Trinucleotide repeat length and progression of illness in Huntington's disease

Thalamic metabolism and symptom onset in preclinical Huntington's disease

The neural basis for the transition from preclinical to symptomatic Huntington's disease (HD) is unknown. We used serial positron emission tomography (PET) imaging in preclinical HD gene carriers (p-HD) to assess the metabolic changes that occur during this period. Twelve p-HD subjects were followed longitudinally with [11C]-raclopride and [18F]-fluorodeoxyglucose PET imaging, with scans at baseline, 18 and 44 months. Progressive declines in striatal D2-receptor binding were correlated with concurrent changes in regional metabolism and in the activity of an HD-related metabolic network. We found that striatal D2 binding declined over time (P < 0.005). The activity of a reproducible HD-related metabolic covariance pattern increased between baseline and 18 months (P < 0.003) but declined at 44 months (P < 0.04). These network changes coincided with progressive declines in striatal and thalamic metabolic activity (P < 0.01). Striatal metabolism was abnormally low at all time points (P < 0.005). By contrast, thalamic metabolism was elevated at baseline (P < 0.01), but fell to subnormal levels in the p-HD subjects who developed symptoms. These findings were confirmed with an MRI-based atrophy correction for each individual PET scan. Increases in network expression and thalamic glucose metabolism may be compensatory for early neuronal losses in p-HD. Declines in these measures may herald the onset of symptoms in gene carriers.

The Impact of Huntington's Disease on Family Life

By assessing a group of adults who grew up in a household with a parent affected by Huntington's disease (HD), the authors explored the hypothesis that HD causes major disruption in family life. High rates of family dysfunction were reported. Adverse parenting in the form of parental and maternal overcontrol and paternal abuse were endorsed for both the HD-positive and HD-negative parent. These results illustrate the impact on all members of a family coping with HD. They are particularly stark, given the overall psychological health of the sample, and suggest that there is an urgent need to use a family perspective when assessing the need for psychosocial intervention in HD.

Striatal and extrastriatal atrophy in Huntington's disease and its relationship with length of the CAG repeat

Huntington's disease (HD) is an autosomal dominant neurodegenerative disorder that affects the striatum most severely. However, except for juvenile forms, relative preservation of the cerebellum has been reported. The objective of the present study was to perform MRI measurements of caudate, putamen, cerebral, and cerebellar volumes and correlate these findings with the length of the CAG repeat and clinical parameters. We evaluated 50 consecutive patients with HD using MRI volumetric measurements and compared them to normal controls. Age at onset of the disease ranged from 4 to 73 years (mean: 43.1 years). The length of the CAG repeat ranged from 40 to 69 (mean: 47.2 CAG). HD patients presented marked atrophy of the caudate and putamen, as well as reduced cerebellar and cerebral volumes. There was a significant correlation between age at onset of HD and length of the CAG repeat, as well as clinical disability and age at onset. The degree of basal ganglia atrophy correlated with the length of the CAG repeat. There was no correlation between cerebellar or cerebral volume and length of the CAG repeat. However, there was a tendency to a positive correlation between duration of disease and cerebellar atrophy. While there was a negative correlation of length of the CAG repeat with age at disease onset and with striatal degeneration, its influence on extrastriatal atrophy, including the cerebellum, was not clear. Extrastriatal atrophy occurs later in HD and may be related to disease duration.

