DNA damage, DNA repair, cell proliferation, and DNA replication: how do gene mutations result?

Abstract

My son enjoys visiting my lab on weekends because the university internet connection is much faster than our home modem connection. He also likes to read DNA sequences to find that single base change that often spells the cause of human genetic disease. And, when he finds a change, he asks me how it occurred. We discuss DNA damage, repair, and replication, but I can never exactly answer that “how” question for him. It takes me back to the 1970s! I vividly remember the end of a backpacking trip in the Great Smoky Mountains National Park, enjoying our first non-freeze-dried meal in 10 days and reading the newspaper article about the discovery of a mouse germ cell “supermutagen.” It was N-ethyl-N-nitrosourea (ENU) inducing specific-locus mutations in mouse spermatogonia cells. Russell et al. (1) concluded that “ENU can serve as a model compound in exploring the effect of such factors as dose-response, dose fractionation, sex and cell stage on the mutagenic action of a chemical.” Subsequent studies showed that ENU is also a powerful rodent carcinogen, inducing mutations in oncogenes (2, 3). But, the question still hangs in the air: Why is ENU such a potent mutagen? It damages DNA, but what happens to yield that gene mutation? Twenty years of research have brought us to the brink of understanding the molecular “how” for ENU and many other mutagens. And now we have a new aid for this quest. In a paper in this issue of PNAS, Bielas and Heddle (4) use ENU as the mutagenic agent in an elegant study of in vitro mutation induction in transgenic mouse cells that carry the bacterial lacI gene. What I find so novel is that the method allows them to determine when the DNA damage is converted or fixed into a heritable mutation, and how much damage is repaired.