Oculopharyngeal muscular dystrophy (OPMD): analysis of the PABPN1 gene expansion sequence in 86 patients reveals 13 different expansion types and further evidence for unequal recombination as the mutational mechanism

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.

Characterization of muscle strength and mobility in oculopharyngeal muscular dystrophy

INTRODUCTION/AIMS

Muscle weakness, and its association with mobility limitations, has received little study in oculopharyngeal muscular dystrophy (OPMD) using quantitative and standardized assessments. The objectives of this study were to (1) document upper and lower limb muscle strength, upper limb functions, fatigue, and mobility capacities; (2) compare them with reference values and across participant age groups; and (3) explore associations between muscle strength, fatigue, and mobility capacities among adults with OPMD.

METHODS

Thirty-four participants were included in this cross-sectional study. The following variables were assessed: quantitative maximal isometric muscle strength, grip and pinch strength, fatigue, walking speed, walking endurance, sit-to-stand, and stair ascent and descent capacities.

RESULTS

Muscle strength was lower for older than younger participants for five muscle groups (P < .05). Walking endurance, sit-to-stand, stairs (ascent and descent), and strength of hip flexion, grip, and pinch were below 80% of reference values in participants ≥56 y old (55.3%-78.2%). Moderate to strong correlations were found between muscle strength and mobility capacities (ρ = 0.42-0.80, P < .05), and between fatigue and either muscle strength or mobility capacities (ρ = 0.42-0.75, P < .05).

DISCUSSION

This study highlights the impact of OPMD on strength, endurance, and functional capacity, among others, with patients being well below reference values even before the age of 65 y. In addition to helping health professionals to offer better clinical guidance, these results will improve clinical trial readiness. The next steps will be to assess the metrological properties of outcome measures and continue to document the disease progression rate.

Oculopharyngeal muscular dystrophy mutations link the RNA-binding protein HNRNPQ to autophagosome biogenesis

Autophagy is an intracellular degradative process with an important role in cellular homeostasis. Here, we show that the RNA binding protein (RBP), heterogeneous nuclear ribonucleoprotein Q (HNRNPQ)/SYNCRIP is required to stimulate early events in autophagosome biogenesis, in particular the induction of VPS34 kinase by ULK1-mediated beclin 1 phosphorylation. The RBPs HNRNPQ and poly(A) binding protein nuclear 1 (PABPN1) form a regulatory network that controls the turnover of distinct autophagy-related (ATG) proteins. We also show that oculopharyngeal muscular dystrophy (OPMD) mutations engender a switch from autophagosome stimulation to autophagosome inhibition by impairing PABPN1 and HNRNPQ control of the level of ULK1. The overexpression of HNRNPQ in OPMD patient-derived cells rescues the defective autophagy in these cells. Our data reveal a regulatory mechanism of autophagy induction that is compromised by PABPN1 disease mutations, and may thus further contribute to their deleterious effects.

Importance of Family History in the Era of Exome Analysis: A Report of a Family with Multiple Concurrent Genetic Diseases

Multiple familial diseases in a single patient often present with overlapping symptomatology that confers difficulty in delineating a clinical diagnosis. Pedigree analysis has been a long-standing practice in the field of medical genetics to discover familial diseases. In recent years, whole exome sequencing (WES) has proven to be a useful tool for aiding physicians in diagnosing and understanding disease etiology. This report shows that pedigree analysis and WES are co-dependent processes in establishing diagnoses in a family with 4 different genetic disorders: Birt-Hogg-Dubé Syndrome, RRM2B-related mitochondrial disease, CDC73-related primary hyperparathyroidism, and familial prostate cancer.

STRs: Ancient Architectures of the Genome beyond the Sequence

Short tandem repeats (STRs) are commonly defined as short runs of repetitive nucleotides, consisting of tandemly repeating 2-6- bp motif units, which are ubiquitously distributed throughout genomes. Functional STRs are polymorphic in the population, and their variations influence gene expression, which subsequently may result in pathogenic phenotypes. To understand STR phenotypic effects and their functional roles, we describe four different mutational mechanisms including the unequal crossing-over model, gene conversion, retrotransposition mechanism and replication slippage. Due to the multi-allelic nature, small length, abundance, high variability, codominant inheritance, nearly neutral evolution, extensive genome coverage and simple assaying of STRs, these markers are widely used in various types of biological research, including population genetics studies, genome mapping, molecular epidemiology, paternity analysis and gene flow studies. In this review, we focus on the current knowledge regarding STR genomic distribution, function, mutation and applications.

