Mitotic and meiotic stability of the CAG repeat in the X-linked spinal and bulbar muscular atrophy gene

Antisense Oligonucleotides (ASOs) in Motor Neuron Diseases: A Road to Cure in Light and Shade

Antisense oligonucleotides (ASOs) are short oligodeoxynucleotides designed to bind to specific regions of target mRNA. ASOs can modulate pre-mRNA splicing, increase levels of functional proteins, and decrease levels of toxic proteins. ASOs are being developed for the treatment of motor neuron diseases (MNDs), including spinal muscular atrophy (SMA), amyotrophic lateral sclerosis (ALS) and spinal and bulbar muscular atrophy (SBMA). The biggest success has been the ASO known as nusinersen, the first effective therapy for SMA, able to improve symptoms and slow disease progression. Another success is tofersen, an ASO designed to treat ALS patients with SOD1 gene mutations. Both ASOs have been approved by the FDA and EMA. On the other hand, ASO treatment in ALS patients with the C9orf72 gene mutation did not show any improvement in disease progression. The aim of this review is to provide an up-to-date overview of ASO research in MNDs, from preclinical studies to clinical trials and, where available, regulatory approval. We highlight the successes and failures, underline the strengths and limitations of the current ASO research, and suggest possible approaches that could lead to more effective treatments.

Sequence composition changes in short tandem repeats: heterogeneity, detection, mechanisms and clinical implications

Short tandem repeats (STRs) are a class of repetitive elements, composed of tandem arrays of 1-6 base pair sequence motifs, that comprise a substantial fraction of the human genome. STR expansions can cause a wide range of neurological and neuromuscular conditions, known as repeat expansion disorders, whose age of onset, severity, penetrance and/or clinical phenotype are influenced by the length of the repeats and their sequence composition. The presence of non-canonical motifs, depending on the type, frequency and position within the repeat tract, can alter clinical outcomes by modifying somatic and intergenerational repeat stability, gene expression and mutant transcript-mediated and/or protein-mediated toxicities. Here, we review the diverse structural conformations of repeat expansions, technological advances for the characterization of changes in sequence composition, their clinical correlations and the impact on disease mechanisms.

X-linked SBMA model mice display relevant non-neurological phenotypes and their expression of mutant androgen receptor protein in motor neurons is not required for neuromuscular disease

X-linked spinal and bulbar muscular atrophy (SBMA; Kennedy's disease) is a rare neuromuscular disorder characterized by adult-onset proximal muscle weakness and lower motor neuron degeneration. SBMA was the first human disease found to be caused by a repeat expansion mutation, as affected patients possess an expanded tract of CAG repeats, encoding polyglutamine, in the androgen receptor (AR) gene. We previously developed a conditional BAC fxAR121 transgenic mouse model of SBMA and used it to define a primary role for skeletal muscle expression of polyglutamine-expanded AR in causing the motor neuron degeneration. Here we sought to extend our understanding of SBMA disease pathophysiology and cellular basis by detailed examination and directed experimentation with the BAC fxAR121 mice. First, we evaluated BAC fxAR121 mice for non-neurological disease phenotypes recently described in human SBMA patients, and documented prominent non-alcoholic fatty liver disease, cardiomegaly, and ventricular heart wall thinning in aged male BAC fxAR121 mice. Our discovery of significant hepatic and cardiac abnormalities in SBMA mice underscores the need to evaluate human SBMA patients for signs of liver and heart disease. To directly examine the contribution of motor neuron-expressed polyQ-AR protein to SBMA neurodegeneration, we crossed BAC fxAR121 mice with two different lines of transgenic mice expressing Cre recombinase in motor neurons, and after updating characterization of SBMA phenotypes in our current BAC fxAR121 colony, we found that excision of mutant AR from motor neurons did not rescue neuromuscular or systemic disease. These findings further validate a primary role for skeletal muscle as the driver of SBMA motor neuronopathy and indicate that therapies being developed to treat patients should be delivered peripherally.

