Presymptomatic testing for late-onset genetic disorders: lessons from Huntington's disease.

Huntington's disease is an inherited, neurodegenerative disorder, usually of adult onset. Since the identification of linked markers, more than 1000 presymptomatic tests have been performed worldwide and multiple ethical issues have been encountered in relation to informed consent, testing of children, exclusion testing during pregnancy, and confidentiality. Further ethical problems are anticipated after identification of the causal mutation (or mutations). As Huntington's disease is a model for other disorders of adult onset for which testing is becoming possible, the successful resolution of these ethical issues is of great importance. A failure to do so might discredit genetic testing as a whole.     

Predictive testing: presymptomatic diagnosis in neurogenetic disorders

Presymptomatic testing is available since 15 years for Huntington disease and it is now possible for a number of other neurogenetic disorders, mostly neurodegenerative disorders. The possibility of determining the genetic status of an at-risk person for the disorder which run in his family raises questions because of the absence of preventive and curative treatments in most instances. In addition, being carrier does not tell you when the disease will start and how it will evolve, impairing the possibilities of planning the future. A pluridisciplinary approach to predictive testing with care before, during and after the test taking into account the medical, social and psychological aspects of the disease is good practice. At the present time, only a minority of at-risk individuals request presymptomatic testing and almost 50 % do not pursue until the results. The consequences of the test may be harmful, more frequently after an unfavorable than after a favorable result. Although the motivations and the outcome in terms of request for prenatal testing after a carrier result are different in Huntington's disease and spinocerebellar ataxias, our experience underlines the benefit of pluridisciplinary care and of time for decision taking. For other disorders like familial Alzheimer's disease, or familial Creutzfeldt-Jakob disease, the experience in presymptomatic testing is still limited but the situation seems similar to Huntington's disease because of the presence of dementia. It will be interesting to study the motivations and the outcome of the tests in disorders like autosomal dominant spastic paraplegias which usually do not reduce the life expectancy. Nevertheless, the overall situation might change greatly when efficient treatments will become available in these disorders.     

Attitudes of young patients with Parkinson's disease towards possible presymptomatic and prenatal genetic testing

OBJECTIVE: To evaluate the opinions and attitudes of young patients with Parkinson's disease (PD) towards possible presymptomatic and prenatal genetic testing for their illness. Background: With progress in understanding of the genetic component in the etiology of PD, presymptomatic genetic testing may become available in subgroups of patients. METHODS: During a survey on sociodemographic and risk factors 111 PD patients (mean age 45 years: mean age at PD onset 36 years) were given a questionnaire with six items about possible presymptomatic and prenatal genetic testing. RESULTS: Fifty-seven patients (5196) had knowledge about presymptomatic and prenatal testing. Eighty patients (72%) would take a presymptomatic test, if they had an autosomal dominant form of PD and if the test were available. The most Important reasons given for taking the test were planning of partnership (40%) and family (48%). When being identified as a carrier of a presumed "Parkinson gene", 78 patients (70%) would decide not to have children. Sixty-three patients (57%) would choose to have prenatal testing. Attitudes were largely independent of sociodemographic and disease variables. CONCLUSIONS: When addressed as hypothetical persons at genetic risk, young patients with PD support possible presymptomatic genetic testing and, to a lesser extent, prenatal testing. Attitudes and reasons to participate in such hypothetical testing do not grossly differ from those of at-risk persons in established single-gene autosomal dominant disorders of late onset.     

Specific enzymatic amplification of a DNA region closely associated with Huntington chorea

Enzymatic DNA amplification has become a new important tool in the analysis of human genome. We report the use of this new technique for the detection of a polymorphic DNA sequence adjacent to the Huntington's disease locus for presymptomatic and prenatal testing of the disease.     

Huntington's disease: seeing the pathogenic process through a genetic lens

Thirteen years ago, the culmination of genetic rather than biochemical strategies resulted in the identification of the root cause of Huntington's disease: an expanded CAG trinucleotide repeat that leads to an elongated polyglutamine tract in the huntingtin protein. Since then, biochemical and cell biological attempts to elucidate pathogenesis have largely focused on N-terminal polyglutamine-containing huntingtin fragments. However, continued application of genetic strategies has suggested that the disease process is, in fact, triggered by the presence of expanded polyglutamine in intact huntingtin. An increased emphasis on the earliest presymptomatic stages of the disease, facilitated by incorporating genetic lessons from human patients into the search for biochemical targets, could provide a route to a rational treatment to prevent or slow the onset of this devastating neurodegenerative disorder.     

Presymptomatic detection or exclusion of prion protein gene defects in families with inherited prion diseases.

