DNA-dependent kinase (p350) as a candidate gene for the murine SCID defect

Restoration of X-ray and etoposide resistance, Ku-end binding activity and V(D) J recombination to the Chinese hamster sxi-3 mutant by a hamster Ku86 cDNA

Ku is a heterodimeric protein composed of 86 and 70 kDa subunits that binds preferentially to the double-stranded ends of DNA. Recent molecular characterization of ionizing-radiation sensitive (IRs) mutants belonging to the XRCC5 complementation group demonstrated the involvement of Ku in DNA double-strand break (DSB) repair and lymphoid V(D)J recombination. Here, we describe the isolation of a full-length hamster cDNA encoding the large subunit of the Ku heterodimer and demonstrate that the stable expression of this cDNA can functionally restore IR, Ku DNA end-binding activity and V(D)J recombination proficiency in the Chinese hamster IRs sxi-3 mutant. Moreover, we also demonstrate that sxi-3 cells are hypersensitive to etoposide, a DNA topoisomerase II inhibitor, and that resistance to this drug was restored by the Ku86 cDNA. These experiments suggest that a defect in the large subunit of the heterodimeric Ku protein is the sole factor responsible for the known defects of sxi-3 cells and our data of further support the role of Ku in DNA DSB repair and V(D)J recombination.

A human RNA polymerase II complex associated with SRB and DNA-repair proteins

We report here the isolation of a human RNA polymerase II complex containing a subset of the basal transcription factors and the human homologues of the yeast SRB (for suppressors of RNA polymerase B) proteins. The complex contains transcriptional coactivators and increases the activation of transcription. In addition, some components of the RNA polymerase II complex participate in DNA repair.

Lack of detectable defect in DNA double-strand break repair and DNA-dependent protein kinase activity in radiosensitive human severe combined immunodeficiency fibroblasts

The initial step of the V(D)J recombination occurs through the generation of a DNA double-strand break (dsb). Defects in the DNA-dependent protein kinase complex (DNA-PK) result in an inability to perform either V(D)J recombination or any dsb repair effectively. The human autosomal T-B-severe combined immunodeficiency (SCID) condition is characterized by an absence of both B and T lymphocytes and is accompanied in some patients by an increase in gamma-ray sensitivity (T-B-RS SCID) comparable to that found in mouse SCID cells. We show here that cells from six patients with T-B-RS SCID had normal DNA-dsb repair kinetics. Furthermore, DNA-PK activity was present in extracts from these human T-B-RS SCID fibroblasts. We therefore conclude that some human T-B-RS SCID disorders are not caused by a defect in an essential DNA-PK component.

Drosophila IRBP/Ku p70 corresponds to the mutagen-sensitive mus309 gene and is involved in P-element excision in vivo

The P family of transposable elements in Drosophila transpose by a cut-and-paste mechanism involving double-strand gap repair. We report here that a Drosophila mutagen-sensitive mutant, mus3O9, contains a mutation in IRBP (inverted repeat binding protein), the Drosophila homolog of the mammalian Ku p70 gene. We show that the repair of double-strand DNA breaks after P-element excision is severely reduced in mus3O9 mutants using an in vivo assay for P-element transposase activity. In addition, excision products recovered from mus3O9 mutant embryos by use of a plasmid-based P-element mobility assay contain large deletions, suggesting that IRBP is involved in the repair of double-strand DNA breaks. Our findings provide the first demonstration that a mutation in the IRBP gene affects double-strand DNA break repair and suggest that DNA repair functions are conserved between Drosophila and mammals.

