Family and molecular data for a fine analysis of age at onset in Huntington disease

The Association between CAG Repeat Length and Age of Onset of Juvenile-Onset Huntington's Disease

There is a known negative association between cytosine-adenine-guanine (CAG) repeat length and the age of motor onset (AMO) in adult-onset Huntington's Disease (AOHD). This relationship is less clear in patients with juvenile-onset Huntington's disease (JOHD), however, given the rarity of this patient population. The aim of this study was to investigate this relationship amongst a relatively large group of patients with JOHD using data from the Kids-JOHD study. Additionally, we analyzed data from the Enroll-HD platform and the Predict-HD study to compare the relationship between CAG repeat length and AMO amongst patients with AOHD to that amongst patients with JOHD using linear regression models. In line with previous reports, the variance in AMO that was predicted by CAG repeat length was 59% (p < 0.0001) in the Predict-HD study and 57% from the Enroll-HD platform (p < 0.0001). However, CAG repeat length predicted 84% of the variance in AMO amongst participants from the Kids-JOHD study (p < 0.0001). These results indicate that there may be a stronger relationship between CAG repeat length and AMO in patients with JOHD as compared to patients with AOHD. These results provide additional information that may help to model disease progression of JOHD, which is beneficial for the planning and implementation of future clinical trials.

Risk factors for the onset and progression of Huntington disease

Huntington's disease (HD) is an autosomal dominant neurodegenerative disorder characterized by chorea, behavioural and psychiatric manifestations, and dementia, caused by a CAG triplet repeat expansion in the huntingtin gene. Systematic review of the literature was conducted to determine the risk factors for the onset and progression of HD. Multiple databases were searched, using terms specific to Huntington disease and to studies of aetiology, risk, prevention and genetics, limited to studies on human subjects published in English or French between 1950 and 2010. Two reviewers independently screened the abstracts and identified potentially relevant articles for full-text review using predetermined inclusion criteria. Three major categories of risk factors for onset of HD were identified: CAG repeat length in the huntingtin gene, CAG instability, and genetic modifiers. Of these, CAG repeat length in the huntingtin gene is the most important risk factor. For the progression of HD: genetic, demographic, past medical/clinical and environmental risk factors have been studied. Of these factors, genetic factors appear to play the most important role in the progression of HD. Among the potential risk factors, CAG repeat length in the mutant allele was found to be a relatively consistent and significant risk factor for the progression of HD, especially in motor, cognitive, and other neurological symptom deterioration. In addition, there were many consistent results in the literature indicating that a higher number of CAG repeats was associated with shorter survival, faster institutionalization, and earlier percutaneous endoscopic gastrostomy.

A Drosophila model of Huntington disease-like 2 exhibits nuclear toxicity and distinct pathogenic mechanisms from Huntington disease

Huntington disease-like 2 (HDL2) and Huntington disease (HD) are adult-onset neurodegenerative diseases characterized by movement disorders, psychiatric disturbances and cognitive decline. Brain tissue from HD and HDL2 patients shows degeneration of the striatum and ubiquitinated inclusions immunoreactive for polyglutamine (polyQ) antibodies. Despite these similarities, the diseases result from different genetic mutations. HD is caused by a CAG repeat expansion in the huntingtin (HTT) gene, while HDL2 results from an expansion at the junctophilin 3 (JPH3) locus. Recent evidence indicates that the HDL2 expansion may give rise to a toxic polyQ protein translated from an antisense mRNA derived from the JPH3 locus. To investigate this hypothesis, we generated and characterized a Drosophila HDL2 model and compared it with a previously established HD model. We find that neuronal expression of HDL2-Q15 is not toxic, while the expression of an expanded HDL2-Q138 protein is lethal. HDL2-Q138 forms large nuclear aggregates, with only smaller puncta observed in the cytoplasm. This is in contrast to what is observed in a Drosophila model of HD, where polyQ aggregates localize exclusively to the cytoplasm. Altering localization of HLD2 with the addition of a nuclear localization or nuclear export sequence demonstrates that nuclear accumulation is required for toxicity in the Drosophila HDL2 model. Directing HDL2-Q138 to the nucleus exacerbates toxicity in multiple tissue types, while confining HDL2-Q138 to the cytoplasm restores viability to control levels. We conclude that while HD and HDL2 have similar clinical profiles, distinct pathogenic mechanisms are likely to drive toxicity in Drosophila models of these disorders.

