Spinocerebellar ataxia 27B (SCA27B), a frequent late-onset cerebellar ataxia

Genetic cerebellar ataxias are still a diagnostic challenge, and yet not all of them have been identified. Very recently, in early 2023, a new cause of late-onset cerebellar ataxia (LOCA) was identified, spinocerebellar ataxia 27B (SCA27B). This is an autosomal dominant ataxia due to a GAA expansion in intron 1 of the FGF14 gene. Thanks to the many studies carried out since its discovery, it is now possible to define the clinical phenotype, its particularities, and the progression of SCA27B. It has also been established that it is one of the most frequent causes of LOCA. The core phenotype of the disease consists of slowly progressive late-onset ataxia with cerebellar syndrome, oculomotor disorders including downbeat nystagmus, and episodic symptoms such as diplopia. Therapeutic approaches have been proposed, including acetazolamide, and 4-aminopyridine, the latter with a better benefit/tolerance profile.

The Canadian Neuromuscular Disease Registry 2010-2019: A Decade of Facilitating Clinical Research Througha Nationwide, Pan-NeuromuscularDisease Registry

We report the recruitment activities and outcomes of a multi-disease neuromuscular patient registry in Canada. The Canadian Neuromuscular Disease Registry (CNDR) registers individuals across Canada with a confirmed diagnosis of a neuromuscular disease. Diagnosis and contact information are collected across all diseases and detailed prospective data is collected for 5 specific diseases: Amyotrophic Lateral Sclerosis (ALS), Duchenne Muscular Dystrophy (DMD), Myotonic Dystrophy (DM), Limb Girdle Muscular Dystrophy (LGMD), and Spinal Muscular Atrophy (SMA). Since 2010, the CNDR has registered 4306 patients (1154 pediatric and 3148 adult) with 91 different neuromuscular diagnoses and has facilitated 125 projects (73 academic, 3 not-for-profit, 3 government, and 46 commercial) using registry data. In conclusion, the CNDR is an effective and productive pan-neuromuscular registry that has successfully facilitated a substantial number of studies over the past 10 years.

Stress in Parents of Children With Genetically Determined Leukoencephalopathies: A Pilot Study

Genetically determined leukoencephalopathies comprise a group of rare inherited white matter disorders. The majority are progressive diseases resulting in early death. We performed a cross-sectional pilot study including 55 parents from 36 families to assess the level of stress experienced by parents of patients with genetically determined leukoencephalopathies, aged 1 month to 12 years. Thirty-four mothers and 21 fathers completed the Parenting Stress Index-4th Edition. One demographic questionnaire was completed per family. Detailed clinical data was gathered on all patients. Statistical analysis was performed with total stress percentile score as the primary outcome. Mothers and fathers had significantly higher stress levels compared with the normative sample; 20% of parents had high levels of stress whereas 11% had clinically significant levels of stress. Mothers and fathers had comparable total stress percentile scores. We identified pediatric behavioral difficulties and gross motor function to be factors influencing stress in mothers. Our study is the first to examine parental stress in this population and highlights the need for parental support early in the disease course. In this pilot study, we demonstrated that using the Parenting Stress Index-4th Edition to assess stress levels in parents of patients with genetically determined leukoencephalopathies is feasible, leads to valuable and actionable results, and should be used in larger, prospective studies.

Diagnostic anoctamin-5 protein defect in patients with ANO5-mutated muscular dystrophy

AIMS

Previously, detection of ANO5 protein has been complicated by unspecific antibodies, most of which have not identified the correct protein. The aims of the study were to specify ANO5 protein expression in human skeletal muscle, and to investigate if the ANO5 protein levels are affected by different ANO5 mutations in anoctaminopathy patients.

METHODS

Four different antibodies were tested for ANO5 specificity. A sample preparation method compatible with membrane proteins, combined with tissue fractionation was used to determine ANO5 expression in cell cultures expressing ANO5, in normal muscles and eight patient biopsies with six different ANO5 mutations in homozygous or compound heterozygous states, and in other dystrophies.

RESULTS

Only one specific monoclonal N-terminal ANO5 antibody was efficient in detecting the protein, showing that ANO5 is expressed as a single 107 kD polypeptide in human skeletal muscle. The truncating mutations c.191dupA and c.1261C>T were found to abolish ANO5 expression, whereas the studied point mutations had variable effects; however, all the ANO5 mutations resulted in clearly reduced ANO5 expression in the patient muscle membrane fraction. Attempts to detect ANO5 using immunohistochemistry were not yet successful.

