A worldwide study of the Huntington's disease mutation. The sensitivity and specificity of measuring CAG repeats

Diagnosis of inherited metabolic disorders affecting the nervous system

Knowledge of the molecular causes for genetic diseases that affect the nervous system is rapidly expanding. Especially striking has been the finding in several autosomal dominant neurodegenerative disorders that unstable expansions of trinucleotide repeats are responsible for the genetic disorder and that the length of the repeat can be correlated with the age of onset and the severity of symptoms. Phenotypic heterogeneity in many disorders associated with enzyme deficiencies can often be linked to the amount of residual enzyme activity occurring with different gene mutations. Making a specific diagnosis of a neurological disorder associated with genetically determined metabolic defects requires access to a laboratory that can assist in arranging for appropriate testing to be carried out. In some disorders such as the aminoacidurias diagnostic metabolic studies can be performed in hospital clinical chemistry laboratories. In others, such as the lysosomal storage diseases, a laboratory that carries out special lipid analyses and white blood cell enzyme assays will be necessary. DNA mutational analyses are becoming commercially available for diagnosing many disorders such as mitochondrial diseases and those conditions associated with expanded trinucleotide repeats. It may be necessary to contact individual research laboratories when confronted with a disorder that has been newly discovered or that is very rare. A computerised directory of specialised laboratories that perform disease specific testing for genetic disorders should be useful in choosing the appropriate diagnostic or research laboratory.

Huntington's disease: CAG genetics expands neurobiology

Huntington's disease, with its progressive uncontrolled movements and characteristic selective neuropathology, has represented a baffling enigma to geneticists and neurobiologists alike. Discovery of the HD gene and its defect has demystified the genetic aspects of the disorder, but has not yet explained its pathogenesis. Attempts to explore this issue suggest that the defect acts as a gain of function, conferring a new deleterious property on the huntingtin protein, and that the gene's normal function may be irrelevant to the disease process.

Mechanisms of DNA expansion

Unstable transmission of repeating segments in genes is now recognized as a new class of mutations causing human disease. Genetic instability observed in disease is termed an "expansion mutation" when the mutation is an increase in the copy number of a repeated unit, commonly a di- or trinucleotide. While the expansion mutation is well characterized in disease, the mechanism by which expansion occurs is not clear. This article focuses on physical properties of expansion at repeating nucleotides that may provide clues to the mechanism. Both biochemical and genetic data indicate that DNA structure is part of the mechanism and the underlying cause for expansion.

Increased apoptosis and early embryonic lethality in mice nullizygous for the Huntington's disease gene homologue

The expansion of CAG triplet repeats in the translated region of the human HD gene, encoding a protein (huntingtin) of unknown function, is a dominant mutation leading to manifestation of Huntington's disease. Targeted disruption of the homologous mouse gene (Hdh), to examine the normal role of huntingtin, shows that this protein is functionally indispensable, since nullizygous embryos become developmentally retarded and disorganized, and die between days 8.5 and 10.5 of gestation. Based on the observation that the level of the regionalized apoptotic cell death in the embryonic ectoderm, a layer expressing the Hdh gene, is much higher than normal in the null mutants, we propose that huntingtin is involved in processes counterbalancing the operation of an apoptotic pathway.

Identification and localization of huntingtin in brain and human lymphoblastoid cell lines with anti-fusion protein antibodies

