X rays induce interallelic homologous recombination at the human thymidine kinase gene

Reduction of Delayed Homologous Recombination by Induction of Radioadaptive Response in RaDR-GFP Mice (Yonezawa Effect): An Old Player With a New Role

Radiotherapy (RT) treats cancer effectively with high doses of ionizing radiation (IR) to killing cancer cells and shrinking tumors while bearing the risk of developing different side effects, including secondary cancer, which is most concerning for long-term health consequences. Genomic instability (GI) is a characteristic of most cancer cells, and IR-induced GI can manifest as delayed homologous recombination (HR). Radioadaptive response (RAR) is capable of reducing genotoxicity, cell transformation, mutation, and carcinogenesis, but the rational evidence describing its contributions to the reduction of radiation risk, in particular, carcinogenesis, remains fragmented. In this work, to investigate the impact of RAR on high-dose, IR-induced GI measured as delayed HR, the frequency of recombinant cells was comparatively studied under RAR-inducible and -uninducible conditions in the nucleated cells in hematopoietic tissues (bone marrow and spleen) using the Rosa26 Direct Repeat-green fluorescent protein (RaDR-GFP) homozygote mice. Results demonstrated that the frequency of recombinant cells was significantly lower in hematopoietic tissues under RAR-inducible condition. These findings suggest that reduction in delayed HR may be at least a part of the mechanisms underlying decreased carcinogenesis by RAR, and application of RAR would contribute to a more rigorous and scientifically grounded system of radiation protection in RT.

Low- and High-LET Ionizing Radiation Induces Delayed Homologous Recombination that Persists for Two Weeks before Resolving

Genome instability is a hallmark of cancer cells and dysregulation or defects in DNA repair pathways cause genome instability and are linked to inherited cancer predisposition syndromes. Ionizing radiation can cause immediate effects such as mutation or cell death, observed within hours or a few days after irradiation. Ionizing radiation also induces delayed effects many cell generations after irradiation. Delayed effects include hypermutation, hyper-homologous recombination, chromosome instability and reduced clonogenic survival (delayed death). Delayed hyperrecombination (DHR) is mechanistically distinct from delayed chromosomal instability and delayed death. Using a green fluorescent protein (GFP) direct repeat homologous recombination system, time-lapse microscopy and colony-based assays, we demonstrate that DHR increases several-fold in response to low-LET X rays and high-LET carbon-ion radiation. Time-lapse analyses of DHR revealed two classes of recombinants not detected in colony-based assays, including cells that recombined and then senesced or died. With both low- and high-LET radiation, DHR was evident during the first two weeks postirradiation, but resolved to background levels during the third week. The results indicate that the risk of radiation-induced genome destabilization via DHR is time limited, and suggest that there is little or no additional risk of radiation-induced genome instability mediated by DHR with high-LET radiation compared to low-LET radiation.

Genome Engineering with TALE and CRISPR Systems in Neuroscience

Recent advancement in genome engineering technology is changing the landscape of biological research and providing neuroscientists with an opportunity to develop new methodologies to ask critical research questions. This advancement is highlighted by the increased use of programmable DNA-binding agents (PDBAs) such as transcription activator-like effector (TALE) and RNA-guided clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR associated (Cas) systems. These PDBAs fused or co-expressed with various effector domains allow precise modification of genomic sequences and gene expression levels. These technologies mirror and extend beyond classic gene targeting methods contributing to the development of novel tools for basic and clinical neuroscience. In this Review, we discuss the recent development in genome engineering and potential applications of this technology in the field of neuroscience.

