Machado-Joseph disease enhances genetic fitness: a comparison between affected and unaffected women and between MJD and the general population

More evidence for widespread antagonistic pleiotropy in polymorphic disease alleles

Introduction

Many loci segregate alleles classified as "genetic diseases" due to their deleterious effects on health. However, some disease alleles have been reported to show beneficial effects under certain conditions or in certain populations. The beneficial effects of these antagonistically pleiotropic alleles may explain their continued prevalence, but the degree to which antagonistic pleiotropy is common or rare is unresolved. We surveyed the medical literature to identify examples of antagonistic pleiotropy to help determine whether antagonistic pleiotropy appears to be rare or common.

Results

We identified ten examples of loci with polymorphisms for which the presence of antagonistic pleiotropy is well supported by detailed genetic or epidemiological information in humans. One additional locus was identified for which the supporting evidence comes from animal studies. These examples complement over 20 others reported in other reviews.

Discussion

The existence of more than 30 identified antagonistically pleiotropic human disease alleles suggests that this phenomenon may be widespread. This poses important implications for both our understanding of human evolutionary genetics and our approaches to clinical treatment and disease prevention, especially therapies based on genetic modification.

A model for the dynamics of expanded CAG repeat alleles: ATXN2 and ATXN3 as prototypes

Background: Spinocerebellar ataxia types 2 (SCA2) and 3 (SCA3/MJD) are diseases due to dominant unstable expansions of CAG repeats (CAGexp). Age of onset of symptoms (AO) correlates with the CAGexp length. Repeat instability leads to increases in the expanded repeats, to important AO anticipations and to the eventual extinction of lineages. Because of that, compensatory forces are expected to act on the maintenance of expanded alleles, but they are poorly understood. Objectives: we described the CAGexp dynamics, adapting a classical equation and aiming to estimate for how many generations will the descendants of a de novo expansion last. Methods: A mathematical model was adapted to encompass anticipation, fitness, and allelic segregation; and empirical data fed the model. The arbitrated ancestral mutations included in the model had the lowest CAGexp and the highest AO described in the literature. One thousand generations were simulated until the alleles were eliminated, fixed, or 650 generations had passed. Results: All SCA2 lineages were eliminated in a median of 10 generations. In SCA3/MJD lineages, 593 were eliminated in a median of 29 generations. The other ones were eliminated due to anticipation after the 650th generation or remained indefinitely with CAG repeats transitioning between expanded and unexpanded ranges. Discussion: the model predicted outcomes compatible with empirical data - the very old ancestral SCA3/MJD haplotype, and the de novo SCA2 expansions -, which previously seemed to be contradictory. This model accommodates these data into understandable dynamics and might be useful for other CAGexp disorders.

Differences in spontaneous speech fluency between Parkinson's disease and spinocerebellar ataxia type 3

Background

The basal ganglia and cerebellum both have a role in speech production although the effect of isolated involvement of these structures on speech fluency remains unclear.

Objective

The study aimed to assess the differences in the articulatory pattern in patients with cerebellar vs. basal ganglia disorders.

Methods

A total of 20 individuals with Parkinson's disease (PD), 20 with spinocerebellar ataxia type 3 (SCA3), and 40 controls (control group, CG) were included. Diadochokinesis (DDK) and monolog tasks were collected.

Results

The only variable that distinguished SCA3 carriers from the CG was the number of syllables in the monolog, with SCA3 patients of a significantly lower number. For patients with PD, the number of syllables, phonation time, DDK, and monolog were significantly lower than for CG. Patients with PD were significantly worse compared to patients with SCA3 in the number of syllables and phonation time in DDK, and phonation time in monolog. Additionally, there was a significant correlation between the number of syllables in the monolog and the MDS-UPDRS III for participants with PD, and the Friedreich Ataxia Rating Scale for participants with SCA3 suggesting a relationship between speech and general motor functioning.

Conclusion

The monolog task is better at discriminating individuals with cerebellar vs. Parkinson's diseases as well as differentiating healthy control and was related to the severity of the disease.

Spinocerebellar ataxia type 2 from an evolutionary perspective: Systematic review and meta-analysis

Dominant diseases due to expanded CAG repeat tracts, such as spinocerebellar ataxia type 2 (SCA2), are prone to anticipation and worsening of clinical picture in subsequent generations. There is insufficient data about selective forces acting on the maintenance of these diseases in populations. We made a systematic review and meta-analysis on the effect of the CAG length over age at onset, instability of transmissions, anticipation, de novo or sporadic cases, fitness, segregation of alleles, and ancestral haplotypes. The correlation between CAG expanded and age at onset was r²  = 0.577, and transmission of the mutant allele was associated with an increase of 2.42 CAG repeats in the next generation and an anticipation of 14.62 years per generation, on average. One de novo and 18 sporadic cases were detected. Affected SCA2 individuals seem to have more children than controls. The expanded allele was less segregated than the 22-repeat allele in children of SCA2 subjects. Several ancestral SCA2 haplotypes were published. Data suggest that SCA2 lineages may tend to disappear eventually, due to strong anticipation phenomena. Whether or not the novel cases come from common haplotypes associated with a predisposition to further expansions is a question that needs to be addressed by future studies.

