Defective nonhomologous end joining blocks B-cell development in FLT3/ITD mice

We have generated an FLT3/ITD knock-in mouse model in which mice with an FLT3/ITD mutation develop myeloproliferative disease (MPD) and a block in early B-lymphocyte development. To elucidate the role of FLT3/ITD signaling in B-cell development, we studied VDJ recombination in the pro-B cells of FLT3/ITD mice and discovered an increased frequency of DNA double strand breaks (DSBs) introduced by the VDJ recombinase. Early pro-B cells from FLT3/ITD mice were found to have a lower efficiency and decreased accuracy of DSB repair by nonhomologous end joining (NHEJ), which is required for rejoining DSBs during VDJ recombination. Reduced NHEJ repair probably results from reduced expression of Ku86, a key component of the classic DNA-PK-dependent NHEJ pathway. In compensation, early pro-B cells from FLT3/ITD cells mice show increased levels of the alternative, and highly error-prone, NHEJ pathway protein PARP1, explaining the increase in repair errors. These data suggest that, in early pro-B cells from FLT3/ITD mice, impairment of classic NHEJ decreases the ability of cells to complete postcleavage DSB ligation, resulting in failure to complete VDJ recombination and subsequent block of B-lymphocyte maturation. These findings might explain the poor prognosis of leukemia patients with constitutive activation of FLT3 signaling.