Ethyl-EPA in Huntington disease: potentially relevant mechanism of action

The pathomechanisms involved in the neuronal dysfunction in Huntington disease (HD) are still unresolved and may be heterogeneous. One potential mechanism might be related to the induction of mitochondrial dysfunction in the CNS. This might lead firstly to neuronal dysfunction and finally to the activation of apoptotic pathways. Several compounds, which should alleviate mitochondrial dysfunction, have been tested in preclinical models as well as in clinical trials of different scale. Recently we reported the efficacy of Ethyl-eicosapentaenoic acid (Ethyl-EPA) in patients with HD. Ethyl-EPA is a polyunsaturated fatty acid from the n-3 group, which is in clinical development for HD and melancholic depression. In our trial with Ethyl-EPA in HD responding patients could be characterized by either a lower CAG repeat number or a chorea-predominant clinical expression of the disease. Here we would like to describe some evidence on the potential mechanism of action of Ethyl-EPA in HD. We specifically focus on pathways, which are known to be influenced in HD and are modified by Ethyl-EPA and which points to an involvement of mitochondrial function as a common target. Some attention is given to the NF-kappa B pathway and the c-Jun amino-terminal kinases (JNK) pathway, which both may lead to an activation of the antiproliferative factor p53 and consequently mitochondrial dysfunction. Further the effects of EPA or Ethyl-EPA in preclinical models of HD are described. The evidence from these studies led to the design of phase III clinical trials, which are ongoing.

Monozygotic Twins Suffering from Huntington’s Disease Show Different Cognitive and Behavioural Symptoms

Monozygotic male twins, carrying the same number of trinucleotide repeats in the IT 15 Huntington disease (HD) gene, showed a different clinical course. Patient 1 presented with anxiety and chorea at the age of 40. Patient 2 showed persecution paranoia and motor impersistence at the age of 42. Both patients were monitored for 30 months using currently recommended motor and behaviour scales. No differences were observed in motor scoring besides small interevaluation fluctuations. However, on the cognitive and behaviour scales, patient 1 showed a significant worsening when compared with patient 2. Our cases support the belief that the motor symptoms and signs in HD are highly dependent on the trinucleotide expansion. However, the differences in the evolution of mental status in our patients suggest that other still unknown environmental factors are important in the phenotypic expression of Huntington's disease.

Selection and slippage creating serine homopolymers

Highly repetitive sequence within proteins is an abundant feature yet is considered by some to be the protein equivalent of "junk DNA." Homopolymer sequences, the most highly repetitive of this group, are typically encoded by trinucleotide repeats at the DNA level. It is thought that many of these sequences are produced by a replicative slippage mechanism. Recent studies suggest that these highly mutable regions within proteins may allow for rapid morphological evolution emerging from the increased variability afforded by such coding structures. However, in a homopolymer, it is difficult to determine if the repeated amino acid is due to slippage at the DNA level or due to selection at the protein level. Here we develop and test a model to detect cases for which the homopolymer tract has clearly been selected for, with no evidence of slippage at the DNA level. The polyserine tract within the phosphatidylserine receptor protein is used as an excellent example of one such case.

The association of CAG repeat length with clinical progression in Huntington disease

OBJECTIVE

To determine whether the rate of clinical progression in Huntington disease (HD) is influenced by the size of the CAG expansion.

METHODS

The dataset consisted of 3,402 examinations of 512 subjects seen through the Baltimore Huntington's Disease Center. Subjects were seen for a mean of 6.64 visits, with mean follow-up of 6.74 years. Subjects were administered the Quantified Neurological Examination, with its subsets the Motor Impairment and Chorea Scores, the Mini-Mental State Examination, and the HD Activities of Daily Living (ADL) Scale.

RESULTS

In an analysis based on the Random Effects Model, CAG length was significantly associated with the rate of progression of all measures except chorea and ADL. There was a significant interaction term between CAG length and disease duration for all measures except chorea. Further graphical exploration of the data supported these linear models and suggested that subjects at the low end of the expanded CAG repeat range may experience a more benign late course.

CONCLUSIONS

CAG repeat length has a small effect on rate of progression that may be clinically important over time. Individuals with the shortest expansions appear to have the best prognosis. These effects of the CAG length may be relevant in the analysis of clinical trials.

Death of neuronal clusters contributes to variance of age at onset in Huntington's disease

Huntington's disease (HD) is a fatal neurodegenerative disease caused by an expanded polyglutamine (polyQ) repeat in the protein huntingtin. Due to selective neuronal loss in the cortex and striatum, HD patients develop various movement disturbances, psychological changes, and dementia. Symptoms usually appear in individuals between 30 and 50 years of age. The principal cause of variability of age at onset (AO) is the length of the polyQ repeat. Several additional genetic factors contributing to the variance have been identified. At least 35% of the variance, however, remains unexplained. Using a stochastic model, we show that the pattern of cell death of striatal neurons might contribute up to 20% of variance of AO.

