CAG size-specific risk estimates for intermediate allele repeat instability in Huntington disease

Ubiquitin specific peptidase 11 knockdown slows Huntington's disease progression via regulating mitochondrial dysfunction and neuronal damage depending on PTEN-mediated AKT pathway

BACKGROUND

Mitochondrial dysfunction and neuronal damage are major sign of cytopathology in Huntington's disease (HD), a neurodegenerative disease. Ubiquitin specific peptidase 11 (USP11) is a deubiquitinating enzyme involved in various physiological processes through regulating protein degradation. However, its specific role in HD is unclear.

METHODS

To interfere with USP11 expression, adeno-associated viruses 2 containing USP11-specific shRNA were injected into the bilateral striatum of 12-week-old R6/1 and WT mice. In vitro, the inducible PC12 cell model of HD was used in which the expression of an N-terminal truncation of huntingtin, with either wild type (Q23) or expanded polyglutamine (Q74) can be induced by the doxycycline. USP11 was knocked down to study its role in HD. The protein expression patterns in Q74 cells were quantified by label-free proteomics to further explore the target protein of USP11. Detecting the association between USP11 and Phosphatase and Tensin Homolog (PTEN) through Co-IP.

RESULTS

Herein, USP11 was found to be upregulated in the striatum of R6/1 mice (an HD model with gradual development of symptoms) in an age-dependent manner. The spontaneous HD was alleviated by silencing USP11, as evidenced by improved locomotor activity and spatial memory, attenuated striatal atrophy in R6/1 mice, reduced accumulation of mutant huntingtin protein, and restored mitochondrial function in vitro and in vivo. The results of label-free proteomics revealed a significant change in the protein expression profile. Through functional enrichment, we focused on PTEN, known as a negative regulator of the AKT pathway. We demonstrated that USP11 downregulation promoted ubiquitination modification of PTEN and activated the AKT pathway, and PTEN overexpression reversed the effects of USP11 knockdown.

CONCLUSIONS

Collectively, USP11 knockdown protects R6/1 mouse neurons from oxidative stress by alleviating mitochondrial dysfunction, thereby preventing the HD progression. This is achieved by inhibiting PTEN expression, which in turn activates the AKT pathway. This study suggests that USP11-PTEN-AKT signaling pathway may be a new attractive therapeutic target for HD.

Regulation of HTT mRNA Biogenesis: The Norm and Pathology

Huntington's disease (HD) is a neurodegenerative disorder caused by the expansion of the CAG repeat in exon 1 of the HTT gene, leading to the formation of a toxic variant of the huntingtin protein. It is a rare but severe hereditary disease for which no effective treatment method has been found yet. The primary therapeutic targets include the mutant protein and the mutant mRNA of HTT. Current clinical trial approaches in gene therapy involve the application of splice modulation, siRNA, or antisense oligonucleotides for RNA-targeted knockdown of HTT. However, these approaches do not take into account the diversity of HTT transcript isoforms in the normal conditions and in HD. In this review, we discuss the features of transcriptional regulation and processing that lead to the formation of various HTT mRNA variants, each of which may uniquely contribute to the progression of the disease. Furthermore, understanding the role of known transcription factors of HTT in pathology may aid in the development of potentially new therapeutic tools based on endogenous regulators.

Exploring the therapeutic potential: Apelin-13's neuroprotective effects foster sustained functional motor recovery in a rat model of Huntington's disease

Huntington's disease (HD) is a hereditary condition considered by the progressive degeneration of nerve cells in the brain, resultant in motor dysfunction and cognitive impairment. Despite current treatment modalities including pharmaceuticals and various therapies, a definitive cure remains elusive. Therefore, this study investigates the therapeutic potential effect of Apelin-13 in HD management. Thirty male Wistar rats were allocated into three groups: a control group, a group with HD, and a group with both HD and administered Apelin-13. Apelin-13 was administered continuously over a 28-day period at a dosage of around 30 mg/kg to mitigate inflammation in rats subjected to 3-NP injection within an experimental HD model. Behavioral tests, such as rotarod, electromyography (EMG), elevated plus maze, and open field assessments, demonstrated that Apelin-13 improved motor function and coordination in rats injected with 3-NP. Apelin-13 treatment significantly increased neuronal density and decreased glial cell counts compared to the control group. Immunohistochemistry analysis revealed reduced gliosis and expression of inflammatory factors in the treatment group. Moreover, Apelin-13 administration led to elevated levels of glutathione and reduced reactive oxygen species (ROS) level in the treated group. Apelin-13 demonstrates neuroprotective effects, leading to improved movement and reduced inflammatory and fibrotic factors in the HD model.

