191st ENMC International Workshop: Recent advances in oculopharyngeal muscular dystrophy research: From bench to bedside 8-10 June 2012, Naarden, The Netherlands

Emerging and established biomarkers of oculopharyngeal muscular dystrophy

Oculopharyngeal muscular dystrophy (OPMD) is a rare, primarily autosomal dominant, late onset muscular dystrophy commonly presenting with ptosis, dysphagia, and subsequent weakness of proximal muscles. Although OPMD diagnosis can be confirmed with high confidence by genetic testing, the slow progression of OPMD poses a significant challenge to clinical monitoring and a barrier to assessing the efficacy of treatments during clinical trials. Accordingly, there is a pressing need for more sensitive measures of OPMD progression, particularly those which do not require a muscle biopsy. This review provides an overview of progress in OPMD biomarkers from clinical assessment, quantitative imaging, histological assessments, and genomics, as well as hypothesis-generating "omics" approaches. The ongoing search for biomarkers relevant to OPMD progression needs an integrative, longitudinal approach combining validated and experimental approaches which may include clinical, imaging, demographic, and biochemical assessment methods. A multi-omics approach to biochemical biomarker discovery could help provide context for differences found between individuals with varying levels of disease activity and provide insight into pathomechanisms and prognosis of OPMD.

Longitudinal Assessment of Strength, Functional Capacity, Oropharyngeal Function, and Quality of Life in Oculopharyngeal Muscular Dystrophy

BACKGROUND AND OBJECTIVES

Oculopharyngeal muscular dystrophy (OPMD) is a late-onset, progressive muscle disease. Disease progression is known to be slow, but details on the natural history remain unknown. We aimed to examine the natural history of OPMD in a large nationwide cohort to determine clinical outcome measures that capture disease progression and can be used in future clinical trials.

METHODS

Patients invited by their treating physicians or identified from the national neuromuscular database and invited family members were examined twice 20 months apart with fixed dynamometry; Medical Research Council (MRC) grading; maximum bite force and isometric tongue strength; Motor Function Measure (MFM); 10-step stair test; maximum swallowing, chewing, and speech tasks; and quality of life assessments.

RESULTS

Disease progression was captured by 8 of 18 measures over 20 months in 43 patients with genetically confirmed OPMD. The largest deterioration was seen in deltoid muscle strength (-27% [range -17% to -37%]), followed by the quadriceps (-14% [range -6 to -23%]), iliopsoas (-12.2%), tongue (-9.9%), and MRC sum score (-2.5%). The 10-step stair test (-12.5%), MFM part D1 (-7.1%), and maximum repetition rate of /pa/ (-5.3%) showed a significant decrease as well (all p < 0.05). The Physical Functioning domain of the Short Form-36 Health Survey significantly deteriorated (p = 0.044). No relationship was found between disease progression and genotype or disease duration (p > 0.05).

DISCUSSION

Despite the slow disease progression of OPMD, this study showed that several outcome measures detected progression within 20 months. Deltoid muscle strength, measured by fixed dynamometry, showed the greatest decline. These longitudinal data provide clinical outcome measures that can be used as biomarkers in future clinical trials.

Diagnostics of short tandem repeat expansion variants using massively parallel sequencing and componential tools

Short tandem repeats (STRs) are scattered throughout the human genome. Some STRs, like trinucleotide repeat expansion (TRE) variants, cause hereditable disorders. Unambiguous molecular diagnostics of TRE disorders is hampered by current technical limitations imposed by traditional PCR and DNA sequencing methods. Here we report a novel pipeline for TRE variant diagnosis employing the massively parallel sequencing (MPS) combined with an opensource software package (FDSTools), which together are designed to distinguish true STR sequences from STR sequencing artifacts. We show that this approach can improve TRE diagnosis, such as Oculopharyngeal muscular dystrophy (OPMD). OPMD is caused by a trinucleotide expansion in the PABPN1 gene. A short GCN expansion, (GCN[10]), coding for a 10 alanine repeat is not pathogenic, but an alanine expansion is pathogenic. Applying this novel procedure in  a Dutch OPMD patient cohort, we found expansion variants from GCN[11] to GCN[16], with the GCN[16] as the most abundant variant. The repeat expansion length did not correlate with clinical features. However, symptom severity was found to correlate with age and with the initial affected muscles, suggesting that aging and muscle-specific factors can play a role in modulating OPMD.

Intranuclear Aggregates Precede Clinical Onset in Oculopharyngeal Muscular Dystrophy

BACKGROUND

Oculopharyngeal muscular dystrophy (OPMD) has long been characterized by a combination of bilateral ptosis and dysphagia and subsequent limb girdle weakness. The role of the typical intranuclear inclusion in the pathophysiology is unresolved.

OBJECTIVE

The aim of this study was to describe the clinical and histopathological features of oculopharyngeal muscular dystrophy (OPMD). We examined this in a Dutch cohort including presymptomatic Ala-expanded-PABPN1 carriers and late symptomatic patients.