Simple sequence in brain and nervous system specific proteins

We examined sequences expressed in the brain and nervous system using EST data. A previous study including sequences thought to have neurological function found a deficiency of simple sequence within such sequences. This was despite many examples of neurodegenerative diseases, such as Huntington disease, which are thought to be caused by expansions of polyglutamine tracts within associated protein sequences. It may be that many of the sequences thought to have neurological function have other additional, non-neurological roles. For this reason, we examined sequences with specific expression in the brain and nervous system, using EST expression data to determine if they too are deficient of simple, repetitive sequences. Indeed, we find this class of sequences to be deficient. Unexpectedly, however, we find sequences expressed in the brain and nervous system to be consistently enriched for histidine-enriched simple sequence. Determining the function of these histidine-rich regions within brain-specific proteins requires more experimental data.

Neurological proteins are not enriched for repetitive sequences

Proteins associated with disease and development of the nervous system are thought to contain repetitive, simple sequences. However, genome-wide surveys for simple sequences within proteins have revealed that repetitive peptide sequences are the most frequent shared peptide segments among eukaryotic proteins, including those of Saccharomyces cerevisiae, which has few to no specialized developmental and neurological proteins. It is therefore of interest to determine if these specialized proteins have an excess of simple sequences when compared to other sets of compositionally similar proteins. We have determined the relative abundance of simple sequences within neurological proteins and find no excess of repetitive simple sequence within this class. In fact, polyglutamine repeats that are associated with many neurodegenerative diseases are no more abundant within neurological specialized proteins than within nonneurological collections of proteins. We also examined the codon composition of serine homopolymers to determine what forces may play a role in the evolution of extended homopolymers. Codon type homogeneity tends to be favored, suggesting replicative slippage instead of selection as the main force responsible for producing these homopolymers.

Using advances in neuroimaging to detect, understand, and monitor disease progression in Huntington's disease

Transgenic mouse models and other screens are being used to identify potential therapeutic agents for use in clinical trials in Huntington's disease (HD). The development of surrogate markers that can be used in clinical therapeutics is an active area of research. Because HD is relatively uncommon and only a portion of available subjects meet inclusion and exclusion criteria, therapeutic trials are limited by the availability of potential subjects as well as the relative insensitivity of the clinical measures used. Neuroimaging methods offer the potential to provide noninvasive, reproducible, and objective methods not only to better understand the disease process but also to follow in clinical studies to determine if a drug is effective in slowing down disease progression or perhaps even in delaying onset. Following is a review of the literature, which highlights the studies that have been published to date.

Movement Disorders

This chapter covers five movement disorders: Parkinson's disease, dystonia, tic disorders, Huntington's disease, and essential tremor. These disorders demonstrate many of the most common challenges encountered in the epidemiologic investigation of movement disorders. Each section includes a description of the disease, followed by a review of descriptive studies (disease incidence, prevalence, and mortality studies), and discussion of genetic and environmental risk factors for the disorder. At the end of each section, directions for future studies are discussed.

Future directions in research with presymptomatic individuals carrying the gene for Huntington's disease

Presymptomatic individuals carrying the gene for Huntington's disease (HD) provide researchers with a unique opportunity of learning more about the neuropathophysiology, symptom onset, behavioural functioning, and mediating factors of this fatal disease. In this review, we attempt to demonstrate that research over the last 8 years, since the isolation of the gene, has remained at large controversial. Although we are aware of some of the factors that can influence age at onset and disease progression, we are still unable to determine exactly when an individual will develop HD symptoms, and how fast these symptoms will progress. In an era rapidly advancing with respect to therapeutic intervention that could forestall the onset and progression of HD, systematic research with improved inclusion criteria is paramount. A greater understanding of the time course of the disease would be beneficial not only in monitoring the effectiveness of future treatments, but also in determining the most appropriate time to administer them. Finally, we present various ethical considerations, as well as put forward various recommendations that could assist in better diagnosing preclinical deficits in presymptomatic individuals.

