Environmental Contaminants, Recombination and Cancer : Investigations on the molecular mechanisms of spontaneous and induced recombination in the hprt gene of Chinese hamster cells

Abstract: Recombination results in rearrangement of the genetic material contained in the chromosomes inherited from the mother and father and this process is also involved in repairing damage to the genome. Occasionally, the machinery that performs recombination malfunctions and this may lead to illegitimate recombination events which inactivate tumor suppressor genes or activate protooncogenes. Recent studies have demonstrated that certain carcinogens (designated recombinogens) can induce genetic recombination without giving rise to mutations in mutation assays. PCBs and heavy metals are examples of recombinogens which are often stable, non-reactive and unable to bind covalently to DNA.The aim of this thesis has been to elucidate basic mechanisms involved in genetic recombination and to investigate how foreign compounds might affect these mechanisms. V79 Chinese hamster cell lines with well-characterized mutations in the hprt gene were used for this purpose. These cell lines exhibit partial gene duplications, including extra exons, in the hprt gene. The two cell lines employed here can revert to the wild-type phenotype by loss of these exons either by homologous or non-homologous recombination. Using this experimental system, these two types of recombination were found to be affected differently by exposure to various agents and, thus, that distinguishing between these two pathways was shown to be important for proper evaluation of recombinogenic effects.A novel finding presented here is that genetic recombination can be induced by brominated flame retardants, indicating that these compounds may be human carcinogens. Several heavy metals and arsenic are recognized as human carcinogens, but the mechanisms underlying their carcinogenicity is not yet fully understood. We show here that arsenic and certain heavy metals induce genetic recombination, which might be of importance in this connection.The basic mechanisms involved in genetic recombination in the hprt gene of V79 Chinese hamster cells were also investigated here. It was found that homologous in contrast to non-homologous recombination is coupled to DNA replication. A model for how recombination can be induced in the vicinity of replication forks is presented.In conclusion, this thesis suggests that investigation of genetic recombination may provide a valuable tool for identifying human carcinogens.