ENU induces mutations in the heart of lacZ transgenic mice

The use of transgenic mouse models as somatic mutation assays allows determination of mutation in all tissues of the mouse, including non-dividing tissues. In this regard, these models can be used to study the possibility that mutations can be induced in mitotically quiescent organs such as the heart. Mutations are generally thought to be associated with mitotic processes of DNA replication. Mutations, however, are also postulated to occur in the absence of mitosis as the result of DNA repair. In order to determine whether or not mutations could be induced in the heart, we analyzed the mutant frequency in the hearts of F(1) (Muta Mouse X SWR) mice that had been treated acutely with 250 mg/kg ENU and sampled at days 10, 35, and 70 post-treatment. A significant increase in mutant frequency at day 70 shows that mutations can be induced in the heart. Since the heart contains small numbers of non-muscle cells, additional mechanisms that could explain these results were also considered. The effect of ENU-induced cell proliferation or a sub-population of rapidly dividing cells is ruled out by C(14)-thymidine uptake studies which showed minimal proliferation. By the same token, the influence of ex vivo mutations (i.e., DNA adducts fixed as mutations during replication in the bacteria) is ruled out by the observed time course of mutations, as well as experimental evidence showing that such mutations are not detected in the lacZ assay.