DNA copy-number instability in low-dose gamma-irradiated TK6 lymphoblastoid clones

Genomic instability that might occur early during low-dose, fractionated radiation exposures may be traceable in radiogenic compared to spontaneous cancers. Using a human 18K cDNA microarray-based comparative genome hybridization protocol, we measured changes in DNA copy number at over 14,000 loci in nine low-dose (137)Cs gamma-irradiated (acute exposure to 10 cGy/day x 21 days) and nine unirradiated TK6 clones and estimated locus-specific copy-number differences between them. Radiation induced copy-number hypervariability at thousands of loci across all chromosomes, with a sevenfold increase in low-level, randomly positioned DNA gains. Recurrent gains at 40 loci occurred among irradiated clones and were distributed nonrandomly across the genome, with the highest densities in 3q, 13q and 20q at sites that were hypodiploid without irradiation. Another nonrandomly distributed set of 94 loci exhibited relative recurrent gains from a hypodiploid state to a diploid state, suggesting hemizygous-to-homozygous transitions. Frequently recurring losses at 57 loci were concentrated on the single X-chromosome but were sparsely distributed at 0-2 loci per autosome. These results suggest induced mitotic homologous recombination as a possible mechanism of low-dose radiation-induced genomic instability. Genomic instability induced in TK6 cells resembled that seen in radiogenic tumors and suggests a way that radiation could induce genomic instability in preneoplastic cells.

High frequency induction of mitotic recombination by ionizing radiation in Mlh1 null mouse cells

Mitotic recombination in somatic cells involves crossover events between homologous autosomal chromosomes. This process can convert a cell with a heterozygous deficiency to one with a homozygous deficiency if a mutant allele is present on one of the two homologous autosomes. Thus mitotic recombination often represents the second mutational step in tumor suppressor gene inactivation. In this study we examined the frequency and spectrum of ionizing radiation (IR)-induced autosomal mutations affecting Aprt expression in a mouse kidney cell line null for the Mlh1 mismatch repair (MMR) gene. The mutant frequency results demonstrated high frequency induction of mutations by IR exposure and the spectral analysis revealed that most of this response was due to the induction of mitotic recombinational events. High frequency induction of mitotic recombination was not observed in a DNA repair-proficient cell line or in a cell line with an MMR-independent mutator phenotype. These results demonstrate that IR exposure can initiate a process leading to mitotic recombinational events and that MMR function suppresses these events from occurring.

"Recombomice": the past, present, and future of recombination-detection in mice

Homology directed repair (HDR) provides an efficient strategy for repairing and tolerating many types of DNA lesions, such as strand breaks, base damage, and crosslinks. Recombinational repair and lesion avoidance pathways that involve homology searching are integral to normal DNA replication. Indeed, it is estimated that at least ten HDR events take place each time a mammalian cell divides. HDR is associated with the transfer and exchange of DNA sequences. Usually, homologous sequences are aligned perfectly and flanking sequences are not exchanged. However, those sequence misalignments and exchanges that do occur can lead to rearrangements that contribute to cancer (e.g. deletions, inversions, translocations or loss of heterozygosity (LOH)). In order to reveal genetic and environmental factors that modulate HDR in mammals, several approaches have been used to detect recombination events in vivo. Here, we briefly review three methods for detecting homologous recombination in mice, namely: sister chromatid exchange (SCE), LOH, and recombination at tandem repeats. We conclude with a more detailed description of the recently developed "Fluorescent Yellow Direct Repeat" (FYDR) mouse model, which exploits enhanced yellow fluorescent protein (EYFP) for detecting mitotic homologous recombination in vivo. Applications of the FYDR mice are described, as well as the broader potential for using fluorescent proteins to detect recombination in various tissues/cell types in vivo.

In vivo Recombination After Chronic Damage Exposure Falls to Below Spontaneous Levels in “Recombomice”