Decreased rates of replicon initiation in mammalian cells

We have designed an assay to measure the rate of initiation of DNA synthesis in Friend erythroleukemia cells and have shown that this parameter is reduced by gamma-radiation and treatment with 4'-demethyl-epipodophyllotoxin-9-(4,6-O-ethylene-beta-D-glucopyranoside) (VP-16). It is concluded, that double-strand breaks in DNA are the immediate cause for this effect. The decrease in the rate of replicon initiation is affected differently by different agents such as cis-diamminedichloroplatinum(II), cycloheximide, staurosporine, and 3-aminobenzamide. The analysis of these results indicates that the observed partial decrease of the rate of DNA initiation is most probably transmitted from the site of damage to the initiation site by one or more phosphorylation/dephosphorylation steps. It does not require de novo synthesis of protein factors, but is probably dependent on poly(ADP-ribosyl)ation of chromatin at the site of DNA breaks.

The DNA-dependent protein kinase is inactivated by autophosphorylation of the catalytic subunit

The DNA-dependent protein kinase (DNA-PK) requires for activity free ends or other discontinuities in the structure of double strand DNA. In vitro, DNA-PK phosphorylates several transcription factors and other DNA-binding proteins and is thought to function in DNA damage recognition or repair and/or transcription. Here we show that in vitro DNA-PK undergoes autophosphorylation of all three protein subunits (DNA-PKcs, Ku p70 and Ku p80) and that phosphorylation correlates with inactivation of the serine/threonine kinase activity of DNA-PK. Significantly, activity is restored by the addition of purified native DNA-PKcs but not Ku, suggesting that inactivation is due to autophosphorylation of DNA-PKcs. Our data also suggest that autophosphorylation results in dissociation of DNA-PKcs from the Ku-DNA complex. We suggest that autophosphorylation is an important mechanism for the regulation of DNA-PK activity.

Involvement of the Saccharomyces cerevisiae HDF1 gene in DNA double-strand break repair and recombination

The HDF1 protein of Saccharomyces cerevisiae shares biochemical properties and structural homology with the 70-kDa subunit of the human autoantigen Ku. The Ku protein, a heterodimer composed of a 70-kDa subunit and an 80-kDa subunit, has been identified as the regulatory subunit of the DNA-dependent protein kinase. This enzyme has recently been shown to be involved in DNA repair and recombination processes in mammalian cells. Here we show that hdf1-disrupted S. cerevisiae strains are strongly sensitive toward the radiomimetic antibiotic bleomycin. In addition, mating-type switching and rates of spontaneous mitotic recombination are strongly reduced. This phenotype is similar to that of mammalian cells lacking components of the DNA-dependent protein kinase holoenzyme, suggesting that HDF1 participates in and exerts equivalent functions in S. cerevisiae.

The half-life of RAG-1 protein in precursor B cells is increased in the absence of RAG-2 expression

Site-specific recombination of immunoglobulin and T cell receptor gene segments in B and T lymphocytes is dependent on the expression of two recombinant activation genes, Rag-1 and Rag-2. Here, we show that RAG-1 protein turnover in pre-B cells depends on the expression of RAG-2. The apparent half-life of RAG-1 protein is increased when RAG-2 is not expressed in differentiating pre-B cells.

Cutting apart V(D)J recombination

The past year has seen major advances in our understanding of the recombination mechanism by which antibody and T cell receptor genes are assembled during lymphoid development. The initial cleavage events can be carried out in vitro by purified RAG1 and RAG/ protein. In addition, a number of genes involved in later steps of the reaction have been cloned, opening the way for an in-depth biochemical analysis of this critical developmental process.

Mechanism of V(D)J recombination

V(D)J recombination can be separated into two basic operations: DNA cleavage and joining of broken ends. Our understanding of both reactions has increased substantially in the past year. Major advances include the development of a cell-free system capable of cleavage and the identification of several proteins involved in both V(D)J recombination and double-strand break repair.

