Huntington disease reduced penetrance alleles occur at high frequency in the general population

Dietary fasting and time-restricted eating in Huntington's disease: therapeutic potential and underlying mechanisms

Huntington's disease (HD) is a devastating neurodegenerative disorder caused by aggregation of the mutant huntingtin (mHTT) protein, resulting from a CAG repeat expansion in the huntingtin gene HTT. HD is characterized by a variety of debilitating symptoms including involuntary movements, cognitive impairment, and psychiatric disturbances. Despite considerable efforts, effective disease-modifying treatments for HD remain elusive, necessitating exploration of novel therapeutic approaches, including lifestyle modifications that could delay symptom onset and disease progression. Recent studies suggest that time-restricted eating (TRE), a form of intermittent fasting involving daily caloric intake within a limited time window, may hold promise in the treatment of neurodegenerative diseases, including HD. TRE has been shown to improve mitochondrial function, upregulate autophagy, reduce oxidative stress, regulate the sleep-wake cycle, and enhance cognitive function. In this review, we explore the potential therapeutic role of TRE in HD, focusing on its underlying physiological mechanisms. We discuss how TRE might enhance the clearance of mHTT, recover striatal brain-derived neurotrophic factor levels, improve mitochondrial function and stress-response pathways, and synchronize circadian rhythm activity. Understanding these mechanisms is critical for the development of targeted lifestyle interventions to mitigate HD pathology and improve patient outcomes. While the potential benefits of TRE in HD animal models are encouraging, future comprehensive clinical trials will be necessary to evaluate its safety, feasibility, and efficacy in persons with HD.

Repeat expansions in AR, ATXN1, ATXN2 and HTT in Norwegian patients diagnosed with amyotrophic lateral sclerosis

Genetic repeat expansions cause neuronal degeneration in amyotrophic lateral sclerosis as well as other neurodegenerative disorders such as spinocerebellar ataxia, Huntington's disease and Kennedy's disease. Repeat expansions in the same gene can cause multiple clinical phenotypes. We aimed to characterize repeat expansions in a Norwegian amyotrophic lateral sclerosis cohort. Norwegian amyotrophic lateral sclerosis patients (n = 414) and neurologically healthy controls adjusted for age and gender (n = 713) were investigated for repeat expansions in AR, ATXN1, ATXN2 and HTT using short read exome sequencing and the ExpansionHunter software. Five amyotrophic lateral sclerosis patients (1.2%) and two controls (0.3%) carried ≥36 repeats in HTT (P = 0.032), and seven amyotrophic lateral sclerosis patients (1.7%) and three controls (0.4%) carried ≥29 repeats in ATXN2 (P = 0.038). One male diagnosed with amyotrophic lateral sclerosis carried a pathogenic repeat expansion in AR, and his diagnosis was revised to Kennedy's disease. In ATXN1, 50 amyotrophic lateral sclerosis patients (12.1%) and 96 controls (13.5%) carried ≥33 repeats (P = 0.753). None of the patients with repeat expansions in ATXN2 or HTT had signs of Huntington's disease or spinocerebellar ataxia type 2, based on a re-evaluation of medical records. The diagnosis of amyotrophic lateral sclerosis was confirmed in all patients, with the exception of one patient who had primary lateral sclerosis. Our findings indicate that repeat expansions in HTT and ATXN2 are associated with increased likelihood of developing amyotrophic lateral sclerosis. Further studies are required to investigate the potential relationship between HTT repeat expansions and amyotrophic lateral sclerosis.

A clinical study and future prospects for bioactive compounds and semi-synthetic molecules in the therapies for Huntington's disease

A neurodegenerative disorder (ND) refers to Huntington's disease (HD) which affects memory loss, weight loss, and movement dysfunctions such as chorea and dystonia. In the striatum and brain, HD most typically impacts medium-spiny neurons. Molecular genetics, excitotoxicity, oxidative stress (OS), mitochondrial, and metabolic dysfunction are a few of the theories advanced to explicit the pathophysiology of neuronal damage and cell death. Numerous in-depth studies of the literature have supported the therapeutic advantages of natural products in HD experimental models and other treatment approaches. This article briefly discusses the neuroprotective impacts of natural compounds against HD models. The ability of the discovered natural compounds to suppress HD was tested using either in vitro or in vivo models. Many bioactive compounds considerably lessened the memory loss and motor coordination brought on by 3-nitropropionic acid (3-NP). Reduced lipid peroxidation, increased endogenous enzymatic antioxidants, reduced acetylcholinesterase activity, and enhanced mitochondrial energy generation have profoundly decreased the biochemical change. It is significant since histology showed that therapy with particular natural compounds lessened damage to the striatum caused by 3-NP. Moreover, natural products displayed varying degrees of neuroprotection in preclinical HD studies because of their antioxidant and anti-inflammatory properties, maintenance of mitochondrial function, activation of autophagy, and inhibition of apoptosis. This study highlighted about the importance of bioactive compounds and their semi-synthetic molecules in the treatment and prevention of HD.

Analysis of HTT CAG repeat expansion among healthy individuals and patients with chorea in Korea

BACKGROUND

Although the epidemiology of Huntington's disease (HD) in Korea differs notably from that in Western countries, the genetic disparities between these regions remain unclear.

OBJECTIVE

To investigate the characteristics and clinical significance of cytosine-adenine-guanine (CAG) repeat size associated with HD in the Korean population.

METHODS

We analyzed the CAG repeat lengths of the HTT gene in 941 healthy individuals (1,882 alleles) and 954 patients with chorea (1,908 alleles) from two referral hospitals in Korea. We presented normative CAG repeat length data for the Korean population and computed the reduced penetrance (36-39 CAG) and intermediate allele (27-35 CAG) frequencies in the two groups. Furthermore, we investigated the relationship between intermediate alleles and chorea development using logistic regression models in individuals aged ≥55 years.

RESULTS

The mean (±standard deviation) CAG repeat length in healthy individuals was 17.5 ± 2.0, with a reduced penetrance allele frequency of 0.05 % (1/1882) and intermediate allele frequency of 0.69 % (13/1882). We identified 213 patients with genetically confirmed HD whose CAG repeat length ranged from 39 to 140, with a mean of 45.2 ± 7.9 in the longer allele. Compared with normal CAG repeat alleles, intermediate CAG repeat alleles were significantly related to a higher risk of developing chorea (age of onset range, 63-84 years) in individuals aged ≥55 years.

CONCLUSIONS

This study provides insights into the specific characteristics of CAG repeat lengths in the HTT gene in the Korean population. The reduced penetrance and intermediate allele frequencies in the Korean general population seem to be lower than those reported in Western populations. The presence of intermediate alleles may increase the risk of chorea in the Korean elderly population, which requires further large-scale investigations.

The relationship between disease-specific psychosocial stressors and depressive symptoms in Huntington's disease

BACKGROUND

Huntington's disease (HD) is an inherited neurodegenerative disease involving motor abnormalities, cognitive decline, and psychological difficulties. Depression is among the most common psychological difficulties in HD. People with HD encounter numerous stressors related to their diagnosis and the impact of HD on their daily lives. Understanding the relationship between HD-specific psychosocial stressors and depression symptoms is critical for optimising treatment and developing a holistic, disease-specific model of depression in HD.

