The same is true of other areas of genetics as well. Known SNPs only account for a small percentage of the risk of many common medical disorders or traits that are thought to be genetic, like heart disease, height, and obesity. Psychiatry, however, is especially barren.
It’s always possible that even bigger studies, looking at even more SNPs, might be able to find more associations. Some recent research has suggested that there are many variants of extremely small effect still to be found for schizophrenia and bipolar disorder. So there’s still life in SNPs, but it’s fair to say that compared to 5 years ago people regard them with rather more skepticism.
CNVs, however, have just taken off — in the nick of time, some say. What’s emerging from these studies, however, may be, in its own way, revolutionary as well.
Psychiatric interest in CNVs was sparked by a landmark paper that appeared in Nature in September 2008. It was authored by an international consortium of schizophrenia researchers, led by employees at an Icelandic company, deCODE Genetics. They found a number of CNVs which seemed to be associated with schizophrenia.
Since then, CNV studies have taken off in the same way as GWAS did 5 years before. There’s now good evidence for the involvement of deletions and duplications in autism, attention-deficit hyperactivity disorder (ADHD), schizophrenia, and intellectual disability (aka mental retardation). By contrast, however, studies in bipolar disorder have been negative.
A typical finding is that, comparing a group of patients to some healthy controls, the rate of CNVs in the patients is higher, and these CNVs are especially likely to disrupt genes involved in brain development and function. For instance, in one recent ADHD study from a group led by Cardiff University’s Nigel Williams and colleagues, published in The Lancet, the authors found large, rare deletions or duplications in 15 percent of the children with the disorder compared to 7 percent of the controls.