Ku86 defines the genetic defect and restores X-ray resistance and V(D)J recombination to complementation group 5 hamster cell mutants

X-ray-sensitive hamster cells in complementation groups 4, 5, 6, and 7 are impaired for both double-strand break repair and V(D)J recombination. Here we show that in two mutant cell lines (XR-V15B and XR-V9B) from group 5, the genetic defects are in the gene encoding the 86-kDa subunit of the Ku autoantigen, a nuclear protein that binds to the double-stranded DNA ends. These mutants express Ku86 mRNA containing deletions of 138 and 252 bp, respectively, and the encoded proteins contain internal, in-frame deletions of 46 and 84 amino acids. Two X-ray-resistant revertants of XR-V15B expressed two Ku86 transcripts, one with and one without the deletion, suggesting that reversion occurred by activation of a silent wild-type allele. Transfection of full-length cDNA encoding hamster Ku86 into XR-V15B cells resulted in a complete rescue of DNA-end-binding (DEB) activity and Ku70 levels, suggesting that Ku86 stabilizes the Ku70 polypeptide. In addition, cells expressing wild-type levels of DEB activity were fully rescued for X-ray resistance and V(D)J recombination, whereas cells expressing lower levels of DEB activity were only partially rescued. Thus, Ku is an essential component of the pathway(s) utilized for the resolution of DNA double-strand breaks induced by either X rays or V(D)J recombination, and mutations in the Ku86 gene are responsible for the phenotype of group 5 cells.

Regulation of thymocyte development from immature progenitors

T lymphocytes differentiate from hematopoietic stem cells that settle in the microenvironment of the thymus. The earliest stages of mouse alpha/beta T-cell differentiation occurring before surface expression of the TCR include three important events: proliferation, commitment to the T lineage, and rearrangement and expression of the TCR loci. Recent evidence suggests that the survival as well as differentiation of early thymocytes depends critically on molecular signals such as those generated by the recently described pre-TCR complex.

Sequence-specific DNA binding by Ku autoantigen and its effects on transcription

DNA-dependent protein kinase (DNA-PK) has been implicated in several nuclear processes including transcription, DNA replication, double-stranded DNA break repair, and V(D)J recombination. Linkage of kinase and substrate on DNA in cis is required for efficient phosphorylation. Recruitment of DNA-PK to DNA is by Ku autoantigen, a DNA-end-binding protein required for DNA-PK catalytic activity. Although Ku is known to translocate along naked DNA, how DNA-end binding by Ku might lead to DNA-PK-mediated phosphorylation of sequence-specific DNA-binding proteins in vivo has not been obvious. Here we report the identification of Ku as a transcription factor that recruits DNA-PK directly to specific DNA sequences. NRE1 (negative regulatory element 1) is a DNA sequence element (-394/ -381) in the long terminal repeat of mouse mammary tumour virus (MMTV) that is important for repressing inappropriate viral expression. We show that direct binding of Ku/DNA-PK to NRE1 represses glucocorticoid-induced MMTV transcription.

Autoantibodies to DNA-dependent protein kinase. Probes for the catalytic subunit

DNA-dependent protein kinase (DNA-PK) is an important nuclear enzyme which consists of a catalytic subunit known as DNA-PKcs and a regulatory component identified as the Ku autoantigen. In the present study, we surveyed 312 patients in a search for this specificity. 10 sera immunoprecipitated a large polypeptide which exactly comigrated with DNA-PKcs in SDS-PAGE. Immunoblot analysis demonstrated that this polypeptide was recognizable by a rabbit antiserum specific for DNA-PKcs. Although the patient sera did not bind to biochemically purified DNA-PKcs in immunoblots or ELISA, they were able to deplete DNA-PK catalytic activity from extracts of HeLa cells in a dose-dependent manner. We conclude that these antibodies should be useful probes for studies which aim to define the role of DNA-PK in cells. Since six sera simultaneously contained antibodies to the Ku protein, these studies suggest that relatively intact forms of DNA-PK complex act as autoantigenic particles in selected patients.

p53 is required for both radiation-induced differentiation and rescue of V(D)J rearrangement in scid mouse thymocytes