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.

Correlation between PABPN1 genotype and disease severity in oculopharyngeal muscular dystrophy

OBJECTIVE

Oculopharyngeal muscular dystrophy (OPMD) is an autosomal dominant adult-onset disease characterized by progressive ptosis, dysphagia, and proximal limb weakness. The genetic cause is an expanded (GCN)n mutation in the PABPN1 gene encoding for the polyadenylate-binding protein nuclear 1. We hypothesized a potential correlation between the size of the (GCN)n expansion and the severity of the phenotype. To do this, we characterized the distribution of the genotypes as well as their correlation with age at diagnosis and phenotypical features in a large cohort of heterozygous and homozygous patients with OPMD in France with a confirmed molecular diagnosis of PABPN1.

METHODS

We explored 354 unrelated index cases recruited between 1999 and 2014 in several neuromuscular centers in France.

RESULTS

This cohort allowed us to characterize the frequency of mutated alleles in the French population and to demonstrate a statistical correlation between the size of the expansion and the mean age at diagnosis. We also confirmed that homozygous patients present with a more severe disease.

CONCLUSIONS

It has been difficult to establish phenotype-genotype correlations because of the rare nature of this disease. Our work demonstrates that patients with OPMD with longer PABPN1 expansion are on average diagnosed at an earlier age than patients with a shorter expansion, confirming that polyalanine expansion size plays a role in OPMD, with an effect on disease severity and progression.

Characterization of PABPN1 expansion mutations in a large cohort of Mexican patients with oculopharyngeal muscular dystrophy (OPMD)

Oculopharyngeal muscular dystrophy (OPMD) is an autosomal-dominant, adult-onset disorder defined by blepharoptosis, dysphagia, and proximal muscle weakness. OPMD arises from heterozygous expansions of a trinucleotide (GCN) tract situated at the 5' region of the polyadenylate RNA binding protein 1 (PABPN1) gene. The frequency of a particular (GCN) expansion in a given population of patients with OPMD is largely influenced by the occurrence of founder mutations. Analysis of large groups of patients with OPMD from different ethnic origins will help to estimate the relative contribution of each expanded allele to the disease. The purpose of this study was to characterize the type of PABPN1 expanded alleles in a large cohort of OPMD individuals from Mexico. Molecular analysis procedures included genomic DNA extraction from blood leukocytes in each patient followed by PCR amplification of PABPN1 exon 1, and direct nucleotide sequencing of PCR products. A total of 102 patients with OPMD were included in the study. Expanded PABPN1 gene alleles were demonstrated in all patients: 65% (66 out of 102) had a (GCN)15 expansion while the remaining 35% (36 out of 102) exhibited a (GCN)13 expansion. This is one of the largest series of molecularly confirmed patients with OPMD in a non-Caucasian population. Ethnic-specific differences in the prevalence of specific PABPN1 expansions must be considered for genetic screening of patients with OPMD.

Pharyngeal Satellite Cells Undergo Myogenesis Under Basal Conditions and Are Required for Pharyngeal Muscle Maintenance

The pharyngeal muscles of the nasal, oral, and laryngeal pharynxes are required for swallowing. Pharyngeal muscles are preferentially affected in some muscular dystrophies yet spared in others. Muscle stem cells, called satellite cells, may be critical factors in the development of pharyngeal muscle disorders; however, very little is known about pharyngeal satellite cells (PSC) and their role in pharyngeal muscles. We show that PSC are distinct from the commonly studied hindlimb satellite cells both transcriptionally and biologically. Under basal conditions PSC proliferate, progress through myogenesis, and fuse with pharyngeal myofibers. Furthermore, PSC exhibit biologic differences dependent on anatomic location in the pharynx. Importantly, PSC are required to maintain myofiber size and myonuclear number in pharyngeal myofibers. Together, these results demonstrate that PSC are critical for pharyngeal muscle maintenance and suggest that satellite cell impairment could contribute to pharyngeal muscle pathology associated with various muscular dystrophies and aging.

Prenatal diagnosis of congenital myopathies and muscular dystrophies

Congenital myopathies and muscular dystrophies constitute a genetically and phenotypically heterogeneous group of rare inherited diseases characterized by muscle weakness and atrophy, motor delay and respiratory insufficiency. To date, curative care is not available for these diseases, which may severely affect both life-span and quality of life. We discuss prenatal diagnosis and genetic counseling for families at risk, as well as diagnostic possibilities in sporadic cases.

