Hereditary conditions in which the loss of heterozygosity may be important

A paternity case with three genetic incompatibilities between father and child due to maternal uniparental disomy 21 and a mutation at the Y chromosome

A parentage case is described that revealed a potentially erroneous exclusion from paternity in three systems, two on chromosome 21 and one on chromosome Y. Follow-up tests, especially of chromosome 21, were subsequently performed. Actually, the child's chromosome 21 showed alleles of maternal but not of paternal origin being consistent with a maternal uniparental disomy of chromosome 21. The third genetic incompatibility was observed at the Y chromosome and attributed to a usual one-step de novo mutation. This case is emphasizing the (generally adopted) requirement that an exclusion from paternity must not be based on the absence of paternal alleles at genetic systems all located on the same chromosome. In fact, the need for extended typing programmes is demonstrated.

Genetic predisposition as a basis for chemoprevention, surgical and other interventions in colorectal cancer

Strategies of cancer prevention are generally developed with the population at large in mind. However, special attention is warranted for those persons with rare genetic traits associated with a greatly elevated risk of developing colorectal cancer (CRC) and some other malignancies: Orphan diseases demand Orphan preventive measures! Recent advances in modern genetics have enhanced our understanding of several genes and the specific germ-line mutations responsible for colorectal carcinogenesis. A number of features provide evidence for a genetic predisposition to CRC. These include typical clinical and histological features of a particular syndrome, a familial aggregation of CRC and associated malignancies, young age at onset of CRC, occurrence of multiple neoplasias and/or unusual localisation of the tumour (e.g., right side of the colon). In hereditary colorectal cancer, genetic testing can easily be demonstrated as cost-effective.

Cell cycle regulation of the endogenous wild type Bloom's syndrome DNA helicase

Bloom's syndrome (BS) is a rare human autosomal recessive disorder characterized by an increased risk to develop cancer of all types. BS cells are characterized by a generalized genetic instability including a high level of sister chromatid exchanges. BS arises through mutations in both alleles of the BLM gene which encodes a 3' - 5' DNA helicase identified as a member of the RecQ family. We developed polyclonal antibodies specific for the NH2- and COOH-terminal region of BLM. Using these antibodies, we analysed BLM expression during the cell cycle and showed that the BLM protein accumulates to high levels in S phase, persists in G2/M and sharply declines in G1, strongly suggestive of degradation during mitosis. The BLM protein is subject to post-translational modifications in mitosis, as revealed by slow migrating forms of BLM found in both demecolcine-treated cells and in mitotic cells isolated from non-treated asynchronous populations. Phosphatase treatment indicated that phosphorylation events were solely responsible for the appearance of the retarded moieties, a possible signal for subsequent degradation. Together, these results are consistent with a role of BLM in a replicative (S phase) and/or post-replicative (G2 phase) process. Oncogene (2000).

Genotoxic activity of four inhibitors of DNA topoisomerases in larval cells of Drosophila melanogaster as measured in the wing spot assay

Four inhibitors of DNA topoisomerases namely nalidixic acid, camptothecin, m-amsacrine and etoposide, have been evaluated for genotoxic effects in the wing spot test of Drosophila melanogaster. This assay assesses somatic recombination and mutational events. We studied nalidixic acid as an inhibitor of bacterial DNA gyrase, camptothecin as a topoisomerase I inhibitor, as well as m-amsacrine and etoposide as topoisomerse II inhibitors. The genotoxic effects were determined from the appearance of wing spots in flies trans-heterozygous for the recessive markers multiple wing hairs (mwh) and flare, as well as in flies heterozygous for mwh and the multiply inverted TM3 balancer chromosome. From our results it appears that whilst nalidixic acid and m-amsacrine were compounds that did not increase the incidence of mutant clones, camptothecin and etoposide proved to be significantly genotoxic in this test, being camptothecin more effective than etoposide. A significant proportion of the total spot induction was due to mitotic recombination, confirming previously reported data. On the other hand, the cotreatments of each topoisomerase inhibitor with the alkylating agent ethyl methanesulfonate (EMS) indicate that, while nalidixic acid, m-amsacrine and etoposide show a tendency to an antagonistic interaction, camptothecin shows an additive effect, suggesting mechanistic differences between the activity of the four inhibitors of DNA topoisomerases studied.

