Genetic testing for Huntington disease

Handbook of Clinical Neurology, Vol. 144 (3rd series) Huntington Disease A.S. Feigin and K.E. Anderson, Editors http://dx.doi.org/10.1016/B978-0-12-801893-4.00010-9 Copyright © 2017 Elsevier B.V. All rights reserved

Chapter 10

Genetic testing for Huntington disease KIMBERLY A. QUAID* Department of Medical and Molecular Genetics, Indiana University School of Medicine, Indianapolis, IN, United States

Abstract In 1983, Huntington disease (HD) became the first disease to be mapped to a previously unknown location on chromosome 4. This discovery meant that we could now identify whether some individuals at risk for HD would develop HD in the future using a method called linkage testing. Testing was first offered through research protocols designed to assess whether testing could be done safely in this population. Testing guidelines were soon developed by the Huntington’s Disease Society of America and the International Huntington Association in collaboration with the World Federation of Neurology. The gene for HD was found in 1993, allowing for direct gene testing for the mutant HTT allele. This chapter will discuss the development of guidelines and recent revisions to the guidelines, prenatal testing, and testing in three complicated situations: (1) the testing of minors; (2) anonymous testing; and (3) testing individuals at 25% risk. Studies examining the outcomes of predictive testing will also be discussed. Outcome studies have shown that testing can be done safely in the context of testing protocols that include neurologic examinations, pretest counseling, psychiatric/psychologic assessment, results in person, and available follow-up support. It appears that anxiety and depression prior to testing are better predictors of psychologic status after testing than the test result itself.

MAPPING THE GENE In 1983, Huntington disease (HD) became the first disease to be mapped to a previously unknown location through the use of restriction enzymes which cleave deoxyribonucleic acid (DNA) at sequence-specific sites (Gusella et al., 1983). Inherited variations of these DNA sequences, also known as restriction fragment length polymorphisms, can be used as genetic markers to map diseases on chromosomes as well as to trace the inheritance of disease within families. Subsequent studies both confirmed and refined the gene’s association with the initial marker on the short arm of chromosome 4 (Folstein et al., 1985; Conneally et al., 1989), and the identification of various additional markers soon followed (Wasmuth et al., 1988; Whaley et al., 1988; Youngman et al., 1988). The discovery of polymorphic markers linked to HD was a significant advance in HD research. Not only did it provide a possible clue for finding the gene

and to understanding the mechanism for how the gene caused brain cells to die; the discovery also meant that predictive testing for those at risk was now possible. Initially, the presymptomatic test procedure used a process known as linkage. Linkage compares the marker DNA of family members who were at risk for HD by virtue of having an affected parent, but who were now old enough to be considered to have only a very slight chance of ever developing HD (known as escapees), with the marker DNA of affected family members. If enough affected and “escapee” family members were available in many, but not all, families, it was possible to determine the genetic pattern that was traveling in that family with the HD gene. Once that pattern was determined, it became possible to assess whether at-risk family members were at either very high or very low risk. Depending on which markers were informative, the low risk was 1–5% and the high risk was 95–99%.

*Correspondence to: Kimberly A. Quaid, Ph.D., Indiana University School of Medicine, Department of Medical and Molecular Genetics, 975 West Walnut Street, Indianapolis IN 46202-5251, United States. Tel: +1-317-278-4039, Fax: +1-317-278-4050, E-mail: [email protected]

K.A. QUAID

114

Predictive testing using linkage had several drawbacks. First, blood or tissue samples were needed from several family members, both affected and elderly unaffected. The exact family members from whom samples were needed varied for each family and needed to be determined on a case-by-case basis using a detailed pedigree or family tree. Initial estimates suggested that only 15% (Harper and Sarafazi, 1985) to 37% (Misra et al., 1988) of families with HD would have the requisite structure or samples available to allow testing using linkage. In those families that did have the necessary structure, gathering the needed samples was time consuming, required delicate negotiations with often estranged relatives, and effectively precluded confidentiality for the person being tested, at least within the family. Second, accurate information about the neurologic status of each person donating a sample, whether or not the individual was affected with HD, was crucial. In some cases, this meant that a neurologic examination and/or medical records had to be obtained for each person. Those seeking testing thus ran the risk that their relatives might learn something about themselves from either their medical records or about their neurologic status that they may not have sought on their own and with which they might not be able to cope (Smith et al., 1998). Third, as previously mentioned, testing using linkage was not 100% accurate. Estimates of the error rate of linkage testing are based on the distance between the gene itself and the markers used for testing. Early tests were estimated to be about 90–95% accurate (Gusella et al., 1983; Folstein et al., 1985; Haines et al., 1986). With the discovery of markers more closely linked to the HD gene, accuracy quickly increased to 99% (Gilliam et al., 1987; Hayden et al., 1988; Wasmuth et al., 1988). Fourth, linkage testing was expensive. Commercial laboratories offering testing charged as much as $500 per sample, and the average test required four to five samples. Testing was also extremely labor-intensive; each case involved long hours of constructing and analyzing pedigrees, collecting the necessary samples, and gathering and reviewing medical records.

DNA BANK After the HD gene was mapped and to assist future linkage testing, the world’s first DNA bank was established at Indiana University under the direction of Dr. Ed Hodes. Families with HD could “bank” samples from affected and elderly unaffected relatives so that they would be available for use in linkage testing when and if their relatives at risk chose to be tested. Individuals at risk who had children were also encouraged to store DNA so that, in the event of their untimely death, the

stored sample could be tested in order to determine the level of risk for the children. These stored samples proved invaluable as linkage testing slowly spread throughout the United States.

PREPARING FOR PREDICTIVE TESTING: FIRST DO NO HARM In preparation for predictive testing for HD, a workshop on presymptomatic diagnosis was convened by Dr. Milton Wexler, President of the Hereditary Disease Foundation and the father of two daughters at risk for HD. Dr. Wexler’s first wife died with HD, as did her three brothers. Invited participants included laypersons with an interest in HD, HD patients and their family members, researchers and clinicians with years of experience taking care of HD patients and their families. Foremost in the minds of many was the fact that the suicide risk in HD families was very high. Completed suicide had been reported to be as high as 13% in HD, reflecting a sevento 12-fold increase from the rate of the general population (Schoenfeld et al., 1984a; Farrer, 1986). After lively discussion, the group decided that testing should first be offered on an experimental basis in order to determine whether or not it could be done safely. They further decided that the release of the genetic probes used for testing should not be widely distributed until the findings of these experimental programs were available. While most thought that this approach demonstrated appropriate caution, many, especially in the genetics community, eager to have access to these probes, felt that it was too paternalistic. The workshop attendees also developed preliminary guidelines for predictive testing protocols, including eligibility criteria and screening procedures. These protocols included a neurologic examination, psychiatric screening, intensive pretest counseling, and posttest follow-up. The experimental protocols were designed to determine: (1) the psychiatric, psychologic, and social consequences of informing people of their genetic status with regard to HD; (2) whether pretest characteristics could distinguish those who adapt well to knowledge of their genetic status from those who adapt poorly; and (3) whether educational and therapeutic interventions could mitigate morbid psychologic and social outcomes (Brandt et al., 1989). In September of 1986, centers at Massachusetts General Hospital in Boston and Johns Hopkins Hospital in Baltimore began offering presymptomatic testing on an experimental basis. Other centers offering testing using similar protocols opened in Australia (Turner et al., 1988), Canada (Bloch et al., 1989; Fox et al., 1989), Wales (Morris et al., 1989), England (Craufurd et al., 1989); Belgium (Evers-Kiebooms, 1990), and

GENETIC TESTING FOR HUNTINGTON DISEASE the Netherlands (Tibben et al., 1990). By the end of 1991, 23 centers were offering presymptomatic testing as part of their clinical service.