Somatic CAG repeat instability in intermediate alleles of the HTT gene and its potential association with a clinical phenotype

Huntington disease (HD) is a neurodegenerative disorder caused by ≥36 CAGs in the HTT gene. Intermediate alleles (IAs) (27-35 CAGs) are not considered HD-causing, but their potential association with neurocognitive symptoms remains controversial. As HTT somatic CAG expansion influences HD onset, we hypothesised that IAs are somatically unstable, and that somatic CAG expansion may drive phenotypic presentation in some IA carriers. We quantified HTT somatic CAG expansions by MiSeq sequencing in the blood DNA of 164 HD subjects and 191 IA (symptomatic and control) carriers, and in the brain DNA of a symptomatic 33 CAG carrier. We also performed genotype-phenotype analysis. The phenotype of symptomatic IA carriers was characterised by motor (85%), cognitive (27%) and/or behavioural (29%) signs, with a late (58.7 ± 18.6 years), but not CAG-dependent, age at onset. IAs displayed somatic expansion that were CAG and age-dependent in blood DNA, with 0.4% and 0.01% of DNA molecules expanding by CAG and year, respectively. Somatic expansions of +1 and +2 CAGs were detected in the brain of the individual with 33 CAGs, with the highest expansion frequency in the putamen (10.3%) and the lowest in the cerebellum (4.8%). Somatic expansion in blood DNA was not different in symptomatic vs. control IA carriers. In conclusion, we show that HTT IAs are somatically unstable, but we found no association with HD-like phenotypes. It is plausible, however, that some IAs, close to the HD pathological threshold and with a predisposing genetic background, could manifest with neurocognitive symptoms.

Highly Potent Peptide Therapeutics To Prevent Protein Aggregation in Huntington's Disease

Huntington's disease (HD) is a neurodegenerative disorder resulting from a significant amplification of CAG repeats in exon 1 of the Huntingtin (Htt) gene. More than 36 CAG repeats result in the formation of a mutant Htt (mHtt) protein. These amino-terminal mHtt fragments lead to the formation of misfolded proteins, which then form aggregates in the relevant brain regions. Therapies that can delay the progression of the disease are imperative to halting the course of the disease. Peptide-based drug therapies provide such a platform. Inhibitory peptides were screened against monomeric units of both wild type (Htt(Q25)) and mHtt fragments, Htt(Q46) and Htt(Q103). Fibril kinetics was studied by utilizing the Thioflavin T (ThT) assay. Atomic force microscopy was also used to study the influence of the peptides on fibril formation. These experiments demonstrate that the chosen peptides suppress the formation of fibrils in mHtt proteins and can provide a therapeutic lead for further optimization and development.

Diamond Quantum Sensing Revealing the Relation between Free Radicals and Huntington's Disease

Huntington's disease (HD) is a well-studied yet rare disease caused by a specific mutation that results in the expression of polyglutamine (PolyQ). The formation of aggregates of PolyQ leads to disease and increases the level of free radicals. However, it is unclear where free radicals are generated and how they impact cells. To address this, a new method called relaxometry was used to perform nanoscale MRI measurements with a subcellular resolution. The method uses a defect in fluorescent nanodiamond (FND) that changes its optical properties based on its magnetic surroundings, allowing for sensitive detection of free radicals. To investigate if radical generation occurs near PolyQ aggregates, stable tetracycline (tet)-inducible HDQ119-EGFP-expressing human embryonic kidney cells (HEK PQ) were used to induce the PolyQ formation and Huntington aggregation. The study found that NDs are highly colocalized with PolyQ aggregates at autolysosomes, and as the amount of PolyQ aggregation increased, so did the production of free radicals, indicating a relationship between PolyQ aggregation and autolysosome dysfunction.

Loss of CAA interruption and intergenerational CAG instability in Chinese patients with Huntington's disease

Huntington's disease (HD) is an autosomal dominant neurodegenerative disorder caused by CAG expansions in huntingtin (HTT) gene, involving motor, cognitive, and neuropsychiatric symptoms. However, genetic modifiers and CAG repeat instability may lead to variations of clinical manifestations, making diagnosis of HD difficult. In this study, we recruited 229 HD individuals from 164 families carrying expanded CAG repeats of HTT, and analyzed loss of CAA interruption (LOI) on the expanded allele and CAG instability during germline transmission. Sanger sequencing and TA cloning were used to determine CAG repeat length and identify LOI variants. Detailed clinical features and genetic testing results were collected. We identified 6 individuals with LOI variants from 3 families, and all probands presented with earlier motor onset age than predicted onset age. In addition, we also presented 2 families with extreme CAG instability during germline transmission. One family showed an expansion from 35 to 66 CAG repeats, while the other family showed both CAG expansion and contraction in lineal three generations. In conclusion, we present the first document of Asian HD population with LOI variant, and we suggest that for symptomatic individuals with intermediate or reduced penetrance allele or negative family history, HTT gene sequencing should be considered in the clinical practice. KEY MESSAGES : We screened the loss of CAA interruption (LOI) variant in a Chinese HD cohort and presented the first document of Asian patients with Huntington's disease carrying LOI variant. We identified 6 individuals with LOI variants from 3 families, and all probands presented with earlier motor onset age than predicted onset age. We presented 2 families with extreme CAG instability during germline transmission. One family showed an expansion from 35 to 66 CAG repeats, while the other family showed both CAG expansion and contraction in lineal three generations. We suggest that for symptomatic individuals with intermediate or reduced penetrance allele or negative family history, HTT gene sequencing should be considered in the clinical practice.

