First-trimester prenatal diagnosis for Huntington's disease with DNA probes

Huntington Disease: Genetics, Prevention, and Therapy Approaches

Huntington's chorea or Huntington disease (HD) is a late-onset autosomal dominant neurodegenerative disorder caused by a trinucleotide repeat expansion. The multidisciplinary study of HD has been the focus of an international collaborating effort of basic and applied research for several decades. HD was the first human genetic disease mapped using linkage analysis of DNA polymorphisms and became a paradigm for scores of genes mapped in the same manner. Presymptomatic and prenatal testing have been available for HD families in the last 30 years, following genetic counseling and careful bioethical guidelines. Nevertheless, with the cure for the disease still elusive the uptake of predictive testing by at risk individuals is low. Current treatment of HD is mostly symptomatic, but ongoing observational studies, clinical trials and development of new gene silencing technologies have provided hopeful results.

Perceptions of genetic discrimination among people at risk for Huntington's disease: a cross sectional survey

OBJECTIVE

To assess the nature and prevalence of genetic discrimination experienced by people at risk for Huntington's disease who had undergone genetic testing or remained untested.

DESIGN

Cross sectional, self reported survey.

SETTING

Seven genetics and movement disorders clinics servicing rural and urban communities in Canada.

PARTICIPANTS

233 genetically tested and untested asymptomatic people at risk for Huntington's disease (response rate 80%): 167 underwent testing (83 had the Huntington's disease mutation, 84 did not) and 66 chose not to be tested.

MAIN OUTCOME MEASURES

Self reported experiences of genetic discrimination and related psychological distress based on family history or genetic test results.

RESULTS

Discrimination was reported by 93 respondents (39.9%). Reported experiences occurred most often in insurance (29.2%), family (15.5%), and social (12.4%) settings. There were few reports of discrimination in employment (6.9%), health care (8.6%), or public sector settings (3.9%). Although respondents who were aware that they carried the Huntington's disease mutation reported the highest levels of discrimination, participation in genetic testing was not associated with increased levels of genetic discrimination. Family history of Huntington's disease, rather than the result of genetic testing, was the main reason given for experiences of genetic discrimination. Psychological distress was associated with genetic discrimination (P<0.001).

CONCLUSIONS

Genetic discrimination was commonly reported by people at risk for Huntington's disease and was a source of psychological distress. Family history, and not genetic testing, was the major reason for genetic discrimination.

Non-disclosing preimplantation genetic diagnosis for Huntington disease

OBJECTIVES

Individuals at risk for Huntington disease face difficult decisions regarding their reproductive options. Most do not wish to pass on the gene for Huntington disease to their children, but may not be prepared themselves to undergo presymptomatic testing and learn their genetic status. For these reasons, many at-risk individuals with a family history of HD would choose a method of genetic diagnosis that would assure them that they can have children unaffected with HD without revealing their own genetic status (non-disclosing). We have shown that, with a carefully designed and executed programme of non-disclosing preimplantation genetic testing, one can successfully assist at-risk couples to have their own biological children who are free from Huntington disease, without forcing parents to confront knowledge of their own genetic status.

METHODS

Couples where one partner was at 50% risk for Huntington disease underwent in vitro fertilization with preimplantation embryo biopsy and molecular analysis for Huntington disease where appropriate.

RESULTS

After extensive counselling and informed consent, 10 couples underwent 13 in vitro fertilization and two frozen embryo transfer cycles in a programme for non-disclosing preimplantation genetic diagnosis for Huntington disease. In 11 cycles, embryos determined to be free of Huntington disease were transferred, resulting in five clinical pregnancies. One set of twins and three singleton pregnancies have delivered. One pregnancy resulted in a first-trimester loss.

CONCLUSIONS

The option of non-disclosing preimplantation genetic diagnosis should be reviewed, along with other relevant medical options, when counselling at-risk Huntington disease families.