No post-genetics era in human disease research

In the 1980s, linkage emerged as a route to discovering genetic defects, spurring the rise of genomics and making gene-based approaches available to previously phenotype-orientated researchers. In the post-genomics era, genetics is fundamental to understanding disease at all stages of the pathogenic process.

Psychiatric symptoms and CAG repeats in neurologically asymptomatic Huntington's disease gene carriers

The putative relationship between the psychiatric profile of a sample of neurologically asymptomatic Huntington's disease gene carriers and CAG repeats was investigated. The psychiatric assessments (by consultant psychiatrist and computerised battery) were undertaken before the genetic testing was carried out. In this way, the informational distortions caused by neurological and cognitive deficits were avoided. The hypothesis that there is a relationship between psychiatric and CAG repeats was tested by seeking direct correlations between psychiatric systems and CAG repeats, and also by correcting the correlation by the number of years above or below the estimated age of onset in Huntington's disease. Scores for irritability and cognitive failures were high in the sample. There was no correlation between any psychiatric variable and CAG repeats. Possible explanations for this lack of correlations are discussed.

The selective vulnerability of nerve cells in Huntington's disease

It is now more than 7 years since the genetic mutation causing Huntington's disease (HD) was first identified. Unstable CAG expansion in the IT15 gene, responsible for disease, is translated into an abnormally long polyglutamine (polyQ) tract near the N-terminus of the huntingtin protein. The presence of expanded polyQ in the mutant protein leads to its abnormal proteolytic cleavage with liberation of toxic N-terminal fragments that tend to aggregate, probably first in the cytoplasm. Subsequent nuclear translocation of the cleaved mutant huntingtin is associated with formation of intranuclear protein aggregates and neurotoxicity, probably involving apoptotic cascades. These processes, which can be experimentally modelled in cultured neuronal and non-neuronal cells, seem to underlie neurodegeneration in HD, and also other polyQ disorders, such as dentatorubro-pallidoluysian degeneration, spinal and bulbar muscular atrophy and the spinocerebellar ataxias. They do not, however, explain why within the corpus striatum and cerebral cortex certain nerve cells are susceptible to disease and others are not. In the human HD brain, vulnerable pyramidal neurones within the deeper layers of the cerebral cortex frequently contain large intranuclear inclusions composed of N-terminal fragments of huntingtin. Such inclusions are, however, rare within neurones of the striatum, even in the medium spiny neurones preferentially lost from this region. While inclusions per se do not seem to be neurotoxic, they may provide a surrogate marker of molecular pathology. Recent studies indicate that the nuclear accumulation of mutant huntingtin interferes with transcriptional events. Of particular importance may be the effect on the genes encoding neurotransmitter receptor proteins, especially those involved with glutamatergic neurotransmission. Such changes may trigger or facilitate a low-grade, chronic excitotoxicity of the glutamatergic cortical projection neurones on their target cells in the striatum, already partly compromised by the toxic effects of the HD mutation. This combination of insults, for anatomical reasons experienced predominantly by striatal projection neurones, would eventually cause their selective demise.

Transplanted fetal striatum in Huntington's disease: phenotypic development and lack of pathology

Neural and stem cell transplantation is emerging as a potential treatment for neurodegenerative diseases. Transplantation of specific committed neuroblasts (fetal neurons) to the adult brain provides such scientific exploration of these new potential therapies. Huntington's disease (HD) is a fatal, incurable autosomal dominant (CAG repeat expansion of huntingtin protein) neurodegenerative disorder with primary neuronal pathology within the caudate-putamen (striatum). In a clinical trial of human fetal striatal tissue transplantation, one patient died 18 months after transplantation from cardiovascular disease, and postmortem histological analysis demonstrated surviving transplanted cells with typical morphology of the developing striatum. Selective markers of both striatal projection and interneurons such as dopamine and c-AMP-related phosphoprotein, calretinin, acetylcholinesterase, choline acetyltransferase, tyrosine hydroxylase, calbindin, enkephalin, and substance P showed positive transplant regions clearly innervated by host tyrosine hydroxylase fibers. There was no histological evidence of immune rejection including microglia and macrophages. Notably, neuronal protein aggregates of mutated huntingtin, which is typical HD neuropathology, were not found within the transplanted fetal tissue. Thus, although there is a genetically predetermined process causing neuronal death within the HD striatum, implanted fetal neural cells lacking the mutant HD gene may be able to replace damaged host neurons and reconstitute damaged neuronal connections. This study demonstrates that grafts derived from human fetal striatal tissue can survive, develop, and are unaffected by the disease process, at least for 18 months, after transplantation into a patient with HD.