Founder Effects of Spinocerebellar Ataxias in the American Continents and the Caribbean

Spinocerebellar ataxias (SCAs) comprise a heterogeneous group of autosomal dominant disorders. The relative frequency of the different SCA subtypes varies broadly among different geographical and ethnic groups as result of genetic drifts. This review aims to provide an update regarding SCA founders in the American continents and the Caribbean as well as to discuss characteristics of these populations. Clusters of SCAs were detected in Eastern regions of Cuba for SCA2, in South Brazil for SCA3/MJD, and in Southeast regions of Mexico for SCA7. Prevalence rates were obtained and reached 154 (municipality of Báguano, Cuba), 166 (General Câmara, Brazil), and 423 (Tlaltetela, Mexico) patients/100,000 for SCA2, SCA3/MJD, and SCA7, respectively. In contrast, the scattered families with spinocerebellar ataxia type 10 (SCA10) reported all over North and South Americas have been associated to a common Native American ancestry that may have risen in East Asia and migrated to Americas 10,000 to 20,000 years ago. The comprehensive review showed that for each of these SCAs corresponded at least the development of one study group with a large production of scientific evidence often generalizable to all carriers of these conditions. Clusters of SCA populations in the American continents and the Caribbean provide unusual opportunity to gain insights into clinical and genetic characteristics of these disorders. Furthermore, the presence of large populations of patients living close to study centers can favor the development of meaningful clinical trials, which will impact on therapies and on quality of life of SCA carriers worldwide.

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.

Population medical genetics: translating science to the community

Rare genetic disorders are currently in the spotlight due to the elevated number of different conditions and significant total number of affected patients. The study of these disorders is extremely helpful for the elucidation of physiological processes related with complex disorders. Isolated populations are instrumental for the study of genetic disorders, considering their homogeneity and high proportion of affected patients in a small geographic area. These favorable conditions lead to the creation of a new discipline, known as “population medical genetics”, which integrates medical genetics, population genetics, epidemiological genetics and community genetics. In order to develop practical activities in this new discipline, the National Institute of Population Medical Genetics (INaGeMP) was created in 2008 in Brazil. INaGeMP has developed several tools and funded numerous research activities. In this review, we highlight three successful projects developed in the first 10 years of INaGeMP activities (2008-2018): a newborn screening pilot study for MPS VI in Northeast Brazil, the study of Machado-Joseph disease in Brazilian families with Azorian ancestry, and the high twinning rate in a small town in southern Brazil. The results of these projects in terms of scientific output and contributions to the affected communities highlight the success and importance of INaGeMP.

Genetic fitness and selection intensity in a population affected with high-incidence spinocerebellar ataxia type 1

Spinocerebellar ataxia type 1 (SCA1) is the major and likely the only type of autosomal dominant cerebellar ataxia in the Sakha (Yakut) people of Eastern Siberia. The prevalence rate of SCA1 has doubled over the past 21 years peaking at 46 cases per 100,000 rural population. The age at death correlates closely with the number of CAG triplet repeats in the mutant ATXN1 gene (r = -0.81); most patients with low-medium (39-55) repeat numbers survived until the end of reproductive age. The number of CAG repeats expands in meiosis, particularly in paternal transmissions; the average total increase in intergenerational transmissions in our cohort was estimated at 1.6 CAG repeats. The fertility rates of heterozygous carriers of 39-55 CAG repeats in women were no different from those of the general Sakha population. Overall, the survival of mutation carriers through reproductive age, unaltered fertility rates, low childhood mortality in SCA1-affected families, and intergenerational transmission of increasing numbers of CAG repeats in the ATXN1 gene indicate that SCA1 in the Sakha population will be maintained at high prevalence levels. The low (0.19) Crow's index of total selection intensity in our SCA1 cohort implies that this mutation is unlikely to be eliminated through natural selection alone.

