Different seasons could affect the function of up to a quarter of human genes, suggests a study published in Nature Communications last week. The activity of particular genes involved in immune defence and inflammation were found to be increased in the winter months. This may exacerbate symptoms of disorders in which an autoimmune response is involved such as rheumatoid arthritis, heart disease and type 1 diabetes.
The study, conducted by an international research team, analysed 16,000 blood and adipose (fat) tissue samples from a variety of countries in both the northern and southern hemispheres. By examining levels of expression at different times of the year, they determined that there was seasonal variation in certain genes, as well as in immune cells, adipose cells and blood composition.
Inverted patterns were identified between the hemispheres, due to the winter and summer months occurring at opposite times of the year. The ARNTL gene in particular is a key contributor to the suppression of inflammation and was found in mice to display reduced activity in the winter months, indicating that the inflammatory response is higher in cold weather.
Seasonal variation may be attributed to environmental factors such as daylight and temperature. Countries close to the equator, where the temperatures are consistently high, experienced similar increases in genetic immunity during the rainy season, when malaria is more prevalent. Conversely, cold countries such as Iceland were found to experience comparatively very little seasonal variation.
Professor John Todd, one of the study authors from the University of Cambridge, argues that there is a potential evolutionary advantage to the changes as they may have helped to fight infection during prehistoric winters, despite acting as a possible risk factors for certain diseases in modern times.
The findings overall support the common belief that people are healthier in summer, though other factors, including nutrition and stress, are also involved in gene function.