The V471A Polymorphism in Autophagy-Related Gene ATG7 Modifies Age at Onset Specifically in Italian Huntington Disease Patients

The cause of Huntington disease (HD) is a polyglutamine repeat expansion of more than 36 units in the huntingtin protein, which is inversely correlated with the age at onset of the disease. However, additional genetic factors are believed to modify the course and the age at onset of HD. Recently, we identified the V471A polymorphism in the autophagy-related gene ATG7, a key component of the autophagy pathway that plays an important role in HD pathogenesis, to be associated with the age at onset in a large group of European Huntington disease patients. To confirm this association in a second independent patient cohort, we analysed the ATG7 V471A polymorphism in additional 1,464 European HD patients of the “REGISTRY” cohort from the European Huntington Disease Network (EHDN). In the entire REGISTRY cohort we could not confirm a modifying effect of the ATG7 V471A polymorphism. However, analysing a modifying effect of ATG7 in these REGISTRY patients and in patients of our previous HD cohort according to their ethnic origin, we identified a significant effect of the ATG7 V471A polymorphism on the HD age at onset only in the Italian population (327 patients). In these Italian patients, the polymorphism is associated with a 6-years earlier disease onset and thus seems to have an aggravating effect. We could specify the role of ATG7 as a genetic modifier for HD particularly in the Italian population. This result affirms the modifying influence of the autophagic pathway on the course of HD, but also suggests population-specific modifying mechanisms in HD pathogenesis.     

Covariate-dependent age-at-onset distributions for Huntington disease.

A combined logistic regression and life-table analysis is presented on age-at-onset data for Huntington disease. Covariates included in the analysis were sex of the at-risk individual, parental age at onset, and sex of transmitting parent. Parental age at onset and parental sex were found to be significant covariates for age at onset in the offspring, and the appropriate logistic regression functions are calculated by maximum likelihood methods. These regression functions permit a more precise evaluation of carrier risks and likelihoods than hitherto was possible by simple computational means. We further introduce a novel method to account for sibship correlations in the significance assessment, using log-likelihood differences between different models.     

Homozygote for Huntington disease.

Four offspring of three different Huntington disease (HD) affected x affected matings were assessed by genetic linkage analysis for possible homozygosity. One individual was found to have a 95% likelihood of being an HD homozygote. The homozygote individual had an age at onset and symptoms which were similar to those of affected HD heterozygote relatives, including some with younger onset. This confirms the observation of Wexler et al. that in HD the homozygote is not more severely afflicted than the heterozygote.     

A test of the hypothesis that age at onset in Huntington disease is controlled by an X-linked recessive modifier.

Data from the Research Roster for Huntington Disease Patients and Families were used to assess the hypothesis that juvenile onset in Huntington disease is determined by an X-linked recessive modifying gene in the affected parent. The observed proportion of affected fathers to affected mothers who had such offspring was not compatible with this hypothesis. Furthermore, neither the excess of affected grandfathers nor the existence of juvenile-onset and adult-onset cases within a sibship would be predicted by this model. We also rejected a more general hypothesis that a major change in gene expression across generations, measured by the presence of juvenile onset and/or major anticipation, is determined by an X-linked modifier. However, the inheritance of a propensity toward juvenile onset via the affected male line could be due to an abnormal pattern of paternal genomic imprinting.     

The normal Huntington disease (HD) allele, or a closely linked gene, influences age at onset of HD.

We evaluated the hypothesis that Huntington disease (HD) is influenced by the normal HD allele by comparing transmission patterns of genetically linked markers at the D4S10 locus in the normal parent against age at onset in the affected offspring. Analysis of information from 21 sibships in 14 kindreds showed a significant tendency for sibs who have similar onset ages to share the same D4S10 allele from the normal parent. Affected sibs who inherited different D4S10 alleles from the normal parent tended to have more variable ages at onset. These findings suggest that the expression of HD is modulated by the normal HD allele or by a closely linked locus.     

Huntington disease in Georgia: age at onset.

