2 Jul 2009
3 July 2009. Three papers appearing in this week's issue of Nature present the much anticipated results of genomewide association studies (GWAS) of schizophrenia, as well as meta-analyses of the three studies together. There are no break-out candidate genes, though there is support for previous linkage findings, several new candidates, as well as statistical modeling that supports the notion of genetic overlap between schizophrenia and bipolar disorder.
Perhaps surprisingly, none of the studies alone identified any genetic marker with significant association to the disease (by the commonly applied genomewide significance benchmark of p Lewis et al., 2003), has also been highlighted by various single candidate gene association studies (see SchizophreniaGene's Chromosome 6 compendium).
The lack of a large new crop of gene candidates is certain to rekindle the debate about the value of expensive large-scale GWAS versus other approaches (see below for historical notes), and while some observers will take these results as a failure of GWAS to deliver, Michael O'Donovan of Cardiff University in Wales, and coauthor of the International Schizophrenia Consortium paper, argues for a different interpretation. "Before today, you could count on the thumb of one hand the number of common variants that have been reliably identified in schizophrenia, so this is a significant increment in knowledge," he said at a press conference Wednesday at the World Conference of Science Journalism (WCSJ) in London. What is needed to follow up on these results is larger samples, O'Donovan argued. While this will be fairly expensive, "that's peanuts compared with the human and economic cost of ignorance about this disease," he said.
Daniel Weinberger of the National Institute of Mental Health, Bethesda, Maryland, sees it differently: “While we had hoped this brute force, clinically agnostic strategy would have been more fruitful, it is clear that the assumption of loading more cases on an already exhausted strategy is likely to only add a few more very small effect genes to the already too small list of very small effect genes," Weinberger wrote in an e-mail to SRF. "We have to critically consider the realistic possibility that the genetic and pathophysiologic heterogeneity of the condition we call schizophrenia may not be well suited for this strategy which assumes a quasi-unitary disease entity as part of its basic experimental logic.”
Although the three separate studies came to the same major conclusion about signals in the chromosome 6p21 region, they did report some different results. The study conducted by the SGENE consortium also found significant association in the combined sample near the neurogranin gene (NRGN) on 11q24.2 and for an intronic SNP in transcription factor 4 (TCF4) on 18q21.2.
And in their paper, the International Schizophrenia Consortium (ISC) deployed a modeling approach to try to estimate the number of genes involved in the disease. Their analysis, they report, supports the polygenic model of the disease whereby a common variation in many hundreds or even thousands of genes contributes to the disease, and also wherein a significant number of these are shared with bipolar disorder.
In October 2005 (on the very weekend SRF was launched!), the World Congress on Psychiatric Genetics in Boston saw a contentious series of exchanges between the proponents of putting most of the genetic funding eggs into a couple of large GWAS baskets, on the one hand, and others who suggested a combination approach that also utilized smaller, multifaceted studies of candidate genes. These latter veterans of psychiatric genetics argued that their approaches had already yielded a considerable number of strong candidate genes found by fine-mapping positional candidate genes under linkage peaks, and they raised concerns that large GWAS would wash out signals at work in smaller populations.
The large-scale GWAS approach was ultimately selected by funders around the world, and great—and rapid—results were anticipated by many in the schizophrenia community. Thus, there was disappointment when the first round of reports failed to deliver either a few genes of large effect or many confirmed genes of small to medium effect (see SRF related news story; SRF news story; SRF related news story).
The first apparent success of the GWAS era was the suggestion that copy number variations (CNVs)—major disruptions in one or more genes in one or a few individuals—could account for schizophrenia in myriad different ways. However, not everyone agreed that this evidence was so clear, or that if it was a factor, that it accounted for a substantial percentage of schizophrenia risk. (see the lively discussion at SRF related news story; SRF news story; SRF related news story). Thus, the community was left anticipating the GWAS studies described in these papers, which looked at the contribution of common SNPs across the genome.
In one of the papers in the current issue of Nature, the ISC, a multinational collaboration led by Pamela Sklar of the Broad Institute, Cambridge, Massachusetts, found no genes with genomewide significance in their sample of 3,322 cases and 3,587 controls of European origin. A similar result was obtained by the SGENE consortium (2,663 cases, 13,498 controls of European origin), led by Kari Stefansson of deCODE Genetics in Reykjavik, Iceland, and by the Molecular Genetics of Schizophrenia (MGS) study (European: 2,681 cases, 2,653 controls; African American: 1,286 cases, 973 controls), led by Pablo Gejman of NorthShore University HealthSystem and Northwestern University, Evanston, Illinois.
