Chronic diseases may be triggered by the activation of certain genes on a seasonal basis, according to a new study. Serious medical conditions, including heart disease and rheumatoid arthritis, can be more common in winter, and this new finding could help explain why this takes place.

Cambridge University researchers examined the DNA of 16,000 people in an effort to understand how the human immune system changes over the course of a year. They examined nearly 23,000 genes, finding that around 25 percent of human genes are more active at some times of year than others. Many of these are related to inflammation and the immune system, which could explain the rise in illnesses during wintertime.

Medical researchers have known for some time that certain diseases, including Type 1 diabetes and multiple sclerosis, affect patients more at some times of year than others. This new study was the first to show that these changes could be due to seasonal changes within immune systems.

"In some ways, it's obvious — it helps explain why so many diseases, from heart disease to mental illness, are much worse in the winter months — but no one had appreciated the extent to which this actually occurred. The implications for how we treat disease like Type 1 diabetes, and even how we plan our research studies, could be profound," John Todd, director of the JDRF/Wellcome Trust Diabetes and Inflammation Laboratory, said.

Blood and fat samples from people around the world were examined as researchers closely compared samples taken at various times of the year. In addition to genes, cells critical to the functioning of the immune system change, along with the chemical makeup of fat and blood. The types of cells within blood samples, as well as the action of genes, were found to be dependent on the season during which the sample was taken from the patient.

Investigators believe this new study could point the way to medical treatments tailored to the current season, providing another way of customizing health care.

Gene cycles, including those of the immune system transcription factor ARNTL, were found to have yearly variations at opposite times of years in subjects from the Northern and Southern Hemispheres. People in Iceland, which sees months of sunlight and darkness, were found to have yet a different pattern. Put together, this lends further evidence to the idea that the cyclical changes are driven by season.

Genes drive human systems toward a more-inflamed state in winter, which could protect bodies from germs as people are driven into closer quarters by the cold, researchers believe.

Study of how human immune systems and gene expressions can be affected by seasonal changes was presented in the journal Nature Communications.

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