I’ve just finished a three year project looking at the reasons behind the differences in xenobiotic toxicokinetics in different multiple strains of mice that have a broad genetic diversity to mimic the broad human diversity. We studied the environmental toxin benzene in 20 different strains of mice that have wide differences in genetic makeup and saw an incredible range of blood levels of benzene and metabolites. We did some genome-wide association mapping and found that the most likely cause for the differences was due to differences in the rates of elimination. It surprised us that we were tracking changes in excretion kinetics. We suspect it might be the same case with people.
We went in with an idea we’d be seeing differences in cytochrome P450 isozymes particularly, the major metabolic enzymes in the liver, and it turned out to be transporters in the kidney.
Xenobiotics are compounds that are manufactured, or made chemically, that are foreign to life systems. They do have a biological effect: benzene is very important because it causes leukaemia, and that’s a large problem for people who work around petroleum products – benzene is a component of gasoline and many petroleum-based products. There’s widespread human exposure.
I was very interested in organic chemistry in undergraduate school. I was interested in synthesizing new pharmaceuticals as a young scientist, and I started a course in pharmacology to define more what pharmacology is and how organic chemistry could make new drugs. I was very fascinated with the role of the metabolism of environmental chemicals, such as those in cigarette smoke and diesel exhaust, and how they could produce toxic side effects like cancer. It’s a chemical reaction that occurs in the body that produces activated reactive intermediates; little chemical synthesis enzymes in your body that take an inert, harmless chemical and make it into one that is potentially toxic or carcinogenic. It evolved from my interest in chemistry and the way chemical reactions in silico have similar reactions in our liver and kidneys, and how they often, instead of making less harmful metabolites, make more harmful ones.
What’s next really does relate to a chemistry background: I’m in a programme that is doing a lot of high throughput screening with tens of thousands of compounds, and the question’s arisen: how can one predict, based on the structure of a compound, what kind of adverse effect it’s likely to produce? So I’ve been delving into the whole realm of quantitative structure-activity relationships (QSARs). The European Union, the OECD, the US EPA have all got models and databases, and so I’m jumping into those and bringing that sort of analysis to the National Toxicology Program high throughput screening initiative under the Tox21 research initiative of the EPA. We’re going to be doing a lot more chemistry, relating structure to toxic effects. That’s going to end up, I think, making fewer and fewer whole animal tests necessary in toxicity testing.
My favourite mentor was the fellow who hired me here at NIH, H. B. ‘Skip’ Matthews. He hired me in 1987 as a Senior Staff Fellow, and he was very instrumental in having my early successes here at the institute and made an atmosphere here that was so wonderful – I reminded him that when we interviewed I told him I’d stay here for two, maybe three years max, and now I’ve been here for twenty-four years… it was so wonderful to work for him I didn’t want to leave.
Skip’s a Southerner, and a Southerner in America has always got a story or a saying or something that fits every occasion, but in the larger arena his basic message was to make sure that all your dealings are transparent, and that your data is transparent and available for anyone to look at; your logic, your writing is transparent so that people know exactly what you’re thinking and why you’re thinking it; and you keep your relationships transparent and open. I paraphrase that to my kids when I say “If you tell people the truth, you don’t have to have a good memory.” If you’re transparent with your boss and transparent with your kids, transparent in life, you can use that as a rule to live by. It’s served him well and I consider that excellent advice.
When I was hired here, I did some very interesting mechanistic toxicology research to answer a particularly pointed question. We had a series of compounds that showed mutagenicity, DNA damage, any number of adverse affects at the molecular level, yet when you exposed animals to them, they didn’t cause cancer. And a mutagen should cause cancer, it’s one of the frameworks of modern molecular biology. Well, I had about thirty chemicals that didn’t do that! They hired me, and said, “Tell us why.” Of course, with my metabolism background I assumed it would be differences in metabolism, and I worked for 18 months, but couldn’t find anything. There was no metabolic reason these compounds shouldn’t be carcinogens. So I went back to the first principles of chemical carcinogenesis that I learned in undergraduate school. There are three phases of chemical carcinogenesis: initiation, promotion and progression – and I was only looking at the first phase. I started evaluating the second phase – promotion – and lo and behold, that was where they failed. They did not promote the initial lesion they’d caused in the DNA, because they weren’t toxic enough to cause cells to start replicating. So the assumption is that the DNA lesion was either repaired, or the damaged cell ultimately died without producing a tumour. I demonstrated it with a variety of different classes, and it was kind of an “ah ha!” moment. You learn a lot when you’re wrong, when your hypothesis just doesn’t work.
Particularly from a therapeutic point of view, what’s unanswered right now is how stem cell therapies are going to be used to reverse or repair damage. And since no good deed goes unpunished I’m really interested to see what kind of adverse effects stem cell therapy may produce in people. Right now everyone’s looking at the benefits, but I’ll bet you anything there’s some unknown toxicities out there.
The old model of principal investigator-initiated research projects is not going to be the favoured model in the Twentyfirst Century. The notion of the lone scientist working in his laboratory on something of interest only to him and his postdoc and grad student has evolved into a larger, multi-focussed workgroups with many different areas of expertise. You’re going to have to learn to talk to people with different specialities from yours, you’re going to have to learn a different language from the one you learned in grad school. Toxicologists will have to learn to talk pathology, basic biochemistry, bioinformatics, statistics. They don’t have to be experts but they do have to be able to work with people of different specialties. Scientists used to have an image of being shy, introverted, sitting in a white coat in their laboratory and not talking to their neighbour, but that’s over. The new paradigm really is going to be that you have to get out there and communicate with scientists just as good as you are, but in a different field. The creative process is going to be not just one person alone but a whole group. That’s a little different to how I was trained in graduate school.
In this clip, Mike talks about toxicology becoming a predictive science, and how it will replace animal testing, and keeping biochemistry in the family.