CONCLUSIONS

The data presented here indicate that the ANO5 protein expression is decreased in ANO5-mutated muscular dystrophy and that most of the non-truncating pathogenic ANO5 mutations likely destabilize the protein and cause its degradation. The method described here allows direct analysis of human ANO5 protein, which can be used in diagnostics, for evaluating the pathogenicity of the potentially harmful ANO5 variants of uncertain significance.

Utility of whole-exome sequencing for those near the end of the diagnostic odyssey: time to address gaps in care

An accurate diagnosis is an integral component of patient care for children with rare genetic disease. Recent advances in sequencing, in particular whole‐exome sequencing (WES), are identifying the genetic basis of disease for 25–40% of patients. The diagnostic rate is probably influenced by when in the diagnostic process WES is used. The Finding Of Rare Disease GEnes (FORGE) Canada project was a nation‐wide effort to identify mutations for childhood‐onset disorders using WES. Most children enrolled in the FORGE project were toward the end of the diagnostic odyssey. The two primary outcomes of FORGE were novel gene discovery and the identification of mutations in genes known to cause disease. In the latter instance, WES identified mutations in known disease genes for 105 of 362 families studied (29%), thereby informing the impact of WES in the setting of the diagnostic odyssey. Our analysis of this dataset showed that these known disease genes were not identified prior to WES enrollment for two key reasons: genetic heterogeneity associated with a clinical diagnosis and atypical presentation of known, clinically recognized diseases. What is becoming increasingly clear is that WES will be paradigm altering for patients and families with rare genetic diseases.

Cognitive impairment and reduced life span of oculopharyngeal muscular dystrophy homozygotes

OBJECTIVE

To assess the evolution and life expectancy in patients with oculopharyngeal muscular dystrophy (OPMD) who are homozygotes for two (GCN)13 expansions in the PABPN1 encoding gene.

BACKGROUND

OPMD is particularly frequent among French Canadians (FCs) and Uzbek Jews (UJs), who carry a same size, (GCN)13, PABPN1 mutation. The high rate of consanguinity among UJs together with late disease onset and normal fertility results in homozygous cases.

METHODS

For 15 to 20 years, we followed 4 FC and 6 UJ homozygotes with OPMD and compared them with their heterozygous parents and siblings. In addition to clinical evaluation, electrodiagnostic tests, psychological tests, and brain imaging studies were performed.

RESULTS

In all (GCN)13-(GCN)13 patients, OPMD started before age 35 years, with bilateral ptosis and dysphagia; external ophthalmoparesis and dysphonia followed within a few years, as well as weakness in proximal limb muscles. All patients had recurrent aspirations and lost weight; 4 patients required surgical interventions to alleviate dysphagia, and 5 required feeding gastrostomies. Most patients were followed by psychiatrists due to cognitive decline, recurrent depression, or psychotic episodes. Six patients died at ages 50, 51, 53, 56, 56, and 57 years. The eldest patient is now 51 years old; she is cachectic and requires special diet and psychiatric care for paranoid psychosis and uninhibited behavior.

CONCLUSIONS

Oculopharyngeal muscular dystrophy progresses faster in homozygote compared with heterozygote patients. It is not restricted to the muscles, but also involves the CNS with cognitive decline and psychotic manifestations and leads to a reduced life expectancy.

[Hereditary ataxias, spastic parapareses and neuropathies in Eastern Canada]

It has been demonstrated, for many inherited diseases, that historical events have shaped the various regional gene pools of Eastern Canada. In so doing, it has given rise to the increased prevalence of some rare diseases due, to founder effects. The following neurogenetic disorders were first identified in patients from Eastern Canada: AOA-2, Arsacs, HSN-2, Arca-1, HMSN/ACC and Arsal. The population of Eastern Canada, we are convinced, will still allow the identification of new rare forms of hereditary ataxias, spastic parapareses and neuropathies as well as contribute to the uncovering of their mutated genes. We have summarized our current knowledge of the various hereditary ataxias, spastic parapareses and neuropathies in Eastern Canada. The study of the more common and homogenous features of these diseases has been largely completed.

A novel founder SCN4A mutation causes painful cold-induced myotonia in French-Canadians

BACKGROUND

Myotonia is observed in classic congenital myotonia caused by CLCN1 mutations and in sodium-channel myotonia (SCM) due to SCN4A mutations.

METHODS

We assessed 66 electrically proven cases of myotonia belonging to 17 French-Canadian families living in the Saguenay Lac St-Jean area of Quebec, a region well known for its genetic founder effects. The CLCN1 gene was sequenced in one affected member of each family. SCN4A exons with known SCM mutations were subsequently sequenced in families where no CLCN1 mutations were found.

