The tell-tale spleen

Most people can hold their breath for about 30-40 seconds, but a population of sea nomads, called the Bajau, who live a marine hunter-gatherer lifestyle over the seas of southeast Asia can routinely manage to remain submerged at considerable depths for minutes at a time on just one lungful of air. And it turns out that their larger than average spleen - the fist-sized immune organ that sits at the top left of your abdomen - is what enables them to do it, as Georgia Mills heard from Melissa Ilardo...

Melissa - What we found was that they’d in fact adapted in a number of ways, but one of those ways is through bigger spleen size. And you might wonder what that has to do with diving but it turns out that when you dive it activates this human dive response and this is present in a number of diving mammals.

So what happens is, first your heart rate slows down, then you have peripheral vaso-constrictions so your blood vessels actually get smaller to preserve the oxygenated blood for your internal organs, and then the last thing that happens is this contraction of the spleen. The spleen holds oxygenated red blood cells, and by contracting it gives you this oxygen boost. We believe that this larger spleen adaptation that we see in the Bajau, where a group of these sea nomads is allowing them to dive for longer.

Georgia - How did you find out these people had large spleens? It’s not exactly an external organ is it?

Melissa - No. I actually took a portable ultrasound machine down with me to Indonesia and I took images of these people’s spleens. It was a little bit of weird request  to meet someone and say hey, can I take a picture of your spleen?

Georgia - A bit awkward?

Melissa - Yeah, a little.

Georgia - Was there a way of finding out whether this was an adaptation that came within life, so just practicing diving makes your spleen grow, or is there something else going on?

Melissa - Yeah, that was actually a really important question that we wanted to address because it could be that simply the activity of diving, because it’s causing this spleen contraction, could be increasing spleen size.

What we did to ask that question was we measured the spleen sizes of Bajau people who were diving and also of those who aren’t diving. At one point in time all Bajau were diving; now it’s about 50/50 in the population at least that we visited. So we were able to get about 50% Bajau people who were diving and 50% who weren’t, and when we compared those spleen sizes they all had about the same sized spleen. So that pointed to the fact that it might be something genetic rather than something that’s happening during their lifetime.

Georgia - Did you find the genes responsible for this?

Melissa - We did, yeah. We performed a selection scan to look at what regions of the genome of the Bajau have been under selection and, in doing that, we found a variant in the region of this gene PDE10A. What PDE10A does, or one of the things that it does, is to affect thyroid hormone levels, and the variant that we see in the Bajau is associated with higher levels of the thyroid hormone T4.

In mice it’s been shown that, if they have extremely low levels of T4, they have a drastic reduction in spleen size; however that effect is actually shown to be reversible through a T4 injection. So it seems to be that the connection is that the Bajau have these higher thyroid hormone levels and that’s leading to an increased spleen size which is then leading to an advantage while they’re diving.

Georgia - What does this tell us then?

Melissa - Well, it tells us a number of things. It tells us that there are ways for the human body to adapt to conditions of acute hypoxia. Hypoxia, or low oxygen is a really important issue in a lot of medical contexts. It’s been studied in other humans before but mostly by looking at these high altitude populations that are adapting to living at chronic levels of low oxygen. This population is instead adapting to these very acute bouts of low oxygen so they’re just suddenly cutting off their oxygen supply and it turns out that the body has ways of avoiding the negative effects of that as well.

Georgia - Is there anything that we could take forward from this then and use medically?

Melissa - Yeah, we’re hoping that there could be. A lot of the insights that they got from looking at these high altitude populations have actually already started to translate into medical applications, so we’re hoping that we might be able to do the same with what we learned from the Bajau and maybe other diving populations. In a lot of critical care conditions when people stop breathing or when they enter these bouts of acute hypoxia, it seems like people react to this very differently. It’s not really clear why certain people react so poorly to these hypoxic conditions when others don’t. It could be that maybe something we learned from our study and others like it is that there’s some kind of genetic predisposition to be able to react in certain ways to these conditions.