PRELIMINARY RESULTS WITH LINKAGE Hayden et al. (1988) reported the results of 20 informative tests on individuals at both 50% risk of developing HD (having an affected parent) and at 25% risk (having an affected grandparent but an unaffected, still at-risk parent). Twelve (60%) received a decreased risk and 8 (40%) received an increased risk of having inherited the HD gene. In the group found to have an increased risk, 2 persons were said to have clinical signs consistent with early-stage HD. Although they did not report the psychologic impact of testing, including persons at 25% risk and those with early symptoms consistent with HD complicates the determination of psychologic risks associated with presymptomatic testing. Meissen et al. (1988) reported that, of 16 DNA tests performed on individuals at 50% risk for HD, 4 were found to have an increased risk, 7 were found to have a decreased risk, and 5 were uninformative. The authors reported that those who were told that they probably had the HD gene were “surprised” or “shocked” by their test results. Half of them (2 people) had periods of severe depression following testing, while the other 2 people had periods of moderate depression at 3-month followup. Despite these results, the authors reported that no psychiatric care had been necessary. Brandt et al. (1989) reported on the first 2 years of their experimental program. DNA analysis was completed for 55 people. Twelve of the tests yielded positive results (high risk), 30 were negative (low risk), and 13 were uninformative. Initial reactions ranged from joy and relief to disappointment, sadness, and demoralization. The researchers reported that there had been no severe depressive reactions thus far. They concluded that people who receive genetic test results appeared to cope well, at least over the short term, when testing was performed in a clinical context that includes education, neurologic examination, psychiatric screening, pretest counseling, psychologic support, and regular follow-up.

THE DEVELOPMENT OF TESTING GUIDELINES In 1985, a committee of representatives of the International Huntington Association (IHA) and the World Federation of Neurology (WFN) Research Group on Huntington’s Chorea was established specifically to produce recommendations for the use of the predictive test for HD (Went, 1990). At their respective meetings in Vancouver in 1989, the two organizations reviewed

115

and adopted these recommendations (IHA-WFN, 1990). These guidelines have been updated in 1994 (IHA-WFN, 1994) and 2012 (MacLeod et al., 2012). Prominent features of the guidelines included: mandatory counseling; the assumption of informed choice on the part of the test taker, including criteria for those unable to make an informed choice; and the restriction of testing to those aged 18 or older, except when a pregnancy was involved. In 1989, the Huntington’s Disease Society of America (HDSA) published Guidelines for Predictive Testing for Huntington’s Disease. These guidelines were the result of a series of meetings with scientists, doctors, health professionals, HD patients and their families, and those who were in a position to offer testing. These guidelines also include mandatory pretest counseling, the assumption of an informed choice on the part of the test taker, including criteria for those deemed unable to make an informed choice, and the restriction of the test to those aged 18 or older, except when a pregnancy was involved (HDSA, 1989). These testing guidelines were revised in 1994 (HDSA, 1994) and again in 2003 (Nance et al., 2003). Later versions of the guidelines specifically addressed three complex situations that may arise in testing: (1) the testing of minors; (2) anonymous testing; and (3) testing persons at 25% risk. New and revised guidelines, Genetic Testing Protocol for Huntington’s Disease, were issued by HDSA in 2016. This most recent iteration was met with some controversy. Critics argued that the use of the term “protocol” rather than “guidelines” appeared to imply strict adherence to a fairly rigid process such as a research protocol rather than a framework of recommended procedures that could be tailored to the unique personnel and structure of each center offering testing (Goodman, 2016). Also cited by critics was a heavy reliance on phone contact rather than face-to-face contact and a change in the protocol such that a neurologic examination, or any contact with a physician, was deemed optional. Others applauded the streamlined approach to testing

TESTING OF MINORS Genetic testing in childhood for disorders that do not manifest until adult life raises a number of challenging ethical issues and has led to several statements by other professional organizations as well as the aforementioned Huntington’s affiliated organizations. These organizations include the American Society of Human Genetics Board of Directors and the American College of Medical Genetics Board of Directors (1995), the American Medical Association Council on Ethical and Judicial Affairs (1995), and the National Society of

116

K.A. QUAID

Genetic Counselors (NSGC, 1995). All have recommended against testing minors for adult-onset diseases. A systematic review of the ethical guidelines and position papers from 1991 to 2005 concerning the presymptomatic and predictive testing of minors concluded that the main justification for such testing was the direct benefit to the minor through either medical intervention or preventive measures. If there are no urgent medical reasons, all guidelines recommend postponing testing until the child can consent to testing as a competent adolescent or as an adult (Borry et al., 2006). A more recent survey by Borry and colleagues (2008) examined the practice and attitudes of geneticists in Europe with regard to the presymptomatic and predictive testing of minors. Most clinical geneticists were unwilling to provide a presymptomatic or predictive test for adult-onset disease unless it would provide a medical benefit (Borry et al., 2008). The testing of “mature minors” is somewhat less controversial (Binedell et al., 1996). The most recent statement from the American Academy of Pediatrics (2013) states that, “Predictive testing for adult onset conditions generally should be deferred unless an intervention initiated in childhood may reduce morbidity or mortality.” However, all guidelines recommend, as a default position, that even mature minors should not be tested unless strong justification is present. Despite this consensus, a number of these tests have been done. An international survey of clinical geneticists indicated that children had been tested for adult conditions for nonmedical reasons a total of 49 times. The most common condition tested for was HD and in 22 (45%) of those cases, the young person tested was immature, defined as under the age of 14 years. The most common reason for testing was because the parents wanted to know. Results were disclosed to only 2 immature minors and in 3 cases parents experienced clinically significant anxiety related to how they would pass the information on to their gene-positive child (Duncan et al., 2005). According to HDSA guidelines, minors should not undergo genetic testing unless there is a medically compelling reason such as a suspicion of HD or the need for a clinical diagnosis. Even under those circumstances, testing should be preceded by a thorough neurologic examination as well as neuropsychologic assessment. Parental anxiety about a child’s risk for developing HD is not considered a medically compelling reason for genetic testing. As we have discovered, the majority of adults at risk for HD do not choose to be tested. For that reason, it is important to protect the ability of a child to decide whether or not to be tested once he or she reaches the age of consent. This presumption against testing children is also to be upheld in the context of adoption where prospective adoptive parents wish to determine a child’s risk for developing HD prior to adoption. This testing would

be neither medically compelling nor necessarily in the child’s best interest and should be resisted. A second study by Duncan and colleagues (2007) presented qualitative interviews with 8 young people who had undergone predictive testing for HD. The authors concluded that predictive genetic testing has the potential to create both harms and benefits for young people at risk, and may have the potential to alleviate pre-existing harms. For some of those interviewed, uncertainty about their genetic status was felt as a barrier to their lives that prevented them from moving forward. There are several indications that the genetic testing of children for late-onset disorders may increase. First, one US survey of genetic service providers indicated that 44% had received requests to test children for adult-onset conditions (Wertz and Reilly, 1997). Second, there has been an uptick in articles advocating the genetic testing of children for late-onset disease. In 2005, Sevick and colleagues argued that the family needs to be empowered to make the best choices given the circumstance of the child and the values of the family. Mary Kay Pelias, a lawyer, argues that the role of parents as decision makers for their minor children has been reinforced in four seminal holdings of the US Supreme Court. She rejects arguments about protecting the future autonomy of children and argues that parents have a right, perhaps even a duty, to exercise their vested authority in making decisions that they believe are in the best interest of their family, including testing children for adult-onset disease (Pelias, 2006). Malpas (2008) examined two psychologic harms that are posited as possibly resulting from testing minors: a limited future and harm to a child’s self-esteem. He concludes that parents generally want what is best for their children and are as likely to overindulge children with a positive test result as deprive them of opportunity; thus, testing is acceptable (Malpas, 2008). However, geneticists responding to surveys regarding genetic testing in minors have reported that parents had requested testing children for HD in order to decide whether or not to save money for their child’s education. If the child had the gene, the parents would not “waste” the family resources (Wertz, 1998). Third are the recent recommendations from the American College of Medical Genetics and Genomics on the reporting of incidental findings in clinical exome and genome sequencing, asserting that recommendations about not testing children for adult-onset disease “can be inconsistent with the general practice of respecting parental decision-making about their children’s health” and raising the issue as to whether standards against testing children can be sustained in an era of comprehensive genomic testing (Green et al., 2013). Finally, HD research has shown that the structural and functional changes in the brains of people with the HD