The murine scid mutation affects both V(D)J recombination and DNA repair. This mutation has been mapped to the gene encoding the catalytic subunit of the DNA-dependent protein kinase (DNA-PK), which is activated by DNA damage in normal cells. In scid mice, antigen receptor gene rearrangements are initiated normally, but impaired joining of coding ends prevents assembly of functional receptor genes, resulting in arrest of B- and T-cell development. Others have shown that exposure of scid mice to genotoxic agents such as gamma-irradiation rescues rearrangement at the T-cell receptor (TCR) beta locus and promotes thymocyte development. Here we demonstrate that irradiation rescues rearrangements at multiple TCR loci, suggesting a general effect on the recombination mechanism. Furthermore, our data show that p53 is required for irradiation-mediated rescue of both thymocyte development and V(D)J recombination. We also find that thymocyte proliferation and differentiation in the absence of DNA damage do not require p53 and are not sufficient to rescue V(D)J recombination. These results suggest that exposure to ionizing radiation facilitates a partial bypass of the scid defect, perhaps by inducing p53-dependent DNA damage response pathways.

Physical map of a YAC contig containing the region of the human gene (HYRC) complementing hyper-radiosensitivity of the scid mouse mutation

We previously mapped the putative human HYRC (the hyper-radiosensitivity of the scid mutation, complementing gene) to human chromosome 8q11.1 by fluorescence in situ hybridization (FISH) using Alu-based PCR products from a mouse-human scid radiation cell hybrid (RD15/5) as probes. From a cosmid library constructed from RD15/5, 57 cosmid clones containing human DNA inserts were isolated. 18 of which were mapped to 8q11. Based on the sequences of plasmid subclones of the 18 cosmids, five novel sequence-tagged-sites (STSs) were made. By a screening of the CEPH-YAC library with these STSs, five yeast artificial chromosome (YAC) clones were isolated. All these YAC clones were confirmed not to be chimeric by FISH, but two of them showed deleted human insert DNAs. Using the other 3 non-deleted YACs, we constructed a physical map covering the HYRC region. We confirmed that the recently isolated gene (the DNA-PKcs gene) which is a strong candidate for HYRC is located within the present contig and spans less than 200 kb. This map will be useful for the analysis of the genomic structure of the DNA-PKcs gene and for isolation of other complementing genes in the HYRC region.

DNA-dependent protein kinase defects are linked to deficiencies in DNA repair and V(D)J recombination

DNA-dependent protein kinase is a nuclear serine/threonine kinase whose catalytic properties are expressed only when the enzyme is bound to DNA ends or other discontinuities in the DNA. DNA-PK comprises two components: one mediates binding to DNA and corresponds to the heterodimeric human autoimmune antigen Ku; the other, DNA-PK catalytic subunit (DNA-PKcs), is a polypeptide of approximately 450 kDa. DNA-PK deficiencies are associated with certain mutant rodent cell lines that display defects in DNA double strand break repair and V(D)J recombination. Specifically, hamster xrs-6 cells lack Ku function, whereas murine scid and hamster V3 cells lack functional DNA-PKcs. Furthermore, the phenotypes of xrs-6 and V3 cells can be corrected by the expression of the genes encoding the 80 kDa component of Ku or DNA-PKcs, respectively. These results imply that DNA-PK is an important component of the DNA double strand break repair/recombination apparatus. Possible roles for DNA-PK in these processes are discussed.

T-cell receptor alpha locus V(D)J recombination by-products are abundant in thymocytes and mature T cells

In addition to the assembled coding regions of immunoglobulin and T-cell receptor (TCR) genes, the V(D)J recombination reaction can in principle generate three types of by-products in normal developing lymphocytes: broken DNA molecules that terminate in a recombination signal sequence or a coding region (termed signal or coding end molecules, respectively) and DNA molecules containing fused recombination signal sequences (termed reciprocal products). Using a quantitative Southern blot analysis of the murine TCR alpha locus, we demonstrate that substantial amounts of signal end molecules and reciprocal products, but not coding end molecules, exist in thymocytes, while peripheral T cells contain substantial amounts of reciprocal products. At the 5' end of the J alpha locus, 20% of thymus DNA exists as signal end molecules. An additional 30 to 40% of the TCR alpha/delta locus exists as remarkably stable reciprocal products throughout T-cell development, with the consequence that the TCR C delta region is substantially retained in alpha beta committed T cells. The disappearance of the broken DNA molecules occurs in the same developmental transition as termination of expression of the recombination activating genes, RAG-1 and RAG-2. These findings raise important questions concerning the mechanism of V(D)J recombination and the maintenance of genome integrity during lymphoid development.

