Genetic Testing for Huntington’s disease Essay
Genetic Testing for Huntington’s disease
Huntington’s disease (HD) is a late onset, progressive type of neurodegenerative, disease which is characterized by progressively worsening physical and mental abnormalities,1 and finally death (Walker, 218). Though symptoms of HD can appear at any age from 1 to 80 years, they most commonly appear in an individual between 30 to 50 years of age and progress over a span of 10 to 25 years (Walker, 218). HD affects the individual’s ability to think, speak and walk. HD is a hereditary disorder which is inherited in an autosomal dominant pattern1. This means that every child born to a person, who has HD, regardless of gender, has a 50% chance of inheriting the gene that causes HD (as shown in figure 1). Since the defect is on autosomes, men and women are equally affected, so either parent may pass on the HD gene to any offspring (Meiser & Dunn, 575). Every person who inherits the HD gene will eventually develop the disease while non carriers will definitely not develop the disease. Thus if one does not inherit the gene, one cannot pass it on.
1 To understand the exact clinical course followed by HD see the article “Huntington’s disease” by Walker, Francis O. in the Lancet. 369, 9557(2007) especially page 218-219.
2 Autosomal dominant inheritance implies that person needs to have only one mutant gene to express this disease phenotypically and he can inherit it from either of his parents (Walker, 218). Also see figure.1
Figure 1. Autosomal Dominant Pedigree Chart
Source: Walker Jeremy. Autosomal Dominant Pedigree Chart. 2006. Accessed 7 April 2007
Genetic Defect Involved in Huntington’s disease.
The gene which undergoes mutation resulting in HD lies on the short arm of chromosome
4 (Walker 220). The normal gene is actually a segment of DNA which contains a strip of trinucleotide sequences usually of the type cytosine-adenine-guanine (CAG) at one end. Mutation of this gene results in expansion of these unstable trinucleotide CAG repeat sequences (Walker 220).Everyone has two copies of the normal gene. The people with HD have mutation of one of these genes. The causative mutation in HD is in the first exon of the IT15 gene on chromosome 42.The mutation results in abnormal production of glutamine (amino acid represented by CAG) which can aggregate to form polyglutamine chains (Walker 222). Development of HD depends on number of CAG repeats contained within the copies of the HD genes. When the number of these CAG repeats is 41 or more, the disease is fully penetrant (i.e. expresses itself completely). Incomplete penetrance is observed in people who have between 36-40 CAG repeats. If number of these CAG repeats is 35 or less, the disease does not manifest itself (Walker, 220). The number of CAG repeats may change as the gene passes from parent to child. Thus the child may show slightly fewer or slightly more repeats compared to the parent. The tendency for a child to inherit the disease is more when the gene is passed to him from father rather than from his mother4.
3To find out more regarding the gene mutation in HD, see the article,’ A novel gene containing a trinucleotide repeat that is expanded and unstable in Huntington’s disease chromosomes’ by Huntington’s Disease Collaborative Research Group in the journal Cell 1993;72:971–83.
4 To find out more about the reason behind this tendency read the article “Huntington’s disease” by Walker, Francis O in the Lancet. 369, 9557(2007) especially page 220.
Genetic Testing for HD
The process of genetic testing involves a blood test to detect the abnormal expansion of CAG repeat sequences (HDSA, 3). The result of the test cannot determine when the symptoms will appear except in cases where number of CAG repeats is found to be very high. For e.g. patients with a high number of CAG repeats ( > 60 )may suffer from juvenile onset HD.5 It cannot be determined whether the person will develop the disease or not if the test shows number of CAG repeats in the intermediate range(36-40). Genetic testing as described by HDSA (3, 6) can be of three types: presymptomatic testing, confirmatory testing and pre-natal testing.
Presymptomatic testing is used to detect the presence of gene in an asymptomatic person,
at “high risk” for the disease (for e.g. a positive family history of HD in parents or grand parents)
The discovery of the gene that causes HD in 1993, led to the development of a genetic test based on direct identification of abnormal gene through a blood test performed in an individual (Meiser, & Dunn, 574). Those “at-risk” for HD are able to find out whether they carry the genes for HD, before the actual symptoms of the disease appear. Presymptomatic testing presents as a difficult choice for the “high risk” patients who want to establish their genetic status before the appearance of any clinical sign as there is no way of preventing or curing this disease.