GENETIC TESTING FOR HUNTINGTON DISEASE mutation occur long before the diagnosable neurologic symptoms appear. For example, in a sample of 21 children at 50% risk for HD ages 7–18, the gene-expanded children had significant abnormalities in brain structure when compared to gene-negative controls, including an excess of white matter and a decrease in cortical gray matter (Nopoulos et al., 2009). Based on these findings, there is a concerted effort to include younger individuals in HD research. As genotyping in children for research purposes becomes more common, there is likely to be a parallel movement for testing in children for other reasons.

ANONYMOUS TESTING Some individuals requesting testing fear discrimination as a result of testing and desire anonymous testing. One concern is that an individual’s desire for anonymity may interfere with the establishment of a supportive relationship between the counselor and the person requesting testing. Some centers may decline to provide anonymous testing while others may consider it on a case-by-case basis. As there is no standardized definition of exactly what it means to be anonymous, the details will need to be worked out in detailed discussions between the individual providing testing and the person requesting testing. One point that needs to be made in these discussions is the fact that, if the individual at risk needs documentation of the testing outcome at some point in the future, for insurance purposes, for example, the test will need to be rerun under the actual name of the person who was tested.

TESTING PERSONS AT 25% RISK Occasionally, a person at 25% risk (having an affected grandparent and an unaffected at-risk parent at 50% risk) will request testing. This situation may arise when the at-risk individual is alienated from his or her at-risk parent, the parent is uninterested in pursuing testing, or the parent is deceased. The problem in this situation is that, if the person at 25% risk does test positive, you will have tested two people, one of whom did not consent to testing. That being said, at the time the initial guidelines were being written, representatives from HD families, citing their personal autonomy, felt very strongly that individuals at 25% risk should be able to be tested if their parent refused to undergo genetic testing. If this situation does arise, every effort should be made to insure that the at-risk individual is aware of the possible consequences of testing both for him- or herself and for family members. Discussion should focus on coming to a satisfactory solution to this potential conflict.

117

REVISITING PROTOCOLS As previously mentioned, with the recent publication by HDSA of new guidelines, there has been some discussion of the need to include a neurologic examination as part of the testing protocol. There are several reasons to require a person requesting testing to have a neurologic exam by a neurologist or physician familiar with the early symptoms of HD, some of them historic, some not. First, during the linkage phase, the accuracy of the test was probably less than that of a diagnosis based on clear clinical features of HD in persons with a welldocumented family history. Second, entering a person into a “presymptomatic” testing protocol implies that that person is not exhibiting symptoms. If symptoms are present, and again using linkage, the test was later found to be uninformative, convincing individuals that they are symptomatic may be difficult. If a diagnosis needs to be made, because of unsafe driving or issues with employment, the process of accepting the diagnosis may be significantly delayed. Third, it is not unusual for individuals at risk to “symptom seek” and to interpret any clumsiness or forgetfulness as a sign of disease onset. Often persons present themselves for testing because they believe themselves to be affected and desire to confirm their suspicions. For many, a normal neurologic exam is enough reassurance that testing can become unnecessary or delayed until a later time. There are also additional benefits. This visit allows the person requesting testing to meet the team, which may consist of a neurologist, clinic nurse, social worker, genetic counselor, and others, depending on the center personnel. It allows a physician to evaluate the individual at risk for anxiety or depression or any other medical condition that may be pertinent to the decision to be tested and may need to be treated prior to testing. Finally, this step provides individuals with one more opportunity to think long and hard about whether testing is right for them (Quaid, 1992). The fact that a relatively high percentage of those who present themselves for testing ultimately choose not to go forward (Bernhardt et al., 2009) suggests that this opportunity is an important one. Tibben (2007) offers a nuanced look at the role of the neurologic exam when a person requests presymptomatic testing but may actually be showing early signs of the disease. He posits that when an individual at risk requests a predictive genetic test, it may be that he or she is not yet ready to learn whether symptoms are already observable. It is not unusual to have a person request presymptomatic testing, only to be found to be clearly affected upon examination. He advises that when an individual at risk clearly does not wish to consider that he or she is possibly affected and that he or she may need a neurologic consultation, we should appreciate the possible

118

K.A. QUAID Exclusion Testing

AA

AB Parent–at-risk

BB

CC

AC

BC

Child has ~50% risk to have inherited HD “No news”

Child unlikely to have inherited HD “Good news”

Fig. 10.1. Exclusion testing.

psychologic defense and respect the request. The request for predictive testing may be seen as the first step in the process towards an awareness that symptoms may be present. In such a case, testing serves as a prelude to accepting a clinical diagnosis.

PRENATAL TESTING In 1985, Harper and Sarafazi proposed offering prenatal exclusion testing as a method whereby a parent who is at 50% risk of carrying the gene for HD can elect to have children who are a high or low risk of being gene carriers by excluding in the fetus the parental allele which is at risk of being linked to the HD gene mutation (Fig. 10.1). This approach had two main advantages. The first is that a previous determination of the genetic linkage phase between the marker and the disease gene in the parent at risk of transmitting the HD gene is not required meaning that the test would be available for most families, including those whose pedigree structure was uninformative for conventional presymptomatic testing using linkage. Second, the test would only indicate whether the fetus was at the same level of risk as the at-risk parent (50%), so a high-risk result would not alter the risk to the parent. One caveat to keep in mind in the use of exclusion testing for HD is the expectation that if the fetus is found to be at high (50%) risk, the pregnancy will be terminated. This expectation exists because of a general reluctance to test a child. If a high-risk pregnancy is continued, and the at-risk parent becomes affected, the parents will know that the child will one day develop HD. This denies the child the right to make his or her own choice with regard to testing. Another uncomfortable fact about the use of exclusion testing is that there is the risk that one may terminate a pregnancy that is not, in fact, at risk.

These hard facts may explain why, even when faced with a high-risk result, some couples choose not to terminate the pregnancy (Tolmie et al., 1995). Uptake for prenatal testing has been low. Data produced by seven genetic centers participating in a European collaborative study estimated that the number of prenatal tests was less than 10% of the number of presymptomatic tests. Worldwide, demand has been low, ranging from 2% (Tyler et al., 1990) to 3% (Adam et al., 1993). The low uptake of prenatal testing in those who are known to carry the mutation may be partially explained by the fact that the average age of persons requesting presymptomatic testing was in the mid to late 30s, suggesting that many undergoing testing have already completed their families or choose not to have further children once their risk is known. Despite the ability to use mutation analysis for prenatal testing, exclusion testing remains a valuable tool for couples who do not wish to learn their own status but who wish to have children who will not develop HD. Fully 34.8% of the tests performed in this collaborative study were exclusion tests (Simpson et al., 2002). One study did look at the effects of predictive testing on subsequent reproduction in this same collaborative study. They found an overall impact of test result on reproduction following testing, with 14% of carriers having one or more subsequent pregnancies, compared to 28% of the noncarrier group. In the carrier group, prenatal testing was used in about two-thirds of the pregnancies. In those who had listed “family planning” as their main reason for being tested, the effect of testing was more pronounced, with 69% of noncarriers having subsequent pregnancies compared to 39% of the carriers. Of the carriers with one or more subsequent pregnancies, the percentage using prenatal diagnosis was only slightly higher than the percentage not using it (Evers-Kiebooms et al., 2002).