Age at onset of motor symptoms was collected on 611 persons affected with Huntington disease (HD) among 3,201 persons "at risk" in 108 kindreds. Life-table estimates correcting for truncated intervals of observation (censoring) produced a median age at onset 5 years older than the observed mean. Risk estimates of HD onset for persons at risk, as calculated by life-table methods, were significantly higher for older ages than were estimates based on the observed distribution of onsets. Age-specific incidence was found to be highest at age 35-64 years, a considerably older age interval than suggested by previous estimates. The offspring of affected males had significantly younger onset than did offspring of affected females, and a trend suggesting and excess of paternal descent among juvenile-onset cases was present. Life-table analysis is contrasted with analyses of (a) the observed distribution of age at onset and (b) remote cohorts age 63 or older at the time of data collection. The implications for risk prediction, genetic counseling, and genetic analysis of HD are discussed.     

Two models for a maternal factor in the inheritance of Huntington disease

Huntington disease is a classic example of an autosomal dominant trait. Over the years, however, a number of investigators have reported anomalies regarding the age of onset of the disease that are inconsistent with this paradigm. We propose two models in which a maternal factor--cytoplasmic in one case, autosomal or X-linked in the other--acts to delay onset in a manner consistent with the previously reported anomalies. Relevant data from the Huntington's Disease Research Roster are presented that reinforce and extend the previous observations.     

Inverse relationship between age at onset of Huntington disease and paternal age suggests involvement of genetic imprinting.

It is well recognized that age at onset of Huntington disease (HD) is strongly influenced by the sex of the affected parent, and this has lead to suggestions that genetic imprinting or maternal specific factors may play a role in the expression of the disease. This study evaluated maternal and paternal ages, birth order, parental age at onset, and sex of the affected parent and grandparent in 1,764 patients in the National HD Roster by using linear-regression techniques which incorporated a weighted least-squares approach to accommodate the correlation among siblings. It was found that paternal age is negatively associated with age at onset of HD, particularly among subjects who inherit the mutant gene from grandfathers. Apparent associations between age at onset and birth order and between age at onset and maternal age were not significant after adjustment for paternal age. The paternal age effect is strongest among juvenile-onset cases and individuals with anticipation of greater than or equal to 10 years, although it is detectable across the entire age-at-onset distribution. The tendency for older fathers, including those not transmitting the HD gene, to have affected offspring with early-onset disease may be consistent with a gene imprinting mechanism involving DNA methylation. Because paternal age in unaffected fathers is also a significant determinant of age at onset, methylation in this context might involve HD modifier genes or the normal HD allele.     

Genotype-phenotype analysis in inherited prion disease with eight octapeptide repeat insertional mutation

A minority of inherited prion diseases (IPD) are caused by four to 12 extra octapeptide repeat insertions (OPRI) in the prion protein gene (PRNP). Only four families affected by IPD with 8-OPRI have been reported, one of them was a three-generation Swedish kindred in which four of seven affected subjects had chorea which was initially attributed to Huntington’s disease (HD). Following the exclusion of HD, this phenotype was labeled Huntington disease-like 1 (HDL1). Here, we provide an update on the Swedish 8-OPRI family, describe the clinical features of one of its affected members with video-recordings, compare the four 8-OPRI families and study the effect of PRNP polymorphic codon 129 and gender on phenotype. Surprisingly, the Swedish kindred displayed the longest survival of all of the 8-OPRI families with a mean of 15.1 years from onset of symptoms. Subjects with PRNP polymorphic codon 129M in the mutated allele had significantly earlier age of onset, longer survival and earlier age of death than 129V subjects. Homozygous 129MM had earlier age of onset than 129VV. Females had a significantly earlier age of onset and earlier age of death than males. Up to 50% of variability in age of onset was conferred by the combined effect of PRNP polymorphic codon 129 and gender. An inverse correlation between early age of onset and long survival was found for this mutation.     

A genetic model for age at onset in Huntington disease.

Although numerous investigators have confirmed excess paternal transmission among juvenile-onset cases of Huntington disease (HD), there are conflicting reports that the late-onset form is inherited more often from the mother than from the father. Results from a survey of age at onset and age at death in 569 patients corroborate earlier findings of delayed onset of HD among offspring of affected mothers at both ends of the onset-age spectrum: 23 of 28 juvenile-onset offspring had affected fathers, and there were 1.6 times more late-onset offspring born to affected mothers than to affected fathers. These patterns, together with data that link age-at-onset variability to familial longevity trends, suggest a model where age at onset is governed, generally, by a set of independently inherited aging genes, but expression of the HD gene may be significantly delayed in individuals who possess a particular maternally transmitted factor.     