The Contribution of Somatic Expansion of the CAG Repeat to Symptomatic Development in Huntington's Disease: A Historical Perspective

The discovery in the early 1990s of the expansion of unstable simple sequence repeats as the causative mutation for a number of inherited human disorders, including Huntington’s disease (HD), opened up a new era of human genetics and provided explanations for some old problems. In particular, an inverse association between the number of repeats inherited and age at onset, and unprecedented levels of germline instability, biased toward further expansion, provided an explanation for the wide symptomatic variability and anticipation observed in HD and many of these disorders. The repeats were also revealed to be somatically unstable in a process that is expansion-biased, age-dependent and tissue-specific, features that are now increasingly recognised as contributory to the age-dependence, progressive nature and tissue specificity of the symptoms of HD, and at least some related disorders. With much of the data deriving from affected individuals, and model systems, somatic expansions have been revealed to arise in a cell division-independent manner in critical target tissues via a mechanism involving key components of the DNA mismatch repair pathway. These insights have opened new approaches to thinking about how the disease could be treated by suppressing somatic expansion and revealed novel protein targets for intervention. Exciting times lie ahead in turning these insights into novel therapies for HD and related disorders.

What is the Pathogenic CAG Expansion Length in Huntington's Disease?

Huntington's disease (HD) (OMIM 143100) is caused by an expanded CAG repeat tract in the HTT gene. The inherited CAG length is known to expand further in somatic and germline cells in HD subjects. Age at onset of the disease is inversely correlated with the inherited CAG length, but is further modulated by a series of genetic modifiers which are most likely to act on the CAG repeat in HTT that permit it to further expand. Longer repeats are more prone to expansions, and this expansion is age dependent and tissue-specific. Given that the inherited tract expands through life and most subjects develop disease in mid-life, this implies that in cells that degenerate, the CAG length is likely to be longer than the inherited length. These findings suggest two thresholds- the inherited CAG length which permits further expansion, and the intracellular pathogenic threshold, above which cells become dysfunctional and die. This two-step mechanism has been previously proposed and modelled mathematically to give an intracellular pathogenic threshold at a tract length of 115 CAG (95% confidence intervals 70- 165 CAG). Empirically, the intracellular pathogenic threshold is difficult to determine. Clues from studies of people and models of HD, and from other diseases caused by expanded repeat tracts, place this threshold between 60- 100 CAG, most likely towards the upper part of that range. We assess this evidence and discuss how the intracellular pathogenic threshold in manifest disease might be better determined. Knowing the cellular pathogenic threshold would be informative for both understanding the mechanism in HD and deploying treatments.

The French national protocol for Kennedy's disease (SBMA): consensus diagnostic and management recommendations

Background

Kennedy’s disease (KD), also known as spinal and bulbar muscular atrophy (SBMA), is a rare, adult-onset, X-linked recessive neuromuscular disease caused by CAG expansions in exon 1 of the androgen receptor gene (AR). The objective of the French national diagnostic and management protocol is to provide evidence-based best practice recommendations and outline an optimised care pathway for patients with KD, based on a systematic literature review and consensus multidisciplinary observations.

Results

The initial evaluation, confirmation of the diagnosis, and management should ideally take place in a tertiary referral centre for motor neuron diseases, and involve an experienced multidisciplinary team of neurologists, endocrinologists, cardiologists and allied healthcare professionals. The diagnosis should be suspected in an adult male presenting with slowly progressive lower motor neuron symptoms, typically affecting the lower limbs at onset. Bulbar involvement (dysarthria and dysphagia) is often a later manifestation of the disease. Gynecomastia is not a constant feature, but is suggestive of a suspected diagnosis, which is further supported by electromyography showing diffuse motor neuron involvement often with asymptomatic sensory changes. A suspected diagnosis is confirmed by genetic testing. The multidisciplinary assessment should ascertain extra-neurological involvement such as cardiac repolarisation abnormalities (Brugada syndrome), signs of androgen resistance, genitourinary abnormalities, endocrine and metabolic changes (glucose intolerance, hyperlipidemia). In the absence of effective disease modifying therapies, the mainstay of management is symptomatic support using rehabilitation strategies (physiotherapy and speech therapy). Nutritional evaluation by an expert dietician is essential, and enteral nutrition (gastrostomy) may be required. Respiratory management centres on the detection and treatment of bronchial obstructions, as well as screening for aspiration pneumonia (chest physiotherapy, drainage, positioning, breath stacking, mechanical insufflation-exsufflation, cough assist machnie, antibiotics). Non-invasive mechanical ventilation is seldom needed. Symptomatic pharmaceutical therapy includes pain management, endocrine and metabolic interventions. There is no evidence for androgen substitution therapy.