METHODS

Fifty-seven adults with the HD gene expansion (33 pre-symptomatic, 24 symptomatic) completed a self-report depression questionnaire and rated how much stress they experienced in relation to 20 psychosocial challenges commonly associated with HD. We examined associations between depression symptoms and each stressor individually, and after clustering using principal components analysis.

RESULTS

Depression symptoms were significantly associated with most of the psychosocial stressors assessed. Clustering with principal components analysis revealed that higher depression scores had significant independent associations with greater stress related to the future implications of HD (β = .44, p = .001) and sleep and psychological difficulties (β = .28, p = .005), but not with stress related to functional limitations (β = .11, p = .33) or interpersonal issues caused by HD (β = .15, p = .21).

CONCLUSIONS

Stressful experiences associated with HD constitute an important risk factor for depression in HD. Our findings support the use of more psychologically informed models of depression in HD and necessitate further research on tailored psychosocial interventions for HD patients with depression.

Data-driven distillation and precision prognosis in traumatic brain injury with interpretable machine learning

Traumatic brain injury (TBI) affects how the brain functions in the short and long term. Resulting patient outcomes across physical, cognitive, and psychological domains are complex and often difficult to predict. Major challenges to developing personalized treatment for TBI include distilling large quantities of complex data and increasing the precision with which patient outcome prediction (prognoses) can be rendered. We developed and applied interpretable machine learning methods to TBI patient data. We show that complex data describing TBI patients' intake characteristics and outcome phenotypes can be distilled to smaller sets of clinically interpretable latent factors. We demonstrate that 19 clusters of TBI outcomes can be predicted from intake data, a ~ 6× improvement in precision over clinical standards. Finally, we show that 36% of the outcome variance across patients can be predicted. These results demonstrate the importance of interpretable machine learning applied to deeply characterized patients for data-driven distillation and precision prognosis.

Arguments for and against the whole-genome sequencing of newborns

Recent decades have brought enormous progress in both genetics and genomics, as well as in information technology (IT). The sequence of the human genome is now known, and although our knowledge is far from complete, great progress has been made in understanding how the genome works. With the developments in storage capacity, artificial intelligence, and learning algorithms, we are now able to learn and interpret complex systems such as the human genome in a very short time. Perhaps the most important goal of learning about the human genome is to understand diseases better: how they develop; how their processes can be prevented or slowed down; and after diseases have developed, how they can be cured or their symptoms alleviated. The vast majority of diseases have a genetic background, i.e., genes, sequence variations, and gene-gene interactions play a role in most diseases to a greater or lesser extent. Accordingly, the first step is to discover which genes, or genomic variants, cause or contribute to the development of a particular disease in a given patient. Given that an individual's genome remains virtually unchanged throughout their life (with one or two exceptions, such as in the case of cancer, which is caused by somatic mutations), it might be considered advantageous to sequence the genome of every person at birth. In this paper, we set out to show the possible benefits of sequencing the entire genome of every human being at birth, while also discussing the main arguments against it.

Genetic variability in sporadic amyotrophic lateral sclerosis

With the advent of gene therapies for amyotrophic lateral sclerosis (ALS), there is a surge in gene testing for this disease. Although there is ample experience with gene testing for C9orf72, SOD1, FUS and TARDBP in familial ALS, large studies exploring genetic variation in all ALS-associated genes in sporadic ALS (sALS) are still scarce. Gene testing in a diagnostic setting is challenging, given the complex genetic architecture of sALS, for which there are genetic variants with large and small effect sizes. Guidelines for the interpretation of genetic variants in gene panels and for counselling of patients are lacking. We aimed to provide a thorough characterization of genetic variability in ALS genes by applying the American College of Medical Genetics and Genomics (ACMG) criteria on whole genome sequencing data from a large cohort of 6013 sporadic ALS patients and 2411 matched controls from Project MinE. We studied genetic variation in 90 ALS-associated genes and applied customized ACMG-criteria to identify pathogenic and likely pathogenic variants. Variants of unknown significance were collected as well. In addition, we determined the length of repeat expansions in C9orf72, ATXN1, ATXN2 and NIPA1 using the ExpansionHunter tool. We found C9orf72 repeat expansions in 5.21% of sALS patients. In 50 ALS-associated genes, we did not identify any pathogenic or likely pathogenic variants. In 5.89%, a pathogenic or likely pathogenic variant was found, most commonly in SOD1, TARDBP, FUS, NEK1, OPTN or TBK1. Significantly more cases carried at least one pathogenic or likely pathogenic variant compared to controls (odds ratio 1.75; P-value 1.64 × 10-5). Isolated risk factors in ATXN1, ATXN2, NIPA1 and/or UNC13A were detected in 17.33% of cases. In 71.83%, we did not find any genetic clues. A combination of variants was found in 2.88%. This study provides an inventory of pathogenic and likely pathogenic genetic variation in a large cohort of sALS patients. Overall, we identified pathogenic and likely pathogenic variants in 11.13% of ALS patients in 38 known ALS genes. In line with the oligogenic hypothesis, we found significantly more combinations of variants in cases compared to controls. Many variants of unknown significance may contribute to ALS risk, but diagnostic algorithms to reliably identify and weigh them are lacking. This work can serve as a resource for counselling and for the assembly of gene panels for ALS. Further characterization of the genetic architecture of sALS is necessary given the growing interest in gene testing in ALS.

Huntingtin CAG-expansion mutation results in a dominant negative effect

Introduction: Huntington's disease (HD) remains an incurable and fatal neurodegenerative disease long after CAG-expansion mutation in the huntingtin gene (HTT) was identified as the cause. The underlying pathological mechanism, whether HTT loss of function or gain of toxicity results from mutation, remains a matter of debate. Methods: In this study, we genetically modulated wild-type or mutant HTT expression levels in isogenic human embryonic stem cells to systematically investigate their contribution to HD-specific phenotypes. Results: Using highly reproducible and quantifiable in vitro micropattern-based assays, we observed comparable phenotypes with HD mutation and HTT depletion. However, halving endogenous wild-type HTT levels did not strongly recapitulate the HD phenotypes, arguing against a classical loss of function mechanism. Remarkably, expression of CAG-expanded HTT in non-HD cells induced HD like phenotypes akin to HTT depletion. Discussion: By corollary, these results indicate a dominant negative effect of mutated HTT on its wild-type counterpart. Complementation with additional copies of wild-type HTT ameliorated the HD-associated phenotypes, strongly supporting a classical dominant negative mechanism. Understanding the molecular basis of this dominant negative effect will guide the development of efficient clinical strategies to counteract the deleterious impact of mutant HTT on the wild-type HTT function.