All forms of cancer are initiated by heritable changes in gene expression. Although point mutations have been studied extensively, much less is known about homologous recombination events, despite its role in causing sequence rearrangements that contribute to tumorigenesis. Although transgenic mice that permit detection of point mutations have provided a fundamental tool for studying point mutations in vivo, until recently, transgenic mice designed specifically to detect homologous recombination events in somatic tissues in vivo did not exist. We therefore created fluorescent yellow direct repeat mice, enabling automated detection of recombinant cells in vivo for the first time. Here, we show that an acute dose of ionizing radiation induces recombination in fluorescent yellow direct repeat mice, providing some of the first direct evidence that ionizing radiation induces homologous recombination in cutaneous tissues in vivo. In contrast, the same total dose of radiation given under chronic exposure conditions suppresses recombination to levels that are significantly below those of unexposed animals. In addition, global methylation is suppressed and key DNA repair proteins are induced in tissues from chronically irradiated animals (specifically AP endonuclease, polymerase β, and Ku70). Thus, increased clearance of recombinogenic lesions may contribute to suppression of homologous recombination. Taken together, these studies show that fluorescent yellow direct repeat mice provide a rapid and powerful assay for studying the recombinogenic effects of both short-term and long-term exposure to DNA damage in vivo and reveal for the first time that exposure to ionizing radiation can have opposite effects on genomic stability depending on the duration of exposure.

Gamma-ray mutagenesis studies in a new human-hamster hybrid, A(L)CD59(+/-), which has two human chromosomes 11 but is hemizygous for the CD59 gene

Kraemer, S. M., Vannais, D. B., Kronenberg, A., Ueno, A. and Waldren, C. A. Gamma-Ray Mutagenesis Studies in a New Human-Hamster Hybrid, A(L)CD59(+/-), which has Two Human Chromosomes 11 but is Hemizygous for the CD59 Gene. Radiat. Res. 156, 10-19 (2001). We have developed a human-CHO hybrid cell line, named A(L)CD59(+/-), which has two copies of human chromosome 11 but is hemizygous for the CD59 gene and the CD59 cell surface antigen that it encodes. Our previous studies used the A(L) and A(L)C hybrids that respectively contain one or two sets of CHO chromosomes plus a single copy of human chromosome 11. The CD59 gene at 11p13.5 and the CD59 antigen encoded by it are the principal markers used in our mutagenesis studies. The hybrid A(L)CD59(+/-) contains two copies of human chromosome 11, only one of which carries the CD59 gene. The incidence of CD59 (-) mutants (formerly called S1(-)) induced by (137)Cs gamma rays is about fivefold greater in A(L)CD59(+/-) cells than in A(L) cells. Evidence is presented that this increase in mutant yield is due to the increased induction of certain classes of large chromosomal mutations that are lethal to A(L) cells but are tolerated in the A(L)CD59(+/-) hybrid. In addition, significantly more of the CD59 (-) mutants induced by (137)Cs gamma rays in A(L)CD59(+/-) cells display chromosomal instability than in A(L) cells. On the other hand, the yield of gamma-ray-induced CD59 (-) mutants in A(L)CD59(+/-) cells is half that of the A(L)C hybrid, which also tolerates very large mutations but has only one copy of human chromosome 11. We interpret the difference in mutability as evidence that repair processes involving the homologous chromosomes 11 play a role in determining mutant yields. The A(L)CD59(+/-) hybrid provides a useful new tool for quantifying mutagenesis and shedding light on mechanisms of genetic instability and mutagenesis.

Interchromosomal gene conversion at an endogenous human cell locus

To examine the relationship between gene conversion and reciprocal exchange at an endogenous chromosomal locus, we developed a reversion assay in a thymidine kinase deficient mutant, TX545, derived from the human lymphoblastoid cell line TK6. Selectable revertants of TX545 can be generated through interchromosomal gene conversion at the site of inactivating mutations on each tk allele or by reciprocal exchange that alters the linkage relationships of inactivating polymorphisms within the tk locus. Analysis of loss of heterozygosity (LOH) at intragenic polymorphisms and flanking microsatellite markers was used to initially evaluate allelotypes in TK(+) revertants for patterns associated with either gene conversion or crossing over. The linkage pattern in a subset of convertants was then unambiguously established, even in the event of prereplicative recombinational exchanges, by haplotype analysis of flanking microsatellite loci in tk(-/-) LOH mutants collected from the tk(+/-) parental convertant. Some (7/38; 18%) revertants were attributable to easily discriminated nonrecombinational mechanisms, including suppressor mutations within the tk coding sequence. However, all revertants classified as a recombinational event (28/38; 74%) were attributed to localized gene conversion, representing a highly significant preference (P < 0.0001) over gene conversion with associated reciprocal exchange, which was never observed.

