The first step in understanding a genetic disease is learning to which extent genetics play a role in it’s development, i.e. Is is really a genetic disease? For many illnesses, it’s not entirely clear what role genetics versus environment play, or how complex their interaction is. There are several tests geneticists can use to uncover a genetic basis of a condition without having to actually analyze DNA.

One way to begin teasing apart the genetic basis of a disease is to understand its heritability. That is, how often does the disease occur in a family, which members of the family have the disease, and how are they related to each other? Patterns of inheritance help to uncover genetic mechanisms. An important thing to keep in mind is that no DNA or genetic analysis is needed, it’s about describing the prevalence of the condition. So detailed observations and knowledge of relationships between people are needed. From this information, geneticists can estimate the Heritability of a trait (read disease). Heritability provides information on what proportion of a trait can be attributed to genetic versus environmental factors.

A recent study (see below) decided to look at two conditions, migraines and depression, and ask what correlation existed in their heritability. Comparison of the heritability scores for depression between patients with migraines and patients without migraines, showed a genetic correlation between depression and migraines (specifically migraines with aura).

Another statistical method that geneticists use to uncover genetic causes of a disease is the Odds Ratio. This asks what are the odds of an event occurring in one group, and what are the odds of that event occurring in another group. It then takes the ratio of these two odds. A ratio above 1.0 indicates that the event is more likely to occur in the first group.

So in the case of depression and migraines, the researchers state :

Odds ratios for depression in patients with migraine were 1.29 for migraines without aura and 1.70 for migraines without aura.

That means the is a high co-occurrence of migraines and depression. This all leads to the conclusion:

There is a bidirectional association between depression and migraine, in particular migraine with aura, which can be explained, at least partly, by shared genetic factors.

So while this form of analysis is very compelling, and indicates a genetic link between the two conditions (high heritability and high odds ratio) it does so without having to actually analyze and DNA. Future steps in this work would be to distinguish genetic markers specific to patients with depression and: migraines with aura, migraines without aura, or without migraines.

See also: Migraine, Depression May Have Genetic Link

Image: Jennerator on Flickr

Citation:

Stam, A., de Vries, B., Janssens, A., Vanmolkot, K., Aulchenko, Y., Henneman, P., Oostra, B., Frants, R., van den Maagdenberg, A., Ferrari, M., van Duijn, C., & Terwindt, G. (2010). Shared genetic factors in migraine and depression. Evidence from a genetic isolate Neurology DOI: 10.1212/WNL.0b013e3181cbcd19