Genetic mapping of a second myotonic dystrophy locus

MBNL1 and CUGBP1 modify expanded CUG-induced toxicity in a Drosophila model of myotonic dystrophy type 1

Myotonic dystrophy type 1 (DM1) is a neuromuscular disorder caused by a CTG expansion in the 3' UTR of the dystrophia myotonica protein kinase (DMPK) gene. It has been hypothesized that the pathogenesis in DM1 is triggered by a toxic gain of function of the expanded DMPK RNA. This expanded RNA is retained in nuclear foci where it sequesters and induces alterations in the levels of RNA-binding proteins (RNA-BP). To model DM1 and study the implication of RNA-BP in CUG-induced toxicity, we have generated a Drosophila DM1 model expressing a non-coding mRNA containing 480 interrupted CUG repeats; i.e. [(CUG)20CUCGA]24. This (iCUG)480 transcript accumulates in nuclear foci and its expression leads to muscle wasting and degeneration in Drosophila. We also report that altering the levels of two RNA-BP known to be involved in DM1 pathogenesis, MBNL1 and CUGBP1, modify the (iCUG)480 degenerative phenotypes. Expanded CUG-induced toxicity in Drosophila is suppressed when MBNL1 expression levels are increased, and enhanced when MBNL1 levels are reduced. In addition, (iCUG)480 also causes a decrease in the levels of soluble MBNL1 that is sequestered in the CUG-containing nuclear foci. In contrast, increasing the levels of CUGBP1 worsens (iCUG)480-induced degeneration even though CUGBP1 distribution is not altered by the expression of the expanded triplet repeat. Our data supports a mechanism for DM1 pathogenesis in which decreased levels of MBNL and increased levels of CUGBP mediate the RNA-induced toxicity observed in DM1. Perhaps more importantly, they also provide proof of the principle that CUG-induced muscle toxicity can be suppressed.

Effect of the [CCTG]n repeat expansion on ZNF9 expression in myotonic dystrophy type II (DM2)

Myotonic dystrophy is caused by two different mutations: a (CTG)n expansion in 3' UTR region of the DMPK gene (DM1) and a (CCTG)n expansion in intron 1 of the ZNF9 gene (DM2). The most accredited mechanism for DM pathogenesis is an RNA gain-of-function. Other findings suggest a contributory role of DMPK-insufficiency in DM1. To address the issue of ZNF9 role in DM2, we have analyzed the effects of (CCTG)n expansion on ZNF9 expression in lymphoblastoid cell lines (n=4) from DM2 patients. We did not observe any significant alteration in ZNF9 mRNA and protein levels, as shown by QRT-PCR and Western blot analyses. Additional RT-PCR experiments demonstrated that ZNF9 pre-mRNA splicing pattern, which includes two isoforms, is unmodified in DM2 cells. Our results indicate that the (CCTG)n expansion in the ZNF9 intron does not appear to have a direct consequence on the expression of the gene itself.

Diseases of unstable repeat expansion: mechanisms and common principles

The list of developmental and degenerative diseases that are caused by expansion of unstable repeats continues to grow, and is now approaching 20 disorders. The pathogenic mechanisms that underlie these disorders involve either loss of protein function or gain of function at the protein or RNA level. Common themes have emerged within and between these different classes of disease; for example, among disorders that are caused by gain-of-function mechanisms, altered protein conformations are central to pathogenesis, leading to changes in protein activity or abundance. In all these diseases, the context of the expanded repeat and the abundance, subcellular localization and interactions of the proteins and RNAs that are affected have key roles in disease-specific phenotypes.

Clinical and molecular aspects of the myotonic dystrophies: a review

Type 1 myotonic dystrophy or DM1 (Steinert's disease), which is the commonest muscular dystrophy in adults, has intrigued physicians for over a century. Unusual features, compared with other dystrophies, include myotonia, anticipation, and involvement of other organs, notably the brain, eyes, smooth muscle, cardiac conduction apparatus, and endocrine system. Morbidity is high, with a substantial mortality relating to cardiorespiratory dysfunction. More recently a second form of multisystem myotonic disorder has been recognized and variously designated as proximal myotonic myopathy (PROMM), proximal myotonic dystrophy (PDM), or DM2. For both DM1 and DM2 the molecular basis is expansion of an unstable repeat sequence in a noncoding part of a gene (DMPK in DM1 and ZNF9 in DM2). There is accumulating evidence that the basic molecular mechanism is disruption of mRNA metabolism, which has far-reaching effects on many other genes, in part through the induction of aberrant splicing, explaining the multisystemic nature of the disease. The unstable nature of the expansion provides a molecular explanation for anticipation. This review emphasizes the clinical similarities and differences between DM1 and DM2. It examines current views about the molecular basis of these disorders, and contrasts them with other repeat expansion disorders that have increasingly been recognized as a cause of neurological disease.