The Huntington disease (HD) phenotype is associated with expansion of a trinucleotide repeat in the IT15 gene, which is predicted to encode a 348-kDa protein named huntington. We used polyclonal and monoclonal anti-fusion protein antibodies to identify native huntingtin in rat, monkey, and human. Western blots revealed a protein with the expected molecular weight which is present in the soluble fraction of rat and monkey brain tissues and lymphoblastoid cells from control cases. In lymphoblastoid cell lines from juvenile-onset heterozygote HD cases, both normal and mutant huntingtin are expressed, and increasing repeat expansion leads to lower levels of the mutant protein. Immunocytochemistry indicates that huntingtin is located in neurons throughout the brain, with the highest levels evident in larger neurons. In the human striatum, huntingtin is enriched in a patch-like distribution, potentially corresponding to the first areas affected in HD. Subcellular localization of huntingtin is consistent with a cytosolic protein primarily found in somatodendritic regions. Huntingtin appears to particularly associate with microtubules, although some is also associated with synaptic vesicles. On the basis of the localization of huntingtin in association with microtubules, we speculate that the mutation impairs the cytoskeletal anchoring or transport of mitochondria, vesicles, or other organelles or molecules.

Origins and evolution of Huntington disease chromosomes

Huntington disease (HD) is one of five neurodegenerative disorders resulting from an expansion of a CAG repeat located within the coding portion of a novel gene. CAG repeat expansion beyond a particular repeat size has been shown to be a specific and sensitive marker for the disease. A strong inverse correlation is evident between CAG length and age of onset. Sporadic cases of HD have been shown to arise from intermediate sized alleles in the unaffected parent. The biochemical pathways underlying the relationship between CAG repeat length and specific cell death are not yet known. However, there is an increasing understanding of how and why specific chromosomes and not others expand into the disease range. Haplotype analysis has demonstrated that certain normal chromosomes, with CAG lengths at the high range of normal, are prone to further expansion and eventually result in HD chromosomes. New mutations preferentially occur on normal chromosomes with these same haplotypes associated with higher CAG lengths. The distribution of different haplotypes on control chromosomes in different populations is thus one indication of the frequency of new mutations for HD within that population. Analysis of normal chromosomes in different populations suggests that genetic factors contribute to expansion and account for the variation in prevalence rates for HD worldwide.

Somatic stability in chorionic villi samples and other Huntington fetal tissues

We have studied different tissues from two affected fetuses with Huntington's disease (HD). In the first case the analysis was performed at 11 weeks of pregnancy; CAG repeats from seven different tissues were compared with the results obtained in the chorionic villi sample (CVS). We found 42 CAG repeats in all samples. In the second case the study was done at 12 weeks; eight tissues (including brain) were studied and compared with the CVS; in all of them, 44 CAG repeats were obtained. Our results show a somatic stability in the different analyzed tissues and suggest that mitotic instability can be a secondary consequence of neuronal degeneration and gliosis. Likewise, our data show great viability in the prenatal diagnosis (PD) of Huntington's disease using samples from any tissue.

The epidemiology of Huntington's disease in Northern Ireland

A survey of Huntington's disease (HD) in Northern Ireland, with a population of 1.5 million, has shown a 1991 prevalence rate of 6.4/100,000. Virtually complete ascertainment was achieved, enabling prevalence rate estimates and age statistics to be calculated over the last 20 years. The prevalence rate is similar to the high prevalence rates of HD found in most European populations, suggesting the presence of either one extremely ancient or a number of separate mutational origins, resulting in a uniform European HD prevalence. The ages at diagnosis and duration of the disease are similar to previous studies, suggesting a consistent effect of the HD gene in all families. Estimates of heterozygote frequency (HF), direct and indirect mutation rate, fertility, and genetic fitness (W) were made. Reliable HF estimates gave values between 10 and 11 x 10(-5). The direct and indirect mutation rates were 0.32 x 10(-6) and 1.05 x 10(-6) respectively. W was increased in the affected HD population but decreased in the at risk population. Fertility in HD is not reduced, but it appears that at risk patients have actively limited their family size. Factors responsible include, among others, the fear of developing HD and genetic counselling of families. This is the first published epidemiological survey to include ascertainment data in a population both before and after isolation of the HD gene, and with the diagnosis in virtually all patients confirmed by DNA mutation testing.