GENETIC TESTING FOR HUNTINGTON DISEASE

THE DISCOVERY OF THE HD GENE In 1993, linkage analysis was replaced by direct mutation analysis following the isolation of the gene responsible for HD (Huntington’s Disease Collaborative Research Group, 1993). Once again, Dr. Milton Wexler played a crucial role. Under his leadership, the Hereditary Disease Foundation was instrumental in organizing the Huntington’s Disease Collaborative Research Group, the group ultimately credited with finding the gene responsible for HD. The HD gene, called huntingtin, is an expanded and unstable DNA segment consisting of the trinucleotide cytosine-adenine-guanine (CAG) repeat that codes for a protein involved in nerve cell function. Healthy individuals will have between 11 and 26 repeats while those who go on to develop HD will have a CAG repeat length of 40 or above. Specifically, test interpretation is as follows: ● ● ● ●

26 CAG repeats and below ¼ normal 27–35 CAG repeats ¼ nonpenetrant with meiotic instability 36–39 CAG repeats ¼ variably penetrant with meiotic instability 40 CAG repeats and above ¼ HD (American College of Medical Genetics/American Society of Human Genetics Huntington Disease Genetic Testing Working Group, 1998).

INTERMEDIATE ALLELES (IAs) There have been isolated case reports that have called into question the test interpretation given above. Kenney et al. (2007) reported on a 65-year-old male with 29 CAG repeats who exhibited symptoms of HD and had autopsy results consistent with HD. Another report involving the Israeli Karaite community, found that some members were carrying 34 repeats or expanded mutations beyond 43 repeats and all shared the typical clinical features of HD (Herishanu et al., 2009). Andrich et al. (2008) described a 75-year-old man with a 15-year history of chorea, dystonia, gait disorder, abnormal saccadic eye movements, and cognitive impairment with a CAG of 34. Two other cases in the context of a family history of HD had signs and symptoms typical of HD with onset at 65 and 68 years of age with CAG repeats of 30 and 31 (Groen et al., 2010). Finally one subject with 33 CAG repeats manifested only subtle initial symptoms of HD but had brain caudate hypometabolism typical of HD (Squitieri et al., 2011). Ha and Jankovic (2011) reviewed all cases of IAs (CAG 27–35) who presented in their clinic as well as those reported in the literature. They described 4 cases with IAs evaluated at their center whose clinical

119

features were highly suggestive of HD (Ha and Jankovic, 2011). These relatively rare isolated cases of the development of HD symptomatology in the context of IAs are difficult to interpret for the purposes of genetic counseling. At the very least, they provide evidence that some individuals with IAs might develop HD. More recently, Killoran and colleagues (2013) reported on the results of a study using data from a large longitudinal cohort of individuals at risk for HD the Prospective Huntington At Risk Observational Study (PHAROS). They compared the clinical characteristics of individuals with IAs (CAG 27–35) to those with alleles below 27 CAG repeats. Fifty of the 983 participants (5.1%) in the PAROS study were found to have an IA and, as a group, did not differ from the controls on either motor or cognitive assessments. However, those participants with IAs exhibited more behavioral changes (suicidal thoughts and apathy) than the controls, thus more closely resembling the study participants with a CAG expansion above 35. On 5 of the other 9 behavioral items and on total behavior, the IA group’s scores were worse than those of controls and expanded participants who themselves scored significantly worse than controls on six behavioral measures (Killoran et al., 2013). Feigin (2013) felt that this study had several important implications. First, it confirmed the high rate of IAs (5–6%) in the general population. Second, the presence of clinical manifestations, although subtle, strongly suggests that IAs can cause HD. Third, Feigin (2013) suggests that perhaps test interpretation should be changed to consider the entire range of 27–39 repeats as indicative of incomplete penetrance, with penetrance approaching 100% above 39 CAG and approaching 0% below 27. Hendricks et al. (2009) attempted to estimate the likelihood that a parent with an IA or high normal CAG repeat (27–35) would transmit to his or her offspring an HD allele in the potentially penetrant range (> 36 CAG repeats). They estimated the conditional probability of an offspring inheriting an expanded penetrant allele given a father with a high normal allele by applying probability definitions and rules to estimates of HD incidence, paternal birth rate, frequency of de novo HD, and frequency of high normal alleles in the general population. They concluded that the estimated probability that a male with a high normal allele would have an offspring with an expanded penetrant allele ranges from 1/6241 to 1/951, even though the man himself might never develop the disease. Subjects carrying 36–39 CAG repeats have been estimated to have at least a 60% chance of being symptomatic at the age of 65 years (Quarrell et al., 2007).

K.A. QUAID

120

DIRECT TESTING Direct testing for the HD mutation through the use of polymerase chain reaction (PCR) has greatly simplified the technical aspects of genetic testing for HD. Testing for HD is now faster, simpler, more accurate, more affordable, and more widely available. The psychologic aspects of testing, however, remain the same. In fact, some have voiced the opinion that direct mutation analysis may actually result in more distress. Absent the need to take extensive family histories and gather blood samples and medical records from various family members, more centers are offering testing and more professionals are ordering tests, many with little or no experience with HD or HD testing.

PREIMPLANTATION GENETIC DIAGNOSIS After the discovery of the HD mutation, another reproductive option, preimplantation genetic diagnosis (PGD), became available for couples at risk for HD. PGD combines advances in genetics with the more established technology of in vitro fertilization (IVF). In IVF, a woman’s monthly reproduction cycle is manipulated through the use of hormones to stimulate her ovaries so that many eggs mature at one time, rather than just one. The eggs are harvested and fertilized in a Petri dish with her partner’s sperm. When the embryo grows to about the eight-cell stage, each embryo is tested for the presence or absence of the genetic mutation that causes HD. Only those embryos that are free of the HD mutation are reintroduced into the woman’s womb and, hopefully, a pregnancy ensues. This procedure ensures that parents can have their own biologic children who will have no risk for developing HD in the future without the need to consider pregnancy termination. There are three main drawbacks to PGD. The first is cost. Charges for IVF include medications for ovary stimulation, charges for harvesting the eggs, charges for the IVF procedure, charges for the biopsy, charges for the genetic analysis, and charges for the reintroduction of the embryos. The charges can add up to upwards of $17 250. While some insurance plans may cover some of the charges, others will not. Second, couples may want to consider the IVF success rate for the clinic they wish to use. Couples considering the use of IVF may want to consult the Society for Assistive Reproductive Technology and Centers for Disease Control and Prevention IVF success rate reports (http://www.cdc.gov/ART/) published in the United States. These reports are published on the web every year and success rates are detailed by age of the mother. The third consideration is that some religious organizations object to the procedure on the

grounds that it involves the destruction of human life. The Roman Catholic Church, for example, takes that position, which may give many couples pause based on their religious convictions.

ATTITUDES TOWARDS GENETIC TESTING Studies done prior to the finding of the HD mutation suggested that there would be high rates of uptake for predictive testing in individuals at risk for HD (Schoenfeld et al., 1984b; Kessler et al., 1987; Markel et al., 1987; Mastromauro et al., 1987; Meissen and Berchek, 1987), with the percentages of those reporting the intention to use the test ranging from a low of 57% (EversKiebooms et al., 1987) to a high of 79% (Kessler et al., 1987). However, current estimates indicate that only 12–15% of individuals at risk have elected to pursue testing (Tassicker et al., 2009; Morrison et al., 2011), although a recent report suggests that uptake may be increasing (Sizer et al., 2012). Interest in genetic testing was negatively associated with being married and positively correlated with the number of affected relatives and the earlier parental age of onset of HD. The most common reasons for wanting to be tested were to be certain, to plan for the future, and to inform children (Mastromauro et al., 1987). The most common reasons cited for choosing not to be tested were an increased risk to children if one were found to be a gene carrier, absence of an effective cure, potential loss of health insurance, financial costs of testing, and the inability to undo the knowledge (Quaid and Morris, 1993).