Clinical Scales Predict Significant Videofluoroscopic Dysphagia in Machado Joseph Disease Patients

Background

Although aspiration is one of the main causes of death in SCA, such as SCA3/Machado Joseph disease (SCA3/MJD), clinical studies on dysphagia are lacking for these diseases. The aims of this study were to characterize dysphagia in SCA3/MJD through videofluoroscopy (VF) of swallowing, correlate VF with disease severity criteria and weight loss, and determine the clinical criteria cutoffs for performing VF in the clinical routine, in order to detect aspiration.

Methods

A cross-sectional study on 34 SCA3/MJD patients was performed. Clinical and molecular data, as well as body mass index (BMI), were obtained. Neurological scales, such as the Scale for the Assessment and Rating of Ataxia (SARA), and the Swallowing Quality of Life (SWAL-QOL) questionnaire were applied. The VF scores, Dysphagia Outcome and Severity Scale (DOSS) and penetration/aspiration scale (PAS), were obtained: Moderate-to-severe scores were grouped as "significant dysphagia."

Results

Overall, 31 of 34 individuals showed abnormal scores at VF. SARA, BMI, and the domain "eating duration" of SWAL-QOL correlated with VF: Their relation to significant dysphagia (DOSS <4 points or PAS >3) was evaluated through receiver operating characteristic curves. A sensitivity of 100% was equivalent to a cutoff of 15 points on SARA score, 23.72 kg/m² on BMI, and 60% on eating duration-SWAL-QOL (P < 0.05).

Conclusion

Significant dysphagia was not related to age at onset, disease duration, or CAG repeat expansion, but with SARA scores, lower BMI, and the domain eating duration of SWAL-QOL. As a guideline for preventing aspiration, we suggest that SARA scores greater than 15 or eating duration-SWAL-QOL lower than 60% should urge VF studies in SCA3/MJD.

Genetic counseling and presymptomatic testing programs for Machado-Joseph Disease: lessons from Brazil and Portugal

Machado-Joseph disease (MJD) is an autosomal dominant, late-onset neurological disorder and the most common form of spinocerebellar ataxia (SCA) worldwide. Diagnostic genetic testing is available to detect the disease-causing mutation by direct sizing of the CAG repeat tract in the ataxin 3 gene. Presymptomatic testing (PST) can be used to identify persons at risk of developing the disease. Genetic counseling provides patients with information about the disease, genetic risks, PST, and the decision-making process. In this study, we present the protocol used in PST for MJD and the relevant observations from two centers: Brazil (Porto Alegre) and Portugal (Porto). We provide a case report that illustrates the significant ethical and psychological issues related to PST in late-onset neurological disorders. In both centers, counseling and PST are performed by a multidisciplinary team, and genetic testing is conducted at the same institutions. From 1999 to 2012, 343 individuals sought PST in Porto Alegre; 263 (77%) of these individuals were from families with MJD. In Porto, 1,530 individuals sought PST between 1996 and 2013, but only 66 (4%) individuals were from families with MJD. In Brazil, approximately 50% of the people seeking PST eventually took the test and received their results, whereas 77% took the test in Portugal. In this case report, we highlight several issues that might be raised by the consultand and how the team can extract significant information. Literature about PST testing for MJD and other SCAs is scarce, and we hope this report will encourage similar studies and enable the implementation of PST protocols in other populations, mainly in Latin America.

Progression rate of neurological deficits in a 10-year cohort of SCA3 patients

Spinocerebellar ataxia 3 is an untreatable CAG repeat expansion disorder whose natural history is not completely understood. Our aims were to describe the progression of neurological manifestations in a long-term cohort of spinocerebellar ataxia 3, and to verify if CAG expanded repeat, gender, and age at onset were associated with the rate of progression. Patients entered the study between 1998 and 2005 and were seen until 2007. On each visit, the validated NESSCA scale, an inventory of 18 neurological manifestations, was applied. Scores observed in each year of disease duration produced a Growth Curve, which was analyzed through the random coefficients model. Scores obtained in some individual items were described through multi-state Markov models. One hundred fifty-six patients (78 families) were recruited; 28 were lost, and 23 died. Mean (sd) ages at onset and at baseline were 32.8 (10.6) and 40.7 (12.8) years; median (range) expanded CAGn was 74 (67-85). Three hundred fifteen NESSCA evaluations were performed, comprising disease durations from zero to 34 years. The 105 patients who completed the study were seen over 5 (sd = 2.4) years at intervals of 2.5 (sd = 1.5) years. The trajectory of NESSCA obtained for the overall group increased by 1.26 points per year. This slope increased by 0.15 points per each additional CAG in the expanded repeat (p < 0.0002) and decreased by 0.03 points per each additional year of age at onset (p = 0.005). NESSCA worsened steadily, producing linear trajectories, which were faster among patients with longer expanded repeats (>74) and with lower ages at onset (<34 years).