Molecular characterization of 15 rearrangements among 90 human in vivo somatic mutants shows that deletions predominate

Molecular Analysis of Mutations in the Human HPRT Gene

The HPRT assay uses incorporation of toxic nucleotide analogues to select for cells lacking the purine scavenger enzyme hypoxanthine-guanine phosphoribosyl transferase. A major advantage of this assay is the ability to isolate mutant cells and determine the molecular basis for their functional deficiency. Many types of analyses have been performed at this locus: the current protocol involves generation of a cDNA and multiplex PCR of each exon, including the intron/exon junctions, followed by direct sequencing of the products. This analysis detects point mutations, small deletions and insertions within the gene, mutations affecting RNA splicing, and the products of illegitimate V(D)J recombination within the gene. Establishment of and comparisons with mutational spectra hold the promise of identifying exposures to mutation-inducing genotoxicants from their distinctive pattern of gene-specific DNA damage at this easily analyzed reporter gene.

Similarity of in vivo somatic mutations at an autosomal adenine phosphoribosyltransferase locus between T- and B-cells in human peripheral blood

In vivo somatic mutations have been detected at several human loci by using clonal cultures of peripheral blood T-cells. It has not been fully understood whether or not the somatic mutations in T-cells are similar to those of other cell types. To address this issue, we cloned, from human peripheral blood, T- and B-cells with mutations at an autosomal adenine phosphoribosyltransferase (APRT) locus. For the efficient detection of somatic mutations at the APRT locus, a blood sample from a human individual heterozygous for germline APRT deficiency was used. T- and B-cells deficient in APRT enzyme activity were cloned from peripheral blood mononuclear cells using a selecting agent, 2,6-diaminopurine. The APRT-deficient mutant frequencies were on the order of 10(-4) in both T- and B-cells. The single-strand conformation polymorphism analysis of the APRT DNA of mutant B-cell clones suggested that the molecular mechanisms leading to the APRT deficiency in B-cells were similar to those in T-cells. Our observations suggest that both the frequency and the mode of in vivo somatic mutations occurring spontaneously at general autosomal loci in B-cells are similar to those in T-cells.

Selection against blood cells deficient in hypoxanthine phosphoribosyltransferase (HPRT) in Lesch-Nyhan heterozygotes occurs at the level of multipotent stem cells

Lesch-Nyhan syndrome is caused by a severe genetic deficiency of hypoxanthine phosphoribosyltransferase (HPRT) and is characterized by central nervous system disorders, gout, and in some cases, macrocytic anemia. Women heterozygous for HPRT deficiency are healthy but their somatic cells are mosaic for enzyme deficiency owing to random inactivation of the X chromosome. Frequencies of red blood cells and T cells deficient in HPRT are significantly lower than the expected 50% in heterozygotes, suggesting that HPRT-negative blood cells are selected against in heterozygotes. To determine at which stage of hematopoiesis such selection occurs, we determined the frequencies of HPRT-negative T, B and erythroid precursor cells in three heterozygotes. Since the cloning efficiencies of T and B cells and colony forming efficiency of burst-forming unit erythroid (BFU-E) for sample from Lesch-Nyhan patients were similar to those of normal cells, HPRT deficiency does not seem to render the differentiated cells less efficient for proliferation. However, the frequencies of HPRT-negative T and B cells, and BFU-E were all less than 10% in each of the three heterozygotes. Although the frequencies of HPRT-negative cells showed tenfold variations between the heterozygotes, each heterozygote had similar frequencies of HPRT-negative cells in the three cell types. These results suggest that HPRT is important at early stages of hematopoiesis, but less so after the cells have differentiated into T cells, B cells and erythroid precursor cells.