Transcriptional effects on double-strand break-induced gene conversion tracts

Transcription stimulates spontaneous homologous recombination, but prior studies have not investigated the effects of transcription on double-strand break (DSB)-induced recombination in yeast. We examined products of five ura3 direct repeat substrates in yeast using alleles that were transcribed at low or high levels. In each strain, recombination was stimulated by DSBs created in vivo at an HO site in one copy of ura3. Increasing transcription levels in donor or recipient alleles did not further stimulate DSB-induced recombination, nor did it alter the relative frequencies of conversion and deletion (pop-out) events. This result is consistent with the idea that transcription enhances spontaneous recombination by increasing initiation. Gene conversion tracts were measured using silent restriction fragment length polymorphisms (RFLPs) at approximately 100bp intervals. Transcription did not alter average tract lengths, but increased transcription in donor alleles increased both the frequency of promoter-proximal (5') unidirectional tracts and conversion of 5' markers. Increased transcription in recipient alleles increased the frequency of bidirectional tracts. We demonstrate that these effects are due to transcription per se, and not just transcription factor binding. These results suggest that transcription influences aspects of gene conversion after initiation, such as strand invasion and/or mismatch repair (MMR).

DNA damaging agents improve stable gene transfer efficiency in mammalian cells

Gene therapy is an evolving discipline which today relies primarily on viral systems for gene transfer. The primary reason that plasmid vectors have not been widely used for gene therapy trials is their relatively low rate of stable gene transfer. We show here that both ionizing irradiation and hydrogen peroxide can each increase the gene transfer efficiency of plasmids. Hydrogen peroxide improves gene transfer in a linear dose-dependent manner. At equitoxic doses, hydrogen peroxide improves gene transfer by 20-fold over untreated cells and approximately 5 times above that seen for radiation, and this improvement correlates with both the total amount of DNA damage induced and the amount of residual damage after 4 hr of repair. These data suggest that DNA damaging agents may be useful to improve human gene therapy.

Modelling haemopoietic stem cell division by analysis of mutant red cells

Data describing the number of human red cells mutated at the glycophorin A locus, measured flow cytometrically, are reported for 752 adults and 49 neonates. The variance increases with age more rapidly than the approximately linear increase in mean. It is postulated that this discrepancy is explained by the known property of asymmetric stem cell division, so that the division of a single mutant stem cell may result in zero, one or two progeny stem cells. A mathematical analysis allows description of this process with three parameters: stem cell number, mean division rate and mutation rate per division. The values of these parameters can not be deduced from the data presented here. However, estimates of either stem cell number or mutation rate from other sources enable deduction of the two other parameters. The mean number of divisions per stem cell per lifetime was estimated to be about 70. This analysis therefore implies that the rate at which blood cell telomeres shorten with age acts as a direct measure of stem cell turnover. Furthermore, it is argued that this low figure implies that mutations occurring during early life, including organogenesis, are relatively important in initiating stem cell-derived malignancy. Finally, the number of human stem cell divisions per lifetime is similar to shorter-lived mammals, suggesting this number is important in the ageing process.