Pathogenesis and potential therapeutic application of stem cells transplantation in Huntington’s disease

Huntington's disease (HD) is a progressive neurodegenerative disorder which is caused due to repetitive CAG or glutamine expression along the coding region of the Huntington gene. This disease results in certain movement abnormalities, affective disturbances, dementia and cognitive impairments. To this date, there is no proper cure for this rare and fatal neurological condition but there have been certain advancements in the field of genetic animal model research studies to elucidate the understanding of the pathogenesis of this condition. Currently, HD follows a certain therapeutic approach which just relieves the symptoms but doesn't cure the underlying cause of the disease. Stem cell therapy can be a breakthrough in developing a potential cure for this condition. In this review, we have discussed the pathogenesis and the efficacy and clinical practicality of the therapeutic application of stem cell transplantation in Huntington's disease. The application of this groundbreaking therapy on genetically altered animal models has been listed and analyzed in brief.

Zebrafish as a model organism for neurodegenerative disease

The zebrafish is increasingly recognized as a model organism for translational research into human neuropathology. The zebrafish brain exhibits fundamental resemblance with human neuroanatomical and neurochemical pathways, and hallmarks of human brain pathology such as protein aggregation, neuronal degeneration and activation of glial cells, for example, can be modeled and recapitulated in the fish central nervous system. Genetic manipulation, imaging, and drug screening are areas where zebrafish excel with the ease of introducing mutations and transgenes, the expression of fluorescent markers that can be detected in vivo in the transparent larval stages overtime, and simple treatment of large numbers of fish larvae at once followed by automated screening and imaging. In this review, we summarize how zebrafish have successfully been employed to model human neurodegenerative diseases such as Parkinson’s disease, Alzheimer’s disease, amyotrophic lateral sclerosis, and Huntington’s disease. We discuss advantages and disadvantages of choosing zebrafish as a model for these neurodegenerative conditions.

Autophagy system as a potential therapeutic target for neurodegenerative diseases

Autophagy is an evolutionally conserved process by which cytoplasmic contents including protein aggregates and damaged organelles such as mitochondria and lysosomes, are sequestered by double-membrane structure, autophagosomes, and delivered to the lysosomes for degradation. Recently, considerable efforts have been made to reveal the role of autophagy in neurodegenerative diseases like Alzheimer's disease, Parkinson's disease and Huntington's disease. Impairment of autophagy aggravates the accumulation of misfolded protein and damaged organelles in neurons, while sufficient autophagic activity reduces such accumulation in nervous system and ameliorates the pathology. Here we summarize recent progress regarding the role of autophagy in several neurodegenerative diseases and the potential autophagy-associated therapies for them.

Modifiers of CAG/CTG Repeat Instability: Insights from Mammalian Models

At fifteen different genomic locations, the expansion of a CAG/CTG repeat causes a neurodegenerative or neuromuscular disease, the most common being Huntington's disease and myotonic dystrophy type 1. These disorders are characterized by germline and somatic instability of the causative CAG/CTG repeat mutations. Repeat lengthening, or expansion, in the germline leads to an earlier age of onset or more severe symptoms in the next generation. In somatic cells, repeat expansion is thought to precipitate the rate of disease. The mechanisms underlying repeat instability are not well understood. Here we review the mammalian model systems that have been used to study CAG/CTG repeat instability, and the modifiers identified in these systems. Mouse models have demonstrated prominent roles for proteins in the mismatch repair pathway as critical drivers of CAG/CTG instability, which is also suggested by recent genome-wide association studies in humans. We draw attention to a network of connections between modifiers identified across several systems that might indicate pathway crosstalk in the context of repeat instability, and which could provide hypotheses for further validation or discovery. Overall, the data indicate that repeat dynamics might be modulated by altering the levels of DNA metabolic proteins, their regulation, their interaction with chromatin, or by direct perturbation of the repeat tract. Applying novel methodologies and technologies to this exciting area of research will be needed to gain deeper mechanistic insight that can be harnessed for therapies aimed at preventing repeat expansion or promoting repeat contraction.

