Effects of Bloom's syndrome fibroblasts on genetic recombination and mutagenesis of herpes simplex virus type 1

The hMre11/hRad50 protein complex and Nijmegen breakage syndrome: linkage of double-strand break repair to the cellular DNA damage response

Nijmegen breakage syndrome (NBS) is an autosomal recessive disorder characterized by increased cancer incidence, cell cycle checkpoint defects, and ionizing radiation sensitivity. We have isolated the gene encoding p95, a member of the hMre11/hRad50 double-strand break repair complex. The p95 gene mapped to 8q21.3, the region that contains the NBS locus, and p95 was absent from NBS cells established from NBS patients. p95 deficiency in these cells completely abrogates the formation of hMre11/hRad50 ionizing radiation-induced foci. Comparison of the p95 cDNA to the NBS1 cDNA indicated that the p95 gene and NBS1 are identical. The implication of hMre11/hRad50/p95 protein complex in NBS reveals a direct molecular link between DSB repair and cell cycle checkpoint functions.

Different capacities for recombination in closely related human lymphoblastoid cell lines with different mutational responses to X-irradiation

WIL2-NS and TK6 are two distinct human lymphoblast cell lines derived from a single male donor. WIL2-NS cells are significantly more resistant to the cytotoxic effects of X-irradiation but considerably more sensitive to induced mutation. In an effort to determine the mechanistic basis for these differences, we analyzed the physical structures of thymidine kinase (tk)-deficient mutants isolated after X-ray treatment of tk heterozygotes derived from TK6 and the more mutable WIL2-NS. Southern analysis showed that while 84% of TK6-derived mutants had arisen by loss of heterozygosity (LOH), all 106 mutants from WIL2-NS derivatives arose with LOH at tk and all but one showed LOH at other linked loci on chromosome 17. We adapted a fluorescence in situ hybridization technique to distinguish between LOH due to deletion, which results in retention of only one tk allele, and LOH due to a mechanism involving the homologous chromosome (e.g., recombination), which results in the retention of two alleles. Among the LOH mutants derived that were analyzed in this way, 9 of 26 from WIL2-NS and 11 of 17 from TK6 cell lines arose by deletion. The remaining mutants retained two copies of the tk gene and thus arose by a mechanism involving the homologous allele. Since many of these mutants arising by a homologous mechanism retained partial heterozygosity of chromosome 17, they must have arisen by recombination or gene conversion, and not chromosome loss and reduplication. Finally, the recombinational capacities of WIL2-NS and TK6 were compared in transfection assays with plasmid recombination substrates. Intermolecular recombination frequencies were greater in WIL2-NS than in TK6. These data are consistent with a model suggesting that a recombinational repair system is functioning at a higher level in WIL2-NS than in TK6; the greater mutability of the tk locus in WIL2-NS results from more frequent inter- and intramolecular recombination events.

Different capacities for recombination in closely related human lymphoblastoid cell lines with different mutational responses to X-irradiation

WIL2-NS and TK6 are two distinct human lymphoblast cell lines derived from a single male donor. WIL2-NS cells are significantly more resistant to the cytotoxic effects of X-irradiation but considerably more sensitive to induced mutation. In an effort to determine the mechanistic basis for these differences, we analyzed the physical structures of thymidine kinase (tk)-deficient mutants isolated after X-ray treatment of tk heterozygotes derived from TK6 and the more mutable WIL2-NS. Southern analysis showed that while 84% of TK6-derived mutants had arisen by loss of heterozygosity (LOH), all 106 mutants from WIL2-NS derivatives arose with LOH at tk and all but one showed LOH at other linked loci on chromosome 17. We adapted a fluorescence in situ hybridization technique to distinguish between LOH due to deletion, which results in retention of only one tk allele, and LOH due to a mechanism involving the homologous chromosome (e.g., recombination), which results in the retention of two alleles. Among the LOH mutants derived that were analyzed in this way, 9 of 26 from WIL2-NS and 11 of 17 from TK6 cell lines arose by deletion. The remaining mutants retained two copies of the tk gene and thus arose by a mechanism involving the homologous allele. Since many of these mutants arising by a homologous mechanism retained partial heterozygosity of chromosome 17, they must have arisen by recombination or gene conversion, and not chromosome loss and reduplication. Finally, the recombinational capacities of WIL2-NS and TK6 were compared in transfection assays with plasmid recombination substrates. Intermolecular recombination frequencies were greater in WIL2-NS than in TK6. These data are consistent with a model suggesting that a recombinational repair system is functioning at a higher level in WIL2-NS than in TK6; the greater mutability of the tk locus in WIL2-NS results from more frequent inter- and intramolecular recombination events.