The recognition of DNA damage

DNA strand breaks are potentially mutagenic and must, therefore, be recognized and repaired. Recent work has identified DNA polymerase epsilon, Ku, and proteins such as DNA-PKcs, Mec1 and Tel1 as key players in DNA damage recognition pathways. Studies on these and other factors have provided important insights into the mechanisms of DNA repair and how DNA damage signals are transduced to the transcription and cell cycle machineries. This work also suggests how deficiencies in DNA damage detection systems can result in genetic instability and cancer.

Frequency and histological appearance of adenomas in multiple intestinal neoplasia mice are unaffected by severe combined immunodeficiency (scid) mutation

Well-characterized murine mutations are powerful analytical tools for the genetic analysis of tumorigenesis. We crossed the multiple intestinal neoplasia (Min) allele of adenomatous polyposis coli (Apc), which produces a profound pre-disposition to intestinal neoplasia, with the severe combined immunodeficiency (scid) mutation, which causes defective double-strand DNA repair and severe immunodeficiency, on the common C57BL/6J genetic background to assay for any combined effect on intestinal tumorigenesis. Several phenotypic traits were exacerbated in an apparently additive manner in the double mutant mice, including reduced immunoglobulin levels, reduced body weight and increased morbidity. However, quantitation and histological evaluation of polyp phenotype indicated that these mutations did not interact to affect either polyp frequency or progression. Thus, neither genome instability nor lack of immunosurveillance conferred by scid contributes to intestinal polyps in this model.

Disruption of DNA-PK in Ku80 mutant xrs-6 and the implications in DNA double-strand break repair

The Chinese hamster ovary (CHO) mutant cell line xrs-6C is highly sensitive to radiation and is deficient in DNA double-strand break (DSB) repair. The repair defect of xrs-6C is complemented by the human DSB repair gene designated as XRCC5. This gene was recently identified as Ku80, which encodes the human autoantigen protein Ku p80. Ku80 protein forms heterodimer with the Ku70 subunit to form a complex that possesses a DNA end-binding activity. Ku70/Ku80 heterodimer can recruit the catalytic p350 subunit of the DNA-dependent protein kinase. It is demonstrated here that, while the Ku70 mRNA expression is normal in the xrs-6C mutant, Ku70 protein is undetectable. However, introduction of human Ku80 gene into the mutant lead to increased expression of Ku70 protein and restored Ku70 binding to DNA ends, suggesting that mutation of the Ku80 gene affected the formation of Ku70/Ku80 dimers and the stability of the Ku70 protein. We also demonstrated that, although p350 protein expression in the mutants was unaffected, the capacity of p350 to bind to DNA ends was impaired in the mutants. After introduction of the human Ku80 into the mutant, the association of p350 with DNA end was restored, accompanied by recovery in cell survival and DNA double-strand break repair. The results in this report show that mutation of the Ku80 gene disrupts formation of the Ku70/Ku80 dimer and compromises the ability of Ku protein to recruit the DNA-PK p350 subunit to DNA double-strand breaks, causing a dysfunction of DNA DSB repair in the cell.

Thermotolerance and heat shock proteins: possible involvement of Ku autoantigen in regulating Hsp70 expression

Here we characterize and compare the phenomenon of thermotolerance and permanent heat resistance in mammalian cells. The biochemical and molecular mechanisms underlying the induction of thermotolerance, and the role that heat shock proteins play in its development and decay are discussed. Finally, we describe a novel constitutive HSE-binding factor (CHBF/Ku) that appears to be involved in the regulation of the heat shock response.