The identification of defects in the prion protein (PrP) gene in families with inherited Creutzfeldt-Jakob disease or Gerstmann-Straussler syndrome allows presymptomatic diagnosis or exclusion of these disorders in subjects at risk. After counseling, PrP gene analysis was performed in three such individuals: two from families with a 144-bp insert and one with a point mutation at codon 102 in the PrP gene. The presence of a PrP gene defect was confirmed in one and excluded in two. Despite the potential problems of using PrP gene analysis in genetic prediction - specifically, uncertainty about penetrance and, generally, problems of presymptomatic testing in any inherited late-onset neurodegenerative disorder - we conclude that it has a role to play in improved genetic counseling for families with inherited prion diseases.     

Familial Creutzfeldt-Jakob Disease: Case report and role of genetic counseling in post mortem testing

Here we present a case of an asymptomatic 53-year-old woman who sought genetic testing for Familial Creutzfeldt-Jakob Disease (fCJD) after learning that her mother had fCJD. The patient's mother had a sudden onset of memory problems and rapidly deteriorating mental faculties in her late 70s, which led to difficulties ambulating, progressive non-fluent aphasia, dysphagia and death within ～1 y of symptom onset. The cause of death was reported as “rapid onset dementia.” The patient's family, unhappy with the vague diagnosis, researched prion disorders online and aggressively pursued causation and submitted frozen brain tissue from the mother to the National Prion Disease Surveillance Center, where testing revealed a previously described 5-octapeptide repeat insertion (5-OPRI) in the prion protein gene (PRNP) that is known to cause fCJD. The family had additional questions about the implications of this result and thus independently sought out genetic counseling.

?While rare, fCJD is likely underdiagnosed due to clinical heterogeneity, rapid onset, early non-specific symptomatology, and overlap in the differential diagnosis of Alzheimer disease and Lewy body dementias. When fCJD is identified, a multidisciplinary approach to return of results that includes the affected patient's provider, genetics professionals, and mental health professionals is key to the care of the family. We present an example case which discusses the psychosocial issues encountered and the role of genetic counseling in presymptomatic testing for incurable neurodegenerative conditions. Ordering physicians should be aware of the basic issues surrounding presymptomatic genetic testing and identify local genetic counseling resources for their patients.     

Huntington's disease: prediction and prevention

The identification of a DNA restriction fragment length polymorphism closely linked to Huntington's disease on the short arm of chromosome 4 has for the first time allowed presymptomatic prediction to be undertaken in first-degree relatives at risk. The late and variable onset of this dominantly inherited disorder makes such prediction a powerful and potentially valuable aid in genetic counselling, but in the absence of effective therapy there are serious ethical reservations concerning such a predictive test. The new developments have stimulated an active and informative debate among professionals and family members on whether and how predictive tests should be used. Guidelines have emerged which should be useful not only for Huntington's disease, but for other serious late-onset neurogenetic disorders. Meanwhile, studies in Wales and elsewhere have not only confirmed the original linkage but have excluded multi-locus heterogeneity as a significant problem. Genetic prediction for the individual at risk remains critically dependent on a suitable family structure, present in only a minority of families in Wales. A more feasible alternative for most families is prenatal exclusion, which can allow risk prediction for a pregnancy without altering the situation for the person at risk. This approach has already been applied in Wales; the experience gained will be useful in full prediction, which is currently being introduced.     

Attitudes and knowledge about presymptomatic genetic testing among individuals at high risk for familial,early-onset Alzheimer's disease

The study was conducted in a large Cuban family with early-onset familial Alzheimer's disease (AD). Fifty-six first-degree relatives of familial cases with AD were interviewed concerning their clinical and genetic knowledge about AD and their attitudes toward the possible use of presymptomatic genetic testing of AD. The individuals had only limited knowledge about their personal risk of developing AD. All 56 family members would use presymptomatic testing to know their own risk of AD. Confronted with a hypothetical reproductive choice, 50% would choose not to have children if they themselves had the mutation. A positive prenatal test would lead 48.2% of the participants to have an abortion, and 19.7% would continue the pregnancy regardless of the positive test result.     

Presymptomatic testing for Huntington's disease in the United Kingdom. The United Kingdom Huntington's Disease Prediction Consortium.

