TopBP1 deficiency impairs V(D)J recombination during lymphocyte development.

TopBP1 was initially identified as a topoisomerase II-β-binding protein and it plays roles in DNA replication and repair. We found that TopBP1 is expressed at high levels in lymphoid tissues and is essential for early lymphocyte development. Specific abrogation of TopBP1 expression resulted in transitional blocks during early lymphocyte development. These defects were, in major part, due to aberrant V(D)J rearrangements in pro-B cells, double-negative and double-positive thymocytes. We also show that TopBP1 was located at sites of V(D)J rearrangement. In TopBP1-deficient cells, γ-H2AX foci were found to be increased. In addition, greater amount of γ-H2AX product was precipitated from the regions where TopBP1 was localized than from controls, indicating that TopBP1 deficiency results in inefficient DNA double-strand break repair. The developmental defects were rescued by introducing functional TCR αβ transgenes. Our data demonstrate a novel role for TopBP1 as a crucial factor in V(D)J rearrangement during the development of B, T and iNKT cells.