Conclusion

The French national Kennedy’s disease protocol provides management recommendations for patients with KD. In a low-incidence condition, sharing and integrating regional expertise, multidisciplinary experience and defining consensus best-practice recommendations is particularly important. Well-coordinated collaborative efforts will ultimately pave the way to the development of evidence-based international guidelines.

Unveiling synapse pathology in spinal bulbar muscular atrophy by genome-wide transcriptome analysis of purified motor neurons derived from disease specific iPSCs

Spinal bulbar muscular atrophy (SBMA) is an adult-onset, slowly progressive motor neuron disease caused by abnormal CAG repeat expansion in the androgen receptor (AR) gene. Although ligand (testosterone)-dependent mutant AR aggregation has been shown to play important roles in motor neuronal degeneration by the analyses of transgenic mice models and in vitro cell culture models, the underlying disease mechanisms remain to be fully elucidated because of the discrepancy between model mice and SBMA patients. Thus, novel human disease models that recapitulate SBMA patients’ pathology more accurately are required for more precise pathophysiological analysis and the development of novel therapeutics. Here, we established disease specific iPSCs from four SBMA patients, and differentiated them into spinal motor neurons. To investigate motor neuron specific pathology, we purified iPSC-derived motor neurons using flow cytometry and cell sorting based on the motor neuron specific reporter, HB9^e438::Venus, and proceeded to the genome-wide transcriptome analysis by RNA sequences. The results revealed the involvement of the pathology associated with synapses, epigenetics, and endoplasmic reticulum (ER) in SBMA. Notably, we demonstrated the involvement of the neuromuscular synapse via significant upregulation of Synaptotagmin, R-Spondin2 (RSPO2), and WNT ligands in motor neurons derived from SBMA patients, which are known to be associated with neuromuscular junction (NMJ) formation and acetylcholine receptor (AChR) clustering. These aberrant gene expression in neuromuscular synapses might represent a novel therapeutic target for SBMA.

Spinal and Bulbar Muscular Atrophy

Spinal and bulbar muscular atrophy, or Kennedy disease, is a slowly progressive X-linked neuromuscular disease caused by a trinucleotide (CAG) repeat expansion in the androgen receptor gene. Affected males typically develop weakness in their mid-40s as well as evidence of androgen insensitivity with reduced fertility and gynecomastia. Diagnosis is often delayed because of decreased awareness of the disease, although genetic testing allows for direct diagnosis. Therapeutic strategies to block the toxicity of the mutant androgen receptor have been unsuccessful thus far, and evaluation of additional candidate therapies is underway.

Expansion of CAG repeats in the spinocerebellar ataxia type 1 (SCA1) gene in idiopathic oligozoospermia patients

PURPOSE

The lengths of CAG repeats in two spinocerebellar ataxia genes, SCA1 and SCA3, were analyzed to determine whether such repeats exist in higher numbers in infertile males.

METHODS

Blood samples were collected from healthy controls, oligozoospermia patients, and azoospermia patients. DNA fragments containing target CAG repeats were amplified by PCR with template DNA purified from the blood samples. CAG repeats in PCR fragments were determined, using ABI PRISM 310 Gene Analyzer.

RESULTS

In SCA1, the distribution of CAG repeats in oligozoospermic males was different from that of the control group: More alleles had a repeat number that exceeded 32. Conversely, for SCA3, the examined oligozoospermia and azoospermia patients exhibited no differences in distribution of CAG repeats in comparison with the control group.

CONCLUSIONS

SCA1 in a subset of oligozoospermia patients has an increased number of CAG repeats.

Clinical aspects of CAG repeat diseases

Seven neurodegenerative disorders are known to be caused by unstable expansions of the trinucleotide CAG within human genes, and more will be discovered in the coming years. These disorders share some clinical similarities, as well as some differences, which are summarized here. These diseases have unusual clinical genetic properties related to the dynamic nature of CAG repeat expansions, including instability of the repeat expansion in meiosis, particularly male meiosis; a strong correlation between onset age and size of the repeat expansion; anticipation (earlier disease onset in succeeding generations); new mutations arising from unstable, mutable alleles with a high-normal CAG repeat number; and reduced penetrance for alleles in the low-affected range. Much more remains to be learned about the molecular biology and clinical pathophysiology of this new class of genetic diseases.

