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Cuervo’s team and collaborators made their findings by examining brain tissue and skin cell lines from Parkinson’s patients transformed into neurons in the laboratory, as well as by studying mice bred to have the normal and mutated forms of the gene.
The function of the healthy LRRK2 protein in the brain remains unclear, but it seems to be involved in the transmission of signals from one nerve cell to another, Cuervo added.
Two previous studies, published in 2008 and 2009, also established a link between Parkinson’s and malfunctions of the cell’s waste disposal systems. Those studies examined a gene called GBA and mutations in it most common among Ashkenazi Jews. GBA is associated with Gaucher’s disease, a rare genetic disorder that only develops when a person inherits two copies of a mutated gene. But scientists began to wonder: Could healthy people with a GBA mutation be at greater risk for Parkinson’s?
“The story of GBA is fascinating,” said Israeli geneticist Avi Orr-Urtreger, who worked on both studies. “It was a surprise that people tried to connect carriers for a Gaucher mutation — who are, after all, healthy people — with Parkinson’s. For a long time the scientific community didn’t believe there was a link between GBA and Parkinson’s putting these people at higher risk until we published our paper.”
Gaucher’s, as scientists have known for some time, is also a disease that involves a malfunctioning of the lysosomes. In this case the cell’s recycling plants fail to properly break down certain fats. The two studies on GBA “started to convince people to look deeper into the lysosomes” to understand Parkinson’s, said Orr-Urtreger, director of the Genetic Institute at Tel Aviv Sourasky Medical Center and an associate professor of pediatrics and human genetics at the Sackler Faculty of Medicine at Tel Aviv University.
In April, Orr-Urtreger published a study in the journal Neurology demonstrating that carriers for yet another, significantly rarer, lysosomal disorder were at higher risk of developing Parkinson’s. This mutation, SMPD1 p.L302P, found among Ashkenazi Jews, results in Niemann-Pick A, a devastating childhood disorder when inherited from both parents. But as Orr-Urtreger found, it is also associated with higher rates of Parkinson’s among healthy carriers.
“Lysosomes are currently very promising in the world of Parkinson’s research,” noted Roy Alcalay, assistant professor of neurology at the Columbia University Medical Center — which also collaborated on the March Nature Neuroscience study — although he cautions that “Parkinson’s is probably more than one disease and people probably get it for different reasons.”
Indeed, having a mutation on a LRRK2 or GBA gene does not mean that a person is destined to develop Parkinson’s. Estimates that a person with the G2019S mutations on LRRK2 will develop Parkinson’s vary widely from as low as 30% to as high as 80%. And while approximately one third of Parkinson’s among Ashkenazi Jews has been associated with either LRRK2 or GBA, two thirds have not been linked to any gene.
Researchers agree that Parkinson’s also has environmental triggers. Vietnam vets who were exposed to Agent Orange, for example, have been found to develop Parkinson’s at a much higher rate than the general population. And it is likely that a person who develops Parkinson’s does so as a result of not one, but several factors. “You need several hits,” said Orr-Urtreger. These can be several genes acting together or various environmental factors or a mix of both. Age alone is a risk factor for the disease.
“Right now drugs are just treating the symptoms of the disease. With LRRK2 we are trying to figure out how we can develop a molecule that will prevent the protein from getting stuck,” said Cuervo. But her reach is greater. “Rather than going one disease at a time, we are looking at keeping this cleaning system going, in general. We have drugs that are promising.”
“I’m optimistic,” added Alcalay. “I think the genes that were discovered give us clues to what causes Parkinson’s and give us a way to manipulate the pathways to slow down Parkinson’s or prevent it. It is also possible that if you discover something that slows down Parkinson’s with these mutations it can also be helpful for the entire Parkinson’s population.”
Talia Bloch writes on culture, religion and science and contributes regularly for the Forward.