The Pleiotropic Face of CREB Family Transcription Factors

cAMP responsive element-binding protein (CREB) is one of the most intensively studied phosphorylation-dependent transcription factors that provide evolutionarily conserved mechanisms of differential gene expression in vertebrates and invertebrates. Many cellular protein kinases that function downstream of distinct cell surface receptors are responsible for the activation of CREB. Upon functional dimerization of the activated CREB to cis-acting cAMP responsive elements within the promoters of target genes, it facilitates signal-dependent gene expression. From the discovery of CREB, which is ubiquitously expressed, it has been proven to be involved in a variety of cellular processes that include cell proliferation, adaptation, survival, differentiation, and physiology, through the control of target gene expression. In this review, we highlight the essential roles of CREB proteins in the nervous system, the immune system, cancer development, hepatic physiology, and cardiovascular function and further discuss a wide range of CREB-associated diseases and molecular mechanisms underlying the pathogenesis of these diseases.

Genetically transitional disease: a new concept in genomic medicine

Traditional classification of genetic diseases as monogenic and polygenic has lagged far behind scientific progress. In this opinion article, we propose and define a new terminology, genetically transitional disease (GTD), referring to cases where a large-effect mutation is necessary, but not sufficient, to cause disease. This leads to a working disease nosology based on gradients of four types of genetic architecture: monogenic, polygenic, GTD, and mixed. We present four scenarios under which GTD may occur; namely, subsets of traditionally Mendelian disease, modifiable Tier 1 monogenic conditions, variable penetrance, and situations where a genetic mutational spectrum produces qualitatively divergent pathologies. The implications of the new nosology in precision medicine are discussed, in which therapeutic options may target the molecular cause or the disease phenotype.

Reduced-penetrance Huntington's disease-causing alleles with 39 CAG trinucleotide repeats could be a genetic factor of amyotrophic lateral sclerosis

BACKGROUND

Expanded HTT alleles with 40 or more CAG repeats were recently found to be a rare cause of frontotemporal dementia and amyotrophic lateral sclerosis (ALS) spectrum diseases. The aim of this study was to investigate the role of HTT repeat expansions in a Taiwanese cohort with ALS.

METHODS

We analyzed the numbers of CAG repeats in exon 1 of HTT in a cohort of 410 Taiwanese patients with ALS and 1514 control individuals by utilizing polymerase chain reaction and amplicon fragment length analysis.

RESULTS

Only one of the 410 ALS patients carried a reduced-penetrance HD-causing allele with 39 CAG repeats, and none had an expanded HTT CAG repeats ≥40. The patient presented with rapidly progressive bulbar-onset ALS with disease onset at the age of 64 years. He had neither chorea nor cognitive impairment. He had a family history of chorea, but no other family member manifested with ALS. None of the 1514 control individuals carried an HTT expanded allele with CAG repeats larger than 37 repeats.

CONCLUSION

The HTT allele with 39 CAG repeats could be a genetic factor linked to ALS susceptibility.

Huntington's disease: Mortality and risk factors in an Australian cohort

INTRODUCTION

There has not been any examination of the risk factors associated with mortality in Huntington's Disease (HD) in an Australian cohort.

METHOD

This retrospective study included inpatients admitted to a specialist neuropsychiatry service in Melbourne, Australia. HD status was based on genetic testing. Risk factors included age of onset, CAG repeat length and neuroimaging. Mortality data was acquired through the Australian Institute of Health and Welfare National Death Index.

RESULTS

The cohort included 83 participants, with 44 (53%) deceased. The median age of death was 59 years and median survival was 18.8 years from onset age (median 41.0 years). CAG repeat length (median 44.0, IQR 42.5, 47.0) was inversely correlated with age of onset (r = -0.73) and age at death (r = -0.80) but was not correlated with mortality status. There was no difference in functional and cognitive assessments, nor brain volumes, in the alive group compared to the deceased group. There were more people who were alive who had a positive family history of a psychiatric condition (p = 0.006) or dementia (p = 0.009). Standardised mortality ratios demonstrated a 5.9× increased risk of death for those with HD compared to the general population.

CONCLUSIONS

This is the first study to examine risk factors of mortality in HD in an Australian cohort. Median survival in our cohort is consistent with previous studies in HD, and markedly reduced compared to the general Australian population. CAG repeat length was not associated with mortality suggesting that non-genetic factors contribute to mortality status and warrant further investigation.

A biological classification of Huntington's disease: the Integrated Staging System

The current research paradigm for Huntington's disease is based on participants with overt clinical phenotypes and does not address its pathophysiology nor the biomarker changes that can precede by decades the functional decline. We have generated a new research framework to standardise clinical research and enable interventional studies earlier in the disease course. The Huntington's Disease Integrated Staging System (HD-ISS) comprises a biological research definition and evidence-based staging centred on biological, clinical, and functional assessments. We used a formal consensus method that involved representatives from academia, industry, and non-profit organisations. The HD-ISS characterises individuals for research purposes from birth, starting at Stage 0 (ie, individuals with the Huntington's disease genetic mutation without any detectable pathological change) by using a genetic definition of Huntington's disease. Huntington's disease progression is then marked by measurable indicators of underlying pathophysiology (Stage 1), a detectable clinical phenotype (Stage 2), and then decline in function (Stage 3). Individuals can be precisely classified into stages based on thresholds of stage-specific landmark assessments. We also demonstrated the internal validity of this system. The adoption of the HD-ISS could facilitate the design of clinical trials targeting populations before clinical motor diagnosis and enable data standardisation across ongoing and future studies.

The evolutionary history of the polyQ tract in huntingtin sheds light on its functional pro-neural activities

Huntington's disease is caused by a pathologically long (>35) CAG repeat located in the first exon of the Huntingtin gene (HTT). While pathologically expanded CAG repeats are the focus of extensive investigations, non-pathogenic CAG tracts in protein-coding genes are less well characterized. Here, we investigated the function and evolution of the physiological CAG tract in the HTT gene. We show that the poly-glutamine (polyQ) tract encoded by CAGs in the huntingtin protein (HTT) is under purifying selection and subjected to stronger selective pressures than CAG-encoded polyQ tracts in other proteins. For natural selection to operate, the polyQ must perform a function. By combining genome-edited mouse embryonic stem cells and cell assays, we show that small variations in HTT polyQ lengths significantly correlate with cells' neurogenic potential and with changes in the gene transcription network governing neuronal function. We conclude that during evolution natural selection promotes the conservation and purity of the CAG-encoded polyQ tract and that small increases in its physiological length influence neural functions of HTT. We propose that these changes in HTT polyQ length contribute to evolutionary fitness including potentially to the development of a more complex nervous system.