History of genetic disease: the molecular genetics of Huntington disease - a history

The Huntington disease gene was mapped to human chromosome 4p in 1983 and 10 years later the pathogenic mutation was identified as a CAG-repeat expansion. Our current understanding of the molecular pathogenesis of Huntington disease could never have been achieved without the recent progress in the field of molecular genetics. We are now equipped with powerful genetic models that continue to uncover new aspects of the pathogenesis of Huntington disease and will be instrumental for the development of therapeutic approaches for this disease.     

Proteomic analysis of protein expression and oxidative modification in r6/2 transgenic mice: a model of Huntington disease

Huntington disease (HD) is a hereditary neurodegenerative disorder characterized by motor, psychiatric, and cognitive symptoms. The genetic defect responsible for the onset of the disease, expansion of CAG repeats in exon 1 of the gene that codes for huntingtin on chromosome 4, has been unambiguously identified. On the other hand, the mechanisms by which the mutation causes the disease are not completely understood yet. However, defects in energy metabolism of affected cells may cause oxidative damage, which has been proposed as one of the underlying molecular mechanisms that participate in the etiology of the disease. In our effort to investigate the extent of oxidative damage occurring at the protein level, we used a parallel proteomic approach to identify proteins potentially involved in processes upstream or downstream of the disease-causing huntingtin in a well established HD mouse model (R6/2 transgenic mice). We have demonstrated that the expression levels of dihydrolipoamide S-succinyltransferase and aspartate aminotransferase increase consistently over the course of disease (10-week-old mice). In contrast, pyruvate dehydrogenase expression levels were found to be decreased in 10-week-old HD transgenic mice compared with young (4-week-old) mice. Our experimental approach also led to the identification of oxidatively modified proteins. Six proteins were found to be significantly oxidized in old R6/2 transgenic mice compared with either young transgenic mice or non-transgenic mice. These proteins are alpha-enolase, gamma-enolase (neuron-specific enolase), aconitase, the voltage-dependent anion channel 1, heat shock protein 90, and creatine kinase. Because oxidative damage has proved to play an important role in the pathogenesis and the progression of Huntington disease, our results for the first time identify specific oxidatively modified proteins that potentially contribute to the pathogenesis of Huntington disease.     

Racial distribution of Dupuytren's disease in Department of Veterans Affairs patients

Dupuytren's disease is a polyclonal fibroproliferative disorder of the palmar fascia of unclear pathogenesis. It has been described as a disease of northern European men and is reportedly rare in other races. A 10-year retrospective study using the Department of Veterans Affairs computer system was conducted to determine the racial distribution of this disorder among patients treated at all Department of Veterans Affairs medical centers. The study also determined demographic and clinical characteristics of black veterans treated for the condition at department medical centers. There were 9938 patients identified between the fiscal years of 1986 and 1995, of whom 412 were black (estimated prevalence of 130 per 100,000 population), 9071 were white (734 per 100,000), 234 were Hispanic white (237 per 100,000), 11 were Native American (144 per 100,000), 8 were Asian (67 per 100,000), and 202 were of unknown race. The characteristics of the disease in blacks are similar to those in whites. In both groups, the disease has a late onset, affects predominantly the ulnar digits, and is associated with other medical conditions, such as alcoholism, smoking, and diabetes. Unlike Dupuytren's disease in whites, however, the disease is rarely bilateral in blacks. The differential prevalence among racial groups suggests a genetic component to the pathogenesis of the disease.     

Stochastic kinetics of intracellular huntingtin aggregate formation

Neurodegeneration in Huntington disease is described by neuronal loss in which the probability of cell death remains constant with time. However, the quantitative connection between the kinetics of cell death and the molecular mechanism initiating neurodegeneration remains unclear. One hypothesis is that nucleation of protein aggregates containing exon I fragments of the mutant huntingtin protein (mhttex1), which contains an expanded polyglutamine region in patients with the disease, is the explanation for the infrequent but steady occurrence of neuronal death, resulting in adult onset of the disease. Recent in vitro evidence suggests that sufficiently long polyglutamine peptides undergo a unimolecular conformational change to form a nucleus that seeds aggregation. Here we use this nucleation mechanism as the basis to derive a stochastic mathematical model describing the probability of aggregate formation in cells as a function of time and mhttex1 protein concentration, and validate the model experimentally. These findings suggest that therapeutic strategies for Huntington disease predicated on reducing the rate of mhttex1 aggregation need only make modest reductions in huntingtin expression level to substantially increase the delay time until aggregate formation.     