Tau protein as a differential biomarker of tauopathies

Microtubule-associated Tau proteins are the basic component of intraneuronal and glial inclusions observed in many neurological disorders, the so-called tauopathies. Many etiological factors, phosphorylation, splicing, and mutations, relate Tau proteins to neurodegeneration. Molecular analysis has revealed that hyperphosphorylation and abnormal phosphorylation might be one of the important events in the process leading to tau intracellular aggregation. Specific set of pathological tau proteins exhibiting a typical biochemical pattern, and a different regional and laminar distribution, could characterize five main classes of tauopathies. A direct correlation has been established between the regional brain distribution of tau pathology and clinical symptoms; for instance progressive involvement of neocortical areas is well correlated to the severity of dementia in Alzheimer's disease, overall suggesting that pathological tau proteins are reliable marker of the neurodegenerative process. Recent discovery of tau gene mutations in frontotemporal dementia with parkinsonism linked to chromosome 17 has reinforced the predominant role attributed to tau proteins in the pathogenesis of neurodegenerative disorders, and underlined the fact that distinct sets of tau isoforms expressed in different neuronal populations could lead to different pathologies. Overall, a better knowledge of the etiological factors responsible for the aggregation of tau proteins in brain diseases is essential for development of future differential diagnosis and therapeutic strategies. They would hopefully find their application against Alzheimer's disease but also in all neurological disorders for which a dysfunction of Tau biology has been identified.

RNA pathogenesis of the myotonic dystrophies

Myotonic dystrophy (dystrophia myotonica, DM) is the most common form of muscular dystrophy in adults. The presence of two genetic forms of this complex multisystemic disease (DM1 and DM2) was unrecognized until the genetic cause of DM1 was identified in 1992. The fact that the DM1 mutation is an untranslated CTG expansion led to extended controversy about the molecular pathophysiology of this disease. When the DM2 mutation was identified in 2001 as being a similarly untranslated CCTG expansion, the molecular and clinical parallels between DM1 and DM2 substantiated the role of a novel mechanism in generating the unusual constellation of clinical features seen in these diseases: the repeat expansions expressed at the RNA level alter RNA processing, at least in part by interfering with alternative splicing of other genes. For example, in both DM1 and DM2, altered splicing of chloride channel and insulin receptor transcripts leads to myotonia and insulin resistance, respectively. Although other mechanisms may underlie the differences between DM1 and DM2, the pathogenic effects of the RNA mechanism are now clear, which will facilitate development of appropriate treatments.

Genetics and molecular pathogenesis of the myotonic dystrophies

Pathogenic repeat expansions were initially identified as causing either a loss of gene product, such as in fragile X mental retardation, or an expansion of a polyglutamine region of a protein, as was first shown in spinobulbar muscular atrophy (Kennedy's disease). The pathogenic effect of the repeat expansion in myotonic dystrophy type 1, however, has been controversial because it does not encode a protein but nonetheless results in a highly penetrant dominant disease. Clinical and molecular characterization of myotonic dystrophy types 1 and 2 have now demonstrated a novel disease mechanism involving pathogenic effects of repeat expansions that are expressed in RNA but are not translated into protein.

Pathogenic RNA repeats: an expanding role in genetic disease

Fragile X mental retardation and Friedreich's ataxia were among the first pathogenic trinucleotide repeat disorders to be described in which noncoding repeat expansions interfere with gene expression and cause a loss of protein production. Invoking a similar loss-of-function hypothesis for the CTG expansion causing myotonic dystrophy type 1 (DM1) located in the 3' noncoding portion of a kinase gene was more difficult because DM is a dominantly inherited multisystemic disorder in which the second copy of the gene is unaffected. However, the discovery that a transcribed but untranslated CCTG expansion causes myotonic dystrophy type 2 (DM2), along with other discoveries on DM1 and DM2 pathogenesis, indicate that the CTG and CCTG expansions are pathogenic at the RNA level. This review will detail recent developments on the molecular mechanisms of RNA pathogenesis in DM, and the growing number of expansion disorders that might involve similar pathogenic RNA mechanisms.

Myotonic dystrophy type 2 and related myotonic disorders

The myotonic dystrophies are a group of dominantly inherited disorders characterized by muscle wasting, myotonia, cataracts, hypogonadism and other system manifestations. Myotonic dystrophy type 1 (DM1) results from an unstable expansion of a CTG repeat in 3' UTR of the DM protein kinase (DMPK) gene on chromosome 19q 13.3. Myotonic dystrophy type 2 (DM2) is caused by an unstable expansion of a CCTG tetraplet repeat in intron 1 of the zinc finger 9 (ZFN9 gene) on chromosome 3q 21.3. However, the clinical diagnosis of DM2 is more complex than that of DM1, and conventional molecular genetic methods used for diagnosis of DM1 are not helpful for DM2. We here describe the detailed clinical, laboratory and biomolecular tests to identify DM2 and related myotonic disorders. At present, foci of accumulated noncoding CCTG repeat RNA (ribonuclear inclusions) in the cell nuclei are thought to interfere with the regulation and expression of several genes at the basis of multisystemic aspects of myotonic dystrophy type 2.

