Only about 1% of epilepsy cases are caused by a mutation in a single gene. Although these cases are rare, they often occur with a very clear family inheritance pattern, so they have been the starting point for scientists to investigate the genetic basis of epilepsy.

Currently, about 60% of cases of epilepsy have no cause identified. Many of these sporadic, isolated cases are likely to have a genetic basis, where a number of genes interact. Each gene has a small effect, which on its own would not cause epilepsy, but all their effects add up to make a person susceptible to seizures. However with no family inheritance pattern, it's difficult to find out which genes are responsible.

In order to investigate these cases, genetic test results from huge numbers of people with epilepsy must be sifted together. Effectively, the smaller the effect you're looking for, the larger the number of cases you've got to look for it in. No single hospital sees enough people with epilepsy to do this kind of study on its own, so collaboration between centres is the key.

The Epilepsy Genetics Consortium is a collaborative research
group which aims to carry out this kind of investigation. In an article published in Lancet Neurology in November 2007, they report a study looking at 279 genes and their association with epilepsy. All of these genes had been implicated by previous research studies as possible causes of common types of epilepsy, or as contributing to its development, or affecting its response to treatment.

In order to study these genes, they examined genetic material from 2717 people with epilepsy and 1118 people who didn't have epilepsy, collected at four independent research centres in the UK, Ireland, Finland and Australia.

No genes were identified as common to all epilepsies, and none were identified as common to all cases of a particular type of epilepsy: there was no 'epilepsy gene' nor, for example, any 'absence epilepsy gene'. However some gene mutations did show an association with certain types of epilepsy, but only ever in one research centre, not in all patients. So, for example, genes found to be significantly associated with one type of epilepsy in patients in the UK were not significantly associated with the condition in patients in Finland, and so on. However, five genes were identified as requiring further study.

It's clear that the genetic basis to apparently random epilepsy cases is complex. A large number of gene variations are involved in causing seizures, but exactly which ones are in each case depends on the population being studied. It's not yet clear whether this is due to genuine genetic differences between populations or to differences in how doctors in the different countries classify the different types of epilepsy.