Oculopharyngeal Muscular Dystrophy

A number sign (#) is used with this entry because of evidence that oculopharyngeal muscular dystrophy (OPMD) is caused by heterozygous mutation in the gene encoding poly(A)-binding protein-2 (PABPN1; 602279) on chromosome 14q11.

Clinical Features

Victor et al. (1962) described a family with oculopharyngeal muscular dystrophy, an autosomal dominant disorder presenting in late life and characterized by dysphagia and progressive ptosis of the eyelids. Nine members of 3 generations were known to be affected. One affected member also had total external ophthalmoplegia and weakness of the limb-girdle muscles.

The combination of ptosis and pharyngeal palsy had been first noted by Taylor (1915), who also commented on the familial nature of the syndrome. Hayes et al. (1963) succeeded in locating Taylor's original family and found that members of 2 subsequent generations had developed the disorder. In a family with this disorder observed in The Johns Hopkins Hospital, the anal and vesical sphincters were also involved (Teasdall et al., 1964). The family reported by Schotland and Rowland (1964) may have had OPMD; 10 members had ptosis, ophthalmoparesis, dysphagia, and weakness and wasting of face, neck, and distal limb muscles.

Morgan-Hughes and Mair (1973) studied 4 patients with what they termed 'oculoskeletal myopathy.' All patients complained of generalized muscle weakness and fatigability. All showed bilateral ptosis with external ophthalmoplegia, facial and sternocleidomastoid weakness, and diffuse wasting in the limbs. Two patients were dysphagic and 1 had pigmentary retinal degeneration.

Scrimgeour and Mastaglia (1984) suggested that a recessive form of oculopharyngeal myopathy with distal myopathy was present in the Melanesian family they studied.

Knoblauch and Koppel (1984) described a family from eastern Switzerland with 7 affected persons in 3 generations. Bilateral ptosis and dysphagia began in the fourth decade.

Becher et al. (2001) studied outpatient records from the University of New Mexico Hospital from 1965 to 2001 and identified 216 persons from 39 Hispanic New Mexican kindreds who had OPMD. In patients with both ocular and pharyngeal muscle weakness, ptosis was just as likely to occur before or concurrent with dysphagia. Proximal limb muscle weakness and gait abnormalities were common and occurred later than ocular or pharyngeal weakness. No decrease in life expectancy was detected. Genetic analysis of 10 individuals from different kindreds revealed an identical polyalanine triplet repeat expansion, (GCG)9, in the PABP2 gene.

Goh et al. (2005) reported a 64-year-old Chinese-Malaysian woman with OPMD who presented with a 6-year history of progressive dysphagia, dysarthria, and ptosis. Her mother and older brother, both of whom were deceased, were believed to be similarly affected. Muscle biopsy of the proband showed randomly distributed angulated fibers with rimmed vacuoles. Genetic analysis of the PABPN1 gene detected an expanded (GCG)9 and a normal (GCG)6 in the 2 alleles. Goh et al. (2005) emphasized that this was a non-Japanese Asian family with OPMD.

Pathogenesis

Morgan-Hughes and Mair (1973) found that triceps biopsies from patients with oculoskeletal myopathy contained isolated or clustered muscle fibers with accumulations of sarcoplasmic matter. The number of abnormal fibers ranged from 18% to 8% with no relation between the number of affected fibers and the severity or duration of the symptoms. Electron microscopy showed degenerative muscle fiber changes in all biopsy samples as well as striking abnormalities of muscle cell mitochondria. The mitochondria were seen to have laminated crystalline inclusions within the cristae. Some mitochondria were large with expanded area between the cristae, and some intercristal spaces were wide and electron dense. The authors stated that similar types of mitochondrial abnormalities have been described in other forms of myopathy.

Tome and Fardeau (1980) identified collections of 8.5-nm tubular filaments within muscle fiber nuclei from patients with OPMD and suggested that the filaments were a characteristic morphologic feature of the disorder.

Pauzner et al. (1991) reported the findings in a Jewish, non-Ashkenazi family derived from Uzbekistan, with affected persons in 4 generations. Electron microscopy of muscle biopsy specimens demonstrated bizarre large mitochondria with abnormal cristae, but no intranuclear inclusion bodies. Ragged-red fibers were not seen.

Uyama et al. (1996) described identical intranuclear tubulofilamentous inclusions within the muscle fibers of affected members of 2 extensive Japanese pedigrees that were segregating late-onset ptosis without external ophthalmoplegia and dysphagia. The authors considered the presence of the intranuclear inclusions to be entirely specific for this disorder and the presence of rimmed vacuoles, absence of ophthalmoplegia, or cardiac dysfunction to be corroborative. Although they observed aggregation of mitochondria, those organelles had an otherwise normal appearance in their biopsies.

