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Molecular Biology of the Huntington’s Disease Genetic Test

What is genetic testing, and how does it work?

Genetic testing can reveal variations in genes that may cause illness or disease. It can be done predictively, to assess a person’s risk of developing a condition, or diagnostically, to confirm a diagnosis. Before deciding to undergo pre-symptomatic genetic testing for Huntington’s disease, a person usually consults with a genetic counselor. The procedure is entirely optional, and the decision to undergo genetic testing can be emotionally difficult. Therefore, it’s important to understand how genetic testing works, its risks and benefits, and consequences of test results. Informed consent is necessary; more information on this process can be found here.

In general, genetic tests are performed on a sample of tissue or fluid. This can be a cheek swab, blood, urine, hair or amniotic fluid sample. Then, the sample is sent to a laboratory, where technicians analyze it and search for a change in protein level or in DNA.

For Huntington’s disease, the genetic test is performed on a blood sample. Once it is sent to the laboratory, technicians perform a DNA test to look at the huntingtin gene, and specifically, to check for the expanded CAG repeat characteristic of HD. The goal of the test is to measure the number of repeats in the huntingtin gene. More information on DNA mutations and the CAG repeat expansion in Huntington’s disease can be found here.

You can also watch a HOPES video on genetic testing here.

How does the genetic test for Huntington Disease work?

Laboratory technicians perform a set of steps to inspect the DNA provided in the blood sample. Let’s take a closer look at each of these steps.

Step 1- The Polymerase Chain Reaction: Making many DNA copies for analysis.

The polymerase chain reaction, or PCR, is used to isolate DNA and make many copies of it. It is needed in order to make lots of copies of the huntingtin gene, allowing scientists to examine it more closely. PCR produces millions of DNA copies in a short amount of time, and includes a few steps as follows.

First, the DNA sample is heated to nearly 100o C. DNA is normally double-stranded in a helix formation, but the heat causes the strands of DNA to separate into single strands. This process is called denaturation.

Then, the sample is cooled a little. Now, primers can bind to each DNA strand. These are small molecules serving as the starting material for a reaction called polymerization. The goal of this reaction is to create more DNA. An enzyme called DNA polymerase makes new DNA strands by adding nucleotides, the structural unit of DNA, to the primer on each strand. It’s like adding building blocks to a pre-existing block tower. As more nucleotides are added, the strand is extended, and eventually, a new copy of the gene is made.

Step 2- Gel Electrophoresis: Separating fragments of DNA based on size.

After creating millions of copies of the huntingtin gene using PCR, we are now ready to separate DNA fragments, in order to inspect them more closely. This can be done using a technique called gel electrophoresis. The principle is simple: DNA fragments are separated based on their size because smaller fragments are able to travel through the gel faster than larger ones. Let’s take a closer look at how exactly gel electrophoresis is done.

First, restriction enzymes attach themselves to DNA and cut it into small fragments. Then, the DNA pieces are placed in small wells in a gel floating horizontally in a buffer solution. This solution is located between two electrodes, one positive and the other negative. Once an electric current is passed through the gel, the fragments of DNA begin to move. DNA is negatively charged, so it is attracted to the positive electrode. The smaller fragments move faster than the larger ones, so they move across a greater distance towards the positive electrode.

Step 3- Inspection of DNA Fragments: How many CAG repeats?

Now that the fragments of DNA have been separated, the technicians are ready to inspect each DNA fragment. They do this to evaluate the number of CAG repeats in the huntingtin gene.

Individuals who do not have HD usually have 28 or fewer repeats. Individuals with HD usually have 40 or more repeats.

Information on test results and what they mean is available here.


For further reading

How is genetic testing done?

Huntington’s Disease and Genetic Testing.

Learning about Huntington’s Disease.