Hunting for Huntington's disease

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.

What is the impact of education on Huntington's disease?

Huntington's disease (HD) is a neurodegenerative disease caused by a cytosine adenosine guanine (CAG) expansion in the huntingtin gene. The length of the triplet repeat is the most important factor in determining age of onset and the severity of the disease, but substantial variability of these parameters is attributed to other factors. To investigate the relationship between the years of education and the age at onset and the severity of the phenotype in patients with HD, we applied multiple linear regression analysis to examine the impact of education on the age at onset and the severity of the clinical scores assessed by the Unified Huntington's Disease Rating Scale (UHDRS) of 891 patients with HD from the multinational observational study "Registry" conducted by the European Huntintgton's Disease Network. The model was adjusted for CAG repeat length and age at the time of assessment. Patients with lengthier education exhibited earlier estimated age at onset but less severe clinical scores (motor = -3.6, P = 0.006; cognitive = 27.0, P < 0.001; behavioral = -3.0, P < 0.001; and functional capacity = 1.1 points, P < 0.001) than those with shorter education, after controlling for age and number of CAG repeats. These differences persisted throughout all quartiles of disease severity. An earlier recognition of symptoms and manifestations among the more educated patients could explain the earlier estimated age at onset in this group. The link between better clinical UHDRS scores and higher education might reflect a beneficial effect of education or its covariates on the course of HD.

Perception, experience, and response to genetic discrimination in Huntington disease: the international RESPOND-HD study

Genetic discrimination-defined as the denial of rights, privileges, or opportunities or other adverse treatment based solely on genetic information (including family history)-is an important concern to patients, healthcare professionals, lawmakers, and family members at risk for carrying a deleterious gene. Data from the United States, Canada, and Australia were collected from 433 individuals at risk for Huntington disease (HD) who have tested either positive or negative for the gene that causes HD and family members of affected individuals who have a 50% risk for developing the disorder but remain untested. Across all three countries, a total of 46.2% of respondents report genetic discrimination or stigma based on either their family history of HD or genetic testing for the HD gene mutation. We report on the overall incidence of discrimination and stigma in the domains of insurance (25.9%), employment (6.5%), relationships (32.9%), and other transactions (4.6%) in the United States, Canada, and Australia combined. The incidence of self-reported discrimination is less than the overall worry about the risk of discrimination, which is more prevalent in each domain. Despite a relatively low rate of perceived genetic discrimination in the areas of health insurance and employment, compared to the perception of discrimination and stigma in personal relationships, the cumulative burden of genetic discrimination across all domains of experience represents a challenge to those at risk for HD. The effect of this cumulative burden on daily life decisions remains unknown.

Transglutaminase activation in neurodegenerative diseases

The following review examines the role of calcium in promoting the in vitro and in vivo activation of transglutaminases in neurodegenerative disorders. Diseases such as Alzheimer's disease, Parkinson's disease and Huntington's disease exhibit increased transglutaminase activity and rises in intracellular calcium concentrations, which may be related. The aberrant activation of transglutaminase by calcium is thought to give rise to a variety of pathological moieties in these diseases, and the inhibition has been shown to have therapeutic benefit in animal and cellular models of neurodegeneration. Given the potential clinical relevance of transglutaminase inhibitors, we have also reviewed the recent development of such compounds.

Perceptions of discrimination among persons who have undergone predictive testing for Huntington's disease

Potential discrimination from genetic testing may undermine technological advances for health care. Researching long-term consequences of testing for genetic conditions that may lead to discrimination is a public health priority. The consequences of genetic discrimination generate social, health, and economic burdens for society by diminishing opportunities for at-risk individuals in a range of contexts. The current study objective was to investigate perceptions of genetic stigmatization and discrimination among persons who completed predictive testing for Huntington's disease (HD). Using semi-structured interviews and computerized qualitative analysis, the perceptions of 15 presymptomatic persons with a positive gene test predicting HD were examined with regard to differential treatment following testing. The sample comprised 11 women and 4 men, mostly married (73%), aged between 22 and 62 years, with an average education of 14.6 years (SD +/- 2.57) and residing in urban, rural and suburban settings of eight U.S. States. Participants reported perceptions of consequences following disclosure of genetic test results in three areas: employment, insurance, and social relationships. Although most employed participants (90%) revealed their test results to their employers, nearly all reported they would not disclose this information to future employers. Most (87%) participants disclosed test results to their physician, but a similar majority (83%) did not tell their genetic status to insurers. Most participants (87%) disclosed test results to family and peers; patterns of disclosure varied widely. Discrimination concerns remain high in this sample and point to the need for more information to determine the extent and scope of the problem.

