Genetic testing for Huntington's disease

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.

Huntington's Disease: Pathological Mechanisms and Therapeutic Strategies

Huntington's disease (HD) is a devastating neurodegenerative disorder that occurs in patients with a mutation in the huntingtin or IT15 gene. Patients are plagued by early cognitive signs, motor deficits, and psychiatric disturbances. Symptoms are attributed to cell death in the striatum and disruption of cortical–striatal circuitry. Mechanisms of cell death are unclear, but processes involving mitochondrial abnormalities, excitotoxicity, and abnormal protein degradation have been implicated. Many factors likely contribute to neuron death and dysfunction, and this has made it difficult to systematically address the pathology in HD. Pharmaceutical therapies are commonly used in patients to treat disease symptoms. These have limited benefit and do not address the inexorable disease progression. Several neuroprotective therapies are being evaluated in animal models of HD as well as in clinical trials. Similarly, cell replacement strategies such as fetal transplantation have been used in the clinic with minimal success, making future cell replacement strategies such as stem cell therapy uncertain. This review describes the disease pathology in HD and addresses many of the past and emerging therapeutic strategies.

Examination of Huntington's disease in a Chinese family

We report brain imaging and genetic diagnosis in a family from Wuhan, China, with a history of Huntington's disease. Among 17 family members across three generations, four patients (II2, II6, III5, and III9) show typical Huntington's disease, involuntary dance-like movements. Magnetic resonance imaging found lateral ventricular atrophy in three members (II2, II6, and III5). Moreover, genetic analysis identified abnormally amplified CAG sequence repeats (> 40) in two members (III5 and III9). Among borderline cases, with clinical symptoms and brain imaging features of Huntington's disease, two cases were identified (II2 and II6), but shown by mutation analysis for CAG expansions in the important transcript 15 gene, to be non-Huntington's disease. Our findings suggest that clinical diagnosis of Huntington's disease requires a combination of clinical symptoms, radiological changes, and genetic diagnosis.

Gene therapy for Huntington's disease

Huntington's disease (HD) is a neurodegenerative disease for which there is no cure. Therapies that are efficacious in animal models have to date shown benefit for humans. One potential powerful approach is gene therapy. The ideal method of administration of gene therapy has been hotly debated and viral vectors have provided one method of long-term and wide-spread delivery to the brain. Trophic factors to protect cells from degeneration and RNAi to reduce mutant huntingtin (mHtt) protein expression are 2 main classes of compounds that demonstrate benefit in animal models. This review will examine some commonly used adeno-associated viral (AAV) vectors and discuss some therapies that hold promise for HD.

Neurturin gene therapy improves motor function and prevents death of striatal neurons in a 3-nitropropionic acid rat model of Huntington's disease

Huntington's disease (HD) is a devastating neurodegenerative disease characterized by the selective loss of neurons in the striatum and cerebral cortex. This study tested the hypothesis that an adenoassociated viral (AAV2) vector encoding for the trophic factor neurturin (NTN) could provide neuroprotection in the rat 3-nitropropionic acid (3NP) model of HD. Rats received AAV2-NTN (CERE-120), AAV2-eGFP or Vehicle, followed 4 weeks later by the mitochondrial toxin 3NP. 3NP induced motor impairments were observed on the rotarod test, the platform test, and a clinical rating scale in all groups. However, each of these deficits was attenuated by AAV2-NTN (CERE-120). Stereological counts revealed a significant protection of NeuN-ir striatal neurons from 3NP toxicity by AAV2-NTN. These data support the concept that AAV2-NTN might be a valuable treatment for patients with Huntington's disease.

Recommendations for the diagnosis and management of Alzheimer's disease and other disorders associated with dementia: EFNS guideline

The aim of this international guideline on dementia was to present a peer-reviewed evidence-based statement for the guidance of practice for clinical neurologists, geriatricians, psychiatrists, and other specialist physicians responsible for the care of patients with dementia. It covers major aspects of diagnostic evaluation and treatment, with particular emphasis on the type of patient often referred to the specialist physician. The main focus is Alzheimer's disease, but many of the recommendations apply to dementia disorders in general. The task force working group considered and classified evidence from original research reports, meta-analysis, and systematic reviews, published before January 2006. The evidence was classified and consensus recommendations graded according to the EFNS guidance. Where there was a lack of evidence, but clear consensus, good practice points were provided. The recommendations for clinical diagnosis, blood tests, neuroimaging, electroencephalography (EEG), cerebrospinal fluid (CSF) analysis, genetic testing, tissue biopsy, disclosure of diagnosis, treatment of Alzheimer's disease, and counselling and support for caregivers were all revised when compared with the previous EFNS guideline. New recommendations were added for the treatment of vascular dementia, Parkinson's disease dementia, and dementia with Lewy bodies, for monitoring treatment, for treatment of behavioural and psychological symptoms in dementia, and for legal issues. The specialist physician plays an important role together with primary care physicians in the multidisciplinary dementia teams, which have been established throughout Europe. This guideline may contribute to the definition of the role of the specialist physician in providing dementia health care.

