1. SECTIONS:
2. preface & acknowledgements
3. genetics at a glance
4. contacts, support & testing
5. testing & pregnancy
6. newborn screening
7. cancer in the family
8. cardiovascular conditions
9. chromosomal conditions
10. clotting & bleeding conditions
11. cystic fibrosis
12. diabetes
13. fragile X syndrome and other causes of developmental delay
14. haemoglobinopathies
15. hereditary haemochromatosis
16. neurofibromatosis
17. neurological conditions
18. psychiatric conditions
19. genetics in practice
20. emerging genetic technologies
21. glossary

Huntington disease

 

Clinical features

1. Huntington disease (HD) is an inherited condition that gives rise to progressive, selective, neuronal cell death.
   
2. Symptoms can be classified into three basic groups:
3. Physical – involuntary, jerky movements or ‘chorea’, abnormal gait, bradykinesia, hyperflexia,
   abnormal eye movements, dysarthria and dysphasia
   
4. Cognitive – impairment including disturbances in verbal fluency, cognitive speed, the retrieval of memories, ability to persist at a task or change cognitive sets, therefore causing difficulties with
   judgment, planning, problem solving and eventually dementia
   
5. Emotional – including personality changes such as impulsiveness, perseveration, disinhibition,
   depression, mood swings and aggression
6. These symptoms usually commence between the ages of 30 and 50 years, and become progressively
   worse over time.
   
7. However, onset can occur in children and people in their later years.
   
8. Not all people will experience all the symptoms, nor will the symptoms appear in any particular order.
   
9. An individual with HD lives on average for 15 to 20 years after developing the first symptoms.

 

Genetics

1. HD follows an autosomal dominant pattern of inheritance. Each child of an individual with HD has a 50% risk of inheriting the mutated gene.
   
2. HD is caused by a mutation in the gene called huntingtin.
   
3. The mutation is the result of an increase in size (expansion) of a certain part of the gene where a
   tri-nucleotide sequence, CAG, is repeated over and over again (known as a triplet repeat). See Contacts,
   support and testing.
   
4. The huntingtin gene normally contains the triplet CAG repeated up to 26 times.
   
5. In people with HD, or those who will develop HD during their lifetime (if they live long enough), the CAG triplet is repeated 40 times or more in one copy of their huntingtin gene.
   
6. Where an individual has the CAG triplet in one copy of their huntingtin gene repeated between 27 and 39 times (intermediate range), careful interpretation is required when assessing the meaning of this result for the individual and their family.
   
7. It is not possible to predict at what age symptoms will appear based on the number of repeats in the mutated huntingtin gene. However, on average the larger the gene expansion the earlier the age of onset.

 

The size of the gene expansion is unstable down generations. Expansions of any size have the potential to increase from one individual to their child, with larger expansions associated with paternal transmission.

 

1. Predictive genetic testing does not require identification of the mutation in an affected family member.

 

Prevalence

HD has a population frequency of approximately 1 in 10,000.

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