Motor onset and diagnosis in Huntington disease using the diagnostic confidence level

Huntington disease (HD) is a neurodegenerative disorder characterized by motor dysfunction, cognitive deterioration, and psychiatric symptoms, with progressive motor impairments being a prominent feature. The primary objectives of this study are to delineate the disease course of motor function in HD, to provide estimates of the onset of motor impairments and motor diagnosis, and to examine the effects of genetic and demographic variables on the progression of motor impairments. Data from an international multisite, longitudinal observational study of 905 prodromal HD participants with cytosine-adenine-guanine (CAG) repeats of at least 36 and with at least two visits during the followup period from 2001 to 2012 was examined for changes in the diagnostic confidence level from the Unified Huntington's Disease Rating Scale. HD progression from unimpaired to impaired motor function, as well as the progression from motor impairment to diagnosis, was associated with the linear effect of age and CAG repeat length. Specifically, for every 1-year increase in age, the risk of transition in diagnostic confidence level increased by 11% (95% CI 7-15%) and for one repeat length increase in CAG, the risk of transition in diagnostic confidence level increased by 47% (95% CI 27-69%). Findings show that CAG repeat length and age increased the likelihood of the first onset of motor impairment as well as the age at diagnosis. Results suggest that more accurate estimates of HD onset age can be obtained by incorporating the current status of diagnostic confidence level into predictive models.

Ascertainment bias causes false signal of anticipation in genetic prion disease

Anticipation is the phenomenon whereby age of onset in genetic disease decreases in successive generations. Three independent reports have claimed anticipation in Creutzfeldt-Jakob disease (CJD) caused by the c.598G > A mutation in PRNP encoding a p.Glu200Lys (E200K) substitution in the prion protein. If confirmed, this finding would carry clear implications for genetic counseling. We analyzed pedigrees with this mutation from four prion centers worldwide (n = 217 individuals with the mutation) to analyze age of onset and death in affected and censored individuals. We show through simulation that selective ascertainment of individuals whose onset falls within the historical window since the mutation's 1989 discovery is sufficient to create robust false signals both of anticipation and of heritability of age of onset. In our data set, the number of years of anticipation observed depends upon how strictly the data are limited by the ascertainment window. Among individuals whose disease was directly observed at a study center, a 28-year difference between parent and child age of onset is observed (p = 0.002), but including individuals ascertained retrospectively through family history reduces this figure to 7 years (p = 0.005). Applying survival analysis to the most thoroughly ascertained subset of data eliminates the signal of anticipation. Moreover, even non-CJD deaths exhibit 16 years anticipation (p = 0.002), indicating that ascertainment bias can entirely explain observed anticipation. We suggest that reports of anticipation in genetic prion disease are driven entirely by ascertainment bias. Guidelines for future studies claiming statistical evidence for anticipation are suggested.

Huntington's disease: how intermediate are intermediate repeat lengths?

BACKGROUND

Huntington's disease (HD) is a devastating heredoneurodegenerative disorder associated with a wide variety of neurological and psychiatric symptoms caused by an expanded CAG repeat in the HTT gene. The expansion mutation in HTT is dominantly transmitted and codes for a protein named huntingtin (htt).