Isolation and molecular characterization of spontaneous mutants of lymphoblastoid cells with extended loss of heterozygosity

The human major histocompatibility complex comprising the HLA class I and II genes provides a versatile source of natural heterozygous loci. This polymorphic genetic system allows analysis of the mechanistic aspects of loss of heterozygosity (LOH), a major phenomenon observed at tumor suppressor genes in human cancer cells. Four lymphoblastoid cell lines, ORI, TK6, WI-L2-NS and VH, were used to adjust current HLA immunoselection protocols to quantify loss of HLA-A2 in human lymphoblastoid cell lines. The modified selection protocol was used to isolate independent spontaneous HLA-A2 mutants from the lymphoblastoid cell line ORI. The frequency of spontaneous loss of HLA-A2 in ORI was 1.7 x 10(-5). By HLA typing 35 spontaneous HLA-A2 mutants, we showed that 74% of the HLA-A2 mutants also lost expression of the HLA-B allele, which is located on the same haplotype as HLA-A2. Microsatellites on both arms of chromosome 6 were used for molecular characterization of the spontaneous HLA-A2 mutants. Loss of heterozygosity at various loci on the p-arm or loss of an entire chromosome 6 was found in 80% of the mutants. Surprisingly, it appeared that a presumed mitotic recombination event in the cell line ORI itself had resulted in homozygosity of all markers distal from the HLA locus up to the telomere. This greatly limited the detection of mitotic recombination, resulting in LOH up to the telomere, on the short arm of chromosome 6 in this cell line. However, gene dosage analysis detected two copies of the remaining D6S265 allele in mutants which showed LOH at various loci along the p-arm. This suggested that recombination resulted in LOH in these mutants. The lymphoblastoid cell line TK6 did contain informative microsatellites along the complete chromosome 6. Mutants of TK6 either retained heterozygosity of all p-arm markers, showed LOH of all p-arm markers or showed loss from a breakpoint up to the telomere. These data indicate that recombination and chromosome loss both are important mechanisms involved in loss of the HLA-A2 allele in vitro. Such mechanisms may be involved in LOH in vivo and contribute to loss of tumor suppressor alleles.

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.

Protection by coffee against somatic genotoxicity in Drosophila: role of bioactivation capacity

The protective effects of coffee against somatic mutation and mitotic recombination induced by cyclophosphamide (CPH), mitomycin C (MMC) and urethane (URE) were evaluated in the standard (ST) and high bioactivation (HB) crosses of the wing spot test in Drosophila melanogaster. These two crosses are characterized by different constitutive levels of cytochrome )-450-dependent enzyme activities. 3-day old larvae transheterozygous for the wing cell markers mwh (multiple wing hairs) and flr3 (flare3) were fed until pupation on medium containing a genotoxin alone or its combination with different concentrations of instant coffee. subsequently, the wings of the resulting adult flies were analysed for detecting single spots (mwh or flr3) originating from mutational or recombinational events as well as twin spots (mwh and flr3) originating exclusively from recombination. The results showed high sensitivity of the HB cross to URE. Co-administration of instant coffee was effective in exerting significant dose-related inhibitory effects on the genotoxicity of URE in the ST and the genetically susceptible HB cross. Similarly, coffee showed significant dose-related inhibitory effects on the genotoxicity of MMC in both crosses. The same protective effect was also observed with one concentration of coffee in combination with CPH. Pretreatment of 2-day-old HB larvae with coffee for 24 hr followed by treatment with URE was also effective in significantly reducing the induction of mutation and recombination. The magnitude of the protective effects of coffee against these three genotoxins was independent of the genotype of the larvae used for treatment, that is it was independent of the bioactivation capacity of these larvae. The study demonstrates the suitability of this assay for obtaining qualitative and quantitative data on the result of interactions among a genotoxin, an inhibitor of genotoxicity and bioactivation capacity of the host.

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.

Colorectal cancer: lessons for genetic counselling and care for families

Cancers of the colon and the rectum are the second leading cause of malignancy in European countries with similar incidence rates for men and women and, therefore, one of the major health concerns. Emphasis is placed on the early detection of a developing neoplasm in order to improve the life expectancy of patients and their quality of life. Colorectal cancer (CRC) is an excellent model for studying the etiology and pathogenesis of a common malignancy and the complex multistage process of carcinogenesis. Abundant clinical and pathological evidence suggests that CRC arises from benign adenomas that proceed through a series of steps to metastatic carcinomas. Following the discovery of oncogenes and, more importantly tumor suppressor genes, Fearon & Vogelstein (1990) proposed a scheme of genetic events which are associated with colorectal tumorigenesis. Genetic linkage studies have recently identified another type of gene for colon cancer susceptibility that seems to act by destabilising the genome.

Genotoxic activity of mebendazole in Aspergillus nidulans

Mebendazole is an anthelmintic drug widely used in Cuba and in Mexico. Its interaction with tubulin interferes with the assemblage of the mitotic apparatus in the parasite cells, thus suggesting a possible genotoxic activity leading to chromosomal malsegregation. The heterozygous diploid strain D30 of Aspergillus nidulans was used to establish the ability of mebendazole to induce mitotic recombination and/or chromosomal non-disjunction, and the haploid strain FGSC #219 of A. nidulans was used to study the ability of mebendazole to induce point mutations in the methG suppressor system. Our results show that mebendazole can induce chromosomal non-disjunction but it fails to promote point mutations.

