Proximal myotonic myopathy. Clinical features of a multisystem disorder similar to myotonic dystrophy

Myotonic dystrophy type 2: molecular, diagnostic and clinical spectrum

BACKGROUND

Myotonic dystrophy types 1 (DM1) and 2 (DM2/proximal myotonic myopathy PROMM) are dominantly inherited disorders with unusual multisystemic clinical features. The authors have characterized the clinical and molecular features of DM2/PROMM, which is caused by a CCTG repeat expansion in intron 1 of the zinc finger protein 9 (ZNF9) gene.

METHODS

Three-hundred and seventy-nine individuals from 133 DM2/PROMM families were evaluated genetically, and in 234 individuals clinical and molecular features were compared.

RESULTS

Among affected individuals 90% had electrical myotonia, 82% weakness, 61% cataracts, 23% diabetes, and 19% cardiac involvement. Because of the repeat tract's unprecedented size (mean approximately 5,000 CCTGs) and somatic instability, expansions were detectable by Southern analysis in only 80% of known carriers. The authors developed a repeat assay that increased the molecular detection rate to 99%. Only 30% of the positive samples had single sizeable expansions by Southern analysis, and 70% showed multiple bands or smears. Among the 101 individuals with single expansions, repeat size did not correlate with age at disease onset. Affected offspring had markedly shorter expansions than their affected parents, with a mean size difference of -17 kb (-4,250 CCTGs).

CONCLUSIONS

DM2 is present in a large number of families of northern European ancestry. Clinically, DM2 resembles adult-onset DM1, with myotonia, muscular dystrophy, cataracts, diabetes, testicular failure, hypogammaglobulinemia, and cardiac conduction defects. An important distinction is the lack of a congenital form of DM2. The clinical and molecular parallels between DM1 and DM2 indicate that the multisystemic features common to both diseases are caused by CUG or CCUG expansions expressed at the RNA level.

Myotonic dystrophy: clinical and molecular parallels between myotonic dystrophy type 1 and type 2

Myotonic dystrophy (DM) is a dominantly inherited disorder with a peculiar pattern of multisystemic clinical features affecting skeletal muscle, the heart, the eye, and the endocrine system. Two genetic loci have been associated with the DM phenotype: DM1 on chromosome 19, and DM2 on chromosome 3. In 1992, the mutation responsible for DM1 was identified as a CTG expansion located in the 3' untranslated region of the dystrophica myotonica-protein kinase gene (DMPK). How this untranslated CTG expansion causes DM1 has been a matter of controversy. The recent discovery that DM2 is caused by an untranslated CCTG expansion, along with other discoveries on DM1 pathogenesis, indicate that the clinical features common to both diseases are caused by a gain of function RNA mechanism in which the CUG and CCUG repeats alter cellular function, including alternative splicing of various genes.

Myotonic dystrophy type 2

Myotonic dystrophy type 2 (DM2) is a clinically but not genetically heterogeneous, multisystem disorder, that is clinically similar to, but distinct from myotonic dystrophy type 1 (DM1). Initially, different phenotypes of DM2 were described by Ricker (proximal myotonic myopathy, PROMM), Ranum (myotonic dystrophy 2, DM2) and Udd (proximal myotonic dystrophy, PDM). Clinical features these three phenotypes had in common were diffuse, proximal or distal weakness, wasting, myotonia, cataract, cerebral, endocrine and cardiac abnormalities. Initially, the clinical differences between DM1 and PROMM seemed unmistakable, but meanwhile it has become apparent that the clinical differences between these entities are blurring. In 1999, Day et al., Meola et al. and Ricker et al. mapped the mutated gene of all three phenotypes to chromosome 3q. In 2001, the three different phenotypes were found to rely on the same mutation in the ZNF9 gene on chromosome 3q21.3. Although DM2 may be clinically heterogeneous, it is by result of a mutation in a single gene. The mutation responsible for DM2 is a CCTG-repeat expansion of 75-11 000 repeats in intron 1 of the ZNF9 gene on chromosome 3q21.3. Because of the clinical heterogeneity, the diagnosis of DM2 should rely on DNA analysis alone.

Dominantly inherited, non-coding microsatellite expansion disorders

Dominantly inherited diseases are generally caused by mutations resulting in gain of function protein alterations. However, a CTG expansion located in the 3' untranslated portion of a kinase gene was found to cause myotonic dystrophy type 1, a multisystemic dominantly inherited disorder. The recent discovery that an untranslated CCTG expansion causes the same constellation of clinical features in myotonic dystrophy type 2 (DM2), along with other recent discoveries on DM1 pathogenesis, have led to the understanding that both DM1 and DM2 mutations are pathogenic at the RNA level. These findings indicate the existence of a new category of disease wherein repeat expansions in RNA alter cellular function. Pathogenic repeat expansions in RNA may also be involved in spinocerebellar ataxia types 8, 10 and 12, and Huntington's disease-like type 2.