Non-disclosure preimplantation genetic diagnosis for Huntington's disease: practical and ethical dilemmas

Prenatal diagnosis of Huntington's Disease (HD) is controversial. Selective abortion is considered unacceptable by some, since, being a late-onset disorder, any child born carrying the HD mutation might still expect many years of disease-free life. The test result itself has implications for the parents and other members of the family who may have decided not to be tested but who know that they may be at risk because a family member is affected. For this reason some potential carriers do not want to know their carrier status and may prefer prenatal exclusion testing. However, since half the fetuses carrying the affected grandparental allele may be normal, aborting these fetuses is also controversial. Preimplantation genetic diagnosis (PGD) has been suggested as an alternative by which asymptomatic individuals who are at high risk of carrying HD can avail themselves of antenatal genetic testing without incurring the emotional, social and financial burdens that might result from the presymptomatic disclosure of their own carrier status. However, non-disclosure testing of embryos in vitro presents specific practical difficulties. Assurance of absolute secrecy is difficult in the large team required for in vitro fertilization biopsy and diagnosis, and changes in practice which may be required to maintain the deception may be unethical.

Huntington's disease

Huntington's disease (HD) is a late-onset degenerative disorder of the central nervous system, caused by a dominantly inherited mutation in a gene on chromosome 4p. The identification of the trinucleotide repeat mutation responsible for this disorder has been an important step towards understanding the molecular pathology of HD, but in the meantime has also made it possible to offer predictive testing and prenatal diagnosis to individuals at high genetic risk. Predictive testing offers obvious benefits for those who receive a favourable result, but also runs the risk of significant psychological and social problems for the families involved. Uptake of testing to date has been limited. Prenatal testing where the pregnancy is at 25% risk carries the same disadvantages as adult predictive testing, because an unfavourable result would also establish that the at-risk parent is a gene carrier; prenatal exclusion testing offers an alternative method of detecting and terminating at-risk pregnancies without revealing the genetic status of the at-risk parent.

The prenatal exclusion test for Huntington's disease: experience in the west of Scotland, 1986-1993

Information about the prenatal exclusion test for Huntington's disease (HD) has been given to an unselected series of couples who attended the genetic counselling clinic from 1986 onwards. Ten couples underwent 13 prenatal tests during this period with expressed intention of stopping a pregnancy if the result indicated a high risk (almost 50%) that the fetus carried the HD gene. Nine fetuses at nearly 50% risk of carrying the HD gene were identified but only six such pregnancies were terminated. In each of three high risk pregnancies which continued, the mother made a "final hour" decision not to undergo the scheduled, first trimester termination. In our experience, late reversal of a previous decision to undergo first trimester pregnancy termination for a genetic indication is uniquely frequent among couples who have undergone the prenatal exclusion test for HD.

Huntington's disease: recent advances in diagnosis and management

Huntington's Disease (HD) is a progressive degenerative disorder of the central nervous system inherited as an autosomal dominant trait. Clinically, the disorder is characterized by choreoathetosis (with age of onset typically in the late thirties or early forties) and neuropsychiatric disturbance. The striatum is particularly vulnerable to the degenerative disease process, with selective loss of medium spiny neurons and decreased levels of associated neurotransmitters, including substance P. GABA, met-enkephalin and dynorphin. Although the underlying pathophysiology is unknown, recent theories concerning pathogenesis have involved mitochondrial abnormalities and excitotoxin-mediated damage. The gene for HD has recently been discovered and characterized as an unstable CAG trinucleotide repeat sequence on the short arm of chromosome 4 (now known as IT15). The direct test now available for the HD gene has facilitated disease diagnosis, particularly for those with unclear family history or chorea of uncertain origin; presymptomatic testing is also available. Management of affected individuals is unsatisfactory as only symptomatic control is available. However, as the effect of the genetic abnormality may soon be known, specific treatment of the disorder may become available in the near future.