Intracellular amyloid-like inclusions formed by mutant proteins result from polyglutamine expansions in Huntington disease (HD; 143100) and polyalanine expansions in the PABP2 gene in OPMD. Bao et al. (2004) found further parallels between these diseases: as had been observed in HD, they demonstrated that HSP70 (601113) and HDJ1 colocalized with PABP2 aggregates in muscle tissue from patients with OPMD and overexpression of HSP70 reduced mutant PABP2 aggregate formation.

Using both immunoelectron microscopy and fluorescence confocal microscopy, Calado et al. (2000) determined that the OPMD-specific nuclear inclusions were stained anti-PABP2 antibodies. In addition, the inclusions were labeled with antibodies directed against ubiquitin (see 191339) and the subunits of the proteasome, and contained a less soluble form of PABP2 as well as poly(A) RNA. The authors suggested that the polyalanine expansions in PABP2 induce a misfolding and aggregation of the protein into insoluble inclusions, similarly to events in neurodegenerative diseases caused by CAG/polyglutamine expansions, and that in OPMD the polyalanine expansions in the PABP2 protein may interfere with the cellular traffic of poly(A) RNA.

Abu-Baker et al. (2003) reported that protein aggregation in a transient transfection cell model of OPMD directly impaired the function of the ubiquitin-proteasome pathway (UPP) as well as molecular chaperone functions. The proteasome inhibitor lactacystin caused significant increase of protein aggregation and toxicity. Overexpression of molecular chaperones HSP40 (DNAJB1; 604572) and HSP70 (see 140559) suppressed protein aggregation and toxicity, and aggregation of mutated PABPN1 protein carrying a polyalanine expansion (PABPN1-ala17; see 602279) proportionally correlated with toxicity. Coexpression of chaperones in this cell model increased the solubility of PABPN1-ala17 and the rate of transfected cell survival.

Inheritance

The transmission pattern of oculopharyngeal muscular dystrophy in the family reported by Victor et al. (1962) was consistent with autosomal dominant inheritance.

Mapping

By linkage analysis of a homogeneous group of OPMD families, Brais et al. (1995) identified a putative disease locus within a 5-cM segment of chromosome 14q11.2-q13. A maximum 2-point lod score of 14.73 at theta = 0.03 was obtained in 3 French Canadian families for linkage with an intronic cardiac beta-myosin heavy chain gene marker (MYH7; 160760). Brais et al. (1995) suggested that the MYH7 gene or the contiguously situated MYH6 gene (160710) could be the site of the mutation in this disorder.

Teh et al. (1997) reported an Australian kindred of German descent with OPMD. Linkage analysis supported the previous assignment to 14q. Studies of repeat expansion found a CAG trinucleotide repeat that did not cosegregate with the disease.

Molecular Genetics

Brais et al. (1998) found that patients with OPMD had an expansion of a wildtype (GCG)6 repeat encoding a polyalanine tract to a pathologic (GCG)8-13 (602279.0001). In addition, a (GCG)7 allele (602279.0002) was found in 2% of the population, consistent with a polymorphism. Patients who were compound heterozygotes for the (GCG)9 mutation and the (GCG)7 polymorphism had a more severe phenotype. Homozygosity for the (GCG)7 allele led to autosomal recessive OPMD. Brais et al. (1998) concluded that the (GCG)7 allele is an example of a polymorphism that can act either as a modifier of a dominant phenotype or as a recessive mutation. The authors postulated that pathologic expansions of the polyalanine tract may cause mutated PABP2 oligomers to accumulate as filament inclusions in nuclei.

In a woman with OPMD, Robinson et al. (2006) identified a heterozygous missense mutation in the PABPN1 gene (G12A; 602279.0003) that generated a contiguous sequence of 13 alanine codons, which is causative of disease in the common triplet repeat expansion mutation. The woman had disease onset at age 61 years and reported 5 affected family members.

Genotype/Phenotype Correlations

Brais et al. (1998) found that patients with OPMD who were heterozygous carriers of the (GCG)9 expansion had longer swallowing times for ice-cold water compared to controls. Three patients who were homozygous for the (GCG)9 expansion had slower swallowing times as well as earlier disease onset, in their thirties. Four patients who were compound heterozygous for the (GCG)9 mutation and the (GCG)7 polymorphism showed an average age at disease onset, but the most severe slowing of swallowing time.