Multiplex PCR analysis of in vivo-arising deletion mutations in the hprt gene of human T-lymphocytes

A multiplex polymerase chain reaction (PCR) procedure was adapted for the rapid and efficient evaluation of deletions of the hypoxanthine guanine phosphoribosyltransferase (hprt) gene in human T-lymphocytes. The hprt clonal assay was used to isolate in vivo-arising hprt-deficient T-cells from six healthy males. Mutant frequencies ranged from 9-27 x 10(-6). Simple crude cellular extracts from 223 mutants were analyzed for hprt gene deletion. Sixteen (7.2%) were found to be due to total gene deletion and 22 (9.9%) were due to partial gene deletion. The relatively high frequency of total gene deletions was caused by replicate isolates of a single mutational event as shown by single-strand conformation polymorphism (SSCP) analysis of rearranged T-cell receptor (TCR)-gamma genes. Eighteen of the 22 partial hprt gene deletion mutants were determined to be of independent origin based on a unique hprt mutation or SSCP-TCR -gamma pattern. One-half (9/18) of the partial deletion mutants involved all or part of exon 4 alone, suggesting that this region of the hprt gene is prone to deletion. The small deletions effecting exon 1 (1 mutant), exon 2 (2 mutants), and exon 4 (6 mutants) would not have been detected by conventional Southern blot analysis and may represent a new, previously unrecognized class of mutations. The ready isolation of such intragenic deletions will allow the characterization of breakpoint junctions and may provide insights into the important processes of DNA breakage and rejoining.

Molecular analysis of mutations in the hprt gene in circulating lymphocytes from normal and DNA-repair-deficient donors

Circulating lymphocytes from patients with the DNA-repair-deficient disorders, xeroderma pigmentosum (XP) and ataxia telangiectasia (A-T) have elevated frequencies of mutants at the hypoxanthine-guanine phosphoribosyltransferase (hprt) locus. We have analysed the DNA sequence of the hprt gene in mutants from normal donors, and compared them with mutants from XP and A-T individuals. In normal donors we found a range of mutations including principally transitions (40%), transversions (32%) and small deletions (20%). In an excision-deficient XP donor from complementation group C the mutation spectrum was similar to that from normal donors, whereas in an XP variant there was a significantly higher frequency (44%) of small deletions. In the two A-T donors, there was a high frequency of large deletions (22 and 75%) compared with only 4% in normal donors.

An improved procedure for the in vitro expansion of human T-lymphocyte clones for mutant analysis

Assays detecting mutants present in human peripheral blood T-lymphocytes have been developed to monitor the population for somatic mutations. In order to investigate the nature of the mutations, colonies are further expanded in vitro by repeated lectin stimulation. To characterise fully each mutant clone, sufficient cells (approximately 10(7)) must be available for several molecular and biochemical techniques to be employed. These techniques, and their importance to the assay for population monitoring, are discussed briefly. We report here that the expansion of mutant colonies to approximately 10(7) cells by repeated lectin stimulation is not effective for all T-cell clones but that an alternative "lectin free" expansion method has enabled us to expand all the clones tested from a variety of normal donors and other individuals of interest.

In vivo mutations in human blood cells: biomarkers for molecular epidemiology

Mutations arising in vivo in recorder genes of human blood cells provide biomarkers for molecular epidemiology by serving as surrogates for cancer-causing genetic changes. Current markers include mutations of the glycophorin-A (GPA) or hemoglobin (Hb) genes, measured in red blood cells, or mutations of the hypoxanthine-guanine phosphoribosyltransferase (hprt) or HLA genes, measured in T-lymphocytes. Mean mutant frequencies (variant frequencies) for normal young adults are approximately: Hb (4 x 10(-8)) < hprt (5 x 10(-6)) = GPA (10 x 10(-6)) < HLA (30 x 10(-6)). Mutagen-exposed individuals show decided elevations. Molecular mutational spectra are also being defined. For the hprt marker system, about 15% of background mutations are gross structural alterations of the hprt gene (e.g., deletions); the remainder are point mutations (e.g., base substitutions or frameshifts). Ionizing radiations result in dose-related increases in total gene deletions. Large deletions may encompass several megabases as shown by co-deletions of linked markers. Possible hprt spectra for defining radiation and chemical exposures are being sought. In addition to their responsiveness to environmental mutagens/carcinogens, three additional findings suggest that the in vivo recorder mutations are relevant in vivo surrogates for cancer mutations. First, a large fraction of GPA and HLA mutations show exchanges due to homologous recombination, an important mutational event in cancer. Second, hprt mutations arise preferentially in dividing T-cells, which can accumulate additional mutations in the same clone, reminiscent of the multiple hits required in the evolution of malignancy.(ABSTRACT TRUNCATED AT 250 WORDS)

