Development of a new Huntington’s disease test has been reported by researchers in Salt Lake City. Here’s what the new test could mean for people who chose to undergo testing for this inherited neurodegenerative disorder.
What is Huntington’s disease?
Huntington’s disease is an inherited condition in which an individual’s nerve cells in the brain degenerate over time. This deterioration causes a progressive loss of motor skills and cognitive functions.
The disease usually first displays signs and symptoms among people in their 30s or 40s, but the disease can develop before age 20 or in older adults as well. Huntington’s disease is rare in young children. Although there are medications that can help manage symptoms of Huntington’s disease, there are no curative treatments, and the condition is ultimately fatal.
A diagnosis of Huntington’s disease is made based mainly on the results of a physical exam, neurological and psychiatric examinations, and consideration of a person’s family medical history. Brain imaging also may be used.
When physicians have patients whose symptoms strongly indicate Huntington’s disease, they often recommend a genetic test, which can detect whether individuals have a mutated Huntingin gene, which is responsible for the disease.
A mutated Huntingtin gene causes an abnormal number of repeated sequences to occur involving three nucleotides or bases: cytosine, adenine, and guanine, or CAG. The greater the number of CAG repeats, the greater a person’s risk of having or developing symptoms of the disease. Therefore, it is critical to have an accurate assessment of the number of repeated sequences.
How does the new test work?
The new method of Huntington disease testing may facilitate genetic testing because of its high sensitivity and specificity. These advantages can help reduce the number of false negative test results, avoid the need for additional testing, and speed up the time to get results back from the lab. Especially important is the ability to get an accurate measure of the number of CAG repeats.
In the new study, the test (called TP PCR, or triple repeat primed polymerase chain reaction) was evaluated using 246 samples that had already been analyzed via other methods. One hundred normal and low-risk samples were included in the sample group, and all the samples were blinded.
Here’s what the investigators found:
- The test correctly sized 240 of the 246 samples
- All 100 of the normal and low-risk samples were correctly sized
- 140 of the 146 samples known to display Huntington’s disease were correctly sized
- Of the 6 remaining samples, the number of repeated sequences differed by only plus or minus one
- Samples that were difficult to analyze using other methods were evaluated solely and accurately using the new test
- The new test also has an ability to identify what is called “true homozygous normal samples,” which allows clinicians to avoid additional, expensive, and time-consuming testing. Other testing methods do not have this advantage.
Genetic testing for Huntington’s disease can be done for two main reasons: for individuals who are showing signs and symptoms of the disease, and people who have a family history of Huntington’s and who may want to know if they will develop the disease. Children who have a parent with Huntington’s have a 50-50 chance of inheriting the disease-causing gene.
Genetic testing can confirm a diagnosis of Huntington’s disease for those in the former category, while people in the latter group can use the information to make decisions about having children or planning their lives. However, genetic testing cannot reveal exactly when symptoms will begin.
Genetic counseling is strongly recommended for anyone who undergoes genetic testing for Huntington’s disease. Typically it is required for anyone without symptoms who elects to undergo testing because of a family history of Huntington's disease.
Jama M et al. Triplet repeat primed PCR simplifies testing for Huntington disease. Journal of Molecular Diagnostics 2013 Feb 13.