5Juvenille onset HD refers to onset of HD at a young age of 18 years or less .Trinucleotide repeats in which number of sequences exceed 28, show instability on replication which grows further with increasing size of the repeat. This can result in production of large number of CAG repeats thus causing the symptoms of HD to appear at an earlier stage (Walker, 220).
Before the discovery of the exact gene responsible for producing HD, the identification of
carriers in an asymptomatic stage was done though linkage analysis based on the detection of a genetic marker for HD.6 Linkage analysis takes advantage of the spatial proximity of a genetic marker and the mutation by examining the pattern of DNA near the gene in both parent and fetus (Meiser, & Dunn, 574). However the triple repeat itself is not analyzed. If the DNA patterns do not match, it can be assumed that fetus did not inherit the HD gene. If the patterns match, it indicates that fetus has same genetic make-up as that of at risk parent. It does not indicate whether the parent (or fetus) actually has the defective gene. Linkage analysis involves examination of blood samples from both affected and unaffected family members over several generations. In this way, the risk that the fetus is carrying the gene for HD can be estimated. The accuracy of this test is limited as genetic recombination can occur between markers and the gene during meiosis. Due to this, the result is given in form or risk estimates only e.g. 90% or more risk (Meiser, & Dunn, 575).
Identification of the exact gene mutation responsible for HD has led to development of tests which can directly detect mutations. This test is highly accurate with sensitivity and specificity rate approaching nearly 100%. This test does not require testing of blood samples from other relatives (Meiser, & Dunn, 575).
6 To get a detailed analysis about genetic markers read the article “Closely linked polymorphic marker (D4S10) to the gene for HD ” by Gusella JF, et al in Nature 1983;306:234–9.
Confirmatory testing is done to confirm the clinical diagnosis of HD in a person exhibiting a few symptoms suggestive of HD. Since the symptoms of HD can be often similar to those seen in other diseases like Alzheimer’s disease etc, it might be difficult to clearly establish a diagnosis. Genetic testing in these cases helps in confirming the diagnosis of HD (HDSA,6).
Pre-natal Genetic Testing
Presymtomatic genetic testing in high risk individuals enables them to make appropriate reproductive choices depending on the results of their test. If the individual is found to be a carrier, he/she can opt for an accurate prenatal diagnosis, established through pre-natal sampling7 or make use of alternatives such as donor gametes, in order ensure that the defective gene would not be transmitted to the baby (Sermon et al, 591). On the other hand some high risk patients may prefer not to know their carrier status, but still might wish to have a baby who does not carry the gene for HD. For these patients, two options are available: exclusion (linkage) testing after prenatal sampling or pre-implantation genetic diagnosis (Sermon et al, 591).Through exclusion testing it is possible for to obtain the fetal risk of inheriting HD by showing whether the marker allele from the unaffected or affected grandparent has been transmitted to the fetus (Ball & Harper, 2818). A disadvantage of this test is that unaffected pregnancies may be terminated if the parent at risk has not inherited the grandparental Huntington gene (Moutou, Nathalie & Viville, 1007). These can lead to serious moral and ethical objections.
7Prenatal sampling involves a procedure of amniocentesis or chorionic villous biopsy in order to retrieve the fetal cells which are then assessed for the presence of gene for HD. (Sermon et al, 591)
Preimplantation genetic diagnosis (PGD)
It is a very early form of prenatal diagnosis in which embryos which are obtained in vitro
through assisted reproductive technology are analyzed for presence of gene for HD. Only the embryos which are found free of genetic disease are transferred inside the mother’s womb
(Sermon, 10). According to sermon (11) the carrier free embryos can be either selected through PCR or FISH techniques, 7 or through the process of exclusion testing (linkage analysis). Both these types are a form of non-disclosure testing as the person undergoing the test need not know his carrier state. However, according to Sermon et al (592) there are ethical and practical problems associated with PGD involving amplification of CAG repeats. These according to them include the issues of confidentiality and issues of performing IVF of PGD on individuals who are perfectly normal and do not carry the HD gene. In exclusion test (as explained earlier) only fetuses or embryos that inherit an allele from the unaffected grandparent are considered to be unaffected. If the fetus has one allele of the affected grandparent, termination of pregnancy is proposed. Thus exclusion (Linkage) testing can lead to unwanted termination of normal fetuses in cases where the parents are not carriers (Moutou, Nathalie & Viville, 1007).