A clinico-genetic study of psychiatric disorder in Huntington's chorea

The introduction in 1985 of a genetic linkage test programme to identify asymptomatic heterozygotes among subjects at 50% initial risk for Huntington's chorea required a review of all cases of Huntington's chorea and their families referred to the Department of Medical Genetics of the Oxford Regional Health Area (population 2.5 million). From a representative sample of these subjects, psychiatric data were collected to estimate the frequency and time of onset of functional psychiatric illness and behaviour disorder. The rationale and method of the linkage test is described. The frequency of functional psychiatric disorder found was compared with that reported for the general population and for Alzheimer's disease. The role in relation to the aetiology of functional psychiatric disorder (1) of the Huntington's chorea gene and (2) of the family disturbance produced, was investigated by comparison between the frequency of functional psychiatric disorder in populations containing different proportions of heterozygotes as shown by (a) the manifestation of Huntington's chorea, and (b) the result of the genetic linkage analysis. In order to investigate the influence of the onset of Huntington's chorea on the production of functional psychiatric disorder the time of onset of the various functional psychiatric disorders was compared between asymptomatic subjects at 50% risk for Huntington's chorea and their cohabiting spouses who were assumed to be at zero risk and who shared their environment. It is concluded that possessing the Huntington's chorea gene: (1) has no influence on the production of functional psychiatric disorder in asymptomatic subjects at risk for Huntington's chorea; and (2) increases the tendency to major depressive disorder in subjects already affected with physical signs of Huntington's chorea.

Chorionic villus sampling: analysis of fetal losses to delivery, placental pathology, and cervical microbiology

A population of 1639 patients were seen for chorionic villus sampling (CVS). Embryonic death was identified at ultrasound in 5.3 per cent of patients. The number of patients undergoing CVS was 1551, with 1416 transcervical procedures and 135 transabdominal procedures. The most common indication for CVS was advanced maternal age. Spontaneous pregnancy losses identified by increased risk of pregnancy loss with increasing aspiration attempts. The total fetal loss for this population was 5.4 per cent with the pregnancy loss estimated due to procedure being 1.2 per cent. Analysis of placentae from patients having CVS and amniocentesis showed no differences. Microbiological assessment prior to CVS was similar to previous publications.

Prenatal diagnosis for Huntington's disease: a molecular and psychological study

Two linked probes were used to determine the Huntington's disease status of the fetus conceived by a woman affected with the condition. The fetus was found to be unaffected with a certainty of 97 per cent. The ethical issues associated with presymptomatic testing were avoided since the mother presented with initial symptoms of Huntington's disease, but other psychological and ethical issues arose. The concerns of an affected woman planning a pregnancy, and the dilemmas involved in decision-making regarding prenatal diagnosis and possible selective abortion were exposed and explored with the patient and her husband.

Relative fertility of unaffected siblings of the Huntington's disease families of Tasmania

The relative fertility of the unaffected siblings of the Huntington's disease families of Tasmania (compared with that of the general population) is calculated to be 0.87. Evidence suggests that these people resolve to restrict their family size. Antenatal testing would allow them the freedom of choice regarding family size which the general population currently enjoys.

Exclusion testing in pregnancy for Huntington's disease

The results of DNA analysis are presented for a series of 90 couples, with one partner at 50% risk for Huntington's disease (HD), who were referred for exclusion testing in pregnancy over a three year period. Thirty-seven couples were studied in detail. The aims of the study were to evaluate attitudes towards prenatal testing, before pregnancy and afterwards, and the effectiveness of our counseling and methods of organising the service. Problems which could arise in relation to presymptomatic testing are documented. It is concluded that exclusion testing is a valuable form of prediction for some couples, particularly where family structure does not permit prediction for the person at risk. The need for intensive counselling was highlighted by the difficulties experienced by many couples in understanding how the test worked. Particular ethical and organisational problems may arise which require careful consideration beforehand and some recommendations are made. The proportion of couples who will continue to request exclusion testing as pre-symptomatic testing becomes more widely applicable remains unknown.