New Avenues for the Treatment of Huntington's Disease

Huntington’s disease (HD) is a neurodegenerative disorder caused by a CAG expansion in the HD gene. The disease is characterized by neurodegeneration, particularly in the striatum and cortex. The first symptoms usually appear in mid-life and include cognitive deficits and motor disturbances that progress over time. Despite being a genetic disorder with a known cause, several mechanisms are thought to contribute to neurodegeneration in HD, and numerous pre-clinical and clinical studies have been conducted and are currently underway to test the efficacy of therapeutic approaches targeting some of these mechanisms with varying degrees of success. Although current clinical trials may lead to the identification or refinement of treatments that are likely to improve the quality of life of those living with HD, major efforts continue to be invested at the pre-clinical level, with numerous studies testing novel approaches that show promise as disease-modifying strategies. This review offers a detailed overview of the currently approved treatment options for HD and the clinical trials for this neurodegenerative disorder that are underway and concludes by discussing potential disease-modifying treatments that have shown promise in pre-clinical studies, including increasing neurotropic support, modulating autophagy, epigenetic and genetic manipulations, and the use of nanocarriers and stem cells.

An update on the neurological short tandem repeat expansion disorders and the emergence of long-read sequencing diagnostics

BACKGROUND

Short tandem repeat (STR) expansion disorders are an important cause of human neurological disease. They have an established role in more than 40 different phenotypes including the myotonic dystrophies, Fragile X syndrome, Huntington's disease, the hereditary cerebellar ataxias, amyotrophic lateral sclerosis and frontotemporal dementia.

MAIN BODY

STR expansions are difficult to detect and may explain unsolved diseases, as highlighted by recent findings including: the discovery of a biallelic intronic 'AAGGG' repeat in RFC1 as the cause of cerebellar ataxia, neuropathy, and vestibular areflexia syndrome (CANVAS); and the finding of 'CGG' repeat expansions in NOTCH2NLC as the cause of neuronal intranuclear inclusion disease and a range of clinical phenotypes. However, established laboratory techniques for diagnosis of repeat expansions (repeat-primed PCR and Southern blot) are cumbersome, low-throughput and poorly suited to parallel analysis of multiple gene regions. While next generation sequencing (NGS) has been increasingly used, established short-read NGS platforms (e.g., Illumina) are unable to genotype large and/or complex repeat expansions. Long-read sequencing platforms recently developed by Oxford Nanopore Technology and Pacific Biosciences promise to overcome these limitations to deliver enhanced diagnosis of repeat expansion disorders in a rapid and cost-effective fashion.

CONCLUSION

We anticipate that long-read sequencing will rapidly transform the detection of short tandem repeat expansion disorders for both clinical diagnosis and gene discovery.

Detecting and targeting neurodegenerative disorders using electrospun nanofibrous matrices: current status and applications

Neurodegeneration is defined as the progressive atrophy and loss of function of neurons; it is present in neurodegenerative disorders such as Multiple Sclerosis, Alzheimer's, Huntington's, and Parkinson's diseases. The detection of such disorders is performed by various imaging modalities while their therapeutic management is quite challenging. Besides, the pathogenesis of neurodegenerative disorders is still under ongoing research due to complex and multi-factorial mechanisms. Currently, targeting the specific proteins responsible for neurodegeneration is of great interest to many researchers. Furthermore, nanotechnology-based approaches for targeting the affected neurons became an emerging field of interest. Nanostructures of various forms have been developed aiming to act as therapeutics for neurodegeneration, in which electrospun nanofibers seem to play an important role as biomedical products for both detection and management of the diseases. Electrospinning is an intriguing method able to produce nanofibers with a wide range of sizes and morphological characteristics. Such nanofibrous matrices can be delivered through different administration routes to target various diseases. In this review, the most recent advancements in electrospun nanofibrous systems that target or detect multiple neurodegenerative diseases have been enlightened and an introduction to the general aspects of neurodegenerative diseases and the electrospinning process has been made. Finally, future perspectives of neurodegeneration targeting were also discussed.

Incidence of Huntington disease in a northeastern Spanish region: a 13-year retrospective study at tertiary care centre

Background

Despite the progress in the knowledge of Huntington disease (HD) in recent years, the epidemiology continues uncertain, so the study of incidence becomes relevant. This is important since various factors (type of population, diagnostic criteria, disease-modifying factors, etc.) make these data highly variable. Therefore, the genetic diagnosis of these patients is important, since it unequivocally allows the detection of new cases.

Methods

Descriptive retrospective study with 179 individuals. Incidence of HD was calculated from the ratio of number of symptomatic cases newly diagnosed per 100,000 inhabitants per year during the period 2007–2019 in Aragon (Spain).