In vivo ionizing irradiations produce deletions in the hprt gene of human T-lymphocytes

The hprt T-lymphocyte cloning assay, which detects mutations occurring in vivo in humans, has been used to examine mutants induced in patients receiving radioimmunoglobulin therapy (RIT) for cancer. Samples from 13 patients before treatment (controls) and 15 samples from 12 patients after treatment were studied for both mutant frequencies and molecular changes in the hprt mutant T-cell clones. Patients were studied up to 48 months after treatment. Post-RIT patients showed increased mutant frequencies as compared to pre-treatment values. T-cell receptor (TCR) gene analysis of mutant T-cell clones demonstrated that 84% arose independently, both pre- and post-treatment, which is the same proportion as seen in normal individuals. However, several individuals did show large sets of mutants with the same TCR gene rearrangement patterns. Molecular analysis of mutants demonstrated a greater proportion of mutations with hprt gene changes on Southern blots after RIT treatment than before (40% versus 20%). RIT increases the proportion of mutations with total rather than partial gene deletions or other gross structural changes compared to normal individuals or pre-treatment patients. These studies are defining the spectrum for radiation-induced hprt gene mutations in vivo in human T-lymphocytes.

Reduction to homozygosity is the predominant spontaneous mutational event in cultured human lymphoblastoid cells

Expression of recessive mutant phenotypes can occur by a number of different mechanisms. Inactivation of the wild-type allele by base-substitution mutations, frameshift mutations or small deletions occurs at both hemizygous and heterozygous cellular loci, while other events, such as chromosome level rearrangements, may not be detected at hemizygous loci because of inviability of the resulting mutants. In order to assess the relative contribution of each type of mutational event, we isolated a human lymphoblastoid cell line that is heterozygous at the adenine phosphoribosyltransferase (aprt) locus. The mutation rate for the expression of the mutant phenotype (aprt(+/-)----aprt-/-) was 1.3 x 10(-5)/cell/generation. Molecular analysis of the DNA from 26 mutant clones revealed that 19% had undergone deletion of the entire wild-type allele. The aprt heterozygote carries a mutation in the coding sequence of the gene that results in the loss of a restriction site. Analysis of aprt-/- mutants for this restriction fragment length difference revealed that 23% of the mutants contained point mutations or small (less than 100 bp) deletions. The remainder of the mutants (58%) resulted from reduction to homozygosity of the mutant allele. We suggest that, as in tumor cells in vivo, reduction to homozygosity is a major mechanism for the expression of recessive mutant phenotypes in cultured human cells.

Fine structure mapping of the hypoxanthine-guanine phosphoribosyltransferase (HPRT) gene region of the human X chromosome (Xq26)

The Xq26-q27 region of the X chromosome is interesting, as an unusually large number of genes and anonymous RFLP probes have been mapped in this area. A number of studies have used classical linkage analysis in families to map this region. Here, we use mutant human T-lymphocyte clones known to be deleted for all or part of the hypoxanthine-guanine phosphoribosyltransferase (hprt) gene, to order anonymous probes known to map to Xq26. Fifty-seven T-cell clones were studied, including 44 derived from in vivo mutation and 13 from in vitro irradiated T-lymphocyte cultures. Twenty anonymous probes (DXS10, DXS11, DXS19, DXS37, DXS42, DXS51, DXS53, DXS59, DXS79, DXS86, DXS92, DXS99, DXS100d, DXS102, DXS107, DXS144, DXS172, DXS174, DXS177, and DNF1) were tested for codeletion with the hprt gene by Southern blotting methods. Five of these probes (DXS10, DXS53, DXS79, DXS86 and DXS177) showed codeletion with hprt in some mutants. The mutants established the following unambiguous ordering of the probes relative to the hprt gene: DXS53-DXS79-5'hprt3'-DXS86-DXS10-DXS177 . The centromere appears to map proximal to DXS53. These mappings order several closely linked but previously unordered probes. In addition, these studies indicate that rather large deletions of the functionally haploid X chromosome can occur while still retaining T-cell viability.