TEST OUTCOMES There have been many studies looking at the psychologic outcomes of testing. Wiggins et al. (1992) used a control group design and found that carriers and noncarriers differed significantly on all psychologic outcome measures at 7–10 days but not at 6 and 12 months of follow-up. They also found that people at risk who requested testing and who did not receive a genetic testing result, because the linkage testing was uninformative, had higher scores for depression and lower scores for well-being at 12 months. The authors concluded that receiving a test result leads to psychologic benefits even if the test is positive by reducing uncertainty and providing an opportunity for appropriate planning. Tibben et al. (1994) looked at hopelessness as a dimension of particular relevance in this population of individuals at risk and found that differences between carriers and noncarriers may persist long term. This finding raises concern given that hopelessness has been identified as a predictor of suicide (Beck et al., 1990).

GENETIC TESTING FOR HUNTINGTON DISEASE The analysis of scores before, compared with after, testing indicates that the psychologic adjustment of noncarriers tends to be unaltered (Tibben et al., 1994) or improves after the receipt of the results (Wiggins et al., 1992; Quaid and Wesson, 1995). Two nonsystematic reviews of the literature have been published looking at outcomes while combining testing conducted through linkage and with mutation analysis. The first review of the literature concluded that much of the evidence with regard to the psychologic effects of testing for HD suggests that noncarriers and carriers differ significantly in short-term but not long-term general psychologic distress. Carriers showed either no changes from psychologic adjustment before testing or only short-term increases in hopelessness. Adjustment to results depends more on psychologic adjustment before testing than on the test result itself. The authors conclude with the reminder that all the studies reviewed followed similar testing protocols, including neurologic examination, psychologic/psychiatric assessment, pretest counseling, results in person, and follow-up. The effects of testing in settings with less intensive counseling and testing protocols or eligibility criteria are unknown (Meiser and Dunn, 2000). In the second review, Hayden and Bombard (2005) looked at both short-term and long-term outcomes of testing. They found that that the highest level of distress was immediately after testing for those found to be carriers. Within the first year, adjustment occurred and levels of stress and depression returned to baseline levels (Bloch et al., 1992). Long-term follow-up demonstrated that psychologic distress was reduced significantly when compared to baseline and that this reduction was sustained throughout a 5-year follow-up period, both for groups with increased and decreased risk results (Almqvist et al., 2003). They also found that roughly 10% of those with a decreased risk result needed additional support because of difficulty coping with their decreased risk (Huggins et al., 1992). The authors conclude that, in general, psychologic health improves for people who undergo predictive testing. Knowing the results of the test, even if it indicates an increased risk, reduces uncertainty and provides an opportunity for appropriate planning. They stress, however, that in their opinion, detailed pretest counseling clearly excluded many individuals who might have suffered an adverse event (Hayden and Bombard, 2005). A systematic review of the literature found comparable results (Duisterhof et al., 2001). In general, test outcomes did not predict psychologic adjustment. Only one study found carriers to be more pessimistic about their future than noncarriers (Codori et al., 1997). The level of psychologic adaptation after testing (anxiety, depression, hopelessness, intrusion, and avoidance) was

121

predicted by the same measures at baseline. The more depressive symptoms reported at baseline, the more distress subjects reported at 1-year follow-up.

EFFECT ON PARTNERS Tibben et al. (1997) found that partners of people undergoing genetic testing showed the same course of distress as carriers. Compared with the partners of noncarriers, carriers’ partners had significantly higher levels of psychologic distress 1 week, 6 months, and 3 years after disclosure. Having children was an additional psychologic risk factor, with the partners of carriers showing significantly higher scores on all psychologic outcome measures both short- and long-term, compared to carriers’ partners without children. These findings are similar to those found by Quaid and Wesson (1995) and suggest that genetic testing for HD has significant effects on the partners of carriers. Richardson and Williams (2004) looked at both tested and untested couples and found that tested couples did experience adverse effects on their relationship soon after testing, but this effect appears to resolve itself over time. Interestingly, in this sample noncarrier couples showed more deterioration in their relationship compared to carrier couples.

ADVERSE EVENTS AFTER TESTING As previously mentioned, prior to the implementation of predictive testing for HD, significant concern was raised about the likelihood of catastrophic events after testing, particularly in those persons receiving an increased risk result. As a result of this concern, a worldwide assessment of the frequency of suicide, suicide attempts, and psychiatric hospitalization after testing was performed (Almqvist et al., 1999). The results indicated that a total of 44 persons (0.97%) of 4527 test participants had a catastrophic event, defined as a successful suicide (5), suicide attempt (21), or hospitalization for psychiatric reasons (18). Eleven out of 13 (84.6%) asymptomatic persons who experienced a catastrophic event in the first year after HD testing received an increased risk result. Factors associated with an increased risk of a catastrophic event included: (1) a psychiatric history 5 years prior to testing; and (2) unemployed status. The authors stress that this study looked at follow-up 1 year after testing with little known about longer-term follow-up and that these results came from established testing centers that follow protocols developed by the HDSA and the WFN/IHA. They further note that 54.4% of all persons who had a catastrophic event were possibly, probably, or recently diagnosed with HD, providing evidence that the risk of suicide is elevated during the period immediately after the diagnosis of HD.

122

K.A. QUAID

A more recent 2-year follow-up study specifically looked at depression and suicidal ideation (as compared to suicide attempt) after predictive testing (Larsson et al., 2006). The authors found that both carriers and noncarriers showed high suicidal ideation before testing, with about 50% of the total group reporting previous thoughts about suicide. Depression score and frequency of suicidal ideation increased for carriers when compared to noncarriers over time. Over time, carriers’ thoughts about suicide increased from “rarely” to “sometimes,” while the frequency of noncarriers’ thoughts decreased. The authors speculate that perhaps suicidal thoughts are a reliable indicator of psychologic health and reflect hopelessness, an inference validated in previous research finding that hopelessness is higher among carriers (Codori et al., 1997; Timmen et al., 2004). They found that being a carrier and having listed HD as a major problem at baseline were predictors of psychologic distress over time (Larsson et al., 2006).

PSYCHOLOGIC IMPACT A recent literature review systematically examined the published evidence exploring the psychologic impact of presymptomatic predictive mutation analysis testing for individuals at risk for HD (Crozier et al., 2015). Eight studies were identified comparing the psychologic impact of HD genetic testing contingent on test result. Three studies with a follow-up of up to 10 years post test have highlighted no significant difference between individuals given a carrier status and those given a noncarrier status (Decruyenaere et al., 1996; Wahlin et al., 2000; Timmen et al., 2004). No study found detrimental effects in those who were found to be noncarriers. In studies where baseline levels for psychologic measures were taken into account, only one study revealed a significantly lower level of depression for noncarriers compared to carriers 2 years after testing (Larsson et al., 2006). No significant differences in levels of suicidal thoughts or behavior were seen between carriers and noncarriers (Wahlin et al., 2000). Horowitz et al. (2001) compared noncarriers with both asymptomatic and symptomatic carriers and found a significant difference in levels of distress 1 year post test. Symptomatic carriers had higher levels of distress than noncarriers but no significant difference between asymptomatic carriers and noncarriers, suggesting that the psychologic difficulties were more a result of symptom onset than genetic status. The authors conclude that their review of the literature suggests little psychologic impact of predictive testing irrespective of test outcome and that living as an HD gene carrier is not in itself clinically distressing. Tibben (2007), however, urges us to consider these optimistic results with some caution, citing the high rates

of dropouts who are lost to follow-up. Timmen et al. (2004) found more pretest distress in those lost to follow-up. If, as the follow-up studies suggest, those who are more distressed prior to testing are more distressed after testing, Tibben may be correct. He reports that information from relatives about the well-being of those dropouts suggest that those who declined participation in follow-up research, both carriers and noncarriers, often have serious problems they do not wish to disclose, indicating that testing may not be as benign as the data suggest (Tibben, 2007).

