Ts65Dn Mouse Hematopoietic Stem Cells To Repair Radiation-induced DNA Damage Disorders

Down syndrome (DS) is the most common chromosomal abnormality in humans caused by the presence of all or part of a third copy of chromosome21. In addition to musculoskeletal and neurodevelopmental abnormalities, pediatric patients with DS also display various hematologic disorders and are at increased risk of acute lymphoblastic leukemia and acute megakaryocytic leukemia. Ts65Dn mice are trisomic for104orthologs of the genes on human chromosome21and are one of the most widely used mouse models for DS research, because they exhibit various deficiencies similar to DS patients. It has been shown that aneuploidy can impair DNA damage repair and induce genomic instability. Therefore, we investigated whether hematopoietic stem cells (HSCs) from Ts65Dn mice are deficient in the repair of DNA double-strand breaks (DSBs), because the deficiency in the repair of DSBs in HSCs can potentially contribute to DS-associated hematological abnormalities and malignancies by impairing HSC self-renewal and inducing hematopoietic genetic instability. Our results showed that Ts65Dn mice had significantly less bone marrow HSCs than wild-type (WT) littermate controls, whereas the levels of bone marrow hematopoietic progenitor cells (HPCs) were similar. The lower level of HSCs in Ts65Dn mice was associated with a significantly higher level of DSBs as determined by γH2AX staining and lower level of clonogenic activity in single cell cultures with SCF, TPO and FL3ligand when compared to the cells from WT controls. Although levels of DSBs in HSCs were similar at1hr after exposure to2Gy y-irradiation in vitro, HSCs from Ts65Dn mice had significantly higher levels of unrepaired DSBs than the cells from WT mice at3and6hr after irradiation. Moreover, after exposure to2Gy y-irradiation in vitro, the reduction in clonogenic function was significantly greater in Ts65Dn HSCs than WT HSCs. In contrast, no significant differences in these parameters were observed in HPCs from Ts65Dn and WT mice with or without irradiation.The molecular mechanism by which extra copy of chromosome21causes HSC defect in repair of DSBs remains to be elucidated. It may be attributable to an increased expression of USP16in Ts65Dn HSCs, because USP16is a deubiquitinase that can regulate DSB repair via the Ring finger protein8-Ring finger protein168(RNF8-RNF168) pathway. These findings suggest that an additional copy of genes on human chromosome21can selectively impair the ability of HSCs to repair DSBs, which may contribute to DS-associated hematological abnormalities and malignancies.