Ionizing radiation and genetic risks. III. Nature of spontaneous and radiation-induced mutations in mammalian in vitro systems and mechanisms of induction of mutations by radiation

This paper (1) presents an analysis of published data on the molecular nature of spontaneously arising and radiation-induced mutations in mammalian somatic cell systems and (2) examines whether the molecular nature and mechanisms of origin of radiation-induced mutations, in mammalian in vivo and in vitro systems, as currently understood, are consistent with expectations based on the biophysical and microdosimetric properties of ionizing radiation. Depending on the test system (CHO cells, human T lymphocytes and human lymphoid cell line TK6), 80-97% of spontaneous HPRT mutations show normal Southern patterns; the remainder is due to gross changes, predominantly partial (intragenic) deletions. Total gene deletions at the HPRT locus are rare except in the TK6 cell line. At the APRT locus in CHO cells, 80-97% of spontaneous mutations are due to base-pair changes, the remainder being, mostly, partial deletions. The latter can extend upstream in the 5' direction but not beyond the APRT gene in the 3' direction. At the human HLA-A locus (T lymphocytes), the percentage of mutations with normal Southern patterns is lower than that for HPRT, and in the range of 50-60%. At the HLA-A locus, mitotic recombination contributes substantially to the mutation spectrum (approximately 30% of mutations recovered) and this is likely to be true of the TK locus in the TK6 cell line as well. With a few exceptions, most of the radiation-induced mutations show altered Southern patterns and are consistent with their being deletions and/or other gross changes (HPRT, 70-90% (CHO); 50-85% (TK6); 50-75% (T lymphocytes); TK, 60-80% (TK6); HLA-A, 80% (T lymphocytes); DHFR, 100% (CHO]. The exceptions are APRT mutations in CHO cells (16-20% of mutants with deletions or other changes) and HPRT mutations in T lymphocytes from A-bomb survivors (15-25%); the latter finding is consistent with the occurrence of in vivo selection against HPRT mutant cells. In cases of HPRT intragenic deletions analyzed (CHO cells and V79 Chinese hamster cells), there is evidence for a non-random distribution of breakpoints. The spontaneous mutation frequencies vary widely, from about 0.04/10(6) cells (sickle cell mutations at the human HBB locus) to 30.8/10(6) cells (HLA-A mutations in T lymphocytes) and are dependent on the locus, the system employed and a number of other factors. Those for the other loci fall between these limits.(ABSTRACT TRUNCATED AT 400 WORDS)

Diagnosis of heterozygous states for adenine phosphoribosyltransferase deficiency based on detection of in vivo somatic mutants in blood T cells: application to screening of heterozygotes

An accurate diagnosis of heterozygotes for autosomal recessive disorders with unknown mutations can be difficult. Using a unique phenomenon occurring in vivo, we designed a method for the diagnosis of heterozygotes for adenine phosphoribosyltransferase (APRT) deficiency which makes way for a qualitative distinction between normal and heterozygous subjects. We cultured peripheral blood mononuclear cells with 2,6-diaminopurine, an APRT-dependent cytotoxin, to search for in vivo mutational cells. Fifteen putative heterozygotes examined were found to possess such mutant cells at rather high frequencies; thus, a false negative diagnosis is unlikely. The analysis of genomic DNA in 82 resistant clones from two of the heterozygotes clarified that 64 (78%) had lost the germinally intact alleles. Thirteen members of APRT-deficient families were examined; eight proved to be heterozygotes. Among 425 individuals from two separate residential areas of Japan, two heterozygotes were found. The authenticity of the heterozygosity was validated by two separate methods for the two heterozygotes; hence, a false positive diagnosis can be ruled out. Our data showed a calculated heterozygote frequency of 0.47% (95% confidence limits; 0.05%-1.7%), a value compatible with that (1.2%) calculated from data concerning the incidence of 2,8-dihydroxyadenine urolithiasis. This novel genetic approach for identifying heterozygotes is now being tested to search for other enzyme deficiencies in humans.

