When you hear the word ‘cluster’ the first thing that comes to mind is probably a group of stars – not MND. During a plenary talk by Dr Ettore Beghi on Saturday 7 December the question was: ‘Is there anything we can learn from clusters in MND?’
What do we mean by clusters?
Dr Beghi began his talk by defining a cluster as “a health related event with temporal or geographical activity.” Meaning: a number of people are diagnosed with a disease in either a specific time period or area. He also highlighted that if rare diseases are found in clusters then this may require environmental factors to be investigated.
We already know that MND is thought to be caused by a combination of subtle genetics, environmental and lifestyle factors. At the MND Association we regularly receive enquiries about apparent ‘clusters of MND’ and I was looking forward to hearing Dr Beghi’s talk on the subject.
BMAA and Guam
Dr Beghi gave specific examples of clusters that have been reported in the scientific literature, which included the well referenced cluster of ALS (the commonest form of MND) Parkinson’s Disease complex (ALS-PDC) in Guam. The indigenous people on this island (the Chamorros) were found to have ALS-PDC at an incidence of around 100 times higher than the rest of the world – however this incidence has decreased dramatically in the last 40 years.
It was thought that this incidence of ALS-PDC may be caused by an inherited form of ALS-PDC, however genetic studies on the Chamorros have not identified any genetic mistakes. Hence the environment, specifically β-N-methylamino-l-alanine (BMAA), is thought to play more of a role in this disease.
Dr Beghi said: “We have no idea how BMAA arrives in the body of these individuals. Also BMAA is found worldwide and cases of ALS-PDC are not as high as they are in Guam”
Read more about BMAA in our previous blog back in September here.
BMAA and Guam was not the only example of a cluster. Dr Beghi gave examples of specific MND clusters in France, America and Italy as well as conjugal MND cases (married couples who both develop MND) in France.
Problems and can we learn anything from clusters?
There are a number of problems with epidemiology research into MND clusters. This is primarily due to there being no definitive cause=effect. Also, clusters occur randomly in nature and are often small sample sizes.
Dr Beghi stressed that “The ALS/PDC cluster in Guam is the only cluster supported by robust epidemiology evidence, however the link between BMAA and ALS is still unproven”
During discussions Dr Beghi stated that: “outside factors are very difficult to prove and any study investigating clusters should not forget the possibility of genetic factors.”
Environmental factors in MND
Gabor Mezei, who was also presenting, stressed from the beginning: “There are no known environmental causes of ALS. There are only ‘suspected’ risk factors.” Dr Weisskopf presented a talk later in the session investigating lead exposure where he looked for specific genetic differences. There was general agreement in the audience when he stated:
“Gene-environmental interactions are of great importance in MND epidemiology research. We shouldn’t just focus on one without the other and we must be careful if we just go looking at one side of this equation.”
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