Severe aplastic anemia (SAA) is a life-threatening bone marrow failure disorder characterized by pancytopenia and a hypocellular bone marrow. Telomeres were reported to be short in up to one-third of patients with SAA.Initially this occurrence was presumed to be secondary to hematopoietic stress. However, the discovery of loss-of-function mutations in genes of the telomerase complex (TERC, TERT) established a genetic etiology for telomere attrition in some patients with marrow failure who did not have the stigmata associated to an inherited bone marrow failure syndrome. These findings implicated telomerase dysfunction in failed hematopoiesis. In family members of probands with SAA, telomerase mutations have been observed which were associated to varying degrees of cytopenias, idiopathic pulmonary fibrosis (IPF) and/or cirrhosis.
Telomere length has been associated with human cancer. Telomere attrition has been implicated in a variety of solid organ malignancies including esophageal and colon adenocarcinoma. In a longitudinal population based study, shorter telomere length associated to a higher cancer mortality risk overtime. It is plausible that a shorter telomere length is not just a biomarker associated to development of cancer, but involved in its pathogenesis. Ample experimental data supports an important role of critically short telomere length in genomic instability. Furthermore, our laboratory data (unpublished) shows that similar chromosome instability occurs in bone marrow cells of mutant patients, confirming the experimental data. Thus, a common molecular mechanism appears to underlie risk for cancer and a range of clinical entities.
In vitro studies suggest that telomere length could, in theory, be modulated with sex hormones.15 Exposure of normal peripheral blood lymphocytes and human bone marrow derived CD34+ cells to androgens increased telomerase activity in vitro and androgens increased low baseline telomerase activity in individuals carrying a loss-of-function TERT mutation to normal levels. In retrospect, the beneficial effects of sex hormones on telomerase activity may be the mechanism by which SAA patients treated over 40 years ago with male hormones showed hematologic improvement in some cases.
In recent years we have seen patients referred to our clinic with varying degree of cytopenia(s) who had significant family history for cytopenia(s), IPF and/or cirrhosis. We have identified very short telomeres in these patients and in some mutations in TERC and TERT. We hypothesize that male hormone therapy might modulate telomere attrition in vivo and ameliorate progression or reverse the clinical consequences of accelerated telomere attrition. Therefore, we propose male hormone therapy in patients with cytopenia(s) and/or IPF who show evidence of telomere dysfunction by a short age adjusted telomere length associated to telomerase gene mutations. The primary biologic endpoint will be delay of telomere attrition over time compared to known rates of telomere erosion in normal individuals and in those who carry mutation in the telomerase genes. The main clinical endpoint will be tolerability of oral danazol over two years. Secondary endpoints will be improvement in blood counts and/or pulmonary function. The small sample size, lack of control groups, and variable clinical course among those with marrow failure and IPF, will not allow for definitive assessment of clinical benefit. Nevertheless, we believe this protocol will provide insight into the possible effects of androgen therapy on telomere attrition in humans and of possible clinical benefit in telomere related disorders, and serve as hypothesis generating for further larger controlled studies.