A muscle stem cell for every muscle: variability of satellite cell biology among different muscle groups

The human body contains approximately 640 individual skeletal muscles. Despite the fact that all of these muscles are composed of striated muscle tissue, the biology of these muscles and their associated muscle stem cell populations are quite diverse. Skeletal muscles are affected differentially by various muscular dystrophies (MDs), such that certain genetic mutations specifically alter muscle function in only a subset of muscles. Additionally, defective muscle stem cells have been implicated in the pathology of some MDs. The biology of muscle stem cells varies depending on the muscles with which they are associated. Here we review the biology of skeletal muscle stem cell populations of eight different muscle groups. Understanding the biological variation of skeletal muscles and their resident stem cells could provide valuable insight into mechanisms underlying the susceptibility of certain muscles to myopathic disease.

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.

Oculopharyngeal muscular dystrophy: phenotypic and genotypic studies in a Chinese population

Oculopharyngeal muscular dystrophy (OPMD) is an autosomal dominant late-onset neuromuscular degenerative disease characterized by ptosis, dysphagia, and proximal muscle weakness. The genetic basis has been identified as an abnormal (GCN) expansion encoding the polyalanine tract in exon 1 of the polyadenylate-binding protein nuclear 1 gene (PABPN1). OPMD is worldwide distributed, but has rarely been reported in East Asians. In this study, we summarized the clinical and genetic characteristics of 34 individuals from 13 unrelated families in Chinese population. In our cohort, the mean age at onset was 47.2 years. Dysphagia, rather than ptosis, was the most common initial symptom. Genetically, we identified seven genotypes in our patients, including one compound heterozygote of (GCN)11/(GCN)12. The genetic heterogeneity implies that there is no single founder effect in Chinese population, and our data also support that the (GCN)11 polymorphism may have a disease-modifying effect. Additionally, the clinical features showed homogeneity within families, which suggests that other genetic factors apart from the already known genotype also play a role in modifying the phenotype.

Polyalanine tract disorders and neurocognitive phenotypes

Expansion of polyalanine tracts cause at least 9 inherited human diseases. Eight of these nine diseases are due to expansions in transcription factors and give rise to congenital disorders, many with neurocognitive phenotypes. Disease-causing expansions vary in length dependingupon the gene in question, with the severity of the associated clinical phenotype generally increasing with length of the polyalanine tract. The past decade has seen considerable progress in the understanding on how these mutations may arise and the functional effect of expanded polyalanine tracts on the resulting protein. Despite this progress, the pathogenic mechanism of expanded polyalanine tracts contributing to the associated disease states remains poorly understood. Gaining insights into the mechanisms that underlie the pathogenesis of different expanded polyalanine tract mutations will be a necessary step on the path to the design of potential treatment strategies for the associated diseases.

Mutation and haplotype analysis of oculopharyngeal muscular dystrophy in Thai patients

Oculopharyngeal muscular dystrophy (OPMD) is an inherited neuromuscular disease associated with a short trinucleotide repeat expansion in Exon 1 of the PABPN1 gene. OPMD is uncommon in East Asian populations, and there have been no previous reports of Thai patients. We studied clinical and molecular genetic features of six unrelated Thai patients with autosomal dominant OPMD. All patients had expansions of the guanine-cytosine-guanine (GCG) repeat ranging from three to seven additional repeats in the PABPN1 gene. Haplotype analysis showed that these mutations might have originated independently. Analysis of the size of the GCG repeat in the PABPN1 gene in 200 Thai control patients showed that 0.5% of the control subjects possessed (GCG)(7), thereby suggesting that the prevalence of autosomal recessive OPMD in the Thai population was approximately 1 in 160,000. In conclusion, our data suggest that OPMD in Thailand may be more common than previously thought.