Genetic instability on chromosome 16 in a human B lymphoblastoid cell line

Mutagenesis at the aprt locus in TK6 human lymphoblasts has been found to occur at an unusually high rate (1.2 x 10(-9)) for a homozygous diploid locus. Evaluation of linked microsatellite polymorphisms demonstrated that loss of heterozygosity (LOH) accompanies conventional intragenic sequence alterations in each APRT- mutant. LOH occurred without allele preference. The extent of loss was highly uniform, ranging from 16q12 to 16qter in 36/38 APRT- mutants. Fluorescence in situ hybridization (FISH), used in conjunction with microsatellite analysis, demonstrated that the loss was not attributable to physical deletion, nondisjunction, or nondisjunction with reduplication of the remaining chromosome. LOH thus appears to be recombinationally mediated. FISH analysis also detected translocations affecting chromosome 16 in 4/20 APRT- mutants examined. APRT- mutants appear to arise as part of a genetic instability phenomenon since three distinct genetic alterations affecting chromosome 16 are recovered in single clones at a detectable rate. These events may be mechanistically related to early events in gene amplification.

DNA-sequence specificity of mutations at the human thymidine kinase locus

We have established a system for the study of DNA-sequence specificity at a functionally heterozygous thymidine kinase (tk) locus in a human lymphoblastoid cell line (TK6). Characterization of the parental locus demonstrated that the 2 tk alleles were fortuitously distinguished by differential gene expression. One round of PCR amplification yielded a specific tk cDNA product only for the functional parental allele. Analysis of cDNA from newly mutated alleles which retain substantial levels of expression is thus simplified. Amplification and sequencing of tk genomic sequences was used for analysis of low expression mutants, and in order to distinguish and characterize deletion and splicing mutations. DNA-sequence analysis of the parental locus identified a frameshift in tk exon 4 of the non-functional parental allele, and surprisingly, an exon 7 frameshift mutation in the functional tk allele. This exon 7 frameshift results in a predicted alteration of the final 21 amino acids of the TK protein, and a C-terminal extension of 131 additional amino acids. Since TK6 is phenotypically TK+, we can infer that this major C-terminal modification does not eliminate enzymatic activity. The system was utilized for the analysis of 36 spontaneous TK- mutants. Loss of heterozygosity accounted for 58% of the mutations, 11% were attributable to intragenic deletions, and the remainder involved point mutations, primarily G:C to A:T transitions.

Evidence for high-frequency allele loss at the aprt locus in TK6 human lymphoblasts

The aprt locus in TK6 human lymphoblasts has been previously shown to have an unusual mutation frequency (5 x 10(-8) and a gene dosage of 2. Measurements of mutation rate (1.2 x 10(-9)) reported here, confirm the mutation-frequency observations. These results are not easily accommodated by models of the gene as functionally homozygous or heterozygous. Characterization of all exon and intron sequences identified no polymorphism which could distinguish two heterozygous aprt alleles. Furthermore, autoradiographs of 16 spontaneous APRT- mutants demonstrate a variety of unique sequences. If aprt were heterozygous, wild-type sequence from the alternate allele would be observed at positions where mutations have occurred. These observations cannot be explained by conventional loss of heterozygosity in which the same mutated allele would be repeatedly recovered. We therefore propose that aprt is a functionally homozygous locus. APRT- mutants may arise by conventional mutation of one allele, and high-frequency loss or conversion of the alternate allele.

Familial and genetic aspects of colorectal carcinogenesis

There is abundant clinical and pathological evidence which suggests that colorectal cancer arises in a sequential manner through a series of events that can be followed during the progression of the disease from early adenoma through to metastatic disease. The molecular events that are associated with the initiation and progression of the disease are gradually being unravelled. As the molecular characterisation of colorectal cancer continues, new mechanisms by which the disease progresses are becoming evident. In this short review, a brief description of current knowledge of colorectal cancer development is presented.

International Commission for Protection against Environmental Mutagens and Carcinogens. Recombination and gene conversion

Recombination is an important aspect of DNA metabolism. It leads to rearrangements of DNA sequences within genomes. Such genome rearrangements seem to be ubiquitous, since they play a role in evolution, human health and biotechnology. In medicine one important aspect of recombination is its role as one possible step in the multistep process of carcinogenesis. Since recombination may occur as a cellular response to DNA damage, the protection of cells from recombination-inducing agents, so-called recombinagen, should eliminate possible deleterious effects resulting from damage-induced DNA recombination. During the last few years, the awareness of the importance of recombination phenomena has substantially increased and the development of assay systems detecting recombinagens has progressed. The need for considering recombinagenic effects as a safety aspect of chemicals has gained ground in the field of genetic toxicology. This paper summarizes present knowledge concerning the occurence, inducibility, detection and toxicological interpretation of DNA recombination.