Deletion mapping of highly conserved transcribed sequence downstream from APRT locus

Comparative analysis of the mutagenic activity of oxaliplatin and cisplatin in the Hprt gene of CHO cells

Oxaliplatin is a platinum-derived antitumor drug that is active against cisplatin-resistant tumors and has lower overall toxicity than does cisplatin. DNA adduct formation is believed to mediate the cytotoxic activity of both compounds; however, the adducts may also be responsible for mutagenic and secondary tumorigenic activities. In this study, we have compared the mutagenicity of oxaliplatin and cisplatin in the Hprt gene of CHO-K1 cells. Both drugs produced dose-related increases in mutant frequency. For 1-hr treatments, oxaliplatin was less mutagenic than cisplatin at equimolar doses, while similar mutant frequencies were induced at equitoxic doses. Sequencing of mutant Hprt genes indicated that the mutation spectra of both oxaliplatin and cisplatin were significantly different from the spontaneous mutation spectrum (P = 0.014 and P = 0.008, respectively). A significant difference was also observed between the spectra of oxaliplatin- and cisplatin-induced mutations (P = 0.033). Although G:C-->T:A transversion was the most common mutation produced by both compounds, oxaliplatin produced higher frequencies of A:T-->T:A transversion than did cisplatin, most commonly at nucleotide 307, and higher frequencies of small deletions/insertions. Also, cisplatin induced tandem base-pair substitutions, mainly at positions 135/136, and a higher frequency of G:C-->A:T transition than did oxaliplatin. These results provide the first evidence that oxaliplatin is mutagenic and that the profiles of cisplatin- and oxaliplatin-induced mutations display not only similarities but also distinctive features relating to the type and sequence-context preference for mutation. Environ.

Oncogenic potential of the DNA replication licensing protein CDT1

The expression of a gene, designated as Retroviral insertion site (Ris)2, was activated by retroviral DNA integration in an immortalized primitive erythroid cell line, EB-PE. Ris2 was also expressed at high levels in all human tumor cell lines analysed. Consistently, NIH3T3 fibroblasts overexpressing Ris2 formed tumors in Rag2 -/- mice when injected subcutaneously. The putative RIS2 protein shows a high sequence similarity to Xenopus CDT1, Drosophila DUP, and human CDT1, a newly identified DNA replication licensing protein, suggesting that Ris2 is a mouse homologue of CDT1. Cells overexpressing Ris2/Cdt1 exhibited a quicker entry into S phase when released from serum starvation compared to controls. Our results suggest that CDT1, an essential licensing protein for DNA replication, can function as an oncogene in mammals.

Combined adenine phosphoribosyltransferase and N-acetylgalactosamine-6-sulfate sulfatase deficiency

We describe a Czech patient with combined adenine phosphoribosyltransferase (APRT) deficiency (2,8-dihydroxyadenine urolithiasis) and N-acetylgalactosamine-6-sulfate sulfatase (GALNS) deficiency (mucopolysaccharidosis Type IVA, Morquio disease A). Adenine and its extremely insoluble derivative, 2,8-dihydroxyadenine, were identified in the urine, and APRT deficiency was confirmed in erythrocytes. There was excessive excretion of keratan sulfate in the urine, and GALNS deficiency was confirmed in leukocytes. GALNS and APRT are both located on chromosome 16q24.3, suggesting that the patient had a deletion involving both genes. PCR amplification of genomic DNA indicated that a novel junction was created by the fusion of sequences distal to GALNS exon 2 and proximal to APRT exon 3, and that the size of the deleted region was approximately 100 kb. The deletion breakpoints were localized within GALNS intron 2 and APRT intron 2. Several other genes, including the alpha subunit of cytochrome B (CYBA), which is deleted or mutated in the autosomal form of chronic granulomatous disease, are located in the 16q24.3 region, but PCR amplification showed that this gene was present in the proband. A patient with hemizygosity for GALNS deficiency and APRT deficiency has been reported from Japan recently. These findings indicate that: (i) APRT is located telomeric to GALNS; (ii) GALNS and APRT are transcribed in the same orientation (centromeric to telomeric); and (iii) combined APRT/GALNS deficiency may be more common than hitherto realized.

Carcinogen-induced loss of heterozygosity at the Aprt locus in somatic cells of the mouse

Genetic events leading to the loss of heterozygosity (LOH) have been shown to play a crucial role in the development of cancer. However, LOH events do not occur only in genetically unstable cancer cells but also have been detected in normal somatic cells of mouse and man. Mice, in which one of the alleles for adenine phosphoribosyltransferase (Aprt) has been disrupted by gene targeting, were used to investigate the potency of carcinogens to induce LOH in vivo. After 7,12-dimethyl-1,2-benz[a]anthracene (DMBA) exposure, a 3-fold stronger mutagenic response was detected at the autosomal Aprt gene than at the X chromosomal hypoxantine-guanine phosphoribosyltransferase (Hprt) gene in splenic T-lymphocytes. Allele-specific PCR analysis showed that the normal, nontargeted Aprt allele was lost in 70% of the DMBA-induced Aprt mutants. Fluorescence in situ hybridization analysis demonstrated that the targeted allele had become duplicated in almost all DMBA-induced mutants that displayed LOH at Aprt. These results indicate that the main mechanisms by which DMBA caused LOH were mitotic recombination or chromosome loss and duplication but not deletion. However, after treatment with the alkylating agent N-ethyl-N-nitrosourea, Aprt had a similar mutagenic response to Hprt while the majority (90%) of N-ethyl-N-nitrosourea-induced Aprt mutants had retained both alleles. Unexpectedly, irradiation with x-rays, which induce primarily large deletions, resulted in a significant increase of the mutant frequency at Hprt but not at Aprt. This in vivo study clearly indicates that, in normal somatic cells, carcinogen exposure can result in the induction of LOH events that are compatible with cell survival and may represent an initiating event in tumorigenesis.