HYPOTHESIS

One hypothesis, according to a multistep mechanism, is that the intergenerational transmission of the normal repeat size causes small, progressive CAG stretch elongations in the general population from one generation to another, until a critical pathological CAG repeat threshold is reached. Mutations may originate in the offspring from paternally transmitted CAG repeats, falling within an intermediate alleles (IA) range of 27 to 35 in repeat length.

CONCLUSIONS

There has been emerging evidence that some individuals with IAs might develop an HD phenotype. This presents a challenge for genetic counseling, because these individuals are often reassured that they are "disease free." However, there are many unanswered questions related to the role of IAs in the development of the HD phenotype and in the pathogenesis of HD.

Genetic background modulates behavioral impairments in R6/2 mice and suggests a role for dominant genetic modifiers in Huntington’s disease pathogenesis

Variability and modification of the symptoms of Huntington’s disease (HD) are commonly observed in both patient populations and animal models of the disease. Utilizing a stable line of the R6/2 HD mouse model, the present study investigated the role of genetic background in the onset and severity of HD symptoms in a transgenic mouse. R6/2 congenic C57BL/6J and C57BL/6J × DBA/2J F1 (B6D2F1) mice were evaluated for survival and a number of behavioral phenotypes. This study reports that the presence of the DBA/2J allele results in amelioration or exacerbation of several HD-like phenotypes characteristic of the R6/2 mouse model and indicates the presence of dominant genetic modifiers of HD symptoms. This study is the first step in identifying genes that confer natural genetic variation and modify the HD symptoms. This identification may lead to novel targets for treatment and help elucidate the molecular mechanisms of HD pathogenesis.

18F-FDG PET uptake in the pre-Huntington disease caudate affects the time-to-onset independently of CAG expansion size

PURPOSE

To test in a longitudinal follow-up study whether basal glucose metabolism in subjects with a genetic risk of Huntington disease (HD) may influence the onset of manifest symptoms.

METHODS

The study group comprised 43 presymptomatic (preHD) subjects carrying the HD mutation. They underwent a (18)F-FDG PET scan and were prospectively followed-up for at least 5 years using the unified HD rating scale to detect clinical changes. Multiple regression analysis included subject's age, CAG mutation size and glucose uptake as variables in a model to predict age at onset.

RESULTS

Of the 43 preHD subjects who manifested motor symptoms, suggestive of HD, after 5 years from the PET scan, 26 showed a mean brain glucose uptake below the cut-off of 1.0493 in the caudate, significantly lower than the 17 preHD subjects who remained symptom-free (P < 0.0001). This difference was independent of mutation size. Measurement of brain glucose uptake improved the CAG repeat number and age-based model for predicting age at onset by 37 %.

CONCLUSION

A reduced level of glucose metabolism in the brain caudate may represent a predisposing factor that contributes to the age at onset of HD in preHD subjects, in addition to the mutation size.

Indexing disease progression at study entry with individuals at-risk for Huntington disease

The identification of clinical and biological markers of disease in persons at risk for Huntington disease (HD) has increased in efforts to better quantify and characterize the epoch of prodrome prior to clinical diagnosis. Such efforts are critical in the design and implementation of clinical trials for HD so that interventions can occur at a time most likely to increase neuronal survival and maximize daily functioning. A prime consideration in the examination of prodromal individuals is their proximity to diagnosis. It is necessary to quantify proximity so that individual differences in key marker variables can be properly interpreted. We take a data-driven approach to develop an index that can be viewed as a proxy for time to HD diagnosis known as the CAG-Age Product Scaled or CAP(S) . CAP(S) is an observed utility variable computed for all genetically at-risk individuals based on age at study entry and CAG repeat length. Results of a longitudinal receiver operating characteristic (ROC) analysis showed that CAP(S) had a relatively strong ability to predict individuals who became diagnosed, especially in the first 2 years. Bootstrap validation provided evidence that CAP(S) computed on a new sample from the same population could have similar discriminatory power. Cutoffs for the empirical CAP(S) distribution can be used to create a classification for mutation-positive individuals (Low-Med-High), which is, useful for comparison with the naturally occurring mutation-negative Control group. The classification is an improvement over the one currently in use as it is based on observed data rather than model-based estimated values. © 2011 Wiley-Liss, Inc.