Molecular genetics: unmasking polyglutamine triggers in neurodegenerative disease

Two decades ago, molecular genetic analysis provided a new approach for defining the roots of inherited disorders. This strategy has proved particularly powerful because, with only a description of the inheritance pattern, it can uncover previously unsuspected mechanisms of pathogenesis that are not implicated by known biological pathways or by the disease manifestations. Nowhere has the impact of molecular genetics been more evident than in the dominantly inherited neurodegenerative disorders, where eight unrelated diseases have been revealed to possess the same type of mutation--an expanded polyglutamine encoding sequence--affecting different genes.

Differences in duration of Huntington's disease based on age at onset

OBJECTIVES

Data from a sample of 2494 patients affected with Huntington's disease (HD), collected as part of the National Research Roster for Huntington Disease Patients and Families, were examined to determine if there was a relation between age at onset and duration of illness.

METHODS

Sufficient data for inclusion in analysis was available from 2068 patients, of whom 828 were deceased and 1240 were living. The median duration of disease was 21.4 years with a range of 1.2 to 40.8 years. Patients were categorised into one of four groups based on their age at onset.

RESULTS

Significant differences in duration based on the age at onset were found (p<0.025), with juvenile and late onset patients with HD having shorter duration of illness compared with those with an onset between 20-49 years.

CONCLUSIONS

Duration of disease is influenced by the age at symptom onset with juvenile and late onset patients having the shortest duration.

Unified Huntington's disease rating scale: a follow up

An objective assessment of the clinical findings in patients with Huntington's disease (HD) is necessary for an evaluation of the longitudinal progression of the disease features. The Unified Huntington's Disease Rating Scale (UHDRS) is a scale to assess clinical performance and functional capacity. The authors examined the 1-year change in UHDRS scores in 78 patients with HD examined either in Leiden, the Netherlands (24 men, 25 women), or in Rochester, New York, United States (12 men, 17 women). A significant decline was seen in motor function, measured with the total motor scale. The total dystonia score increased significantly; the total chorea score did not. The frequency of behavioral disorders tended to increase. The scores on independence scale, functional assessment, total functional capacity, and symbol digit decreased significantly. No relation was observed between the UHDRS items and the age at onset or duration of illness. Thirteen patients with 2-year follow up showed a clear increase in score on the total motor scale and a decline on the independence scale and in total functional capacity. The UHDRS may also be used as a tool for determining therapeutic intervention. Annual evaluation of the total motor scale in every patient gives a clear description of the motor progression of the disease. The authors suggest performing a total UHDRS evaluation every second year for every HD patient as part of the routine longitudinal evaluation.

Advances in Huntington's disease: implications for experimental therapeutics

The gene mutation causing Huntington's disease was identified in 1993 as an expanded trinucleotide repeat within the coding region for a 348-kd protein called huntingtin. The mechanism by which this cytosine-adenosine-guanosine repeat produces the progressive signs and symptoms of Huntington's disease remains uncertain, but recent advances have begun to provide insights into this process. Promising developments include transgenic mouse models of Huntington's disease with neuronal intranuclear inclusions, the identification of differential neuronal features which might account for the selective vulnerability of neurons seen in Huntington's disease and further evidence for the role of excitotoxicity and impaired mitochondrial energy production. These observations have suggested new therapeutic strategies, and have lent further support for experimental therapeutics aimed at improving mitochondrial function and reducing excitotoxic injury.