Genetic criteria for Huntington's disease pathogenesis

Genetic analysis aims to identify the variations in DNA sequence whose functional consequences produce heritable variations in phenotype. In one of the first successes of unbiased molecular genetic analysis in human disease, the Huntington's disease (HD) gene was mapped and cloned without any prior knowledge of the nature of its protein product or of the molecular defect that underlies the characteristic phenotype of the disorder. However, while the cloning of HD and recognition of its trinucleotide repeat expansion spawned a plethora of approaches to investigating HD through its distinctive neuropathology, the role for genetic strategies in HD research did not end there. The use of genetic analysis has remained a critical tool for defining the characteristics of the mechanism that triggers the pathogenic process, permitting the investigation of early events that occur long before traditionally recognized pathology. Delineation of these events can reveal molecular targets for development of therapies that prevent onset of HD. Most recently, an extension of genetic analysis to the identification of non-linked genetic variations that alter the course of HD pathogenesis has offered the promise of identifying modifier genes to reveal biological pathways active throughout the disease process and to provide valid targets for pharmacological intervention. Thus, unbiased genetic strategies have not only provided a crucial entrée into molecular investigation of HD via a root cause that was previously unsuspected, they also represent a continuing route to accelerate the ultimate goal of developing an effective treatment for HD.

Precise genetic mapping and haplotype analysis of the familial dysautonomia gene on human chromosome 9q31

Familial dysautonomia (FD) is an autosomal recessive disorder characterized by developmental arrest in the sensory and autonomic nervous systems and by Ashkenazi Jewish ancestry. We previously had mapped the defective gene (DYS) to an 11-cM segment of chromosome 9q31-33, flanked by D9S53 and D9S105. By using 11 new polymorphic loci, we now have narrowed the location of DYS to <0.5 cM between the markers 43B1GAGT and 157A3. Two markers in this interval, 164D1 and D9S1677, show no recombination with the disease. Haplotype analysis confirmed this candidate region and revealed a major haplotype shared by 435 of 441 FD chromosomes, indicating a striking founder effect. Three other haplotypes, found on the remaining 6 FD chromosomes, might represent independent mutations. The frequency of the major FD haplotype in the Ashkenazim (5 in 324 control chromosomes) was consistent with the estimated DYS carrier frequency of 1 in 32, and none of the four haplotypes associated with FD was observed on 492 non-FD chromosomes from obligatory carriers. It is now possible to provide accurate genetic testing both for families with FD and for carriers, on the basis of close flanking markers and the capacity to identify >98% of FD chromosomes by their haplotype.

Synteny conservation of the Huntington's disease gene and surrounding loci on mouse Chromosome 5

The mouse homologs of the Huntington's disease (HD) gene and 17 other human Chromosome (Chr) 4 loci (including six previously unmapped) were localized by use of an interspecific cross. All loci mapped in a continuous linkage group on mouse Chr 5, distal to En2 and I16, whose human counterparts are located on Chr 7. The relative order of the loci on human Chr 4 and mouse Chr 5 was maintained, except for a break between D5H4S115E and Idua/rd, with relocation of the latter to the opposite end of the map. The mouse HD homolog (Hdh) mapped within a cluster of seven genes that were completely linked in our data set. In human these loci span a approximately 1.8 Mb stretch of human 4p16.3 that has been entirely cloned. To date, there is no phenotypic correspondence between human and mouse mutations mapping to this region of synteny conservation.

Mouse Huntington's disease gene homolog (Hdh)

The incurable neurodegenerative disorder, Huntington's disease (HD), is caused by an expanded, unstable CAG repeat encoding a stretch of polyglutamine in a 4p16.3 gene (HD) of unknown function. Near the CAG repeat is a polyproline-encoding CCG repeat that shows more limited allelic variation. The mouse homologue, Hdh, has been mapped to chromosome 5, in a region devoid of mutations causing any comparable phenotype. We have isolated overlapping cDNAs from the Hdh gene and compared their sequences with the human transcript. The consensus mouse coding sequence is 86% identical to the human at the DNA level and 91% identical at the protein level. Despite the overall high level of conservation, Hdh possesses an imperfect CAG repeat encoding only seven consecutive glutamines, compared to the 13-36 residues that are normal in man. Although no evidence for polymorphic variation of the CAG repeat was seen, a nearby CCG repeat differed in length by one unit between several strains of laboratory mouse and Mus spretus. The absence of a long CAG repeat in the mouse is consistent with the lack of a spontaneous mouse model of HD. The information presented concerning the sequence of the mouse gene should facilitate attempts to create such a model.

Structure and expression of the Huntington's disease gene: evidence against simple inactivation due to an expanded CAG repeat

Huntington's disease, a neurodegenerative disorder characterized by loss of striatal neurons, is caused by an expanded, unstable trinucleotide repeat in a novel 4p16.3 gene. To lay the foundation for exploring the pathogenic mechanism in HD, we have determined the structure of the disease gene and examined its expression. The HD locus spans 180 kb and consists of 67 exons ranging in size from 48 bp to 341 bp with an average of 138 bp. Scanning of the HD transcript failed to reveal any additional sequence alterations characteristic of HD chromosomes. A codon loss polymorphism in linkage disequilibrium with the disorder revealed that both normal and HD alleles are represented in the mRNA population in HD heterozygotes, indicating that the defect does not eliminate transcription. The gene is ubiquitously expressed as two alternatively polyadenylated forms displaying different relative abundance in various fetal and adult tissues, suggesting the operation of interacting factors in determining specificity of cell loss. The HD gene was disrupted in a female carrying a balanced translocation with a breakpoint between exons 40 and 41. The absence of any abnormal phenotype in this individual argues against simple inactivation of the gene as the mechanism by which the expanded trinucleotide repeat causes HD. Taken together, these observations suggest that the dominant HD mutation either confers a new property on the mRNA or, more likely, alters an interaction at the protein level.