Immune deficiency in SCID mice

Since the discovery of SCID mice in 1983, numerous studies utilizing these mice were carried out. These investigations can be classified into two major groups. First, the analysis of the immune defect has revealed defective V(D)J recombination and defective DNA double-strand break repair, and has lead to the identification of the candidate gene for SCID mice. Second, the use of SCID mice to explore ways to introduce a murine or xenogeneic immune system into SCID mice by taking advantage of the immune deficiency of the mice has provided an animal model to examine the in vivo function of transferred human or murine immune cells. In this review, we summarize the recent advances made in these two areas of SCID mouse research.

The use of SCID mice in biotechnology and as a model for human disease

The use of SCID (severe combined immunodeficient) mice in medical research and biotechnology has increased tremendously in recent years. This review outlines the major characteristics of these animals and the impediments that they pose to the engraftment of human cells and tissues. The development of our SCID mice pretreatment protocol (anti-asialo GM1 antisera and radiation) is described, and the results of xenotransplantation studies of human cells and tissues in these pretreated animals are outlined. Wherever possible, data from transplantation studies (of human tissues and cells) in pretreated and nonpretreated animals are compared. The potential of our pretreated SCID mice for medical research and biotechnology is discussed.

Modulating the radiation response

This review describes some current and future approaches designed to modulate the response of tumors and normal tissues to cell killing by ionizing radiation. The emerging knowledge of tumor, cellular and molecular biology is providing a better understanding of the clinical results with the hypoxic cell sensitizers and novel approaches to radiation sensitization and protection.

Insights into T cell development and signal transduction provided by TCR-zeta chain deficient mice

The T cell antigen receptor (TCR) transduces signals that mediate different responses depending on the stage of development of the T cell and the nature of the ligand it engages. The presence of multiple signal transducing subunits (CD3-gamma-delta,-epsilon and zeta chain) suggests the potential to control these responses by altering the subunit composition of the TCR. zeta chain represents an especially important signalling molecule as it contains multiple signalling motifs within its cytoplasmic tail. The generation and analysis of zeta deficient (zeta-/-) and zeta-transgenic mice has provided insight into the role of zeta as well as the CD3 subunits in TCR surface expression, T cell activation and thymocyte development. Herein, we discuss the results from such experiments which suggest distinct roles for zeta chain and the CD3 components at different stages of T cell development.

Cellular and molecular analysis of lymphoid development using Rag-deficient mice

The establishment of a functional immune system with diverse antigen receptors is dependent on the V(D)J recombination activating gene products Rag-1 and Rag-2. These two proteins constitute the key lymphoid components required for the activation of antigen receptor rearrangement. Both Rag-1 and Rag-2 are required for the catalysis of the initial stages of V(D)J recombination. Thus, functional disruption of either the Rag-1 or Rag-2 genes by homologous recombination, leads to immunodeficiency due to lymphoid arrest at a stage prior to the recombination of the antigen receptor loci. In Rag-deficient mice, both B- and T-cell differentiation is eliminated due to the absence of antigen receptors. Lymphoid development can be restored by the introduction of rearranged antigen receptor transgenes that give rise to monoclonal populations of fully mature B- or T-cells. The absence of the major conventional populations of B- and T-cells from the Rag-deficient mice provided an excellent background for studying the molecular and cellular mechanisms of lymphoid differentiation. The Rag-deficient background has been used as a system for: the functional analysis of Rag-1 and Rag-2; studying the developmental functions of antigen receptors and other molecules of the immune system; the molecular analysis of the early stages of the B- and T-cell lineages; the co-development of lymphocytes with stroma cells; the identification of minor subpopulations of the developing immune system; the involvement of lymphoid populations in the onset of pathogenesis. In addition, the development of the "blastocyst complementation assay" methodology, based on the phenotype of the Rag-/- mice, allowed the functional analysis of numerous lymphoid specific components.