RESULTS

Six families, 33% of cases (22/66), presenting classic congenital myotonia phenotypes were found to carry two previously identified CLCN1 mutations. In the other 11 families comprising 66% of cases (44/66), a new dominant SCN4A mutation in exon 24 (M1476I) was uncovered and segregated with a variable SCM phenotype. Although all carriers of this novel mutation had electrical myotonia, some were asymptomatic (25%) and age at onset was variable in the others (5 to 67, mean 21). Cold aggravated myotonia was observed in 41% of cases and painful myotonia in 18%. Additional features observed include aggravation of symptoms with pregnancies (7%), localized muscle swelling (2%), myotonic reactions to anesthesia (2%), and food-induced paralysis (2%).

CONCLUSIONS

This cohort is the largest described with a variable sodium-channel myotonia phenotype caused by a single SCN4A mutation. The clinical variability observed in this cohort underlines the phenotypic heterogeneity of SCN4A mutations and suggests that variants in other genes likely modulate clinical expression.

Ataxia-oculomotor apraxia 2 patients show no increased sensitivity to ionizing radiation

Mutations in senataxin have been described recently in 24 cases of French-Canadian descent with ataxia-oculomotor apraxia 2. This recessive ataxia is associated with an elevation in alpha-fetoprotein as in ataxia-telangiectasia. Because ataxia-telangiectasia cells are highly radiosensitive, we used a colony survival assay to measure the radiosensitivity of lymphoblastoid cell lines derived from five French-Canadian patients with ataxia-oculomotor apraxia 2. Two were homozygous for the common French-Canadian L1976R SETX missense mutation; the three others were compound heterozygotes for the common mutation and three different missense mutations. Overall, lymphoblastoid cell lines derived from these cases did not show significant variation from a normal response to 1 Gray of ionizing radiation but the two patients who were homozygous for the common L1976R mutation fell in the intermediate or non-diagnostic range.

A novel autosomal recessive limb-girdle muscular dystrophy with quadriceps atrophy maps to 11p13-p12

Limb-girdle muscular dystrophies (LGMD) are a heterogeneous group of pathologies. We have identified a cohort of 14 French-Canadian patients from eight different families displaying a novel form of LGMD with an autosomal recessive inheritance. These patients share some features with previously described cases of 'quadriceps myopathy' that evolved into an LGMD. All demonstrate quadriceps femoris asymmetrical atrophy. Creatine kinase values were variable from normal to 6000 U/l. Clinical evaluations and MRI studies demonstrate a variable intrafamilial and interfamilial phenotype. Asymmetrical muscle involvement was clinically observed and confirmed by imaging. MRI studies suggest that the hamstrings and the adductor magnus are the first limb muscles to demonstrate fatty infiltration. Muscle pathology shows no sign of active inflammation but increased endomysial connective tissue associated with basal lamina duplication and collagen disorganization. A genome-wide scan using the two largest families uncovered linkage to marker D11S1360 on chromosome 11p12 [multipoint logarithm of the odds (LOD) score of 2.78]. Further genotyping for the eight families confirmed linkage to this new LGMD locus (multipoint LOD score of 4.56). Fine mapping subsequently defined a less than 3.3 cM candidate interval on 11p13-p12. Haplotype analysis of carrier chromosomes suggests that the most frequent mutation may account for up to 81.3% of French-Canadian mutations. In this study, we describe the chromosomal locus of a new form of recessive LGMD with prominent quadriceps femoris atrophy.

A new form of congenital muscular dystrophy with joint hyperlaxity maps to 3p23-21

Congenital muscular dystrophies (CMDS) are a heterogeneous group of disorders. A growing number of CMDS have been found to be associated with joint hyperlaxity. We recruited 14 French-Canadian cases belonging to 11 families affected by a novel autosomal recessive congenital muscular dystrophy with hyperlaxity (CMDH). All cases come from the southwestern part of Quebec, suggesting a new French-Canadian founder effect. All patients present muscle weakness, proximal contractures coexisting with distal joint hyperlaxity. Pathological and genetic studies have excluded that mutations in the three genes coding for collagen VI subunits are responsible for this disease. A genome-wide scan established linkage of two CMDH families to a region on chromosome 3p23-21. Further linkage analysis confirmed that all families are linked to the same region (log of the odds score of 5.3). Haplotype analysis defines a 1.6-cM candidate interval and suggests that two common mutations may account for 78% of carrier chromosomes. This study describes and maps a new form of recessive CMD with joint hyperlaxity distinct from Ullrich and Bethlem myopathies with a founder effect in the French-Canadian population.

