DNA methods for detecting and analyzing mutations in vivo

On key lesions and all that: a tribute to Paul Lohman

This paper is a tribute to Paul Lohman at the occasion of his retirement from the position of Professor in the Medical Faculty at the Leiden University in The Netherlands and as Director of its Department of Radiation Genetics and Chemical Mutagenesis. Paul's contributions to the science of genetic toxicology are discussed in the context of more recent insights as to how mammalian cells process DNA damage, and how this may lead to cancer and, possibly, aging. Starting with his work on the characterization of UV-induced DNA repair in cultured cells from xeroderma pigmentosum patients and the development of methodology for monitoring the removal of UV-induced lesions in human cells, the concept of the key lesion is introduced. Among the myriad of DNA lesions that can be induced in DNA as a consequence of exposure to a range of natural or synthetic mutagens, key lesions are the ones responsible for subsequent adverse effects, for example, because they give rise to mutation. The development of methods using immunofluorescence microscopy to detect and identify such key lesions and quantitate them at the single cell level, is one of the highlights of Paul's career. Based on the perceived need to evaluate mutational end points in vivo in relation to specific lesions identified by his immunofluorescence methods, Paul subsequently made crucial contributions to the development of the first transgenic mouse model to measure mutations in chromosomally integrated reporter genes. In parallel to his experimental work, Paul greatly contributed to genetic toxicology at the theoretical level by his work on the development and evaluation of methods for assessment or prediction of risks of exposure to environmental mutagens. Finally, Paul has served the discipline of genetic toxicology in a more administrative role in various ways, both locally as one of the founders of the Medical Genetics Center South-West Netherlands and internationally by playing a prominent role in organizations such as ICPEMC. Here, his numerous contributions to the journal Mutation Research, both as author on many papers and as Executive Managing Editor should not go unmentioned.

Gene and sequence specificity of DNA damage induction and repair: consequences for mutagenesis

The field of DNA repair has been expanded enormously in the last 20 years. In this paper, work on gene and sequence specificity of DNA damage induction and repair is summarized in the light of the large and broad contribution of Phil Hanawalt to this field of research. Furthermore, the consequences of DNA damage and repair for mutation induction is discussed, and the contribution of Paul Lohman to the development of assays employing transgenic mice for the detection of gene mutations is highlighted.

Transgenic mice as model systems for studying gene mutations in vivo

Transgenic mice carrying bacterial reporter genes have been developed to study spontaneous or induced mutations in vivo. Mutations can be analysed upon the efficient retrieval of reporter genes from mouse genomic DNA into a suitable bacterial host. These systems allow, for the first time, the direct correlation of mutational mechanisms in vivo with their ultimate physiological endpoints, for example, cancer or ageing.

Unscheduled DNA synthesis induced by the antitumor drug vincristine in germ cells of male mice

The cytotoxic and genotoxic effects of vincristine (VCR) on germ cells of male mice were investigated. Several parameters (the scale and the time course of unscheduled and scheduled DNA synthesis in spermatocytes and spermatids and the number of sperm present in caudal epididymides) were analyzed. Our results show that intraperitoneal administration of a single dose of VCR resulted in: (1) damage to DNA in spermatocytes and spermatids; (2) a reduction in the rate of germ-cell development; and (3) killing of the non-proliferating spermatid cells. Damage of DNA in germ cells indicates that VCR may have potential genetic hazards to patients who receive it in antitumor therapy.

High somatic mutation frequencies in a LacZ transgene integrated on the mouse X-chromosome

To study spontaneous and induced mutagenesis in vivo we recently constructed a series of transgenic mice harboring different numbers of bacteriophage lambda shuttle vectors, provided with a LacZ mutational target gene, integrated in their genome. The transgenic mice enabled analysis of spontaneous and induced mutation frequencies in postmitotic tissues like liver and brain. The obtained data indicated spontaneous mutation frequencies in the order of 10(-5)-10(-6). Here we report a 25-100 times higher spontaneous mutation frequency in liver and brain DNA of mice from strain 35.5, with the lambda-gt10LacZ concatemer integrated on the X-chromosome. These results indicate the presence of a mutational 'hot spot' in the mammalian somatic genome in vivo.

DNA damage metabolism and aging

As a result of permanent exposure to low levels of various endogenous and exogenous genotoxic agents, large numbers of lesions are continuously induced in the DNA of cells of living organisms. Such lesions could lead to dysfunction of cells and tissues, and they might well be the underlying cause of the age-related reduction of homeostatic capacity and the increased incidence of cancer and other diseases of old age. The rate of damage induction as well as the persistence of the lesions depends on the activity, efficiency and reliability of a wide variety of molecular defense systems. However, a certain degree of imperfection seems to be a general characteristic of most of these defense systems and this could lead to a gradual accumulation of DNA alterations during aging. Even when the original lesions are quickly removed, they can still lead to secondary changes in the DNA, such as DNA-sequence changes and changes in gene expression. This process would be accelerated in case of the occurrence of an age-related decline in the efficiency of these molecular defense systems. This review deals with the present knowledge on the occurrence of 'spontaneous' DNA damage in aging organisms, its potential sources, the influence of preventive and processive cellular defense mechanisms and its consequences in terms of DNA-sequence changes, DNA conformational and configurational changes and changes in gene expression. In general, it can be concluded from the data discussed here that, in spite of a number of discrepancies and conflicting results, an age-related accumulation of DNA alterations occurs at all levels, e.g., chemical structure, DNA-sequence organization and gene expression.

Utility of short-term tests for genetic toxicity

By definition, short-term tests (STTs) for genetic toxicity detect genotoxic agents, not carcinogens specifically. However, there is sufficient evidence, based on mechanistic considerations alone, to say that genotoxic agents are potential carcinogens. STTs have high statistical power, are almost always replicated, can be performed rather easily under various sets of experimental conditions, are relatively inexpensive, and detect a variety of endpoints relevant to carcinogenesis. In addition, several STTs have shown considerable utility in evaluating the genotoxic effects of real-world, environmental complex mixtures as well as the antimutagenic effects of various pure compounds and complex mixtures. STTs are likely to continue to be refined, resulting in STTs that are increasingly more relevant to human mutation and disease. Their utility should not be judged solely against the questionable standard of a rodent carcinogenicity assay.

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.

A search for DNA alterations in the aging mammalian genome: an experimental strategy

In order to experimentally test the hypothesis that at the basal level senescence is caused by instabilities in DNA, techniques are required that allow the sensitive detection, quantification and characterization of DNA damage and DNA sequence changes in various organs and tissues of naturally aging mammals. In this article a strategy is presented that should allow the analysis of DNA damage metabolism in aging mammals from the original changes in the chemical structure of DNA (DNA damages) via their processing (DNA repair) to the molecular endpoints in terms of gene mutations and DNA rearrangements. With respect to the detection of DNA damage, a short overview is provided of recently emerged biochemical and immunochemical methods that can be applied immediately to study the spectrum of DNA damages in various organs and tissues of aging animals or humans. By contrast, before one is able to study low frequency changes in DNA sequence organization in somatic tissues, formidable problems have yet to be overcome. This is mainly due to the fact that the scale of highly advanced recombinant DNA techniques recently developed is almost totally devoted to the analysis of heritable genetic factors. In order to be able to study low frequency changes in DNA sequence organization in somatic tissues, we have initiated experimental approaches based on recently emerged shuttle vector technology in combination with transgenic mice, and on new electrophoretic separation principles. These approaches and their potentialities for testing somatic mutation theories of aging are discussed.