X-ray-induced mutation to 6-thioguanine resistance in cultured human diploid fibroblasts.

X-ray induced mutation to 6-thioguanine (6TG)-resistance was studied in early passage cultures of human diploid fibroblasts. The appearance of phenotypic induced mutants in irradiated cell populations was linearly related to the number of post-irradiation cell doublings and to the duration of the growth period prior to mutant selection; the maximum yield of X-ray induced mutants was observed when cells surviving radiation had completed 3--4 douplings (6--7 days growth) in non-selective medium. The maximum induced mutation frequency was linearly related to X-ray dose and the mutation rate was estimated to be 3.1-10(-7) mutations per viable cell per rad. The data obtained for X-ray induced mutations in cultured human diploid fibroblasts were compared with (a) similar experimental data obtained with established cell cultures and (b) with theoretical predictions of X-ray mutation rates in human germ cells.

The isolation and preliminary characterisation of 6-thioguanine-resistant mutants of human diploid fibroblasts.

Mutant clones of human diploid fibroblasts deficient in the enzyme, hypoxanthine-guanine phosphoribosyl transferase (HGPRT) were selected by their ability to grow in medium containing the cytotoxic purine analogue, 6-thioguanine (6TG). The optimal condtions for mutant selection were 6TG concentrations between 1 and 5 mug ml1 and cell plating densities approximately 10(3) cells cm-2. Nine spontaneous and four radiation-induced 6TG-resistant mutants had less than 2% of the parental strain HGPRT activity and were unable to grow in medium containing azaserine. These mutants were phenotypically stable during greater than 25 population doublings in non-selective medium and five mutants that were examined showed no gross change from the normal human karyotype. Evidence is presented to show that 6TG is a better selective agent than 8-aza-guanine (8AG) for HGPRT-deficient mutants of human diploid fibroblasts.

Mutation and inactivation of cultured mammalian cells exposed to beams of accelerated heavy ions. III. Human diploid fibroblasts.

The induction of inactivation and mutation to thioguanine-resistance in cultured human diploid fibroblasts was studied after exposure to ionising radiations with LET's in the range 20--470 keV micrometer-1. Unique r.b.e. values were obtained for inactivation and mutation induction with nine different qualities of radiation. The plot of r.b.e. verus LET gave humped curves for both endpoints; r.b.e. maxima were in the LET range 90--200 keV micrometer-1 but the maximum r.b.e. value for mutation induction was almost twice that for inactivation. The accuracy of estimates of mutation induction are discussed with regard to possible selective effects against mutants during post-irradiation growth.

The effectiveness of restriction endonucleases in cell killing and mutation.

The use of restriction endonucleases (RE) to study the importance of DNA break end structures in differential cellular response has proved controversial. The number of DNA cut sites and the accessibility of RE are recognized examples of confounding factors. We have eliminated these factors by comparing the effectiveness of isoschizomers. Additionally, we considered for the first time the tolerance of the enzymes to cellular conditions. Cell killing and mutation were compared to the overall cutting ability of the enzymes in an "intracellular" buffer. We found that the activity of each RE combined with its lifetime, under simulated cellular conditions, were the overriding factors in determining effectiveness.

The repair of potentially lethal damage in x-irradiated cultures of normal and ataxia telangiectasia human fibroblasts.

The repair of potentially lethal damage (PLD) after X-rays was studied in plateau phase cultures of nine normal and five ataxia telangiectasia (AT) strains of human fibroblasts. In the normal strains PLD repair was complete after 6 hours of post-irradiation incubation. There were differences in the form of the survival curves of normal strains after maximum PLD repair but the extent of post-irradiation recovery was similar in all strains. In contrast all AT strains were almost completely deficient in PLD repair even when post-irradiation incubation was extended to 18 or 24 hours. The relevance of the PLD repair-deficiency in cultured AT strains to in vivo radiotherapeutic observations in AT patients is briefly discussed.

Ataxia-telangiectasia: a human mutation giving high-frequency misrepair of DNA double-stranded scissions.

The ability of three normal and one radiosensitive Ataxia-telangiectasia (A-T) human cell lines to rejoin restriction endonuclease-induced double-stranded (ds) DNA scissions was investigated using gene-transfer techniques with recombinant plasmid as target DNA. The results of cellular experiments using gene transfer frequencies as a measure of DNA rejoining strongly suggested that the A-T cell line had a greatly elevated frequency of misrepair of double-stranded DNA scissions. Southern blot analysis of DNA from plasmid-transformed cells confirmed this and further suggested that the misrepair in the A-T cell line took the form of large deletions and/or rearrangements at or around the scission. We postulate a disequilibrium in A-T between rejoining and exonuclease digestion of DNA termini as a possible basis for the misrepair and discuss this mechanism in relation to the major clinical features of the disease.