Mutations induced by benzo[a]pyrene diolepoxide at the hprt locus in human T-lymphocytes in vitro

Human T-lymphocytes have been treated with benzo[a]pyrene diolepoxide (BPDE) in vitro and T-cell clones mutated in the hprt gene have been isolated. The mutant frequencies in BPDE-treated T-cell cultures were on average 24-fold higher than those of untreated cultures. Thus, BPDE is a potent inducer of gene mutation in this system. In order to examine which types of mutations are induced by BPDE in human cells, 41 spontaneous and 44 BPDE-induced mutant clones have been characterized using the Southern blot technique. In addition, rearrangements of the T-cell-receptor beta and gamma loci have been used to determine the proportion of isolated clones that are unique, and thus likely to represent independent mutational events. Out of 23 independent spontaneous mutants 4 had large hprt alterations that could be detected on Southern blots. Two of these alterations, deletions of exons 2-6, have been confirmed using PCR of hprt cDNA and direct sequencing of the PCR product. All 33 independent BPDE-induced mutants had normal hprt restriction patterns which indicates that BPDE is mainly a point mutagen in this system.

6-Thioguanine-resistant T-lymphocyte autoradiographic assay. Determination of variation frequencies in individuals suspected of radiation exposure

We used the autoradiographic assay to assess human in vivo somatic cell gene mutation at the hypoxanthine guanine phosphoribosyl transferase (HGPRT) locus in T-lymphocytes. Cells able to incorporate tritiated thymidine in vitro in the presence of 6-thioguanine were enumerated in order to determine 6-thioguanine-resistance (TGr) variant frequencies in cryopreserved lymphocytes from 17 normal control individuals, from 3 persons suspected to have been exposed to 60Co in an accident in Cd. Juárez (Mexico), studied 24 months after the accident, and from 4 individuals who were in Kiev during the radiation accident in Chernobyl (U.S.S.R.); 2 of them were studied 1 month after the accident, and again 1 year after the first sampling, the other 2 were studied 13 months after the accident. The data obtained indicate that this assay may be useful in any laboratory of cytogenetics for human population monitoring and that its use following accidental exposure to ionizing radiation should be further evaluated.

Review of the molecular characteristics of gene mutations of the germline and somatic cells of the human

Molecular analyses of the limited number of de novo germinal mutations identified in humans indicate that an array of alterations in gene structure can be generated. Similar conclusions are derived from the large data set obtained from molecular analyses of alleles that segregate in the human population and cause genetic diseases. The molecular alterations include nucleotide substitutions as well as insertions, deletions and other rearrangements of the DNA. The lesions may be located in the coding or the noncoding regions of genes or may involve the flanking sequences. The insertions and deletions involve fragments ranging from single nucleotides to many kilobases, and involve both unique sequences and repetitive elements. The nature of the lesions observed to date as either de novo mutations or segregating variants suggests there are locus-specific characteristics of the alterations in DNA structure that are recovered as genetic diseases. Differences in mutation spectra among genetic loci appear to reflect both the structure of the target sequences and the relationship between gene structure and gene function. No induced germinal mutations have been identified, thus no data are available that reveal the relationships between mutagenic exposures and the molecular fingerprints of the lesion induced in the human germ cell and transmitted to the subsequent generations. In contrast, the prospects for analyzing the roles of genetic target, exposure history and individual responsiveness to exposure in creating particular molecular lesions in somatic cells are excellent, both for alterations of single nucleotides and for major alterations of gene structure.(ABSTRACT TRUNCATED AT 400 WORDS)

Molecular genetic analysis of recessive mutations at a heterozygous autosomal locus in human cells

We have investigated the genotypic changes that lead to expression of a recessive allele at a heterozygous autosomal locus in a human cell line. Mutant clones lacking thymidine kinase activity were derived from a B-cell lymphoblastoid line initially heterozygous at the tk locus, and restriction mapping was performed to detect intragenic structural alterations in the tk gene. In addition, informative molecular markers located elsewhere on chromosome 17 were analysed in order to detect large-scale (multilocus) events. We report that among 325 spontaneous and induced mutants, allele loss was more common than intragenic rearrangements or point mutations; in many cases, loss of heterozygosity appears to have extended well beyond the locus under selection. Cytogenetic analysis of a subset of these mutants showed that expression of the recessive TK-deficient phenotype and the associated loss of heterozygosity for chromosome 17 markers was not typically associated with detectable chromosomal changes.

