An Emory project studying schizophrenia genetics is a good example of how geneticists are shifting from examining small, common mutations to “rare variants” when studying complex diseases.

From studies of twins, doctors have known for a long time that heredity plays a big role in causing schizophrenia. But dissecting out which genes are the most important has been a challenge.

Three landmark studies on schizophrenia genetics published this summer illustrate the limitations of “genome wide association” studies. New York Times science reporter Nicholas Wade summarized the results in this way:

“The principal news from the three studies is that schizophrenia is caused by a very large number of errant genes, not a manageable and meaningful handful.”

The limitations from this type of study comes from the type of markers geneticists are looking at, says Steve Warren, chair of the human genetics department at Emory.

Genome wide association studies usually follow SNPs — single nucleotide polymorphisms. This is a one-letter change somewhere in the genetic code that is found in a fraction of the population. It’s not a big change in the genome, and in many cases, it will have a small effect on disease risk.

Researchers looking for the genes behind complex diseases such as schizophrenia and autism are starting to shift their efforts away from genome wide association studies, Warren says.

Think of a SNP like a misspelling of a word in a certain place in a book, he says. In contrast, the “rare variants” geneticists are starting to study more intensively are more like printers’ errors or missing pages. The rapid sequencing technology that allows scientists to investigate these changes easily is just now coming on line, he says.

One example of these rare variants is DiGeorge syndrome, a deletion that gets rid of dozens of genes on one copy of chromosome 22. Children who have this chromosomal alteration often have anatomical changes to their heart and palate. But it also substantially increases the risk of schizophrenia – to about 25% lifetime risk. That’s a lot more than any of the SNPs identified this summer.

Working with several Emory colleagues, researcher Brad Pearce is planning to examine the genes missing in DiGeorge syndrome in several groups of patients: people with DiGeorge, patients with “typical” schizophrenia and people at high risk of developing schizophrenia.

An article in this spring’s Emory Health describes genetic research on autism. Several of the researchers mentioned there, such as geneticist Joe Cubells and psychiatrist Opal Ousley, are involved in this schizophrenia project as well, because deletions on chromosome 22 also lead to an increased risk of autism.

Pearce’s project is funded through American Recovery and Reinvestment Act money from the NIH.