V(D)J rearrangement is the molecular mechanism by which an almost infinite array of specific immune receptors are generated. Defects in this process result in profound immunodeficiency as is the case in the C.B-17 SCID mouse or in RAG-1 (recombination-activating gene 1) or RAG-2 deficient mice. It has recently become clear that the V(D)J recombinase most likely consists of both lymphoidspecific factors and ubiquitously expressed components of the DNA double-strand break repair pathway. The deficit in SCID mice is in a factor that is required for both of these pathways. In this report, we show that the factor defective in the autosomal recessive severe combined immunodeficiency of Arabian foals is required for (i) V(D)J recombination, (ii) resistance to ionizing radiation, and (iii) DNA-dependent protein kinase activity.During early lymphoid differentiation distinct gene segments called variable (V), diversity (D), and joining (J) are joined to form the coding sequences of immunoglobulin and T-cell antigen receptor variable regions. This process depends upon site-specific somatic recombination and results in the random assortment of various combinations of V, D, and J gene segments (reviewed in refs. 1-3). The rearrangement process involves two double-stranded DNA cuts and subsequent religations. This results in the formation of two new DNA joints-coding joints, which contain the coding information, and signal joints, which contain the two recombination signal sequences (ref. 3 and references therein). In 1989 and 1990, two highly conserved genes were discovered, RAG-1 and RAG-2 (recombination-activating genes 1 and 2), which are clearly essential for V(D)J recombinase activity (4, 5) and have recently been directly implicated in initiation of V(D)J recombination (6).In 1983, Bosma et al. (7) described a spontaneous mutation in C.B-17 mice which phenotypically resembled a human lymphoid deficiency disease, severe combined immunodeficiency (SCID). In 1986, it became clear that the deficiency in C.B-17 mice is in the step of coding-joint ligation of V(D)J recombination (8-10). In 1990 it was reported that the mutation in SCID mice not only affects V(D)J recombination but also impairs the more general process of double-strand break repair (DSBR) (11, 12). Thus, this was the first suggestion that the V(D)J recombinase might utilize ubiquitous DNA repair factors to carry out the V(D)J recombination reaction.DSBR is vital to all organisms because it ensures integrity of chromosomes. Studies with mammalian cells that are hypersensitive to agents that induce chromosomal breaks have defined three distinct complementation groups that are deficient both in rejoining double-strand breaks and in their ability to support RAG-induced V(D)J recombination (xrs, XR-1,The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact. and murine SCID; refs. 13 and 14). Thus, the eme...

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