Electrodiagnostic approach to the patient with suspected myopathy

Uncovering the cause of a suspected myopathy may be challenging. However, a careful approach starts with utilizing the wealth of available information regarding the clinical and laboratory features of myopathy. Electrodiagnostic testing is then obtained (in most cases). Recognition of the pattern of EMG findings in light of the clinical and laboratory features should narrow the differential diagnosis and dictate the next steps in the evaluation. Histopathologic or molecular studies, or both may follow. Ultimately, this approach usually allows the clinician to make the correct diagnosis.

Proximal myotonic myopathy: a syndrome with a favourable prognosis?

Cardiac involvement in myotonic dystrophy type 1 (DM1) is well known. In contrast, the severity and frequency of cardiac abnormalities in proximal myotonic myopathy (PROMM) are still unclear. To identify similarities and differences in the rate of progression of muscle weakness and cardiac disturbances in these two disorders, 16 patients with PROMM (3q-unlinked PROMM: n=10; uniformative for linkage: n=6) were compared to 33 patients with moderately severe myotonic dystrophy type 1 (DM1). There was no significant difference in disease duration between PROMM and DM1. Patients underwent serial manual muscle strength testing, EKG, 24-h Holter monitoring, 2D-echocardiography. Muscle weakness progressed slowly in both groups. Most DM1 patients developed conduction defects. No significant atrioventricular disturbances on initial and follow-up examinations were found in PROMM patients. One patient developed right bundle branch block. Many families with PROMM appear to have more benign cardiac manifestations and less severe prognosis compared to DM1. Further studies of subgroups of PROMM (linked to the 3q21 locus and without linkage) are necessary to determine whether the cardiac conduction disturbances are more common in a specific genotype of PROMM.

Idiopathic painful sensory neuropathy

Unexplained painful burning feet are common complaints among the elderly. Conventional elettrodiagnostic testing may be normal or only minimally abnormal and the lack of a universally accepted diagnostic test for small fiber dysfunction has posed a significant impediment to evaluation of these patients. The increasing application of newer diagnostic techniques, including quantitative sensory testing, tests of sudomotor function, and skin biopsy for intrnepidermal nerve fiber quantification, has permitted recognition and characterization of the idiopathic painful sensory neuropathies This article reviews the clinical features, evaluation, and management of this syndrome.

Clinical and genetic heterogeneity in myotonic dystrophies

This review of myotonic dystrophies primarily concentrates on the clinical and genetic findings that can distinguish a novel form of myotonic dystrophy, myotonic dystrophy type 2 (DM2); proximal myotonic myopathy (PROMM); and proximal myotonic dystrophy (PDM) from myotonic dystrophy type 1 (DM1). The multisystemic nature of these disorders leads to a spectrum of symptoms and signs. Careful clinical evaluation of patients with DM2/PROMM shows that the similarities among the multisystemic myotonic disorders outweigh the differences. An important point in the comparison of the phenotypes of DM1 and DM2/PROMM is that no severe congenital type of DM2/PROMM has yet been described. Genetic linkage analyses show that myotonic dystrophies can be divided into three types: the conventional Steinert type linked to chromosome 19q13.3 (DM1); DM2/PROMM and PDM linked to chromosome 3q21.3; and families not linked to either chromosomal site. Although the diagnosis may be clinically suspected, it depends on DNA analysis.

Myotonic dystrophies

This review demonstrates genetic and phenotypic heterogeneity in all of the multisystemic myotonic disorders collectively called 'myotonic dystrophies' according to the new nomenclature: myotonic dystrophy type 1, myotonic dystrophy type 2, proximal myotonic myopathy and proximal myotonic dystrophy. Only two loci have so far been assigned (19q 13.3 in myotonic dystrophy type 1, and 3q 21.3 in myotonic dystrophy type 2). Although the diagnosis of these disorders may be suspected clinically, it needs to be confirmed by DNA analysis.

Hypothyroidism unmasking proximal myotonic myopathy

No specific diagnostic test is available to identify patients with proximal myotonic myopathy and to distinguish them from common disorders causing similar complaints. We describe three patients from three separate families who were initially diagnosed as having hypothyroid myopathy. Proximal weakness, stiffness and myotonia have persisted in each patient (2-10 years) despite the restoration of the euthyroid state. A familial pattern of autosomal dominant inheritance for proximal weakness, myotonia, and cataracts was clearly identified in one family and was likely in the other two families. DNA testing showed normal size of CTG repeat in the gene for myotonic dystrophy. The clinical presentation of these three patients strongly suggests that hypothyroidism can unmask PROMM in asymptomatic individuals who carry the genetic abnormality. Other cases of 'hypothyroid myopathy' may represent examples of unmasked PROMM.