METHODS

We performed a prospective, observational study in OPMD patients and adult children of genetically confirmed OPMD patients. The study includes a structured history, a detailed neurological examination, muscle histology and biochemical analysis. Forty patients and 18 adult children participated in this study, among whom were six presymptomatic mutation carriers. One patient died during the study and had given permission to autopsy.

RESULTS

In addition to the characteristic OPMD symptoms including ptosis and dysphagia, other symptoms such as limb girdle and axial weakness, and external ophthalmoplegia were frequently observed. Intranuclear aggregates were observed in the biopsies of presymptomatic carriers. Biochemical analysis of the biopsies of the presymptomatic carriers showed no mitochondrial dysfunction. The autopsy showed that muscle weakness correlated with histopathological findings in five different muscles in an individual patient.

CONCLUSIONS

The main findings of this nationwide study are the presence of intranuclear aggregates before clinical onset and the absence of mitochondrial changes in Ala-expanded-PABPN1 carriers. This indicates that the expression of Ala-expanded-PABPN1 causes the formation of nuclear aggregates before the onset of muscle weakness. Normal results of biochemical analysis in presymptomatic carriers suggest that possible mitochondrial dysfunction occurs later. Furthermore we confirmed that limb girdle weakness occurs frequently in Dutch OPMD patients. This study thus expands the OPMD research towards characterization of presymptomatic carriers.

Oculopharyngeal Weakness, Hypophrenia, Deafness, and Impaired Vision: A Novel Autosomal Dominant Myopathy with Rimmed Vacuoles

Background:

Myopathies with rimmed vacuoles are a heterogeneous group of muscle disorders with progressive muscle weakness and varied clinical manifestations but similar features in muscle biopsies. Here, we describe a novel autosomal dominant myopathy with rimmed vacuoles in a large family with 11 patients of three generations affected.

Methods:

A clinical study including family history, obstetric, pediatric, and development history was recorded. Clinical examinations including physical examination, electromyography (EMG), serum creatine kinase (CK), bone X-rays, and brain magnetic resonance imaging (MRI) were performed in this family. Open muscle biopsies were performed on the proband and his mother. To find the causative gene, the whole-exome sequencing was carried out.

Results:

Disease onset was from adolescence to adulthood, but the affected patients of the third generation presented an earlier onset and more severe clinical manifestations than the older generations. Clinical features were characterized as dysarthria, dysphagia, external ophthalmoplegia, limb weakness, hypophrenia, deafness, and impaired vision. However, not every patient manifested all symptoms. Serum CK was mildly elevated and EMG indicated a myopathic pattern. Brain MRI showed cerebellum and brain stem mildly atrophy. Rimmed vacuoles and inclusion bodies were observed in muscle biopsy. The whole-exome sequencing was performed, but the causative gene has not been found.

Conclusions:

We reported a novel autosomal dominant myopathy with rimmed vacuoles characterized by dysarthria, dysphagia, external ophthalmoplegia, limb weakness, hypophrenia, deafness, and impaired vision, but the causative gene has not been found and needs further study.

The Dutch patients' perspective on oculopharyngeal muscular dystrophy: A questionnaire study on fatigue, pain and impairments

Research on oculopharyngeal muscular dystrophy focuses mainly on genetic and pathophysiological aspects. Clinically, oculopharyngeal muscular dystrophy is often considered as a disease with a relatively mild initial disease course with no or only mild functional disabilities. However the occurrence of fatigue, pain and functional impairments other than dysphagia has never been studied systematically. The aim of this study is therefore to assess the prevalence of fatigue, pain, and functional limitations, and the social participation and psychological well-being of oculopharyngeal muscular dystrophy patients. We performed a questionnaire study on fatigue, pain, functional impairments, social participation and psychological distress in 35 genetically confirmed oculopharyngeal muscular dystrophy patients with an average disease duration of 11.6 years. We showed that 19 (54%) of the patients experienced severe fatigue and also 19 (54%) experienced pain. Limitations in daily life activities and social participation were detected in 33 (94%) of the patients. Many patients reported pelvic girdle weakness and limitations in ambulation. Fatigue severity was related to functional impairments, while pain and disease duration were not. Psychological distress was not different from healthy adults. In conclusion, fatigue and pain are present among approximately half of the patients, and almost all patients are impaired in daily life activities, social participation and ambulation. These data should be taken into account in symptomatic management of oculopharyngeal muscular dystrophy.

PABPN1 (GCN)11 as a Dominant Allele in Oculopharyngeal Muscular Dystrophy -Consequences in Clinical Diagnosis and Genetic Counselling

Oculopharyngeal muscular dystrophy (OPMD) is mainly characterized by ptosis and dysphagia. The genetic cause is a short expansion of a (GCN)10 repeat encoding for polyalanine in the poly(A) binding protein nuclear 1 (PABPN1) gene to (GCN)12–17 repeats. The (GCN)11/Ala11 allele has so far been described to be either a polymorphism or a recessive allele with no effect on the phenotype in the heterozygous state. Here we report the clinical and histopathological phenotype of a patient carrying a single (GCN)11/Ala11 heterozygous allele and presenting an atypical form of OPMD with dysphagia and late and mild oculomotor symptoms. Intranuclear inclusions were observed in his muscle biopsy. This suggests a dominant mode of expression of the (GCN)11/Ala11 allele associated with a partial penetrance of OPMD.