Clinical and genetic analysis of a family with PROMM

PROMM (proximal myotonic myopathy) and DM2 (myotonic dystrophy Type 2) are autosomal dominant multisystem disorders that have both been linked to chromosome 3q. Recently, the genetic basis of DM2 has been defined by a '(CCTG)(n)' expansion mutation in intron 1 of the ZNF9 gene. We identified and studied a multigenerational family in which five members had clinical features consistent with PROMM. Two affected members were available for detailed clinical, electrophysiological, radiological and genetic analysis. Our study confirms that the PROMM phenotype is associated with DM2-(CCTG)(n) expansion mutations. In addition, our results may extend the clinical spectrum of manifestations to include vestibular symptoms.

Syndromic immunodeficiencies: genetic syndromes associated with immune abnormalities

In syndromic immunodeficiencies, clinical features not directly associated with the immune defect are prominent. Patients may present with either infectious complications or extra-immune medical issues. In addition to the immunologic abnormality, a wide range of organ systems may be affected. Patients may present with disturbances in skeletal, neurologic, dermatologic, or gastrointestinal function or development. These conditions can be caused by developmental abnormalities, chromosomal aberrations, metabolic disorders, or teratogens. For a number of these conditions, recent advances have resulted in an enhanced understanding of their genetic basis. The finding of immune deficits in a number of defined syndromes with congenital anomalies suggests that an underlying genetic syndrome should be considered in those patients in whom a significant non-immune feature is present.

Insulin receptor splicing alteration in myotonic dystrophy type 2

Myotonic dystrophy (DM) is caused by either an untranslated CTG expansion in the 3' untranslated region of the DMPK gene on chromosome 19 (dystrophia myotonica type 1 [DM1]), or an untranslated CCTG tetranucleotide repeat expansion in intron 1 of the ZNF9 gene on chromosome 3 (dystrophia myotonica type 2 [DM2]). RNA-binding proteins adhere to transcripts of the repeat expansions that accumulate in the nucleus, and a trans-dominant dysregulation of pre-mRNA alternative splicing has been demonstrated for several genes. In muscle from patients with DM1, altered insulin-receptor splicing to the nonmuscle isoform corresponds to the insulin insensitivity and diabetes that are part of the DM phenotype; because of insulin-receptor species differences, this effect is not seen in mouse models of the disease. We now demonstrate that comparable splicing abnormalities occur in DM2 muscle prior to the development of muscle histopathology, thus demonstrating an early pathogenic effect of RNA expansions.

Myotonic dystrophy: RNA pathogenesis comes into focus

Myotonic dystrophy (DM)--the most common form of muscular dystrophy in adults, affecting 1/8000 individuals--is a dominantly inherited disorder with a peculiar and rare 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 dystrophia myotonica-protein kinase gene (DMPK). How this untranslated CTG expansion causes myotonic dystrophy type 1(DM1) has been controversial. The recent discovery that myotonic dystrophy type 2 (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. We discuss the pathogenic mechanisms that have been proposed for the myotonic dystrophies, the clinical and molecular features of DM1 and DM2, and the characterization of murine and cell-culture models that have been generated to better understand these diseases.

New methods for molecular diagnosis and demonstration of the (CCTG)n mutation in myotonic dystrophy type 2 (DM2)

Myotonic dystrophy types 1 and 2 are autosomal dominant, multisystemic disorders with many similarities in their clinical manifestations. Myotonic dystrophy type 1 is caused by a (CTG)n expansion in the 3' untranslated region of the DMPK gene in 19q13.3 and myotonic dystrophy type 2 by a (CCTG)n expansion in intron 1 of ZNF9 in 3q21.3. However, the clinical diagnosis of myotonic dystrophy type 2 is more complex than that of myotonic dystrophy type 1, and conventional molecular genetic methods used for diagnosing myotonic dystrophy type 1 are insufficient for myotonic dystrophy type 2. Herein we describe two in situ hybridization protocols for the myotonic dystrophy type 2 mutation detection. Chromogenic in situ hybridization was used to detect both the genomic expansion and the mutant transcripts in muscle biopsy sections. Chromogenic in situ hybridization can be used in routine myotonic dystrophy type 2 diagnostics. Fluorescence in situ hybridization on extended DNA fibers was used to directly visualize the myotonic dystrophy type 2 mutation and to estimate the repeat expansion sizes.