Results

50 (27.9%) incident cases of HD (CAG repeat length ≥ 36) were identified from a total of 179 persons studied. The remaining 129/179 (72.1%) were HD negative (CAG repeat length < 36). 29 (58.0%) females and 21 (42.0%) males were confirmed as HD cases. The overall incidence was 0.648 per 100,000 patient-years. 11/50 positive HD cases (22.0%) were identified by performing a predictive test, without clinical symptoms. The minimum number of CAG repeats found was 9 and the most common CAG length among HD negative individuals was 16.

Conclusions

Our incidence lied within the range reported for other Caucasian populations. Implementation of new techniques has allowed to determine the exact number of CAG repeats, which is especially important in patients with triplet expansions in an HD intermediate and/or incomplete penetrance allele, both in diagnostic, predictive and prenatal tests.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12881-020-01174-z.

New developments in Huntington's disease and other triplet repeat diseases: DNA repair turns to the dark side

Huntington’s disease (HD) is a fatal, inherited neurodegenerative disease that causes neuronal death, particularly in medium spiny neurons. HD leads to serious and progressive motor, cognitive and psychiatric symptoms. Its genetic basis is an expansion of the CAG triplet repeat in the HTT gene, leading to extra glutamines in the huntingtin protein. HD is one of nine genetic diseases in this polyglutamine (polyQ) category, that also includes a number of inherited spinocerebellar ataxias (SCAs). Traditionally it has been assumed that HD age of onset and disease progression were solely the outcome of age-dependent exposure of neurons to toxic effects of the inherited mutant huntingtin protein. However, recent genome-wide association studies (GWAS) have revealed significant effects of genetic variants outside of HTT. Surprisingly, these variants turn out to be mostly in genes encoding DNA repair factors, suggesting that at least some disease modulation occurs at the level of the HTT DNA itself. These DNA repair proteins are known from model systems to promote ongoing somatic CAG repeat expansions in tissues affected by HD. Thus, for triplet repeats, some DNA repair proteins seem to abandon their normal genoprotective roles and, instead, drive expansions and accelerate disease. One attractive hypothesis—still to be proven rigorously—is that somatic HTT expansions augment the disease burden of the inherited allele. If so, therapeutic approaches that lower levels of huntingtin protein may need blending with additional therapies that reduce levels of somatic CAG repeat expansions to achieve maximal effect.

Distribution of the CAG Triplet Repeat in ATXN1, ATXN3, and CACNA1A Loci in Peruvian Population

Spinocerebellar ataxia subtypes 1, 3, and 6 (SCA1, MJD/SCA3, and SCA6) are among the most prevalent autosomal dominant cerebellar ataxias worldwide, but their relative frequencies in Peru are low. Frequency of large normal (LN) alleles at spinocerebellar ataxia-causative genes has been proposed to be associated with disease prevalence. To investigate the allelic distribution of the CAG repeat in ATXN1, ATXN3, and CACNA1A genes in a Peruvian mestizo population and examine their association with the relative frequency of SCA1, MJD/SCA3, and SCA6 across populations. We genotyped 213 healthy mestizo individuals from Northern Lima, Peru, for ATXN1, ATXN3, and CACNA1A using polymerase chain reaction (PCR) and polyacrylamide gel electrophoresis (PAGE). We compared the frequency of LN alleles and relative disease frequency between populations. We also tested 40 samples for CAT repeat interruptions within the CAG tract of ATXN1. We found no association between disease frequency and population frequency of LN alleles at ATXN1 and ATXN3. All 40 ATXN1 samples tested for CAT interruptions were positive. Frequency of LN alleles at CACNA1A correlates with SCA6 frequency across several populations, but this effect was largely driven by data from a single population. Low frequency of SCA1 and MJD/SCA3 in Peru is not explained by frequency of LN alleles at ATXN1 and ATXN3, respectively. The observed correlation between CACNA1A LN alleles and SCA6 frequency requires further assessment.

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 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.

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.

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.

HTT haplogroups in Finnish patients with Huntington disease

Objective

To study genetic causes of the low frequency of Huntington disease (HD) in the Finnish population, we determined HTT haplogroups in the population and patients with HD and analyzed intergenerational Cytosine-Adenosine-Guanosine (CAG) stability.

Methods

A national cohort of patients with HD was used to identify families with mutant HTT (mHTT). HTT haplogroups were determined in 225 archival samples from patients and from 292 population samples. CAG repeats were phased with HTT haplotypes using data from parent-offspring pairs and other mHTT carriers in the family.