Spinocerebellar ataxia type 6: gaze-evoked and vertical nystagmus, Purkinje cell degeneration, and variable age of onset

Spinocerebellar ataxia type 6 (SCA6) was recently identified as a form of autosomal dominant cerebellar ataxia associated with small expansions of the trinucleotide repeat (CAG)n in the gene CACNL1A4 on chromosome 19p13, which encodes the alpha1 subunit of a P/Q-type voltage-gated calcium channel. We describe clinical, genetic, neuroimaging, neuropathological, and quantitative oculomotor studies in four kindreds with SCA6. We found strong genetic linkage of the disease to the CACNL1A4 locus and strong association with the expanded (CAG)n alleles in two large ataxia kindreds. The expanded alleles were all of a single size (repeat number) within the two large kindreds, numbering 22 and 23 repeat units. It is noteworthy that the age of onset of ataxia ranged from 24 to 63 years among all affected individuals, despite the uniform repeat number. Radiographically and pathologically, there was selective atrophy of the cerebellum and extensive loss of Purkinje cells in the cerebellar cortex. In addition, clinical and quantitative measurement of extraocular movements demonstrated a characteristic pattern of ocular motor and vestibular abnormalities, including horizontal and vertical nystagmus and an abnormal vestibulo-ocular reflex. These studies identify a distinct phenotype associated with this newly recognized form of dominant SCA.

Juvenile Huntington disease in the Netherlands

Juvenile Huntington disease (JHD) patients are distinguished from adult patients by an age at onset of less than 20 years. Investigating patients in our own database, we examined the proposition derived from studies in world literature that JHD should not be viewed as a separate clinical entity but rather as a manifestation of the rigid variant of the disease. Of 53 patients with JHD recorded in the Leiden Roster for Huntington Disease, relationships between sex, age at onset, duration of illness, maternal or paternal inheritance, motor symptom, first clinical features, and characteristics during the disease course, were obtained from the patients' files, and investigated. Although chorea is present in JHD, patients more often developed rigidity. Paternal inheritance, early dementia, epilepsy/myoclonus, and tremor during the disease course are confined for the most part to the rigid cases. A shorter duration of illness was evident in male patients with rigid JHD who inherited the disease from their father and developed their first disease feature at a younger age. The recognition of JHD as a distinct clinical entity does not appear to be warranted. Therefore, we propose, in accordance with other investigators, that rigid JHD be considered a clinical variant with special features.

The likelihood of being affected with Huntington disease by a particular age, for a specific CAG size

Prior studies describing the relationship between CAG size and the age at onset of Huntington disease (HD) have focused on affected persons. To further define the relationship between CAG repeat size and age at onset of HD, we now have analyzed a large cohort of affected and asymptomatic at-risk persons with CAG expansion. This cohort numbered 1,049 persons, including 321 at-risk and 728 affected individuals with a CAG size of 29-121 repeats. Kaplan-Meier analysis has provided curves for determining the likelihood of onset at a given age, for each CAG repeat length in the 39-50 range. The curves were significantly different (P < .0005), with relatively narrow 95% confidence intervals (95% CI) (+/-10%). Penetrance of the mutation for HD also was examined. Although complete penetrance of HD was observed for CAG sizes of > or = 42, only a proportion of those with a CAG repeat length of 36-41 showed signs or symptoms of HD within a normal life span. These data provide information concerning the likelihood of being affected, by a specific age, with a particular CAG size, and they may be useful in predictive-testing programs and for the design of clinical trials for persons at increased risk for HD.

CAG repeat number governs the development rate of pathology in Huntington's disease

We compared the number of CAG repeats, the age at death, and the severity of neuropathology in 89 Huntington's disease brains. We found a linear correlation between the CAG repeat number and the quotient of the degree of atrophy in the striatum (the brain region most severely affected in Huntington's disease) divided by age at death, with an intercept at 35.5 repeats. The largest CAG repeat length, therefore, at which no pathology is expected to develop is 35.5. These results imply that striatal damage in Huntington's disease is almost entirely a linear function of the length of the polyglutamine stretch beyond 35.5 glutamines multiplied by the age of the patient. Thus, it is predicted that the pathological process develops linearly from birth. Analysis of other measures of striatal function could test this hypothesis and might determine when treatment for CAG repeat diseases should start.

