Perhaps an explanation is in order as to why we suggested in last week's post on C.P. Snow's Two Cultures that science can't be the solution to many of the problems people believe it can solve. This may already be obvious, but we were interested to see no other commentaries last week that addressed this, so we thought perhaps we should clarify what we meant, at least with respect to disease.
As we've discussed already in this blog, determining disease causation can be difficult. For more than 100 years, epidemiology has struggled with this, and now genetics faces the same problem. Infectious disease should be easy to explain--find the infectious agent and you've found the cause. (Though, even confirming the agent isn't always straightforward.) But, what really 'causes' HIV/AIDS? The virus? If so, why is it that poor and minority populations are at highest risk? In the same vein, what 'causes' cholera, or malaria, or lymphatic filariasis, or river blindness, or any number of diseases that could be killing people in the rich world, but aren't? Is it the parasite, or poverty and the lack of political will to clean up the water and eliminate mosquitoes?
Similarly, what 'causes' the complex chronic diseases that are the significant killers in rich countries? In some ways, science knows a lot less about this--it's easier to find the proximate cause of infectious disease, but often harder to eliminate the more distal causes. With complex diseases, it's the distal causes (poor diet and lack of exercise, say) that are malleable rather than the proximate (genes and/or environmental triggers). People often talk about 'gene by environment interaction', but that is hard to measure, or even to understand. What is it about fat in the diet that increases risk of breast cancer, for example? And, while public health measures, such as clean water, can control many infectious diseases, public health is less likely to effectively prevent or control chronic diseases; that often turns out to be the responsibility of the individual, but health education is notoriously ineffective, and legislating health behavior is notoriously unpopular.
Since the end of World War II, the prevalence of type II diabetes, obesity and gallbladder disease have grown to epidemic proportions among Native American and Mexican American populations. The epidemiology, or patterns of disease in these populations make it seem possible that a simple genetic 'cause' might be responsible--perhaps not a single gene, but a small number. But, no such gene or causal pattern has emerged, even after 40 years of searching, and even if a simple genetic explanation is found, the ancestors of the people now suffering from these diseases had the same gene or genes but they didn't have diabetes or obesity or gallbladder disease. So, can we still say that these putative genes 'cause' disease? This seems to be a textbook case of gene by environment interaction, but if so, what are the environmental triggers, and how do they change gene action?
The same can be said of genes 'for' asthma or heart disease or breast cancer or other kinds of cancers, diseases that have increased in prevalence in the last two, three, four decades. Hundreds of millions of dollars have been spent on the search for these genes, and only a few, with small effect, have been found. And, even when they are found, they might explain some increased risk, but knowing them can't prevent or cure the disease.
Science is useful in many ways, but science alone can't prevent disease.