Incomplete Penetrance and Variable Expressivity: From Clinical Studies to Population Cohorts

The same genetic variant found in different individuals can cause a range of diverse phenotypes, from no discernible clinical phenotype to severe disease, even among related individuals. Such variants can be said to display incomplete penetrance, a binary phenomenon where the genotype either causes the expected clinical phenotype or it does not, or they can be said to display variable expressivity, in which the same genotype can cause a wide range of clinical symptoms across a spectrum. Both incomplete penetrance and variable expressivity are thought to be caused by a range of factors, including common variants, variants in regulatory regions, epigenetics, environmental factors, and lifestyle. Many thousands of genetic variants have been identified as the cause of monogenic disorders, mostly determined through small clinical studies, and thus, the penetrance and expressivity of these variants may be overestimated when compared to their effect on the general population. With the wealth of population cohort data currently available, the penetrance and expressivity of such genetic variants can be investigated across a much wider contingent, potentially helping to reclassify variants that were previously thought to be completely penetrant. Research into the penetrance and expressivity of such genetic variants is important for clinical classification, both for determining causative mechanisms of disease in the affected population and for providing accurate risk information through genetic counseling. A genotype-based definition of the causes of rare diseases incorporating information from population cohorts and clinical studies is critical for our understanding of incomplete penetrance and variable expressivity. This review examines our current knowledge of the penetrance and expressivity of genetic variants in rare disease and across populations, as well as looking into the potential causes of the variation seen, including genetic modifiers, mosaicism, and polygenic factors, among others. We also considered the challenges that come with investigating penetrance and expressivity.

Sporadic Huntington's disease in the Philippines: a case report

Huntington's disease (HD) is an autosomal dominant neurodegenerative disorder with core clinical features of choreoathetosis, cognitive deficits and behavioral changes. It is a rare disorder, primarily affecting the Caucasian population, and rarely Asians. To date, there are only two reported, genetically proven familial HD cases in the Philippines. We present the case of a 39-year-old Filipino male with a 10-year history of progressive behavior and personality changes followed by cognitive decline and choreoathetotic movements. Neuroimaging showed atrophy of both caudate and putamen with putaminal rim sign. Genetic testing revealed a 47 CAG trinucleotide repeats in the Huntingtin gene; family history is negative. This is the first, genetically proven, sporadic and the third HD case in the Philippines. Despite its rarity, this report highlights the importance of including HD as a possible cause of adult-onset chorea among Filipinos.

Age of Onset of Huntington's Disease in Carriers of Reduced Penetrance Alleles

BACKGROUND

Age of manifest Huntington's disease (HD) onset correlates with number of CAG repeats in the huntingtin gene. Little is known about onset with 36 to 39 repeats, the "reduced penetrance" (RP) range.

OBJECTIVES

We provide allele-specific estimates of HD penetrance (diagnostic confidence level of 4) for RP allele carriers.

METHODS

We analyzed 431 pre-manifest RP allele carriers from Enroll-HD, the largest prospective observational HD study. Cumulative penetrance (CP) was estimated from Kaplan-Meier curves.

RESULTS

No one with 36 repeats (n = 25) phenoconverted. CP for 38 repeats (n = 120) was 32% (95% confidence interval [CI] 0%-55%) and 51% (CI, 10%-73%) by ages 70 and 75, respectively, and 68% (CI, 46%-81%) and 81% (CI, 58%-92%) by ages 70 and 75 for 39 repeats (n = 253). CP was not estimable at those ages for 37 repeats (n = 33).

CONCLUSIONS

Differences by RP-range repeat length did not reach significance with a 3-year median follow-up duration among censored individuals. © 2021 International Parkinson and Movement Disorder Society.

Huntingtin gene CAG repeat size affects autism risk: Family-based and case-control association study

The Huntingtin (HTT) gene contains a CAG repeat in exon 1, whose expansion beyond 39 repeats consistently leads to Huntington's disease (HD), whereas normal-to-intermediate alleles seemingly modulate brain structure, function and behavior. The role of the CAG repeat in Autism Spectrum Disorder (ASD) was investigated applying both family-based and case-control association designs, with the SCA3 repeat as a negative control. Significant overtransmission of "long" CAG alleles (≥17 repeats) to autistic children and of "short" alleles (≤16 repeats) to their unaffected siblings (all p < 10^-5 ) was observed in 612 ASD families (548 simplex and 64 multiplex). Surprisingly, both 193 population controls and 1,188 neurological non-HD controls have significantly lower frequencies of "short" CAG alleles compared to 185 unaffected siblings and higher rates of "long" alleles compared to 548 ASD patients from the same families (p < .05-.001). The SCA3 CAG repeat displays no association. "Short" HTT alleles seemingly exert a protective effect from clinically overt autism in families carrying a genetic predisposition for ASD, while "long" alleles may enhance autism risk. Differential penetrance of autism-inducing genetic/epigenetic variants may imply atypical developmental trajectories linked to HTT functions, including excitation/inhibition imbalance, cortical neurogenesis and apoptosis, neuronal migration, synapse formation, connectivity and homeostasis.

Frequency of the loss of CAA interruption in the HTT CAG tract and implications for Huntington disease in the reduced penetrance range

Purpose

In some Huntington disease (HD) patients, the “loss of interruption” (LOI) variant eliminates an interrupting codon in the HTT CAG-repeat tract, which causes earlier age of onset (AOO). The magnitude of this effect is uncertain, since previous studies included few LOI carriers, and the variant also causes CAG size misestimation. We developed a rapid LOI detection screen, enabling unbiased frequency estimation among manifest HD patients. Additionally, we combined published data with clinical data from newly identified patients to accurately characterize the LOI’s effect on AOO.

Methods

We developed a LOI detection polymerase chain reaction (PCR) assay, and screened patients to estimate the frequency of the LOI variant and its effect on AOO.

Results

Mean onset for LOI carriers (n = 49) is 20.4 years earlier than expected based on diagnosed CAG size. After correcting for CAG size underestimation, the variant is still associated with onset 9.5 years earlier. The LOI is present in 1.02% of symptomatic HD patients, and in 32.2% of symptomatic reduced penetrance (RP) range patients (36–39 CAGs).

Conclusion

The LOI causes significantly earlier onset, greater than expected by CAG length, particularly in persons with 36–39 CAG repeats. Detection of this variant has implications for HD families, especially for those in the RP range.

Genetic testing for neurodegenerative diseases: Ethical and health communication challenges

Advances in genomic science are informing an expansion of genetic testing for neurodegenerative diseases, which can be used for diagnostic and predictive purposes and performed in both medical and consumer genomics settings. Such testing-which is often for severe and incurable conditions like Huntington's, Alzheimer's, and Parkinson's diseases-raises important ethical and health communication challenges. This review addresses such challenges in the contexts of clinical, research, and direct-to-consumer genetic testing; these include informed consent, risk estimation and communication, potential benefits and psychosocial harms of genetic information (e.g., genetic discrimination), access to services, education and workforce needs, and health policies. The review also highlights future areas of likely growth in the field, including polygenic risk scores, use of genetic testing in clinical trials, and return of individual research results.