Muscle pathology in 57 patients with myotonic dystrophy type 2

We evaluated muscle biopsies from 57 patients with genetically confirmed myotonic dystrophy type 2/proximal myotonic myopathy (DM2/PROMM). Light microscopy showed myopathic together with "denervation-like" changes in almost all biopsies obtained from four different muscles: increased fiber size variation, internal nuclei, small angulated fibers, pyknotic nuclear clumps, and predominant type 2 fiber atrophy. Quantitative morphometry in 18 biopsies that were immunostained for myosin heavy chain confirmed a predominance of nonselective type 2 fiber atrophy. These histological changes were similar in all patients regardless of the site of biopsy, the predominant clinical symptoms and signs, and the clinical course. It is likely that, in a number of undiagnosed patients, DM2 is the underlying disorder. With a better understanding of the histopathological pattern in DM2, biopsies from patients with undiagnosed neuromuscular disorders can now be reevaluated.

Improvement of the diagnostic procedure in proximal myotonic myopathy/myotonic dystrophy type 2

Proximal myotonic myopathy/myotonic dystrophy type 2 (PROMM/DM 2) is caused by an expansion of the (TG)n(TCTG)n(CCTG)n repeat tract in intron 1 of the ZNF9 gene located on chromosome 3q21. Because these expansions show a marked mitotic instability, expanded alleles are often difficult to detect. In order to improve the diagnostic procedure, we applied a combination of pulsed-field gel electrophoresis and semi-quantitative Southern blot analysis with a novel hybridization probe. The combination of these methods led to unequivocal results in about 98% of cases with a clinical diagnosis of PROMM/DM 2. Furthermore, we report the genotype/phenotype correlation in a patient lacking a normal ZNF9 allele and a further proband with a "grey zone" allele.

Executive dysfunction and avoidant personality trait in myotonic dystrophy type 1 (DM-1) and in proximal myotonic myopathy (PROMM/DM-2)

A previous study in proximal myotonic myopathy (PROMM/DM-2) and myotonic dystrophy type 1 (DM-1) using brain positron emission tomography demonstrated a reduced cerebral blood flow in the frontal and temporal regions associated with cognitive impairment. The objective was to investigate further cognitive and behavioural aspects in a new series of patients with DM-1 and PROMM/DM-2. Nineteen patients with genetically determined PROMM/DM-2 and 21 patients with moderately severe DM-1 underwent neuropsychological testing and neuropsychiatric interviews. DM-1 and PROMM/DM-2 patients had significantly lower scores on tests of frontal lobe function compared to controls. Neuropsychiatric interviews demonstrated an avoidant trait personality disorder in both patient groups. Brain single photon emission computed tomography showed frontal and parieto-occipital hypoperfusion. The results suggest that there is a specific cognitive and behavioural profile in PROMM/DM-2 and in DM-1, and that this profile is associated with hypoperfusion in frontal and parieto-occipital regions of the brain.

Molecular genetics of spinocerebellar ataxia type 8 (SCA8)

We previously reported that a transcribed but untranslated CTG expansion causes a novel form of ataxia, spinocerebellar ataxia type 8 (SCA8) (Koob et al., 1999). SCA8 was the first example of a dominant spinocerebellar ataxia that is not caused by the expansion of a CAG repeat translated into a polyglutamine tract. This slowly progressive form of ataxia is characterized by dramatic repeat instability and a high degree of reduced penetrance. The clinical and genetic features of the disease are discussed below.

Myotonic dystrophy type 2: human founder haplotype and evolutionary conservation of the repeat tract

Myotonic dystrophy (DM), the most common form of muscular dystrophy in adults, can be caused by a mutation on either chromosome 19 (DM1) or 3 (DM2). In 2001, we demonstrated that DM2 is caused by a CCTG expansion in intron 1 of the zinc finger protein 9 (ZNF9) gene. To investigate the ancestral origins of the DM2 expansion, we compared haplotypes for 71 families with genetically confirmed DM2, using 19 short tandem repeat markers that we developed that flank the repeat tract. All of the families are white, with the majority of Northern European/German descent and a single family from Afghanistan. Several conserved haplotypes spanning >700 kb appear to converge into a single haplotype near the repeat tract. The common interval that is shared by all families with DM2 immediately flanks the repeat, extending up to 216 kb telomeric and 119 kb centromeric of the CCTG expansion. The DM2 repeat tract contains the complex repeat motif (TG)(n)(TCTG)(n)(CCTG)(n). The CCTG portion of the repeat tract is interrupted on normal alleles, but, as in other expansion disorders, these interruptions are lost on affected alleles. We examined haplotypes of 228 control chromosomes and identified a potential premutation allele with an uninterrupted (CCTG)(20) on a haplotype that was identical to the most common affected haplotype. Our data suggest that the predominant Northern European ancestry of families with DM2 resulted from a common founder and that the loss of interruptions within the CCTG portion of the repeat tract may predispose alleles to further expansion. To gain insight into possible function of the repeat tract, we looked for evolutionary conservation. The complex repeat motif and flanking sequences within intron 1 are conserved among human, chimpanzee, gorilla, mouse, and rat, suggesting a conserved biological function.

