Adult-Onset PLEC-Related Congenital Myasthenic Syndrome-Myopathy Overlap with Upper Limb Predominant Weakness

Adult-Onset PLEC-Related Congenital Myasthenic Syndrome-Myopathy Overlap with Upper Limb Predominant Weakness | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Short Report Adult-Onset PLEC-Related Congenital Myasthenic Syndrome-Myopathy Overlap with Upper Limb Predominant Weakness Angel Jose, Aina Azily, Katie Doyle, Karen Stals, Hayley Lees, and 4 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-8602044/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 18 Apr, 2026 Read the published version in neurogenetics → Version 1 posted 13 You are reading this latest preprint version Abstract Congenital myasthenic syndromes (CMS) are genetically heterogeneous disorders in which precise molecular diagnosis is essential for management. Pathogenic variants in PLEC are a rare cause of CMS and are frequently associated with epidermolysis bullosa simplex. We report an adult-onset case with upper limb–predominant weakness, combined myopathic and myasthenic features, and cardiomyopathy. Standard CMS gene panel testing and exome sequencing were initially non-diagnostic. Trio exome sequencing identified a second deep intronic PLEC variant, and RNA analysis demonstrated intron retention with frameshift, enabling variant reclassification and definitive diagnosis. This case expands the phenotypic spectrum of PLEC-related disease and highlights the diagnostic value of transcript-level analysis. PLEC Congenital myasthenic syndrome Deep intronic variant RNA analysis Genetic diagnosis Figures Figure 1 Figure 2 Introduction Congenital myasthenic syndromes (CMS) are a heterogeneous group of inherited disorders of neuromuscular transmission caused by pathogenic variants in more than 40 genes.( 1 ) Establishing a molecular diagnosis is critical, as prognosis and treatment response vary significantly by genetic subtype. While next-generation sequencing has improved diagnostic yield, pathogenic variants may remain undetected due to technical limitations, particularly when variants lie outside coding regions or require functional validation. Pathogenic variants in PLEC represent a rare cause of CMS and are frequently associated with epidermolysis bullosa simplex (EBS), reflecting the essential role of plectin in maintaining cytoskeletal integrity at the neuromuscular junction (NMJ) and in mechanically stressed tissues.( 2 ) Reported cases of PLEC -related CMS typically present in infancy or early childhood with dermatological manifestations followed by progressive neuromuscular involvement.( 3 – 5 ) Here, we report an unusual adult-onset case of PLEC -related CMS with upper limb–predominant weakness, cardiomyopathy and non-diagnostic initial genetic testing. A definitive diagnosis was achieved only through RNA analysis of a deep intronic variant, underscoring the importance of phenotype-driven investigation and functional studies in resolving cryptic Mendelian disease. Case Presentation The patient first presented at the age of 26 with a 3-year history of progressive upper limb weakness, predominantly affecting shoulder abduction, elbow flexion and finger and wrist extension, with no ocular, bulbar or respiratory symptoms. The patient had a diagnosis of EBS from birth, but normal developmental and motor milestones. There was no family history of neuromuscular problems and no consanguinity. On examination there was mild fatiguable ptosis, bilateral facial weakness and proximal predominant upper limb weakness with reduced upper limb deep tendon reflexes (Fig. 1 ). Lower limb strength, sensation and axial strength were all normal. Over the following 6 years, he developed mild ophthalmoplegia and progressive facial and upper limb weakness, while lower limb strength remained entirely normal. Nerve conduction studies showed markedly reduced median and ulnar compound muscular action potential (CMAP) amplitudes motor responses and moderately reduced peroneal responses with preserved velocities and normal sensory studies. Electromyography demonstrated spontaneous activity and small amplitude, long duration polyphasic motor units with early recruitment in bilateral biceps brachii, right vastus lateralis and right tibialis anterior muscles. Repetitive