A new autosomal recessive spastic ataxia associated with frequent white matter changes maps to 2q33-34

Recessive ataxias are a heterogeneous group of diseases. We identified a group of 23 French-Canadian cases belonging to 17 families affected by an autosomal recessive spastic ataxia associated with frequent white matter changes. The fact that 59% of these families have a genealogical relationship to the Portneuf County of Quebec suggests that this is a new form of ataxia with a regional founder effect. All cases present with cerebellar ataxia and spasticity. There is great intrafamilial and interfamilial variability, as illustrated by the spectrum of age of diagnosis (range: 2-59 years, mean: 15.0) and the presence of white matter changes on MRI in 52.4% of cases. The more severe cases have spasticity from birth, scoliosis, dystonia and cognitive impairment and were considered cases of cerebral palsy. Brain MRI constantly shows cerebellar atrophy, which in some cases may be associated with cortical atrophy, leucoencephalopathy and corpus callosum thinning. A genome wide scan uncovered linkage of three families to marker D2S2321 localized on chromosome 2q33-34. Linkage analysis confirmed that all families are linked to the same region [multipoint log of the odds (LOD) score of 5.95]. Haplotype analysis and allele sharing suggest that one common mutation may account for 97% of carrier chromosomes in Quebec. The uncovering of the mutated gene may point to a common pathway for pyramidal and cerebellar degeneration as both are often observed in recessive ataxias and complicated paraplegias.

Two mutations in the HSN2 gene explain the high prevalence of HSAN2 in French Canadians

BACKGROUND

Hereditary sensory and autonomic neuropathy type 2 (HSAN2; MIM 201300) is a rare recessive neuropathy typically diagnosed in the first decade. The 1973 study of a French Canadian family led to the definition of HSAN2.

OBJECTIVES

To demonstrate that the apparent higher prevalence of HSAN2 in Quebec is due to the presence of two HSN2 mutations and that carriers of different mutations appear to have a similar phenotype.

METHODS

Through attending physicians, the authors recruited French Canadian patients with HSAN2. Exclusion of linkage to the known HSAN loci and linkage to the HSAN2 was performed using standard methods. Sequencing of the HSN2 gene was used to uncover the causal mutations.

RESULTS

A large cluster of HSAN2 patients comprising 16 affected individuals belonging to 13 families was identified. The mode of inheritance is clearly autosomal recessive. All patients originated from southern Quebec, and 75% are from the Lanaudière region. Whereas linkage to the HSAN1, 3, and 4 loci was excluded, linkage to the 12p13.33 HSAN2 locus was confirmed. Sequencing of the HSN2 gene uncovered two French Canadian mutations and a novel nonsense mutation in a patient of Lebanese origin, all predicted to lead to truncations of the HSN2 protein. The comparison of clinical variables between patients with different genotypes does not suggest any difference in phenotype.

CONCLUSIONS

Two founder mutations are responsible for the apparently higher prevalence of HSAN2 in French Canadians. Genotype-phenotype correlation does not suggest any significant clinical variability.

Population history and its impact on medical genetics in Quebec

Knowledge of the genetic demography of Quebec is useful for gene mapping, diagnosis, treatment, community genetics and public health. The French-Canadian population of Quebec, currently about 6 million people, descends from about 8500 French settlers who arrived in Nouvelle-France between 1608 and 1759. The migrations of those settlers and their descendants led to a series of regional founder effects, reflected in the geographical distribution of genetic diseases in Quebec. This review describes elements of population history and clinical genetics pertinent to the treatment of French Canadians and other population groups from Quebec and summarizes the cardinal features of over 30 conditions reported in French Canadians. Some were discovered in French Canadians, such as autosomal recessive ataxia of the Charlevoix-Saguenay (MIM 270550), agenesis of corpus callosum and peripheral neuropathy (MIM 218000) and French-Canadian-type Leigh syndrome (MIM 220111). Other conditions are particularly frequent or have special genetic characteristics in French Canadians, including oculopharyngeal muscular dystrophy, hepatorenal tyrosinaemia, cystic fibrosis, Leber hereditary optic neuropathy and familial hypercholesterolaemia. Three genetic diseases of Quebec First Nations children are also discussed: Cree encephalitis (MIM 608505), Cree leukoencephalopathy (MIM 603896) and North American Indian childhood cirrhosis (MIM 604901).