The genetic defect causing Huntington's disease: repeated in other contexts?

Images

Clinical presentation and patterns of regional cerebral atrophy related to the length of trinucleotide repeat expansion in patients with adult onset Huntington's disease

We correlated trinucleotide CAG repeat numbers in the huntingtin gene with the regional brain atrophy and clinical phenotype in 23 adult autopsy cases of Huntington's disease (HD). CAG repeat number (39-56, mean 45.4 +/- 4.6) correlated inversely (P < 0.0001) with age at onset and death, but not with disease duration or initial symptoms. Cross-sectional areas of the striatum, pallidum, thalamus, amygdala, hippocampus, and the cortical grey and white matter within the frontal, temporal and parietal lobes at four levels (genu of the corpus callosum, amygdala, accumbens, hippocampus) were measured morphometrically from the coronal brain slices using image analysis. None of these morphometric variables correlated with number of CAG repeats. Thus, tissue atrophy in advanced HD is unrelated to the underlying genetic defect.

Which diagnostic procedures in the elderly? The case of late-onset Huntington's disease

The typical adult-onset form of Huntington's disease (HD) is a clinical condition in which the latest advances of genetic research can be usefully applied during the course of the diagnostic process; not so clear are the guidelines for the much less frequent late-onset variant. We have recently seen three patients in their late sixties who had been misdiagnosed for up to 10 years due to the apparently isolated, mild, and slowly progressive nature of their hyperkinetic movements or cognitive disorders. Only after the results of DNA sequencing on a blood sample became available could the appropriate diagnosis of late-onset HD be reached. By contrast, neuroimaging studies lacked sufficient sensitivity and specificity. Appropriate neurogeriatric assessment in these cases should follow specific guidelines and should always include selected high-technology procedures.

Huntington disease--another chapter rewritten

To those of us who began life when humans had 48 chromosomes and who began working in genetics when the (by then 46) chromosomes had no bands and chromosome 4 could not reliably be distinguished from chromosome 5, the mere ability to diagnose and correlate the clinical phenotypes of genetic disorders with their molecular genotypes is a source of continuing astonishment and pleasure. Indeed, molecular genetic analysis of neurogenetic disorders such as Huntington disease (HD) has provided a steady stream of challenges and surprises to all who believe the genetic principles that they were taught about these disorders. The paper by Rubinsztein et al. in this issue of the journal highlights yet another surprise, which was adumbrated even in the initial paper announcing the discovery of the HD gene: incomplete penetrance of HD gene mutations.

Mental retardation: genetic findings, clinical implications and research agenda

The most important genetic advances in the field of mental retardation include the discovery of the novel genetic mechanism responsible for the Fragile X syndrome, and the imprinting involved in the Prader-Willi and Angelman syndromes, but there have also been advances in our understanding of the pathogenesis of Down syndrome and phenylketonuria. Genetic defects (both single gene Mendelizing disorders and cytogenetic abnormalities) are involved in a substantial proportion of cases of mild as well as severe mental retardation, indicating that the previous equating of severe mental retardation with pathology, and of mild retardation with normal variation, is a misleading over-simplication. Within the group in which no pathological cause can be detected, behaviour genetic studies indicate that genetic influences are important, but that their interplay with environmental factors, which are also important, is at present poorly understood. Research into the joint action of genetic and environmental influences in this group will be an important research area in the future.

Psychiatric symptoms and CAG expansion in Huntington's disease

The mutation responsible for Huntington's disease (HD) is an elongated CAG repeat in the coding region of the IT15 gene. A PCR-based test with high sensitivity and accuracy is now available to identify asymptomatic gene carriers and patients. An inverse correlation between CAG copy number and age at disease onset has been found in a large number of affected individuals. The influence of the CAG repeat expansion on other phenotypic manifestations, especially specific psychiatric symptoms has not been studied intensively. In order to elucidate this situation we investigated the relation between CAG copy number and distinct psychiatric phenotypes found in 79 HD-patients. None of the four differentiated categories (personality change, psychosis, depression, and nonspecific alterations) showed significant differences in respect to size of the CAG expansion. In addition, no influence of individual sex on psychiatric presentation could be found. On the other hand in patients with personality changes maternal transmission was significantly more frequent compared with all other groups. Therefore we suggest that clinical severity of psychiatric features in HD is not directly dependent on the size of the dynamic mutation involved. The complex pathogenetic mechanisms leading to psychiatric alterations are still unknown and thus genotyping does not provide information about expected psychiatric symptoms in HD gene carriers.

