Erosion of Telomeric Single-Stranded Overhang in Patients with Aplastic Anemia Carrying Telomerase Mutations.

Abstract 1079

Poster Board I-101

Loss-of-function mutations in telomerase complex genes reduce telomerase activity and shorten overall telomere length in leukocytes, and they can clinically manifest as bone marrow failure (aplastic anemia and dyskeratosis congenita), familial pulmonary fibrosis, and hepatic cirrhosis. The double-stranded tandem telomeric TTAGGG repeats are followed by a 3' G-rich single-stranded overhang, a crucial structural component responsible for protective t-loop formation. We investigated the length of telomeric overhangs in 25 healthy individuals from 0 to 76 years, 16 patients with aplastic anemia, and 13 of their immediate relatives using a non-denaturing in-gel method and the telomere-oligonucleotide ligation assay (T-OLA). Among healthy controls, overall telomeric length of leukocytes shortened as a function of age. It was longest in umbilical cord blood samples and eroded as a function of a third-order polynomial until 76 years (R²=0.9517; Spearman=0.90; P<0.0001). However, the lengths of single-stranded overhangs were constant with age, as determined by both non-denaturing and the T-OLA methods. In contrast, most patients with marrow failure carrying a telomerase gene mutation showed marked erosion of telomeric overhangs associated with critically short telomeres (P<0.05). Seven telomerase complex-mutant aplastic patients had overhang lengths below the 5% percentile established for healthy volunteers. In aplastic patients with normal genotypes, normal overall telomere lengths, and who effectively responded to immunosuppressive therapy, telomeric overhangs were maintained and were all within the range established for control individuals. In conclusion, telomeric overhang erosion does not participate in physiological aging but eroded telomeric overhangs and abnormal telomere structure appear in pathologic shortening of telomeres, especially caused by loss-of-function telomerase mutations. Disrupted telomere structure caused by short telomeric overhangs may contribute to the mechanisms of abnormal hematopoietic compartment senescence and chromosomal instability in human bone marrow failure.