The genetics of Juvenile Huntington’s disease

A gene is a piece of biological information you inherit from your parents. It is present in every cell of the body and tells cells what to do. Genes control cells by producing proteins; each gene is really a kind of recipe for making a certain protein.

Genes are usually attached to a chromosome (a strand of DNA containing many different genes). Each human cell contains around 25,000 genes and most have 23 pairs of chromosomes. Chromosomes come in pairs because we inherit one from each parent.

It is the same gene that determines whether you will develop Huntington’s disease or Juvenile Huntington’s disease. Everyone has it and it’s called the Huntington’s gene. It’s found on chromosome pair number 4. However, people that develop either adult or Juvenile Huntington’s have an important difference or ‘fault’ in their Huntington’s gene.

Some facts about genetics and Juvenile Huntington’s:

  • If you have Juvenile or adult Huntington’s disease, this means you inherited a faulty version of the Huntington’s gene from one of your parents, and the recipe for the protein the gene produces is incorrect.
  • The faulty version of the gene repeats a particular coding sequence known as CAG (cytosine-adenine-guanine) too many times - a bit like adding too much of one ingredient in a recipe. This means the protein it makes damages nerve cells in the brain.
  • If you have 40 or more CAG repeats, it is certain that you will develop Huntington’s at some point. If you have more than 50, it is highly likely that you will get Juvenile Huntington’s (in other words, you will start to experience symptoms before the age of 21). But not everyone with a high CAG count develops Juvenile Huntington’s.
  • Less than 10% of people with the faulty Huntington’s gene have more than 50 CAG repeats, making Juvenile Huntington’s a very rare disease.
  • If you do have more than 50 repeats, there is a 90% chance you got the faulty gene from your father, as CAG repeats tend to be more unstable when passed on from the man. We don’t yet know for sure why this is, but it is thought to be because the gene becomes more unstable in sperm.
  • Every child conceived naturally to a parent who has the faulty gene has a 50 % chance of inheriting it and the disease. If both parents have the faulty gene the child has a 75 % chance of inheriting it. The number of CAG repeats produced by the gene will determine whether the child will start to show symptoms under the age of 21, or later in adult life.
  • Juvenile Huntington’s is what’s known as an “autosomal dominant disorder”. In plain English, this means that if you inherit the faulty gene, you may live for several years without any symptoms, but you will go on to develop the disease at some point.

Diagnosis

If a child who has a parent with the Huntington's gene starts to show symptoms of Juvenile Huntington’s, and if there is a strong suspicion that they may have it, a diagnostic blood test may be done to see if they do have it.

However, because Juvenile Huntington’s is so rare and because its symptoms can be hard to recognise, it can often take a long time before a definite diagnosis is established. Not knowing can be stressful, but support is available to help you through this situation.

We have an experienced specialist adviser on Juvenile Huntington’s, Helen Santini. She will be happy to answer any questions you have. You can reach her on helen.santini@hda.org.uk or 01279 507656.