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Pear). reverted by the ectopic expression ofBRCA1. Taken together, our data show for the first time a disruption of the FA/BRCA pathway inBCR/ABLcells, suggesting that this defective pathway should BDA-366 play an important role in the genomic instability of CML by the co-occurrence of centrosomal amplification and DNA repair deficiencies. == Introduction == Chronic Myeloid Leukemia (CML) is a clonal hematopoietic disorder generated by a t(9;22)(q34;q11) translocation resulting in aBCR/ABLoncogene[1],[2]that encodes for a tyrosine kinase BCR/ABL-p210 oncoprotein[3]. Although several genetic defects are accumulated in CML cells during the progression from the chronic phase towards the accelerated and blast crisis phases (see review in[4]), studies in mice transplanted withBCR/ABLtransduced cells demonstrated that this oncogene is the causative agent of CML[5]. In addition to a differentiation arrest, failures in the genomic surveillance and DNA repair of CML cells account for the natural malignant progression of the disease (see review in[6]). Although the mechanisms by which BCR/ABL interferes with the genomic stability of the cell are still poorly understood, the effects of this oncoprotein upon DNA damage, apoptosis and DNA repair are considered critical processes facilitating the accumulation of mutations during the progress to blast crisis (see review in[7]). Moreover, increasing evidence has been published showing that BCR/ABL induces reactive oxygen species (ROSs) causing oxidative damage to BDA-366 CML cells[8], and therefore a variety of DNA lesions, including the highly mutagenic double strand breaks (DSBs)[9],[10]. These effects, together with the reported effect of this oncoprotein on the efficacy and/or the fidelity of different DNA repair mechanisms[9],[11],[12]contribute to explain the mutator phenotype of CML cells. Concerning the mechanisms by which BCR/ABL affects the repair of the DSBs, previous studies have shown that this oncoprotein interferes both with the nonhomologous end joining (NHEJ) pathway and with pathways that utilize homologous templates. Regarding the effects of BCR/ABL on classic NHEJ, Deutschet alobserved that the catalytic subunit of DNA-dependent protein kinase (DNA-PKcs), a key protein in this major DNA repair system in mammalians, was down-regulated in CML cells[13]. In addition to NHEJ, BDA-366 BCR/ABL has also been involved in the aberrant regulation of the two pathways that utilize homologous templates, the faithful homology directed repair (HDR) and the mutagenic single strand annealing (SSA). Interestingly, previous studies have shown that BRCA1, a critical protein for preserving the genomic integrity by promoting homologous recombination[14], is nearly undetectable in CML cells[15]. On the other hand, BDA-366 more recent studies have shown that BCR/ABL specifically promotes the repair of DSBs through SSA, a mutagenic pathway that involve sequence repeats[16],[17]. Because the Fanconi anemia (FA) pathway is believed to control BDA-366 several DNA repair pathways, and therefore the genomic stability of the cell (see review in[18]), we ought to investigate the integrity of this pathway in CML cells. Thirteen FA proteins have been identified in the FA pathway, each of them participating in one of the three FA protein complexes. The upstream complex the FA core complex – is integrated by eight FA proteins (FANCA, FANCB, FANCC, FANCE, FANCF, FANCG, FANCL, FANCM) and two FA associated proteins Rabbit Polyclonal to MAN1B1 (FAAP24 and FAAP100). A second complex is formed by FANCD2 and FANCI, which work together in the FA-ID complex. Because of the E3 ligase activity (FANCL) of the FA core complex, FANCD2 and FANCI can be monoubiquitinated and then loaded onto chromatin, forming large nuclear foci in response to DNA damage or replication arrest. Finally, monoubiquitinated FANCD2/FANCI interact with downstream FA proteins such as FANCJ/BRIP1, FANCN/PALB2 and FANCD1/BRCA2, which form stable complexes with proteins participating in HDR, like BRCA1 and RAD51[19],[20]. The results presented in this study demonstrate for the first time that CML cells are characterized by a defective FA/BRCA pathway, downstream FANCD2 monoubiquitination. In particular.