Results

The allele frequencies of HTT haplotypes in the Finnish population differed from those in 411 non-Finnish European subjects (p < 0.00001). The frequency of haplogroup A was lower than that in Europeans and haplogroup C was higher. Haplogroup A alleles were significantly more common in patients than in controls. Among patients with HD haplotypes A1 and A2 were more frequent than among the controls (p = 0.003). The mean size of the CAG repeat change was +1.38 units in paternal transmissions being larger than that (−0.17) in maternal transmissions (p = 0.008). CAG repeats on haplogroup A increased by 3.18 CAG units in paternal transmissions, but only by 0.11 units in maternal transmissions (p = 0.008), whereas haplogroup C repeat lengths decreased in both paternal and maternal transmissions.

Conclusions

The low frequency of HD in Finland is partly explained by the low frequency of the HD-associated haplogroup A in the Finnish population. There were remarkable differences in intergenerational CAG repeat dynamics that depended on HTT haplotype and parent gender.

Proteostasis in Huntington's disease: disease mechanisms and therapeutic opportunities

Many neurodegenerative diseases are characterized by impairment of protein quality control mechanisms in neuronal cells. Ineffective clearance of misfolded proteins by the proteasome, autophagy pathways and exocytosis leads to accumulation of toxic protein oligomers and aggregates in neurons. Toxic protein species affect various cellular functions resulting in the development of a spectrum of different neurodegenerative proteinopathies, including Huntington's disease (HD). Playing an integral role in proteostasis, dysfunction of the ubiquitylation system in HD is progressive and multi-faceted with numerous biochemical pathways affected, in particular, the ubiquitin-proteasome system and autophagy routes for protein aggregate degradation. Unravelling the molecular mechanisms involved in HD pathogenesis of proteostasis provides new insight in disease progression in HD as well as possible therapeutic avenues. Recent developments of potential therapeutics are discussed in this review.

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.

Huntington disease

Huntington disease is a monogenic neurodegenerative disorder that displays an autosomal-dominant pattern of inheritance. It is characterized by motor, psychiatric, and cognitive symptoms that progress over 15-20 years. Since the identification of the causative genetic mutation in 1993 much has been discovered about the underlying pathogenic mechanisms, but as yet there are no disease-modifying therapies available. This chapter reviews the epidemiology, genetic basis, pathogenesis, presentation, and clinical management of Huntington disease. The principles of genetic testing are explained. We also describe recent developments in the ongoing search for therapeutics and for biomarkers to track disease progression.

Methods for Assessing DNA Repair and Repeat Expansion in Huntington's Disease

Huntington's disease (HD) is caused by a CAG repeat expansion in the HTT gene. Repeat length can change over time, both in individual cells and between generations, and longer repeats may drive pathology. Cellular DNA repair systems have long been implicated in CAG repeat instability but recent genetic evidence from humans linking DNA repair variants to HD onset and progression has reignited interest in this area. The DNA damage response plays an essential role in maintaining genome stability, but may also license repeat expansions in the context of HD. In this chapter we summarize the methods developed to assay CAG repeat expansion/contraction in vitro and in cells, and review the DNA repair genes tested in mouse models of HD. While none of these systems is currently ideal, new technologies, such as long-read DNA sequencing, should improve the sensitivity of assays to assess the effects of DNA repair pathways in HD. Improved assays will be essential precursors to high-throughput testing of small molecules that can alter specific steps in DNA repair pathways and perhaps ameliorate expansion or enhance contraction of the HTT CAG repeat.

Clinical Features of Huntington's Disease

Huntington's disease (HD) is the most common monogenic neurodegenerative disease and the commonest genetic dementia in the developed world. With autosomal dominant inheritance, typically mid-life onset, and unrelenting progressive motor, cognitive and psychiatric symptoms over 15-20 years, its impact on patients and their families is devastating. The causative genetic mutation is an expanded CAG trinucleotide repeat in the gene encoding the Huntingtin protein, which leads to a prolonged polyglutamine stretch at the N-terminus of the protein. Since the discovery of the gene over 20 years ago much progress has been made in HD research, and although there are currently no disease-modifying treatments available, there are a number of exciting potential therapeutic developments in the pipeline. In this chapter we discuss the epidemiology, genetics and pathogenesis of HD as well as the clinical presentation and management of HD, which is currently focused on symptomatic treatment. The principles of genetic testing for HD are also explained. Recent developments in therapeutics research, including gene silencing and targeted small molecule approaches are also discussed, as well as the search for HD biomarkers that will assist the validation of these potentially new treatments.

Genetic counseling and testing for Huntington's disease: A historical review

This manuscript describes the ways in which genetic counseling has evolved since John Pearson and Sheldon Reed first promoted "a genetic education" in the 1950s as a voluntary, non-directive clinical tool for permitting individual decision making. It reviews how the emergence of Huntington's disease (HD) registries and patient support organizations, genetic testing, and the discovery of a disease-causing CAG repeat expansion changed the contours of genetic counseling for families with HD. It also reviews the guidelines, outcomes, ethical and laboratory challenges, and uptake of predictive, prenatal, and preimplantation testing, and it casts a vision for how clinicians can better make use of genetic counseling to reach a broader pool of families that may be affected by HD and to ensure that genetic counseling is associated with the best levels of care. © 2016 Wiley Periodicals, Inc.

