They found that people who began smoking before age 17 and who possessed two copies of the high-risk haplotype had from a 1.6-fold to almost 5-fold increase in risk of heavy smoking as an adult.

One haplotype for the nicotine receptor put European-American smokers at greater risk of heavy nicotine dependence as adults, but only if they began daily smoking before the age of 17.

The high-risk haplotype is common in the three study populations, and European-American populations in general, ranging in frequency from 38 percent to 41 percent.

"We know that people who begin smoking at a young age are more likely to face severe nicotine dependence later in life," said Robert Weiss, a professor of human genetics at the U. and lead author of the study. "This finding suggests that genetic influences expressed during adolescence contribute to the risk of lifetime addiction severity produced from the early onset of tobacco use."

Exactly what constituted severe nicotine dependance and heavy use were defined by several guidelines, including how difficult participants said it was to quit, how often they had stopped and then started smoking again and how soon after waking in the morning they smoked their first cigarette of the day. The range varied from several hours later to before their feet touched the floor. That group almost invariably reported they had been smoking since they were teenagers, the researchers said.

The findings are significant, said Nora Volkow, director of NIDA, and adds to the recent "explosion in the understanding of how small genetic variations can impact all aspects of health — including addiction.

"As we learn more about how both genes and environment play a role in smoking, we will be able to better tailor both prevention and cessation programs to individuals," Volkow said.

An NIDA tobacco use study last year found that 7.1 percent of eighth-graders, 14 percent of 10th-graders and 21.6 percent of 12th-graders had used cigarettes at least once in the month prior to being surveyed.

Although Utah has the lowest percentage of youth smokers in the country, and although cigarette use among teens has declined slightly in recent years, more than 3 million 12- to 17-year-olds nationwide smoke, according to the NIDA study.

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E-mail: jthalman@desnews.com