CAG-repeat length and the age of onset in Huntington disease (HD): a review and validation study of statistical approaches

CAG-repeat length in the gene for HD is inversely correlated with age of onset (AOO). A number of statistical models elucidating the relationship between CAG length and AOO have recently been published. In the present article, we review the published formulae, summarize essential differences in participant sources, statistical methodologies, and predictive results. We argue that unrepresentative sampling and failure to use appropriate survival analysis methodology may have substantially biased much of the literature. We also explain why the survival analysis perspective is necessary if any such model is to undergo prospective validation. We use prospective diagnostic data from the PREDICT-HD longitudinal study of CAG-expanded participants to test conditional predictions derived from two survival models of AOO of HD. A prior model of the relationship of CAG and AOO originally published by Langbehn et al. yields reasonably accurate predictions, while a similar model by Gutierrez and MacDonald substantially overestimates diagnosis risk for all but the highest risk participants in this sample. The Langbehn et al. model appears accurate enough to have substantial utility in various research contexts. We also emphasize remaining caveats, many of which are relevant for any direct application to genetic counseling.

Protein aggregation as a paradigm of aging

The process of physiological decline leading to death of the individual is driven by the deteriorating capacity to withstand extrinsic and intrinsic hazards, resulting in damage accumulation with age. The dynamic changes with time of the network governing the outcome of misfolded proteins, exemplifying as intrinsic hazards, is considered here as a paradigm of aging. The main features of the network, namely, the non-linear increase of damage and the presence of amplifying feedback loops within the system are presented through a survey of the different components of the network and related cellular processes in aging and disease.

Distinct brain volume changes correlating with clinical stage, disease progression rate, mutation size, and age at onset prediction as early biomarkers of brain atrophy in Huntington's disease

Searching brain and peripheral biomarkers is a requisite to cure Huntington's disease (HD). To search for markers indicating the rate of brain neurodegenerative changes in the various disease stages, we quantified changes in brain atrophy in subjects with HD. We analyzed the cross-sectional and longitudinal rate of brain atrophy, quantitatively measured by fully-automated multiparametric magnetic resonance imaging, as fractional gray matter (GM, determining brain cortex volume), white matter (WM, measuring the volume of axonal fibers), and corresponding cerebral spinal fluid (CSF, a measure of global brain atrophy), in 94 gene-positive subjects with presymptomatic to advanced HD, and age-matched healthy controls. Each of the three brain compartments we studied (WM, GM, and CSF) had a diverse role and their time courses differed in the development of HD. GM volume decreased early in life. Its decrease was associated with decreased serum brain-derived-neurotrophic-factor and started even many years before onset symptoms, then decreased slowly in a nonlinear manner during the various symptomatic HD stages. WM volume loss also began in the presymptomatic stage of HD a few years before manifest symptoms appear, rapidly decreasing near to the zone-of-onset. Finally, the CSF volume increase began many years before age at onset. Its volume measured in presymptomatic subjects contributed to improve the CAG-based model of age at onset prediction. The progressive CSF increase depended on CAG mutation size and continued linearly until the last stages of HD, perhaps representing the best marker of progression rate and severity in HD (R(2)= 0.25, P < 0.0001).

