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The E2A gene products, E12 and E47, are critical for proper early B-cell development and commitment to the B-cell lineage. Here we reveal a new role for E2A in T-lymphocyte development. Loss of E2A activity results in a partial block at the earliest stage of T-lineage development. This early T-cell phenotype precedes the development of a T-cell lymphoma which occurs between 3 and 9 months of age. The thymomas are monoclonal and highly malignant and display a cell surface phenotype similar to that of immature thymocytes. In addition, the thymomas generally express high levels of c-myc. As assayed by comparative genomic hybridization, each of the tumor populations analyzed showed a nonrandom gain of chromosome 15, which contains the c-myc gene. Taken together, the data suggest that the E2A gene products play a role early in thymocyte development that is similar to their function in B-lineage determination. Furthermore, the lack of E2A results in development of T-cell malignancies, and we propose that E2A inactivation is a common feature of a wide variety of human T-cell proliferative disorders, including those involving the E2A heterodimeric partners tal-1 and lyl-1.The E2A gene encodes two basic helix-loop-helix (HLH) transcription factors, E12 and E47 (32). E12 and E47, members of the class I HLH proteins, are characterized by their broad expression pattern and their ability to bind DNA either as homodimers or as heterodimers with tissue-specific HLH proteins (9,22,33,40,43). Class I HLH proteins share several highly conserved domains. The HLH domain mediates homoand/or heterodimerization, and the basic region constitutes the sequence-specific DNA binding domain (11,22,32,50). In addition, two distinct domains located in the N-terminal portion of the class I HLH proteins have been shown to be required for transactivation (1,26,39).E12 and E47 arise through differential splicing to the exon that encodes for the HLH domain. Within the HLH domain, their amino acid sequences differ by 20% (32). Both E12 and E47 have the ability to form heterodimers with class II HLH members, including the myogenic regulators (9, 23). However, E12 and E47 have distinct biochemical properties. E47 homodimers bind with high affinity to DNA, whereas an inhibitory domain present in E12 prevents those homodimers from high-affinity DNA binding (46).E2A polypeptides bind to E-box sites present in a wide variety of tissue-specific enhancers, including the insulin, muscle creatine kinase, and immunoglobulin (Ig) intronic and 3Ј enhancers (16,22,28,32,35,38,52). In B cells, it is homodimers of the E2A gene products that bind to E2-box sites present in the Ig enhancers (2,34,43). That E2A gene products play a crucial role in B lymphocyte development has been demonstrated recently by the generation of E2A-deficient mice and transgenic mice overexpressing Id1, an inhibitor of E2A (3,47,56). In the absence of E2A activity, even the earliest committed B-cell precursors are undetectable in the bone marrow (3,47,56). In addition, E2A-deficient mice l...

The E2A gene products, E12 and E47, are required for proper B cell development. Mice lacking the E2A gene products generate only a very small number of B220+ cells, which lack immunoglobulin DJ(H) rearrangements. We have now generated mice expressing either E12 or E47. B cell development in mice expressing E12 but lacking E47 is perturbed at the pro-B cell stage, and these mice lack IgM+B220+ B cells in both bone marrow and spleen. IgM+B220+ B cells can be detected, albeit at significantly reduced levels, in the bone marrow and spleen of mice lacking E12. Ectopic expression of both E12 and E47 in a null mutant background shows that E12 and E47 act in concert to promote B lineage development. Taken together, the data indicate that both E12 and E47 allow commitment to the B cell lineage and act synergistically to promote B lymphocyte maturation.

In mammalian meiotic prophase, the initial steps in repair of SPO11-induced DNA double-strand breaks (DSBs) are required to obtain stable homologous chromosome pairing and synapsis. The X and Y chromosomes pair and synapse only in the short pseudo-autosomal regions. The rest of the chromatin of the sex chromosomes remain unsynapsed, contains persistent meiotic DSBs, and the whole so-called XY body undergoes meiotic sex chromosome inactivation (MSCI). A more general mechanism, named meiotic silencing of unsynapsed chromatin (MSUC), is activated when autosomes fail to synapse. In the absence of SPO11, many chromosomal regions remain unsynapsed, but MSUC takes place only on part of the unsynapsed chromatin. We asked if spontaneous DSBs occur in meiocytes that lack a functional SPO11 protein, and if these might be involved in targeting the MSUC response to part of the unsynapsed chromatin. We generated mice carrying a point mutation that disrupts the predicted catalytic site of SPO11 (Spo11YF/YF), and blocks its DSB-inducing activity. Interestingly, we observed foci of proteins involved in the processing of DNA damage, such as RAD51, DMC1, and RPA, both in Spo11YF/YF and Spo11 knockout meiocytes. These foci preferentially localized to the areas that undergo MSUC and form the so-called pseudo XY body. In SPO11-deficient oocytes, the number of repair foci increased during oocyte development, indicating the induction of S phase-independent, de novo DNA damage. In wild type pachytene oocytes we observed meiotic silencing in two types of pseudo XY bodies, one type containing DMC1 and RAD51 foci on unsynapsed axes, and another type containing only RAD51 foci, mainly on synapsed axes. Taken together, our results indicate that in addition to asynapsis, persistent SPO11-induced DSBs are important for the initiation of MSCI and MSUC, and that SPO11-independent DNA repair foci contribute to the MSUC response in oocytes.