Telomere attrition has been implicated in a variety of solid organ malignancies. In Barrett’s esophagus, there was an increased risk to develop esophageal adenocarcinoma in individuals who had shorter leukocyte telomere length at first clinical presentation.31
Furthermore, as in our study, telomere length was an independent predictor for progression to esophageal cancer after correction for other covariates.31
Premature shortening of telomeres of leukocytes and in colonic epithelia in a few patients has been correlated to cancer progression in ulcerative colitis.21, 32
Our current data are consistent with findings in other inflammatory diseases which predispose to cancer, in that the risk for clonal evolution was increased in those with pre-treatment, age-adjusted telomere length in the lower quartile. However, in contrast to many investigations of predisposition to gastrointestinal tumors, we determined telomere length months to years preceding malignant transformation, and in those hematopoietic cells directly subject to dysplasia and leukemia. Indeed, bone marrow cells from patients with short telomeres in vitro showed increased number of telomere-free chromosomal ends, aneuploidy, and chromosomal translocations not seen in cells of patients with longer telomeres (our unpublished data).
Our study has strengths and limitations. The strengths include the homogeneous cohort enrolled into our research protocols with specified diagnostic criteria, immunosuppressive drug administration, supportive care, and prospectively determined and defined clinical outcomes. We monitor our patients indefinitely at specified intervals, and our protocols require periodic assessment for adverse events like relapse and clonal evolution. Our study is limited due to the retrospective nature of the analysis and the relatively small number of patients, which did not allow for validation testing in a separate cohort or for reliable subgroup analysis. As a research facility and quaternary referral center, our patient population also may not be representative and our results not necessarily generalizable. Therefore, our results need to be replicated to validate the observed associations and to determine reliable telomere length thresholds that could be incorporated in treatment algorithms.
In conclusion, our data show that in a cohort of patients with SAA receiving immunosuppressive therapy, telomere length was not associated with response, but was associated with risk of relapse, clonal evolution, and overall survival.