Fork stalling and template switching as a mechanism for polyalanine tract expansion affecting the DYC mutant of HOXD13, a new murine model of synpolydactyly

Polyalanine expansion diseases are proposed to result from unequal crossover of sister chromatids that increases the number of repeats. In this report we suggest an alternative mechanism we put forward while we investigated a new spontaneous mutant that we named "Dyc" for "Digit in Y and Carpe" phenotype. Phenotypic analysis revealed an abnormal limb patterning similar to that of the human inherited congenital disease synpolydactyly (SPD) and to the mouse mutant model Spdh. Both human SPD and mouse Spdh mutations affect the Hoxd13 gene within a 15-residue polyalanine-encoding repeat in the first exon of the gene, leading to a dominant negative HOXD13. Genetic analysis of the Dyc mutant revealed a trinucleotide expansion in the polyalanine-encoding region of the Hoxd13 gene resulting in a 7-alanine expansion. However, unlike the Spdh mutation, this expansion cannot result from a simple duplication of a short segment. Instead, we propose the fork stalling and template switching (FosTeS) described for generation of nonrecurrent genomic rearrangements as a possible mechanism for the Dyc polyalanine extension, as well as for other polyalanine expansions described in the literature and that could not be explained by unequal crossing over.

Study of a Taiwanese family with oculopharyngeal muscular dystrophy

BACKGROUND

Oculopharyngeal muscular dystrophy (OPMD) is a late onset autosomal dominant muscle disorder. OPMD is caused by a short trinucleotide repeat expansion encoding an expanded polyalanine tract in the polyadenylate binding-protein nuclear 1 (PABPN1) gene. We identified and characterized a PABPN1 mutation in a Taiwanese family with OPMD.

METHODS

The phenotypic and genotypic characteristics of all subjects were evaluated in a Taiwanese OPMD family. Genetic alterations in the PABPN1 gene were identified using PCR and DNA sequencing.

RESULTS

Ten subjects with OPMD (6 symptomatic and 4 asymptomatic) within the Taiwanese family carried a novel mutation in the PABPN1 gene. The normal (GCG)6(GCA)3GCG sequence was replaced by (GCG)6(GCA)(GCG)4(GCA)3GCG due to an insertion of (GCG)4GCA into the normal allele in the Taiwanese OPMD subjects.

CONCLUSIONS

In contrast to a single GCG expansion in most of OPMD patients in the literature, an insertion of (GCG)4GCA in the PABPN1 gene was found in the Taiwanese OPMD subjects. The identification of this mutation appears to support the molecular mechanism of unequal cross-over of two PABPN1 alleles.

RNA regulons: coordination of post-transcriptional events

Recent findings demonstrate that multiple mRNAs are co-regulated by one or more sequence-specific RNA-binding proteins that orchestrate their splicing, export, stability, localization and translation. These and other observations have given rise to a model in which mRNAs that encode functionally related proteins are coordinately regulated during cell growth and differentiation as post-transcriptional RNA operons or regulons, through a ribonucleoprotein-driven mechanism. Here I describe several recently discovered examples of RNA operons in budding yeast, fruitfly and mammalian cells, and their potential importance in processes such as immune response, oxidative metabolism, stress response, circadian rhythms and disease. I close by considering the evolutionary wiring and rewiring of these combinatorial post-transcriptional gene-expression networks.

Variability of the recessive oculopharyngeal muscular dystrophy phenotype

Oculopharyngeal muscular dystrophy (OPMD) is usually transmitted as an autosomal-dominant trait and characterized by an expansion from 6 to 8 or more GCG/GCA repeats in the poly-(A) binding protein nuclear 1 (PABPN1) gene on chromosome 14q11. Autosomal-recessive OPMD with a homozygous (GCG)7 expansion of PABPN1 has only been described in two Canadian patients, who showed a comparably mild phenotype, suggesting that it is less severe than the dominant form. We clinically and genetically characterized the first two reported cases of autosomal-recessive OPMD in Europe. Remarkably, both patients revealed severe and diverse phenotypes, with an unusual onset and atypical clinical course in one patient. Former studies found a 1%-2% frequency of the (GCG)7 allele, which theoretically produces an incidence of 1:10,000 of autosomal-recessive OPMD in the general population. We conclude that the apparent rarity of the autosomal-recessive form of OPMD may be due to the fact that genetic testing is generally administered only to patients with typical clinical features or a positive family history.

Identification of a novel mutation in a Korean patient with oculopharyngeal muscular dystrophy

Oculopharyngeal muscular dystrophy (OPMD) is a late-onset muscle disorder characterized by progressive dysphagia and bilateral ptosis. Mutations in the polyadenylate binding protein nuclear 1 (PABPN1) gene have been found to cause OPMD. The typical mutation is a stable trinucleotide repeat expansion in the first exon of the PABPN1 gene, in which (GCG)(6) is the normal repeat length. We investigated a Korean patient with OPMD and identified a novel mutation: a heterozygous insertion of a 9-bp sequence [(GCG)(GCA)(GCA); c.27_28insGCGGCAGCA] instead of the (GCG) repeat expansion, resulting in an in-frame insertion of three alanines (p.A10insAAA). To the best of our knowledge, this is the first report of a genetically confirmed case of OPMD in Korea.

