Molecular structural analysis of 417 HPRT mutations induced by restriction endonucleases in Chinese hamster ovary (CHO) cells

Contact dermatitis

Contact dermatitis (CD) is among the most common inflammatory dermatological conditions and includes allergic CD, photoallergic CD, irritant CD, photoirritant CD (also called phototoxic CD) and protein CD. Occupational CD can be of any type and is the most prevalent occupational skin disease. Each CD type is characterized by different immunological mechanisms and/or requisite exposures. Clinical manifestations of CD vary widely and multiple subtypes may occur simultaneously. The diagnosis relies on clinical presentation, thorough exposure assessment and evaluation with techniques such as patch testing and skin-prick testing. Management is based on patient education, avoidance strategies of specific substances, and topical treatments; in severe or recalcitrant cases, which can negatively affect the quality of life of patients, systemic medications may be needed.

A Bystander Effect Observed in Boron Neutron Capture Therapy: A Study of the Induction of Mutations in the HPRT Locus

PURPOSE

To investigate bystander mutagenic effects induced by alpha-particles during boron neutron capture therapy, we mixed cells that were electroporated with borocaptate sodium (BSH), which led to the accumulation of (10)B inside the cells, and cells that did not contain the boron compound. The BSH-containing cells were irradiated with alpha-particles produced by the 10B(n,alpha)7Li reaction, whereas cells without boron were affected only by the 1H(n,gamma)2H and 14N(n,rho)14C reactions.

METHODS AND MATERIALS

The lethality and mutagenicity measured by the frequency of mutations induced in the hypoxanthine-guanine phosphoribosyltransferase locus were examined in Chinese hamster ovary cells irradiated with neutrons (Kyoto University Research Reactor: 5 MW). Neutron irradiation of 1:1 mixtures of cells with and without BSH resulted in a survival fraction of 0.1, and the cells that did not contain BSH made up 99.4% of the resulting cell population. The molecular structures of the mutations were determined using multiplex polymerase chain reactions.

RESULTS

Because of the bystander effect, the frequency of mutations increased in the cells located nearby the BSH-containing cells compared with control cells. Molecular structural analysis indicated that most of the mutations induced by the bystander effect were point mutations and that the frequencies of total and partial deletions induced by the bystander effect were less than those induced by the original neutron irradiation.

CONCLUSION

These results suggested that in boron neutron capture therapy, the mutations caused by the bystander effect and those caused by the original neutron irradiation are induced by different mechanisms.

Intermediate DNA repair activity associated with the 322delG allele of the fanconi anemia complementation group C gene

Fanconi anemia (FA) is an autosomal recessive disorder associated with pancytopenia and cancer susceptibility. The disorder is heterogeneous, with at least nine complementation groups having been identified. Several recent studies have suggested that defective plasmid DNA end-joining is a consistent feature of FA cells. It was therefore surprising to discover a strain of fibroblasts from an FA patient that possessed wild-type plasmid DNA end-joining activity. Unlike other FA strains, these fibroblasts have wild-type levels of homologous DNA recombination activity and are relatively insensitive to restriction endonuclease-induced death. Interestingly, while end-joining in a number of FA fibroblast strains belonging to complementation groups A, C, and D2 was approximately 70% precise, end-joining in this latter strain of fibroblasts was more than 95% imprecise. Analysis revealed that these latter cells harbored an allele of the FA C gene, referred to as 322delG, that encodes an amino-terminal truncated protein. The relative rarity of this allele precluded the analysis of other FA fibroblast strains; however, studies revealed that overexpression of this allele in normal cells recapitulated the DNA end-joining phenotype seen in the 322delG FA fibroblast strain. These results indicate that DNA end-joining in fibroblasts expressing the 322delG allele of the FA-C gene in fibroblasts is highly imprecise; however, the DNA repair efficiency of these cells is more normal than that commonly associated with FA fibroblasts. This conclusion is intriguing, since a number of reports have suggested that patients harboring this allele exhibit a milder clinical course than do individuals with other alleles of the FA-C gene.

HPRT mutants induced in bystander cells by very low fluences of alpha particles result primarily from point mutations

We have shown previously that damage signals may be transmitted from irradiated cells to nonirradiated cells in monolayer cultures, leading to changes in gene expression and an enhanced frequency of mutations in these "bystander" cells. The present study was designed to test the hypothesis that mutations occurring in bystander cells result from a different mechanism than those occurring in irradiated cells, and thus show differences in molecular structure. Structural changes in the HPRT gene of Chinese hamster ovary (CHO) cells were determined by multiplex PCR analysis. A total of 790 mutant clones derived from monolayer cultures exposed to mean doses of 0, 0.5 or 10 cGy of alpha-particle radiation (0, 3% or 44%, respectively, of nuclei traversed by one or more alpha particles) were examined. Whereas mutations induced by 10 cGy included a high frequency of deletions, nearly all mutations occurring in bystander cells in cultures irradiated with 0.5 cGy involved point mutations, confirming our hypothesis that they are induced by a different mechanism.