The XRCC4 gene encodes a novel protein involved in DNA double-strand break repair and V(D)J recombination

The XR-1 Chinese hamster ovary cell line is impaired in DNA double-strand break repair (DSBR) and in ability to support V(D)J recombination of transiently introduced substrates. We now show that XR-1 cells support recombination-activating gene 1- and 2-mediated initiation of V(D)J recombination within a chromosomally integrated substrate, but are highly impaired in ability to complete the process by forming coding and recognition sequence joins. On this basis, we isolated a human cDNA sequence, termed XRCC4, whose expression confers normal V(D)J recombination ability and significant restoration of DSBR activity to XR-1, clearly demonstrating that this gene product is involved in both processes. The XRCC4 gene maps to the previously identified locus on human chromosome 5, is deleted in XR-1 cells, and encodes a ubiquitously expressed product unrelated to any described protein.

Repair of x-ray-induced DNA double-strand breaks in specific Not I restriction fragments in human fibroblasts: joining of correct and incorrect ends

An assay that allows measurement of absolute induction frequencies for DNA double-strand breaks (dsbs) in defined regions of the genome and that quantitates rejoining of correct DNA ends has been used to study repair of dsbs in normal human fibroblasts after x-irradiation. The approach involves hybridization of single-copy DNA probes to Not I restriction fragments separated according to size by pulsed-field gel electrophoresis. Induction of dsbs is quantitated from the decrease in the intensity of the hybridizing restriction fragment and an accumulation of a smear below the band. Rejoining of dsbs results in reconstitution of the intact restriction fragment only if correct DNA ends are joined. By comparing results from this technique with results from a conventional electrophoresis assay that detects all rejoining events, it is possible to quantitate the misrejoining frequency. Three Not I fragments on the long arm of chromosome 21 were investigated with regard to dsb induction, yielding an identical induction rate of 5.8 X 10(-3) break per megabase pair per Gy. Correct dsb rejoining was measured for two of these Not I fragments after initial doses of 80 and 160 Gy. The misrejoining frequency was about 25% for both fragments and was independent of dose. This result appears to be representative for the whole genome as shown by analysis of the entire Not I fragment distribution. The correct rejoining events primarily occurred within the first 2 h, while the misrejoining kinetics included a much slower component, with about half of the events occurring between 2 and 24 h. These misrejoining kinetics are similar to those previously reported for production of exchange aberrations in interphase chromosomes.

Equine severe combined immunodeficiency: a defect in V(D)J recombination and DNA-dependent protein kinase activity

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 lymphoid-specific 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.

Localization, interaction, and RNA binding properties of the V(D)J recombination-activating proteins RAG1 and RAG2

The RAG1 and RAG2 gene products are indispensable for activating somatic rearrangement of antigen receptor gene segments. The two proteins form a stable complex in primary thymocytes as well as when expressed in adherent cells. In both cell types, most cells localize RAG proteins at the periphery of the nucleus. However, when overexpressed in fibroblast cells, RAG1 is found largely in the nucleolus. Nucleolar localization of RAG1 is mediated by several domains containing stretches of basic amino acids, indicating that RAG1 has affinity for RNA or ssDNA. The RAG1 interacting proteins SRP1 and Rch1 directly bind to the nuclear localization signals of RAG1, which mediate the nuclear and nucleolar translocation of the protein. RAG1 appears to have a binary structure, each half containing multiple regions that can act as NLSs, binding sites for the SRP1/Rch1 family, and RNA binding domains.

DNA-dependent protein kinase is one of a subset of autoantigens specifically cleaved early during apoptosis

Proteolytic cleavage of key substrates appears to be an important biochemical mechanism underlying the apoptotic process, and the centrality of interleukin 1 beta-converting enzyme (ICE)-like proteases as mediators of apoptosis has been suggested. The identification of the relevant substrates of the ICE protease family during apoptosis therefore constitutes a major challenge. Using human autoantibodies, we demonstrate here that a subset of autoantigens is specifically cleaved early during apoptosis. One of these cleaved molecules is identified as the catalytic subunit of the DNA-dependent protein kinase. The time courses of all proteolytic cleavages are identical and coincide with the onset of morphologic apoptosis. Furthermore, all cleavages share the same inhibition characteristics, which implicate an ICE-like activity(ies). We propose that cleavage of these autoantigens targets these molecules for an autoimmune response by revealing immunocryptic fragments in a proimmune apoptotic setting. Study of the immunogenicity of these fragments may yield insights into the autoimmune targeting of molecules. Moreover, the autoantibodies described will be valuable tools for the elucidation of mechanistically important proteolytic steps along the apoptotic pathway.