(GCG)11 founder mutation in the PABPN1 gene of OPMD Uruguayan families

The dominant oculo-pharyngeal muscular dystrophy mutation consists of an expanded (GCN)(12-17) in the coding region of the PolyA Binding Protein Nuclear 1 gene. A founder effect has been demonstrated in Canadian and Bukhara Jewish populations with relatively high prevalence of this disease. Since the oculo-pharyngeal muscular dystrophy prevalence was remarkably high in Southern Uruguay, a founder effect was hypothesized. To identify the ancestral haplotype we determined the (GCN) repeat number and the variants of four intragenic SNPs in Uruguayan OPMD families and a control sample. All families carrying the mutation (GCG)(11)(GCA)(3)(GCG) shared a common ancestral haplotype and the age of the mutation was estimated in 37-53 generations by a composite likelihood method. One family carrying the (GCG)(9)(GCA)(3)(GCG) allele had a different haplotype. The genealogical and molecular data suggested that the common ancestors were Canary Islands' settlers that arrived in Uruguay in the XIX century.

[Oculopharyngeal muscular dystrophy: study of patients from seven Spanish families with different GCG expansions in PABP2 gene]

INTRODUCTION

Autosomal dominant oculopharyngeal muscular dystrophy (OPMD), with late onset due to ptosis and/or dysphagia, is caused by short (GCG)8-13 triplet-repeat expansions in the polyadenylation binding protein 2 (PABP2) gene, which is localized in chromosome 14q11. The severity of the dominant OPMD as well as the number of expansions that cause the disease are variable. (GCG)9 is mentioned as the most frequent and the genotype/phenotype has still not been well-determined.

OBJECTIVE

To describe the type of expansions (GCG)n found in Spanish families with OPMD, establishing if there is variability of them and the possible geno-phenotypical correlations.

METHODS

Clinicopathological and molecular studies have been performed in 15 consecutive patients, belonging to seven Spanish families with OPMD. The muscular biopsy study under electronmicroscopy shows intranuclear inclusions (INIs) in all the examined patients (one patient per family). The genetic findings confirm the cause of the disease in all the affected members and in one clinically asymptomatic member of one recently examined family: three families (six, one and one studied members, respectively) present the (GCG)9 expansion, two families (one studied member each one) present the (GCG)10 expansion and two families (one and four studied members respectively) present the (GCG)11 expansion. In these 15 patients with a short GCG expansion causing OPMD, clinical tests for OPMD and a follow-up study of their clinical course have been carefully assessed: in patients with the (GCG)9 expansion major abnormalities appeared in extrinsic ocular mobility and more precocious presentation of limb girld (lumbopelvic preferentially) weakness leading to a great disability before the seventh decade of life under the seventies in some patients and sometimes leading to death. In patients with (GCG)10 and (GCG)11 expansions, eye movements are always preserved and the limb girld muscles weakness did not appear before the seventh decade. No correlation seems to exist between age of onset of the ptosis or dysphagia and the different (GCG)n expansions and the surgical treatment of ptosis, performed in eight patients, showed good results independently of the (GCG)n mutation.

CONCLUSIONS

Although further clinical and genetic studies are necessary to establish a strict genotype/phenotype correlation in OPMD, we concluded that the (GCG)9 expansion involve more severe phenotypes than those related to the (GCG)10 or (GCG)11 expansions. Therefore, genetic testing could benefit prognosis in asymptomatic individuals.

Oculopharyngeal muscular dystrophy: a late-onset polyalanine disease

Oculopharyngeal muscular dystrophy (OPMD) is a muscle disease of late onset associated with progressive ptosis of the eyelids, dysphagia, and unique tubulofilamentous intranuclear inclusions (INIs). OPMD is usually transmitted as an autosomal dominant trait (OMIM 164300). A rarer allelic autosomal recessive form has also been observed (OMIM 257950). Both forms are caused by short (GCG)8-13 expansions in the polyadenylate-binding protein nuclear 1 gene (PABPN1) located on chromosome 14q11.1. The mutations cause the lengthening of an N-terminal polyalanine domain. Both slippage and unequal recombination have been proposed as the mutation mechanisms. The size of the mutation has not yet been conclusively shown to inversely correlate with the severity of the phenotype. Mutated PABPN1 proteins have been shown to be constituents of the INIs. The INIs also contain ubiquitin, proteasome subunits, HSP 40, HSP 70, SKIP, and abundant poly(A)-mRNA. The exact mechanism responsible for polyalanine toxicity in OPMD is unknown. Various intranuclear inclusion dependent and independent mechanisms have been proposed based on the major known function of PABPN1 in polyadenylation of mRNA and its shuttling from the nucleus to the cytoplasm. OPMD is one of the few triplet-repeat diseases for which the function of the mutated gene is known. Because of the increasing number of diseases caused by polyalanine expansions and the pathological overlap with CAG/polyglutamine diseases, what pathological insight is gained by the study of OPMD could lead to a better understanding of a much larger group of developmental and degenerative diseases.