nerve stimulation (RNS) was carried out at 3Hz in abductor pollicis brevis only, due to poor patient tolerance. This revealed a borderline 10–11% decrement, suggestive of an NMJ defect. There was no post-exercise facilitation. Creatinine kinase was elevated at 1757 U/L. MRI of the brain and spine were normal. Cardiac investigations identified asymptomatic dilated cardiomyopathy with mild LV impairment (Fig. 1 c). Given the combination of myopathy, NMJ dysfunction and epidermolysis bullosa simplex, a Plectinopathy was considered. Targeted CMS gene panel testing was performed and identified a single heterozygous canonical splice-site variant in PLEC (c.3162 + 1G > A), initially classified as a variant of uncertain significance (VUS). Chromosomal microarray analysis was normal. Subsequent trio exome sequencing confirmed paternal inheritance of this variant and identified a second, maternally-inherited deep intronic PLEC variant (c.800-20T > G). Despite its rarity in population databases, this variant was initially classified as a VUS in the absence of functional evidence. Owing to strong clinical suspicion, RNA analysis from PAXgene samples demonstrated intron 8 retention resulting in a frameshift (Fig. 2 ). This functional evidence enabled reclassification of both variants as pathogenic under ACMG/ACGS variant interpretation criteria, establishing a definitive molecular diagnosis. Treatment with pyridostigmine was ineffective, while salbutamol was associated with gradual clinical improvement. Discussion Plectin is a cytolinker protein essential for maintaining structural integrity in tissues exposed to mechanical stress, including skeletal and cardiac muscle, skin, and the NMJ.( 6 ) Consistent with this multifunctional role, pathogenic variants in PLEC give rise to a spectrum of overlapping phenotypes encompassing EBS, myopathy, cardiomyopathy and CMS.( 2 ) In contrast, the present case demonstrated adult-onset myopathic and myasthenic features with a striking upper limb–predominant pattern of weakness, slow disease progression and complete sparing of the lower limbs despite prolonged follow-up. Interestingly, one recent case report described adult-onset myopathy with distal upper limb weakness due to PLEC mutations, although this occurred in the absence of NMJ dysfunction.( 7 ) The phenotype observed in our patient therefore appears distinct and broadens the recognised spectrum of PLEC -related CMS. This case illustrates that PLEC -related disease cannot be readily categorised as purely myopathic or purely synaptic, but rather represents a disorder in which both components contribute to clinical weakness. Several features indicate a primary defect of neuromuscular transmission in this patient. These include fatiguable weakness and a reproducible decrement on RNS, and improvement with salbutamol. Importantly, prior ultrastructural and electrophysiological studies in PLEC -related CMS demonstrated reduced miniature endplate potentials and progressive destruction of postsynaptic junctional folds.( 8 ) The observed clinical response to salbutamol in our case, further supports a postsynaptic NMJ contribution to disease pathogenesis. While improvement was gradual and incomplete, the preferential response to a β₂-adrenergic agonist mirrors therapeutic responses seen in several CMS subtypes characterised by postsynaptic structural instability. Notably, we and others have previously shown that salbutamol partially rescues postsynaptic architecture, including restoration of junctional folds.( 9 ) The response in this patient suggests that β₂-agonists may partially compensate for plectin deficiency at the NMJ by enhancing postsynaptic stability or remodelling, thereby improving neuromuscular transmission despite an underlying structural myopathy. In addition, this patient exhibited persistent hyperCKaemia, myopathic EMG changes, and cardiomyopathy, supporting a concomitant structural muscle disease. Cardiac involvement is not a typical feature of most CMS subtypes but has been reported in plectinopathies, particularly in association with EBS–muscular dystrophy overlap syndromes.