Targeted base substitutions and small deletions induced by neocarzinostatin at the APRT locus in plateau-phase CHO cells

Treatment of confluence-arrested CHO-D422 cells for 48 h with low concentrations (0.5-3 nM) of the radiomimetic antibiotic neocarzinostatin resulted in an increase in up to 11-fold in the frequency of mutations at the hemizygous APRT locus. Analysis by PCR and DNA sequencing revealed that the mutations were a mixture of base substitutions, small deletions, and large-scale rearrangements. base substitutions occurred preferentially at sequence positions where the drug is known to produce abasic sites with closely opposed strand breaks, e.g., AGT, TGT and AGC, where the abasic site occurs at the underlined base and the strand break occurs opposite the first base in each triplet. These results suggest that the substitutions were produced by replicative bypass of the abasic sites, perhaps during attempted repair of the accompanying strand break. Single-base deletions, which comprised nearly half of all deletions, were targeted to these same sequence positions, suggesting that they may have been generated either by replicative bypass of the abasic sites, or by end-joining repair of double-strand breaks, which are induced the same sites. Quantitative analysis of neocarzinostatin-induced damage to APRT DNA in vitro confirmed the association between lesions involving concommitant damage to both DNA strands, and mutations. The results are consistent the hypothesis that agents which induce such bistranded DNA damage can produce biologically significant levels of mutagenesis even in nondividing cells.

Repair of site-specific double-strand breaks in a mammalian chromosome by homologous and illegitimate recombination

In mammalian cells, chromosomal double-strand breaks are efficiently repaired, yet little is known about the relative contributions of homologous recombination and illegitimate recombination in the repair process. In this study, we used a loss-of-function assay to assess the repair of double-strand breaks by homologous and illegitimate recombination. We have used a hamster cell line engineered by gene targeting to contain a tandem duplication of the native adenine phosphoribosyltransferase (APRT) gene with an I-SceI recognition site in the otherwise wild-type APRT+ copy of the gene. Site-specific double-strand breaks were induced by intracellular expression of I-SceI, a rare-cutting endonuclease from the yeast Saccharomyces cerevisiae. I-SceI cleavage stimulated homologous recombination about 100-fold; however, illegitimate recombination was stimulated more than 1,000-fold. These results suggest that illegitimate recombination is an important competing pathway with homologous recombination for chromosomal double-strand break repair in mammalian cells.

Clonal analysis of delayed karyotypic abnormalities and gene mutations in radiation-induced genetic instability

Many tumors exhibit extensive chromosomal instability, but karyotypic alterations will be significant in carcinogenesis only by influencing specific oncogenes or tumor suppressor loci within the affected chromosomal segments. In this investigation, the specificity of chromosomal rearrangements attributable to radiation-induced genomic instability is detailed, and a qualitative and quantitative correspondence with mutagenesis is demonstrated. Chromosomal abnormalities preferentially occurred near the site of prior rearrangements, resulting in complex abnormalities, or near the centromere, resulting in deletion or translocation of the entire chromosome arm, but no case of an interstitial chromosomal deletion was observed. Evidence for chromosomal instability in the progeny of irradiated cells also included clonal karyotypic heterogeneity. The persistence of instability was demonstrated for at least 80 generations by elevated mutation rates at the heterozygous, autosomal marker locus tk. Among those TK- mutants that showed a loss of heterozygosity, a statistically significant increase in mutation rate was observed only for those in which the loss of heterozygosity encompasses the telomeric region. This mutational specificity corresponds with the prevalence of terminal deletions, additions, and translocations, and the absence of interstitial deletions, in karyotypic analysis. Surprisingly, the elevated rate of TK- mutations is also partially attributable to intragenic base substitutions and small deletions, and DNA sequence analysis of some of these mutations is presented. Complex chromosomal abnormalities appear to be the most significant indicators of a high rate of persistent genetic instability which correlates with increased rates of both intragenic and chromosomal-scale mutations at tk.

Benzo[a]pyrenediol-epoxide induces loss of heterozygosity in Chinese hamster ovary cells heterozygous at the aprt locus

Benzo[a]pyrenediol-epoxide (BPDE), a metabolite of the ubiquitous environmental carcinogen benzo[a]pyrene (B[a]P), has been implicated as a point mutagen. However, as mutational events other than point mutations are also often associated with cancer, we have investigated whether BPDE can induce other classes of mutation. This was done by analyzing mutation at the aprt and hprt loci, both in hemizygous (D422) and heterozygous (D423) Chinese hamster ovary (CHO) cell strains. Southern blotting analysis indicated that BPDE is not an effective producer of either deletions or insertions in the hemizygous environment. The analysis of mutation in the aprt heterozygote was done to investigate the frequency of loss of heterozygosity (LOH) events following BPDE treatment. Using PCR to produce an artificial restriction fragment length polymorphism in the functional aprt allele, BPDE was found to induce LOH in about one-quarter of the mutants recovered. While the precise mechanism of this phenomenon remains obscure, it is likely to have important implications, since similar events involving homologous recombination in somatic cells may have an impact in tumorigenesis.