Herpes simplex virus type 1 recombination: the Uc-DR1 region is required for high-level a-sequence-mediated recombination

The a sequences of herpes simplex virus type 1 are believed to be the cis sites for inversion events that generate four isomeric forms of the viral genome. Using an assay that measures deletion of a beta-galactosidase gene positioned between two directly repeated sequences in plasmids transiently maintained in Vero cells, we had found that the a sequence is more recombinogenic than another sequence of similar size. To investigate the basis for the enhanced recombination mediated by the a sequence, we examined plasmids containing direct repeats of approximately 350 bp from a variety of sources and with a wide range of G+C content. We observed that all of these plasmids show similar recombination frequencies (3 to 4%) in herpes simplex virus type 1-infected cells. However, recombination between directly repeated a sequences occurs at twice this frequency (6 to 10%). In addition, we find that insertion of a cleavage site for an a-sequence-specific endonuclease into the repeated sequences does not appreciably increase the frequency of recombination, indicating that the presence of endonuclease cleavage sites within the a sequence does not account for its recombinogenicity. Finally, by replacing segments of the a sequence with DNA fragments of similar length, we have determined that only the 95-bp Uc-DR1 segment is indispensable for high-level a-sequence-mediated recombination.

In-vivo assessment of DNA ligation efficiency and fidelity in cells from patients with Fanconi's anemia and other cancer-prone hereditary disorders

We developed a host cell DNA ligation assay, in which we transfected linearized plasmid pZ189 into human lymphoblasts or fibroblasts in order to assess the efficiency and accuracy of DNA ligation within these host cells. We used cell lines from patients with Fanconi's anemia and other chromosome breakage or instability syndromes (Bloom's syndrome, ataxia telangiectasia, Werner's syndrome). With the Fanconi's anemia lymphoblast line GM8010 we did not find a reduced, but a slightly hypermutable DNA ligation. Mutation analysis revealed a unique 7.9-12.5-fold increase in insertions or complex mutations. With cells from the other chromosome breakage/instability syndromes we also found a hypermutable and/or reduced DNA ligation. An impaired DNA ligation might be a common molecular mechanism of genetic instability in these disorders.

Two types of DNA ligase I activity in lymphoblastoid cells from patients with Bloom's syndrome

DNA ligases I and II were separated by hydroxylapatite (HA) column chromatography in cell-free extracts of lymphoblastoid cell lines (LCLs) derived from two unrelated patients with Bloom's syndrome (BS) and two healthy individuals. The specific activity of ligase I from the crude extract was consistently lower in GM3403, a BS LCL from an Ashkenazi Jewish patient, than in normal control LCLs. By contrast, the level of ligase I activity in BSL-2KA, another BS LCL derived from a Japanese patient, was equivalent to those in normal LCLs, although GM3403 and BSL-2KA shared the feature of exceedingly high frequency of spontaneous sister-chromatid exchange. The levels of total ligase activity in crude extracts without the separation into the two forms, however, were approximately two-fold higher for the two BS LCLs than for the normal LCLs. Partial purification by chromatography on a DEAE-cellulose 23 column and a phosphocellulose column did not affect the superiority of the two BS LCLs over the normal LCLs in the specific activity of the total ligases. Nonetheless, subsequent application to an HA column again resulted in much less elevation of the specific activity of ligase I for GM3403 than for BSL-2KA and control LCLs. The levels of ligase II activity, accounting for 4-13% of total ligase activity, were similar among the LCLs examined. Irrespective of the extent of purification, essentially no difference in the heat lability of DNA ligase I was detected among the four LCLs. These findings suggest that there may exist among BS LCLs at least two types of subtle abnormality of DNA ligase I itself and/or a putative substance modulating the enzyme function.

Molecular evidence that homologous recombination occurs in proliferating human somatic cells

A strategy has been developed to detect and characterize certain heritable genomic alterations that occur as cells proliferate in vitro. Multiple subclones of cells were isolated from two clonal lymphoblastoid cell lines--one from a boy with Bloom's syndrome (BS), a cancer-predisposing condition known to feature excessive somatic mutation, the other from a normal man. The DNAs from the cell lines were hybridized to a panel of probes that can detect restriction fragment length polymorphisms, and the patterns of polymorphism in the primary clones were compared with that in each of the secondary clones. In one of the BS secondary clones three loci, positioned distally on the long arm of chromosome 3 and that are heterozygous in the donor and all other cell lines derived from the primary clone, had lost heterozygosity and apparently had become homozygous; in contrast, heterozygous loci more proximal on 3q had retained their heterozygosity, as had those on 3p. Taking into account the pattern of chromosome instability uniquely characteristic of BS, the most plausible explanation for the alterations in the altered clone is that somatic recombination had occurred in vitro, via homologous chromatid interchange. Such spontaneous recombinational events in nonneoplastic, nonmutagenized cells may contribute to the high cancer incidence in BS and, by analogy, to cancer that arises in the general population.

