Dentatorubral-Pallidoluysian Atrophy

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Dentatorubral-pallidoluysian atrophy

Definition

Dentatorubral-pallidoluysian atrophy (DRPLA) is a disorder of ataxia (loss of balance), choreoathetosis (involuntary rapid, irregular, jerky movements or slow, writhing movements that flow into one another), and dementia (inability to clearly think; confusion, poor judgement; failure to recognize people, places, and things; personality changes) in adults, and ataxia, myoclonus (involuntary spasms of a muscle or muscle group), epilepsy (seizures), and loss of intellectual function (mental retardation) in children.

Description

DRPLA has also been referred to as Haw River syndrome and Natito-Oyanagi disease. The typical age of onset of DRPLA is 30, but it can present in people as young as one year of age and as late as 62 years of age, with differences in presentation between children and adults. In patients under the age of 20, DRPLA presents as seizures, ataxia, myoclonus, as well as progressive (worsening) mental deterioration. In patients over the age of 20, DRPLA is suspected when a person develops ataxia, choreoathetosis, dementia, and psychiatric disturbances (delusions, hallucinations). A positive family history (a relative with similar symptoms or one already diagnosed) confirms the diagnosis. DRPLA is sometimes initially thought to be Huntington disease .

A possible diagnosis of DRPLA can be devastating for a family to experience—their once healthy child, or young adult, will begin to have seizures, involuntary movements, loss of control over voluntary movement, and delusions—perhaps no longer being able to identify family members. Diagnosing DRPLA is complicated and requires a knowledgeable physician with expertise in both neurology and genetics. Usually an individual diagnosed with DRPLA already has a parent with the disease, however, if the disorder was not diagnosed properly, or the parent died prior to the onset of symptoms, or the parent has very late onset of the disease, there may not be a documented family history of DRPLA.

Genetic profile

DRPLA is an autosomal dominant condition which means that both males and females are equally likely to have the disease, and an individual with the variant gene has a 50/50 chance to pass the condition to any child. The DRPLA gene is located on chromosome number 12 and has a section of DNA where the DNA alphabet is repeated in triplets, called CAG repeats. Normally a person has 6 to 35 CAG repeats in the DRPLA gene. In patients with DRPLA, there are 49 to 88 repeats which causes the gene's protein product, Atrophin 1, to be toxic to cells. Although scientists do not understand the exact mechanism, the number of repeats expands when the gene is transmitted from parent to child. The size of the repeat transmitted to the next generation depends upon the size of the parent's repeat and the sex of the transmitting parent.

There is an inverse correlation between the age of onset and the size of the expanded CAG repeats. In other words, the younger the age of onset, the larger the number of CAG repeats:

Onset before age 21—repeat range of 63–69 (average of 68).
Onset from 21–40 years—repeat range of 61–69 (average of 64).
Onset after 40 years—repeat range of 54–63 (average of 63). Although there is significant overlap, the inverse correlation exists.

DRPLA as well as other genetic conditions, exhibits a phenomenon known as anticipation. Anticipation means that the disease increases in severity and presents at a younger age of onset with each successive generation. For example, when the CAG repeat is inherited from the father, DRPLA can manifest itself 28 years earlier than the father began having symptoms, while if transmitted from the mother, DRPLA can present 15 years earlier than the previous generation.

Demographics

DRPLA has been reported to occur most often in the Japanese population, although it has been described in other ethnic groups including those in Europe and North America. The prevalence of DRPLA in the Japanese population is estimated to be 2–7 in 1,000,000, which is similar to the prevalence of Huntington disease in this population. A CAG repeat size of 17 or higher (usually 20–35) is more common in healthy Japanese individuals than Caucasians, which may explain why DRPLA is more common in the Japanese. In other words, a larger repeat size in a parent increases the possibility that the DNA will become unstable and expand when transmitted to the next generation. Even though DRPLA is rare in the United States, a large African-American family in North Carolina has DRPLA, where the condition is also called the Haw River syndrome.

Signs and symptoms

The cardinal features of DRPLA are involuntary movements (usually in the face, neck, tongue and hands) and dementia (inability to clearly think; confusion; poor judgement; failure to recognize people, places, and things; personality changes) regardless of the age of onset. A history of ataxia, epilepsy, and mental retardation in children, combined with a positive family history, are often the presenting signs of this condition in an individual under 20 years of age. Seizures are always present in patients under 20, but are not as common in patients age 20–40, and rarely seen in patients with onset after 40. Adult onset DRPLA (after 20) presents with ataxia, choreoathetosis, dementia, and psychiatric disturbances.

Diagnosis

A diagnosis of DRPLA exists when there is a positive family history of the disease, characteristic clinical findings, and DNA testing that reveals an expansion in the CAG repeat of the DRPLA gene. Genetic testing to examine the CAG repeats in the DRPLA gene can be performed from a small blood sample. A few reports have described DRPLA as sporadic (occurring by chance) in some families. Upon closer examination, the asymptomatic fathers had a mildly expanded CAG repeat size. Therefore, it is always important to evaluate both parents of an affected individual even if they appear to have no symptoms of DRPLA. Testing of asymptomatic children is not appropriate since it takes away the child's right to want to know, or not know this information, raises the possibility of stigmatization (labeling someone a certain way and making assumptions about them) within a family, as well as the threat of educational and employment discrimination. Children with symptoms, however, usually benefit from having a diagnosis established.

For pregnancies at 50% risk, prenatal diagnosis is available via either CVS (chorionic villus sampling) or amniocentesis . CVS is a biopsy of the placenta performed in the first trimester of pregnancy under ultrasound guidance. Ultrasound is the use of sound waves to visualize the developing pregnancy. The genetic makeup of the placenta is identical to the fetus (developing baby) and therefore the DRPLA gene can be studied from this tissue. There is approximately a 1 in 100 chance for miscarriage with CVS. Amniocentesis is a procedure done under ultrasound guidance where a long thin needle is inserted into the mother's abdomen, into the uterus, to withdraw a couple of tablespoons of amniotic fluid (fluid surrounding the developing baby) to study. The DRPLA gene can be studied using cells from the amniotic fluid. Other genetic tests, such as a chromosome analysis, may also be performed on either a CVS or amniocentesis. A small risk of miscarriage (1 in 200 to 1 in 400) is associated with amniocentesis.