Oligomerization of polyalanine expanded PABPN1 facilitates nuclear protein aggregation that is associated with cell death

Oculopharyngeal muscular dystrophy (OPMD) is an adult-onset disorder characterized by progressive eyelid drooping, swallowing difficulties and proximal limb weakness. The autosomal dominant form of this disease is caused by short expansions of a (GCG)(6) repeat to (GCG)(8-13) in the PABPN1 gene, which results in the expansion of a polyalanine stretch from 10 to 12-17 alanines in the N-terminus of the protein. Mutated PABPN1 (mPABPN1) is able to induce nuclear protein aggregation and form filamentous nuclear inclusions, which are the pathological hallmarks of OPMD. PABPN1, when bound to poly(A) RNA, forms both linear filaments and discrete-sized, compact oligomeric particles in vitro. In the absence of poly(A) RNA, PABPN1 can form oligomers. Here we report that: (i) oligomerization of PABPN1 is mediated by two potential oligomerization domains (ODs); (ii) inactivating oligomerization of mPABPN1 by deletions of 6-8 amino acids in either of the ODs prevents nuclear protein aggregation; (iii) expression of mPABPN1 in COS-7 cells is associated with cell death; and (iv) preventing nuclear protein aggregation by inactivating oligomerization of mPABPN1 significantly reduces cell death. These findings suggest that oligomerization of PABPN1 plays a crucial role in the formation of OPMD nuclear protein aggregation, while the expanded polyalanine stretch is necessary but not sufficient to induce OPMD protein aggregation, and that the nuclear protein aggregation might be toxic and cause cell death. These observations also imply that inactivation of oligomerization of mPABPN1 might be a useful therapeutic strategy for OPMD.

Oculopharyngeal MD among Bukhara Jews is due to a founder (GCG)9 mutation in the PABP2 gene

OBJECTIVE

To determine whether all cases of oculopharyngeal muscular dystrophy (OPMD) among Bukhara Jews share the same founder mutation.

BACKGROUND

Autosomal dominant OPMD is caused by a (GCG)8-13 repeat expansion in the polyadenylation binding protein 2 (PABP2) gene. The disease has a worldwide distribution but is particularly prevalent in Bukhara Jews and in French Canadians, in whom it was introduced by three sisters in 1648.

METHODS

We established the size of the PABP2 mutation in 23 Bukhara Jewish patients belonging to eight unrelated families. In all families, we constructed haplotypes for the carrying chromosomes composed of the alleles for eight chromosome 14q polymorphic markers.

RESULTS

All patients share a (GCG)9 PABP2 mutation and a four-marker haplotype. Furthermore, a shared intron single nucleotide polymorphism (SNP) in the PABP2 gene 2.6Kb from the mutation was not observed in 22 families with (GCG)9 mutations from nine different countries. The smaller size of the chromosomal region in linkage disequilibrium around the mutation in Bukhara Jews, as compared with French Canadians, suggests a founder effect that occurred more than 350 years ago. Based on the Luria-Delbrück corrected "genetic clock," we estimate that the mutation appeared or was introduced once in the Bukhara Jewish population between AD 872 and 1512 (mean, AD 1243).

CONCLUSION

OPMD among Bukhara Jews is the result of a shared, historically distinct, PABP2 (GCG)9 mutation that likely arose or was introduced in this population at the time they first settled in Bukhara and Samarkand during the 13th or 14th centuries.

PABP2 polyalanine tract expansion causes intranuclear inclusions in oculopharyngeal muscular dystrophy

Oculopharyngeal muscular dystrophy is caused by expansion of a (GCG)n trinucleotide repeat in the poly(A) binding protein 2 (PABP2) gene. The pathological hallmark of oculopharyngeal muscular dystrophy is the accumulation of intranuclear inclusions in muscle fibers. To test whether the polyalanine expansion of PABP2 directly leads to the formation of the nuclear aggregates, both normal and expanded PABP2 cDNAs were expressed in COS-7 cells. We find that expression of mutated PABP2 protein is sufficient for its accumulation as intranuclear inclusions.

