Have you ever wondered why some health conditions such as arthritis worsen in the winter months? You may put it down to the colder weather, but a new study finds it may actually be down to seasonal changes in gene expression.

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Researchers found that the activity of almost one quarter of our genes varies by season.

Led by researchers from the University of Cambridge in the UK, the study reveals that the activity of almost one quarter of our genes varies depending on the season – some are more active in the winter months, while others are more active in the summer.

What is more, the researchers found that seasonality appears to affect the composition of immune cells in our blood and adipose tissue, or fat.

The study was recently published in the journal Nature Communications.

Study author Prof. John Todd, director of the JDRF/Wellcome Trust Diabetes and Inflammation Laboratory at the Cambridge Institute for Medical Research at the University of Cambridge, and colleagues says their findings may explain why some conditions worsen in winter months and may have important implications for the treatment of such conditions.

To reach their findings, Prof. Todd and colleagues analyzed the cell composition and gene expression in samples of blood and adipose tissue from more than 16,000 people living in countries in the northern and southern hemispheres, including the UK, Iceland, the US, Australia and The Gambia.

The researchers found that of the 22,822 genes identified from the samples, the activity of 5,136 differed depending on the time of year the samples were taken.

In addition, they found that the types of immune cells found in blood and adipose tissue also differed depending on whether the samples were gathered in the winter or summer months.

In samples taken during European winter, for example, the team identified greater levels of genes associated with cardiovascular and autoimmune diseases in the blood. The researchers say this may explain why cardiovascular conditions and autoimmune diseases – such as type 1 diabetes and multiple sclerosis – are more prevalent in winter.

“This is a really surprising – and serendipitous – discovery as it relates to how we identify and characterize the effects of the susceptibility genes for type 1 diabetes,” says study author Prof. Todd, adding:

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.”

The researchers identified seasonal differences in gene expression and cell composition across all populations studied, identifying opposing patterns between those living in the northern and southern hemispheres.

Icelandic residents, however, showed much weaker seasonal patterns. The team says this may be explained by the almost 24 hours of daylight in summer and 24 hours of darkness in winter that occurs in Iceland.

The findings also revealed an increase in blood immune cells among people from The Gambia that correlated with the rainy season from June-October – the season in which malaria and other infectious diseases are more prevalent.

“We know that humans adapt to changing environments,” says study author Dr. Chris Wallace, also of the JDRF/Wellcome Trust Diabetes and Inflammation Laboratory. “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.”

One gene in particular that caught the team’s interest was ARNTL – a gene that has been associated with suppression of inflammation in mice studies.

ARNTL was found to be less active in the winter but more active in the summer. They hypothesize that if this gene suppresses inflammation in humans, then people in the northern hemisphere would experience higher levels of inflammation in winter.

As such, the team says that in winter, individuals with inflammation-related conditions would reach the “threshold” at which symptoms worsen a lot sooner, and they suggest drugs that target the cause of inflammation could lead to more effective treatment in the winter months.

The researchers also found that some genes associated with vaccination response were more active in winter, which suggests that individuals may benefit more from vaccines if they are administered in winter when the immune system is ready to respond.

While the team is unable to explain what drives the seasonal variation in gene expression and cell composition, they hypothesize that it may be down to differences in exposure to daylight and the temperature of our environment.

Whatever the reasons, Prof. Todd says “it is perhaps understandable that people want to head off for some ‘winter sun’ to improve their health and well-being.”

Commenting on the team’s findings, Prof. Mike Turner, head of infection and immunobiology at the Wellcome Trust, says the study “provides real evidence” supporting the popular notion that we are healthier in the summer months. He adds:

Seasonal variation to this extent is a fascinating find – the activity of many of our genes, as well as the composition of our blood and fat tissue, varies depending on the seasons. Although we are still unclear of the mechanism that governs this variation, one possible outcome is that treatment for certain diseases could be more effective if tailored to the seasons.”