Cerebellar Predominant Increase in mRNA Expression Levels of Sirt1 and Sirt3 Isoforms in a Transgenic Mouse Model of Huntington's Disease

The potential role of Sirt1 and Sirt2 subtypes of Sirtuins (class III NAD⁺-dependent deacetylases) in the pathogenesis of Huntington’s disease (HD) has been extensively studied yielding some controversial results. However, data regarding the involvement of Sirt3 and their variants in HD are considerably limited. The aim of this study was to assess the expression pattern of Sirt1 and three Sirt3 mRNA isoforms (Sirt3-M1/2/3) in the striatum, cortex and cerebellum in respect of the effect of gender, age and the presence of the transgene using the N171-82Q transgenic mouse model of HD. Striatal, cortical and cerebellar Sirt1-Fl and Sirt3-M1/2/3 mRNA levels were measured in 8, 12 and 16 weeks old N171-82Q transgenic mice and in their wild-type littermates. Regarding the striatum and cortex, the presence of the transgene resulted in a significant increase in Sirt3-M3 and Sirt1 mRNA levels, respectively, whereas in case of the cerebellum the transgene resulted in increased expression of all the assessed subtypes and isoforms. Aging exerted minor influence on Sirt mRNA expression levels, both in transgene carriers and in their wild-type littermates, and there was no interaction between the presence of the transgene and aging. Furthermore, there was no difference between genders. The unequivocal cerebellar Sirtuin activation with presumed compensatory role suggests that the cerebellum might be another key player in HD in addition to the most severely affected striatum. The mitochondrially acting Sirt3 may serve as an interesting novel therapeutic target in this deleterious condition.

Evaluating drug targets through human loss-of-function genetic variation

Naturally occurring human genetic variants that are predicted to inactivate protein-coding genes provide an in vivo model of human gene inactivation that complements knockout studies in cells and model organisms. Here we report three key findings regarding the assessment of candidate drug targets using human loss-of-function variants. First, even essential genes, in which loss-of-function variants are not tolerated, can be highly successful as targets of inhibitory drugs. Second, in most genes, loss-of-function variants are sufficiently rare that genotype-based ascertainment of homozygous or compound heterozygous 'knockout' humans will await sample sizes that are approximately 1,000 times those presently available, unless recruitment focuses on consanguineous individuals. Third, automated variant annotation and filtering are powerful, but manual curation remains crucial for removing artefacts, and is a prerequisite for recall-by-genotype efforts. Our results provide a roadmap for human knockout studies and should guide the interpretation of loss-of-function variants in drug development.

Investigation of intermediate CAG alleles of the HTT in the general population of Rio de Janeiro, Brazil, in comparison with a sample of Huntington disease-affected families

BACKGROUND

Huntington disease (HD) (MIM: 143100) is a severe autosomal dominant neurodegenerative disease caused by the expansion of CAG trinucleotides (>35) in the HTT.

OBJECTIVE

To investigate the frequency of intermediate CAG alleles (IAs) in individuals residing in Rio de Janeiro city with no familial history of HD (general population, GP) in comparison with a sample of individuals from families presenting with HD who were previously investigated by our group (affected sample, AS).

RESULTS

The frequency of normal CAG alleles was 96.2%, while that of IAs was 3.6%, and that of reduced penetrance alleles was 0.2% in the GP (n = 470 chromosomes); 7.2% (17/235 individuals) of the GP presented an IA in heterozygosis with a normal allele. There was no statistically significant difference between the frequencies of the IAs in the GP and in the AS (p = .9). The most frequent haplotype per normal allele was (CAG)17-(CCG)7 (101/461) and per IA was (CAG)27-(CCG)7 (6/17) in the GP. These haplotypes were also the most frequent in the normal and IA chromosomes of the AS, respectively.

CONCLUSION

The genetic profiles of the IAs obtained from GP and AS were rather similar. It is important to investigate the frequencies of the IAs because expansions arise from a step-by-step mechanism in which, during intergenerational transmission, large normal alleles can generate IAs, which are then responsible for generating de novo HD mutations. The genetic investigation of IAs in the GP was also important because it was focused on the population of Rio de Janeiro, an understudied group. CCG7 was the most frequent CCG allele in linkage disequilibrium with normal, intermediate, and expanded CAG alleles, similar to the Western Europe population. However, a more robust investigation, in conjunction with haplogroup determination (A, B, or C), will be required to elucidate the ancestral origin of the HTT mutations in Brazilians.

Risk Assessment for Huntington's Disease for (Future) Offspring Requires Offering Preconceptional CAG Analysis to Both Partners

Amongst the main reasons people at risk for Huntington's disease (HD) have for undergoing predictive genetic testing are planning a family and prevention of passing on an expanded CAG-repeat to future offspring. After having received an unfavourable test result, a couple may consider prenatal testing in the foetus or preimplantation genetic diagnostic testing (PGD) in embryos. Testing of the foetus or embryos is possible by means of direct testing of the expanded repeat. Optimal reliability in testing the foetus or embryos requires the establishment of the origin of the repeats of both parents in the foetus. For PGD the analysis is combined with or sometimes solely based on identification of the at-risk haplotype in the embryo. This policy implies that in the context of direct testing, the healthy partner's CAG repeat lengths in the HD gene are also tested, but with the expectation that the repeat lengths of the partner are within the normal range, with the proviso that the partner's pedigree is free of clinically confirmed HD. However, recent studies have shown that the expanded repeat has been observed more often in the general population than previously estimated. Moreover, we have unexpectedly observed an expanded repeat in the non-HD partner in four cases which had far-reaching consequences. Hence, we propose that in the context of reproductive genetic counselling, prior to a planned pregnancy, and irrespective of the outcome of the predictive test in the HD-partner, the non-HD partner should also be given the option of being tested on the expanded allele. International recommendations for predictive testing for HD should be adjusted.

Prevalence of Non-psychiatric Comorbidities in Pre-symptomatic and Symptomatic Huntington's Disease Gene Carriers in Poland

Huntington's disease (HD) is monogenic neurodegenerative disorder caused by CAG expansions within the Huntingtin gene (Htt); it has a prevalence of 1 in 10,000 worldwide and is invariably fatal. Typically, healthy individuals have fewer than 35 CAG repeats, while the CAG expansions range from 36 to ~200 in HD patients. The hallmark of HD is neurodegeneration, especially in the striatal nuclei, basal ganglia and cerebral cortex, leading to neurological symptoms that involve motor, cognitive, and psychiatric events. However, HD is a complex disorder that may also affect peripheral organs, so it is possible that HD patients could be affected by comorbidities. Hence, we investigated the prevalence of comorbid conditions in HD patients (pre-symptomatic and symptomatic groups) and compared the frequency of those conditions to a control group. Our groups represent 65% of HD gene carriers registered in Poland. We identified 8 clusters of comorbid conditions in both HD groups, namely: musculoskeletal, allergies, cardiovascular, neurological, gastrointestinal, thyroid, psychiatric, and ophthalmologic. We found that HD patients have a significantly higher percentage of co-existing conditions in comparison to the control group. Among the 8 clusters of diseases, musculoskeletal, psychiatric, and cardiovascular events were significantly more frequent in both pre- and symptomatic HD patients, while neurological and gastrointestinal clusters showed significantly higher occurrence in the HD symptomatic group. A greater recognition of comorbidity in HD might help to better understand health outcomes and improve clinical management.

