Natural selection at the MJD locus: phenotypic diversity, survival and fertility among Machado-Joseph Disease patients from the Azores

The Homogeneous Azorean Machado-Joseph Disease Cohort: Characterization and Contributions to Advances in Research

Machado-Joseph disease (MJD)/spinocerebellar ataxia type 3 (SCA3) is the most common autosomal dominant ataxia worldwide. MJD is characterized by late-onset progressive cerebellar ataxia associated with variable clinical findings, including pyramidal signs and a dystonic-rigid extrapyramidal syndrome. In the Portuguese archipelago of the Azores, the worldwide population cluster for this disorder (prevalence of 39 in 100,000 inhabitants), a cohort of MJD mutation carriers belonging to extensively studied pedigrees has been followed since the late 1990s. Studies of the homogeneous Azorean MJD cohort have been contributing crucial information to the natural history of this disease as well as allowing the identification of novel molecular biomarkers. Moreover, as interventional studies for this globally rare and yet untreatable disease are emerging, this cohort should be even more important for the recruitment of trial participants. In this paper, we profile the Azorean cohort of MJD carriers, constituted at baseline by 20 pre-ataxic carriers and 52 patients, which currently integrates the European spinocerebellar ataxia type 3/Machado-Joseph disease Initiative (ESMI), a large European longitudinal MJD cohort. Moreover, we summarize the main studies based on this cohort and highlight the contributions made to advances in MJD research. Knowledge of the profile of the Azorean MJD cohort is not only important in the context of emergent interventional trials but is also pertinent for the implementation of adequate interventional measures, constituting relevant information for Lay Associations and providing data to guide healthcare decision makers.

Selective forces acting on spinocerebellar ataxia type 3/Machado-Joseph disease recurrency: A systematic review and meta-analysis

Spinocerebellar ataxia type 3/Machado-Joseph disease (SCA3/MJD) is a dominant neurodegenerative disease caused by the expansion of a CAG repeat tract in ATXN3. Anticipation and worsening of clinical picture in subsequent generations were repeatedly reported, but there is no indication that SCA3/MJD frequency is changing. Thus, we performed a systematic review and meta-analysis on phenomena with potential effect on SCA3/MJD recurrency in populations: instability of CAG repeat transmissions, anticipation, fitness, and segregation of alleles. Transmission of the mutant allele was associated with an increase of 1.23 CAG repeats in the next generation, and the average change in age at onset showed an anticipation of 7.75 years per generation; but biased recruitments cannot be ruled out. Affected SCA3/MJD individuals had 45% more children than related controls. Transmissions from SCA3/MJD carriers showed that the expanded allele was segregated in 64% of their children. In contrast, transmissions from normal subjects showed that the minor allele was segregated in 54%. The present meta-analysis concluded that there is a segregation distortion favoring the expanded allele, among children of carriers. Therefore, further studies on transmissions and anticipation phenomena as well as more observations about fertility are required to clarify these selective forces over SCA3/MJD.

Mortality Statistics and their Contribution to Improving the Knowledge of Rare Diseases Epidemiology: The Example of Hereditary Ataxia in Europe

Official mortality statistics provide population-based data and serve to improve epidemiological knowledge of rare diseases (RDs), by helping with the description of the natural history of the disease. They are an important complement of registries and estimates of disease burden and costs. At the same time, they heighten both the visibility of these diseases and the interest in their study and the search for treatments that may increase survival. This chapter contains a European analysis of hereditary ataxia mortality, which considers the time trend in different countries and the geographical variability in risk of death. Despite the limitations of applying this data source to RDs, mortality statistics share criteria which facilitate international comparisons and are of great utility for obtaining sufficiently uniform and robust time series for analysis of low-prevalence diseases.

Fertility in essential tremor. Data from population-based and clinical sources

BACKGROUND

The view of essential tremor (ET) as a degenerative disorder, which is now gaining support from postmortem studies, is at odds with traditional views of ET as a 'super-healthy' condition characterized by increased longevity and fertility. Longevity has recently been re-examined in ET, yet fertility, measured by number of offspring, has never been critically assessed in this disease. The objective was to determine whether ET cases and controls differ in terms of number of children.

METHODS

Family history data were collected on ET cases and controls from 2 distinct sources, a population and a clinical (referred) sample.

RESULTS

In the population, the number of children was similar in 59 cases vs. 72 controls [mean +/- SD (median) = 2.3 +/- 2.9 (1) vs. 2.2 +/- 1.8 (2); p = 0.26]. In the referred sample, number of children was similar in 184 cases vs. 241 controls [2.0 +/- 1.5 (2) vs. 1.9 +/- 1.7 (2); p = 0.33]. In adjusted analyses, results were similar.

CONCLUSIONS

Fertility, measured by number of children, was similar in cases and controls. With a newer understanding of the biological substrate of ET emerging from postmortem studies, it is important to critically reassess this and other fundamental biological questions about the disease.

Analysis of segregation patterns in Machado-Joseph disease pedigrees

Machado-Joseph disease (MJD), also known as spinocerebellar ataxia type 3 (SCA3), is an autosomal dominant neurodegenerative disorder of late onset, which is considered the most common form of SCA worldwide. The main goal of this study was to investigate the presence of segregation ratio distortion (SRD) during transmissions of ATXN3 alleles by MJD patients, evaluating the putative role of SRD in the epidemiological representation of the disease. Sixty-two complete sibships, each with one clinically affected parent, totalling 330 transmissions were selected according to defined criteria and used for segregation analysis. Onset data from MJD patients with Azorean origin was used for residual risk estimates according to different ages. Residual risk values were applied to unaffected offspring to calculate the probability of inheriting the expanded allele. The proportion of offspring that received the expanded or the normal allele from the affected parent was calculated to determine the presence of SRD during transmissions of ATXN3 alleles by MJD patients. Segregation of ATXN3 alleles was in accordance with the expected Mendelian proportions (chi (2) = 0.982, P = 0.322). However, there was a tendency favouring the transmission of the normal alleles. Thus, SRD is not a potential mechanism on the basis of MJD epidemiological representation.

Population Genetics of Wild-Type CAG Repeats in the Machado-Joseph Disease Gene in Portugal

OBJECTIVE

To gain insights on the molecular mechanisms of mutation that led to the emergence of expanded alleles in the MJD gene, by studying the behavior of wild-type alleles and testing the association of its distribution with the representation of the disease.

METHODS

The number of CAG motifs in the MJD gene was determined in a representative sample of 1000 unrelated individuals. Associations between the repeat size and the epidemiological representation of MJD were tested.

RESULTS

The allelic profile of the total sample was in the normal range (13-41 repeats), with mode (CAG)23. No intermediate alleles were present. Allelic size distribution showed a negative skew. The correlation between the epidemiological representation of MJD in each district and the frequency of small, medium and large normal alleles was not significant. Further correlations performed grouping the districts also failed to produce significant results.

CONCLUSIONS

The absence of association between the size of the repeats and the representation of MJD demonstrates that prevalence is not an indirect reflection of the frequency of large normal alleles. Globally the results obtained are in accordance with a model that postulates the occurrence of a few mutations on the basis of most of the MJD cases worldwide.