Cigarette smoking is a major cause of illness and death worldwide. But it is a complex behavior, and how much people smoke, how hard they find it to quit, and the impact of long-term smoking on health varies greatly among individuals. A substantial portion of this variability is genetic. Two years ago, deCODE discovered the first common, single-letter variation (SNP) in the sequence of the human genome, on chromosome 15q25, associated with nicotine addiction and risk of lung cancer.

Today, deCODE scientists and academic colleagues from 23 institutions in a dozen countries build on this work with the discovery of common SNPs on chromosomes 8p11 and 19q13 that among smokers increase the number of cigarettes smoked per day (CPD), a measure of nicotine addiction, and increase risk of lung cancer.

The study, published today in the online edition of Nature Genetics, analyzes detailed genotypic and smoking data from more than 130,000 participants. Both of the new SNPs are common, and in smokers each copy carried associates with a small increase in smoking quantity – about half a cigarrette per day – but an approximately 10% increase in risk of lung cancer compared to non carriers. This is about one third of the increase in lung cancer risk conferred by the SNP on chromosome 15q25. But taken together these variants can identify a sizeable proportion of smokers whose health is at even greater risk than average from their habit, information which may serve as an additional spur to smoking cessation. The study, ‘Sequence variants at CHRNB3-CHRNA6 and CYP2A6 affect smoking behavior,’ can be found at www.nature.com/ng.

“Smoking is bad for anyone’s health. It is even worse for some, and today’s discoveries continue to strengthen our ability to identify who those people are and give them a compelling additional reason to quit. We plan to incorporate these SNPs into our testing products to do that. What we do not yet know is exactly how this additional risk is conferred. To some degree these variants suggest that those for whom nicotine is more addictive are driven to smoke more, increasing their exposure to environmental risk. But given the quite substantial corresponding increases in risk of lung cancer it may also be that they make people more susceptible to the noxious effects of tobacco smoke. What is clear is that these variants — which are all near genes that encode nicotine metabolizing enzymes and receptors — are giving us a solid starting point for finding answers to advance personal and public health,” said Kari Stefansson, executive chairman and president of research at deCODE and senior author on the paper.

deCODE wishes to thank all those who participated in and contributed to this study. It was supported in part by the National Institutes of Health (R01-DA017932), and the European Comission’s GENADDICT project (LSHM-CT-2004-005166) and ENGAGE smoking consortium (HEALTH-F4-2007-201413).

Scientists from deCODE genetics have identified a clear link between one genetic variant and susceptibility to nicotine dependence and will publish their results in the April 3 issue of Nature. Moreover, in part because of its impact on smoking behavior, each copy of the risk variant of this SNP confers an approximately 30% increase in risk of lung cancer and a 20% increase in risk of peripheral arterial disease (PAD), a common and debilitating constriction of the arteries to the legs.

deCODE scientists came upon the genetic variant by closely examining the genetic makeup of more than 10,000 smokers. They then followed up with an analysis of 32,000 patients and controls from Iceland, New Zealand, Austria, Sweden, Italy, the Netherlands and Spain for lung cancer and PAD, two common diseases strongly associated with smoking.

Kari Stefansson, deCODE CEO, expressed the importance of the discovery: “These findings provide an example of the power of human genetics for shedding light on the most complex health challenges. Not only have we made a convincing link between a single genetic variant and a behavioral disorder – greater smoking quantity and addiction to nicotine – but also demonstrated how this risk factor translates into risk of lung cancer and PAD.”

Stefansson also pointed out that deCODE’s genetic profile service, deCODEme, will test for the gene immediately.

Details of the smoking gene study, which was funded in part by the European Commission, and from the National Institute of Drug Abuse of the U.S. National Institutes of Health, are available at www.nature.com.