OBJECTIVE--To evaluate the United Kingdom Huntington''s disease presymptomatic testing programme. DESIGN--Postal questionnaire survey to collect data on all tests performed by clinical genetics centres between 1987 and 1990. SETTING--Genetic centres providing presymptomatic testing in the United Kingdom. SUBJECTS--248 subjects at risk of Huntington''s disease who had presymptomatic testing at their request. MAIN OUTCOME MEASURES--Sex, age, prior risk, and risk after testing. RESULTS--The risk of carrying the Huntington disease gene was reduced for 151 (61%) of the applicants and raised for 97 (39%). 158 (64%) of the subjects were female and 90 (36%) male. The median age at which the results were given was 32.5 years. CONCLUSIONS--The demand for testing was lower than expected and may have reached its peak in 1990. The excess of low risk results was not fully explained by the age effect. All the genetics centres concerned have agreed a common service protocol which requires extensive pre-test counselling and post-test follow up. The worth of the procedure remains to be decided. The availability of a large body of pooled data from all the United Kingdom testing centres, which individually are likely to have only a few results, will form a valuable resource for monitoring the long term psychosocial impact of testing.     

Location cloning strategy for characterizing genetic defects in Huntington's disease and Alzheimer's disease

The recognition that DNA polymorphisms are widespread in the human genome and can be used as high quality genetic markers has introduced a new strategy for approaching inherited disorders for which no protein defect has been identified. Genetic linkage analysis can establish the chromosomal position of the genetic defect, providing a potential opportunity for isolating the disease gene and characterizing its product in the absence of any knowledge of its biochemical function. The first step in this location cloning approach has been successful in mapping the Huntington's disease gene to chromosome 4, and has implicated chromosome 21 as the site of a defect in familial Alzheimer's disease. An intensive effort is under way to narrow the region containing the disease gene and identify the defect in each of these disorders. This review will present the success that has been achieved and the problems that remain and will assess the current status of the location cloning strategy with regard to Huntington's disease and familial Alzheimer's disease.     

Analysis of the IT15 gene in Huntington's disease families

Direct molecular genetic testing carried out in 59 Huntington's disease patients belonging to 46 families from Bashkortostan revealed the (CAG)n repeat expansion in exon 1 of the IT15 gene in 57 of them. By use of this analysis the disease status was not confirmed in two patients with atypical form of the disease and negative family history. The (CAG)n repeat expansion was identified in 27 out of 127 asymptomatic at-risk individuals. Analysis of the mutant (CAG)n allele inheritance demonstrated extremely high instability and high mutation rate predominantly leading to the appearance of the alleles with increasing number of (CAG)n repeats in subsequent generations. The instability was mostly observed in cases of paternal transmission. Almost complete linkage disequilibrium between the (CCG)7 mutant alleles and the del2642 deletion was demonstrated. Three major haplotypes revealed, (CCG)7/del-, (CCG)7/del+, and (CCG)10/del-, implied the existence of at least three sources of the origin of Huntington's disease in Bashkortostan. The identified haplotype frequency distribution patterns displayed similarities with those in European populations. The contribution of a number of genetic factors to the age of onset of Huntington's disease was analyzed.     

Attitudes of families affected by adrenoleukodystrophy toward prenatal diagnosis, presymptomatic and carrier testing, and newborn screening

Families affected by adrenoleukodystrophy (ALD) and adrenomyeloneuropathy (AMN) were surveyed to elicit attitudes toward prenatal, presymptomatic and carrier testing, and newborn screening in order to determine the level of support that these families have for current and future genetic testing protocols. Identifying attitudes toward genetic testing, including newborn screening, is especially important because of new data regarding therapeutic options and the possible addition of ALD to newborn screening regimens. The Kennedy Krieger Institute (KKI) database identified 327 prospective participants. Families that were willing to participate in the study received an anonymous questionnaire for completion. Frequencies were generated using SPSS software for Windows. Questionnaires were returned from 128 families for a response rate of 39%. Sons who were at risk for inheriting the ALD gene would be tested by 93% of respondents, and 89.3% would ideally have this testing performed prenatally or in the newborn period. Eighty-nine percent would test an at-risk daughter and 51.2% would ideally have this testing performed prenatally or shortly after birth. ALD newborn screening for males and females was supported by 90% of respondents. If newborn screening for ALD/AMN commences, or there is a new diagnosis of ALD, genetic professionals need to be prepared to have extensive conversations with families regarding the benefits and limitations of current therapeutic and genetic testing options.     

Disclosure of Huntington's disease to family members: the dilemma of known but unknowing parties

Predictive genetic testing presents unique issues in the legal and ethical debate concerning disclosure of information within the physician-patient relationship. A duty to disclose information to family members has been found when the disclosure is likely to result in the ability to mitigate the damaging effects of the disease. When evaluating the situation where a individual is at risk of Huntington's disease, the analysis must be different, as shown in this paper, and necessitates an ethical and legal examination of the consequences of receipt of the information on family members, those known but unknowing parties who are at risk of inheriting a genetic disease. This paper analyzes the potential legal duty of a physician to disclose or withhold genetic information from the family members of patients. Existing statutes governing genetic information do not directly address the interests of family members. Courts that have ruled on the duty to disclose medical or genetic information have analyzed these issues using traditional concepts of tort law. Yet the situation presented by Huntington's disease is unique and demands a different framework for analysis, given the late onset and lack of curative or ameliorative treatment. This paper also analyzes the ethical standards to be invoked when considering violating the privacy of a patient or a family member. The principles of autonomy and self-determination of family members are considered, versus the risk of harm and the privacy interest in not knowing potentially devastating information.     