Radiation carcinogenesis

Research on radiation carcinogenesis during the past 2 decades has focused on cellular and molecular mechanisms for the effects of radiation in mammalian cells. This paper will review several of these areas of research, as they may relate specifically to the induction of cancer by ionizing radiation. Knowledge of the critical DNA damage of biologic importance, and how this damage is repaired, will be discussed in relation to its role in the induction of mutations by radiation. The search for the initiating event in radiation carcinogenesis, as well as other genetic events that may be involved, is discussed in terms of the possible role of the activation of oncogenes or tumor suppressor genes and the loss of cell-cycle checkpoints. Finally, evidence will be described indicating that important genetic consequences of radiation may arise in cells that in themselves receive no direct nuclear irradiation. It has been shown that radiation can, by itself, induce a type of genomic instability in cells, which enhances the rate at which mutations and other genetic changes arise in the descendants of the irradiated cell after many generations of replication. Preliminary evidence has been presented that irradiation targeted to the cytoplasm yields a significant increase in the frequency of mutations. Finally, genetic events including the induction of mutations and changes in gene expression may occur in neighboring cells that receive no direct radiation exposure at all. This 'bystander effect' involves gap junction mediated cell-cell communication, and activation of the p53 damage response pathway. The possible role of these phenomena in radiation carcinogenesis is discussed.

The tumor suppressor p53 can reduce stable transfection in the presence of irradiation

The tumor suppressor p53 is believed to play an essential role in maintaining genome stability. Although it is currently unknown how p53 is involved in this important biological safeguard, several previous publications indicate that p53 can help to maintain genome integrity through the recombination-mediated DNA repair process. The integration of linearized plasmid DNA into the host chromosome utilizes the same repair process, and the frequency can be measured by clonogenic assays in which cells that were stably transfected by plasmid integration can be scored by their colony-forming abilities. To gain insight into whether p53 has a direct role in plasmid integration into the host chromosome, we determined the frequency of stable transfection with CHO cells expressing either wild-type or mutant p53 in the presence and absence of irradiation. We found that low-dose irradiation ( approximately 50 to 100 cGy) increased stable transfection frequencies in CHO cells regardless of their p53 status. However, the increase of transfection frequency was significantly lower in CHO cells expressing wild-type p53. Our data thus suggest that wild-type p53 can suppress plasmid DNA integration into the host genome. This p53 function may play a direct and significant role in maintaining genome stability.

Mutant p53 proteins stimulate spontaneous and radiation-induced intrachromosomal homologous recombination independently of the alteration of the transactivation activity and of the G1 checkpoint

We report here a systematic analysis of the effects of different p53 mutations on both spontaneous and radiation-stimulated homologous recombination in mouse L cells. In order to monitor different recombination pathways, we used both direct and inverted repeat recombination substrates. In each line bearing one of these substrates, we expressed p53 proteins mutated at positions: 175, 248 or 273. p53 mutations leading to an increased spontaneous recombination rate also stimulate radiation-induced recombination. The effect on recombination may be partially related to the conformation of the p53 protein. Moreover, p53 mutations act on recombination between direct repeats as well as between inverted repeats indicating that strand invasion mechanisms are stimulated. Although all of the p53 mutations affect the p53 transactivation activity measured on the WAF1 and MDM2 gene promoters, no correlation between the transactivation activity and the extent of homologous recombination can be drawn. Finally, some p53 mutations do not affect the G1 arrest after radiation but stimulate radiation-induced recombination. These results show that the role of p53 on transactivation and G1 cell cycle checkpoint is separable from its involvement in homologous recombination. A direct participation of p53 in the recombination mechanism itself is discussed.

Analysis of intrachromosomal homologous recombination in mammalian cell, using tandem repeat sequences

In all the organisms, homologous recombination (HR) is involved in fundamental processes such as genome diversification and DNA repair. Several strategies can be devised to measure homologous recombination in mammalian cells. We present here the interest of using intrachromosomal tandem repeat sequences to measure HR in mammalian cells and we discuss the differences with the ectopic plasmids recombination. The present review focuses on the molecular mechanisms of HR between tandem repeats in mammalian cells. The possibility to use two different orientations of tandem repeats (direct or inverted repeats) in parallel constitutes also an advantage. While inverted repeats measure only events arising by strand exchange (gene conversion and crossing over), direct repeats monitor strand exchange events and also non-conservative processes such as single strand annealing or replication slippage. In yeast, these processes depend on different pathways, most of them also existing in mammalian cells. These data permit to devise substrates adapted to specific questions about HR in mammalian cells. The effect of substrate structures (heterologies, insertions/deletions, GT repeats, transcription) and consequences of DNA double strand breaks induced by ionizing radiation or endonuclease (especially the rare-cutting endonuclease ISce-I) on HR are discussed. Finally, transgenic mouse models using tandem repeats are briefly presented.