A family with PROMM not linked to the recently mapped PROMM locus DM2

Proximal myotonic myopathy is an autosomal dominantly inherited multisystem disorder, clinically similar to but genetically distinct from myotonic dystrophy (DM). A recently mapped second locus for myotonic dystrophy was thought to be an attractive candidate locus for PROMM, and this hypothesis was supported by reports of linkage to this locus in some PROMM families. We present a large German pedigree with PROMM in which linkage to this locus could be excluded, showing that PROMM is genetically heterogeneous.

The short exercise test is normal in proximal myotonic myopathy

OBJECTIVES

Proximal myotonic myopathy (PROMM) is a multisystem disorder that may mimic myotonic dystrophy (MD). Previously we demonstrated that the 60 s exercise test was normal in two siblings with PROMM. The test enabled distinction of PROMM from MD, as there is a well documented immediate post-exercise compound muscle action potential (CMAP) amplitude decline in MD.

METHODS

We now performed exercise testing using several exercise durations in 8 PROMM patients from 6 kinships, and one MD patient, extending our previous observations. Repetitive stimulation and needle electromyography findings were also recorded.

RESULTS

The 10 (n = 8), 30 (n = 5), and 60 (n = 5) s, and the 5 min (n = 1) exercise tests were normal in all PROMM patients. Specifically, the maximum post-exercise CMAP amplitude decline was 8%. In contrast, the MD patient had CMAP amplitude declines of 48% (10 s exercise test) and 26% (30 s exercise test). The distribution of repetitive stimulation and motor unit duration abnormalities were variable and less diagnostically useful.

CONCLUSIONS

The 10, 30, and 60 s exercise tests help distinguish PROMM from MD. As the 10 s exercise test is rapid and easily tolerated, we recommend this test for clinical testing.

Proximal myotonic myopathy: clinical and molecular investigation of a Norwegian family with PROMM

Proximal myotonic myopathy (PROMM) was first described in 1994 as a multisystem disorder with similarity to myotonic dystrophy (DM), but without the abnormal (CTG)n expansion in the DM protein kinase (DMPK) gene. The inheritance is autosomal dominant and the clinical features include myotonia, proximal muscle weakness and cataract. Linkage analysis in nine German PROMM families has indicated the possibility of linkage to DM2 locus on chromosome 3. We report a Norwegian PROMM family in which the proband was clinically diagnosed as DM but without the (CTG)n expansion. Using an intragenic marker we showed that the DMPK gene did not segregate with the disease in this family. All family members are heterozygous for the R894X mutation in CLCN1 gene. Linkage analysis could not be performed, but haplotyping probably excludes the DM2 locus as the disease locus in this family. The present family emphasises that myalgia is a prominent symptom in PROMM and the clinical differences may be explained by genetic heterogeneity. This family will be reinvestigated along with the identification of candidate genes or regions in larger PROMM families.

Fluctuating clinical myotonia and weakness from Thomsen's disease occurring only during pregnancies

Advances in molecular genetics are allowing better phenotype to genotype correlation of the non-dystrophic myotonic disorders. We report a 32-year-old woman, who first noted myotonia that was associated with weakness during her first pregnancy. The work-up disclosed that she had Thomsen's disease which is not known to be associated with weakness. In addition, her myotonia was of the fluctuating type and occurred (symptomatically) only during two pregnancies. We discuss the evaluation of myotonia in the pregnant woman which led to the diagnosis of Thomsen's disease and we conclude that in exceptional cases, fluctuating myotonia and weakness occurs in autosomal dominant chloride channel myotonia (Thomsen's disease).

Proximal myotonic myopathy (PROMM) presenting as myotonia during pregnancy

Proximal myotonic myopathy is a recently described autosomal dominant condition characterized by proximal myopathy, cataracts, intermittent myotonia, and myalgia. We report a further family with this condition. The proband and her two sisters presented with myotonia during pregnancy which resolved after each delivery. Two sisters experienced myalgia between each pregnancy. This relationship between pregnancy and symptom exacerbation suggests an intriguing hormonal influence in PROMM.

