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Researchers say a link between vitamin D and a gene known to cause multiple sclerosis has been identified.

The study in this week's PLoS Genetics journal could explain one of the key environmental risk factors for multiple sclerosis, also known as MS.

The prevalence of MS is higher in countries further away from the equator, which is thought to be related to the amount of sunshine exposure and vitamin D3 production.

Neurologist Dr Bill Carroll, from Perth's Sir Charles Gairdner Hospital, says this study is the first to link the environmental and genetic risk factors that cause MS.

"What they've been able to do with this study is show that vitamin D3 is closely related to the part of the genetics of the immune system that we think is most closely related to susceptibility to MS," he said.

Dr Carroll is also chair of Multiple Sclerosis Research Australia's research management council.

Response element

The researchers focused on a gene known as HLA-DRB1, which is part of a family of genes that make up the major histocompatibility complex. This complex plays a critical role in the body's immune system and autoimmunity.

Previous studies have identified that a variant of HLA-DRB1 increases the risk of MS, but there is also strong geographic variation in risk that appears to be linked to sun exposure.

The team of researchers were able to track down a region of the HLA-DRB1 gene that contained a vitamin D "response element".

This suggests that vitamin D is directly involved in the expression of the immune system gene.

"The relationship between environment and genes has not been able to be correlated until this piece of evidence," Dr Carroll said.

Turning on itself

The discovery has implications for other autoimmune conditions such as Type 1 diabetes and inflammatory bowel disease, Dr Carroll says, as these diseases also have similar geographic distribution.

MS is an autoimmune disease that results in the breakdown of myelin, a protective fatty sheath that surrounds nerves.

Symptoms of MS vary from person to person, but can include tremors, paralysis and memory loss.

Australian researchers have previously shown that people living in Tasmania are five times more likely to develop MS than those living in Queensland.

"If you're in the northern hemisphere and you're born at the end of the northern hemisphere winter, born in May, you have 20 per cent greater chance of developing MS than if you're born at end of the northern hemisphere summer in November," Dr Carroll said.

According to Multiple Sclerosis Research Australia there are more than 18,000 Australians diagnosed with MS - three-quarters of those are women.