Confirmation of the type 2 myotonic dystrophy (CCTG)n expansion mutation in patients with proximal myotonic myopathy/proximal myotonic dystrophy of different European origins: a single shared haplotype indicates an ancestral founder effect

Myotonic dystrophy (DM), the most common form of muscular dystrophy in adults, is a clinically and genetically heterogeneous neuromuscular disorder. DM is characterized by autosomal dominant inheritance, muscular dystrophy, myotonia, and multisystem involvement. Type 1 DM (DM1) is caused by a (CTG)(n) expansion in the 3' untranslated region of DMPK in 19q13.3. Multiple families, predominantly of German descent and with clinically variable presentation that included proximal myotonic myopathy (PROMM) and type 2 DM (DM2) but without the DM1 mutation, showed linkage to the 3q21 region and were recently shown to segregate a (CCTG)(n) expansion mutation in intron 1 of ZNF9. Here, we present linkage to 3q21 and mutational confirmation in 17 kindreds of European origin with PROMM and proximal myotonic dystrophy, from geographically distinct populations. All patients have the DM2 (CCTG)(n) expansion. To study the evolution of this mutation, we constructed a comprehensive physical map of the DM2 region around ZNF9. High-resolution haplotype analysis of disease chromosomes with five microsatellite and 22 single-nucleotide polymorphism markers around the DM2 mutation identified extensive linkage disequilibrium and a single shared haplotype of at least 132 kb among patients from the different populations. With the exception of the (CCTG)(n) expansion, the available markers indicate that the DM2 haplotype is identical to the most common haplotype in normal individuals. This situation is reminiscent of that seen in DM1. Taken together, these data suggest a single founding mutation in DM2 patients of European origin. We estimate the age of the founding haplotype and of the DM2 (CCTG) expansion mutation to be approximately 200-540 generations.

Assessment of cardiovascular autonomic function in myotonic dystrophy type 2 (DM2/PROMM)

BACKGROUND

Proximal myotonic myopathy is an autosomal dominant multisystem disorder with a recently defined CCTG expansion on chromosome 3 in the major subgroup (myotonic dystrophy type 2). Cardiac rhythm disturbances have been described in patients with this disease, but it is not known whether myotonic dystrophy type 2/proximal myotonic myopathy patients suffer from dysautonomia and whether cardiac arrhythmias relate to autonomic dysfunction.

OBJECTIVES

To investigate cardiovascular autonomic function in myotonic dystrophy type 2/proximal myotonic myopathy patients with and without cardiac arrhythmias.

PATIENTS AND METHODS

Standard autonomic function tests (heart rate responses to Valsalva manoeuvre, deep breathing and active change of posture, and blood pressure responses to active change of posture and sustained handgrip), resting heart rate variability in the time- and frequency-domain, and the corrected QT interval length were determined in 16 patients with genetically defined myotonic dystrophy type 2/proximal myotonic myopathy and compared to the results obtained in 16 age- and sex-matched healthy control subjects.

RESULTS

Standard autonomic tests yielded similar results in both groups. Measures of heart rate variability tended to be lower in myotonic dystrophy type 2/proximal myotonic myopathy patients compared to healthy controls, but reached statistical significance only for the number of R-R intervals exceeding 50 ms (p50) and the power spectrum density in the low-frequency range (low-frequency power). Four patients (25%) suffered from mild cardiac rhythm disturbances encompassing paroxysmal tachycardia, sinoatrial block, right bundle branch block, ventricular premature beats and bradycardia. The autonomic responses of these patients were essentially similar compared to those without cardiac arrhythmias, apart from a decreased heart rate response to deep breathing in the patients with cardiac arrhythmias.

CONCLUSIONS

We found no major abnormalities of cardiovascular autonomic function in patients with myotonic dystrophy type 2/proximal myotonic myopathy, neither in the whole study group nor in the subgroup of patients with cardiac rhythm abnormalities.

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 syndromes

PURPOSE OF REVIEW

To highlight recent advances in understanding the clinical manifestations and molecular genetics of myotonic syndromes, with particular emphasis on the myotonic dystrophies.

