Fanconi Anemia D2 Protein Contributes to BCR/ABL-Mediated Transformation of Hematopoietic Cells.

Homologous recombination (HR), involving RAD51 protein, plays an important role in the response of BCR/ABL-positive leukemia cells to numerous DNA double-strand breaks (DSBs) induced by reactive oxygen species (ROS) or genotoxic treatment. Fanconi D2 protein (FANCD2), a member of the Fanconi protein family, is monoubiquitinated on K561 and phosphorylated by ATM on S222 in response to DSBs. The K561 monoubiquitinated form of FANCD2 interacts with RAD51 during HR, and phosphorylation of FANCD2 on S222 is important for activation of S phase checkpoint in response to DNA damage. Our studies detected an enhanced interaction between RAD51 and FANCD2 in BCR/ABL-positive leukemia cells in comparison to normal counterparts. In addition, although the expression of FANCD2 was stimulated by BCR/ABL and growth factors, higher levels of FANCD2 monoubiquitination was detectable in CML patient cells at chronic phase and in blast crisis, and in BCR/ABL-transformed cells in comparison to non-transformed cells. This effect was reversed after inhibition of BCR/ABL kinase with STI571. Therefore, monoubiquitination of FANCD2 may play a role in BCR/ABL-mediated leukemogenesis. BCR/ABL kinase displayed an impaired transformation potential in FANCD2-/- murine bone marrow cells in comparison to +/+ counterparts. In addition, expression of BCR/ABL kinase, but not the kinase-deficient K1172R mutant, inhibited the proliferation rate of FANCD2-/- human lymphoblast cell line. Growth ability of BCR/ABL-positive FANCD2-/- cells could be rescued by co-expression of the wild-type and S222A mutant of FANCD2, but not the K561R mutant. This observation suggested that K561 monoubiquitination, but not S222 phosphorylation might play an important role in BCR/ABL-mediated transformation. Since BCR/ABL cells employ RAD51-dependent HR to repair numerous DSBs induced by ROS, elevated expression of monoubiquitinated FANCD2 may facilitate this process. This hypothesis is supported by the observation that BCR/ABL-positive FANCD2-/- cells accumulate more DNA damage than +/+ counterparts as indicated by enzymatic assays converting oxidative DNA lesions into gaps detectable by comet assay. In addition, enhanced oxidative DNA damage in BCR/ABL-positive FANCD2-/- cells produced a variety of DNA lesions including abasic sites, and single- and double-strand breaks assessed by neutral comet assay. Moreover, BCR/ABL-positive FANCD2-/- cells accumulated higher numbers of DSBs detected by γ-H2AX immunostaining and displayed discrete apoptosis. In conclusion we hypothesize that monoubiquitination of FANCD2 may play a role in the initial steps of BCR/ABL dependent leukemogenesis, probably due to its ability to interact with RAD51 and facilitate HR repair of an excess of spontaneous DSBs induced by ROS.