Seasonal Genes

Is it remarkable that the Sun, a ball of nuclear-powered plasma 93 million miles away, can turn our skin into a benign bronze, a raspberry red, and occasionally, a malignant mass? That this faraway energy source can produce changes on a cellular level inside of our bodies? Today, researchers have revealed in the journal Nature that the Sun and the tilt of the Earth may also be responsible for thousands of changes in the way our genes are expressed as we circle around the Sun

It’s easy to imagine how local factors cause bodily responses such as the inflammation from a bacterial infection or even even the feeling of satiation from a full meal. Once a signal is received (from, say, adjacent cells, inside our tummies, or millions of miles distant), an RNA molecule wends a meandering path on the way to creating the hard-working proteins of our cells. From its DNA template inside the nucleus, messenger RNA must leave the double-membrane of the nuclear envelope, through the salty mélange of the cytoplasm, and into the endoplasmic reticulum, where, with the help of existing proteins and a helper called tRNA, a single wayfaring messenger RNA can create hundreds or thousands of copies of a functioning protein that our cells use for everything from food consumption to signaling and immunity. Understanding when and why the genes for these RNA molecules turn on and off has been central to the last fifty years of molecular and cellular biology.

In the study, Xaquin Castro Dopico of the University of Cambridge identified more than 4,000 RNA molecules in our white blood cells (part of our immune system) and adipose tissue (science-code for fat cells) whose expression correspond with seasonal changes. Similar populations in the northern versus southern hemispheres have opposite responses, confirming that this is somehow linked to the seasons and not a random correlation.

“We know that humans adapt to changing environments,” said Chris Wallace, one of the co-authors. “Our paper suggests that human immune systems adapt to show different seasonal variation in equatorial regions with fewer distinct seasons compared to regions at higher and lower latitudes with more pronounced differences between winter and season.” In climates such as northern Europe, an increased immune response during the cold winter months may be an evolved system designed to protect from infection during flu season. Closer to the equator in The Gambia, there is increased immune expression during the rainy season, June-October, “during which time the immune system faces different pathogenic challenges,” say the authors, “such as an increased infectious disease burden, including malaria.” (Interestingly the pattern isn’t seen in Iceland, which according to the authors “could be due to near-24-h[our] daylight during summer.”)

The root cause of these changes is not yet clear. But perhaps the seasonality and long-distance signaling affecting our cells is not so surprising when one considers that plants flower and drop their leaves based on the time of year, cicadas and bears emerge from hibernation based on temperature, and even human cardiac and psychiatric health seems to coincide with long, dark winters spent indoors—although it may make for a great time to stay in a well-lit lab and get a lot of genetic research done!

Image: Marko Kivelä/Flickr