Autism: towards an integration of clinical, genetic, neuropsychological, and neurobiological perspectives

Autism constitutes one of the best validated child psychiatric disorders. Empirical research has succeeded in delineating the key clinical phenomena, in demonstrating strong genetic influences on the underlying liability, and in identifying basic cognitive deficits. A range of neurobiological abnormalities has also been found, although the replicability of specific findings has not been high. An understanding of the causal processes leading to autism, and accounting for the marked variability in its manifestations, requires an integration across these different levels of enquiry. Although this is not yet possible, a partial integration provides a useful strategy for identifying key research questions, the limitations of existing hypotheses, and future research directions that are likely to prove fruitful. The research findings for each research level are critically reviewed in order to consider how to move towards an integration across levels.

Relationship between trinucleotide repeats and neuropathological changes in Huntington's disease

The discovery of the Huntington's disease (HD) gene has provided the impetus to determine the association between the triplet repeat sequences and clinical manifestations of the disease. The present study is directed toward determining the relationship between the triplet repeat sequences and severity of the neurodegenerative process. Nineteen HD postmortem cases were evaluated for neuropathological changes as well as for the number of trinucleotide repeat sequences, each in a blinded fashion. Each case was assigned a gross grade according to the scale of Vonsattel and colleagues (1985); neuronal counts were then performed on both the caudate and the putamen. For 7 of the postmortem cases, blood had been collected prior to death and was analyzed for the HD gene. For the 12 remaining cases for which blood was unavailable, DNA from the frontal neocortex and striatum was extracted from frozen or formalin-fixed paraffinized tissue and subsequently analyzed for the HD gene. When correlation was made for age at death, greater numbers of trinucleotide repeats were associated with greater neuronal loss, in both the caudate (r = 0.9641, p < 0.001) and the putamen (r = 0.9652, p < 0.001). When correction was made for disease duration, the correlation was again significant, for both the caudate (r = 0.6396, p < 0.01) and the putamen (r = 0.6710, p < 0.001). This suggests that in HD, longer trinucleotide repeat length is associated with a faster rate of deterioration and greater pathological severity. A comparison of trinucleotide repeat length in different brain regions in 4 of the HD postmortem cases associated with greater numbers of repeats consistently demonstrated fewer repeats in the cerebellum than in the frontal cortex, striatum or blood.

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.

Complete penetrance of Creutzfeldt-Jakob disease in Libyan Jews carrying the E200K mutation in the prion protein gene

BACKGROUND

Creutzfeldt-Jakob disease (CJD) is a prion disease which is manifest as a sporadic, inherited, and transmissible neurodegenerative disorder. The mean age at onset of CJD is approximately 60 years, and as such, many people destined to succumb undoubtedly die of other illnesses first. The delayed onset of CJD has complicated the analysis of inherited forms of the illness and led to the suggestion that mutations in the prion protein (PrP) gene are necessary but not sufficient for prion disease despite genetic linkage; indeed, an environmental factor such as a ubiquitous virus has been proposed as a second necessary factor.

MATERIALS AND METHODS

To examine what appeared to be incomplete penetrance, we applied a life-table analysis to clinical and pedigree data from a cluster population of Libyan Jews in which the E200K mutation is prevalent. The study population included 42 affected and 44 unaffected members of 13 Libyan Jewish families, all of whom possessed the E200K mutation.

RESULTS

The calculated value using life table analysis is 0.77 at age 70 which increases to 0.89 if a mutation carrier survives to age 80 and 0.96 if age 80 is surpassed.

CONCLUSIONS

These data argue that the E200K mutation alone is sufficient to cause prion disease and does so in an age-dependent manner.

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.