Genome-wide studies, known as GWAS, look at common genetic variants among different people to find associations with diseases or traits. They compare the DNA of those with a condition to others without it.
The genetic details are analyzed, looking for gene variants that show up more frequently in people with the disease than in those without it. Variations in the individual make-up of a genome are identified by noting areas where there are changes and variations that could heighten the risk of disease.
For these studies, the International OCD Foundation Genetic Collaborative, which includes more than 20 research groups in nine countries, analyzed about 480,000 gene variants called SNPs (single-nucleotide polymorphisms) in 1,465 people with OCD, more than 5,500 without OCD, and in others including both parents of someone with the condition.
The Tourette Syndrome Association International Consortium for Genetics and the Tourette Syndrome GWAS Consortium, which includes 22 groups in seven nations, analyzed 484,000 SNPs in about 1,500 individuals against 5,250 people without Tourette syndrome.
McMahon explained that the purpose of the GWAS approach is to look across the whole genome for common forms of variation, looking for genetic differences present in a quarter, a third, or half the population. "That's where the statistical power lies," he noted.
The researchers identified a signal within a gene in people with Tourette syndrome. That gene encodes a protein in the cerebellum, an area of the brain involved in motor control. They also found that a strong signal close to another gene, active in the brain during childhood and adolescence, was associated with those with OCD, which often emerges during that particular time of life. But none of the discoveries were closely enough associated to identify risk factors.
Dr. James Leckm
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