Multiple manifestations of X-ray-induced genomic instability in Chinese hamster ovary (CHO) cells

Carcinogenesis is postulated to follow a multistep cascade in which the first genetic event may destabilize cells and thereby facilitate the induction of subsequent mutations within the same cell. It has recently been shown that exposure to ionizing radiation can in itself induce a persistent, heritable genetic instability in cells. To further investigate this phenomenon, we utilized a mutationally unstable population derived from a single Chinese hamster ovary (CHO) cell that survived X irradiation. We exposed these cells to a second dose of radiation, selected hypoxanthine phosphoribosyl transferase (HPRT) mutant subclones, and identified the type of mutations involved. We found complete deletions, continuous tract partial deletions, single-exon deletions, discontinuous-exon deletions ("skip mutations"), and point mutations (changes of less than 100 bp) among the isolated HPRT mutants. We hypothesized that the skip mutation clones might be more likely to demonstrate genomic instability. To test this hypothesis, mutant subclones were screened for three markers of genetic instability: alteration of minisatellite sequences, change in telomere length, and induction of chromosomal aberrations. Clones with skip mutations and single-exon deletions possessed elevated frequencies of minisatellite alterations and chromosomal aberrations, particularly rings and dicentrics. All mutant clones showed longer telomere terminal restriction fragment lengths than did wild-type cells. These results are consistent with the hypothesis that irradiation may induce a global instability phenotype, since the multiple alterations observed are mechanistically distinct, heritable cellular modifications that arose in the clonogenic progeny of the irradiated cells. Skip mutations may be one manifestation of this instability, but their presence was not specifically associated with the other genetic alterations.

Molecular structural analysis of HPRT mutations induced by thermal and epithermal neutrons in Chinese hamster ovary cells

Chinese hamster ovary (CHO) cells were exposed to thermal and epithermal neutrons, and the occurrence of mutations at the HPRT locus was investigated. The Kyoto University Research Reactor (KUR), which has been improved for use in neutron capture therapy, was the neutron source. Neutron energy spectra ranging from nearly pure thermal to epithermal can be chosen using the spectrum shifters and thermal neutron filters. To determine mutant frequency and cell survival, cells were irradiated with thermal and epithermal neutrons under three conditions: thermal neutron mode, mixed mode with thermal and epithermal neutrons, and epithermal neutron mode. The mutagenicity was different among the three irradiation modes, with the epithermal neutrons showing a mutation frequency about 5-fold that of the thermal neutrons and about 1.5-fold that of the mixed mode. In the thermal neutron and mixed mode, boron did not significantly increase the frequency of the mutants at the same dose. Therefore, the effect of boron as used in boron neutron capture therapy (BNCT) is quantitatively minimal in terms of mutation induction. Over 300 independent neutron-induced mutant clones were isolated from 12 experiments. The molecular structure of HPRT mutations was determined by analysis of all nine exons by multiplex polymerase chain reaction. In the thermal neutron and mixed modes, total and partial deletions were dominant and the fraction of total deletions was increased in the presence of boron. In the epithermal neutron mode, more than half of the mutations observed were total deletions. Our results suggest that there are clear differences between thermal and epithermal neutron beams in their mutagenicity and in the structural pattern of the mutants that they induce. Mapping of deletion breakpoints of 173 partial-deletion mutants showed that regions of introns 3-4, 7/8-9 and 9-0 are sensitive to the induction of mutants by neutron irradiation.

Dimethyl sulfoxide protects against thermal and epithermal neutron-induced cell death and mutagenesis of Chinese hamster ovary (CHO) cells

PURPOSE

To investigate the protective effects of dimethyl sulfoxide (DMSO) on cell killing and mutagenicity at the HPRT locus in Chinese hamster ovary (CHO) cells against thermal and epithermal neutrons produced at the Kyoto University Research (KUR) reactor.

METHODS AND MATERIALS

DMSO was added to cells 15 min before irradiation and removed 15 min after irradiation. Cells were irradiated by thermal and epithermal neutrons with or without boron at 10 ppm. The biological endpoint of cell survival was measured by colony formation assay. The mutagenicity was measured by the mutant frequency in the HPRT locus. A total of 378 independent neutron-induced mutant clones were isolated in separate experiments. The molecular structure of HPRT mutations was determined by analysis by multiplex polymerase chain reaction of all nine exons.

RESULTS

The D(0) values of epithermal and thermal neutrons in three different modes, i.e., thermal, epithermal, and mixtures of thermal and epithermal, were 0.8-1.2 Gy. When cells were treated with DMSO, the D(0) values increased to 1.0-2.3, especially in the absence of boron. DMSO showed a protective effect against mutagenesis of the HPRT locus induced by epithermal and thermal neutron irradiation. After DMSO treatment, the mutagenicity was decreased, especially when the cells were irradiated in epithermal neutron mode. Molecular structure analysis indicated that total and partial deletions were dominant and the incidence of total deletions was increased in the presence of boron in the thermal neutron and mixed modes. In the epithermal neutron mode, more than half of the mutations were total deletions. When cells were treated with DMSO, the incidence of total deletions by thermal neutron irradiation with boron and epithermal irradiation decreased.

CONCLUSIONS

Our results suggest that DMSO has various protective effects against cytotoxic and mutagenic effects of thermal and epithermal neutrons, and that the extent of protection is reflected by the percentage of absorbed dose distribution for each neutron irradiation mode.

Mutations induced by DNA double-strand breaks: the influence of genomic site

The transgenic CHO cell line PL61, carrying a recombinant SV40-gpt gene, was treated with restriction endonucleases to assess mutagenesis from defined DNA double-strand breaks. Mutations in gpt were measured under two conditions: a stringent condition where selection ensured that the closely-linked neo gene was retained functionally intact, or a relaxed condition without the requirement for neo gene function. Despite testing 18 different restriction endonucleases with various numbers of potential break-sites within the transgene, mutations were only found under relaxed selection conditions. These mutations commonly led to complete loss of the transgene, suggesting that large deletions predominate when selection is relaxed. It is argued, in comparison to mutation data for other genomic sites in CHO cells, that variations in the 'effective target size' for mutagenesis may explain the response of the transgene under different conditions.