Prevalence of Carriers of Intermediate and Pathological Polyglutamine Disease-Associated Alleles Among Large Population-Based Cohorts

Importance

Nine hereditary neurodegenerative diseases are known as polyglutamine diseases, including Huntington disease, 6 spinocerebellar ataxias (SCAs) (SCA1, SCA2, SCA3, SCA6, SCA7, and SCA17), dentatorubral-pallidoluysion atrophy, and spinal bulbar muscular atrophy.

Objective

To determine the prevalence of carriers of intermediate and pathological polyglutamine disease-associated alleles among the general population.

Design, Setting, and Participants

This observational cross-sectional study included data from 5 large European population-based cohorts that were compiled between 1997 and 2012, and the analyses were conducted in 2018. In total, 16 547 DNA samples were obtained from participants of the 5 cohorts. Individuals with a lifetime diagnosis of major depression were excluded (n = 2351). In the remaining 14 196 participants without an established polyglutamine disease diagnosis, the CAG repeat size in both alleles of all 9 polyglutamine disease-associated genes (PDAGs) (ie, ATXN1, ATXN2, ATXN3, CACNA1A, ATXN7, TBP, HTT, ATN1, and AR) was determined.

Exposure

The number of CAG repeats in the alleles of the 9 PDAGs.

Main Outcomes and Measures

The number of individuals with alleles within the intermediate or pathological range per PDAG, as well as differences in sex, age, and body mass index between individuals carrying alleles within the normal or intermediate range and individuals carrying alleles within the pathological range of PDAGs.

Results

In the 14 196 analyzed participants (age range, 18-99 years; 56.3% female), 10.7% had a CAG repeat number within the intermediate range of at least 1 PDAG. Moreover, up to 1.3% of the participants had a CAG repeat number within the disease-causing range, predominantly in the lower pathological range associated with elderly onset. No differences in sex, age, or body mass index were found between individuals with CAG repeat numbers within the pathological range and individuals with CAG repeat numbers within the normal or intermediate range.

Conclusions and Relevance

These results indicate a high prevalence of individuals carrying intermediate and pathological ranges of polyglutamine disease-associated alleles among the general population. Therefore, a substantially larger proportion of individuals than previously estimated may be at risk of developing a polyglutamine disease later in life or bearing children with a de novo mutation.

Tracking Expansions of Stable and Threshold Length Trinucleotide Repeat Tracts In Vivo and In Vitro Using Saccharomyces cerevisiae

Trinucleotide repeat (TNR) tracts are inherently unstable during DNA replication, leading to repeat expansions and/or contractions. Expanded tracts are the cause of over 40 neurodegenerative and neuromuscular diseases. In this chapter, we focus on the (CAG)_n and (CTG)_n repeat sequences that, when expanded, lead to Huntington's disease (HD) and myotonic dystrophy type 1 (DM1), respectively, as well as a number of other neurodegenerative diseases. TNR tracts in most individuals are relatively small and stable in terms of length. However, TNR tracts become increasingly prone to expansion as tract length increases, eventually leading to very long tracts that disrupt coding (e.g. HD) or noncoding (e.g., DM1) regions of the genome. It is important to understand the early stages in TNR expansions, that is, the transition from small, stable lengths to susceptible threshold lengths. We describe PCR-based in vivo assays, using the model system Saccharomyces cerevisiae, to determine and characterize the dynamic behavior of TNR tracts in the stable and threshold ranges. We also describe a simple in vitro system to assess tract dynamics during 5' single-stranded DNA (ssDNA) flap processing and to assess the role of different DNA metabolism proteins in these dynamics. These assays can ultimately be used to determine factors that influence the early stages of TNR tract expansion.

Brain Microbiota in Huntington's Disease Patients

One of the most important challenges facing medical science is to better understand the cause of neuronal pathology in neurodegenerative diseases. Such is the case for Huntington's disease (HD), a genetic disorder primarily caused by a triplet expansion in the Huntingtin gene (HTT). Although aberrant HTT is expressed from embryogenesis, it remains puzzling as to why the onset of disease symptoms manifest only after several decades of life. In the present study, we investigated the possibility of microbial infection in brain tissue from patients with HD, reasoning that perhaps mutated HTT could be deleterious for immune cells and neural tissue, and could facilitate microbial colonization. Using immunohistochemistry approaches, we observed a variety of fungal structures in the striatum and frontal cortex of seven HD patients. Some of these fungi were found in close proximity to the nucleus, or even as intranuclear inclusions. Identification of the fungal species was accomplished by next-generation sequencing (NGS). Interestingly, some genera, such as Ramularia, appeared unique to HD patients, and have not been previously described in other neurodegenerative diseases. Several bacterial species were also identified both by PCR and NGS. Notably, a curved and filamentous structure that immunoreacts with anti-bacterial antibodies was characteristic of HD brains and has not been previously observed in brain tissue from neurodegenerative patients. Prevalent bacterial genera included Pseudomonas, Acinetobacter, and Burkholderia. Collectively, our results represent the first attempt to identify the brain microbiota in HD. Our observations suggest that microbial colonization may be a risk factor for HD and might explain why the onset of the disease appears after several decades of life. Importantly, they may open a new field of investigation and could help in the design of new therapeutic strategies for this devastating disorder.

A genetic association study of glutamine-encoding DNA sequence structures, somatic CAG expansion, and DNA repair gene variants, with Huntington disease clinical outcomes

BACKGROUND

Huntington disease (HD) is caused by an unstable CAG/CAA repeat expansion encoding a toxic polyglutamine tract. Here, we tested the hypotheses that HD outcomes are impacted by somatic expansion of, and polymorphisms within, the HTT CAG/CAA glutamine-encoding repeat, and DNA repair genes.

METHODS

The sequence of the glutamine-encoding repeat and the proportion of somatic CAG expansions in blood DNA from participants inheriting 40 to 50 CAG repeats within the TRACK-HD and Enroll-HD cohorts were determined using high-throughput ultra-deep-sequencing. Candidate gene polymorphisms were genotyped using kompetitive allele-specific PCR (KASP). Genotypic associations were assessed using time-to-event and regression analyses.

FINDINGS

Using data from 203 TRACK-HD and 531 Enroll-HD participants, we show that individuals with higher blood DNA somatic CAG repeat expansion scores have worse HD outcomes: a one-unit increase in somatic expansion score was associated with a Cox hazard ratio for motor onset of 3·05 (95% CI = 1·94 to 4·80, p = 1·3 × 10^-6). We also show that individual-specific somatic expansion scores are associated with variants in FAN1 (pFDR = 4·8 × 10^-6), MLH3 (pFDR = 8·0 × 10^-4), MLH1 (pFDR = 0·004) and MSH3 (pFDR = 0·009). We also show that HD outcomes are best predicted by the number of pure CAGs rather than total encoded-glutamines.

INTERPRETATION

These data establish pure CAG length, rather than encoded-glutamine, as the key inherited determinant of downstream pathophysiology. These findings have implications for HD diagnostics, and support somatic expansion as a mechanistic link for genetic modifiers of clinical outcomes, a driver of disease, and potential therapeutic target in HD and related repeat expansion disorders.

FUNDING

CHDI Foundation.

