June 24, 2011 -- Researchers have identified two new genetic variants linked to Parkinson's disease and say they now know how big a role heredity plays in the neurodegenerative disorder.
About a dozen genetic associations with Parkinson's have been confirmed, and many more remain to be discovered, says researcher Nicholas Eriksson, PhD, of the California-based direct-to-consumer gene testing company 23andMe.
In their new study, published this week in the journal PLoS Genetics, Eriksson and colleagues estimated that about a quarter of the variation in susceptibility to the disease is due to genetic factors.
"Each new genetic variant we find gets us a little bit closer to being able to see the full picture of how genes impact this disease," Eriksson tells WebMD. "Roughly 10 genetic variants that contribute to Parkinson's had been found and we added another two to the list."
Role of Genes in Parkinson's
The exact causes of Parkinson's disease are not known, but researchers now believe that both environmental triggers and genetic influences play a role.
Genetic variants have been implicated in the small percentage of cases that occur in people under the age of 50, known as early-onset Parkinson's. But much less is known about the role of genes in late-onset disease.
In an effort to better understand genetic influences in both early- and late-onset Parkinson's, Eriksson, study researcher Chuong B. Do, PhD, and colleagues conducted a novel genome-wide study involving around 3,400 Parkinson's patients and close to 30,000 people without the disease who were 23and Me clients.
The Parkinson's patients were recruited with the help of the Michael J. Fox Foundation, the Parkinson's Institute, and the National Parkinson's Foundation. The study was funded solely by the genetic testing company, however.
Genome-wide association studies became possible following the completion of the Human Genome Project and other landmark projects around the middle of the decade, which provided tools that allow researchers to look for genetic contributions to common diseases.
These tools include computerized human genome sequence databases, human genetic variation mapping, and continuously evolving technologies that simplify the analysis of genetic variations that contribute to disease.
Identifying Genetic Variants
The newly published study included the largest Parkinson's patient group ever recruited for a genome-wide association study.
One of the newly identified genetic variants, SCARB2, is associated with a known Parkinson's disease pathway involving protein degradation.
The other, SREBF1, is not associated with any known Parkinson's pathway.
"This variant is involved in lipid metabolism," Do tells WebMD. "Its association with Parkinson's is not really clear, which is what makes it exciting because it highlights a new area to look at."
Based on their own predictive model, the researchers estimate that around 7% of the genetic variants associated with Parkinson's disease have been identified. That means that more than 90% have not.
While the genetic variants, or mutations, identified to date explain only a small percentage of Parkinson's cases, the gene studies have provided clinically relevant information, Do says.
He points out that one identified mutation is associated with a 50% lifetime risk for developing the disease.
"This one variant accounts for a very small percentage of the total disease burden, but for people who do have the variant it is quite significant," he says.