A review of Huntington's disease

Huntington's disease is an autosomal dominant neuropsychiatric disorder which usually afflicts people with dependents. Psychiatric disorders figure prominently in the differential diagnosis, and may complicate the clinical picture, sometimes even antedating the onset of neurological signs. Diagnosis is still usually clinical, for which some knowledge of the disease is required. Positron emission tomography, where available, can assist in early detection. Recent scientific breakthroughs have considerably improved the accuracy of predictive genetics, and the psychological outcome of such testing may be better than previously, but considerable ethical dilemmas have yet to be fully addressed. Genetic treatment is being considered. Drug treatments offer crude symptomatic improvement. Counselling and support services are essential, as are voluntary self-help organisations. Specific day and inpatient facilities are sparse.

Clinical ethics committee

An informal clinical ethics committee was set up to advise on ethical problems in prenatal diagnosis in Leeds. It was used twice in six months but was not called on again in the subsequent year, and we describe this experience. In North America similar committees are often used to advise on clinical moral dilemmas, and we review the published evidence from there and discuss some of the advantages and problems. Our committee's advice may have altered clinicians' actions considerably, but perhaps doctors in Britain are not yet ready to surrender this aspect of clinical autonomy.

Truth-telling in clinical practice

Disclosure by doctors to patients has changed during the past 30 years in the direction of honesty, but deception and nondisclosure are still common. Clear ethical and legal precedents and guidelines regarding truth-telling exist. Physicians should not protect patients from potentially upsetting information; they will be held responsible should patients be injured due to failure to disclose.

Presymptomatic testing for autosomal dominant spinocerebellar ataxia type 1

Presymptomatic testing was done on four people from a large family in which an autosomal dominant form of spinocerebellar ataxia was segregating. Earlier genetic analysis had shown that in this family the disorder was tightly linked to an informative microsatellite polymorphism on chromosome 6p. Two subjects with prior risks of 50% of developing the disease had final risks after testing of 2%; the other two with prior risks of 25% had final risks of 1%. Chromosome 6p linked spinocerebellar ataxia may now be added to Huntington's disease as a late onset disorder in which genetic linkage may be used to carry out presymptomatic testing.

Assay by polymerase chain reaction (PCR) of multi-allele polymorphisms in the Huntington's disease region of chromosome 4

The Huntington's disease-linked D4S115 marker has been converted from a DNA blot assay to a more sensitive and rapid polymerase chain reaction (PCR) assay. PCR amplification of a tandem repeat at D4S115 revealed 7 allelic fragments, ranging in size from approximately 610 to 915 bp, differing in their apparent copy number of a approximately 55 bp core repeat. This repeat unit differs strikingly in sequence from the repeat units of other multi-allele markers from chromosome region 4p 16.3, arguing that the VNTR (Variable Number of Tandem Repeats) loci clustered in this region did not arise from a common ancestral sequence. The D4S115 marker can be assayed simultaneously with PCR products from D4S125, D4S95 and D4S43 on a single agarose gel, providing a rapid scan for successful amplification of these difficult-to-assay VNTRs, and for inheritance of the entire candidate Huntington's disease region. This approach should help to increase the speed, informativeness and accuracy of presymptomatic and prenatal linkage testing in this devastating disorder.

One hundred requests for predictive testing for Huntington's disease

In Grampian region, the first 100 requests for predictive testing for Huntington's disease have been analysed. Forty-five presymptomatic predictive test results have been given, eleven families have made a decision to undergo prenatal exclusion tests, and so far eleven pregnancies have been investigated using chorion villus biopsy. This paper presents the protocol used and a descriptive analysis of those receiving results, those choosing to leave the programme, and those unable to complete it because of an unsuitable family structure or unco-operative relatives.

Huntington's disease and the ethics of genetic prediction

What ethical justification can be found for informing a person that he or she will later develop a lethal disease for which no therapy is available? This question has been discussed during the past twenty years by specialists concerned with the prevention of Huntington's Disease, an incurable late-onset hereditary disorder. Many of them have played an active role in developing experimental testing programmes for at-risk persons. This paper is based on a corpus of 119 articles; it reviews the development of their reflection and includes an outline of the ethical problems identified and the solutions adopted in pre-clinical protocols. Seen in a broader perspective, the experience of presymptomatic testing for Huntington's Disease has given medical geneticists the opportunity to clarify their ethical position in the as yet little explored field of predictive medicine.