Many diseases have a genetic basis and researchers often focus in identifying the genes that are implicated in these diseases, in order to be able to detect it as early possible, to prevent if from developing and also to investigate possible treatments. The candidate gene approach has been widely used to try to identify genes but what is it and how does it work exactly?
Candidate genes studies are typically case/control studies where a number of people carrying a certain trait or disease (“cases”) and a number of people not carrying this same disease (“controls”) are selected and analyzed. This kind of genetic studies will focus on a few mutations in a few genes (studies published up to now rarely exceed 200 mutations and 100 genes). So the basic idea is to analyze the presence of the mutation in cases and controls. If the frequency of the mutation in cases is higher than the one in controls, and statistics can confirm this (X2 test), then the mutation can be called a risk marker for the disease. This also means that a person who carries the mutation will have a higher risk of developing the disease.
The candidate gene approach thus directly tests the effects of mutations located in specific genes for their “association” to the disease, which is why they are also called association studies. These studies are relatively cheap and quick to perform but on the other hand can only permit the identification of mutations that are common (present in more than 1% of the population) and with a small effect on the disease. Moreover, this approach is based on the selection of candidate genes, which are genes that have been in some way related to the disease previously and is thus based on previous findings. The candidate gene approach cannot reveal the association of totally new mutations, unlike other types of genetic studies such as linkage mapping or genome-wide association studies, that will screen the entire genome and not only a few selected genes.