Researchers have shed light on the genetic basis of crippling migraines.

Nauseating, often debilitating, headaches affect 15 to 20 per cent of adults in developed countries, yet they have been stubbornly hard to explain.

But a new study Finnish shows why some families are susceptible to migraines and how genetics may influence the type of migraine they get.

“For a long time, we have been wondering why common disease (like migraines) aggregates in families,” said Aarno Palotie of the Institute for Molecular Medicine in Finland and the Broad Institute of MIT and Harvard.

Previous migraine research identified two ways migraines might run in families.

Three genes (CACNA1A, ATP1A2 and SCN1A) are associated with hemiplegic migraine, a severe form of migraine that induces stroke-like symptoms.

These genes fall under a Mendelian inheritance model, meaning the two copies of a gene a child inherits from his or her parents determine a trait or disease. Sickle cell anaemia could be thought of as Mendelian.

Migraines could also run in families as a polygenic trait, which means that a group of genes collectively influence that characteristic.

Like being dealt a bad hand in a card game, each common genetic variant may have only a small individual effect, but the collection of common variants builds up to influence the trait - or disease risk - a person inherits. Height is an example of a polygenic trait.

The new study showed that the polygenic common variants were driving migraine risk more than the three Mendelian genes.

If a person's migraines started at an earlier age, if a person experienced more severe migraines, or if migraines ran in a person's family, the data showed that it is more likely that person had a greater genetic burden of common polygenic variants to blame.

“The strength of the common variants in these families surprised us,” Dr Palotie said.

On the other hand, the three rare but powerful Mendelian genes linked to migraine did not seem to influence migraine risk as much as the researchers expected.

Out of the large family study, researchers identified 45 families with hemiplegic migraine and sequenced their genes. Migraines in only four out of 45 families were driven by the rare Mendelian genes--more cases were driven by common variants.

“This really shows, in a very big sample set, that common variants are very important factors in aggregation of migraines in the family,” Dr Palotie said.

More genome sequencing and larger studies are expected help researchers find both more common variants linked to migraines and more Mendelian variants.

The study is accessible here.