RECENT FINDINGS

Myotonic syndromes include the non-dystrophic myotonias, caused by mutations in genes encoding the chloride or sodium channels that are specific to skeletal muscle, and the myotonic dystrophies. Previous studies have shown that myotonic dystrophy type 1 is caused by the expansion of a CTG repeat in the gene. Recently, it was discovered that myotonic dystrophy type 2 (proximal myotonic myopathy) is also caused by a DNA expansion mutation. In both types of myotonic dystrophy the expanded repeat is transcribed and the RNA produced from the mutant allele is retained in nuclear inclusions. Recent studies suggest that the mutant RNA has a toxic effect on muscle fibers by interfering with the essential functions of the myonucleus, such as RNA processing.

SUMMARY

It now appears likely that myotonic dystrophy is the first instance of a genetic disease in which the harmful effect of a mutation involves the production of a pathogenic RNA. However, the exact mechanism is not understood, and it is unclear whether this RNA-mediated disease process is also responsible for the manifestations of myotonic dystrophy in non-muscle tissues.

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.

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.

Intolerance to neuroleptics and susceptibility for malignant hyperthermia in a patient with proximal myotonic myopathy (PROMM) and schizophrenia

We report a patient with proximal myotonic myopathy who was treated with neuroleptics because of exacerbating schizophrenia. Under therapy with fluanxol, the patient developed muscle stiffness and oculogyric cramps. Treatment with both amisulpride and olanzapine lead to markedly elevated serum creatine kinase levels. An in-vitro contracture test was positive for halothane. Thus, in patients with all kinds of multisystemic myotonic myopathies, a susceptibility for malignant hyperthermia and intolerance towards neuroleptics should be taken into account.

A biologia molecular contribuindo para a compreensão e a prevenção das doenças hereditárias

O fim do seqüenciamento do genoma humano levanta inúmeras questões: Como o projeto genoma humano vai influenciar nossas vidas? Como a medicina tem se beneficiado do estudo dos genes? Quais são as aplicações práticas imediatas e o que se espera para o futuro? Quais são as implicações éticas? Este capítulo ilustra como as doenças genéticas têm contribuído para a compreensão do genoma humano. Ajuda-nos a entender como nossos genes funcionam quando normais e por que causam doenças quando alterados. Do ponto de vista prático, o estudo dos genes tem permitido o diagnóstico molecular para um número crescente de patologias, o que é fundamental para evitar outros exames invasivos, identificar casais em risco, e prevenir o nascimento de novos afetados. Além disso, discute-se quais são as perspectivas futuras em relação ao tratamento destas e de outras patologias genéticas incluindo a clonagem para fins terapêuticos e a utilização de células-tronco. Finalmente aborda as implicações éticas relacionadas ao uso de testes genéticos. Os benefícios de cada teste, principalmente para doenças de início tardio para as quais ainda não há tratamento, têm que ser discutidos exaustivamente com os consulentes antes de sua aplicação.

Triplet repeats, RNA secondary structure and toxic gain-of-function models for pathogenesis

Ten years after the discovery of human diseases caused by trinucleotide repeat expansions, searching for mechanistic links between gene mutation and pathological phenotype remains a fundamental and unsolved issue. Evidence accumulated so far indicates that the pathogenesis of repeat disorders is complex and multi-factorial. Diseases caused by CAG expansions coding for polyglutamine tracts have been extensively studied, and in most cases a toxic gain-of-function of the mutant protein was demonstrated. Most recently, tracking the effects of repeats along the pathway of gene expression is providing additional clues to understand how a triplet repeat expansion can cause disease. Expanded repeats form DNA secondary structures that confer genetic instability, and most likely contribute to alter the local chromatin configuration leading to transcriptional silencing. At the level of RNA, the expanded repeat may either interfere with processing of the primary transcript, resulting in deficit of the corresponding protein, or interact with RNA-binding proteins altering their normal activity. The latter mechanism, termed RNA gain-of-function, has no precedents in human genetics. Recent evidence suggests that expanded RNAs and associated RNA-binding proteins are potential contributors to the pathogenesis of several triplet repeat diseases.

Myotonic dystrophy--a multigene disorder

Myotonic dystrophy (DM1) is the most common form of adult muscular dystrophy with an estimated incidence of 1/8000 births. The mutation responsible for this condition is an expanded CTG repeat within the 3' untranslated region of the protein kinase gene DMPK. Strong nucleosome positioning signals created by this expanded repeat cause a reduction in gene expression within the region. This "field effect" is further confounded by the retention of DMPK expansion containing transcripts, which acquire a toxic gain of function. Thus, the various manifestations exhibited by DM1 patients can be explained as a result of gene silencing, nuclear retention and sequestration of nuclear factors by the CUG containing transcript.