The contribution of homologous recombination in preserving genome integrity in mammalian cells

Although it is clear that mammalian somatic cells possess the enzymatic machinery to perform homologous recombination of DNA molecules, the importance of this process in mitigating DNA damage has been uncertain. An initial genetic framework for studying homologous recombinational repair (HRR) has come from identifying relevant genes by homology or by their ability to correct mutants whose phenotypes are suggestive of recombinational defects. While yeast has been an invaluable guide, higher eukaryotes diverge in the details and complexity of HRR. For eliminating DSBs, HRR and end-joining pathways share the burden, with HRR contributing critically during S and G2 phases. It is likely that the removal of interstrand cross-links is absolutely dependent on efficient HRR, as suggested by the extraordinary sensitivity of the ercc1, xpf/ercc4, xrcc2, and xrcc3 mutants to cross-linking chemicals. Similarly, chromosome stability in untreated cells requires intact HRR, which may eliminate DSBs arising during DNA replication and thereby prevent chromosome aberrations. Complex regulation of HRR by cell cycle checkpoint and surveillance functions is suggested not only by direct interactions between human Rad51 and p53, c-Abl, and BRCA2, but also by very high recombination rates in p53-deficient cells.

Single and coincident intragenic mutations attributable to gene conversion in a human cell line

Two polymorphic sites are located within the heterozygous TK1 locus in the human lymphoblastoid cell line TK6: an inactivating frameshift in exon 4 of the nonfunctional allele and a phenotypically silent frameshift in exon 7 of the functional allele. Through the use of these intragenic polymorphisms and microsatellite markers that flank TK1, we demonstrate that partial gene conversion accounts for 3/75 (0.04) spontaneous and 9/163 (0.06) X-ray-induced TK1- mutants, thus comprising a significant component of forward mutations at this locus. In all cases, the conversion tract is < 1 cM, rendering double exchange a remote alternate explanation for these results. Sequence analysis of full length TK1 cDNA provides rigorous exclusion of deletion events as a mechanism for generation of these allelotypes. Detailed examination of allelotypes in TK1- mutants identified two mechanisms for the generation of coincident sequence alterations that sometimes accompanied gene conversions. Mutations within the conversion tract were attributed to either error-prone gap filling synthesis during recombinational repair or mismatch repair within a heteroduplex region following branch migration. These findings suggest that a proportion of point mutations may not be targeted to sites of DNA base damage, but rather may arise as secondary consequences from the repair of DNA strand breaks.

Cell cycle dependence of radiation-induced homologous recombination in cultured monkey cells

A lacZ transgene recombination system that reports homologous recombination events involving duplicated lacZ segments was used to study recombination in monkey cells exposed to ionizing radiation at different points in the cell cycle. With this system, recombination events can be detected in single cells by histochemical staining soon after exposure of cells to DNA-damaging treatment. Ionizing radiation rapidly induced recombination 5-10-fold in cells that were at the mitosis stage of the cell cycle. Irradiation either of cells at other points in the cell cycle or of nonsynchronized cells had less of an effect on recombination between lacZ segments.

Ionizing radiation greatly improves gene transfer efficiency in mammalian cells

The vast majority of clinical protocols involving gene therapy today rely on viral vectors for gene transduction. The primary reason that plasmid vectors have not been widely used for gene therapy trials is their relatively low rate of stable gene transfer. We show here that ionizing radiation can improve plasmid transfection efficiency in both normal and neoplastic human and mouse cells. As high as 1,400-fold improvement in transfection efficiency can be seen in primary human fibroblasts treated with 9 Gy. Radiation improves transfection efficiency in a dose-dependent manner of only linearized plasmid DNA in transformed or immortalized cells, but of both linearized and supercoiled plasmid in normal human fibroblasts. The gene transfer dose-response curves are linear for neoplastic cell lines and exponential for primary cell lines. This suggests that radiation can improve gene integration by at least two mechanisms, one that may require free DNA ends and one that does not. The 2-hr delay described here, from the time of irradiation to the beginning of enhanced gene integration, suggests an inducible process that becomes active after the bulk of the radiation damage has been repaired. Our data further suggest that radiation may be useful to target human gene therapy using plasmid vectors.