Southern-blot analyses of human T-lymphocyte mutants induced in vitro by gamma-irradiation

G0 phase cultures of human peripheral blood T-lymphocytes from a single individual were exposed to 300 rad of gamma-irradiation from a 137Cs source and cultured in vitro for 8 days to allow phenotypic expression. Thioguanine-resistant (TGr) mutants were isolated by a cell cloning assay in microtiter plates. These mutants were studied by Southern blot analysis to define the gross structural alterations in the hypoxanthine-guanine phosphoribosyl transferase (hprt) gene by use of an hprt cDNA probe. A similar analysis of the T-cell receptor (TCR) gene rearrangement patterns was employed to define the independent nature of each mutant colony by use of TCR beta and gamma cDNA probes. 74 mutants were isolated in 5 separate experiments. TCR gene rearrangement analysis showed these to represent 24 independent mutations, of which 18 contained hprt structural alterations. These alterations included simple deletions (10/18) as well as more complex rearrangements resulting in molecular weight changes of restriction fragments representing both the 5' and 3' regions of the hprt gene (4/18 and 4/18, respectively). These results demonstrate that gamma-irradiation primarily induces TGr mutations through gross structural alterations in the hprt gene and that these alterations are randomly distributed across the gene. This approach to mutation analysis will provide information on the types of alterations induced by this irradiation, especially the extent of deletions involving the hprt gene. These results also demonstrate the feasibility of employing in vitro exposure of human T-lymphocytes to a single mutagenic agent as an aid to understanding the mechanisms of mutations occurring in vivo in humans.

Increased frequency of 6-thioguanine-resistant lymphocytes in peripheral blood of workers employed in cyclophosphamide production

The frequency of 6-thioguanine-resistant peripheral blood lymphocytes has been determined by autoradiography in a control population and a population of cyclophosphamide-exposed individuals. The mean variant frequency in a non-exposed population was found to be 2.76 +/- 1.48 X 10(-5). Subpopulations of smokers and non-smokers revealed statistically significant differences in the variant frequencies, i.e. 3.52 +/- 1.55 X 10(-5) and 2.07 +/- 1.05 X 10(-5) respectively. In 20 out of a total of 23 individuals employed in cyclophosphamide synthesis and manufacturing, the variant frequency of 6-thioguanine-resistant lymphocytes was found to be higher than the maximum individual frequency found in the control population. The mean variant frequency in the cyclophosphamide-exposed population was 13.64 +/- 13.56 X 10(-5), a statistically significant increase as compared to the mean control frequency. There was no correlation between variant frequency and duration of employment suggesting that this test reflects the actual exposure and not a cumulative effect.

In vivo hprt mutant frequencies in T-cells of normal human newborns

Mutation at the hypoxanthine-guanine phosphoribosyl transferase locus (hprt; HPRT enzyme) in the human fetus was studied by clonal assay of placental cord blood samples from full-term newborns. Conditions for determining hprt mutant frequencies, as defined for adults, were also optimal for studies in newborns. The mean mutant frequency for 45 normal human newborns (37 male, 8 female) was 0.64 X 10(-6) (SD = 0.41 X 10(-6); median value = 0.58 X 10(-6). These values are approx. 10-fold lower than corresponding adult hprt mutant frequency values. Factors such as limiting-dilution cloning efficiencies, delay prior to study of sample, sex, cryopreservation or technician performing the assay did not significantly affect assay results. Maternal smoking did not result in elevated mutant frequency values. Most wild-type and mutant clones studied were CD4 surface antigen positive (helper/inducer). All hprt mutants analyzed lacked HPRT activity.

Molecular analyses of in vivo hprt mutations in human T-lymphocytes. III. Longitudinal study of hprt gene structural alterations and T-cell clonal origins

The hprt clonal assay detects mutations occurring in vivo in the hypoxanthine-guanine phosphoribosyltransferase (hprt) gene of human T-lymphocytes. Analysis of 94 wild-type and 326 hprt mutant clones from 3 normal males was performed using Southern blotting with hprt and T-cell receptor (TCR) gene probes. Gross structural alterations of the hprt gene occurred in approximately 14% of the in vivo derived mutants. Breakpoints were randomly distributed across the gene with one possible mutational "hot spot" observed. Most hprt mutants were independent as judged by TCR gene rearrangement patterns indicating that the measured hprt mutant frequency is a good measure of the actual hprt mutation frequency. However, sibling mutants (generally doublets and triplets except for one nonamer) were detected. Information on the timing in vivo of the hprt mutational events and the persistence in vivo of sibling mutants was also obtained.