DOCUMENTING TRENDS THROUGH LONG-TERM FOLLOW-UP A series of articles have looked at long-term follow-up, ranging from 8 to 12 years. In one center in Germany, actual demand for the predictive test declined from 1993 to 2004. The authors reported that, at the outset of the counseling procedure, 71% wanted to make use of the test, yet the actual demand for the test declined from 67% to 38% over the years. Over the entire time that the data were gathered, only 52% finally opted for blood withdrawal – a dropout rate of 48% (Bernhardt et al., 2009). Over 12 years, a program in Mexico had 75 requests for testing. Four people did not meet the inclusion criteria and 5 dropped out before receiving their test results. Sixty-three percent were female and they had a preponderance of negative test results (62%) (Alonso et al., 2009). Several studies have looked at the testing experience in entire countries. Harper and colleagues (2000) reported on 2937 tests completed in a period between 1987 and 1997 in the United Kingdom. A total of 93.1% of those were at 50% risk, with a significant excess of females requesting testing (58.3%). Results yielded 41.4% high-risk results, including 29.4% in subjects aged 60 or over. The number of tests performed had leveled out to about 500 per year (Harper et al., 2000). Creighton and his colleagues (2003) examined genetic testing in Canada between 1987 and 2000. They collected data from 15 of the 22 centers offering testing. At total of 1061 predictive tests were completed, with 45.0% resulting in a high-risk result. They, too, had a preponderance of females (60.2%) and estimated that uptake was about 18% of the estimated at-risk population in Canada. They observed that it appeared that the rate of testing was declining and that very few prenatal tests (15) were requested (Creighton et al., 2003). In Greece, a large-scale genetic and epidemiologic study was carried out between January 1995 and December 2008. The researchers found that the uptake of predictive testing was 8.6%, with a total of 256 predictive tests completed. Six prenatal tests were requested.

GENETIC TESTING FOR HUNTINGTON DISEASE They also reported that 21/256 tested individuals were children, with a mean age of 13.2 years and a range of 1–17 years. Eleven of the children were found to be mutation carriers (Pana et al., 2011). Australian researchers looked at testing results for a 10-year period, 1994–2003. They reported on 2036 tests performed using direct mutation analysis and found 38% of their tests were positive, 56% were negative, and 6% were in the mutable normal CAG repeat range (27–35) or in the reduced penetrance range (36–39). During this period, 63 prenatal tests were performed and 13 children were born following PGD for HD. In this paper, they included examples of challenging counseling cases and conclude with the following statement: Over the 10 years on which this report is based, we have remained convinced of the vital importance of respecting and following the international guidelines and protocols for predictive testing. While it is recognized that there is a need for some individual flexibility, for example, in the number of pre-result counseling sessions, we believe that it is unwise to significantly depart from the protocol as this is likely to lead to poorer post-result psychosocial outcomes for tested individuals (Tassicker et al., 2006).

SUMMARY In 1983, HD became the first disease mapped to a previously unknown genetic location on chromosome 4. This discovery made it possible to offer predictive or presymptomatic testing to individuals at risk for HD as well as prenatal testing using a process known as linkage. In response, both the HDSA and the WFN in collaboration with the IHA published guidelines for testing as well as recommended testing protocols that include a neurologic examination, psychiatric/psychologic assessment, pretest counseling, results in person, and follow-up. Ten years later, the discovery of the gene responsible for HD, hungtingtin, made it possible to offer direct mutation analysis as a means of testing. The vast majority of studies looking at the psychologic outcomes of testing have taken place in centers that follow the recommended testing protocols. These studies have found, in general, that while identified carriers and their partners may be more distressed than noncarriers in the short term, over time they achieve equilibrium and do not appear to be more distressed than noncarriers. Some have urged caution at this interpretation, citing the high levels of dropouts in follow-up studies. Baseline levels of distress, depression, and anxiety, prior to testing, appear to be better predictors of distress than test results.

123

Several trends in testing have been identified. More women than men request testing. Although there have been speculations about the reason for the preponderance of females, further research should specifically examine this issue. The number of requests for prenatal testing has been and remains low. In some countries, requests for predictive testing appear to be declining. Whether this decline is real, or is in response to the continuing lack of effective treatments, much less a cure or means of prevention, remains to be seen.

ACKNOWLEDGMENT Sections of this chapter have been reproduced from Smith DH, Quaid KA, Dworkin RB, et al. (1998) Early warning: cases and ethical guidance for presymptomatic testing in genetic disease. Bloomington, IN: Indiana University Press, with permission from Indiana University Press.

REFERENCES Adam S, Wiggins S, Whyte P et al. (1993). Five year study of prenatal testing for Huntington’s disease: demand, attitudes, and psychological assessment. J Med Genet 30: 549–556. Almqvist EW, Bloch M, Brinkman R et al. (1999). A worldwide assessment of the frequency of suicide, suicide attempts, or psychiatric hospitalization after predictive testing for Huntington disease. Am J Hum Genet 64: 1293–1304. Almqvist EW, Brinkman R, Wiggins S et al. (2003). Psychological consequences and predictors of adverse events five years after testing for Huntington disease. Clin Genet 64: 300–309. Alonso ME, Ochoa A, Sosa AL et al. (2009). Presymptomatic diagnosis in Huntington’s disease: the Mexican experience. Genet Test and Mol Bio 13: 717–720. American Academy of Pediatrics Committee on Bioethics, Committee on Genetics; American College of Medical Genetics Social Ethical and Legal Issues Committee (2013). Ethical and policy issues in genetic testing and screening of children. Pediatrics 131: 620–622. American College of Medical Genetics/American Society of Human Genetics Huntington Disease Genetic Testing Working Group (1998). ACMG/ASHG statement on laboratory guidelines for Huntington disease genetic testing. Am J Hum Genet 62: 1243–1247. American Medical Association Council on Ethical and Judicial Affairs (1995). Testing children for genetic status. Code of Medical Ethics, Report 66, American Medical Association, Chicago, IL. American Society of Human Genetics Board of Directors the American College of Medical Genetics Board of Directors (1995). Points to consider: ethical, legal, and psychosocial implications of genetic testing in children and adolescents. American Society of Human Genetics Board of Directors. Am Journal of Hum Genet 57: 1233.

