Diagnostic DNA testing for Huntington's disease
Direct DNA testing for autosomal dominant hereditary neurodegenerative Huntington's disease can be diagnostic, presymptomatisk or fetuses.
The article is aimed at physicians outside the medical genetic departments consider DNA testing for diagnosing a disease in patients with symptoms or signs that may be consistent with Huntington's disease. It provides a brief overview of the practical relevant clinical, epidemiological, molecular genetic and legal aspects of diagnostic DNA testing for Huntington's disease. We emphasize the need for adequate information before sampling and after the test is performed and close contact with the genetic center that performs testing for Huntington's disease and providing genetic counseling.
The diagnostic tests are the treating physician is responsible for informing the patient about the results of DNA research and to ensure adequate follow-up. The applicant may wish to request assistance from an expert in clinical genetics. A positive diagnostic DNA test for Huntington's disease in a patient can have far-reaching consequences for family members, and these should be offered genetic counseling. Since symptom-free family members with disease risk may request presymptomatisk testing in the future, it is indicated for confirmation of diagnosis by DNA examination of any person being investigated because of clinical signs of Huntington's disease, although the diagnosis is considered clinically safe.
Huntington's disease is a rare, inherited neurodegenerative disorder with autosomal dominant inheritance. The most common symptoms and signs can be divided into three main groups:
- Movement disorders, usually with motor agitation with involuntary athetoid movements, or rigidity and øyemotilitetsforstyrrelser. Eventually, it may act management difficulties with unsteady gait. Explicit swallowing difficulties and the increased motor activity can lead to an increasing nutritional deficiencies and wasting
- Development of often serious psychiatric disorder and change in personality
- A gradually increasing cognitive impairment
Age of onset, symptoms, signs and progrediering can vary considerably, even within the same family. The disease occurs typically from about 40-years old. There is a rare juvenile form in childhood and adolescence, and also cases with onset of moderate symptoms and signs in 60 - to 70-years old. In the juvenile form is described more frequent occurrence of rigidity, dystonia and early epilepsy than in patients who become ill during adulthood. Some patients have predominantly motor signs, with unsteady gait and clumsiness, and less severe neuropsychological and psychiatric outcomes.
Age at onset of symptoms and signs are in practice difficult to schedule in the individual patient. This is also the number of years of life after stroke onset is difficult to define precisely, both in the individual patient and in larger patient groups, but it is in the literature is estimated to be between 10 - 25 years (1).
Prevalence is the most studied European populations is estimated at 4 to 8 per 100 000 (2). It is after this between 180 and 360 patients with Huntington's disease. Also, patients with other psychiatric or neurological diagnoses can after DNA testing proved to be suffering from Huntington's disease, so that future prevalence surveys may show higher numbers than the former. The degree of openness about the disease within families and between branches of the same family may differ, and lack of transparency can contribute to the diagnosis is not determined by individual family members, or be very late.
Inheritance, penetrance and antesipering
The autosomal dominant inheritance means that each child of a person who has gene for Huntington's disease, has a 50% risk of inheriting it, regardless of gender. Huntington's disease skips not over a generation.
Penetransen is age-dependent and virtually complete at a normal life. In the juvenile form of the plant to the disease most commonly inherited through the father. This form of amplification of the disease, with earlier onset and possibly with more severe clinical course from one generation to the next is called antesipering. Antesiperingen is a clinical expression of the molecular instability in parts of the gene during the formation of sex cells. Antesiperingen in Huntington's disease occurs mainly when the gene is inherited from the father, because the instability is most pronounced in the formation of male gametes (3).
Legislation around the testing
Act on medical applications of biotechnology of 5 August 1994 (4) regulates the use of genetic testing. The applicant of genetic testing, such as DNA testing for Huntington's disease, will in practice face three different test situations: diagnostic examination, presymptomatisk examination and prenatal diagnosis.
A diagnostic evaluation may be ordered by any doctor and any hospital for patients with symptoms or signs suggestive of Huntington's disease. The diagnostic tests for the detection or investigation of the disease is the applicant responsible for advance information to patients about a possible positive test result will mean, notification of test results and follow-up when the test result is available.
Offer genetic counseling to the patient and family should be included as part of the follow-up. This is especially true when the disease is detected in the family for the first time, because the implications of a positive test may be significant for the family and other relatives.
Presymptomatisk study of Huntington's disease occurs in the Department of Medical Genetics, when there is an application for testing of a person without clear symptoms or signs of Huntington's disease, but with the estimated risk for future development of the disease. The law requires when it is given extensive genetic counseling before testing, the examination procedure in the period until the answer is and after that the answer is given. The result of a test presymptomatisk notified personally and conveyed either to the referring physician or other agencies, unless this is necessary for diagnosis and treatment. According lovpresisering only authorized institutions that can commandeer presymptomatiske laboratory investigations.
Fetal Diagnosis can be offered when one parent has gene for Huntington's disease. After genetic counseling, prenatal diagnosis can be performed in 10 pregnancy by placental examination (chorionbiopsi), or in 13 - 14 pregnancy after amniocentesis (amniocentese). It is therefore important that the medical-genetic department contacted as early as possible in pregnancy.