Spontaneous and ionizing radiation induced mutations involve large events when selecting for loss of an autosomal locus

The mouse P19H22 embryonal carcinoma cell line contains two distinct chromosome 8 homologs, one derived from Mus musculus domesticus (M. domesticus) and the other derived from Mus musculus musculus (M. musculus). It also contains a deletion for the M. musculus aprt allele, which is located on chromosome 8. In this study, cells with spontaneous or induced aprt deficiencies were isolated from P19H22 and examined to determine the nature of the mutational events that had occurred. Ultraviolet radiation (UV), ethyl methanesulfonate (EMS), and two forms of ionizing radiation, 137Cs and 252Cf, were used for mutation induction. DNA preparations from the aprt deficient cells were initially screened with a Southern blot analysis and separated into two broad classes: those that had lost the M. domesticus aprt allele and those that had retained it. The overwhelming majority (> 95%) of the spontaneous and ionizing radiation-induced mutants exhibited aprt gene loss, indicating that relatively large events had occurred and that homozygosity for the deleted region was not a lethal event. Loss of heterozygosity for syntenic markers was found to be a common event in cells exhibiting aprt gene loss. In contrast, a majority of the UV-induced mutants (61%) and a substantial minority of the EMS-induced mutants (38%) retained the aprt gene. A sequence analysis confirmed that base-pair substitutions were responsible for this class of mutation. Gene inactivation associated with hypermethylation of the promoter region was found to be a rare event and was not induced by any of the mutagenic agents tested. The results demonstrate the suitability of the P19H22 cell line for mutational studies, particularly those that are large in nature.

Linear dose-response relationship and no inverse dose-rate effect observed for low X-ray dose-induced mitotic recombination in Drosophila melanogaster

Mitotic recombination has emerged lately as a surprisingly common cause of recessive functional gene loss in mammalian cells and has been implicated in tumour suppressor gene loss in human neoplasms. In an assay, primarily monitoring mitotic recombination in Drosophila melanogaster, the ability of low dose acute- and chronic X-ray irradiation to induce clonal expression of recessive mutations of formally heterozygous loci was investigated. Mosaic spots of recessive wing-hair misshape mutations (mwh and flr) and of hair-into-bristles transforming mutation (zw3tic) were enhanced by a factor of two over control level following irradiation of heterozygous larvae to doses as low as 0.01, 0.03 or 0.1 Gy X-rays. The frequencies of mosaic spots induced with eight doses in the interval 0.01-2.0 Gy was linearly related to the dose. The regression lines show no significant intercept at zero dose. During the entire larval developmental period exposure of the exponentially growing target cell population to conditions of chronic irradiation at dose-rate of 15.7 x 10(-5) Gy/min provided no evidence of an inverse dose-rate effect as reported in yeast. In Drosophila, the probability of mitotic recombination per induced DNA double-strand break appears to be at least one order of magnitude higher than in man.

Molecular analysis of hypoxanthine phosphoribosyltransferase gene deletions induced by alpha- and X-radiation in human lymphoblastoid cells