124

K.A. QUAID

Andrich J, Arning L, Wieczorek S et al. (2008). Huntington’s disease as caused by 34 CAG repeats. Mov Disord 23: 879–881. Beck AT, Brown G, Berchick RJ et al. (1990). Relationship between hopelessness and ultimate suicide: a replication with psychiatric outpatients. Am J Psychiatry 147: 190–195. Bernhardt C, Schwan AM, Kraus P et al. (2009). Decreasing uptake of predictive testing for Huntington’s disease in a German centre: 12 years’ experience (1993–2004). Eur J Hum Genet 17: 295–300. Binedell J, Soldan JR, Scourfield J et al. (1996). Huntington’s disease predictive testing: the case for an assessment approach to requests from adolescents. J Med Genet 33: 912–918. Bloch M, Fahy M, Fox S et al. (1989). Predictive testing for Huntington’s disease: II. Demographic lifestyle patterns, attitudes and psychosocial assessments of the first 51 test candidates. Am J Med Genet 32: 217–224. Bloch M, Adam S, Wiggins S et al. (1992). Predictive testing for Huntington disease in Canada: the experience of those receiving an increased risk. Am J Med Genet 42: 499–507. Borry P, Stultiens L, Nys H (2006). Presymptomatic and predictive genetic testing in minors: a systematic review of guidelines and position papers. Clin Genet 70: 374–381. Borry P, Goffin T, Nys H (2008). Attitudes regarding predictive genetic testing in minors: a survey of European clinical geneticists. Am J Med Genet Part C (Seminars in Medical Genetics) 148C: 78–83. Brandt J, Quaid KA, Folstein SE et al. (1989). Presymptomatic diagnosis of delayed-onset disease with linked DNA markers. JAMA 261: 3108–3114. Codori AM, Slavney PR, Young C et al. (1997). Predictors of psychological adjustment in genetic testing for Huntington’s disease. Health Psychol 16: 36–50. Conneally PM, Haines JL, Tanzi RE et al. (1989). Huntington disease: no evidence for locus heterogeneity. Genomics 5: 304–308. Craufurd D, Dodge A, Kerzin-Storrar L et al. (1989). Uptake of presymptomatic predictive testing for Huntington’s disease. Lancet 334: 603–605. Creighton S, Almqvist EW, MacGregor D et al. (2003). Predictive, pre-natal and diagnostic genetic testing for Huntington’s disease: the experience in Canada from 1987 to 2000. Clin Genet 63: 462–475. Crozier S, Robertson N, Dale M (2015). The psychological impact of predictive genetic testing for Huntington’s disease: a systemic review of the literature. J Genet Couns 24: 29–39. Decruyenaere M, Evers-Kiebooms G, Boogaerts A et al. (1996). Prediction of psychological functioning one year after the predictive test for Huntington disease and the impact of the test result on reproductive decision-making. J Med Genet 33: 737–743. Duisterhof M, Trijsburg RW, Niermeijer MF et al. (2001). Psychological studies in Huntington’s disease: making up the balance. J Med Genet 38: 852–861.

Duncan RE, Savulescu J, Gilliam L et al. (2005). An international survey of predictive testing in children for adult onset conditions. Genet in Med 7: 390–396. Duncan RE, Gillam L, Savulescu J et al. (2007). “Holding your breath”: interviews with young people who have undergone predictive genetic testing for Huntington disease. Am J Med Genet Part A143A: 1984–1989. Evers-Kiebooms G (1990). Predictive testing for Huntington’s disease in Belgium. J Psych Obstet and Gyn 11: 61–72. Evers-Kiebooms G, Cassiman JJ, Van de Berghe H (1987). Attitudes towards predictive testing in Huntington disease: a recent survey in Belgium. J Med Genet 24: 275–279. Evers-Kiebooms G, Nys K, Harper P et al. (2002). Predictive DNA-testing for Huntington disease and reproductive decision-making: a European collaborative study. Eur J Hum Genet 10: 167–176. Farrer LA (1986). Suicide and attempted suicide in Huntington’s disease: implications for preclinical testing of persons at risk. Am J Med Genet 24: 305–311. Feigin A (2013). Redefining the genetic risk for Huntington disease. Neurology 80: 2004–2005. Folstein SE, Phillips JA, Meters DA et al. (1985). Huntington’s disease: two families with differing clinical features show linkage to the G8 probe. Science 229: 776–779. Fox S, Bloch M, Fahy M et al. (1989). Predictive testing for Huntington’s disease: I Description of a pilot project in British Columbia. Am J Med Genet 32: 211–216. Gilliam TC, Bucan M, MacDonald ME et al. (1987). A DNA segment encoding two genes very tightly linked to Huntington’s disease. Science 238: 950–952. Goodman L (2016). Some thoughts on the HDSA protocol for genetic testing in HD. Available online at http://hddrugworks.org/dr-goodmans-blog/guidelines-andprotocols. Green RC, Berg JS, Grody WW et al. (2013). ACMG recommendations for reporting of incidental findings in clinical exome and genome sequencing. Genet in Med 15: 565–574. Groen JL, de Bie RM, Foncke EM et al. (2010). Late-onset Huntington disease with intermediate CAG repeats: true or false? J Neurol Neurosurg Psychiatry 81: 2280230. Gusella JF, Wexler NS, Connealy PM et al. (1983). A polymorphic DNA marker genetically linked to Huntington’s disease. Nature 306: 234–238. Ha AD, Jankovic J (2011). Exploring the correlates of intermediate CAG repeats in Huntington disease. Postgrad Med 123: 116–121. Haines JL, Tanzi R, Wexler NS et al. (1986). No evidence of linkage heterogeneity between Huntington’s disease and G8(D4S10). Am J Hum Genet 39: 156. Harper P, Sarafazi M (1985). Genetic prediction and family structure in Huntington’s chorea. Brit Med J 290: 1929–1931. Harper PS, Lim C, Craufurd D et al. (2000). Ten years of presymptomatic testing for Huntington’s disease: the experience of the UK Huntington’s disease prediction consortium. J Med Genet 37: 567–571.

GENETIC TESTING FOR HUNTINGTON DISEASE Hayden MR, Bombard Y (2005). Psychological effects of predictive testing for Huntington disease. Adv in Neur 96: 226–239. Hayden MR, Hewitt J, Wasmuth JJ et al. (1988). A polymorphic DNA marker that represents a conserved expressed sequence in the region of the Huntington disease gene. Am J Hum Genet 42: 125–131. Hendricks AE, Latourelle JC, Lunetta KL et al. (2009). Estimating the probability of de novo HD cases from transmissions of expanded penetrant CAG alleles in the Huntington disease gene from male carriers of high normal alleles (27–35 CAG). American Journal of Medical Genetics A 149A: 1375–1381. Herishanu YO, Parvari R, Pollack Y et al. (2009). Huntington’s disease in subjects from an Israeli community carrying alleles of intermediate and expanded CAG repeats in the HTT gene: Huntington’s disease or phenocopies? J Neurol Sci 277: 143–146. Horowitz MJ, Field N, Zanko A et al. (2001). Psychological impact of news of genetic risk for Huntington disease. Am J Med Genet 103: 188–192. Huggins M, Bloch M, Wiggins S et al. (1992). Predictive testing for Huntington disease in Canada: adverse effects and unexpected results in those receiving a decreased risk. Am J Med Genet 42: 508–515. Huntington’s Disease Collaborative Research Group (1993). A novel gene containing a trinucleotide repeat that is expanded and unstable on Huntington’s disease chromosomes. Cell 72: 971–983. Huntington’s Disease Society of America (1989). Guidelines for predictive testing for Huntington’s disease, Huntington Disease Society of America, New York. Huntington’s Disease Society of America (1994). Guidelines for genetic testing for Huntington’s disease, Huntington Disease Society of America, New York. Huntington’s Disease Society of America (2016). Genetic testing protocol for Huntington’s disease, Huntington’s Disease Society of America, New York. International Huntington Association and World Federation of Neurology (1990). Ethical issues policy statement on Huntington’s disease molecular genetics predictive test. J Med Genet 27: 34–38. International Huntington Association and the World Federation of Neurology Research Group on Huntington’s Chorea (1994). Guidelines for the molecular genetics predictive test in Huntington’s disease. J Med Genet 31: 555–559. Kenney C, Powell S, Jankovic J (2007). Autopsy proven Huntington’s disease with 29 trinucleotide repeats. Mov Disord 22: 127–130. Kessler S, Field T, Worth L (1987). Attitudes of persons at risk for Huntington’s disease toward predictive testing. Am J Med Genet 26: 259–270. Killoran A, Biglan K, Jankovic J et al. (2013). Characterization of the Huntington intermediate CAG repeat expansion phenotype in PHAROS. Neurology 80: 2022–2027. Larsson MU, Luszez MA, Bui TH (2006). Depression and suicidal ideation after predictive testing for Huntington’s