A phenotypic-genetic study of a group of Polish patients with spinal and bulbar muscular atrophy

We studied phenotype-genotype correlation in a group of Polish males with spinal and bulbar muscular atrophy (SBMA) and in female carriers. Eleven males with suspected SBMA phenotype and three suspected female carriers were examined. Male patients presented with the predominant signs of progressive, symmetrical distal limb weakness with amyotrophy, facial muscular weakness with orofacial fasciculations, nasal voice and slight dysphagia, gynaecomastia, decreased potency, as well as hand tremor and distal peripheral sensory disturbances in a few cases. One of the carriers presented with a 30-year history of fasciculations and minimal distal weakness and cramps in the legs, while the other two were asymptomatic. DNA analysis revealed expanded size of CAG repeats in Xq11-12 in the AR gene in 10 out of 11 men (range 45-52 CAG repeats) and in the women (range 46-48 CAG repeats). There was no correlation between CAG repeat size and the age of disease onset and duration of the disease. A rare, predominantly distal distribution of weakness and amyotrophy was found in our group of the SBMA patients (8 out of 11 cases) from three unrelated kindreds and also in the remaining two sporadic cases. The extended CAG repeats within families were stable.

Molecular diagnosis of inheritable neuromuscular disorders. Part II: Application of genetic testing in neuromuscular disease

Molecular genetic advances have led to refinements in the classification of inherited neuromuscular disease, and to methods of molecular testing useful for diagnosis and management of selected patients. Testing should be performed as targeted studies, sometimes sequentially, but not as wasteful panels of multiple genetic tests performed simultaneously. Accurate diagnosis through molecular testing is available for the vast majority of patients with inherited neuropathies, resulting from mutations in three genes (PMP22, MPZ, and GJB1); the most common types of muscular dystrophies (Duchenne and Becker, facioscapulohumeral, and myotonic dystrophies); the inherited motor neuron disorders (spinal muscular atrophy, Kennedy's disease, and SOD1 related amyotrophic lateral sclerosis); and many other neuromuscular disorders. The role of potential multiple genetic influences on the development of acquired neuromuscular diseases is an increasingly active area of research.

Somatic instability of the androgen receptor CAG repeat in a normal female

The polyglutamine repeat disorders represent a family of degenerative neurological diseases which are characterized by expansions of tandemly repeated CAG repeats. Investigations have demonstrated that in Huntington disease, dentatorubral pallidoluysian atrophy, and the spinocerebellar ataxias type 1, 2, and 3, the polyglutamine expansions show both meiotic and mitotic instability. However, previous studies have suggested that the polyglutamine motif within exon 1 of the androgen receptor gene (AR) which expands in cases of spinobulbar muscular atrophy differs in that it is apparently mitotically stable. During linkage analysis in a family with FG syndrome, a rare condition mapped to Xq12-q22.1, we detected the presence of an unusually small AR allele segregating within the pedigree. Sequencing, cytogenetic analysis, and PCR of flanking markers indicate that this allele arose by a somatic contraction of seven CAG repeats in the proband's mother, representing the first report of mitotic instability of an AR CAG repeat of normal size.

The polymorphic CAG repeat of the androgen receptor gene: a potential role in breast cancer in women over 40

Previous investigations into the relationship of CAG-repeat lengths in the androgen receptor (AR) gene to female breast cancer (BC) have yielded somewhat confusing results. Decreased AR transactivational activity lowers androgen:estrogen balance, and may thereby effect functional hyperestrogenicity. This may promote the pathogenesis of BC. To elucidate whether longer CAG repeats of the AR gene (AR), which correlate with lower transactivational activity of the AR, are associated with BC in women over 40, we examined the distribution of CAG-repeat lengths in BC tissue from this population. The BC tissue was histologically graded as: Grade 1, well differentiated (WD); Grade 2, moderately differentiated (MD); and Grade 3, poorly-differentiated (PD). Analysis showed significant differences as compared to controls when CAG lengths greater than 21 were examined, and that alleles with > or = 26 repeats were 2.4-fold more frequent in BC samples than in constitutional samples from a normal population. A significant shift to greater CAG-repeat lengths, appeared in WD and MD tumors only. Our results give some indication as to the progression of BC by suggesting that hypotransactive ARs with long polyglutamine (polyGln) tracts may have a role in the initiation and/or progression of BC. PD tumors tended to have shorter than normal CAG-repeat lengths. In this case it is hypothesized that the ARs have now become hypertransactive, possibly coinciding with the estrogen resistance that is associated with PD tumors. Whether this shift is of germline or somatic origin was not clear, though the appearance in 14% of the BC samples of a third CAG-repeat length indicates that it may be somatic.

