Despite the success of genome-wide association studies, many experts rightly point out the need to remember that most common diseases have a significant environmental component.
For example, cigarette smoking increases one’s risk of lung cancer 10- to 20-fold, according to the National Cancer Institute. But other studies have found that even after controlling for smoking, having first-degree relatives with lung cancer increases one’s own risk by a modest amount, suggesting at least a small genetic component.
Lung Cancer Risk
A trio of papers published today in Nature and Nature Genetics has found a SNP on chromosome 15 that is linked to lung cancer risk.
Two of the studies, by Hung et al. and Amos et al., matched lung cancer cases and healthy controls for smoking behavior and compared their genotypes across 300,000 SNPs. Both studies found that one of the variants increased a subjects’ odds of lung cancer by about 1.3 times compared to those without the variant; having two copies increased subjects’ odds by 1.8 times.
A 1.8-fold increase may not seem like much compared to the 10- to 20-fold increase seen for smokers. But if this genetic effect is independent of the effect of smoking, smokers with variant might find themselves at 18- to 36-fold increased risk compared to nonsmokers without the variant.
Cigarettes Per Day
A third study questions whether the SNP’s effect on lung cancer risk is truly independent of smoking. Thorgeirsson et al. found evidence that the same SNP is actually linked to nicotine dependence. On average, being homozygous for this variant increases the number of cigarettes subjects smoked by about one cigarette per day. The authors also reported that most of the increased risk of lung cancer conferred by this variant was due to its effect on smoking. Thus, an environmental component itself appears to have a genetic component.
However, the effect of SNP on smoking quantity was seen only if a subject smoked at all. This variant wasn’t connected with whether subjects smoke, only with how much they smoke once they start.
So in the nature vs. nurture debate, perhaps there is room for free will after all.