According to the papers, when the three groups combined the European samples of more than 8,000 cases and 19,000 controls, and applied varying methods of statistical analysis, their findings all converged on a swath at chromosome 6p21. Originally tagged as a region of interest more than 30 years ago (Smeraldi et al., 1976) and later supported by linkage studies (see Lewis et al., 2003), this stretch of DNA contains a host of major histocompatibility complex genes, coding for proteins involved in immune functions. This stretch also contains many other genes of other function (see Sanger Institute overview of chromosome 6).
The results on chromosome 6p21 might tantalize with the possibility of linking genetics to previous epidemiologic findings regarding schizophrenia and season of birth effects, autoimmune disease, and prenatal infection (see SRF related news story; SRF news story). However, as the Associated Press reported, Stefansson noted the presence of other, non-immune genes in this region, and warned, "It's guilt by association; it's not really a link."
Some different approaches and results
The ISC attempted to glean some information by combining the tens of thousands of markers that had even nominal (i.e., not statistically significant) association with the disease. With the understanding that this "polygenic score" would probably contain a vast majority of false positives, the statistics team led by Shaun Purcell of the Broad Institute nonetheless hoped it would allow them make some observations about the overall common genetic landscape of the disorder. In particular, they wanted to test whether the data supported the polygenic theory that hundreds or even thousands of genes can influence the risk of schizophrenia, notably advanced by Gottesman and Shields (1967). According to the authors, the resulting modeling, strongly supports "a polygenic basis to schizophrenia that 1) involves common SNPs, 2) explains at least one-third of the total variation in liability, 3) is substantially shared with bipolar disorder, and 4) is largely not shared with several non-psychiatric diseases."
The question of a genetic link between bipolar disorder and schizophrenia has been debated since the disease categories were created, and O'Donovan and colleagues at Cardiff University have applied a particular focus on this question (see SRF Live Discussion). Their research agenda was supported earlier this year by a large Swedish epidemiology study that left little doubt about the shared heritability of the disorders (see SRF related news story).
Outside the MHC regions, the SGENE group identified significant results in their meta-analyses for an SNP just upstream from the neurogranin gene (NRGN) on 11q24.2, coding for a synaptic protein, and an intronic SNP in transcription factor 4 (TCF4) on 18q21.2.
The MGS group's paper is notable in that it is the first report of an African American schizophrenia GWAS sample. Although none of the genes identified reached genomewide significance, it will be interesting to follow this line of research as researchers try to determine whether different SNPs and/or genes are at play in different populations.
Where to now?
If 8,000 subjects were not enough to identify more than a handful of markers indicating nearby schizophrenia risk loci (most of which were in a region already suspect), what would it take to find a substantial number of the many common variants presumably affecting disease risk? David Collier of the Institute of Psychiatry in London, and a member of the SGENE collaboration, told SRF at the WCSJ in London that he thought genomewide statistical significance might not begin to emerge until samples of 100,000 cases and more than 100,000 controls had been collected. He said that assembling such samples was a challenge, but not impossible, especially since groups like the Wellcome Trust are assembling large control samples to study a range of diseases.
Thus, the field is left with some important questions, beginning with, Is this the time for a course correction in regard to studying the role of common variants, i.e., to steer the ship away from massive GWAS samples and toward other approaches that incorporate endophenotypes (see SRF live discussion) or that attempt to tease apart the complex and varying phenotype of the disease itself (see SRF related news story)? Or is this the time for a steady hand on the GWAS ship's wheel—in for a penny, in for a pound, to mix some metaphors?
Furthermore, how much effort should be spent on probing the contribution of copy number variation, or on resequencing the many strong candidate genes already existing to find new common or rare variants? As Collier told SRF, GWAS of the type reported this week will explain only the risk due to common polymorphisms at the population level, which the ISC estimates at one-third or more of the contribution to the disease, though this number may be more or less. The remainder of genetic variation, said Collier, will come from some combination of CNVs and rare variants.
How best to probe gene effects that only emerge under the influence of variation in other genes (epistasis)? Are family studies going to yield more fruit than case-control studies? The schizophrenia community awaits a consensus response to these questions from the genetics community, and SRF invites readers to begin this discussion.—Hakon Heimer (with additional reporting by Peter Farley).
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