Am I genetically predisposed to being a smoker?
-An undergraduate from California
Genes can definitely influence whether someone will become a smoker. They aren't the whole story, but they can make someone more likely to start and keep smoking.
How? To get at this question, let's split up becoming a smoker into two parts -- taking a risk and becoming addicted. This obviously isn't the whole story but it will give some idea about how genes can impact a behavior like smoking.
As almost everyone knows, smoking is a huge health risk. If you decide to smoke anyway, you are definitely a risk taker.
Being a risk taker has been linked to a certain DNA change in the DRD4 (Dopamine Receptor D4) gene. The idea is that if you have this mutation, you will do riskier things. Like maybe taking up smoking.
How did scientists stumble across this risk taking gene? The first hint was some earlier studies that linked risk taking to a brain chemical called dopamine. As its name implies, the DRD4 gene is involved with dopamine.
The other hint came from the fact that the DRD4 gene comes in so many different versions. The idea was that each version corresponded to a certain level of risk taking. Enough different versions and you can end up with accountants and stuntmen, people with 30 year fixed and ARM mortgages, hang gliders and chess players.
The researchers studied two common versions of DRD4 -- one that has a 4 repeat and one that has a 7 repeat*. What they found was that people with 7 repeats scored higher on the standard test for risk taking, the TPQ Novelty Seeking scale, than did those with 4 repeats.
So, maybe if you have 7 repeats in your DRD4 gene you'll be more likely to take the risk of becoming a smoker. Before running out and trying to get tested, it should be noted that this finding has been a tough one to prove.
Lots of other groups have tried to replicate the results with limited success. Some have been able to show the same thing, others haven't. As you can imagine, something like risk taking can be hard to pin down.
Most likely there are many genes involved in establishing someone's level of risk taking. Not to mention other influences like where you grew up, what your family did on the weekends, etc. But still, genes are probably involved in risk taking.
While the involvement of genes in risk taking is murky, their role in addiction is not. How quickly you become addicted to smoking can obviously influence whether or not you keep smoking.
Once you've taken the risk and picked up a cigarette, how hard is it for you to not pick up a second. Or a third. Or...
When people are addicted to smoking, we usually think of them as being addicted to nicotine. And nicotine works by causing dopamine, that same brain chemical we talked about earlier, to be released in the brain.
Dopamine is involved in feelings of pleasure. Whenever someone takes a puff of a cigarette, the nicotine stimulates the brain to make dopamine. And it feels good.
The feelings of pleasure quickly wear off though. So, the smoker needs to take another puff. And another. And another.
One of the reasons the effects of nicotine wear off so quickly is because of a gene called CYP2A6. The CYP2A6 gene is involved in ridding our body of nicotine.
There are some versions of the CYP2A6 gene that don't work, though. People with these defective copies of CYP2A6 break down nicotine more slowly than people with working copies.
And these people are less likely to become addicted to nicotine. Why getting more bang for the buck would make someone less likely to become addicted isn't obvious, though.
Maybe the dizziness and nausea that happen when you smoke your first cigarette lasts longer for these people. And because of this, they don't stick with it long enough to become addicted.
These folks also suffer less from the harmful side effects of nicotine because they need fewer puffs to keep their dopamine high. Fewer cigarettes mean a lower risk for cancer.
OK, so are you genetically predisposed to becoming a smoker? Maybe, if you have two working copies of the CYP2A6 gene and 7 repeats in your DRD4 gene. As long as the science is right, that is.
There are most likely other as yet undiscovered genes involved in becoming a smoker. And we've totally ignored nurture in our discussion of nature. Peer pressure, family, tobacco advertising, how cool Humphrey Bogart looked in Casablanca are all important as well.
But wait a minute...how much of peer pressure is genetic? We'll save that for another answer.
*A repeat is a DNA sequence that, well, repeats itself. Something like the CAG in CAGCAGCAG. These kinds of repeating sequences are a common place to develop DNA changes that increase or decrease the number of repeats. Diseases like Huntington's and Kennedy syndrome are examples of diseases caused by these mutations.