What Is Huntington's Disease? - Symptoms, Genetic Cause & Treatment

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  • 0:02 What is Huntington's Disease?
  • 1:09 Genetics of…
  • 1:49 Why Is Huntington's…
  • 3:23 CAG Repeats and…
  • 5:57 Stability of CAG Repeats
  • 6:56 Huntington's Disease Treatment
  • 7:31 Lesson Summary
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Lesson Transcript
Instructor: Joshua Anderson
In this lesson, you'll learn about a neurodegenerative disorder called Huntington's disease. You'll learn what a CAG repeat is, and how the number of these repeats can be used to predict whether or not a person will get Huntington's disease.

What Is Huntington's Disease?

Today, we're going to talk about Huntington's disease. So, let's start with the question: What is Huntington's disease? Huntington's disease is an autosomal dominant degenerative neurological disease. Usually, people who have Huntington's disease don't start showing any symptoms until they are in their 30s or 40s. Mood changes, depression, irritability, or paranoia are often the first signs of the disease and are followed by a decline in coordination and an unsteady gait.

Movements will become increasingly uncoordinated and jerky in nature as the disease progresses, and the mental capabilities of the person will continue to slowly decline. Eventually, the person may not remember who they are or recognize close family members. This progressive loss of cognitive capabilities is called dementia. Huntington's disease progresses slowly but steadily, and patients generally live for about 20 years after the first symptoms appear.

Genetics of Huntington's Disease

The genetics of Huntington's disease is a little different than most of the other diseases we've talked about so far. First off, Huntington's disease is an autosomal dominant disease, which means that it is a disease caused by a gene on an autosome and only one copy of the causative gene is needed to cause the disease. Since Huntington's disease is autosomal, it affects males and females equally. And, because Huntington's disease is a dominant condition, each child of an affected person has a 50% chance of inheriting the disease gene and eventually developing Huntington's disease.

Why Is Huntington's Disease Dominant?

So, why is Huntington's disease dominant? We've seen examples of genetic conditions, like Tay-Sachs disease, that are recessive because having only one dysfunctional copy of a gene still leaves a person with one functional copy of the gene. In Tay-Sachs disease, one wild-type gene can make enough functional protein for a cell to function normally, and the person won't be affected unless both copies of the gene are dysfunctional. You may remember that this phenomenon is called haplosufficiency, and it occurs in situations where a single copy of a functional gene is enough to maintain normal function.

Huntington's disease is very different. The actual gene name for the causative gene is huntingtin (HTT), and it encodes a protein with the same name. I don't know why the gene and protein are spelled with an 'i' and the disease is spelled with an 'o,' but it's not a typo - that's the way it really is!

The protein coded by a Huntington's disease allele doesn't code for a protein that has simply lost its normal function; instead it codes for a protein that slowly damages brain cells. You can think of it like a slow-acting poison that poisons and kills brain cells. The exact mechanism by which the disease form of the huntingtin protein damages cells is unknown, but we do know a lot about how HTT alleles can be used to predict whether or not a person gets Huntington's disease with remarkable accuracy.

CAG Repeats and Huntington's Disease

The HTT gene contains a three-nucleotide sequence, CAG, that is repeated several times in a row. Geneticists refer to it as a trinucleotide repeat because it is a three-nucleotide sequence that is repeated several times back-to-back. It is also sometimes referred to more specifically as a CAG repeat.

You may remember that, within a gene, a three-nucleotide sequence can serve as a codon, which codes for a specific amino acid in a protein. In this case, CAG is a codon for glutamine, which is represented by the letter Q when the amino acid sequence of a protein is written out. In a normal HTT allele, there is a CAG repeat, but it is repeated less than 36 times, which results in a part of the protein consisting of several glutamines in a row. This region of the HTT gene that contains the CAG repeat is called the polyQ region because in the protein sequence it appears as a stretch of multiple Qs.

It should be noted that this CAG repeat is a normal feature of the gene, and alleles with less than 36 repeats code for normally-functioning huntingtin protein. People who have two alleles which have, say, 20 repeats and 32 repeats, respectively, will have normally-functioning huntingtin protein and will not develop Huntington's disease.

However, alleles with more than 36 repeats are considered mutant alleles and are designated mHTT. Mutant proteins with 36 or more glutamines in a row increase the rate of deterioration of certain types of neurons in the brain. The rate of this deterioration and the resulting severity of Huntington's disease is linked to the length of the repeat.

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