Myotonic dystrophy type 2 caused by a CCTG expansion in intron 1 of ZNF9

Myotonic dystrophy (DM), the most common form of muscular dystrophy in adults, can be caused by a mutation on either chromosome 19q13 (DM1) or 3q21 (DM2/PROMM). DM1 is caused by a CTG expansion in the 3' untranslated region of the dystrophia myotonica-protein kinase gene (DMPK). Several mechanisms have been invoked to explain how this mutation, which does not alter the protein-coding portion of a gene, causes the specific constellation of clinical features characteristic of DM. We now report that DM2 is caused by a CCTG expansion (mean approximately 5000 repeats) located in intron 1 of the zinc finger protein 9 (ZNF9) gene. Parallels between these mutations indicate that microsatellite expansions in RNA can be pathogenic and cause the multisystemic features of DM1 and DM2.

Proximal myotonic myopathy and proximal myotonic dystrophy: two different entities? The phenotypic variability of proximal myotonic syndromes

Multisystemic myotonic myopathies are characterised by a variable pattern of symptoms and signs and a variable degree of disease severity. Proximal myotonic dystrophy has been described as an entity distinct from proximal myotonic myopathy because of severe proximal muscle weakness and dystrophic changes on magnetic reasonace imaging and on muscle histopathology. We describe two siblings, one of them presenting with a proximal myotonic myopathy phenotype, the other with a proximal myotonic dystrophy-like phenotype. The variability of disease expression in these two siblings suggests that a proximal myotonic dystrophy-like variant may occur in proximal myotonic myopathy.

Myotonic dystrophy and myotonic dystrophy protein kinase

Myotonic dystrophy protein kinase (DMPK) was designated as a gene responsible for myotonic dystrophy (DM) on chromosome 19, because the gene product has extensive homology to protein kinase catalytic domains. DM is the most common disease with multisystem disorders among muscular dystrophies. The genetic basis of DM is now known to include mutational expansion of a repetitive trinucleotide sequence (CTG)n in the 3'-untranslated region (UTR) of DMPK. Full-length DMPK was detected and various isoforms of DMPK have been reported in skeletal and cardiac muscles, central nervous tissues, etc. DMPK is localized predominantly in type I muscle fibers, muscle spindles, neuromuscular junctions and myotendinous tissues in skeletal muscle. In cardiac muscle it is localized in intercalated dises and Purkinje fibers. Electron microscopically it is detected in the terminal cisternae of SR in skeletal muscle and the junctional and corbular SR in cardia muscle. In central nervous system, it is located in many neurons, especially in the cytoplasm of cerebellar Purkinje cells, hippocampal interneurons and spinal motoneurons. Electron microscopically it is detected in rough endoplasmic reticulum. The functional role of DMPK is not fully understood, however, it may play an important role in Ca2+ homeostasis and signal transduction system. Diseased amount of DMPK may play an important role in the degeneration of skeletal muscle in adult type DM. However, other molecular pathogenetical mechanisms such as dysfunction of surrounding genes by structural change of the chromosome by long trinucleotide repeats, and the trans-gain of function of CUG-binding proteins might be responsible to induce multisystemic disorders of DM such as myotonia, endocrine dysfunction, etc.

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.

A novel locus for autosomal dominant, non-syndromic hearing impairment (DFNA18) maps to chromosome 3q22 immediately adjacent to the DM2 locus

Investigating a large German pedigree with non-syndromic hearing impairment of early onset and autosomal dominant mode of inheritance, linkage to known DFNA loci was excluded and in a subsequent genomic scan the phenotype was mapped to a 10-cM interval on chromosome 3q22; a maximum two-point lod score of 3.77 was obtained for the marker D3S1292. The new locus, DFNA18, is excluded from neighbouring deafness loci, DFNB15 and USH3, and it overlaps with the recently described DM2/PROMM locus. As hearing loss has been described as one feature of the PROMM phenotype, the DFNA18 gene might also be responsible for hearing loss in DM2/PROMM.

Skeletal muscle-specific calpain, p94, and connectin/titin: their physiological functions and relationship to limb-girdle muscular dystrophy type 2A

The skeletal muscle-specific calpain homologue, p94 (also called calpain 3), is essential for normal muscle function. A mutation of the p94 gene causes limb-girdle muscular dystrophy type 2A (LGMD2A), which is one type of autosomal recessive inherited disease characterized by progressive muscular degeneration. In myofibrils, p94 specifically binds to connectin/titin, and the activity of p94 is probably suppressed by this binding. Thus, we postulate that a signal transduction pathway exists, involving p94 and connectin/titin to modulate functions of skeletal muscle, and LGMD2A occurs when this signalling pathway is not properly regulated by p94. LGMD2A mutants of p94 also reveal significant information on the factors that relate structure to function in this molecule.

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.