Improved engraftment of human hematopoietic cells in severe combined immunodeficient (SCID) mice carrying human cytokine transgenes

We have generated immunodeficient scid-/scid- (SCID)-transgenic mice expressing the genes for human interleukin 3, granulocyte/macrophage-colony stimulating factor, and stem cell factor. We have compared engraftment and differentiation of human hematopoietic cells in transgenic SCID mice with two strains of nontransgenic SCID mice. Human bone marrow cells carrying the CD34 antigen or human umbilical cord blood were injected into sublethally irradiated recipients. Human DNA was detected by polymerase chain reaction in peripheral blood and bone marrow of 14 of 28 transgenic SCID mice after transplantation, but in only 2 of 15 nontransgenic SCID littermates at a 10-fold lower level. Bone marrow cultures 8 wk after transplantation of cord blood gave rise to human burst-forming unit erythroid, colony-forming unit granulocyte/macrophage, or granulocyte/erythroid/macrophage/megakaryocyte colonies. Engraftment was observed for up to 6 mo in transgenic SCID mice, twice as long as nontransgenic littermates or previous studies in which transplanted SCID mice were given daily injections of growth factors. We conclude that the level and duration of engraftment of human cells in SCID mice can be improved by expression of human cytokine transgenes and that transgenic SCID mice are an efficient model system for the study of human hematopoiesis.

Engraftment and humoral immunity in SCID and RAG-2-deficient mice transplanted with human peripheral blood lymphocytes

SCID and RAG-2 deficient mice were transplanted intraperitoneally with human peripheral blood lymphocytes (hu-PBL-SCID and hu-PBL-RAG mice). Seven days after transplantation the mice were immunized with a pneumococcal polysaccharide vaccine. Flow cytometry analysis of cells from the peritoneal cavity and the spleen after 8-10 weeks revealed that human cells had more limited engraftment in RAG than in SCID recipient mice, and that more human cells were found in the spleen than in the peritoneal cavity. Functionality of the human cells recovered from these two locations was explored by the counting of human immunoglobulin secreting cells (hu-ISC). A total of 83% of the hu-PBL-SCID mice and 29% of the hu-PBL-RAG mice had detectable hu-ISC in the peritoneal cavity and/or the spleen. The kinetic profiles of human immunoglobulins in the mouse sera during the experiment showed donor dependency. More than 90% of the hu-PBL-SCID mice had detectable levels of human IgG, IgM and IgA, while 78% had detectable levels of IgE, whereas detectable levels of IgG, IgM, IgA and IgE were measured in 37%, 64%, 68% and 23% of the hu-PBL-RAG mice, respectively. Forty-seven per cent of immunized hu-PBL-SCID mice showed a human antipneumococcal IgG level that was significantly above the background level in non-immunized mice, while none of the hu-PBL-RAG mice produced any detectable levels of human antipneumococcal IgG. In short, human PBL showed a better engraftment and a better antibody response when transplanted into SCID mice than into RAG mice.

Gene for the catalytic subunit of mouse DNA-dependent protein kinase maps to the scid locus

The gene encoding the catalytic subunit of DNA-dependent protein kinase (DNA-PKcs) has been proposed recently as a candidate gene for the mouse severe combined immune deficiency (scid) locus. We have used a partial cDNA clone for human DNA-PKcs to map the mouse homologue using a large interspecific backcross panel. We found that the mouse gene for DNA-PKcs does not recombine with scid, consistent with the hypothesis that scid is a mutation in the mouse gene for DNA-PKcs.