The RecQ family helicase BLM is critically involved in the maintenance of genomic stability, and BLM mutation causes the heritable disorder Bloom’s syndrome. Affected individuals suffer from a predisposition to a multitude of cancer types and an ill-defined immunodeficiency involving low serum Ab titers. To investigate its role in B cell biology, we inactivated murine Blm specifically in B lymphocytes in vivo. Numbers of developing B lymphoid cells in the bone marrow and mature B cells in the periphery were drastically reduced upon Blm inactivation. Of the major peripheral B cell subsets, B1a cells were most prominently affected. In the sera of Blm-deficient naive mice, concentrations of all Ig isotypes were low, particularly IgG3. Specific IgG Ab responses upon immunization were poor and mutant B cells exhibited a generally reduced Ab class switch capacity in vitro. We did not find evidence for a crucial role of Blm in the mechanism of class switch recombination. However, a modest shift toward microhomology-mediated switch junction formation was observed in Blm-deficient B cells. Finally, a cohort of p53-deficient, conditional Blm knockout mice revealed an increased propensity for B cell lymphoma development. Impaired cell cycle progression and survival as well as high rates of chromosomal structural abnormalities in mutant B cell blasts were identified as the basis for the observed effects. Collectively, our data highlight the importance of BLM-dependent genome surveillance for B cell immunity by ensuring proper development and function of the various B cell subsets while counteracting lymphomagenesis.

b-catenin, an oncogene, and P53, a tumor suppressor, are common targets of mutation in human cancers. It has been observed that P53 is often inactivated in tumors involving b-catenin activation. In an attempt to model this situation in vivo, we crossed the previously characterized MMTV-DN-b-catenin mouse with the P53 knockout mouse. Female multiparous mice that carry the MMTV-DN-bcatenin transgene and that are heterozygous for P53 (Tg DN-bCat / þ , P53 þ /À) display an increased tumor burden (2.05 vs 1.31 tumors/animal), with a generally more advanced pathology, and increased metastatic rate (39 vs 0%) relative to transgenic female mice that are wild type for P53 (Tg DN-bCat / þ , P53 þ / þ ). These differences were not due to complete loss of P53 as only one of 21 tumors demonstrated loss of heterozygosity at the P53 locus. Furthermore, no mutations were present in tumors retaining a single wild-type allele. Tg DN-bCat / þ , P53À/À male mice developed testicular teratomas and survived an average of 65 days, whereas non-Tg DN-bCat , P53À/À males survived an average of 84 days. Sixty-two percent of Tg DN-bCat , P53À/À mice developed testicular teratomas, whereas only 10% of the non-Tg DN-bCat , P53À/À mice developed these tumors. These results indicate that the level of P53 and the tissue of origin are important factors in determining outcome of cancer caused by oncogene activation.

Oncogenesis is a progressive process often involving collaboration between various oncogenes and tumor suppressors. To identify those genes that collaborate with oncogenic ras, we took advantage of the Tg.AC transgenic mouse, a line that harbors the v-Ha-ras transgene and spontaneously develops an array of malignant tumors. By crossing Tg.AC mice on an inbred FVB background to other inbred strains, F1 mice were created that could be analysed using genome wide, single nucleotide polymorphism (SNP) screens. Loss of heterozygosity (LOH) in tumors and tumor cell lines marked a somatic event, possibly the inactivation of tumor suppressor gene(s). LOH could also represent DNA damage, a sign of genomic instability in the pretransformed cell. Nonetheless, the screens showed no evidence of such generalized genomic instability. Instead, they revealed a single region of LOH on chromosome 4 that occurred via somatic recombination/gene conversion, generating a region of isoparental disomy. This LOH provided a clue that linked v-Ha-ras to the inactivation of the Ink4a locus in 25 of 32 tumor cell lines. This collaboration is seen regardless of tumor type or genetic background. In contrast, tumors that develop in bitransgenic mice bearing both the v-Ha-ras gene and a heterozygous mutant p53 allele tend to retain the Ink4a locus and instead lose the p53 wild-type allele. This suggests that different strategies can be selected to collaborate with v-Ha-ras in tumorigenesis.

Thalassemia is a disease caused by a variety of mutations affecting both the adult and embryonic alpha- and beta-globin loci. A mouse strain carrying an embryonic zeta-globin gene disrupted by the insertion of a PGK-Neo cassette displays an alpha-thalassemia-like syndrome. Embryonic survival of this zeta-null mouse is variable and strongly influenced by genetic background, the 129/SvEv mouse strain displaying a more severe phenotype than C57BL/6. We have identified two modifying loci on C57BL/6 chromosomes 2 and 5, which affect the penetrance of embryonic lethality in the 129/SvEv mouse. Through this work, we were able to observe an interesting effect on somatic recombination events in thalassemic embryos. We show that these events can occur on multiple chromosomes in very early embryonic cells, prior to their allocation to the germline. Our results demonstrate that somatic recombination events can be transmitted to subsequent generations.