Emotional and functional impact of DNA testing on patients with symptoms of Huntington's disease

The potential impact of DNA testing on asymptomatic subjects at risk for Huntington's disease (HD) has been addressed by numerous studies, but the effect of revealing the genetic results to patients with a clinically established diagnosis of HD has not been previously evaluated. We studied 36 patients, with equal distribution of men and women, mean age 53.9 (SD 12.3) years (range 25-76) and mean duration of symptoms of 11.2 (SD 7.7) years (range 2-33), whose clinical diagnosis of HD was confirmed by expanded CAG repeats (> 40). Coping strategies and depression levels were assessed before the results of DNA testing were imparted. The assessments were repeated two weeks and three months after the results were explained to the patients and their relatives and were compared to the baseline assessments. This group of HD patients was compared with 10 patients who had similar symptoms but the diagnosis of HD was excluded by normal CAG repeats (< 30). Although some patients with HD expressed a subjective reaction to the positive result (four were "surprised", one was "frustrated", and one "devastated"), there were no differences in any psychological scores including Beck Depression Inventory, functional capacity, symptom interference, independence scale, and other measures of mood and behaviour two weeks and three months later. Similarly, no change was noted in any of these measures in the non-HD group. These results suggest that mood and coping strategies are unaffected by DNA confirmation of diagnosis in symptomatic patients with HD.

Targeted disruption of the Huntington's disease gene results in embryonic lethality and behavioral and morphological changes in heterozygotes

Huntington's disease (HD) is an incurable neuropsychiatric disease associated with CAG repeat expansion within a widely expressed gene that causes selective neuronal death. To understand its normal function, we have created a targeted disruption in exon 5 of Hdh (Hdhex5), the murine homolog of the HD gene. Homozygotes die before embryonic day 8.5, initiate gastrulation, but do not proceed to the formation of somites or to organogenesis. Mice heterozygous for the Hdhex5 mutation display increased motor activity and cognitive deficits. Neuropathological assessment of two heterozygous mice shows significant neuronal loss in the subthalamic nucleus. These studies show that the HD gene is essential for postimplantation development and that it may play an important role in normal functioning of the basal ganglia.

Dentatorubral-pallidoluysian atrophy: clinical features are closely related to unstable expansions of trinucleotide (CAG) repeat

Dentatorubral-pallidoluysian atrophy is an autosomal dominant neurodegenerative disease characterized by various combinations of ataxia, choreoathetosis, myoclonus, epilepsy, and dementia as well as a wide range of ages at onset. A specific unstable trinucleotide repeat expansion in a gene on the short arm of chromosome 12 was recently identified as the pathogenic mutation for this disease. We investigated how the degree of expansion of the CAG repeat effects the clinical manifestations of dentatorubral-pallidoluysian atrophy. The size of the expanded alleles was well correlated with the age at onset (r = -0.696, p < 0.001). Patients with the progressive myoclonus epilepsy phenotype had larger expansions (62-79 repeats) and an earlier age at onset (onset before age 21). Furthermore, most of the patients with the progressive myoclonus epilepsy phenotype inherited their expanded alleles from their affected fathers. On the other hand, patients with the non-progressive myoclonus epilepsy phenotype showed smaller expansions (54-67 repeats) and a later age at onset (onset at or after age 21). Detailed analyses of clinical features demonstrated that ataxia, involuntary movement of either myoclonus or choreoathetosis, and intellectual decline are cardinal features of dentatorubral-pallidoluysian atrophy, with myoclonus and epilepsy being observed more frequently in patients with an earlier age at onset. Thus the wide variation in clinical manifestations of dentatorubral-pallidoluysian atrophy can now be clearly explained based on the degree of CAG repeat expansion, which strongly indicates that the expanded alleles are intimately involved in the neuronal degeneration in dentatofugal and pallidofugal systems.

