The 'ataxia telangiectasia mutated' (Atm) gene maintains genomic stability by activating a key cell-cycle checkpoint in response to DNA damage, telomeric instability or oxidative stress. Mutational inactivation of the gene causes an autosomal recessive disorder, ataxia-telangiectasia, characterized by immunodeficiency, progressive cerebellar ataxia, oculocutaneous telangiectasia, defective spermatogenesis, premature ageing and a high incidence of lymphoma. Here we show that ATM has an essential function in the reconstitutive capacity of haematopoietic stem cells (HSCs) but is not as important for the proliferation or differentiation of progenitors, in a telomere-independent manner. Atm-/- mice older than 24 weeks showed progressive bone marrow failure resulting from a defect in HSC function that was associated with elevated reactive oxygen species. Treatment with anti-oxidative agents restored the reconstitutive capacity of Atm-/- HSCs, resulting in the prevention of bone marrow failure. Activation of the p16(INK4a)-retinoblastoma (Rb) gene product pathway in response to elevated reactive oxygen species led to the failure of Atm-/- HSCs. These results show that the self-renewal capacity of HSCs depends on ATM-mediated inhibition of oxidative stress.

We have investigated the function of TIE2/TEK receptor tyrosine kinase in the development of definitive hematopoiesis. In the vitelline artery at 9.5 days postcoitum (d.p.c.), TIE2+ hematopoietic cells aggregated and adhered to TIE2+ endothelial cells. Soluble TIE2-Fc chimeric protein inhibited the development of hematopoiesis and angiogenesis in the para-aortic splanchnopleural mesoderm (P-Sp) explant culture, and TIE2-deficient mice showed severely impaired definitive hematopoiesis. An in vitro study revealed that Angiopoietin-1 but not Angiopoietin-2 promoted the adhesion to fibronectin (FN) through integrins in TIE2-transfected cells and primary TIE2+ cells sorted from 9.5 d.p.c. P-Sp. Adhesion of TIE2+ cells induced by Angiopoietin-1 enhanced the proliferation of hematopoietic progenitor cells.

The ribosomal protein S19 (RPS19) is located in the small (40S) subunit and is one of 79 ribosomal proteins. The gene encoding RPS19 is mutated in approximately 25% of patients with Diamond-Blackfan anemia, which is a rare congenital erythroblastopenia. Affected individuals present with decreased numbers or the absence of erythroid precursors in the bone marrow, and associated malformations of various organs are common. We produced C57BL/6J mice with a targeted disruption of murine Rps19 to study its role in erythropoiesis and development. Mice homozygous for the disrupted Rps19 were not identified as early as the blastocyst stage, indicating a lethal effect. In contrast, mice heterozygous for the disrupted Rps19 allele have normal growth and organ development, including that of the hematopoietic system. Our findings indicate that zygotes which are Rps19 ؊/؊ do not form blastocysts, whereas one normal Rps19 allele in C57BL/6J mice is sufficient to maintain normal ribosomal and possibly extraribosomal functions.The ribosomal proteins constitute a major component of cellular proteins and are known to be mandatory for cellular growth. A reduced level of one ribosomal protein is rate limiting for the assembly of ribosomes and may constitute a bottleneck for protein synthesis in tissues with a high proliferative activity. Recently, Draptchinskaia et al. found that the gene encoding ribosomal protein S19 (RPS19) was mutated in 25% of patients with Diamond-Blackfan anemia (DBA) (5). DBA is a rare, congenital, and chronic anemia that is characterized by the absence or decreased numbers of erythroid precursors in the bone marrow but an otherwise normal cellularity (1, 4). Approximately 30% of affected patients with DBA show one or several dysmorphic features including growth retardation, hand and/or limb malformations, urogenital anomalies, and congenital heart defects (1, 2, 10). Clinical expression in DBA is highly variable (14,21), and the role of RPS19 in the pathogenesis of the disease is presently unknown.RPS19 is part of the 40S ribosomal unit and shows a high degree of sequence conservation across mammalian species at the protein level (5). In addition to its implication in erythropoiesis, there are indications that RPS19 has extraribosomal functions.A previous study has shown that a dimer of RPS19 may mediate chemotaxis (13). Free RPS19 has also been shown to interact with fibroblast growth factor 2 (FGF-2), and a possible role for RPS19 in embryonic development has been suggested (17). We hypothesized that an Rps19 null mutant would be deleterious for development and that mice heterozygous for Rps19 may present with hematological abnormalities. To clarify the role of RPS19 in erythropoiesis, we created a null mutation for murine Rps19 by homologous recombination in embryonic stem (ES) cells. The targeted Rps19 was introduced on a C57BL/6J background. We present here the results from our studies of this mouse model. MATERIALS AND METHODSConstruction of the Rps19 targeting vector. An Rps19 intron probe was used t...

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Diamond-Blackfan anemia (DBA) is a congenital red cell aplasia in which 25% of the patients have a mutation in the ribosomal protein S19 (RPS19) gene. To study effects of RPS19 deficiency in hematopoiesis we transduced CD34 ؉ umbilical cord blood (CB) and bone marrow (