Is MND hereditary?
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This article explains our current understanding of the role of genetics in MND.
This is a common question and concern, and the answer has become more complicated by recent discoveries. This is only a summary of some of the issues and the field is changing rapidly.
Genes and disease
Many neurological diseases, including MND, are thought to arise through a complex mix of our genetic 'blueprint', responsible for the development and maintenance of our nervous system, and interactions with other factors such as environmental exposures and random damaging events in cells that tend to occur more commonly as we age. Although we have learned a great deal about the genetic factors that might determine whether someone is born with a risk of MND later in life, environmental and age-related factors have been more difficult to identify. This means that people seem to develop MND because of a series of factors amounting to 'bad luck', rather than because of major lifestyle factors, such as their occupation or diet.
Familial and sporadic MND
Only one in 20 patients with MND will be aware of another close family member (parent, brother or sister, grandparent, aunt, uncle or cousin) with the disease, or with the related condition frontotemporal dementia (FTD). In this case, which we call familial MND, the genetic contribution is the major factor in causing the disease, but it is clearly not the whole story, since it still requires many years of the aging process in most cases, before the disease reveals itself. Sometimes it is not possible to know with certainty if MND is familial, due to adoption or because medical problems in the family have not been talked about openly. We have also found that occasionally MND presenting with walking problems has been misdiagnosed as multiple sclerosis (MS), and FTD has been diagnosed as Alzheimer's disease or just 'dementia'.
So, more than 90 percent of patients will assume that their case is a 'one off'. We call this sporadic MND, and studies so far suggest that the overall risk for their children is only very slightly increased compared with the background population. Since the absolute lifetime risk of any individual developing MND is roughly 0.25 percent (i.e. one in 400 people get MND), a small increase in risk still means that the chance of developing the disease for anyone with a relative with sporadic MND is very low indeed.
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We can easily test for abnormalities in two genes in MND patients who have also had a close family member with the condition. These genes are called SOD1 and C9orf72. There are several other genes which have been associated with MND, but these are very rare, and currently routine testing is not available. Those with a faulty C9orf72 gene more commonly have family members with dementia. Those carrying this faulty gene have a 50 percent (1 in 2) chance of passing it on to their children. However, these children may live long lives and then die of more common causes such as heart disease or cancer, before they get MND or FTD. For this reason we do not recommend testing family members without symptoms of disease, as this can cause unnecessary worry. Occasionally those who have a family member with MND, and are planning to have their own children, want to consider screening their embryos for the faulty gene to ensure only those without the gene are implanted. However, this situation is unusual and needs a detailed discussion with a genetic counsellor. It is important to bear in mind that the prospect for treatment and even prevention of conditions like MND is likely to increase in their children's lifetimes.
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Should all MND patients be tested for the faulty C9orf72 gene?
If we test the DNA, in a research setting, of every MND patient who does not have a family history of MND or FTD, we find that a small number (1 in 15) have a faulty C9orf72 gene after all. This means that their children have a 50 percent risk of carrying the same faulty gene. While this undoubtedly means they then carry a much higher risk of MND or FTD later in life, there is still uncertainty about when this might occur, and importantly it may not be in their natural lifetime.
Genetics will not explain the cause of MND in every patient, and the result does not yet influence treatment. A positive test can help to understand a little bit more about why MND has arisen, and can help provide information to other family members, especially those thinking about planning their own families. However, a positive result can also cause anxiety for other family members, and a negative result is not fully reassuring, as we do not yet have a complete scientific understanding about the contribution of faulty genes.
Genetic testing is therefore generally only for those who have symptoms of MND or FTD, and where the neurologist thinks there is a high chance of a positive result providing clear and useful information. It is an individual decision for you to make, based on a discussion with your neurologist, and often also a genetic counsellor trained to cover the wider issues.
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Finally, we think that studying families where there is a known faulty gene will provide us with a revolutionary way to see the very earliest changes and develop better tests and treatment for MND. These vital studies can be done without the need for a person to know their own genetic result. The Oxford MND Centre is undertaking ground-breaking work with families affected by MND, studying unaffected relatives of patients.
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If you would like to know more about the genetic factors causing motor neuron disease, or are worried about your own risk because MND has occurred in your family, in the first instance you should ask the advice of your GP. Depending on where you live it may be possible for you to receive advice from a Consultant specialising in Clinical Genetics or from a neurologist who has a specialist interest in MND. At the Oxford MND Centre we are happy to receive referrals from anywhere in the UK, from GPs or neurologists, of patient with familial MND who might want to know more about their disease or to participate in our research programme.
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