CAG repeat lengths > or =335 attenuate the phenotype in the R6/2 Huntington's disease transgenic mouse

With spontaneous elongation of the CAG repeat in the R6/2 transgene to > or =335, resulting in a transgene protein too large for passive entry into nuclei via the nuclear pore, we observed an abrupt increase in lifespan to >20 weeks, compared to the 12 weeks common in R6/2 mice with 150 repeats. In the > or =335 CAG mice, large ubiquitinated aggregates of mutant protein were common in neuronal dendrites and perikaryal cytoplasm, but intranuclear aggregates were small and infrequent. Message and protein for the > or =335 CAG transgene were reduced to one-third that in 150 CAG R6/2 mice. Neurological and neurochemical abnormalities were delayed in onset and less severe than in 150 CAG R6/2 mice. These findings suggest that polyQ length and pathogenicity in Huntington's disease may not be linearly related, and pathogenicity may be less severe with extreme repeats. Both diminished mutant protein and reduced nuclear entry may contribute to phenotype attenuation.

Functional genomics approaches to neurodegenerative diseases

Many of the neurodegenerative diseases that afflict humans are characterised by the protein aggregation in neurons. These include complex diseases like Alzheimer's disease and Parkinson's disease, and Mendelian diseases caused by polyglutamine expansion mutations [like Huntington's disease (HD) and various spinocerebellar ataxias (SCAs), like SCA3]. A range of functional genomic strategies have been used to try to elucidate pathways involved in these diseases. In this minireview, I focus on how modifier screens in organisms from yeast to mice may be of value in helping to elucidate pathogenic pathways.

NR2A and NR2B receptor gene variations modify age at onset in Huntington disease in a sex-specific manner

In addition to the pathogenetic CAG repeat expansion other genetic factors play a significant role in determining age at onset (AO) in Huntington disease (HD), e.g. variations in the NR2A and NR2B glutamate receptor subunit genes (GRIN2A, GRIN2B). In order to expand these findings we fine-mapped a larger HD patient panel (n = 250) using densely spaced markers flanking the originally associated SNPs in GRIN2A and GRIN2B. In GRIN2A association fine-mapping based on eight additional SNPs confirmed intron 2 as the region of strongest association. In GRIN2B fine-mapping with seven additional SNPs consolidated C2664T as causal genetic variation. Gender stratification of patients revealed differences in the variability in AO attributable to the CAG repeat number and highly significant differences in the AO association with the C2664T and rs8057394/ rs2650427 variations. Addition of the corresponding genotype variations to the effect of CAG repeat lengths resulted in a significant increase of the R2 values only in females. The sex-specific effect for C2664T is underscored by differences in the genotype and allele frequencies observed for female versus male HD patients (P = 0.01) caused by decreased CC frequency in females. Overall, female HD patients homozygous for the CC genotype tended to have later AO compared to the other two genotypes. Stratification of the results by presumed menopausal status demonstrated that the significant findings were predominantly observed in pre-menopausal patients. We speculate that altered hormone levels herald protective effects of this genotype. Together, GRIN2A and GRIN2B genotype variations explain 7.2% additional variance in AO for HD.

Reduced penetrance alleles for Huntington's disease: a multi-centre direct observational study

OBJECTIVE

To obtain penetrance data for Huntington's disease when DNA results are in the range of 36-39 CAG repeats and assess the consistency of reporting the upper allele from two reference centres.

METHOD

Data were collected anonymously on age of onset or age last known to be unaffected from a cohort of individuals with results in this range. DNA samples were re-analysed in two reference centres. Kaplan-Meier analysis was used to construct an age of onset curve and penetrance figures.

RESULTS

Clinical data and concordant DNA results from both reference centres were available for 176 samples; penetrance figures (and 95% confidence intervals) for this cohort, at age 65 and 75 years, were 63.9% (55.5% to 73.2%) and 74.2% (64.2% to 84.2%), respectively. Inclusion of 28 additional subjects for whom repeat DNA results were unavailable, obtained from only one reference centre, or discrepant by one repeat within this range, gave penetrance data (including 95% confidence intervals) at ages 65 and 75 years of 62.4% (54.4% to 70.4%) and 72.7.% (63.3% to 82.1%), respectively. 238 duplicate results were available from the reference centres; 10 (4.2%) differed by one CAG repeat in the reporting of the upper allele and in two (0.84%) of these cases the discrepancy was between 39 and 40 repeats.