Hprt mutations and karyotype abnormalities in T-cell clones from healthy subjects and melphalan-treated ovarian carcinoma patients

In vivo mutations at the locus for hypoxanthine phosphoribosyl transferase (hprt) were studied in 6-thioguanine (TG)-resistant T-lymphocyte clones from healthy male and female subjects and ovarian carcinoma patients treated with melphalan. Southern blot analysis of 108 clones showed alterations in 14% (4/29) of the clones from healthy males, 4.3% (2/47) of the clones from healthy females and 3.1% (1/32) of the clones from melphalan-treated patients. 2 of the 7 abnormal clones had a total deletion of the hprt gene; the others had partial deletions. Karyotype analysis of 82 clones revealed 1 clonal abnormality in 29 mutant clones from healthy males (3.6%). Loss or structural aberration of 1 X-chromosome occurred in 6% of the clones from healthy females. The frequency of karyotypic abnormalities (excluding those affecting one of the X-chromosomes) was significantly higher in clones from patients (37%) as compared to healthy females (5.9%). No aberration was found to affect the hprt locus at Xq27 in any of the 82 clones studied.

Molecular analysis of spontaneous and ethyl methanesulphonate-induced mutations of the hprt gene in hamster cells

Independent spontaneous or ethyl methanesulphonate (EMS)-induced mutants lacking HPRT enzyme activity were analysed for changes in hprt gene structure. Of 21 spontaneous mutants, 6 had total gene deletions, 2 had partial gene deletions, and 13 were indistinguishable from wild-type by Southern analysis. In contrast a sample of 23 EMS-induced mutants, each of which showed potentially interesting characteristics (e.g. high reversion frequency, X-chromosome rearrangement), showed no detectable hprt gene changes. RNA isolated from 59 mutants with presumptive point mutations (13 spontaneous, 46 EMS-induced) was analysed on dot blots for changes in the amount of hprt mRNA. A wide range of mRNA levels was found, from mutants with undetectable amounts to those with more than wild-type amounts. However, Northern blots of all these mutant RNAs revealed only one (EMS-induced) mutation with a change in hprt mRNA size. Taken with our previously-published data on these mutants, it is argued that they represent a broad range of mutational types, and that the hprt gene mutation system provides a sensitive means of distinguishing mutational spectra of different DNA-damaging agents.

Specific-locus mutations induced in eukaryotes (especially mammalian cells) by radiation and chemicals: a perspective

In the course of discovering the first mutagen (X-rays) just over 60 years ago, Herman J. Muller asked whether X-rays induced single-gene mutations and/or chromosomal (multiple-gene) mutations. To a large extent, his question has set the agenda for mutagenesis research ever since. We explore historically the answers to this question, with special emphasis on recent developments in the field of mammalian cell mutagenesis. Studies indicate that ionizing radiation and many chemical mutagens/carcinogens induce both gene and chromosomal mutations; however, only certain genetic systems permit the recovery and analysis of both classes of mutations. Few chemical mutagens induce only gene mutations in mammalian cells; instead, most mutagens appear to induce both classes of mutations, with chromosomal mutations (especially multilocus deletions) predominating at high doses. These results have implications regarding the mechanisms of mutagenesis, the role of chromosomal mutations in carcinogenesis and hereditary disease, and the type of data required for risk assessment of physical and chemical mutagens/carcinogens.