Trinucleotide (CAG) repeat expansion in chromosomes of Spanish patients with Huntington's disease

Huntington's disease (HD) is a neurodegenerative and hereditary disease characterized by progressive movement disorders and mental and behavioral abnormalities. The HD gene is an expanding and unstable trinucleotide repeat (CAG repeat sequences). We studied 77 individuals from 38 families with HD in an attempt to obtain information for genetic counselling and differential diagnosis. Our results indicate that individuals with more than 40 repeats will be affected by the disease, whereas those with fewer than 30 will be healthy. There can be some overlap between 30 and 40 repeats, and one should be careful when interpreting these results.     

Risk prediction for clinical phenotype in myotonic dystrophy type 1: data from 2,650 patients

Myotonic dystrophy type 1 (DM1) is a multisystem disorder that affects skeletal and smooth muscle as well as the eye, heart, endocrine system, and central nervous system. DM1 is caused by expansion of a CTG trinucleotidedaggerrepeat in the gene DMPK. Clinical findings in DM1 span a continuum from mild to severe. Although the CTG repeat correlates with the disease phenotype, caution is used in predicting disease severity on the basis of CTG repeat number. This study reports an extensive genotype-phenotype study to evaluate the clinical validity and clinical utility of the molecular genetic test. Data were analyzed by multiple logistic regression, used to estimate the odds ratio (OR) and correlation coefficients for patients phenotype in respect to the categorical variables expansion class, gender, familiarity, and the continuous variables age and disease duration. We assessed disease expression by clinical evaluation and the molecular genetic test in 2,650 patients identified by accurate clinical diagnosis and family segregation. We were able to estimate OR and correlation coefficients for patients phenotype according to CTG number. A genotype-phenotype correlation was established to derivate a clinical predictive risk on the basis of molecular data. This study demonstrates that measurement of triplet expansions in patients' DNA can be considered as a useful tool for DM1 phenotype assessment and presymptomatic testing.     

Long-range mapping in the research and diagnosis of genetic disease

This paper reviews current genetic and molecular biological methods that may be used in the so-called "reverse genetics" approach. These methods are the mapping, isolation, and study of the chromosomal DNA containing a previously unidentified gene responsible for a genetic disease, beginning with its chromosomal localization. In principle, the reverse genetics methodology follows the same path for different diseases studied. An overall outline of the steps to be undertaken is given and discussed. Several stages are illustrated with reference to current research in the fields of Duchenne muscular dystrophy, Huntington's disease, and polycystic kidney disease.     

Presymptomatic testing for Huntington's disease

Summary Presymptomatic testing for Huntington's disease (HD) is possible through the use of restriction fragment length polymorphisms (RFLPs) at the closely linked D4S10 locus. Recombination between the HD and D4S10 loci will occur in 4%–5% of meioses, and is a well-recognised complication of predictive testing. Recombination between RFLPs within the D4S10 locus is a rare event and can usually be ignored. We report a case where such an intra-locus recombination frustrated attempts to predict the chance of a high-risk individual inheriting the HD gene.     

Fine-scale mapping of the gene responsible for multiple endocrine neoplasia type 1 (MEN 1).

We have constructed a high-resolution genetic linkage map in the vicinity of the gene responsible for multiple endocrine neoplasia type 1 (MEN1). The mutation causing this disease, inherited as an autosomal dominant, predisposes carriers to development of neoplastic tumors in the parathyroid, the endocrine pancreas, and the anterior lobe of the pituitary. The 12 markers on the genetic linkage map reported here span nearly 20 cM, and linkage analysis of MEN1 pedigrees has placed the MEN1 locus within the 8-cM region between D11S480 and D11S546. The markers on this map will be useful for prenatal or presymptomatic diagnosis of individuals in families that segregate a mutant allele of the MEN1 gene.     

Genetic counseling on a molecular basis for Huntington's chorea

The gene responsible for Huntington Disease has been mapped at the end of the short arm of chromosome 4. Different polymorphic probes from the immediate vicinity of the gene are available and provide accurate tools for the identification of carriers of the gene. The presymptomatic testing of adults or the prenatal testing (exclusion testing) of the unborn child must be considered with great caution, it requires a multidisciplinary approach and will benefit from intensive collaboration with the lay organizations. Presymptomatic testing will presumably find a much wider application when it can be coupled to an effective preventive treatment.