PROMM: the expanding phenotype. A family with proximal myopathy, myotonia and deafness

We describe a family with a proximal myopathy, subclinical EMG myotonia, cataracts and deafness. Transmission through two generations and down the male line confirms autosomal dominant inheritance. There was no abnormal expansion of the CTG triplet repeat in the last exon of the dystrophia myotonica protein kinase (DMPK) gene associated with myotonic dystrophy. Heteroduplex analysis of all but the promoter region of the DMPK gene has excluded point mutations in this gene as an underlying cause for this myotonic disorder. The family was not sufficiently informative to exclude linkage to the sodium channel gene SCN4A or the chloride channel gene CLC1. This family clearly fulfils the recently established diagnostic criteria for PROMM (proximal myotonic myopathy) and in addition shows consistent severe deafness as a hitherto undescribed feature of PROMM. We discuss the diagnostic criteria of PROMM in relation to this family and other recent papers, all of which would now fulfil the aforementioned diagnostic criteria for PROMM.

Fragile-X syndrome and myotonic dystrophy: parallels and paradoxes

Fragile-X syndrome and myotonic dystrophy are caused by triplet repeat expansions embedded in CpG islands in the transcribed non-coding regions of the FMR1 and the DMPK genes, respectively. Although initial reports emphasized differences in the mechanisms by which the expanded triplet repeats caused these diseases, results published in the past year highlight remarkable parallels in the likely molecular etiologies. At both loci, expansion is associated with altered chromatin, aberrant methylation, and suppressed expression of the adjacent FMR1 and DMAHP genes, implicating epigenetic mediation of these genetic diseases.

Molecular basis of genetic heterogeneity: role of the clinical neurologist

Advances in molecular genetics have disclosed many different explanations for allelic heterogeneity, how different clinical syndromes arise from mutations in the same gene. The converse, how similar clinical syndromes arise from mutations of different genes on different chromosomes is called locus heterogeneity. Both, however, give rise to some disease-defining mutations, as in childhood spinal muscular atrophy or Duchenne muscular dystrophy. Nevertheless, new problems have been created, including what might be called "diagnosis by the number," diverse syndromes from mutations in the same gene without current explanation, or siblings with different clinical syndromes. These discoveries have transformed the clinical neurology of heritable diseases. They also provide clinicians with new responsibilities and opportunities in defining clinical syndromes and influencing the evolution of our clinical language.

Proximal myotonic dystrophy--a family with autosomal dominant muscular dystrophy, cataracts, hearing loss and hypogonadism: heterogeneity of proximal myotonic syndromes?

We describe a family with an autosomal dominant, multisystem disorder, consisting of late-onset proximal muscular dystrophy, electrophysiological myotonia, cataracts, late-onset deafness and male hypogonadism. Four patients were available for clinical examinations. Examination of asymptomatic family members revealed another patient with bilateral cataracts but without definite muscle disorder. Five deceased members of the family had proximal muscle weakness, reportedly or confirmed in medical records. Molecular examination of genomic DNA showed no expansion of the unstable (CTG)n trinucleotide repeat on chromosome 19q13.3 associated with myotonic dystrophy (DM). Linkage to two loci implicated in other myotonic disorders, the muscle chloride channel (CLCN1) gene, and the muscle sodium channel (SCN4A) gene, was assessed and excluded. The clinical findings differ from those described in proximal myotonic myopathy (PROMM), in terms of the more severe muscle involvement with atrophy of affected muscles and the hearing loss. These findings suggest phenotypic and probably genetic heterogeneity among the proximal myotonic syndromes.

Carrell-Krusen Symposium Invited Lecture-1997. Myotonic disorders in childhood: diagnosis and treatment

The recent discoveries that mutations in the genes for the skeletal muscle sodium and chloride channels are responsible, respectively, for paramyotonia/hyperkalemic periodic paralysis and for myotonia congenita of Thomsen have made the classification, diagnosis, and treatment of these disorders much easier. The discovery that myotonic dystrophy results from an unstable [CTG]n trinucleotide expansion has permitted the accurate diagnosis of both symptomatic and asymptomatic individuals, and has led to major advances in preventive treatment, including prenatal and genetic counseling. Diseases that resemble the inherited myotonic disorders are now easier to identify, and as a result of genetic testing a new clinical disorder that is similar to but distinct from myotonic dystrophy has emerged. This new disorder, proximal myotonic myopathy, does not appear to be linked to the genes for the sodium or chloride channels, and has cataracts, myotonia, weakness, and no abnormal expansion of the [CTG]n repeat in the gene for myotonic dystrophy. This review discusses the diagnosis and treatment of these myotonic disorders.

