Sex Hormones Up-Regulate Telomerase Activity of Normal Human Hematopoietic Cells and Restore Telomerase Activity in Carriers of Telomerase Complex Mutations.

Mutations in components of the telomerase complex (TERT and TERC) are risk factors for acquired aplastic anemia. Telomerase is a reverse transcriptase responsible for the maintenance of telomeres, repetitive nucleotide sequences capping the chromosome ends. Patients with marrow failure with TERT or TERC mutations have low telomerase activity, short telomere length in leukocytes, reduced hematopoietic function, and they do not respond adequately to immunosuppressive therapy. The constitutional aplastic anemia dyskeratosis congenita is also caused by mutations in TERC (autosomal recessive type) or in an additional component of the telomerase complex, dyskerin, encoded by the DKC1 gene (X-linked type). The majority of patients with dyskeratosis congenita and some patients with acquired aplastic anemia benefit from androgen therapy, but the mechanism by which this hormone stimulates hematopoiesis is not well understood. We investigated whether sex hormones stimulated telomerase activity in hematopoietic tissue. Peripheral blood lymphocytes from either healthy controls or mutation carriers were cultured with phytohemagglutinin and interleukin-2 for 1–7 days in the presence of methyltrienolone, estradiol, hydroxycortisone, and/or tamoxifen. Telomerase enzymatic activity was measured using a fluorescent telomere-repeating amplification protocol (TRAP), and TERT mRNA expression was assayed by real-time PCR. Androgens and estradiol up-regulated telomerase activity in lymphocytes from normal subjects in a dose-dependent fashion (controls, 124±24 total product generated [TGP] units per reaction; methyltrienolone 1 μM, 242±16 TGP units/reaction; methyltrienolone 10 μM, 757±87 TGP units/reaction; P<0.01). Increased telomerase activity correlated to higher TERT mRNA levels (P<0.05). Steroidal effects on telomerase activity were neutralized by tamoxifen, an estrogen antagonist. TERT mutation carriers had lower telomerase activity compared to controls (P<0.05), but their enzymatic function was restored to normal levels when cultured with methyltrienolone. As a control, hydrocortisone at 1 μM inhibited telomerase activity without influencing cell cycle or inducing apoptosis. We also investigated the effects of androgen on hematopoietic progenitor cells. Bone marrow CD34⁺ cells from normal subjects were immunomagnetically separated and cultured for eight days in StemSpan supplemented with Flt-3 ligand, stem cell factor, and interleukins 3 and 6, in the presence methyltrienolone. Up-regulation of telomerase activity was observed in CD34⁺ cells cultured with methyltrienolone (P<0.05), the increase correlated with greater cell numbers, a higher proportion of CD34⁺ cells after eight days of culture (from 10% to 15% at 10 μM methyltrienolone and to 19% at 100 μM), without changes in cell cycle or cell differentiation, based on CD45, CD71, and CD33 expression. These results indicate that sex hormones stimulate TERT expression via the estrogen receptors, leading to increased telomerase activity in hematopoietic cells and increased proliferation of hematopoietic progenitors. Androgens were able to restore telomerase activity to normal levels in TERT mutation carriers. Sex hormone effects on telomerase in normal cells may have implications for the normal aging process and in hematological malignancy (leukemic cells have high telomerase activity), and our results suggest that androgens might be useful in the treatment of patients bearing telomerase complex mutations.