Blood samples are sent to the medical genetic department that the patient belong to. It must be accompanied by clear medical information about signs and symptoms suggestive of Huntington's disease, and the necessary related information. Without medical information is not otherwise possible for the department to distinguish a diagnostic from a presymptomatisk test situation, which is necessary because of the law.
The regional medical genetic departments offer genetic counseling to patients with Huntington's disease and their families. Hospital's Center for rare diseases and syndromes provide information to patients, families and agencies who come into contact with patients with Huntington's disease.
The gene for Huntington's disease
The gene for Huntington's disease is on the short arm of chromosome 4 From the coupling studies of families had been known location for ten years before the gene was cloned and characterized in 1993 (4). This led to the more uncertain link-based DNA studies (5) could be replaced by direct examination of mutations in the gene.
The gene codes for a normally occurring protein of unknown function that is called huntingtin and is expressed in most tissues examined. The first of the gene 67 coding regions (exon 1) contains an area with a number of repetitions of a sequence of three bases cytosine, adenine and guanine (CAG), hence the term "trinucleotide repeats". Normally, there are 9 to 35 CAG-repeats (6). There is thus a normal genetic variation in the number of CAG-repeats - (CAG) n - in this part of huntingtingenet. The test can show that such a person. has a version of the gene (allele) with 17 CAG-repeats and one with 21 CAG-repeats (one gene inherited from the father and one from the mother), that is a normal number of CAG-repeats in both chromosomes 4
Patients with Huntington's disease is a pathological increase in the number of 36 or more CAG-repeats. Alleles with 36 to 39 CAG-repeats does not always clinically manifest Huntington's disease, but alleles with 40 or more CAG repeats, always involves Huntington's disease. It is now widely accepted that this increased number of CAG-repeats in huntingtingenet is a prerequisite for the development of the disease. A normal number of iterations therefore rules in practice, Huntington's disease.
Number of CAG-repeats in huntingtingenet may change between generations, and this tendency to instability is clearly most pronounced in men (6).
Instability also occurs in alleles with normal number of iterations, especially in the range 27 to 35 CAG-repeats. These normal, but unstable, alleles account for only 1 - 2% of the alleles in published larger series (7 - 9). It is believed that these alleles in rare cases can expand and be candidates for nymutasjoner. Nymutasjoner is very rare. Instability can also lead to a contraction, ie a reduction of the number of CAG-repeats within the pathological range, for example. from 39 to 37
Population studies show a clear tendency to an earlier onset and more severe progression of increasing the number of repetitions (10). However, there is considerable individual variation in age of onset and clinical course of patients with the same pathological number of CAG-repeats, even within families. An abnormal number of CAG-repeats of a healthy person is thus not when the disease will break out.
DNA laboratory receiving the blood sample must be informed that the requested assistance to make a disease diagnosis (diagnostic test). If in doubt about the existence of a diagnostic or presymptomatisk test situation, obtained the necessary clarifications from the applicant before the analysis can take place.
After receipt of the blood sample and concentration determination of extracted DNA performed by the polymerase (polymerase chain reaction, PCR) an amplification of the area in huntingtingenet that contains the variable number of CAG-repeats. After electrophoresis of PCR product, whose size depends on the number of CAG-repeats, one can calculate the number of CAG-repeats in both alleles. Cloned PCR fragments with known number of CAG-repeats are used as reference material.
A normal result, for example. be 18 and 23 CAG-repeats, while a pathological result eg. can be 19 and 42 CAG-repeats (a PCR product from each person's two hunting things energy, ie, one from each chromosome 4).
The method does not distinguish between the PCR product from one allele and the product of two alleles with the same number of CAG-repeats. In some cases, read it, therefore, only a single number of CAG-repeats. This happens when the investigated inherited alleles of the same number of CAG-repeats from both parents, and therefore are homozygous. This will usually occur with the common alleles, with for example 19-CAG repeats. In such situations should be DNA from the parents or children could be examined to confirm that the investigated can be homozygous. Also by the direct DNA test can therefore access to DNA from relatives contribute to the quality assurance of the survey and the interpretation of test results.
Before ordering a DNA test for Huntington's disease must be clarified whether there is a diagnostic or presymptomatisk test situation.
By submitting blood for DNA examination of a person with symptoms or signs of Huntington's disease (diagnostic test) must be provided relevant clinical information. The applicant is responsible for communicating the results to the patient and to ensure adequate follow-up. The patient and family should be offered genetic counseling at the regional department of medical genetics.
DNA study will show whether the investigated person has an abnormal number of CAG-repeats in one of his hunting things thousands. Detection of the gene in each patient does not say how the disease will be progressive, or the symptoms will occur. Although today we can offer a direct DNA-examination, access to blood samples from family members contribute to the quality of the survey.
It is discovered still cases of Huntington's disease in families where the disease has not previously been acknowledged or discussed. Negative family history should not be given too much emphasis on clinical suspicion. The diagnosis of Huntington's disease should be considered even in elderly people with moderate symptoms or signs.