Nuclear inclusions in oculopharyngeal muscular dystrophy consist of poly(A) binding protein 2 aggregates which sequester poly(A) RNA

Oculopharyngeal muscular dystrophy (OPMD) is an adult-onset disease characterized by progressive eyelid drooping, swallowing difficulties and proximal limb weakness. The autosomal dominant form of the disease is caused by short (GCG)(8-13) expansions in the PABP2 gene. This gene encodes the poly(A) binding protein 2 (PABP2), an abundant nuclear protein that binds with high affinity to nascent poly(A) tails, stimulating their extension and controlling their length. In this work we report that PABP2 is detected in filamentous nuclear inclusions, which are the pathological hallmark of OPMD. Using both immunoelectron microscopy and fluorescence confocal microscopy, the OPMD-specific nuclear inclusions appeared decorated by anti-PABP2 antibodies. In addition, the inclusions were labeled with antibodies directed against ubiquitin and the subunits of the proteasome and contained a form of PABP2 that was more resistant to salt extraction than the protein dispersed in the nucleoplasm. This suggests 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. No significant differences were observed in the steady-state poly(A) tail length in OPMD and normal myoblasts. However, the nuclear inclusions were shown to sequester poly(A) RNA. This raises the possibility that in OPMD the polyalanine expansions in the PABP2 protein may interfere with the cellular traffic of poly(A) RNA.

CAG tract of MJD-1 may be prone to frameshifts causing polyalanine accumulation

Machado-Joseph disease (MJD) is one of several disorders caused by the expansion of a coding CAG repeat (exp-CAG). The presence of intranuclear inclusions (INIs) in patients and cellular models of exp-CAG-associated diseases has lead to a nuclear toxicity model. Similar INIs are found in oculopharyngeal muscular dystrophy, which is caused by a short expansion of an alanine-encoding GCG repeat. Here we propose that transcriptional or translational frameshifts occurring within expanded CAG tracts result in the production and accumulation of polyalanine-containing mutant proteins. We hypothesize that these alanine polymers deposit in cells forming INIs and may contribute to nuclear toxicity. We show evidence that supports our hypothesis in lymphoblast cells from MJD patients, as well as in pontine neurons of MJD brain and in in vitro cell culture models of the disease. We also provide evidence that alanine polymers alone are harmful to cells and predict that a similar pathogenic mechanism may occur in the other CAG repeat disorders.

Oculopharyngeal muscular dystrophy

Autosomal dominant oculopharyngeal muscular dystrophy (OPMD) is an adult-onset disease with worldwide distribution. It usually presents in the fifth or sixth decades with progressive dysphagia, eyelid ptosis, and proximal limb weakness. Unique intranuclear filament inclusions in skeletal muscle fibers are its morphological hallmark. Surgical correction of the ptosis and cricopharyngeal myotomy are the only therapies available. Autosomal dominant OPMD is caused by short (GCG)8-13 riplet-repeat expansions in the polyadenylation binding protein 2 (PABP2) gene, which is localized in chromosome 14q11. Autosomal recessive OPMD is caused by a double dose of a (GCG)7 PABP2 allele. The GCG expansions cause lengthening of a predicted polyalanine tract in the protein. The expanded polyalanine domains may cause polyalanine nuclear toxicity by accumulating as nondegradable nuclear filaments.

Homozygotes for oculopharyngeal muscular dystrophy have a severe form of the disease

Autosomal dominant oculopharyngeal muscular dystrophy (OPMD) usually begins with ptosis or dysphagia during the fifth or sixth decade of life. We studied 7 patients with OPMD symptoms starting before the age of 36 years. All were found to be homozygotes for the dominant (GCG)9 OPMD mutation. On average, disease onset was 18 years earlier than in heterozygotes, and patients had a significantly larger number of muscle nuclei containing intranuclear inclusions (INIs) (9.4 vs 4.9%).

Vocal cord and pharyngeal weakness with autosomal dominant distal myopathy: clinical description and gene localization to 5q31

Distal myopathy refers to a heterogeneous group of disorders in which the initial manifestations are weakness and atrophy of the hands and feet. We report a family segregating an autosomal dominant distal myopathy, with multiple affected individuals in whom vocal cord and pharyngeal weakness may accompany the distal myopathy, without involvement of the ocular muscles. To our knowledge, this pedigree displays a distinct distal myopathy with the added features of pharyngeal and vocal cord dysfunction (VCPDM) that has not been previously reported. We mapped the MPD2 gene for VCPDM to chromosome 5q within a 12-cM linkage interval between markers D5S458 and D5S1972 in a large pedigree (a maximum LOD score of 12.94 at a recombination fraction of 0 for D5S393) and combined genome screening and DNA pooling successfully adapted to fluorescent markers. This technique provides for the possibility of fully automated genome scans.