Options for prenatal testing for Huntington's disease using linked DNA probes

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Predictive testing for Huntington's disease with linked DNA markers

Availability of new DNA markers, more tightly linked to the Huntington's disease (HD) locus than the original G8 (D4S10) probes, has improved predictive accuracy for both presymptomatic and prenatal exclusion testing. 50 predictive tests were carried out on high-risk individuals. 6 of these were on first-trimester chorionic villus biopsy specimens; in 2 cases the HD gene was not transmitted to the fetus while in 4 cases no exclusion could be made. The remaining 44 tests were on adults with either 25 or 50% risk of manifesting the disease; 19 had a greatly increased risk and 25 a substantially decreased risk of HD. Family structures in Scotland are suitable for testing about 75% of potentially affected individuals, and the new generation of DNA markers makes virtually all families fully informative.

Different options for prenatal testing for Huntington's disease using DNA probes

The discovery of DNA markers closely linked to the gene for Huntington's disease (HD) has allowed development of predictive and prenatal testing programmes for HD. This report describes four different approaches to prenatal testing for HD which have arisen during a pilot predictive and prenatal testing program in British Columbia, Canada. In the first approach (exclusion testing), the at risk parent cannot or prefers not to learn of his/her HD status. Two other approaches involve definitive testing of a fetus when a parent is determined to be at increased risk to have inherited the HD gene or is affected with Huntington's disease. The fourth approach is a stepwise combination of the above two methods which we refer to as 'exclusion-definitive' testing. These different approaches introduce a variety of challenging counselling and ethical issues. The role of each approach to prenatal testing in the management of Huntington's disease awaits the results of this and other predictive and prenatal testing programmes.

Huntington's chorea: general practitioners' knowledge about individuals' at-risk status

We assessed general practitioners' knowledge about the at-risk status of offspring of persons suffering from Huntington's chorea. Of 97 practitioners who were questioned about only one at-risk individual, 16 were aware of the person's at-risk status, while nine of 26 practitioners who were questioned about more than one at-risk person were aware of it. The results indicate that prophylactic action can be achieved more efficiently by registration of families in which Huntington's chorea occurs, followed by genetic counselling.

Prenatal exclusion testing for Huntington disease using the polymerase chain reaction

Prenatal exclusion of Huntington disease (HD) may be carried out by analysis of cosegregating DNA markers on a first-trimester chorionic villus sample. The conventional Southern blot method is time-consuming and requires microgram quantities of DNA and milligram quantities of villus tissue. The use of the polymerase chain reaction (PCR) to amplify genomic DNA by a factor of 10(7) or more makes it possible to do analyses on very small samples in a few hours and without recourse to Southern blotting or hybridization with radioactive probes. We report on a fetus at risk of HD; prenatal testing was carried out by using the PCR to amplify a polymorphic DNA sequence adjacent to the HD locus. The risk of the fetus inheriting the HD gene could not be excluded and the pregnancy was terminated. This represents an example of gene tracking by using amplification of a restriction fragment length polymorphism at some distance from the relevant mutation.

Prenatal exclusion testing for Huntington's disease: a problem of too much information

At eight weeks of pregnancy a couple were informed that the prospective father's mother had died of Huntington's disease (HD). There were no living affected members in the immediate family to confirm the diagnosis. By inspection of the local genetic register, it was established that it was indeed HD segregating in the extended family. Genotyping of the prospective mother and father, the father's unaffected father, and his unaffected maternal grandmother was carried out using a battery of polymorphic DNA markers, including a new probe which has a very low recombination rate with the HD locus. Analysis of DNA from a chorionic villus sample taken at 10 weeks of pregnancy showed that the fetus must have inherited a chromosome from its father's affected mother. Its risk of developing HD was 47%. If the genotype of the unaffected maternal grandmother was taken into account, the risk was reduced to 42%. Neither risk was considered acceptable by the prospective parents and the pregnancy was terminated at 12 weeks' gestation. Prospects for future pregnancies are good, with a 50% chance of having a child whose risk of inheriting the HD gene is less than 1.5%. In retrospect it was noted that although genotyping of the maternal grandmother had refined the fetal risk, it had also nearly contributed to an inadvertent and unwanted predictive test for HD on the father. This case makes the point that in prenatal exclusion testing, linkage information must be generated with considerable care.

