Summary:Huntington's disease (HD) is a dominantly transmitted neurodegenerative disorder with wide variation in onset age but with an average age at onset of 40 years. Children of HD gene carriers have a 50% chance of inheriting the disease.
Huntington's is a dominant genetic disease. With these diseases, you are almost never an invisible carrier like you can be with recessive genetic diseases. You usually can't pass on a gene that causes the disease because you don't have it.
In around 3% of cases of Huntington's disease, there's no obvious family history of the condition. This could be because relatives with the condition died before they were diagnosed with it. In rare cases, it's due to a new fault in the gene that causes Huntington's disease.
Huntington's disease is a progressive brain disorder caused by a single defective gene on chromosome 4 — one of the 23 human chromosomes that carry a person's entire genetic code. This defect is "dominant," meaning that anyone who inherits it from a parent with Huntington's will eventually develop the disease.
Huntington's disease does not skip generations. Each child of a parent with Huntington's has a 50% chance of inheriting the defective gene. If a child does not inherit the gene, he or she cannot pass it on. If the child does inherit the gene, he or she can pass it on and will develop the disease.
Dominant Disorders
If either parent is a carrier of a dominant mutation, that parent is affected by the mutation and will have the disease, and their child, daughter or son, has a 50% chance of receiving the gene mutation and being affected by the disease.
Both men and women can get it. If a parent has the Huntington's disease gene, there's a: 1 in 2 (50%) chance of each of their children developing the condition – affected children are also able to pass the gene to any children they have.
The symptoms usually start at 30 to 50 years of age, but can begin earlier than this (juvenile Huntington's disease) or much later. Once they start, the symptoms usually get gradually worse.
People with 36 to 39 CAG repeats may or may not develop the signs and symptoms of Huntington disease, while people with 40 or more repeats almost always develop the disorder.
Every child of a parent with HD has a 50/50 chance of inheriting the expanded gene that causes the disease. If the child has not inherited this expanded gene, he or she will never develop the disease and cannot pass it on to their children.
In the United States, Huntington's disease occurs in about one of every 10,000 to 20,000 people. It affects males and females equally and crosses all ethnic and racial boundaries. Typically, symptoms begin between age 30 and 55.
There have been reports that juvenile onset Huntington's chorea is almost always inherited from the father, and that late-onset Huntington's chorea is inherited more often from the mother than from the father.
Huntington's disease is autosomal dominant, meaning inheritance of just a single copy of an abnormal chromosome from a biological parent is enough to cause it. If one parent carries the abnormal gene, each of their biological children has a 50 percent chance of Huntington's disease inheritance.
Therefore someone can be without symptoms or presymptomatic for a number of years. In the past, there was no way to test for the abnormal gene, but now a blood test can determine whether or not an individual carries the gene for HD.
Familial prion disease may produce a diverse range of phenotypes, even within the same pedigree. It may resemble HD with prominent personality change, psychiatric symptoms and cognitive decline, chorea, rigidity, and dysarthria. Limb and truncal ataxia and seizures may be present.
Nine patients have died, and seven have been lost to follow up. The range of disease duration was between 2 and 17 years, the oldest living to age 91.
As the disease progresses, a variety of motor, emotional/behavioral, and cognitive symptoms are experienced, including unsteadiness, trouble holding onto things, trouble walking, changes in sleeping patterns, delusions and hallucinations, intellectual decline, and memory loss.
Onset occurring ≤20 years is classified arbitrarily as juvenile-onset HD (JHD). The youngest symptomatic child documented had an age of onset of 18 months,3 with other children developing symptoms in their teens, meaning that some individuals with JHD may now be in their twenties.
HD is inherited as an autosomal dominant disorder, meaning that each child of an affected parent has a 50:50 chance of inheriting the genetic mutation that causes HD, and men and women are equally likely to inherit the mutation.
There's a 25% chance of this happening with every pregnancy. Most often, people don't know that they are carriers. As a result, it's now possible to genetically screen prospective parents to determine whether they are carriers for any of the more commonly seen autosomal recessive disorders.