Proof of genetic heterogeneity in the proximal myotonic myopathy syndrome (PROMM) and its relationship to myotonic dystrophy type 2 (DM2)

Recently, myotonic dystrophy type 2 has been described as a separate disease entity that is distinctive from classical Steinert's disease since it lacks a CTG repeat expansion on chromosome 19q. A gene locus for myotonic dystrophy type 2 has been mapped to chromosome 3q. Independently, proximal myotonic myopathy has been recognized as yet another form of a multisystem myotonic disorder. Its relationship to myotonic dystrophy type 2 remains to be clarified. In our linkage study of 17 German proximal myotonic myopathy families nine of them mapped to the myotonic dystrophy type 2 locus (LOD score 18.9). However, two families with a typical proximal myotonic myopathy phenotype were excluded from this locus (LOD score -7.4). These results confirm genetic heterogeneity in the proximal myotonic myopathy syndrome. Furthermore, in the majority of the proximal myotonic myopathy families the disease phenotype may be caused by allelic mutations in the putative myotonic dystrophy type 2 gene.

Hyperparathyroidism in a patient with proximal myotonic myopathy (PROMM)

We describe a 60-year-old woman with progressive proximal leg weakness and dystrophic changes of the quadriceps and the adductor muscles on magnetic resonance imaging. She also suffered from primary hyperparathyroidism. A biopsy of the left deltoid muscle showed vacuoles as an unusual histopathological finding.

Deconstructing myotonic dystrophy

Triplet repeat diseases are disorders in which there is expansion of a repeat sequence of three nucleotides in the affected gene. Although the pathology usually results from production of a defective protein, myotonic dystrophy (DM) has proved to be a puzzle because the expanded repeats appear in a non-coding region of the affected DMPK gene. In a Perspective, Tapscott explains how findings from a new mouse model of DM (Mankodi et al.) could solve this paradox.

Recruitment of human muscleblind proteins to (CUG)(n) expansions associated with myotonic dystrophy

Myotonic dystrophy (DM1) is an autosomal dominant neuromuscular disorder associated with a (CTG)(n) expansion in the 3'-untranslated region of the DM1 protein kinase (DMPK) gene. To explain disease pathogenesis, the RNA dominance model proposes that the DM1 mutation produces a gain-of-function at the RNA level in which CUG repeats form RNA hairpins that sequester nuclear factors required for proper muscle development and maintenance. Here, we identify the triplet repeat expansion (EXP) RNA-binding proteins as candidate sequestered factors. As predicted by the RNA dominance model, binding of the EXP proteins is specific for dsCUG RNAs and proportional to the size of the triplet repeat expansion. Remarkably, the EXP proteins are homologous to the Drosophila muscleblind proteins required for terminal differentiation of muscle and photoreceptor cells. EXP expression is also activated during mammalian myoblast differentiation, but the EXP proteins accumulate in nuclear foci in DM1 cells. We propose that DM1 disease is caused by aberrant recruitment of the EXP proteins to the DMPK transcript (CUG)(n) expansion.

Triplet repeat expansion in neuromuscular disease

Expansions of unstable trinucleotide repeats cause at least 15 inherited neurologic diseases. Here we review what has been learned of three neuromuscular diseases caused by this type of mutation. X-linked spinal and bulbar muscular atrophy is a motor neuronopathy caused by a CAG repeat expansion in the androgen receptor gene. The mutated protein has an expanded polyglutamine tract, forms intranuclear aggregates, and mediates neurodegeneration through a toxic gain-of-function mechanism. Oculopharyngeal muscular dystrophy is a dominantly inherited myopathy caused by a GCG/polyalanine expansion in the gene encoding poly(A)-binding protein 2. Myotonic dystrophy is a clinically variable multisystem disease caused by a CTG expansion in the 3' untranslated region of the myotonin gene. For each of these disorders, we summarize the clinical and pathologic features and review current understanding of the molecular mechanisms underlying their pathogenesis.

Proximal myotonic myopathy: clinical, electrophysiological and pathological findings in a family

Proximal myotonic myopathy (PROMM) is an autosomal dominant muscle disorder characterized by proximal weakness, myotonia, muscle pain and cataract. It resembles Steinert myotonic dystrophy (MD), but weakness is proximal, without facial muscle involvement, and the chromosome 19 CTG trinucleotide repeat expansion characteristic of MD is not present. We describe a further family with PROMM. Affected members complained of weakness of lower limbs or of myotonia. EMG revealed diffuse myotonic discharges. Muscle histology showed dystrophic abnormalities. The PROMM phenotype varies, even in the same pedigree, and may mimic MD or limb-girdle muscle dystrophy. EMG is particularly useful, since it may disclose myotonic discharges even in the absence of overt myotonia. Thus far it is not known whether PROMM is a single entity, or if it represents a heterogeneous group of disorders. This question will probably soon be settled through genetic analysis.