Predictive testing for Huntington disease over 24 years: Evolution of the profile of the participants and analysis of symptoms

BACKGROUND

Huntington disease (HD) is a devastating neurodegenerative autosomal dominant genetic condition. Predictive testing (PT) is available through a defined protocol for at-risk individuals. We analyzed the over-24-years evolution of practices regarding PT for HD in a single center.

METHODS

We gathered data from the files of all individuals seeking PT for HD in Lyon, France, from 1994 to 2017.

RESULTS

448 out of 567 participants had exploitable data. Age at consultation dichotomized over 24 years toward an eightfold increase in individuals aged >55 (2/94 vs. 30/183; 2% to 16%; p < .0001) and twice as many individuals aged 18-20 (3/94 vs. 12/183; 3%-7%; p < .05). Motives for testing remained stable. The rate of withdrawal doubled over 24 years (9/94 vs. 38/183; 9%-21%; p < .02). Independently of the time period, less withdrawal was observed for married, accompanied, at 50% risk, and symptomatic individuals, and in those able to explicit the motives for testing or taking the test to inform their children. We also assessed the consistency between the presence of subtle symptoms compatible with HD found before the test by the team's neurologist, and the positivity of the molecular test. The concordance was 100% (17/17) for associated motor and cognitive signs, 87% (27/31) for isolated motor signs, and 70% (7/10) for isolated cognitive signs. Furthermore, 91% (20/22) of individuals who requested testing because they thought they had symptoms, were indeed found carriers.

CONCLUSION

This over-24 years study underlines an increasing withdrawal from protocol and a dichotomization of participants' age. We also show a strong concordance between symptoms perceived by the neurologist or by the patient, and the subsequent positivity of the predictive molecular test.

Effect of Apolipoprotein E Genotypes on Huntington's Disease Phenotypes in a Han Chinese Population

Huntington's disease (HD) is an autosomal dominant degenerative disease that mainly encompasses movement, cognition, and behavioral symptoms. The apolipoprotein E (APOE) gene is thought to be associated with many neurodegenerative diseases. Here, we enrolled a cohort of 223 unrelated Han Chinese patients with HD and 1241 unrelated healthy controls in Southeastern China and analyzed the correlation between APOE genotypes and HD phenotypes. The results showed that the frequency of the E4 allele (7.1%) in HD patients was statistically less than that in controls (12.0%) (P =0.004). In addition, we divided patients into motor-onset and non-motor-onset groups, and analyzed the relationship with APOE genotypes. The results, however, were negative. Furthermore, the age at onset (AAO), defined as the age at the onset of motor symptoms, was compared in each APOE genotype subgroup and multivariate regression analysis was used to exclude the interference of CAG repeat length on AAO, but no association was found between APOE genotypes and AAO. Finally, we analyzed adult-onset HD to exclude the interference caused by juvenile HD (n = 13), and the results were negative. Therefore, our study suggests that APOE may not be a genetic modifier for HD, especially for adult-onset HD among Chinese of Han ethnicity. To the best of our knowledge, this is the first study of the correlation between APOE genotypes and HD phenotypes in a Han Chinese population.

Length of Uninterrupted CAG, Independent of Polyglutamine Size, Results in Increased Somatic Instability, Hastening Onset of Huntington Disease

Huntington disease (HD) is caused by a CAG repeat expansion in the huntingtin (HTT) gene. Although the length of this repeat is inversely correlated with age of onset (AOO), it does not fully explain the variability in AOO. We assessed the sequence downstream of the CAG repeat in HTT [reference: (CAG)n-CAA-CAG], since variants within this region have been previously described, but no study of AOO has been performed. These analyses identified a variant that results in complete loss of interrupting (LOI) adenine nucleotides in this region [(CAG)n-CAG-CAG]. Analysis of multiple HD pedigrees showed that this LOI variant is associated with dramatically earlier AOO (average of 25 years) despite the same polyglutamine length as in individuals with the interrupting penultimate CAA codon. This LOI allele is particularly frequent in persons with reduced penetrance alleles who manifest with HD and increases the likelihood of presenting clinically with HD with a CAG of 36-39 repeats. Further, we show that the LOI variant is associated with increased somatic repeat instability, highlighting this as a significant driver of this effect. These findings indicate that the number of uninterrupted CAG repeats, which is lengthened by the LOI, is the most significant contributor to AOO of HD and is more significant than polyglutamine length, which is not altered in these individuals. In addition, we identified another variant in this region, where the CAA-CAG sequence is duplicated, which was associated with later AOO. Identification of these cis-acting modifiers have potentially important implications for genetic counselling in HD-affected families.

Genetic Counseling in Huntington's Disease: Potential New Challenges on Horizon?

Huntington's disease (HD) is a rare, hereditary, neurodegenerative and dominantly transmitted disorder affecting about 10 out of 100,000 people in Western Countries. The genetic cause is a CAG repeat expansion in the huntingtin gene (HTT), which is unstable and may further increase its length in subsequent generations, so called anticipation. Mutation repeat length coupled with other gene modifiers and environmental factors contribute to the age at onset in the offspring. Considering the unpredictability of age at onset and of clinical prognosis in HD, the accurate interpretation, a proper psychological support and a scientifically sound and compassionate communication of the genetic test result are crucial in the context of Good Clinical Practice and when considering further potential disease-modifying therapies. We discuss various genetic test scenarios that require a particularly careful attention in psychological and genetic counseling and expect that the counseling procedures will require a constant update.

A Study of Triplet-Primed PCR for Identification of CAG Repeat Expansion in the HTT Gene in a Cohort of 503 Indian Cases with Huntington's Disease Symptoms

BACKGROUND

Huntington's disease (HD) is an autosomal-dominant neurodegenerative disorder with an average age at onset of 40 years. It is a polyglutamine (polyQ) disorder that is caused by an increase in the number of CAG repeats in the huntingtin (HTT) gene. Genetic tests that accurately determine the number of CAG repeats are performed for confirmation of diagnosis, predictive testing of persons at genetic risk for inheriting HD, and prenatal testing. The aim of our study was to evaluate efficacy of triplet-primed polymerase chain reaction (TP-PCR) for routine diagnosis of HD in suspected cases from India.

METHODS

We evaluated a combination of CAG flanking PCR and triplet-primed PCR for estimation of CAG repeats in 503 cases with clinical suspicion of HD.

RESULTS

There were 250 cases (49.7%) that showed the presence of expanded alleles, with 241 (47.9%) being fully penetrant alleles and nine (1.8%) in the reduced penetrance category. There were seven juvenile cases with an age of onset of < 20 years, with the longest allele comprising 106 CAG repeats found in an 8-year-old male patient. The results demonstrated an inverse (R = - 0.67) relationship between CAG length and age at clinical onset.

CONCLUSION

Our study on pan-Indian cases is one of the largest studies reported so far in India and focuses on the most accurate and comprehensive molecular diagnostic evaluation of HD.