A newly-described myotonic disorder (proximal myotonic myopathy--PROMM): personal experience and review of the literature

The aim of this study is to describe the essential characteristics of a family affected by the newly-described proximal myotonic myopathy (PROMM) The clinical, laboratory and genetic findings are described and compared with those reported in the literature, and the clinical spectrum of the manifestations that are similar to but distinct from myotonic dystrophy (MD) is also explored. This has practical implications because the cases so far described suggest that the long-term prognosis with PROMM seems to be more favourable than that of patients with MD.

Myotonic dystrophy phenotype without expansion of (CTG)n repeat: an entity distinct from proximal myotonic myopathy (PROMM)?

Myotonic dystrophy (DM) is associated with an expansion of an unstable (CTG)n repeat in the 3' untranslated region of the DM protein kinase (DMPK) gene on chromosome 19q13.3. We studied six patients from two families who showed no expansions of the repeat, in spite of their clinical diagnosis of DM. These patients had multi-systemic manifestations that were distinguishable from those seen in other myotonic disorders, including proximal myotonic myopathy (PROMM). In one additional family, two symptomatic members showed no expanded (CTG)n repeats, while their affected relatives had the expanded repeats. DM haplotype analysis failed to exclude the DMPK locus as a possible site of mutation in each family; however, DMPK mRNA levels were normal. We conclude that a mutation(s) other than the expanded (CTG)n repeat can cause the DM phenotype. The mutation(s) in these families remain(s) to be mapped and characterized.

AAEM minimonograph #46: neurogenic muscle hypertrophy

Muscle hypertrophy occurs uncommonly in several neurogenic disorders including neuropathies, radiculopathies, spinal muscular atrophy, and post-polio syndrome. Its pathogenesis varies in different circumstances. In the presence of generalized myokymia and neuromyotonia (Isaacs' syndrome), symmetrical hypertrophy appears to be the result of continuous spontaneous electrical stimulation of myofibers and, in some cases, results in type 1 myofiber preponderance. Focal hypertrophy occurring with radiculopathies and mononeuropathies was associated with complex repetitive discharges (CRDs) in approximately half the cases. CRDs may play a role in the pathogenesis of myofiber hypertrophy by continuous myofiber stimulation, but in some cases, with and without CRDs, myofiber hypertrophy may be related to mechanical events. Muscle enlargement seen in old polio appears to involve a significant degree of pseudohypertrophy, although some myofiber hypertrophy occurs. The symmetrical occurrence of hypertrophy in genetically determined disorders, such as spinal muscular atrophy, and hereditary motor and sensory neuropathy types 1 and 2 may have both a genetic and a mechanical basis in addition to pseudohypertrophy in some cases.

A family with an unusual myotonic and myopathic phenotype and no CTG expansion (proximal myotonic myopathy syndrome): a challenge for future molecular studies

Myotonic dystrophy (DM) is a well-defined autosomal dominant disorder characterized by myotonia, muscle weakness, cardiac conduction defects, cataracts, and endocrine abnormalities. Recently a newly recognized disorder, similar to but distinct from DM, has been observed with multisystem findings including intermittent myotonia, proximal myopathy, and occasional cardiac conduction disturbances. This disorder has been called proximal myotonic myopathy (PROMM). No history of anticipation is present and there is no linkage to the gene locus for DM or to the loci for the muscle sodium or chloride channels. This report describes a family with a normal size of the CTG trinucleotide repeat expansion of the DM gene in which affected individuals have myotonia (intermittent, exacerbated by cold), bilateral cataracts, mild hypogonadism and mild temporal atrophy. Affected individuals also have proximal muscle weakness, facial involvement, nonspecific abnormalities on muscle biopsy, normal cardiac conduction, and no glucose intolerance. The absence of trinucleotide repeat expansion in the DM gene is consistent with this family being affected by a disorder distinct from DM, possibly a form of PROMM.

Proximal myotonic myopathy: mini-review of a recently delineated clinical disorder

This mini-review describes proximal myotonic myopathy, a recently delineated, dominantly inherited disorder that is similar to but distinct from myotonic dystrophy. Proximal myotonic myopathy is not linked to the gene locus for myotonic dystrophy or to the loci of the genes of the muscle sodium and chloride channels associated with other myotonic disorders. Patients often present with myotonia and peculiar muscle pain in early adulthood and develop weakness of the thigh muscles later in life. Cataracts that are indistinguishable from those in myotonic dystrophy also occur commonly. The gene defect responsible for proximal myotonic myopathy awaits discovery. Because of the clinical similarities between proximal myotonic myopathy and myotonic dystrophy, clarification of the genetic differences will not only shed light on the pathomechanism of proximal myotonic myopathy, but may also increase our understanding of myotonic dystrophy.