Ability of structurally related polycyclic aromatic carcinogens to induce homologous recombination between duplicated chromosomal sequences in mouse L cells

To investigate the role of DNA damage in the induction of homologous recombination in mammalian cells, a series of structurally related, polycyclic aromatic carcinogens, i.e., 1-nitrosopyrene (1-NOP), N-acetoxy-2-acetylaminofluorene (N-AcO-AAF), and 4-nitroquinoline 1-oxide (4-NQO), were compared for their ability to cause intrachromosomal homologous recombination between two herpes simplex virus thymidine kinase (Htk) genes stably integrated in the genome of a tk- mouse L cell strain 333 M. Each Htk gene contains an 8-bp XhoI linker inserted at a unique site so that expression of a functional Htk enzyme requires a productive recombinational event between the two nonfunctional genes. Each carcinogen caused a dose-dependent increase in the frequency of recombination. The results were compared to what had been found previously for a structurally related carcinogen, (+/-)-7 beta, 8 alpha-dihydroxy-9 alpha, 10 alpha-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene (BPDE). As a function of concentration, BPDE was the most active agent, followed by 4-NQO, and 1-NOP, and then N-AcO-AAF. When compared on the basis of equal cell killing, the most efficient carcinogen was 1-NOP, followed by N-AcO-AAF and BPDE, and then 4-NQO. Use of tritium-labeled compounds to determine the frequency of recombination as a function of the number of adducts initially bound to DNA showed that the most effective agent was BPDE, followed by 1-NOP and 4-NQO, and then N-AcO-AAF (ratio, 6.6:2.5:1.8:1.0). To determine if these differences in recombinagenic effectiveness reflected different rates of removal of the adducts from DNA, we measured the percentage of DNA adducts removed during the 24-h period post treatment and found that 1-NOP, 4-NQO and N-AcO-AAF residues were removed at approximately the same rate, i.e., 25%-30% off. Cellular analysis of a series of independent recombinants indicated that approximately 82% of the recombinational events induced by each agent were consistent with gene conversion. DNA-DNA hybridization analysis confirmed this, and showed that each recombinant tested contained an XhoI-resistant (wild-type) Htk gene; with the majority retaining the Htk gene duplication, consistent with nonreciprocal transfer of wild-type genetic information. In the rest, only a single copy of the Htk gene remained, reflecting a single reciprocal exchange within a chromatid or a single unequal exchange between sister chromatids.

Bloom's syndrome. XII. Report from the Registry for 1987

Bloom's syndrome has been known as a clinical entity for 34 years. Careful records of cases diagnosed throughout the world have been maintained since its recognition as an entity, and most instances of cytologically verified Bloom's syndrome have been accessioned to what has been referred to as the Bloom's Syndrome Registry since the mid-1960s. Presented here is the fourth in a series of progress reports from the Registry of information accumulated during this long-term surveillance of affected families, along with mention of selected recent advances that have been made in the understanding of the syndrome. 130 persons had been accessioned to the Registry by the end of 1987; 96 of these were alive, their mean age being 18.9 years. Although a number of clinical complications occur in Bloom's syndrome, the most important is malignant neoplasia. In the 130 persons in the Registry, 57 malignant neoplasms had been detected, the mean age at diagnosis being 24.8 years. Neoplasia in Bloom's syndrome is noteworthy not only because of its frequency and exceptionally early age of emergence but for its variety of histological types and sites of origin.

Evidence for increased in vivo mutation and somatic recombination in Bloom's syndrome

The glycophorin A assay was used to estimate the frequency of mutations that accumulate in vivo in somatic cells of persons with Bloom's syndrome (BS). This assay measures the frequency in persons of blood type MN of variant erythrocytes that lack the expression of one allelic form of glycophorin A, presumably due to mutational or recombinational events in erythroid precursor cells. Samples of blood from persons with BS showed dramatic 50- to 100-fold increases in the frequency of variants of three types, those with a hemizygous phenotype, those with a homozygous phenotype, and those with what appears to be partial loss of the expression of one locus. The high frequency of homozygous variants, genetic evidence for altered allelic segregation of a specific biochemical locus, provides evidence for increased somatic crossing-over in vivo in BS. An increased generation of functional hemizygosity and homozygosity in their somatic cells may play an important role in the extreme cancer risk of persons with BS.