Gene Mutation That Causes Huntington's Can Occur Spontaneously

J. Eric Ahlskog, M.D., Mayo Clinic

DEAR MAYO CLINIC:

I am 29 and was recently diagnosed with Huntington's disease, having had symptoms for six years. My parents don't have symptoms but they have also not had genetic testing. Could this be a unique incidence of Huntington's starting only with me? Are my siblings also at risk of having the disease?

ANSWER:

Huntington's disease is a genetic disorder that can only be confirmed by genetic testing. Most of the time, the gene mutation that causes Huntington's disease is passed from parent to child. But, in rare cases, the gene mutation may arise spontaneously, and a person can develop the disease without a parent having the gene mutation.

Huntington's disease is a progressive, degenerative disorder that causes certain nerve cells in the brain to waste away, causing uncontrolled movements, emotional disturbances and mental deterioration over time. The signs and symptoms of Huntington's disease can vary significantly from person to person. Huntington's disease usually develops slowly, and the severity of signs and symptoms is related to the degree of nerve cell loss.

Huntington's disease is an inherited disorder caused by a single abnormal gene. It's referred to as an autosomal dominant disorder because only one copy of the defective gene, inherited from either parent, is needed to produce the disease. When one parent has the faulty gene, the chance that a child will inherit the defect is 50 percent. Because you've been diagnosed with this disease, one might expect that one of your parents likewise has the defective gene; however, it's also possible that your gene mutation arose spontaneously and was not inherited from one of your parents.

The risk that your siblings have Huntington's disease obviously depends upon whether one of your parents also has the gene. If so, then each sibling has about a 50 percent risk of carrying that same gene. If neither of your parents has the gene mutation, then it would be assumed your case is spontaneous, and your siblings would not be at increased risk of developing Huntington's.

To precisely determine if your siblings are at risk requires genetic testing. If your parents are tested and the gene mutation isn't found, then your siblings shouldn't be at risk. The most straightforward way for your siblings to confirm their status, though, would be for them to undergo the genetic test themselves.

The test involves taking a blood sample and having it analyzed to determine the presence of the faulty gene. If one of your parents has the gene mutation, it means, of course, that not only are your siblings at risk, but the parent with the mutation should eventually develop the disease. For this reason, if they go ahead with genetic testing, it's important to do so in a setting that offers both pre-test counseling and post-test support.

If your parents choose not to undergo genetic testing for the gene mutation that causes Huntington's disease, your siblings could individually undergo testing to definitively determine whether they will develop the disease.

Deciding whether to be tested for the gene mutation is a personal decision. For some people, the uncertainty of whether they carry the faulty gene is stressful and distracting. For others, the knowledge that they will develop the condition is burdensome. If your family members are uncertain about undergoing the test, they may want to consider contacting a genetic counselor who can discuss with them the implications of a positive or negative test result, walk them through the testing process, and help weigh the pros and cons prior to making a decision about genetic testing.

-- J. Eric Ahlskog, M.D., Ph.D., Neurology, Mayo Clinic, Rochester, Minn.

Medical Edge from Mayo Clinic is an educational resource and doesn't replace regular medical care.

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