CONCLUSION

When DNA results are in this range, a conservative approach is to say that there is at least a 40% chance the person will be asymptomatic at age 65 years and at least a 30% chance the person will be asymptomatic at age 75 years.

No evidence of association between BDNF gene variants and age-at-onset of Huntington's disease

Huntington's disease (HD) is a late-onset, autosomal dominant neurodegenerative disease caused by a CAG trinucleotide expansion. The number of repeats on the HD chromosome explains most of the variability in age of onset, but genetic factors other than the HD gene are responsible for part of the residual variance. Based on the role played by the brain derived neurotrophic factor (BDNF) in neurodysfunction and neurodegeneration in HD, we searched for novel polymorphisms in the neuron restrictive silencer element located in the BDNF promoter. Then, the effect of the Val66Met variant in determining age of onset was tested in a large sample of HD carriers by using a multivariate regression approach. The CAG repeat number accounted for 62% of the variance. After correction for the predominant effect of the CAG expansion, no multiple regression model provided evidence of association between the Val66Met genotype and variation in age-at-onset. Additional studies are warranted to further investigate BDNF as genetic modifier of the HD phenotype.

The search for cerebral biomarkers of Huntington's disease: a review of genetic models of age at onset prediction

The mutation causing Huntington's disease is an expanded CAG trinucleotide repeat number beyond 35 in the 5' translated region of the gene. The mutation penetrance varies widely and depends on the CAG expansion length, the low pathological triplet range (36-41) showing a very low penetrance, possibly associated with late ages at onset. No research has so far yielded biomarkers for accurately predicting either age at onset or disease progression in at risk individuals. Specific markers able to follow-up mutation carrier subjects from the pre-symptomatic stages of life are crucial for testing experimental neuroprotective preventive therapies. Nevertheless, the factor accounting for the largest percentage of age at onset variation is the expanded repeat number within the gene. Over the years, this factor has helped in setting up models for genetically predicting age at onset. Once available for practical application in clinics, such models allowed phenotype-genotype correlations that were hitherto inconceivable. In this review, we discuss how these genetic models have been applied in clinical practice and comment on their potential value in searching for cerebral biomarkers of disease onset and severity and in designing trials of therapeutic drugs.

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.

Juvenile Huntington's disease: does a dosage-effect pathogenic mechanism differ from the classical adult disease?

Huntington's disease (HD) is caused by a CAG repeat mutation translating as a polyglutamine (poly(Q)) expansion in the huntingtin protein, whose main pathogenic mechanism is a gain of toxic function. In the case of large expansions beyond 60 repeats onset may result in juvenile HD (JHD, onset before 20 years of age). However, the triplet number does not represent the only onset modifier even in case of large expansions, mechanisms other than the size of the mutation contribute to the phenotype. In this review we discuss the possibility that some of the pathogenic mechanisms contributing to age at onset and progression may differ in the early onset HD compared with the classical adult pathology.

Adenosine A2A receptor dysfunction correlates with age at onset anticipation in blood platelets of subjects with Huntington's disease

Huntington's disease (HD) may manifest at an earlier age in affected offspring than in transmitting parents. Earlier onset in successive generations (anticipation) only partially depends on intergenerational parent-child elongation of the CAG expanded mutation. An aberrant amplification of adenosine A(2A) receptor signaling documented in peripheral blood cells of subjects with HD implies that this cellular dysfunction may be related to clinical and genetic features. Prompted by evidence of higher receptor densities in siblings of HD subjects with stronger onset anticipation, in this study we investigated a possible relationship between A(2A) receptor densities and age at onset. We measured adenosine A(2A) receptor densities in blood cell platelets from 32 patients with HD and healthy control siblings, and sought a possible linear correlation between maximum platelet A(2A) receptor binding (B(max)) values for the whole cohort of HD subjects and anticipation in years. The increased B(max) values for the 32 subjects with HD (220 in patients vs. 137 in healthy control subjects, P = 0.0001) correlated significantly with anticipation in years (r2, 0.48, P = 0.0001 by linear correlation analysis). An increased platelet A(2A) receptor B(max) may belong in a cascade of toxic events leading to earlier onset of HD: as such it could be a useful marker of onset anticipation.