Genetic linkage between Huntington's disease and D4S10 (G8) in Scottish families

Genetic linkage between Huntington's disease (HD) and polymorphic DNA markers at the D4S10 locus has been investigated in 16 Scottish families. A maximum lod score of 3.499 at a recombination fraction of 0.07 was found, with 95% confidence limits of 0.02 and 0.22. Only one obvious recombinant was detected, and the wide confidence limits probably reflect the large number of unaffected individuals whose risk could only be estimated empirically.

Presymptomatic testing for Huntington's disease. A case complicated by recombination within the D4S10 locus

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.

Considerations in using linkage analysis as a presymptomatic test for Huntington's disease

The polymorphic locus D4S10 that is genetically linked to the locus for Huntington's disease (HD) has made possible a presymptomatic test for those at risk. Because the symptoms of this progressively debilitating and fatal illness are not usually manifest until adulthood, the outcome of the test will influence major decisions about career, marriage, and procreation. Several differential diagnoses must be considered before using the test if HD is not confirmed in at least one family member. Review of a large number of pedigrees has shown that 40% of persons at risk do not have appropriate family structure for a linkage test. Furthermore, uncooperative or inaccessible relatives may make this test infeasible for many others who wish to be tested. Linkage phase, which must be known in the affected parent for an informative test, can be determined using one or more of 12 probe-enzyme combinations for D4S10. Although the polymorphism information content (PIC) value for any one RFLP is less than 40%, the PIC value for the haplotype of the two G8 HindIII, pK083 EcoRI, and R7 BglII RFLPs is greater than 88%. We have developed a scheme to incorporate linkage data and age at onset information adjusted for censored observations, sex of affected parent, and familial correlation for age at onset, using the computer program MLINK for calculation of risk of having HD. Simulated experiments showed that proper age at onset adjustment is crucial to the calculation of the probability of risk. A formal presymptomatic testing protocol, including pre- and post-test counselling, psychological testing, and paternity testing is recommended. Many of these considerations are illustrated in several actual test cases.

Looking to the east for herbal remedies

The genetic defect in Huntington's disease (HD), an inherited neuropsychiatric disorder of unknown etiology, has not been defined. The discovery of linkage between HD and the DNA marker D4S10(G8) raised the possibility of isolating the disease gene on the basis of its chromosomal location, in addition to providing a limited presymptomatic test for the late onset disorder. But it has been difficult to isolate other DNA markers nearer to the HD gene, and this has hampered attempts to identify the disease locus and limited the applicability and accuracy of predictive testing. Recently, several new DNA markers from the region of the genome near the HD gene have been isolated using a directed cloning strategy. We describe here the characterization of one of these new markers, D4S95, a highly polymorphic locus which displays no recombination with the HD gene in the families tested. The high degree of polymorphism at this locus and its proximity to the HD gene make it extremely useful for predictive testing and as a new starting point for attempts to clone the disease gene.

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.

Predictive testing for Huntington's disease with use of a linked DNA marker

The probability of carrying the gene for Huntington's disease can in many cases be estimated in the children of affected persons by identifying a specific DNA marker that is genetically linked to the gene. We studied 47 persons at 50 percent risk of inheriting Huntington's disease who requested a presymptomatic or prenatal genetic-linkage test between September 1986 and January 1988. The participants were given pre-test counseling and psychological and neurologic evaluations. Nineteen persons later voluntarily withdrew from the protocol, including one who would have been excluded anyway, and one person was from a family that was too small to allow testing. Three D4S10 restriction-fragment-length polymorphisms produced by the HindIII, EcoRI, and Bg/I enzymes were used for all tests, and the probability that a subject was a Huntington's disease carrier was calculated. The accuracy of the test was compromised by a 4 percent recombination frequency between D4S10 and the Huntington's disease gene. Fifteen presymptomatic tests and one prenatal test were completed. Four yielded positive results, seven yielded negative results, and five were uninformative; seven persons are awaiting test results. All participants with positive tests experienced intermittent depression, but none required hospitalization, and no suicide threats were reported. Five participants received a diagnosis of Huntington's disease on the basis of the neurologic assessment. We conclude that some persons in the early stages of Huntington's disease may seek presymptomatic testing rather than neurologic evaluations.