Information on this page are from the Huntington Disease Youth Organization website: http://en.hdyo.org/jhd/articles/52
When George Huntington first identified Huntington disease in 1872, he suggested that the disease affects adults only. It is now known that Huntington disease can occur in individuals at any age in their life – young or old. When someone who is 20 or younger develops symptoms of Huntington’s disease, they are classed as having Juvenile Huntington’s disease (JHD). So effectively ‘JHD’ means a young person with Huntington’s disease.
Why do people get JHD rather than Huntington Disease?
Huntington’s disease is hereditary – that means that it can be passed down, from parent to child, through genes in our DNA. Genes are passed to you from your parents – that’s why you might have hair like your dad, or brown eyes like your mum. We have thousands of genes, and they are all sectioned into different segments in our DNA. These segments are known as chromosomes and in total we have 23 pairs of chromosomes. The reason we have ‘pairs’ of chromosomes is because we inherit one from each parent.
In 1993, A team of researchers discovered the gene responsible for Huntington’s disease on chromosome 4. They found that one part of the gene repeated itself over and over again, like a stutter or when you hold a key down too long on a computer keeeeeeeeeeeeeeeeeeeeeeeeeyboard. This repeat is what causes the development of Huntington’s disease and is known as a ‘CAG repeat’.
Higher CAG repeats
Everybody has two copies of the Huntington’s disease gene, whether they’re at risk for Huntington’s disease or not. One copy was inherited from each parent. And every Huntington’s disease gene contains a CAG repeat in that gene.
What causes the development of the disease is the number of times the CAG is repeated. Put simply, people who develop Huntington’s disease have longer CAG repeats than those that don’t. To make things easier to follow, we will split the CAG repeats into different ranges.
26 And lower (Unaffected range) – As we mentioned, everybody has a CAG repeat in their Huntington’s disease genes (even those not in a family affected by Huntington’s disease). Most people who don’t have Huntington’s disease have around 10-20 CAG repeats. In fact, anything up to 26 CAG repeats is fine and will not cause any problems. But, once you go above that, there may be complications.
40 Repeats and higher (disease range) – Anyone with 40 CAG repeats or above is, unfortunately, certain to develop Huntington’s disease in their lifetime, with a 50% risk of passing the gene on to each of their children as well.
Repeats in the middle range between 27 and 39 are quite rare. They’re a bit less straightforward, and in fact scientists don’t understand them fully, so they’re sometimes referred to as the ‘grey area’.
Most people that have Huntington’s disease have a CAG repeat between 40-50. For these people, it is hard to tell when their symptoms will begin. For example, two people with the same CAG repeat of 45, could get symptoms of Huntington’s disease at completely different ages, decades apart. But JHD tends to be different, people with JHD usually have higher CAG repeats – around 50+ repeats. This difference in CAG repeats results in people getting symptoms at a younger age, earlier in life.
Is JHD common?
Less than 10% of cases of Huntington’s disease come in the Juvenile form. When you consider that Huntington’s disease itself is rare, this highlights just how rare JHD actually is.
For reasons which have only recently become clearer, most people (around 90%) with JHD inherit the gene from their father. The reason for this is that the CAG repeats tend to be less stable when passed from the male. In general, CAG repeats usually increase a little when passed from generation to generation. So after a few generations of the gene being passed on, the likelihood of getting symptoms earlier in life also increases. However, this ‘increase’ seems to be larger when the gene is passed from the male (the father), as opposed the female – where the CAG repeat usually stays around the same. It’s important to state that this is not always the case. As with anything regarding Huntington’s disease, it is very unpredictable.
Symptoms of JHD
Support for young people with JHD is vital, and their needs differ from typical adult-onset Huntington’s disease, in both the symptoms and the ways that it changes the lives of the individuals and families that it affects. JHD presents unique challenges to affected individuals, their caregivers, and the various professionals who are called upon to assist them.
Typical early symptoms of Juvenile HD
- Stiffness of the legs
- Clumsiness of arms and legs
- Decline in cognitive function
- Changes in behaviour
- Seizures (In 25-30% of cases, there is a tendency to epileptic seizures – something almost never seen in adults with HD.)
- Speech problems
- Behavioural disturbances
It is thought that the progression of JHD may be quicker than that of adult-onset Huntington’s disease in some younger people with JHD, although as with adults there is always a huge variation in how someone’s condition progresses so it is difficult to know for certain how fast each person’s condition will progress.
Find more on www.hdyo.org