Levels of mutant huntingtin influence the phenotypic severity of Huntington disease in YAC128 mouse models

Huntington disease (HD) is a devastating neuropsychiatric disease caused by expansion of a trinucleotide repeat (CAG) in the HD gene. Neuropathological changes include the appearance of N-terminal huntingtin fragments, decreased brain weight and apoptotic neuronal loss in a select subset of neurons located in the striatum. There is still controversy over whether homozygosity for the mutation in HD is associated with a more severe phenotype. In humans, resolution of this issue has been complicated by the small number of homozygous patients and difficulty in the definition of reliable phenotypic endpoints. In order to definitively determine whether there is a correlation between phenotypic severity and expression levels of mutant huntingtin, we undertook a behavioral and neuropathological assessment of YAC128 mice with varying levels of mutant huntingtin. The results reveal a clear relationship between levels of mutant huntingtin and phenotype defined by earlier age of onset, more rapid progression, enhanced striatal volume loss, acceleration of nuclear huntingtin fragment accumulation and increased sensitivity to NMDAR-mediated excitotoxicity. These results provide clear evidence in vivo supporting a more severe phenotype associated with increased levels of mutant huntingtin as seen in homozygotes for HD.

Age at onset variance analysis in spinocerebellar ataxias: a study in a Dutch-French cohort

In dominant spinocerebellar ataxias (SCAs), the issue of whether non-CAG dependent factors contribute to onset age remains unsettled. Data on SCA genotype, onset age, normal/expanded CAG repeat length, sex of the patient and transmitting parent, and family details were available from 802 patients. Based on the model [log(10) (age at onset) = k - b CAG(exp) + epsilon], we examined changes in adjusted R(2) and residual standard error following incorporation of the other factors in this model. The expanded repeat explained 44.3 to 74.9% of onset age variance, although this was less than 50% in SCA3 and SCA6, implicating a large effect of non-CAG factors. The relation between onset age and CAG repeat was similar for SCA1, 3, 6, and 7, but different for SCA2, pointing to different polyglutamine effects in SCA2. For SCA2 and SCA3, 17.1 and 45.5% of onset age variance, respectively, were explained by currently (unidentified) familial factors. We found a significant contribution of the nonexpanded allele in SCA1 and SCA6. Besides polyglutamine motif (determined by the expanded CAG repeat length), we identified the following age at onset modifiers: protein context in SCA2; familial factors in SCA2 and SCA3; and the nonexpanded CAG repeat in SCA1 and SCA6.

Venezuelan kindreds reveal that genetic and environmental factors modulate Huntington's disease age of onset

Huntington's disease (HD) is an autosomal dominant neurodegenerative disease caused by a triplet (CAG) expansion mutation. The length of the triplet repeat is the most important factor in determining age of onset of HD, although substantial variability remains after controlling for repeat length. The Venezuelan HD kindreds encompass 18,149 individuals spanning 10 generations, 15,409 of whom are living. Of the 4,384 immortalized lymphocyte lines collected, 3,989 DNAs were genotyped for their HD alleles, representing a subset of the population at greatest genetic risk. There are 938 heterozygotes, 80 people with variably penetrant alleles, and 18 homozygotes. Analysis of the 83 kindreds that comprise the Venezuelan HD kindreds demonstrates that residual variability in age of onset has both genetic and environmental components. We created a residual age of onset phenotype from a regression analysis of the log of age of onset on repeat length. Familial correlations (correlation +/- SE) were estimated for sibling (0.40 +/- 0.09), parent-offspring (0.10 +/- 0.11), avuncular (0.07 +/- 0.11), and cousin (0.15 +/- 0.10) pairs, suggesting a familial origin for the residual variance in onset. By using a variance-components approach with all available familial relationships, the additive genetic heritability of this residual age of onset trait is 38%. A model, including shared sibling environmental effects, estimated the components of additive genetic (0.37), shared environment (0.22), and nonshared environment (0.41) variances, confirming that approximately 40% of the variance remaining in onset age is attributable to genes other than the HD gene and 60% is environmental.

