People with a combination of genes linked to lower vitamin D levels appear to be at greater risk of developing multiple sclerosis, according to the authors of a recent study. The paper’s researchers used data from a very large genetic study of 14,000 people with MS (specifically looking at a chemical produced from vitamin D) and compared their genetic backgrounds to people without the disease. The results showed that people with decreased vitamin D due to their genetics were twice as likely to develop multiple sclerosis – although the risk was still very low.
The finding sheds further light on a previously suggested link between MS and sunshine levels, a common source of vitamin D. But what does this mean for our understanding of the causes of MS? And more importantly, does this help us in the quest to develop new and better treatments for the disease?
Multiple sclerosis (MS) is a complex disease, with many different symptoms and a propensity to come and go (relapse and remit as doctors would say). But when it strikes it can be devastating, often completely incapacitating the individuals who suffer from it. MS is one of the most common neurological disorders in the UK, affecting around 100,000 people.
Although we have known about MS for almost 150 years – it was first described in 1868 – it still isn’t clear as to what causes the disease. We do know that the symptoms are the result of a process called demyelination, where nerve cells lose a protective sheaf (made out of a protein called myelin). This means that they can no longer pass on the messages that control muscles and relay information back to the brain. Sometimes the myelin can be replaced by the body repairing itself, causing the symptoms to go into remission, but as the disease progresses, and more myelin is lost, the nerves cells themselves can die, leading to permanent disability.
At least part of the process that causes someone to start losing myelin from their nerve cells is autoimmunity. This is where the immune system, which normally protects us against threats from outside our body, starts attacking our own tissue – although again why this occurs isn’t clear.
Like many human neurological diseases, it is thought that MS develops as a result of the interplay between an individual’s genetics and their environment – so partly due to the genes inherited from someone’s parents and partly due to things that happen to them during their lives.
For example, large studies of MS, documenting where it occurs and how many people are affected, have revealed that there are big differences between the incidence (or rate at which the disease occurs) in different countries and even within countries.
In UK, there are many more cases in Scotland than there are in Cornwall, once you adjust for the size of the population. This seems to hold true across the northern hemisphere – the further away from the equator you are, the greater the number of MS cases (although not south of the equator, for reasons we don’t fully understand). One theory to explain this is that low exposure to sunlight can increase the risk of developing MS – so because Scotland is further away from the equator, with less hours of daylight than Cornwall, this might explain the difference.
As one of the big consequences of exposure to sunlight is the production of Vitamin D, this has led some scientists to suggest that Vitamin D (or lack of it) could be one of the contributing factors when someone develops MS. Whether this is really the case, however, is still far from clear.
This is where the study, published in the Public Library of Science Medicine, comes in. Although it suggests that a combination of gene variants that can lead to decreased vitamin D (in this particular population sample) is correlated with an increased risk of MS, it doesn’t provide us with direct evidence that one causes the other.
However, taken together with other evidence pointing to a role for vitamin D in MS it does suggest that this might be significant, although without providing an explanation as to why.
This last point is an important one, as it is only when we have a clear understanding of the mechanics as to why there is a correlation between vitamin D and the risk of developing MS will we make progress on the road to developing new ways to slow down or prevent the disease. This is particularly the case for vitamin D and MS, as the straightforward approach of providing increased vitamin D as a supplement to people with MS is one that has already been tested, and was found not to make a consistent significant difference.
This doesn’t necessarily mean that there isn’t a link between vitamin D and MS. For example, it could be that having lower vitamin D levels makes it more likely that you will develop MS, but has no impact after that – and so treating people who already have MS with vitamin D would make no difference. If this were the case, then perhaps increasing vitamin D levels in people at risk of developing MS might be a way to intervene. Unfortunately we don’t have a way to accurately predict who will develop MS (people could potentially be tested for the genetic variants above but the risk is still very small). Perhaps the most important contribution of this study is that it provides us with pointers as to what we need to be doing at in the earliest stages of the disease, before symptoms have even become apparent.