Increased somatic cell mutant frequency in atomic bomb survivors

Frequencies of mutant T-cells in peripheral blood, which are deficient in hypoxanthine guanine phosphoribosyltransferase (HPRT) activity, were determined for atomic bomb survivors by direct clonal assay using a previously reported method (Hakoda et al., 1987). Results from 30 exposed survivors (more than 1 rad exposed) and 17 age- and sex-matched controls (less than 1 rad exposed) were analyzed. The mean mutant frequency (Mf) in the exposed (5.2 X 10(-6); range 0.8-14.4 X 10(-6)) was significantly higher than in controls (3.4 X 10(-6); range 1.3-9.3 X 10(-6)), which was not attributable to a difference in non-mutant cell-cloning efficiencies between the 2 groups, which were virtually identical. An initial analysis of the data did not reveal a significant correlation between individual Mfs and individual radiation dose estimates when the latter were defined by the original, tentative estimates (T65D), even though there was a significant positive correlation of Mfs with individual frequency of lymphocytes bearing chromosome aberrations. However, reanalysis using the newer revised individual dose estimates (DS86) for 27 exposed survivors and 17 controls did reveal a significant but shallow positive correlation between T-cell Mf values and individual exposure doses. These results indicate that HPRT mutation in vivo in human T-cells could be detected in these survivors 40 years after the presumed mutational event.

A novel class of unstable 6-thioguanine-resistant cells from dog and human kidneys

Thioguanine-resistant primary clones were grown from single cell suspensions obtained from dog and human kidneys by enzymatic digestion. In medium containing a relatively high concentration (10 micrograms/ml) of thioguanine, thioguanine-resistant primary clones arose from each source at frequencies ranging from 10(-4) to 10(-5). A reduction in total hypoxanthine uptake was found in the thioguanine-resistant primary clones which had developed in thioguanine medium, consistent with a reduction in hypoxanthine phosphoribosyltransferase activity. When these thioguanine-resistant primary clones were subsequently grown in the absence of thioguanine and assayed for the thioguanine-resistant phenotype and hypoxanthine phosphoribosyltransferase activity, it was found that most were now thioguanine-sensitive and yielded cell-free extracts with substantial amounts of hypoxanthine phosphoribosyltransferase activity. In contrast, thioguanine-resistant human clones grown continuously in the presence of thioguanine yielded cell-free extracts with little or no detectable hypoxanthine phosphoribosyltransferase activity. Southern blot analysis demonstrated no structural alterations in the hypoxanthine phosphoribosyltransferase gene in thioguanine-resistant primary human kidney clones. These results suggest that a novel mechanism(s) for thioguanine resistance and the control of hypoxanthine phosphoribosyltransferase expression may occur in dog and human kidney cells.

The aprt heterozygote/hemizygote system for screening mutagenic agents allows detection of large deletions

Frequencies of mutation at the hprt and aprt loci in various CHO cell lines were measured after exposure of the cells to ionizing radiation. In D423 and AA8-16, which are aprt+/- heterozygotes, the ratio of hprt- mutants to aprt- mutants ranged from 0.11 to 0.36. In D422 and AA8-5, which are aprt+/0 cell lines in which only one copy of the gene and its flanking sequences is present these ratios were greater than 5. In contrast, chemical mutagenesis generated mutations at both loci, in all cell lines, at equal frequencies. Southern blot analysis of DNA from hprt- and aprt- mutants of one of the aprt+/- heterozygous lines showed some apparently unaltered genes, some rearrangements and some complete deletions of hprt among hprt- mutants, but only complete deletions of aprt-linked sequences among aprt- mutants. These results strongly suggest that X-ray-induced mutational events are frequently larger than 40 kb (the length of the hprt gene) and that the difference among the frequencies observed at the two loci in the two types of cell lines were due to the presence of essential sequences close the respective target genes. The combined use of these cell lines in screening environmental mutagens should allow qualitative as well as quantitative analysis of the mutagenic potential of environmental agents.

A method for specific cloning and sequencing of human hprt cDNA for mutation analysis

A method has been developed for the specific amplification and cloning of human hprt cDNA which can be used for mutant sequence analysis. Messenger RNA is isolated from TK6 lymphoblasts and is used to produce a first strand cDNA with reverse transcriptase primed with oligo dT. Second strand synthesis and subsequent amplification of hprt sequences is accomplished using Thermus aquaticus DNA polymerase and hprt-specific primers in the polymerase chain reaction (PCR) procedure. Convenient restriction enzyme sites have been built into the 5' ends of the PCR primers to allow cloning of the hprt fragments in M13mp19. Dideoxy sequencing of hprt with specific primers can be carried out using either the PCR reaction product or fragments cloned in M13mp19 as substrate. This general cloning/sequencing method can be used to analyze hprt mutation in human cells obtained both in vitro and in vivo.