Trinucleotide repeats that expand in human disease form hairpin structures in vitro

We show that repeating units from all reported disease genes are capable of forming hairpins of common structure and threshold stability. The threshold stability is roughly -50 kcal per hairpin and is influenced by the flanking sequence of the gene. Hairpin stability has two components, sequence and length; only DNA of select sequences and the correct length can form hairpins of threshold energy. There is a correlation among the ability to form hairpins of threshold stability, the sequence selectivity of expansion, and the length dependence of expansion. Additionally, hairpin formation provides a potential structural basis for the constancy of the CCG region of the Huntington's disease gene in individuals and explains the stabilizing effects of AGG interruptions in FMR1 alleles.

A comparison of the Huntington's disease associated trinucleotide repeat between Chinese and white populations

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Cellular localization of the Huntington's disease protein and discrimination of the normal and mutated form

Huntington's disease (HD) results from the expansion of a polyglutamine encoding CAG repeat in a gene of unknown function. The wide expression of this transcript does not correlate with the pattern of neuropathology in HD. To study the HD gene product (huntingtin), we have developed monoclonal antibodies raised against four different regions of the protein. On western blots, these monoclonals detect the approximately 350 kD huntingtin protein in various human cell lines and in neural and non-neural rodent tissues. In cell lines from HD patients, a doublet protein is detected corresponding to the mutated and normal huntingtin. Immunohistochemical studies in the human brain using two of these antibodies detects the huntingtin in perikarya of some neurons, neuropiles, varicosities and as punctate staining likely to be nerve endings.

Exclusion mapping of the benign hereditary chorea gene from the Huntington's disease locus: report of a family

A Greek family is presented in which seven members suffered from benign hereditary chorea (a rare autosomal dominant non-progressive chorea without dementia). All patients and three informative healthy relatives were submitted to DNA analysis using probes from loci linked to Huntington's disease. The results confirm one previous suggestion that these two disorders are not allelic and that they should be considered as two distinct genetic entities.

Diagnosis of "sporadic" Huntington's disease

The diagnosis of Huntington's disease (HD) in patients with progressive chorea and mental impairment, but without similarly affected relatives, remains uncertain and impedes genetic counseling. Twenty patients with suspected HD, but with no family history of the disease underwent molecular analysis of the CAG repeat in the IT15 gene for HD. Eighteen patients displayed the HD expanded allele and two had CAG repeats in the normal range. Neuropsychological tests could be performed in 12 of the 20 patients. Of these 10 with the expanded allele presented the deficits typical of HD, but not the two patients without the HD mutation. This study shows that a neuropsychological pattern is specific to patients with the expanded CAG and that most isolated patients with suspected HD are in fact affected.

Structural analysis of the 5' region of mouse and human Huntington disease genes reveals conservation of putative promoter region and di- and trinucleotide polymorphisms

We have previously cloned and characterized the murine homologue of the Huntington disease (HD) gene and shown that it maps to mouse chromosome 5 within a region of conserved synteny with human chromosome 4p16.3. Here we present a detailed comparison of the sequence of the putative promoter and the organization of the 5' genomic region of the murine (Hdh) and human HD genes encompassing the first five exons. We show that in this region these two genes share identical exon boundaries, but have different-size introns. Two dinucleotide (CT) and one trinucleotide intronic polymorphism in Hdh and an intronic CA polymorphism in the HD gene were identified. Comparison of 940-bp sequence 5' to the putative translation start site reveals a highly conserved region (78.8% nucleotide identity) between Hdh and the HD gene from nucleotide -56 to -206 (of Hdh). Neither Hdh nor the HD gene have typical TATA or CCAAT elements, but both show one putative AP2 binding site and numerous potential Sp1 binding sites. The high sequence identity between Hdh and the HD gene for approximately 200 bp 5' to the putative translation start site indicates that these sequences may play a role in regulating expression of the Huntington disease gene.

Huntington's disease

Early in 1993, an unstable, expanded trinucleotide repeat in a novel gene of unknown function was identified on HD chromosomes. This discovery unleased a flurry of experimentation that has established the expanded CAG repeat the almost universal cause of the characteristic neurologic symptoms and pathology of this neurodegenerative disorder of midlife onset. The biochemical basis for the specific neuronal loss of HD remains uncertain, but the genetic lesion probably acts via its consequent polyglutamine segment in the protein product, huntingtin. This review will describe the basic parameters of the HD repeat's behavior and the knowledge that has accumulated concerning its potential mechanisms of action.