Restriction map of a YAC and cosmid contig encompassing the oculopharyngeal muscular dystrophy candidate region on chromosome 14q11.2-q13

As part of our effort to clone positionally the oculopharyngeal muscular dystrophy (OPMD) gene, we constructed a YAC contig, a cosmid contig, and an EcoRI restriction map of the OPMD candidate region. The YAC contig spans more than 2 Mb and encompasses the loci D14S283 and D14S990 and the cardiac alpha and beta myosin heavy chain genes (MYH6 and MYH7). A 700-kb cosmid contig containing the D14S990 and the myosin genes and a long-range restriction map covering the region between D14S990 and the MYH6 and MYH7 gene cluster were established. A detailed EcoRI restriction map of the cosmid contig was determined, and five putative CpG islands were identified. Based on these data, the four loci were mapped within an approximately 600-kb region with the following centromere to telomere order: D14S283, D14S990, MYH6, and MYH7. The YAC and cosmid contigs will facilitate the identification of genes lying within the OPMD candidate interval.

Short GCG expansions in the PABP2 gene cause oculopharyngeal muscular dystrophy

Autosomal dominant oculopharyngeal muscular dystrophy (OPMD) is an adult-onset disease with a world-wide distribution. It usually presents in the sixth decade with progressive swallowing difficulties (dysphagia), eyelid drooping (ptosis) and proximal limb weakness. Unique nuclear filament inclusions in skeletal muscle fibres are its pathological hallmark. We isolated the poly(A) binding protein 2 gene (PABP2) from a 217-kb candidate interval on chromosome 14q11 (B.B. et al., manuscript submitted). A (GCG)6 repeat encoding a polyalanine tract located at the N terminus of the protein was expanded to (GCG)8-13 in the 144 OPMD families screened. More severe phenotypes were observed in compound heterozygotes for the (GCG)9 mutation and a (GCG)7 allele that is found in 2% of the population, whereas homozygosity for the (GCG)7 allele leads to autosomal recessive OPMD. Thus the (GCG)7 allele is an example of a polymorphism which can act either as a modifier of a dominant phenotype or as a recessive mutation. Pathological expansions of the polyalanine tract may cause mutated PABP2 oligomers to accumulate as filament inclusions in nuclei.

Using the full power of linkage analysis in 11 French Canadian families to fine map the oculopharyngeal muscular dystrophy gene

Oculopharyngeal muscular dystrophy (OPMD) is a late onset autosomal dominant muscular dystrophy with a high prevalence in the French Canadian population. We report linkage analysis with 7 chromosome 14q polymorphic markers in 11 large French Canadian families. An observed recombination in one family establishes D14S283 as the new centromeric flanking marker, therefore reducing the previously reported candidate interval from 5cM to 2cM. The highest two-point LOD score was 26.05 at theta = 0.01 for MYH7.1. Multipoint analysis suggested that the OPMD genes lies within a 1.5cM region around D14S990. This study of large French Canadian families underlines the great power of this population to fine map disease genes.

A pilot study on upper esophageal sphincter dilatation for the treatment of dysphagia in patients with oculopharyngeal muscular dystrophy

Upper esophageal sphincter (UES) dilatation was done for the treatment of moderate to severe dysphagia with a Maloney bougie in 14 patients with oculopharyngeal muscular dystrophy (OPMD) or with an achalasia dilator in three patients. The severity of dysphagia prior to UES dilatation was evaluated by a 15-point dysphagia score, a pharyngeal and esophageal manometry and a radionuclide pharyngoesophageal transit study. Using actuarial life table, the improvement rate after dilatation with Maloney bougie was 64.3% (95% CI 39.2-89.4) at 3- and 6-month follow-ups, and was 55.7% (95% CI 28.9-82.5) at 12- and 18-month follow-ups. At 3-month post-dilatation, we observed a significant reduction of the mean dysphagia score from 9.6 to 7.2 (P = 0.05). No significant manometric or radionuclide factors were found to predict effective dilatation. The results of this pilot study showed that UES dilatation with Maloney bougie or achalasia dilator may be an effective treatment of moderate dysphagia in patients with OPMD. However, further studies with larger sample sizes are needed to corroborate these results and to assess long-term outcome.

Recent studies on oculopharyngeal muscular dystrophy in Québec

In 1990, we launched a major study to ascertain the clinical picture of OPMD in Québec and to identify large families for linkage analysis. In 14 patients, the chromosomes were karyotyped to eliminate any deletion or translocation. Relevant family information and clinical data were computerized and correlations were sought for the age of onset, the identification of the first symptom and the distribution of weakness. A simple test to detect dysphagia was validated. Twenty-one families have taken part in the study, which led to our localization of the gene in 1995 [Brais B, Xie Y-G, Sanson M, et al. Hum Mol Genet 1995; 4:429-434]. At least one case in each family underwent muscle biopsy to confirm the presence of the typical nuclear filaments found in OPMD. Electrodiagnostic and pathologic studies were also conducted to better understand the disease process. An illustrative case is presented.