V(D)J recombination in mammalian cell mutants defective in DNA double-strand break repair

Steps along the pathway of V (D)J recombination

The mechanism of lymphoid-specific gene rearrangement (V(D)J recombination) is discussed, with a focus on the existence of broken DNA intermediates. Older evidence in support of this idea includes the sequence alteration at the recombined junctions and the presence of aberrant recombinants. More recently, broken DNA molecules have been directly detected in recombinationally active cells. The signal sequence ends have normal blunt-ended DNA breaks, but the coding ends have a hairpin (self-joined) structure that provides an explanation for the self-complementary P nucleotide insertions often found after V(D)J joining in the antigen receptor genes.

Characterization of coding ends in thymocytes of scid mice: implications for the mechanism of V(D)J recombination

We previously identified possible intermediates in V(D)J recombination at the TCR delta locus and characterized molecules with signal ends and with covalently sealed (hairpin) coding ends in thymocytes of scid mice by Southern blotting. Here, we use a sensitive ligation-mediated PCR assay to demonstrate that all coding ends detected in scid thymocytes are covalently sealed. Neither coding nor signal ends exhibit loss or addition of nucleotides. These data imply that hairpin formation is coupled to the initial cleavage at the signal/coding border, and that the cleavage step in V(D)J recombination is conservative. In scid/+ or wild-type thymocytes, hairpin coding ends are at least 1000-fold less abundant than signal ends. These results provide insight into the mechanism of V(D)J recombination.

The study of responses to 'model' DNA breaks induced by restriction endonucleases in cells and cell-free systems: achievements and difficulties

The use of restriction endonucleases (RE) as a means of implicating DNA double-strand breaks (dsb) in cellular responses is reviewed. The introduction of RE into cells leads to many of the responses known to be characteristic of radiation damage--cell killing, chromosomal aberration, oncogenic transformation, gene mutation and amplification. Additionally, radiosensitive cell lines are hypersensitive to RE, including those from the human disorder ataxia-telangiectasia. However, quantitation of response and comparisons of the effectiveness of different RE are difficult, partly because of unknown activity and lifetime of RE in the cell. RE-induced dsb have also been used to reveal molecular mechanisms of repair and misrepair at specific sites in DNA. Dsb have been implicated in recombination processes including those leading to illegitimate rejoining (formation of deletions and rearrangements) at short sequence features in DNA. Also model dsb act as a signal to activate other cellular processes, which may influence or indirectly cause some responses, including cell death. In these signalling responses the detailed chemistry at the break site may not be very important, perhaps explaining why there is considerable overlap in responses to RE and to ionizing radiations.

The B cell antigen receptor in B-cell development

During B-cell development, immature and mature forms of the B cell antigen receptor complex are deployed in a regulated fashion; thus, B cell antigen receptor complexes play essential roles in the transit of cells through ontogeny. The past year has seen progress in our understanding of how antigen receptor gene assembly is controlled and in defining the requirements for antigen receptor mediated signaling at specific developmental stages. The discovery that a defective protein tyrosine kinase is responsible for X-linked agammaglobulinemia in man and X-linked immunodeficiency in the mouse is particularly interesting, as it may provide the means to link a specific intracellular signaling pathway with a particular step in B-cell development.

The SCID mouse: relevance as an animal model system for studying human disease

The simultaneous description some 5 years ago of two methods for the partial reconstitution of a human immune system in severe combined immune-deficient (SCID) mice (collectively, human:SCID mice) was met with great enthusiasm. At the time, it was hoped that human:SCID mice would provide experimental animal model systems for studying human disease and the human immune system. Many of these hopes have been borne out. Importantly, the experimental results obtained from these chimeric human/animal studies appear to be relevant to human disease and immune function. In spite of these glowing achievements, the SCID mouse may not represent the optimal experimental system with which to address these questions. The incomplete penetrance ("leakiness") of the scid mutation and the recent discovery that the mutation is not lymphoid specific, but rather affects a general DNA repair pathway, will only serve to complicate the interpretation of already complex biological interactions. Recently other immune-deficient mice have been described that appear to overcome one or both of these problems and thus these mice could represent improved hosts for the adaptive transfer of a human immune system. The current status of the SCID mouse in light of these new findings is discussed.