Two of the largest studies yet carried out on the genetics of schizophrenia in Chinese populations have turned up three genetic loci, or chromosomal regions, previously not known to be related to the disease.
These genome-wide association studies (GWAS), done independently and published in Nature Genetics on October 30, also begin to redress a geographical imbalance: until now, GWAS have focused mainly on Western populations.
Roughly 1 in 100 people will suffer from schizophrenia in their lifetimes, which is considered largely heritable (up to 80%). But this genetic influence seems to be produced by hundreds of variations in DNA, each of which increases risk by a small amount. Researchers have so far found some 20 such variants, but have been unable to pin down the exact genes that are affected by those variations or molecular mechanisms that cause the disease.
In one study, a group led by Wei Huang, a geneticist at the Chinese National Human Genome Centre in Shanghai, and Dai Zhang a neuroscientist at Peking University in Beijing, compared the genomes of 746 people with schizophrenia with those of 1,599 controls. They found that previously unknown variations in a region of chromosome 11 -- 11p11.2 -- were linked with the disease. The correlation was verified in follow-up studies of another 4,027 people with schizophrenia and 5,603 controls.
In another study, a team led by Lin He and Yongyong Shi, of the Bio-X Institutes at Shanghai Jiao Tong University, compared the genomes of 3,750 people with schizophrenia with those of 6,468 controls. They turned up two culprit regions: 8p12 and 1q24.2. The association with schizophrenia of the two regions was validated in another study of 4,383 people with schizophrenia and 4,539 controls.
"[These genetic loci] represent new potential targets to help understand schizophrenia. As we have only a relatively small number, this represents an important increase," says Pamela Sklar, chief of psychiatric genomics at the Mount Sinai School of Medicine in New York, who was not involved in either study.
Something old, something new
Both studies also reinforce previous findings in studies of European populations that a region of chromosome 6 associated with the major histocompatibility complex (MHC), a gene family involved in the immune system and autoimmunity, is involved in schizophrenia.
"This definitely strengthens the case that one or more genetic factors in this broad region are involved in disease risk," says Shaun Purcell, who develops statistical and computational tools for genetic studies at Massachusetts General Hospital in Boston.
Pablo Gejman, a psychiatry researcher at the University of Chicago's Pritzker School of Medicine, agrees. "The replication of the MHC locus is of great potential significance," he says. "The MHC is a well replicated common variant locus for schizophrenia and suggests aetiological mechanisms of disease that have not been previously considered with sufficient focus."
But there is much work still to be done. "None of the studies by themselves pinpoint specific genes or causal alleles in this large, complex [MHC] region," says Purcell. The next step might be to integrate the Chinese and European data, he adds. "It would be great to see these large data sets combined."
Grist for the mill
The three newly implicated loci will point to future studies, but researchers will have to start nailing down the genes and specific molecular mechanisms that drive the disease. "The connections with underlying gene expression will need deeper exploration to verify the connection with schizophrenia pathogenesis," says Sklar.
Lin He says the next step is to identify the risk genes affected by the variants in the interesting regions. "After carrying out fine mapping studies and functional validations, we might have the chance to report the right risk gene in an associated region soon," he says.
It is still unclear whether the genetics at work in schizophrenia differ between Asian- and European-descended populations, although the latest studies may be a first step towards uncovering the answer. "These are the first large GWAS studies in Asians," says Sklar, adding that "the extent to which population-specific genetic factors exist is an important and open question".
"This is a largely understudied topic," says Gejman. "GWAS studies are potentially helpful, but the studied samples will need to be much larger to understand the genetic architecture of schizophrenia in the Chinese population, as well as in European populations."
This article is reproduced with permission from the magazine Nature. The article was first published on October 31, 2011.