( 3 ) The identification of asymptomatic cardiomyopathy in this patient underscores the importance of systematic cardiac evaluation in individuals with suspected PLEC -related disease. Importantly, this cardiac phenotype may remain clinically silent without targeted investigation and genetic clarification. This case also highlights the diagnostic limitations of standard genetic testing approaches. Initial CMS panel testing and exome sequencing identified only a single pathogenic PLEC variant, delaying molecular diagnosis despite strong clinical suspicion. The second pathogenic allele was a deep intronic variant that escaped detection by conventional analyses and required RNA studies to demonstrate aberrant splicing. Functional evidence of intron retention and frameshift was essential for variant reclassification, emphasising the continued importance of transcript-level analysis in resolving cryptic Mendelian disease. Without a phenotype-driven approach and targeted functional studies, this patient would likely have remained genetically unresolved. Establishing a precise molecular diagnosis in CMS has direct therapeutic implications. While acetylcholinesterase inhibitors are effective in many CMS subtypes, treatment response varies and may be limited or adverse in specific genetic contexts.( 10 ) In this case, pyridostigmine was ineffective, whereas salbutamol was associated with gradual clinical improvement, consistent with previous reports in selected CMS subtypes. These observations further reinforce the importance of accurate genetic classification to guide management and surveillance. In summary, this case expands the phenotypic spectrum of PLEC-related CMS to include adult-onset, upper limb–predominant weakness with cardiomyopathy and demonstrates the diagnostic value of RNA analysis in uncovering pathogenic deep intronic variants. It underscores the need for sustained clinical suspicion and functional investigation when genetic findings do not initially align with a compelling phenotype. Declarations Written informed consent was obtained from the patient for publication of this case report and any accompanying clinical information and images. The authors declare no competing interests. Acknowledgements and Funding GMcM receives support from the Neuromuscular Study Group, the Irish Institute of Clinical Neurosciences Brain Research Fellowship, Muscular Dystrophy UK (25GRO-PG12-0865-1) and the Academy of Medical Sciences (SGCL033\1116). JMcC is supported in his academic work by a generous donation by Mr K McNeany. Author Contribution A.J. and A.A. led data collection and drafted the manuscript. K.S. and H.L. performed the genetic analyses. C.McK. led the genetic investigations and contributed to interpretation of the genetic results alongside K.S. and H.L. J.McC. performed the clinical assessments and contributed to clinical interpretation. A.J.McK. provided critical review and revision of the manuscript. G.M. contributed to study conception, interpretation of neuromuscular junction mechanisms, and critical revision of the manuscript for intellectual content. All authors reviewed and approved the final manuscript. References Ohno K, Ito M, Ohkawara B (2025 June) Review of 40 genes causing congenital myasthenic syndromes. J Hum Genet 18:1–10 Winter L, Wiche G (2013) The many faces of plectin and plectinopathies: pathology and mechanisms. Acta Neuropathol (Berl) 125(1):77–93 Mroczek M, Durmus H, Töpf A, Parman Y, Straub V Four Individuals with a Homozygous Mutation in Exon 1f of the PLEC Gene and Associated Myasthenic Features. Genes 2020 June 27;11(7):716 Forrest K, Mellerio JE, Robb S, Dopping-Hepenstal PJC, McGrath JA, Liu L et al (2010) Congenital muscular dystrophy, myasthenic symptoms and epidermolysis bullosa simplex (EBS) associated with mutations in the PLEC1 gene encoding plectin. Neuromuscul Disord NMD 20(11):709–711 Maselli RA, Arredondo J, Cagney O, Mozaffar T, Skinner S, Yousif S et al (2011) Congenital myasthenic syndrome associated with epidermolysis bullosa caused by homozygous mutations in PLEC1 and CHRNE. Clin Genet 80(5):444–451 Castañón MJ, Walko G, Winter L, Wiche G (2013 July) Plectin-intermediate filament partnership in skin, skeletal muscle, and peripheral nerve. Histochem Cell Biol 140(1):33–53 Llansó L, Reyes-Leiva D, Segarra-Casas A, Xuclà-Ferrarons T, Gallardo E, Blanco R et al (2025) Adult-onset distal myopathy with predominant hand involvement