Mitochondrial DNA linked to Personality type
Psychologists have known for many years that some sorts of personality type are associated with certain health outcomes. But these are associations and it could be that lifestyle factors common to the psychology and the disease are what link the two. But, speaking with Chris Smith, Luigi Ferrucci, who directs the US National Institute of Ageing, has made a surprising discovery: a connection between personality type and mitochondrial DNA in the bloodstream…
Luigi - Some people are more resistant to stress than others. And our question was why, what is the mechanism by which, you know, aptitude and personality mediate the effect on health.
Chris - And what did you measure to find out?
Luigi - One of the things that we have recently discovered, not us, but the entire world is that the mitochondria, which are the source of energy in the cell also the hub for many of the environmental stress, you know, and that makes sense because you know where the energy is is probably the best way to kind of shift energy from one function to another and under stress that become extremely important. So we hypothesised that people with certain personality will have different characteristic of mitochondria and these different characteristic of mitochondria will mediate the relationship between personality and health outcome. And in this case we look for mortality because mortality and survival are very, very solid and strong health outcomes. There is a biomarker of mitochondria that you can measure in the blood. When you do a genome sequencing in the blood, there are many, many, many molecule of mitochondrial DNA, and the amount of mitochondrial DNA is an indicator of mitochondrial volume and mitochondrial health. So we hypothesised that lower mitochondrial DNA copy number in the blood will be associated with different personalities.
Chris - Who did you look at though? You've mentioned what the possible association is, but you haven't said in whom. So who are you looking at?
Luigi - So when you look this association study, the problem is that we're never sure that this result is by chance. So in order to make sure that the result was strong, we measure this both personality and mitochondrial DNA copy number in two different populations. One is the Baltimore longitudinal study of aging and one is the Sardinia study, a study of aging in Sardinia. So we wanted to measure the same thing in two population that are an ocean apart.
Chris - So you've got from these study participants, you've got their personality type. Yes. And you've got blood samples and you are now asking are there any associations between what's going on in terms of the number of mitochondria in the blood and Yeah, well-documented psychological tests that put people into different categories of, of what their personality is?
Luigi - Personality is measured with a highly standardised questionnaire and divide people and measures different aspect of personality. And there is a very strong and wide literature suggesting of this personality trait. Neuroticism is associated with adverse health outcome, higher risk of cardiovascular disease and higher risk of cancer. And in fact what we found is that neuroticism were strongly associated with lower level of mitochondrial DNA copy number. It was amazing that the result were exactly the same in these two very, very independent and very, very different populations.
Chris - Is this one snapshot in time? Because obviously what we don't know is whether this is a chicken or egg thing. Is it cause or effect?
Luigi - That's a very good question, but has also a simple answer. Personality does not change over time. So even if we were doing longitudinal studies, because the predictor does not change over time, we will not find anything different on what we have found in the cross-sectional study.
Chris - How do you explain this then?
Luigi - We were sceptical. We do know from the literature that there is an association between neuroticism and mortality. Can we do an analysis to see whether this association is mediated by mitochondrial DNA copy number? So we did something that statistically can help you understand whether three variables one after the other are related. So that personality affect the DNA copy number and through this effect affects mortality. So that if you adjust the analysis or mitochondrial DNA copy number, the association between neuroticism and the mortality basically fade - disappear - because it's all explained by DNA copy number. And that's exactly what we found there was a very, very strong mediation.
Chris - So what do you think is going on? Why is there this connection?
Luigi - Well the my answer is hypothetical. We don't have any proof or what is the mechanism right now, but our hypothesis is that the stress affect mitochondria and the mitochondria become dysfunctional and the mitochondria that become dysfunctional tend to be eliminated by the mechanism that's called mitophagy because otherwise they will create damage to the cell. And because of that we find that people that are more sensitive to stress tend to have lower mitochondria. As I mentioned, this is something that you know, is, is an hypothesis that drive our interpretation of this study, but will have to be demonstrated using possibly animal models.