Evidence for T-Cell Oligoclonal Expansion in Aplastic Anemia Associated with Telomerase Complex Mutations: Pathophysiological and Clinical Implications.

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Aplastic anemia is characterized by peripheral blood pancytopenia and a hypocellular bone marrow. In the majority of patients, bone marrow destruction is immune-mediated by a type-1 T cell response, dominated by oligoclonal expansion of CD8+ cells and targeting hematopoietic stem cells. Some patients with acquired aplastic anemia are heterozygous for mutations in genes encoding the major components of the telomerase complex, telomerase reverse transcriptase (encoded by TERT; Yamaguchi et al., N Engl J Med 2005; 352:1413) and the RNA component (encoded by TERC; Fogarty et al., Lancet 2003; 362:1628). These mutations lead to shortened telomeres of leukocytes, low telomerase activity and reduced hematopoietic function. Relatives carrying the same mutations also have short telomeres and reduced hematopoietic function, but they do not inevitably develop overt marrow failure. In contrast, dyskeratosis congenita, a constitutional type of marrow failure, is caused by mutations in TERC (autosomal dominant type) or in the DKC1 gene, which encodes an additional component of the telomerase complex, dyskerin. In addition to marrow failure, patients with dyskeratosis congenita also often have physical anomalies, such as leukoplakia, nail dystrophy, and hyperpigmentation as well as hepatic or pulmonary fibrosis. In dyskeratosis congenita, family members bearing the genetic mutation present variable degrees of physical abnormalities or organ damage. However, in patients with acquired aplastic anemia carrying telomerase mutations without physical anomalies, it is unclear whether these mutations are sufficient for the development of marrow failure. We hypothesized that telomerase complex mutations reduce the size of the hematopoietic stem cell compartment and affect its regenerative capacity, making carriers more vulnerable to environmental insults or autoimmune damage. We analyzed the distribution of the T-cell repertoire (T-cell receptor [TCR] Vβ subfamily) and expansion of particular Vβ subsets by flow cytometry in peripheral blood of six aplastic anemia patients carrying telomerase complex mutations (five with TERT mutations, one with a TERC mutation), two patients with Fanconi anemia, and 15 healthy subjects. The expression of 22 Vβ subfamilies were evaluated in CD8+CD28− cells, and expansion was defined when the percentage of a given Vβ subfamily was above two standard deviations based on the control group (Risitano et al. Lancet 2004; 364:355). We also evaluated interferon-γ levels in serum of nine telomerase mutant patients and 10 controls by ELISA. Expanded Vβ subsets were observed in all six aplastic patients carrying telomerase complex mutations analyzed. Five patients (with TERT mutations) had two clones expanded and one (with TERC mutation) showed three overrepresented clones. The Vβ subsets 9, 13.6, 17, and 20 were expanded in more than one patient. Oligoclonal Vβ expansion was not observed in Fanconi anemia. Telomerase mutant patients also had significantly increased interferon-γ serum levels in comparison to controls (27 pg/mL; range, 0–95 vs. 7.6, 0–16, respectively; P<0.02). These results indicate that an immune process targeting hematopoiesis may operate in patients carrying telomerase complex gene mutations. Limited responsed to immunosuppression may reflect the poor hematopoietic reserve rather than a nonimmunologic mechanism of marrow destruction.