Population Genetics

Barbeau (1966) showed that all of the numerous reported French Canadian cases could be traced back to a single ancestor who emigrated from France in the 1600s. By genealogic reconstruction, Brais et al. (1999) demonstrated that the expanded (GCG)9 PABPN1 allele in French Canadian patients with OPMD was likely introduced by 3 French sisters in 1648.

De Braekeleer (1991) estimated the frequency to be more than 1/7,500 in the Saguenay-Lac-Saint-Jean region of Quebec Province. Brais et al. (1998) cited data on estimates of the frequency of the OPMD mutation: 1 in 1,000 in the province of Quebec, approximately 1 in 200,000 in France, and 1 in 700 in Bukhara Jews living in Israel.

Blumen et al. (2000) identified a common (GCG)9 expanded PABPN1 allele and a shared haplotype among 23 Bukhara Jewish patients from 8 unrelated families with OPMD. The mutation likely arose or was introduced into the population during the 13th or 14th centuries AD, when that specific Jewish population settled in Bukhara or Samarkand.

Rodriguez et al. (2005) found that affected members of 21 Uruguayan families with OPMD had an expanded (GCG)11 PABPN1 allele. Haplotype analysis indicated a founder effect, and the (GCG)11 allele was estimated in 37 to 53 generations. The disease allele was likely brought to Uruguay by a family of Canary Island settlers during immigration from Europe between the 18th and 19th centuries. The ancestral mutation may have originated between the 10th and 14th centuries in Old World Europe.

Animal Model

Hino et al. (2004) generated transgenic mice expressing the human PABPN1 gene. Transgenic mice lines expressing normal PABPN1 with a 10-alanine stretch did not reveal myopathic changes, whereas lines expressing high levels of expanded PABPN1 with a 13-alanine stretch showed an apparent myopathy phenotype, especially in old age. Pathologic studies in the latter mice disclosed intranuclear inclusions consisting of aggregated mutant human PABPN1 product. Some TUNEL-positive nuclei were shown around degenerating fibers and in a cluster in the lesion in necrotic muscle fibers. The degree of myopathic change was more prominent in eyelid and pharyngeal muscles, and muscle weakness in limbs was apparent from the fatigability test. Nuclear inclusions seemed to develop gradually with aging, at least after 1 week of age, in transgenic mouse muscle.

In COS-7 cells, Davies et al. (2006) found that trehalose (TREH; 275360) reduced aggregate formation and toxicity of mutant PABPN1 with a repeat expansion. Oral administration of trehalose to OPMD transgenic mice attenuated muscle weakness, reduced aggregate formation, and decreased the number of apoptotic nuclei in skeletal muscle. Trehalose is thought to act by binding and stabilizing partially folded polyglutamine proteins. The findings suggested that therapy that reduces aggregation may be an effective treatment for OPMD.

Chartier et al. (2009) noted that intrabodies are antibodies that are modified to be expressed intracellularly and target specific antigens in subcellular locations. They are commonly generated by artificially linking the variable domains of antibody heavy and light chains. Mammals of the Camelidae family, including llamas, produce natural single-chain antibodies, which when engineered combine the advantages of being single-chain, small-sized, and very stable. Using the Drosophila model of OPMD, the authors showed the in vivo efficiency of llama intrabodies against PABPN1. Among 6 anti-PABPN1 intrabodies expressed in muscle nuclei, the 3F5 intrabody was a strong suppressor of OPMD muscle degeneration in Drosophila, leading to nearly complete rescue. Expression of the 3F5 intrabody affected PABPN1 aggregation and restored muscle gene expression.

Trollet et al. (2010) used a transgenic mouse model of OPMD (A17.1) to perform transcriptomic studies combined with a detailed phenotypic characterization of this model at 3 time points. The transcriptomic analysis revealed a massive gene deregulation in the A17.1 mice, among which was a significant deregulation of pathways associated with muscle atrophy. One-third of the progressive genes were also associated with muscle atrophy. Functional and histologic analysis of skeletal muscle in this mouse model confirmed a severe and progressive muscular atrophy associated with a reduction in muscle strength. Moreover, muscle atrophy in the A17.1 mice was restricted to fast glycolytic fibers, which contained a large number of intranuclear inclusions. The soleus muscle and, in particular, oxidative fibers, were spared even though they contained intranuclear inclusions, albeit to a lesser degree. The authors concluded that there was a fiber-type specificity of muscle atrophy in this OPMD model.