DNA mismatch repair, genome instability and cancer in zebrafish

Abstract

The objective of this study was to find out whether the zebrafish can be an appropriate model for studying DNA repair and cancer. For this purpose three fish lines were used that lack components of an important mechanism for the repair of small DNA damage: DNA mismatch repair. These fish are therefore hereditary predisposed to develop cancer. Several mismatch repair enzymes also have a function in meiosis, the reduction cell division that is necessary for the formation of germ cells. Male zebrafish with mutations in the mismatch repair gene mlh1 were found to be sterile and to display an arrest in spermatogenesis at metaphase I. In contrast, female mutants are fully fertile, but their progeny shows high rates of dysmorphology and mortality within the first days of development, which was found to be caused by aneuploidy resulting from meiosis I chromosomal missegregation. Surprisingly, the small percentage of progeny that develops normally has a complete triploid genome, consisting of both sets of maternal and one set of paternal chromosomes. The three different fish families with defects in the mismatch repair genes mlh1, msh2 and msh6 were indeed found to be more sensitive to the development of cancer. They develop a range of tumour types at low frequencies. Predominantly neurofibromas were observed, indicating that zebrafish mimic distinct features of the human disease resulting from lack of mismatch repair, and are complementary to mouse models. The advantage of experimental accessibility of zebrafish was used in combination with an in vivo assay for mutation detection to study the in vivo effect of alkylation damage on lethality and mutation frequency in developing embryos. Consistent with the damage-sensing role of the MMR system, mutant embryos lacking the MMR enzyme MSH6 displayed lower lethality than wild types after exposure to ENU and MNU. In line with this, alkylation-induced somatic mutation frequencies were found to be much higher in wild type embryos than in the msh6 loss-of-function mutants. These mutations were found to be chromosomal aberrations, likely caused by chromosomal breaks that in turn result from stalled replication forks. Additionally, we found that mismatch repair deficiency does not improve ENU mutagenesis in the zebrafish germ line. Altogether, this indicates that the action of the mismatch repair system in zebrafish with regard to the formation of germ cells, the growth of tumours and the response to chemotherapeutic agents is similar to that in human and mouse. This means that the zebrafish can be considered a useful model. However, within all these areas there are clear differences, which are equally important as they enable us to study new aspects of cancer - research that is not possible in mouse and human.