The cortical neuritic pathology of Huntington's disease

We have studied the brains of 10 patients with clinically and pathologically defined Huntington's disease and graded the degree of striatal pathology according to the Vonsattel grading system. Sections from nine cerebral cortical areas (Brodmann areas 8, 10, 24, 33, 28, 38, 7, 39, 18), the cerebellum, hypothalamus, medulla and caudate nucleus were stained with antibodies to ubiquitin and ubiquitin C-terminal hydrolase (PGP 9.5). Dystrophic neurites, immunoreactive with ubiquitin and PGP 9.5 were detected in all cortical areas, in layers 3, 5 and 6, of all brains studied. No dystrophic neurites were found in subcortical areas or cerebellum. Sections from cortical areas 8 and 24 from the two brains with the most and least ubiquitin-immunoreactive neurites were stained with antibodies to beta-amyloid precursor protein, tau, glial fibrillary acidic protein, neurofilament protein, alpha B crystallin, GABA, cholecystokinin and somatostatin. The dystrophic neurites were found to also react with beta-amyloid precursor protein. Electron microscopy showed the abnormal neurites to contain granulofilamentous material. Granular deposits with a diameter of 40-100 nm were interspersed between randomly orientated 'fuzzy' or coated, straight or slightly curved filaments measuring 10-15 nm in diameter. These structures have not been seen in control brain and differ from age-related neuritic degeneration and neurites associated with amyloid. Immunohistochemically these structures most resemble CA 2/3 neurites seen in Lewy body disease, and, ultrastructurally, the intraneuronal filamentous inclusions in motor neuron disease.(ABSTRACT TRUNCATED AT 250 WORDS)

Genetics and molecular biology of Huntington's disease

In 1993, the genetic abnormality responsible for Huntington's disease was identified as a trinucleotide-repeat expansion in a novel gene. Much has been learned about the molecular genetics of Huntington's disease and the possible effects of the trinucleotide expansion in the development of this disease and other neurological disorders. The Huntington's disease locus is widely expressed throughout the brain and in many non-neural tissues. Current speculation about the pathogenesis of neuronal death concentrates on a 'gain of function' effect in which the abnormal protein has acquired a new and lethal property. Future research will define the normal function of the Huntington's disease locus, test hypotheses regarding the putative gain of function, and explore the factors that determine neuronal susceptibility to the effects of the abnormal allele.

Trinucleotide repeat expansion in neurological disease

Expansion of trinucleotide repeats is now recognized as a major cause of neurological disease. At least seven disorders result from trinucleotide repeat expansion: X-linked spinal and bulbar muscular atrophy (SBMA), two fragile X syndromes of mental retardation (FRAXA and FRAXE), myotonic dystrophy, Huntington's disease, spinocerebellar ataxia type 1 (SCA1), and dentatorubral-pallidoluysian atrophy (DRPLA). The expanded trinucleotide repeats are unstable, and the phenomenon of anticipation, i.e., worsening of disease phenotype over successive generations, correlates with increasing expansion size. In this review, we compare the clinical and molecular features of the trinucleotide repeat diseases, which may be classified into two types. Fragile X and myotonic dystrophy are multisystem disorders usually associated with large expansions of untranslated repeats, while the four neurodegenerative disorders, SBMA, Huntington's disease, SCA1, and DRPLA, are caused by smaller expansions of CAG repeats within the protein coding portion of the gene. CAG repeats encode polyglutamine tracts. Polyglutamine tract expansion thus appears to be a common mechanism of inherited neurodegenerative disease. Although polyglutamine tract lengthening presumably has a toxic gain of function effect in the CAG trinucleotide repeat disorders, the basis of this neuronal toxicity remains unknown.