8PCR or polymerase chain reaction is a genetic technique which allows exponential amplification of small DNA fragments. FISH or Fluorescent-in-situ hybridization is a technique which allows the visualization of chromosomal regions. For more information regarding their use in PGD see the article” Current concepts in preimplantation genetic diagnosis (PGD): a molecular biologist’s view” by Sermon, Karen. in the journal Human Reproduction Update, 8, 1(2002): 11-20.
The results of the European collaborative study conducted by Evers-Kiebooms et al (167-
176) show that predictive DNA-testing can help many couples to fulfill their wishes of having children without any risk of transmitting the HD mutation to them. Genetic testing for HD has an important impact on subsequent reproductive and family planning decisions among the carriers and non-carriers of the Huntington mutation. Overall, an effective impact of the predictive test result on subsequent reproduction was seen in this study: 14% of the carriers, in comparison to 28% of the non-carriers had one or more subsequent pregnancies in a span of three years. In the entire carrier group, prenatal diagnosis was performed in about two thirds of pregnancies while one couple chose to have PGD, resulting in pregnancies free from HD gene (Evers-Kiebooms et al, 174).
Most Common Reasons for Taking the Test
The most common reasons which motivate the individuals at high risk to take this test are: the need for certainty or relief from uncertainty, making reproductive decisions, informing their children about their risk for HD and making decisions on practical matters like financial issues, career, employment etc. [Meiser and Dunn (575);Gerry et al (168)].
Important Barriers against Taking the Test
Despite the widespread availability of the test, it has been seen that only about 20% of
individuals at high risk, opt for this test (Evers-Kiebooms et al, 168).The possible causes are: emotional and psychological consequences of a positive result, helplessness on the part of the patient as well as the physician due to the lack of curative treatment, concerns about the reaction of their children and partner, potential loss of health insurance etc (Evers-Kiebooms et al, 168). There has also been a lot of concern regarding the ethical issues related to genetic testing for HD.
Current Protocol for Genetic Testing
Huntington’s Disease Society of America HDSA (1-7) has devised a protocol which is to be followed before conducting genetic testing for HD. These include the following:
Age: The test is not performed in individuals less than 18 years of age, unless there is onset of symptoms before this age (Walker, 220).
Time period: The process of testing involves pre-test and post-test counseling sessions, neurological examination (which helps in identifying any symptom suggestive of HD), psychiatric and psychological examination before the actual blood test. The blood test is followed by discussion of results and a period of follow-up. Sessions of testing process are usually spread out over a period of one month or more. This is to ensure that the person is given adequate time to absorb all information, understand the implication of the test and make his choices accordingly (HDSA, 3).The person can withdraw from testing at any point of time.
Pre-test Genetic counseling: Testing should never be forced upon an individual. Every person has his or her own circumstances to consider. Pre-test counseling sessions give adequate time to the person and enable him to develop a plan for his life depending on whether he would have a positive or a negative result (Ball and Harper, 2818). The person needs to confront the emotional, social and economic challenge related to the disease The patient needs to understand the implications that a positive test can have on his relationship with his spouse, siblings (especially in case of identical twins), parents (they might be suffering from the disease too) and his children (who may have inherited the gene for HD from him). (HDSA, 3). After thinking about all aspects of the situation, the person can give free informed decision about having or not having a predictive test. (Ball and Harper, 2818).
Ethical issues related to genetic testing for HD
Many serious concerns have been raised regarding whether it is ethical to offer genetic testing for HD. Since the disease is eventually fatal and has no cure, it has been feared that the test results could precipitate many psychological problems like depression, anxiety, suicidal tendencies etc. It can also result in extreme degree of stress and cause strained relationships with other family members (especially siblings, spouse etc). Genetic disorders are often associated with social stigmatization and discrimination in employment and in seeking insurance claims (Almqvist et al, 1293). A review of existing evidence by Meiser and Dunn (574) suggests that there is a significant difference among non-carriers and carriers regarding the level of general psychological distress suffered after knowing the test results, only in short term, but not in long term.. Meisner and Dunn (575) identified no obvious contraindications for testing people at risk. According to them the psychological impact of testing may depend on whether testing was based on linkage analysis or mutation detection9. Meisner and Dunn (576) also observed that those individuals who were psychologically well adjusted before undergoing the test due to adequate counseling services showed much better psychological response to distress after knowing the results of the test in comparison to individuals who were not adequately counseled.. Thus this study further emphasizes the need for an intensive counseling protocol before the test.