The IL-1 alpha and beta genes are closely linked (less than 70 kb) on mouse chromosome 2.

The murine IL-1 alpha and IL-1 beta genes encode structurally and evolutionarily related cytokines that exert a regulatory role in numerous physiological processes including hemopoiesis. Previous studies have shown these genes to be closely linked in the F region of mouse chromosome 2. Here we show, using pulsed-field gel electrophoresis, that the IL-1 alpha and beta genes of the CBA/H mouse are very closely linked and contained within a SmaI genomic fragment of approximately 70 kb. From conventional and PFGE analyses we suggest that IL-1 beta lies 5' to IL-1 alpha and that the two genes are in the same orientation and separated by approximately 50 kb. The apparent clustering of such hemopoietic genes is discussed in relation to evolutionary tandem gene duplication and possible associations with chromosomal fragile sites and leukemogenesis.

Preliminary molecular studies on two chromosome 2 encoded genes, c-abl and beta 2M, in radiation-induced murine myeloid leukaemias.

The majority of radiation-induced murine myeloid leukaemias are characterized by deletion and/or translocation of an interstitial region of chromosome 2, and there is evidence that such events may occur very early in myeloid leukaemogenesis. Analyses presented and discussed here on the structure and function of two possibly relevant chromosome 2 encoded genes (c-abl and beta 2M) lead to the preliminary conclusion that neither are directly involved nor activationally changed by the characteristic chromosome 2 rearrangements.

A phenotype map of the mouse X chromosome: models for human X-linked disease.

The identification of many of the transcribed genes in man and mouse is being achieved by large scale sequencing of expressed sequence tags (ESTs). Attention is now being turned to elucidating gene function and many laboratories are looking to the mouse as a model system for this phase of the genome project. Mouse mutants have long been used as a means of investigating gene function and disease pathogenesis, and recently, several large mutagenesis programs have been initiated to fulfill the burgeoning demand of functional genomics research. Nevertheless, there is a substantial existing mouse mutant resource that can be used immediately. This review summarizes the available information about the loci encoding X-linked phenotypic mutants and variants, including 40 classical mutants and 40 that have arisen from gene targeting.

DNA-mediated gene transfer into human diploid fibroblasts derived from normal and ataxia-telangiectasia donors: parameters for DNA transfer and properties of DNA transformants.

We investigate the feasibility of DNA-mediated gene transfer into human diploid fibroblasts derived from patients with the radiation sensitive syndrome ataxia-telangiectasia (A-T) and from a normal donor. Although they are markedly different in their growth characteristics, both normal and A-T strains give similar frequencies for DNA transfer in a model system using the recombinant plasmid pSV2 -gpt. pSV2 -gpt DNA transformants arise with a frequency between 10(-5) and 10(-4) per viable cell. Analysis of such transformants, although possible, is severely handicapped by the limited clonal life span of diploid human cells. Despite these problems it may be concluded that diploid human fibroblasts are competent recipients for DNA-mediated gene transfer and the putative repair deficiency of A-T does not markedly effect the efficiency of this process.

The use of recombinant DNA plasmids for the determination of DNA-repair and recombination in cultured mammalian cells.

Using the recombinant plasmid pSV2gpt and DNA transfer techniques, cell mediated DNA ligation and recombination of plasmid DNA have been demonstrated in four human cell lines. Data suggesting the involvement of a possible defect in the cellular equilibrium between ligation and exonuclease digestion of double strand DNA scissions in an ataxia-telangiectasia (A-T) cell line is discussed. The same A-T line was grossly proficient in DNA recombination but it will be necessary to distinguish between recombination in coding and non-coding plasmid sequences.

The sex-linked fidget mutation abolishes Brn4/Pou3f4 gene expression in the embryonic inner ear.

We have demonstrated that the phenotype of the mouse mutant sex-linked fidget ( slf ) is caused by developmental malformations of the inner ear that result in hearing loss and vestibular dysfunction. Recently, pilot mapping experiments suggested that the mouse Brn4 / Pou3f4 gene co-segregated with the slf locus on the mouse X chromosome. These mapping data, in conjunction with the observation that the vertical head-shaking phenotype of slf mutants is identical to that observed in mice with a targeted deletion of the Brn4 gene, suggested that slf is a mutant allele of the Brn4 gene. In this paper, we have identified the nature of the slf mutation, and demonstrated that it is an X chromosomal inversion with one breakpoint close to Brn4. This inversion selectively eliminates the expression of the Brn4 gene in the developing inner ear, but not the neural tube. Finally, these results demonstrate that the slf mutation is a good mouse model for the most prevalent form of X-linked congenital deafness in man, which is associated with mutations in the human Brn4 ortholog, POU3F4.