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

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.

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.

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.

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.

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.

Epidemiology of myotonic dystrophy in Italy: re-apprisal after genetic diagnosis

Before the discovery of the myotonic dystrophy (DM) gene, the DM epidemiological rates could not be accurately estimated. The aim of this study was to calculate the DM prevalence rates in Padova (North-East Italy) and in four provinces of North-West Tuscany (Central Italy) and, as of 30 June 1999, to do so using molecular genetic testing. A minimum prevalence rate of 9.31x10(-5) inhabitants was found, consistent with epidemiological rates worldwide, and more than two times as high as those of two previous studies conducted in the same areas during the era prior to molecular genetic testing. This study, the first in Italy since the discovery of the DM gene, underlines the importance of direct genetic diagnosis of DM, especially in detecting mildly affected patients, a fundamental step in correctly estimating the risk of disease transmission in affected families.

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.