125

disease: a two-year follow-up study. J Genet Couns 15: 361–374. MacLeod R, Tibben A, Frontali M et al. (2012). Recommendations for the predictive gene test in Huntington’s disease. Clin Genet 83: 221–231. Malpas PJ (2008). Predictive genetic testing of children for adult-onset diseases and psychological harm. J Med Ethics 34: 275–278. Markel DS, Young AB, Penney JB (1987). At-risk persons’ attitudes toward presymptomatic and prenatal testing of Huntington disease in Michigan. Am J Med Genet 26: 295–305. Mastromauro C, Myers RH, Berkman B (1987). Attitudes toward presymptomatic testing in Huntington disease. Am J Med Genet 26: 271–282. Meiser B, Dunn S (2000). Psychological effect of genetic testing for Huntington’s disease: an update of the literature. J Neurol Neurosurg Psychiatry 69: 574–578. Meissen GJ, Berchek RL (1987). Intended use of predictive testing by those at risk for Huntington disease. Am J Med Genet Supplement 57: A246. Meissen GJ, Myers RH, Mastromauro CA et al. (1988). Predictive testing for Huntington’s disease with use of a linked DNA marker. NE J of Med 318: 535–542. Misra VP, Baraitser M, Harding AE (1988). Genetic prediction in Huntington disease: what are the limitations posed by pedigree structure? Mov Disord 3: 233–236. Morris MJ, Tyler A, Lazarous L et al. (1989). Problems in genetic testing for Huntington’s disease (September 9, 1989). Lancet 601–603. Morrison PJ, Harding-Lester S, Bradley A (2011). Uptake of Huntington disease predictive testing in a complete population. Clin Genet 80: 281–286. Nance M, Myers R, Wexler A et al. (2003). Genetic testing for Huntington’s disease: revised HDSA guidelines, Huntington’s Disease Society of America, New York. National Society of Genetic Counselors (1995). Prenatal and childhood testing for adult onset disorders. Pers in Genet Counseling 17: 5. Nopoulos P, Byars J, Axelson E et al. (2009). Abnormal brain structure in children at risk for Huntington’s disease: evidence for abnormal brain development. Clin Genet 76 (Suppl.1): 51. Pana M, Karadima G, Vassos E et al. (2011). Huntington disease in Greece: the experience of 14 years. Clin Genet 80: 586–590. Pelias MK (2006). Genetic testing of children for adult-onset diseases: is testing in the child’s best interests? The Mt Sinai J Med 73: 605–608. Quaid KA (1992). Presymptomatic testing for Huntington Disease: recommendations for counseling. J Genet Couns 1: 277–302. Quaid KA, Morris M (1993). Reluctance to undergo predictive testing: the case of Huntington’s disease. Am J Med Genet 45: 41–45. Quaid KA, Wesson MK (1995). Exploration of the effects of predictive testing on intimate relationships. Am J Med Genet 57: 46–51.

126

K.A. QUAID

Quarrell OW, Rigby AS, Barron L et al. (2007). Reduced penetrance for Huntington’s disease: a multi-centre direct observational study. J Med Genet 44. e68. Richardson F, Williams K (2004). Impact on couple relationships of predictive testing for Huntington disease: a longitudinal study. A J Med Genet 126A: 161–169. Schoenfeld M, Myers RH, Cupples A et al. (1984a). Increased rate of suicide among patients with Huntington’s disease. J Neur Neurosurg Psych 47: 1283–1287. Schoenfeld M, Myers RH, Berkman B et al. (1984b). Potential impact of a predictive test on the gene frequency of Huntington disease. Am J Med Genet 18: 423–429. Sevick MA, Nativio DG, McConnell T (2005). Genetic testing of children for late onset disease. Cambridge Quart Healthcare Ethics 14: 47–56. Simpson SA, Zoeteweij MW, Nys K et al. (2002). Prenatal testing for Huntington disease: a European collaborative study. Eur J Hum Genet 10: 689–693. Sizer E, Haw T, Wessels TM et al. (2012). The utilization and outcome of diagnostic, predictive, and prenatal testing for Huntington disease in Johannesburg, South Africa. Genet Test and Mol Biomarkers 16: 58–62. Smith DH, Quaid KA, Dworkin RB et al. (1998). Early warning: cases and ethical guidance for presymptomatic testing in genetic diseases, Indiana University Press, Bloomington, IN. Squitieri F, Esmaelizadeh M, Ciarmiello A et al. (2011). Caudate glucose hypometabolism in a subject carrying an unstable allele of intermediate CAAG33 repeat length in the Huntington’s disease gene. Mov Disord 26: 925–927. Tassicker RJ, Marshall PK, Liebeck TA et al. (2006). Predictive and pre-natal testing for Huntington disease in Australia: results and challenges encountered during a 10 year period (1994–2003). Clin Genet 70: 480–489. Tassicker RJ, Teltscher B, Trembath MK et al. (2009). Problems assessing uptake of Huntington disease predictive testing and a proposed solution. Eur J Hum Genet 17: 66–70. Tibben A (2007). Predictive testing for Huntington’s disease. Brain Res Bull 72: 165–171. Tibben A, Vegter-van der Vlis M, Niermeijer MF et al. (1990). Testing for Huntington’s disease with support for all parties. Lancet 335: 553. Tibben A, Duivenvoorden HJ, Niermeijer MF et al. (1994). Psychological effects of presymptomatic DNA testing for Huntington’s disease in the Dutch program. Psychosom Med 56: 526–532.

Tibben A, Timmen R, Bannick E et al. (1997). Three year followup after presymptomatic testing for Huntington disease in tested individuals and partners. Health Psychol 16: 20–35. Timmen R, Roos R, Maat-Kievit A et al. (2004). Adverse effects of predictive testing for Huntington disease underestimated: long term effects 7–10 years after the test. Health Psychol 2: 189–197. Tolmie JL, Davidson HR, May HM et al. (1995). The prenatal exclusion test for Huntington disease: experience in the west of Scotland, 1986–1993. J Med Genet 32: 97–101. Turner DR, Haan EA, Jacka E et al. (1988). Prenatal and adult presymptomatic testing for Huntington’s disease. Med J Austral 148: 567–573. Tyler A, Quarrell OW, Lazarou LP et al. (1990). Exclusion testing in pregnancy for Huntington’s disease. J Med Genet 27: 488–495. Wasmuth JJ, Hewitt J, Smith B et al. (1988). A highly polymorphic locus very tightly linked to the Huntington disease gene. Nature 332: 734–736. Went L (1990). Ethical issues policy statement on Huntington’s disease molecular genetics predictive test. International Huntington Association. World Federation of Neurology. J Med Genet 27: 34–38. Wertz D (1998). Eugenics is alive and well: a survey of genetic professionals around the world. Sci in Context 11: 493–510. Wertz DC, Reilly PR (1997). Laboratory policies and practices for the genetic testing of children: a survey of the Helix network. Am J Hum Genet 61: 1163–1168. Whaley WL, MacDonald Me Michiels F et al. (1988). Mapping of D4S98/S114/S113 confines the Huntington‘s defect to a reduced physical region at the telomere of chromosome 4. Nucleic Acids Res 16: 11769–11780. Wahlin TBR, Backmann L, Lundin A et al. (2000). High suicidal ideation in persons testing for Huntington’s disease. Acta Neurol Scand 102: 150–161. Wiggins S, Whyte P, Huggins M et al. (1992). The psychological consequences of predictive testing for Huntington’s disease. New England Journal of Medicine 327: 1402–1405. Youngman S, Shaw DJ, Bucan M et al. (1988). D4S90 (D5) a DNA segment in close proximity to Huntington’ss disease, is the most terminally located probe on the short arm of chromosome 4. Abstract, Am J Hum Genet 43: 651.