Pathogenetic Role of Impaired Telomere Repair in Myelodysplastic Syndrome.

Abstract 2777

Poster Board II-753

Telomeres are repeated nucleotide sequences at the ends of chromosomes that serve to preserve genetic integrity in hematopoietic progenitors (HPs). Abnormal telomere maintenance resulting from mutations in telomerase or other telomere repair genes have an established pathogenetic role in a subset of acquired and congenital forms of bone marrow failure (BMF). Excess telomere shortening of myeloid elements was reported previously in MDS, however it is unclear whether such abnormalities in telomere maintenance play a pathogenetic role in disease susceptibility or represent a disease specific phenotype arising from accelerated cellular proliferation of the myeloid compartment. Alterations in telomere maintenance and telomerase activity in T cells may offer insight into genetic susceptibility since these cells are not derived from the malignant clone. We examined telomere length and replication history in neutrophils and T cells in newly diagnosed MDS cases (n=66) and compared results to unrelated, age-matched controls (n=63) lacking a prior history of cancer. Telomere length in peripheral blood mononuclear cells was assessed by quantitative real-time PCR (qRT-PCR) and found to be significantly shorter among MDS cases compared to controls. Peripheral blood concentration of the senescence protein stathmin was analyzed in a subset of 48 patients and 48 controls and found to be higher in MDS cases compared to controls after adjustment for age and sex (mean 4.24 pg/ml ± 7.25 in cases vs mean 2.45 pg/ml ± 2.84 in controls, p=0.06) with no change observed with age. Mechanisms of cellular senescence and telomere attrition in T-cells may be related to antigen-driven or homeostatic proliferation. Therefore, telomere length was compared to an independent genetic marker of proliferation, i.e., the concentration of T cell receptor excision circles (TRECs). TRECs are episomal DNA fragments excised during TCR gene rearrangement within the thymus that do not transfer to daughter cells. Therefore, TREC DNA copy number has been shown to be diluted during each round of division and tends to be reduced with age due to impaired generation of new thymic T-cell emigrants. In controls, TREC concentration declined with age consistent with cellular proliferation and loss of thymic function. For every year increase in age, log TREC values decreased by 0.05 DNA copies (p=0.0012). More aggressive proliferation was evident in T cells from cases compared to controls with a 2-fold more rapid decline in TREC copy number each year (0.099 unit decrease in TREC copies per year among cases, p<0.0001; 0.05 copies per year among controls, p=0.012). Changes in TREC levels with age were compared to corresponding changes in telomere length. In cases, TRECs and telomere length in CD4+ and CD8+ T cells tended to be positively correlated suggesting that the rate of telomere repair may be insufficient for the degree of proliferation (CD4: r=0.59, p=0.06 CD8: r=0.50, p-value=0.11). Among controls, no statistically significant associations were observed between age and either CD4 or CD8 telomere length. These results suggest that telomere repair is activated to preserve the proliferative capacity of peripheral T-cells and prolong survival after thymic involution. To determine if T cell proliferative function is altered, antigen-induced proliferation was compared among MDS patients and controls. After stimulation of the T cell receptor, proliferation was reduced in cases compared to controls, supporting pre-mature replicative senescence in MDS. These results suggest that altered T cell function in MDS may arise from accelerated age-related telomere erosion arising from an impairment in telomere repair and high rates of replicative stress. This defect in lymphoid cells may represent a genetic marker of disease susceptibility which merits prospective investigation.