The HTT CAG-Expansion Mutation Determines Age at Death but Not Disease Duration in Huntington Disease

Huntington disease (HD) is caused by an expanded HTT CAG repeat that leads in a length-dependent, completely dominant manner to onset of a characteristic movement disorder. HD also displays early mortality, so we tested whether the expanded CAG repeat exerts a dominant influence on age at death and on the duration of clinical disease. We found that, as with clinical onset, HD age at death is determined by expanded CAG-repeat length and has no contribution from the normal CAG allele. Surprisingly, disease duration is independent of the mutation’s length. It is also unaffected by a strong genetic modifier of HD motor onset. These findings suggest two parsimonious alternatives. (1) HD pathogenesis is driven by mutant huntingtin, but before or near motor onset, sufficient CAG-driven damage occurs to permit CAG-independent processes and then lead to eventual death. In this scenario, some pathological changes and their clinical correlates could still worsen in a CAG-driven manner after disease onset, but these CAG-related progressive changes do not themselves determine duration. Alternatively, (2) HD pathogenesis is driven by mutant huntingtin acting in a CAG-dependent manner with different time courses in multiple cell types, and the cellular targets that lead to motor onset and death are different and independent. In this scenario, processes driven by HTT CAG length lead directly to death but not via the striatal pathology associated with motor manifestations. Each scenario has important ramifications for the design and testing of potential therapeutics, especially those aimed at preventing or delaying characteristic motor manifestations.     

A universal mechanism ties genotype to phenotype in trinucleotide diseases

Trinucleotide hereditary diseases such as Huntington disease and Friedreich ataxia are cureless diseases associated with inheriting an abnormally large number of DNA trinucleotide repeats in a gene. The genes associated with different diseases are unrelated and harbor a trinucleotide repeat in different functional regions; therefore, it is striking that many of these diseases have similar correlations between their genotype, namely the number of inherited repeats and age of onset and progression phenotype. These correlations remain unexplained despite more than a decade of research. Although mechanisms have been proposed for several trinucleotide diseases, none of the proposals, being disease-specific, can account for the commonalities among these diseases. Here, we propose a universal mechanism in which length-dependent somatic repeat expansion occurs during the patient's lifetime toward a pathological threshold. Our mechanism uniformly explains for the first time to our knowledge the genotype–phenotype correlations common to trinucleotide disease and is well-supported by both experimental and clinical data. In addition, mathematical analysis of the mechanism provides simple explanations to a wide range of phenomena such as the exponential decrease of the age-of-onset curve, similar onset but faster progression in patients with Huntington disease with homozygous versus heterozygous mutation, and correlation of age of onset with length of the short allele but not with the long allele in Friedreich ataxia. If our proposed universal mechanism proves to be the core component of the actual mechanisms of specific trinucleotide diseases, it would open the search for a uniform treatment for all these diseases, possibly by delaying the somatic expansion process.     

Factors related to onset age of Huntington disease.

One prominent feature of Huntington disease (HD) is the variable age at which the characteristic neurological or psychiatric symptoms appear. Ages of manifestation varying from 4 to 65 years are found in a sample of 95 HD pedigrees compiled since 1968 from the Southeastern United States. Significant parent-child correlations of age of onset indicate consistency of onset age within nuclear families. However, an average intrafamily range of 9 years and an average intrapedigree range of 12 years reveal substantial variability of onset age within these groups. Of the nine cases of juvenile-onset HD identified in this sample, seven were of paternal descent. The preponderance of juvenile patients inheriting the HD gene from a father confirms similar findings from other studies. In addition, a trend toward earlier onset in all offspring of paternal transmission suggests that the juvenile-onset phenomenon is only the tail of a shift in the curve of onset ages for this group. A trend toward earlier onset in successive generations was noted. This "anticipation" may reflect the finding that persons of early onset in prior generations are selectively nonreproductive as a result of manifestation of the disorder. By identifying familial factors influencing onset age of HD, it may be possible to more effectively evaluate environmental factors that influence the onset of the disorder.