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

OBJECTIVE

To directly estimate the frequency and penetrance of CAG repeat alleles associated with Huntington disease (HD) in the general population.

METHODS

CAG repeat length was evaluated in 7,315 individuals from 3 population-based cohorts from British Columbia, the United States, and Scotland. The frequency of ≥36 CAG alleles was assessed out of a total of 14,630 alleles. The general population frequency of reduced penetrance alleles (36-39 CAG) was compared to the prevalence of patients with HD with genetically confirmed 36-39 CAG from a multisource clinical ascertainment in British Columbia, Canada. The penetrance of 36-38 CAG repeat alleles for HD was estimated for individuals ≥65 years of age and compared against previously reported clinical penetrance estimates.

RESULTS

A total of 18 of 7,315 individuals had ≥36 CAG, revealing that approximately 1 in 400 individuals from the general population have an expanded CAG repeat associated with HD (0.246%). Individuals with CAG 36-37 genotypes are the most common (36, 0.096%; 37, 0.082%; 38, 0.027%; 39, 0.000%; ≥40, 0.041%). General population CAG 36-38 penetrance rates are lower than penetrance rates extrapolated from clinical cohorts.

CONCLUSION

HD alleles with a CAG repeat length of 36-38 occur at high frequency in the general population. The infrequent diagnosis of HD at this CAG length is likely due to low penetrance. Another important contributing factor may be reduced ascertainment of HD in those of older age.

Prenatal testing in Huntington disease: after the test, choices recommence

The objective of this study was (1) to determine the impact of prenatal diagnosis (PND) for Huntington disease (HD) on subsequent reproductive choices and family structure; and (2) to assess whether children born after PND were informed of their genetic status. Out of 354 presymptomatic carriers of HD gene mutation, aged 18-45 years, 61 couples requested 101 PNDs. Fifty-four women, 29 female carriers and 25 spouses of male carriers, accepted to be interviewed (0.6-16.3 years after the last PND, median 6.5 years) on their obstetrical history and information given to children born after PND. Women were willing to undergo two or more PNDs with a final success rate of 75%. Reproductive decisions differed depending on the outcome of the first PND. If favourable, 62% couples decided against another pregnancy and 10% chose to have an untested child. If unfavourable, 83% decided for another pregnancy (P<0.01), and the majority (87%) re-entered the PND procedure. In contrast, after a second PND, only 37% asked for a PND and 30% chose to have an untested child. Thirty-three percent had both, tested and untested children. Among children born after PND, 10 years and older, 75% were informed of their genetic status. The decision to prevent transmission of the HD mutation is made anew with each pregnancy. Couples may need more psychological support after PND and pre-counselling sessions should take into account the effect of the outcome of a first PND on subsequent reproductive choices.

CAG repeat size in Huntingtin alleles is associated with cancer prognosis

The abnormal expansion of a ≥36 CAG unit tract in the Huntingtin gene (HTT) leads to Huntington's disease (HD), but has also been associated with cancer: the incidence of cancer is lower in HD patients than in age-matched controls, but HD-causing variants of HTT accelerate the progression of breast tumors and the development of metastases in mouse models of breast cancer. To investigate the relationship between HTT CAGs and cancer, data concerning 2407 women with BRCA1 or BRCA2 mutations that predispose to breast and ovarian cancers and 431 patients with breast cancer without family histories were studied; the size of the CAG expansions on both HTT alleles was determined in each subject. The proportion of individuals carrying a CAG expansion in a pathological range for HD was 10 times more frequent than previously reported in the literature. In carriers of BRCA2 mutations, the length of the HTT CAG tract was correlated with lower incidence of ovarian cancer. Among carriers of BRCA1 mutations who developed a breast cancer, its onset occurred 2.4 years earlier in individuals with intermediate HTT alleles (≥27) than in those with a CAG tract <27. Finally, in patients with sporadic HER2 breast cancer, metastasis increased by a factor of 11.10 per 10 additional CAG repeats in HTT. We concluded that whereas long CAG length could be associated with lower cancer incidence, it could also be paradoxically associated with cancer severity (age of apparition and metastasis development).