The gender effect in juvenile Huntington disease patients of Italian origin

We analyzed a population of juvenile Huntington disease (HD) subjects of Italian origin (n = 57). The main aim of this study was to analyze the gender effect of the affected parent on age at onset and clinical presentation of offspring with juvenile HD. We also analyzed molecular features of the disease, including CAG mutation length and GluR6 gene polymorphism, according to the affected parent's gender. The mutation length was longer in paternally than in maternally transmitted HD juvenile patients (P = 0.025), nevertheless a similar mean early onset in the two groups (P > 0.05). This data was even enforced by that obtained from the whole cohort of patients included in the databank (n = 600) where, in the presence of increased mean parent-child CAG repeat change in paternal vs. maternal meiotic transmissions (+7.3 vs. +0.7 CAG, P = 0.0002), the mean parent-child year-of-onset change was similar in the two groups (-10.4 and -7.0 years, P > 0.05). A lower TAA-triplet in GluR6 was associated with an earlier age at onset in juvenile patients (P = 0.031, R2 = 0.10). When we added the GluR6 effect on age at onset to the CAG expanded number effect (P = 0.0001, R2 = 0.68) by multiple regression approach, the coefficient of determination R2 increased to 0.81. This effect in addition to the expanded CAG repeat number, found in juvenile and not in adult patients, was slightly enforced by paternal compared to maternal transmissions (R2=0.82). Our findings suggest the occurrence of a weaker effect of the paternal mutation on juvenile age at onset in our population, possibly amplified by other genetic factors, such as the TAA-triplet length in the GluR6 gene.

Unusual early-onset Huntingtons disease

Huntington's disease is an autosomal dominant progressive neurodegenerative disorder characterized by involuntary movements, cognitive decline, and behavioral disorders leading to functional disability. In contrast to patients with adult onset, in which chorea is the major motor abnormality, children often present with spasticity, rigidity, and significant intellectual decline associated with a more rapidly progressive course. An unusual early-onset Huntington's disease case of an 11-year-old boy with severe hypokinetic/rigid syndrome appearing at the age of 2.5 years is presented. Clinical diagnosis was confirmed by polymerase chain reaction study of the expanded IT-15 allele with a compatible size of 102 cytosine-adenosine-guanosine repeats L-Dopa mildly ameliorated rigidity, bradykinesia, and dystonia. We conclude that Huntington's disease should be included in the differential diagnoses of regressive syndromes of early childhood.

Onset and pre-onset studies to define the Huntington's disease natural history

Huntington's disease's (HD) clinical history has not been defined yet. However, many aspects of the most confusing clinical stages, i.e., the first and last disease phases, including the symptom progression and the disease duration, have been better approached after discovery of the responsible gene. The existence of accurate genetic tests, available for affected and pre-symptomatic subjects (i.e., mutation carriers) and the possibility to study transgenic in vivo models, are actually helping us to understand some of the aspects of HD clinical presentation. HD may present with motor symptoms other than chorea, the psychiatric manifestations may represent part of the clinical picture and cognitive deterioration may occur very early in the disease and depend on early cortical involvement. Pre-onset studies are of crucial importance in understanding the temporal sequence of the clinical events. This is also very important for future therapeutic strategies in those diseases initiating late in the life, such as HD.