9When testing was performed using linkage analysis the individual experienced less psychological distress compared to when the direct detection of the mutation was performed. This is so as the linkage test gives only an estimated risk about the presence of HD gene in an individual. Such result estimates have an increased scope for optimism. Also, people who present for linkage testing may have had greater social and family support at the time of testing as the test involves taking blood samples from multiple family members (Meisner and Dunn 576).
The risk for suicide in patients with positive test result for HD is estimated to be about
7 to 200 times higher than the risk found in general population (Paulsen et al, 725). Findings of study by Paulsen et al (727) showed that the frequency of suicidal ideation was about 9.1% in HD gene positive persons with a normal neurological examination, 19.8% in HD gene positive persons with non-specific motor abnormalities and 23.5% in persons with “possible Huntington’s disease.” To study the psychological impact of genetic testing for HD, Almqvist, et al (1293-1304) in their study investigated the frequency of catastrophic events (like suicide, suicidal attempts and psychiatric hospitalization) in persons in whom genetic test for HD had been performed. Of the 4,527 persons undergoing testing, there were 5 cases of completed suicides, 20 suicide attempts and 18 psychiatric hospitalizations, resulting in an overall catastrophic event rate of 0.97%. All the five cases of completed suicide were seen in HD positive individuals The risk factors which predisposed to the occurrence of catastrophic events in the study were found to be: previous history of psychiatric disorders, history of unemployment and clinical status of the disease10 (Almqvist et al., 1295).
On the contrary, some authors argue that receiving a positive test result may provide psychological benefits. This is so as the result would remove all uncertainty from the mind of an individual and give him an opportunity to appropriately plan his life (Bird, 1290). Besides the patient, the genetic testing for HD testing can also have a direct impact on the psychological
10 The incidence of the catastrophic events was less in the asymptomatic stage but increased at the time of onset of clinical symptoms. Higher incidence was seen amongst the recently diagnosed cases with HD. (Almqvist et al, 1295) health of his care-taker, who lands up taking care of patient when his symptoms appear (Bird, 1290). Thus it is of utmost importance that the legal and ethical issues related to genetic testing of HD are adequately discussed and appropriate safeguards and legislation is devised by the government and health care authorities (Ball and Harper (2819).
How the US Policy must approach the issues of reproduction and insurance coverage for carriers of HD United States needs to centrally organize and co-ordinate testing and collection of all information related to individuals undergoing genetic testing (Quaid, 785). One main reason why the number of “high risk” individuals tested in the United States remains small, is limited accessibility and availability of testing centers (Quaid, 785). The US government should ensure that adequate number of testing centres are present in each state depending on the population Another important consideration related to genetic testing for HD is that no curative treatment is available. Some drugs like teterabenezine can provide only symptomatic relief. More funding and encouragement of the research projects dedicated towards discovering an effective form of treatment for this disorder is also required (Quaid, 785). At the same time, one needs to consider significant emotional and psychological effects that the results of the test can have on the person and his family. US government should ensure that pre-testing and post-testing counseling is provided at all centres. The US policy should aim at providing correct and accurate information related to the disease and removing all myths related to it. Correct education and awareness of the disease will encourage people to undergo genetic testing and also remove the stigma related to the disease. US government should set up well defined centralized ‘Genetic testing protocol’ and ensure that it is rigorously followed by all the testing centres.
Knowledge about the carrier state can help the couples in making decisions related to childbirth. With the discovery of pre-natal and pre implantation genetic diagnosis, it is possible for mutation positive individuals to make sure that their children will not inherit the disease (Sermon, 591). At present very few people, just about 5% of the individuals at risk of HD take advantage of prenatal testing (Walker 220). People need to be encouraged, supported and guided in order to undertake these tests. Knowledge and awareness about the disease needs to be disseminated to general people. More funds and resources must be allocated to ensure that the HD testing centres, particularly HDSA testing centres have facilities for prenatal testing as well as PGD. Government needs to ensure that facilities of pre-natal testing and PGD are easily available in all states and accessible to normal public at low costs.
At present different insurance providers have different policies regarding the pre-symptomatic testing. One needs to check with their individual insurance provider. Some insurance providers have been known to deny any health claims by an individual found to be positive on genetic testing for HD. The US government needs to make sure that such discrimination for genetic diseases like HD is not carried out. A positive result can also be considered as a ‘pre-existing condition’ due to which it may become difficult to obtain future insurance claims. (HDSA, 4) There is a need for federal law which would prohibit any kind of discrimination by the insurance companies related to genetic diseases like HD.
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