Multiple founder effects in spinal and bulbar muscular atrophy (SBMA, Kennedy disease) around the world

SBMA (spinal and bulbar muscular atrophy), also called Kennedy disease, is an X-chromosomal recessive adult-onset neurodegenerative disorder caused by death of the spinal and bulbar motor neurones and dorsal root ganglia. Patients may also show signs of partial androgen insensitivity. SBMA is caused by a CAG repeat expansion in the first exon of the androgen receptor (AR) gene on the X-chromosome. Our previous study suggested that all the Nordic patients with SBMA originated from an ancient Nordic founder mutation, but the new intragenic SNP marker ARd12 revealed that the Danish patients derive their disease chromosome from another ancestor. In search of relationships between patients from different countries, we haplotyped altogether 123 SBMA families from different parts of the world for two intragenic markers and 16 microsatellites spanning 25 cM around the AR gene. The fact that different SBMA founder haplotypes were found in patients from around the world implies that the CAG repeat expansion mutation has not been a unique event. No expansion-prone haplotype could be detected. Trinucleotide diseases often show correlation between the repeat length and the severity and earlier onset of the disease. The longer the repeat, the more severe the symptoms are and the onset of the disease is earlier. A negative correlation between the CAG repeat length and the age of onset was found in the 95 SBMA patients with defined ages at onset.

Spinal and bulbar muscular atrophy (SBMA): somatic stability of an expanded CAG repeat in fetal tissues

Spinal and bulbar muscular atrophy (SBMA) is a rare X-linked motor neuron degenerative disease caused by an expanded trinucleotide repeat. Unlike most other trinucleotide repeat diseases, SBMA shows limited meiotic instability, and evidence thus far indicates absence of somatic instability in adults. Data regarding the presence of fetal tissue somatic mosaicism is unavailable. We present a family in which a woman whose father had SBMA requested prenatal testing. After informed consent. molecular genetic evaluation showed the male fetus to carry the SBMA repeat elongation. Testing of fetal tissues after elective pregnancy termination showed no somatic mosaicism in the CAG repeat length. This is the first report of molecular genetic analysis of multiple tissues in an affected fetus, and only the second report of prenatal diagnosis in SBMA.

Discordant repeat size and phenotype in Kennedy syndrome

Previous reports in the literature have described correlation of increasing repeat length with severity of the phenotype, in Kennedy syndrome. We describe male siblings with different repeat lengths, with lack of expression of the phenotype in the sibling with the longer repeat length. The phenotype was identical to motor neurone disease. There is variability of expression in Kennedy syndrome and repeat length even in siblings cannot be taken as a conclusive indicator of severity. CAG repeat length cannot be used to predict the natural history of Kennedy disease. The diagnosis of Kennedy syndrome should be considered in male patients presenting with atypical motor neurone disease.

Analysis of spinocerebellar ataxia type 2 in Gunma Prefecture in Japan: CAG trinucleotide expansion and clinical characteristics

We analyzed 13 patients with spinocerebellar ataxia type 2 (SCA2) in seven unrelated families who live in Gunma Prefecture, Japan (population approx. 2,000,000), and documented the clinical and molecular properties correlated with the CAG repeat expansion. Twelve of the 13 patients and one presymptomatic female were genetically examined, and the CAG repeat number of the expanded and normal alleles was 40.8+/-4.8 (mean+/-S.D., n=13) and 22+/-0 (n=13), respectively. The repeat size of the expanded alleles was inversely correlated with the patients' age at onset. Paternal anticipation was observed, accompanied by an increase of the CAG repeat size. The patients presented here were clinically characterized by a relatively higher frequency of slow saccades, hyporeflexia, hypotonia, and tremor. A number of peaks in the expanded allele on polyacrylamide gel electrophoresis showed the presence of cell mosaicism in SCA2 as well. In Gunma Prefecture, SCA2, Machado-Joseph disease and spinocerebellar ataxia type 6 are almost equally present and at higher frequencies than spinocerebellar ataxia type 1 and hereditary dentatorubropallidoluysian atrophy, which are rare. Thus, the difference of frequency of autosomal dominant spinocerebellar ataxias may be present in Japan.