Muscle weakness and speech in oculopharyngeal muscular dystrophy

PURPOSE

We documented speech and voice characteristics associated with oculopharyngeal muscular dystrophy (OPMD). Although it is a rare disease, OPMD offers the opportunity to study the impact of myopathic weakness on speech production in the absence of neurologic deficits in a relatively homogeneous group of speakers.

METHODS

Twelve individuals with OPMD and 12 healthy age-matched controls underwent comprehensive assessment of the speech mechanism including spirometry (respiratory support), nasometry (resonance balance), phonatory measures (pitch, loudness, and quality), articulatory measures (diadochokinetic rates, segment duration measures, spectral moments, and vowel space), tongue-to-palate strength measures during maximal isometric and speechlike tasks, quality-of-life questionnaire, and perceptual speech ratings by listeners.

RESULTS

Individuals with OPMD had substantially reduced tongue strength compared to the controls. However, little impact on speech and voice measures or on speech intelligibility was observed except for slower diadochokinetic rates.

CONCLUSIONS

Despite having less than half the maximal tongue strength of healthy controls, the individuals with OPMD exhibited minimal speech deficits. The threshold of weakness required for noticeable speech impairment may not have been reached by this group of adults with OPMD.

Hip flexion weakness is associated with impaired mobility in oculopharyngeal muscular dystrophy: a retrospective study with implications for trial design

Oculopharyngeal muscular dystrophy (OPMD) is a rare myopathy for which validated outcome measures are lacking, posing a barrier to clinical trials. Our goal was to identify factors associated with impaired mobility in OPMD in order to guide development of surrogate endpoints in future clinical trials. One hundred forty-four individuals with OPMD were included in this retrospective, single-center study. We made novel use of parametric time-to-event analysis to model age at initial use of assistive device for ambulation. We hypothesized that limb weakness and other markers of disease severity are associated with earlier use of assistive devices. 23.6% of individuals (34/144) progressed to use of assistive devices (mean age 66.0 ± 9.6 y). Earlier age at assistive device was associated with hip flexion Medical Research Council grade ≤3 (p <0.0001), earlier disease onset (p <0.0001), and lack of blepharoptosis surgery (p = 0.011). Markers of dysphagia severity were not associated with earlier progression to assistive devices. Our study is the first to show a statistical association between hip flexion weakness and impaired mobility in OPMD, indicating that hip flexion strength could be explored as a surrogate endpoint for use in clinical trials. Since severity of disease features may be discordant within individuals, composite outcome measures are warranted.

Oculopharyngeal muscular dystrophy as a paradigm for muscle aging

Symptoms in late-onset neuromuscular disorders initiate only from midlife onward and progress with age. These disorders are primarily determined by identified hereditable mutations, but their late-onset symptom manifestation is not fully understood. Here, we review recent research developments on the late-onset autosomal dominant oculopharyngeal muscular dystrophy (OPMD). OPMD is caused by an expansion mutation in the gene encoding for poly-adenylate RNA binding protein1 (PABPN1). The molecular pathogenesis for the disease is still poorly understood. Despite a ubiquitous expression of PABPN1, symptoms in OPMD are limited to skeletal muscles. We discuss recent studies showing that PABPN1 levels in skeletal muscles are lower compared with other tissues, and specifically in skeletal muscles, PABPN1 expression declines from midlife onward. In OPMD, aggregation of expanded PABPN1 causes an additional decline in the level of the functional protein, which is associated with severe muscle weakness in OPMD. Reduced PABNPN1 expression in muscle cell culture causes myogenic defects, suggesting that PABPN1 loss-of-function causes muscle weakness in OPMD and in the elderly. Molecular signatures of OPMD muscles are similar to those of normal muscle aging, although expression trends progress faster in OPMD. We discuss a working hypothesis that aging-associated factors trigger late-onset symptoms in OPMD, and contribute to accelerated muscle weakness in OPMD. We focus on the pharyngeal and eyelid muscles, which are often affected in OPMD patients. We suggest that muscle weakness in OPMD is a paradigm for muscle aging.

RNA-binding protein misregulation in microsatellite expansion disorders

RNA-binding proteins (RBPs) play pivotal roles in multiple cellular pathways from transcription to RNA turnover by interacting with RNA sequence and/or structural elements to form distinct RNA-protein complexes. Since these complexes are required for the normal regulation of gene expression, mutations that alter RBP functions may result in a cascade of deleterious events that lead to severe disease. Here, we focus on a group of hereditary disorders, the microsatellite expansion diseases, which alter RBP activities and result in abnormal neurological and neuromuscular phenotypes. While many of these diseases are classified as adult-onset disorders, mounting evidence indicates that disruption of normal RNA-protein interaction networks during embryogenesis modifies developmental pathways, which ultimately leads to disease manifestations later in life. Efforts to understand the molecular basis of these disorders has already uncovered novel pathogenic mechanisms, including RNA toxicity and repeat-associated non-ATG (RAN) translation, and current studies suggest that additional surprising insights into cellular regulatory pathways will emerge in the future.