Siblings with recessive oculopharyngeal muscular dystrophy

Oculopharyngeal muscular dystrophy (OPMD) is a late onset myopathy usually presenting in the 5th or 6th decade of life with progressive ptosis, dysphagia and proximal muscle weakness. It is usually dominantly inherited; however, a rare recessive form has also been described although documentation of such cases in the literature is very sparse. Here we report two siblings with recessive OPMD, in one of whom the clinical picture is complicated by ankylosing spondilitis and pneumonia. They exhibit later onset and milder symptoms than is typical for patients with dominantly inherited OPMD. This and the possibility that OPMD may be masked by symptoms of other diseases of the elderly may account for the paucity of cases of recessive OPMD reported in the literature.

Oculopharyngeal muscular dystrophy: a point mutation which mimics the effect of the PABPN1 gene triplet repeat expansion mutation

BACKGROUND

Oculopharyngeal muscular dystrophy (OPMD) is an autosomal dominant late onset neuromuscular disease characterised by proximal muscle weakness, ptosis, and swallowing difficulty. The only causative mutation described to date is a triplet repeat expansion consisting of two to seven additional base triplets in a repeat sequence in exon 1 of the polyadenine binding protein nuclear 1 (PABPN1) gene. This results in an increase in length of a polyalanine tract in the PABPN1 protein from 10 to 12-17 residues.

OBJECTIVE

Description of another mutation in a case of OPMD.

METHODS

Sequence analysis of exon 1 of the PABPN1 gene was undertaken on 202 patients referred for a possible diagnosis of OPMD but negative for the triplet repeat expansion mutation.

RESULTS

A case was identified with typical symptoms of OPMD, negative for the repeat expansion mutation but with a missense mutation in PABPN1 close to the 3' end of the normal polyalanine codon repeat sequence.

CONCLUSIONS

The single base mutation changes a glycine codon to an alanine codon and results in an increase in the number of contiguous polyalanine codons. This mimics the effect of the common triplet repeat expansion mutation and represents a previously undescribed mechanism of mutation.

Genetic heterogeneity in 30 German patients with oculopharyngeal muscular dystrophy

Oculopharyngeal muscular dystrophy (OPMD) is due to short elongations of a polyalanine tract in the poly(A) binding protein nuclear 1 (PABPN1) gene. Originally GCG expansions in which (GCG)(6) is extended to (GCG)(7-13) were found. Subsequently five further genotypes with additional GCA- and GCG-trinucleotides were identified in single OPMD patients. This indicated larger genetic heterogeneity and showed that unequal crossing-over and not replication slippage must be the underlying mechanism of elongation.We performed sequencing of the PABPN1 gene in 30 German OPDM index patients to determine the exact genotype. The original GCG expansion ranging from (GCG)(8) to (GCG)(11) was found in 22 patients. In 8 patients, however, three different elongated alleles other than classical (GCG)(7-13) were observed. Two of these genotypes had already been identified in Japanese patients. One genotype was recently identified showing (GCG)(6) followed by inserted (GCA)(3)GCG in four unrelated patients. This study further supports the theory of unequal crossing over as the molecular mechanism leading to elongation. It shows that other genotypes than classical (GCG)(7-13) are rather common in German OPMD patients. The data imply that there is no single founder effect in German OPMD patients.

An investigation into FOXE1 polyalanine tract length in premature ovarian failure

Premature ovarian failure (POF) is a common condition affecting 1% of women worldwide. There is strong evidence for genetic involvement in POF as many cases are familial, and mutations in several genes have been associated with POF. We investigated variation in FOXE1 polyalanine tract length, following the observation that polyalanine tract deletions are seen in the closely related FOXL2 in patients with POF. In addition, polyalanine tract expansions in FOXL2 are often seen in patients with blepharophimosis-ptosis-epicanthus inversus syndrome (BPES), a rare eyelid disorder often associated with POF. The FOXE1 polyalanine tract shows marked variation in its length between POF patients and normal controls, existing as an allele of 12, 14, 16, 17 or 19 alanine residues. We found evidence to suggest that variation in FOXE1 polyalanine tract length predisposes to POF.