Genetic Modification of Huntington Disease Acts Early in the Prediagnosis Phase

Age at onset of Huntington disease, an inherited neurodegenerative disorder, is influenced by the size of the disease-causing CAG trinucleotide repeat expansion in HTT and by genetic modifier loci on chromosomes 8 and 15. Stratifying by modifier genotype, we have examined putamen volume, total motor score (TMS), and symbol digit modalities test (SDMT) scores, both at study entry and longitudinally, in normal controls and CAG-expansion carriers who were enrolled prior to the emergence of manifest HD in the PREDICT-HD study. The modifiers, which included onset-hastening and onset-delaying alleles on chromosome 15 and an onset-hastening allele on chromosome 8, revealed no major effect in controls but distinct patterns of modification in prediagnosis HD subjects. Putamen volume at study entry showed evidence of reciprocal modification by the chromosome 15 alleles, but the rate of loss of putamen volume was modified only by the deleterious chromosome 15 allele. By contrast, both alleles modified the rate of change of the SDMT score, but neither had an effect on the TMS. The influence of the chromosome 8 modifier was evident only in the rate of TMS increase. The data indicate that (1) modification of pathogenesis can occur early in the prediagnosis phase, (2) the modifier loci act in genetic interaction with the HD mutation rather than through independent additive effects, and (3) HD subclinical phenotypes are differentially influenced by each modifier, implying distinct effects in different cells or tissues. Together, these findings indicate the potential benefit of using genetic modifier strategies for dissecting the prediagnosis pathogenic process in HD.

Neuro-Cardio Mechanisms in Huntington's Disease and Other Neurodegenerative Disorders

Although Huntington's disease is generally considered to be a neurological disorder, there is mounting evidence that heart malfunction plays an important role in disease progression. This is perhaps not unexpected since both cardiovascular and nervous systems are strongly connected - both developmentally and subsequently in health and disease. This connection occurs through a system of central and peripheral neurons that control cardiovascular performance, while in return the cardiovascular system works as a sensor for the nervous system to react to physiological events. Hence, given their permanent interconnectivity, any pathological events occurring in one system might affect the second. In addition, some pathological signals from Huntington's disease might occur simultaneously in both the cardiovascular and nervous systems, since mutant huntingtin protein is expressed in both. Here we aim to review the source of HD-related cardiomyopathy in the light of recently published studies, and to identify similarities between HD-related cardiomyopathy and other neuro-cardio disorders.

The molecular epidemiology of Huntington disease is related to intermediate allele frequency and haplotype in the general population

Huntington disease (HD) is the most common monogenic neurodegenerative disorder in populations of European ancestry, but occurs at lower prevalence in populations of East Asian or black African descent. New mutations for HD result from CAG repeat expansions of intermediate alleles (IAs), usually of paternal origin. The differing prevalence of HD may be related to the rate of new mutations in a population, but no comparative estimates of IA frequency or the HD new mutation rate are available. In this study, we characterize IA frequency and the CAG repeat distribution in fifteen populations of diverse ethnic origin. We estimate the HD new mutation rate in a series of populations using molecular IA expansion rates. The frequency of IAs was highest in Hispanic Americans and Northern Europeans, and lowest in black Africans and East Asians. The prevalence of HD correlated with the frequency of IAs by population and with the proportion of IAs found on the HD-associated A1 haplotype. The HD new mutation rate was estimated to be highest in populations with the highest frequency of IAs. In European ancestry populations, one in 5,372 individuals from the general population and 7.1% of individuals with an expanded CAG repeat in the HD range are estimated to have a molecular new mutation. Our data suggest that the new mutation rate for HD varies substantially between populations, and that IA frequency and haplotype are closely linked to observed epidemiological differences in the prevalence of HD across major ancestry groups in different countries.

Genetic modifiers of Mendelian disease: Huntington's disease and the trinucleotide repeat disorders

In the decades since the genes and mutations associated with the commoner Mendelian disorders were first discovered, technological advances in genetic analysis have made finding genomic variation a much less onerous task. Recently, the global efforts to collect subjects with Mendelian disorders, to better define the disorders and to empower appropriate clinical trials, along with improved genetic technologies, have allowed the identification of genetic variation that does not cause disease, but substantially modifies disease presentation. The advantage of this is it identifies biological pathways and molecules, that, if modified in people, might alter disease presentation. In Huntington's disease (HD), caused by an expanded CAG repeat tract in HTT, genetic variation has been uncovered that is associated with change in the onset or progression of disease. Some of this variation lies in genes that are part of the DNA damage response, previously suggested to be important in modulating expansion of the repeat tract in germline and somatic cells. The genetic evidence implicates a DNA damage response-related pathway in modulating the pathogenicity of the repeat tracts in HD, and possibly, in other trinucleotide repeat disorders. These findings offer new targets for drug development in these currently intractable disorders.

Possible Role of the Polyglutamine Elongation in Evolution of Amyloid-Related Evolvability

The polyglutamine (polyQ) diseases, such as Huntington's disease and the spinocerebellar ataxias, are characterized by the accumulation of elongated polyQ sequences (epolyQ) and mostly occur during midlife. Considering that polyQ disorders have not been selected out in evolution, there might be important physiological functions of epolyQ during development and/or reproduction. In a similar context, the physiological functions of neurodegeneration-associated amyloidogenic proteins (APs), such as β-amyloid in Alzheimer's disease and α-synuclein in Parkinson's disease, remain elusive. In this regard, we recently proposed that evolvability for coping with diverse stressors in the brain, which is beneficial for offspring, might be relevant to the physiological functions of APs. Given analogous properties of APs and epolyQ in terms of neurotoxic amyloid-fibril formation, the objective of this paper is to determine whether evolvability could also be applied to the physiological functions of epolyQ. Indeed, APs and epolyQ are similar in many ways, including functional redundancy of non-amyloidogenic homologues, hormesis conferred by the heterogeneity of the stress-induced protein aggregates, the transgenerational prion-like transmission of the protein aggregates via germ cells, and the antagonistic pleiotropy relationship between evolvability and neurodegenerative disease. Given that epolyQ is widely expressed from microorganisms to human brain, whereas APs are only identified in vertebrates, evolvability of epolyQ is considered to be much more primitive compared to those of APs during evolution. Collectively, epolyQ may be not only be important in the pathophysiology of polyQ diseases, but also in the evolution of amyloid-related evolvability.

Measuring Dynamic Behavior of Trinucleotide Repeat Tracts In Vivo in Saccharomyces cerevisiae

Trinucleotide repeat (TNR) tracts are inherently unstable during DNA replication, leading to repeat expansions and/or contractions. Expanded tracts are the cause of over 40 neurodegenerative and neuromuscular diseases. In this chapter, we focus on the (CNG)_n repeat sequences that, when expanded, lead to Huntington's disease (HD), myotonic dystrophy type 1 (DM1), and a number of other neurodegenerative diseases. We describe a series of in vivo assays, using the model system Saccharomyces cerevisiae, to determine and characterize the dynamic behavior of TNR tracts that are in the early stages of expansion, i.e., the so-called threshold range. Through a series of time courses and PCR-based assays, dynamic changes in tract length can be observed as a function of time. These assays can ultimately be used to determine how genetic factors influence the process of tract expansion in these early stages.