Mutations caused by exposure to X-radiation and to radon and its decay products were compared in the hprt gene of a human lymphoblastoid cell line. Thirty-one X-radiation-induced, 29 radon-induced, and 24 spontaneous mutants were recovered from cell cultures under identical conditions except for the exposure to radiation. Seven spontaneous point mutations were recovered and DNA sequenced. These mutations included three C:G-->T:A transitions. These spontaneous point mutations were located in the exon or splice donor regions of five of the nine hprt exons. Four X-radiation-induced and three radon-induced point mutations were also analyzed by DNA sequencing. The frequency of induced mutants at the D0 doses for radon and X-radiation respectively were 5 x 10(-6) and 4.5 x 10(-6). Deletions were the predominant mutations recovered from both radon- and X-irradiated cells. Eighty-one percent of the mutants from X-radiation-treated cultures, 86% of the radon-treated cultures, and 63% of the spontaneous mutants involved deletions. Deletions involving exon and intron DNA, as well as intron DNA alone, were found to inactivate the hprt gene and result in a selectable HPRT- phenotype. Among the deletion mutants, however, only 21% of the spontaneous mutants versus 55% of both the X-radiation- and radon-induced mutants exhibited loss of the entire hprt gene. More X-radiation-induced deletions than radon-induced deletions extended further than 800 bp in the telomeric direction from the hprt gene (six of 17 versus two of 17). The results show that at the human hprt locus of TK-6 cells the predominant kind of mutation indicative of exposure to both high LET alpha-radiation and low LET X-radiation is a large deletion, spanning the entire hemizygous hprt gene and extending into flanking sequences.

Single-base deletion induced by benzo[a]pyrene diol epoxide at the adenine phosphoribosyltransferase locus in human fibrosarcoma cell lines

Benzo[a]pyrene diol epoxide (BPDE), a metabolic product of benzo[a]pyrene, is one of the most widely distributed environmental carcinogens. In this study, we demonstrate that BPDE produces a dose-dependent increase in the frequency of APRT gene reversion in the APRT-deficient cell line, HTD114, which contains single nucleotide insertions at different positions in each APRT allele. The highest reversion frequency observed after BPDE exposure was 3.3 +/- 0.9 x 10(-5), at least 10(3)-fold greater than the spontaneous frequency. Reversion of either mutant allele was observed to be a consequence of a frame-restoring loss of a single nucleotide. A similar frequency of BPDE-induced reversion at APRT also was observed in a cell line containing only one type of the mutant alleles of HTD114, thus eliminating the possibility that gene conversion plays a major role in APRT gene reversion in HTD114 cells. Therefore, the data demonstrate that BPDE can function as an effective frameshift mutagen in human cells.

Demonstration by DNA fingerprint analysis of genomic instability in mouse BALB 3T3 cells during cell transformation

We employed DNA fingerprint analysis to monitor DNA rearrangements in BALB 3T3 cells transformed spontaneously or by treatment with 3-methylcholanthrene (MCA) and UV-C. The effect of 12-O-tetradecanoyl-phorbol-13-acetate (TPA) in combination with MCA was also examined. Twenty-three spontaneously transformed cells, 28 induced transformed cells (18 by 1 microgram/ml MCA, six by 5 micrograms/ml MCA, and four by UV-C), and 31 non-transformed subclones were isolated from parental BALB 3T3 A31-1-1 cells. The DNAs were digested with HinfI and subjected to DNA fingerprint analysis with three multi-locus minisatellite probes, Per-6, Core, and Ins. Per-6 was the most effective probe for detecting DNA rearrangements. Rearranged bands detected by the Per-6 probe were observed in 9/31 (29%) of non-transformed subclones, 14/23 (61%) of spontaneously transformed cells, 16/18 (89%) of cells transformed by 1 microgram/ml of MCA, 6/6 (100%) of cells transformed by 5 micrograms/ml MCA, and 4/4 (100%) of UV-C-transformed cells. Higher numbers of DNA rearrangements (> or = 3) occurred most frequently in the induced transformed cells. TPA enhanced the frequency of DNA rearrangements in cells transformed by MCA. These data indicate that (1) genomic DNA in BALB 3T3 cells is unstable and susceptible to rearrangement, (2) its instability is elevated during cell transformation, and (3) MCA and UV-C induce DNA rearrangements, and TPA enhances the effect of the former, probably via the recombination process. DNA fingerprint analysis is valuable for monitoring genomic instability during cell transformation.