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      Myotonic disorders: A review article

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          Abstract

          The myotonic disorders are a heterogeneous group of genetically determined diseases that are unified by the presence of myotonia, which is defined as failure of muscle relaxation after activation. The presentation of these disorders can range from asymptomatic electrical myotonia, as seen in some forms of myotonia congenita (MC), to severe disability with muscle weakness, cardiac conduction defects, and other systemic features as in myotonic dystrophy type I (DM1). In this review, we describe the clinical features and pathophysiology of the different myotonic disorders, their laboratory and electrophysiologic findings and briefly review the currently available treatments.

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          Molecular basis of myotonic dystrophy: expansion of a trinucleotide (CTG) repeat at the 3' end of a transcript encoding a protein kinase family member.

          J. D. Brook, M. McCurrach, H Harley … (1992)
          Using positional cloning strategies, we have identified a CTG triplet repeat that undergoes expansion in myotonic dystrophy patients. This sequence is highly variable in the normal population. PCR analysis of the interval containing this repeat indicates that unaffected individuals have been 5 and 27 copies. Myotonic dystrophy patients who are minimally affected have at least 50 repeats, while more severely affected patients have expansion of the repeat containing segment up to several kilobase pairs. The CTG repeat is transcribed and is located in the 3' untranslated region of an mRNA that is expressed in tissues affected by myotonic dystrophy. This mRNA encodes a polypeptide that is a member of the protein kinase family.
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            Myotonic dystrophy type 2 caused by a CCTG expansion in intron 1 of ZNF9.

            C Liquori (2001)
            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.
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              Myotonic dystrophy type 2: molecular, diagnostic and clinical spectrum.

              J W Day, K Ricker, J Jacobsen … (2003)
              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. Three-hundred and seventy-nine individuals from 133 DM2/PROMM families were evaluated genetically, and in 234 individuals clinical and molecular features were compared. 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). 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.
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                Author and article information

                Journal
                Journal ID (nlm-ta): Iran J Neurol
                Journal ID (iso-abbrev): Iran J Neurol
                Journal ID (publisher-id): IJNL
                Title: Iranian Journal of Neurology
                Publisher: Tehran University of Medical Sciences (Tehran, Iran )
                ISSN (Print): 2008-384X
                ISSN (Electronic): 2252-0058
                Publication date (Print): 5 January 2016
                Volume: 15
                Issue: 1
                Pages: 46-53
                Affiliations
                [1] 1 Department of Neurology, Brigham and Women’s Hospital, Harvard Medical School, Boston, USA
                Author notes
                Corresponding Author: Mohammad Kian Salajegheh msalajegheh@ 123456partners.org
                Article
                Publisher ID: IJNL-15-46
                PMC ID: 4852070
                PubMed ID: 27141276
                SO-VID: 563f89df-94c3-4906-9313-5aa72dd82172
                Copyright © Copyright © 2015 Iranian Neurological Association, and Tehran University of Medical Sciences
                License:

                This is an Open Access article distributed under the terms of the Creative Commons Attribution License, ( http://creativecommons.org/licenses/by/3.0/) which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

                History
                Date received : 25 August 2015
                Date accepted : 8 November 2015
                Categories
                Subject: Review Article

                Keywords: myotonia,myotonic dystrophy,myotonia congenita,paralysis periodica paramyotonia,hyperkalemic periodic paralysis
                Data availability:
                Keywords: myotonia, myotonic dystrophy, myotonia congenita, paralysis periodica paramyotonia, hyperkalemic periodic paralysis

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