Ask a Geneticist

by Sarah Pierce, Stanford University

My roommate has a direct link with Parkinson's disease with her father. Is there a test to see if she has the this disease? Are there any studies going on relative to this subject? Has anyone been looking at [very] early signs?

-A curious adult from Florida

Predicting who will get Parkinson's is usually impossible. The exceptions are a couple of very rare forms that are caused by mistakes in a single gene. These forms can be tested for and if you have that mistake, you'll get the disease.

So how would your friend know whether her father has this rare form?

When Parkinson's is caused primarily by genes, then there are usually lots of cases in the family. This is because other family members will also have the Parkinson's genes. People who get Parkinson's before the age of 50 are also more likely to have the purely genetic kind.

Usually, though, you can't tell if you're going to get Parkinson's or not. If it runs in the family then your chances go up but it is nearly impossible to know for sure. Why is it so hard to predict?

For a couple of reasons. First, more than one gene is most likely involved. This makes it hard to figure out which genes are responsible. To show why this is tricky, imagine a big group of people eating at a buffet. Suddenly, people start getting sick, and you are responsible for figuring out why.

If just the chicken is making people sick, it probably won't take you that long to figure out that the chicken is the problem. But what if both the chicken and the fish are bad? Remember, it is a buffet, so everyone has tried many different things. You may talk to someone who ate chicken and someone who ate fish and be thrown off by the fact that they both also tried the pork.

The second reason why Parkinson's is hard to predict based on your genes is that having certain versions of these genes really only increases your chances of getting Parkinson's. Most forms of Parkinson's aren't like a simple disease like cystic fibrosis (CF). If you get two copies of the CF gene, you will get CF.

For Parkinson's, having certain genes just means that something in your environment is more likely to trigger the disease. In other words, having the Parkinson's version of a gene doesn't mean you'll automatically get the disease.

This is because the environment plays an important role in getting Parkinson's too. It is a combination of your genes and what happens to you that cause Parkinson's.

For example, some scientists think that Muhammad Ali got Parkinson's because he had so many head injuries when he was a boxer. But not all boxers get Parkinson's. It is possible that Muhammad Ali also has a bad gene that made it easier for him to get the disease. This gene could make his body worse at coping with head injuries.

No one knows for sure what will trigger Parkinson's. Things like pesticides, infections, and head injuries have all been implicated.

If a person doesn't have one of the two Parkinson's genes that can be tested for, there is currently no way to tell they have the disease until symptoms develop.

The symptoms include tremors, stiffness, difficulty moving, and problems with balance. There is no perfect test. A patient is diagnosed with Parkinson's when they have these symptoms, and the doctor can't find any other explanation for them.

You may have noticed that the symptoms of Parkinson's all involve problems with movement. These symptoms happen because a certain type of cell in your brain dies.

So any early diagnostic test might require looking for early signs of these cells dying. As you can guess, this is not easy!

But scientists are trying.

One new type of test works by labeling the brain cells that die in Parkinson's patients with something that shows up in a brain scan. Doctors can use this test to see if these cells are dying. In the past doctors were only able to check for cell death in an autopsy.

Another new test that scientists are trying uses cocaine eye drops. The same cell death that leads to Parkinson's also seems to affect the way people's eyes react to the drops!

It will probably never be possible to tell who will get Parkinson's based only on genetic tests. But knowing what genes are involved is still important.

In the future, doctors may use genetic tests to decide who is at risk and should get checked regularly using new tests like the brain scan. Also, studying the genes that make people susceptible will help scientist understand what more about how Parkinson's works. This will help them develop even better treatments and tests.

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