Elongated telomeres in scid mice

Severe combined immunodeficiency (scid) mice are deficient in the enzyme DNA-PK (DNA-dependent protein kinase) as a result of the mutation in the gene encoding the catalytic subunit (DNA-PKcs) of this enzyme. DNA-PKcs is a member of the phosphatidylinositol 3-kinase superfamily, which includes the human protein ATM (ataxia telangiectasia mutated) and the yeast protein Tel1. Using Q-FISH (quantitative fluorescence in situ hybridization), we show here that scid mice from four different genetic backgrounds have, on average, 1.5-2 times longer telomeres than those of corresponding wild-type mice. Our results point to the possibility that DNA-PKcs may, directly or indirectly, be involved in telomere length regulation in mammalian cells.     

Rearrangement of antigen receptor genes is defective in mice with severe combined immune deficiency

A process unique to lymphocyte differentiation is the rearrangement of genes encoding antigen-specific receptors on B and T cells. A mouse mutant (C.B-17scid) with severe combined immune deficiency, i.e., that lacks functional B and T cells, shows no evidence of such gene rearrangements. However, rearrangements were detected in Abelson murine leukemia virus-transformed bone marrow cells and in spontaneous thymic lymphomas from C.B-17scid mice. Most of these rearrangements were abnormal: approximately 80% of Igh rearrangements deleted the entire Jh region, and approximately 60% of TCR beta rearrangements deleted the entire J beta 2 region. The deletions appeared to result from faulty D-to-J recombination. No such abnormal rearrangements were detected in transformed tissues from control mice. The scid mutation may adversely affect the recombinase system catalyzing the assembly of antigen receptor genes in developing B and T lymphocytes.     

Defective repair of radiation-induced chromosomal damage in scid/scid mice.

The murine severe combined immunodeficiency (scid) mutation interferes with normal recombination of immunoglobulin and T-cell receptor genes. This immunologic defect results in a lack of fully differentiated B and T cells in scid/scid mice. Animals homozygous for the scid mutation also display increased sensitivity to the damaging effects of ionizing radiation. We report here our observations of high frequencies of radiation-induced chromatid interchanges and intrachanges in bone marrow cells and fibroblasts from scid/scid mice. The presence of these aberrant chromosome structures suggests that a delay in strand rejoining underlies the increased sensitivity of scid/scid mice to ionizing radiation. The scid mutation may provide important clues for understanding the relationship between mitotic recombination and DNA repair in higher eukaryotic cells.     

Mutations in the p53 and scid genes do not cooperate in lymphomagenesis in doubly heterozygous mice

Analysis of double mutant mice of the p53 and scid genes, which have a combination of cell cycle checkpoint/apoptosis and DNA repair defects, shows that the latter defect synergistically enhances lymphoma development with loss of the former function. These mice lack the ability to eliminate lymphocytes predisposed to neoplastic transformation resulting from faulty antigen receptor gene rearrangement. Here we examine the cooperativity in double heterozygotes of p53 and scid in which normal development of lymphocytes is not impaired. MSM mice carrying a p53-knockout allele were crossed with BALB/c mice heterozygous for the scid locus and 129 offspring were obtained. They were subjected to gamma-ray irradiation, 84 thymic lymphomas being generated. The tumors and host mice were genotyped of p53 and scid. Among 42 mice developing p53-deficient lymphomas, scid/+ and +/+ genotypes did not provide difference in onset and latency. Besides, allelic loss of the Scid gene occurred at a high frequency in those lymphomas but the loss exhibited no allelic bias. The results suggest that the scid/+ genotype is not a modifier of loss of p53 function in the double heterozygotes.     

Biased T-cell receptor delta element recombination in scid thymocytes.

Thymocytes in mutant mice with severe combined immunodeficiency (scid thymocytes) show ongoing recombination of some T-cell receptor delta gene elements, generating signal joints quantitatively and qualitatively indistinguishable from those in wild-type fetal thymocytes. Excised D delta 2-J delta 1 and D delta 1-D delta 2 rearrangements are detectable at levels equivalent to or greater than those in thymocytes from wild-type mice on fetal day 15. Signal junctional modification, shown here to occur frequently in wild-type adult but not newborn excised D delta 2-J delta 1 junctions, can occur normally in adult scid thymocytes. Excised D delta 1-D delta 2 scid junctions, similar to wild-type thymocytes, include pseudonormal coding junctions as well as signal junctions. Inversional D delta 1-D delta 2 rearrangements, generating conventional hybrid junctions, are also reproducibly detectable in scid thymus DNA. These hybrids, unlike those reported for artificial recombination constructs, do not show extensive nucleotide loss. In contrast to the normal or high incidences of D delta 1-, D delta 2-, and J delta 1-associated signal junctions in scid thymocytes, V delta 1, V gamma 3, and V gamma 1.2 signal products are undetectable in scid thymocytes or are detectable at levels at least 10-fold lower than the levels in wild-type fetal thymocytes. These findings confirm biased T-cell receptor element recombination by V(D)J recombinase activity of nontransformed scid thymocytes and indicate that analysis of in vivo-mediated gene rearrangements is important for full understanding of how the scid mutation arrests lymphocyte development.     

T-cell antigen receptor studies in mice expressing the lpr genetic defect

Mice having the lpr genetic defect bear an expanding lymphoid T-cell population with unusual cell surface characteristics. Using SDS-PAGE analysis, the T cells from lpr mice were shown to have two forms of the T-cell antigen receptor. A conventional (nonreduced) 90-kDa receptor and a lighter 70- to 85-kDa form were both detected. The 70- to 85-kDa antigen receptor was also shown to be present on lpr thymocytes. Only the normal 90-kDa receptor was found in MRL-++ mice. Treatment of the receptor with N-glycanase shows that the 70- to 85-kD form may be a product of abnormal glycosylation. The low-molecular-weight antigen receptor on lpr T cells is unusual and might contribute in some way to the disordered immunoregulation and autoimmunity that occur spontaneously in these mice.