Huntington's Disease: Relationship Between Phenotype and Genotype

Huntington's disease (HD) is an autosomal dominant inherited neurodegenerative disease with the typical manifestations of involuntary movements, psychiatric and behavior disorders, and cognitive impairment. It is caused by the dynamic mutation in CAG triplet repeat number in exon 1 of huntingtin (HTT) gene. The symptoms of HD especially the age at onset are related to the genetic characteristics, both the CAG triplet repeat and the modified factors. Here, we reviewed the recent advancement on the genotype-phenotype relationship of HD, mainly focus on the characteristics of different expanded CAG repeat number, genetic modifiers, and CCG repeat number in the 3' end of CAG triplet repeat and their effects on the phenotype. We also reviewed the special forms of HD (juvenile HD, atypical onset HD, and homozygous HD) and their phenotype-genotype correlations. The review will aid clinicians to predict the onset age and disease course of HD, give the genetic counseling, and accelerate research into the HD mechanism.

Huntington disease

Huntington disease is devastating to patients and their families - with autosomal dominant inheritance, onset typically in the prime of adult life, progressive course, and a combination of motor, cognitive and behavioural features. The disease is caused by an expanded CAG trinucleotide repeat (of variable length) in HTT, the gene that encodes the protein huntingtin. In mutation carriers, huntingtin is produced with abnormally long polyglutamine sequences that confer toxic gains of function and predispose the protein to fragmentation, resulting in neuronal dysfunction and death. In this Primer, we review the epidemiology of Huntington disease, noting that prevalence is higher than previously thought, geographically variable and increasing. We describe the relationship between CAG repeat length and clinical phenotype, as well as the concept of genetic modifiers of the disease. We discuss normal huntingtin protein function, evidence for differential toxicity of mutant huntingtin variants, theories of huntingtin aggregation and the many different mechanisms of Huntington disease pathogenesis. We describe the genetic and clinical diagnosis of the condition, its clinical assessment and the multidisciplinary management of symptoms, given the absence of effective disease-modifying therapies. We review past and present clinical trials and therapeutic strategies under investigation, including impending trials of targeted huntingtin-lowering drugs and the progress in development of biomarkers that will support the next generation of trials. For an illustrated summary of this Primer, visit: http://go.nature.com/hPMENh.

Germline transmission in transgenic Huntington's disease monkeys

Transgenic nonhuman primate models are an increasingly popular model for neurologic and neurodegenerative disease because their brain functions and neural anatomies closely resemble those of humans. Transgenic Huntington's disease monkeys (HD monkeys) developed clinical features similar to those seen in HD patients, making the monkeys suitable for a preclinical study of HD. However, until HD monkey colonies can be readily expanded, their use in preclinical studies will be limited. In the present study, we confirmed germline transmission of the mutant huntingtin (mHTT) transgene in both embryonic stem cells generated from three male HD monkey founders (F0) and in second-generation offspring (F1) produced via artificial insemination by using intrauterine insemination technique. A total of five offspring were produced from 15 females that were inseminated by intrauterine insemination using semen collected from the three HD founders (5 of 15, 33%). Thus far, sperm collected from the HD founder (rHD8) has led to two F1 transgenic HD monkeys with germline transmission rate at 100% (2 of 2). mHTT expression was confirmed by quantitative real-time polymerase chain reaction using skin fibroblasts from the F1 HD monkeys and induced pluripotent stem cells established from one of the F1 HD monkeys (rHD8-2). Here, we report the stable germline transmission and expression of the mHTT transgene in HD monkeys, which suggest possible expansion of HD monkey colonies for preclinical and biomedical research studies.

A new mutation for Huntington disease following maternal transmission of an intermediate allele

New mutations for Huntington disease (HD) originate from CAG repeat expansion of intermediate alleles (27-35 CAG). Expansions of such alleles into the pathological range (≥ 36 CAG) have been exclusively observed in paternal transmission. We report the occurrence of a new mutation that defies the paternal expansion bias normally observed in HD. A maternal intermediate allele with 33 CAG repeats expanded in transmission to 48 CAG repeats causing a de novo case of HD in the family. Retrospectively, the mother presented with cognitive decline, but HD was never considered in the differential diagnosis. She was diagnosed with dementia and testing for HD was only performed after her daughter had been diagnosed. This observation of an intermediate allele expanding into the full penetrance HD range after maternal transmission has important implications for genetic counselling of females with intermediate repeats.

Trinucleotide expansion in disease: why is there a length threshold?

Trinucleotide repeats (TNRs) expansion disorders are severe neurodegenerative and neuromuscular disorders that arise from inheriting a long tract (30-50 copies) of a trinucleotide unit within or near an expressed gene (Figure 1a). The mutation is referred to as 'trinucleotide expansion' since the number of triplet units in a mutated gene is greater than the number found in the normal gene. Expansion becomes obvious once the number of repeating units passes a critical threshold length, but what happens at the threshold to render the repeating tract unstable? Here we discuss DNA-dependent and RNA-dependent models by which a particular DNA length permits a rapid transition to an unstable state.