as a rare phenotype of plectinopathy. J Neuromuscul Dis 12(6):865–869 Selcen D, Juel VC, Hobson-Webb LD, Smith EC, Stickler DE, Bite AV et al (2011) Myasthenic syndrome caused by plectinopathy. Neurology 76(4):327–336 McMacken GM, Spendiff S, Whittaker RG, O’Connor E, Howarth RM, Boczonadi V et al Salbutamol modifies the neuromuscular junction in a mouse model of ColQ myasthenic syndrome. Hum Mol Genet 2019 July 15;28(14):2339–2351 McMacken G, Abicht A, Evangelista T, Spendiff S, Lochmüller H (2017) The Increasing Genetic and Phenotypical Diversity of Congenital Myasthenic Syndromes. Neuropediatrics 48(4):294–308 Additional Declarations No competing interests reported. Cite Share Download PDF Status: Published Journal Publication published 18 Apr, 2026 Read the published version in neurogenetics → Version 1 posted Editorial decision: Revision requested 09 Feb, 2026 Reviews received at journal 25 Jan, 2026 Reviewers agreed at journal 24 Jan, 2026 Reviews received at journal 23 Jan, 2026 Reviewers agreed at journal 23 Jan, 2026 Reviews received at journal 23 Jan, 2026 Reviewers agreed at journal 22 Jan, 2026 Reviewers agreed at journal 22 Jan, 2026 Reviewers agreed at journal 22 Jan, 2026 Reviewers invited by journal 22 Jan, 2026 Editor assigned by journal 22 Jan, 2026 Submission checks completed at journal 22 Jan, 2026 First submitted to journal 14 Jan, 2026 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-8602044","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Short Report","associatedPublications":[],"authors":[{"id":580647591,"identity":"6c0688cb-2b09-46c0-99a3-9625ead3157b","order_by":0,"name":"Angel Jose","email":"","orcid":"","institution":"Queen’s University Belfast","correspondingAuthor":false,"prefix":"","firstName":"Angel","middleName":"","lastName":"Jose","suffix":""},{"id":580647592,"identity":"5a9ad9e9-d4a3-4bbe-9866-fe8517cf320a","order_by":1,"name":"Aina Azily","email":"","orcid":"","institution":"Queen’s University Belfast","correspondingAuthor":false,"prefix":"","firstName":"Aina","middleName":"","lastName":"Azily","suffix":""},{"id":580647593,"identity":"61a62483-3d03-4962-82e0-17b25e287258","order_by":2,"name":"Katie Doyle","email":"","orcid":"","institution":"Ulster Hospital, South Eastern Health and Social Care Trust","correspondingAuthor":false,"prefix":"","firstName":"Katie","middleName":"","lastName":"Doyle","suffix":""},{"id":580647594,"identity":"004a5fd7-a344-4f81-9286-dfe8e0e4e58b","order_by":3,"name":"Karen Stals","email":"","orcid":"","institution":"Royal Devon University Healthcare NHS Foundation Trust","correspondingAuthor":false,"prefix":"","firstName":"Karen","middleName":"","lastName":"Stals","suffix":""},{"id":580647595,"identity":"af211a5b-02f1-41c4-b61f-c85941dbfd37","order_by":4,"name":"Hayley Lees","email":"","orcid":"","institution":"Royal Devon University Healthcare NHS Foundation Trust","correspondingAuthor":false,"prefix":"","firstName":"Hayley","middleName":"","lastName":"Lees","suffix":""},{"id":580647596,"identity":"19570a89-b881-4ad4-925e-4cd8ad72a2d5","order_by":5,"name":"Caoimhe McKenna","email":"","orcid":"","institution":"Belfast City Hospital, Belfast Health and Social Care Trust","correspondingAuthor":false,"prefix":"","firstName":"Caoimhe","middleName":"","lastName":"McKenna","suffix":""},{"id":580647597,"identity":"ee99fe25-1eec-4f5d-8c9b-fd829a8bb06b","order_by":6,"name":"Amy Jayne McKnight","email":"","orcid":"","institution":"Queen’s University Belfast","correspondingAuthor":false,"prefix":"","firstName":"Amy","middleName":"Jayne","lastName":"McKnight","suffix":""},{"id":580647598,"identity":"05723ac4-9bb6-42ed-94f4-6689cd6d0c25","order_by":7,"name":"John McConville","email":"","orcid":"","institution":"Ulster Hospital, South Eastern Health and Social Care Trust","correspondingAuthor":false,"prefix":"","firstName":"John","middleName":"","lastName":"McConville","suffix":""},{"id":580647599,"identity":"cafeccfa-826e-44d3-af0d-a71687076e7d","order_by":8,"name":"Grace McMacken","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA3ElEQVRIiWNgGAWjYBADAwYG5gMMjA1gDjOxWtgSSNbCY0CcFnkH7sRPN2rsjPmlez7e5t3BIM/fwGNsgE+L4QHezdI5x5LNJOec3WzNe4bBcMYBHuMEvFoaeLcx57AdsDG4kbtNmreNgXEDA4/xAcJa/h2wsb+R8wykxZ6gFnkGoJbctgNmBhI5bCAtiSAteB1mwAz0S25fsrHEjTRjy7lnJJJnHGYrxut9+fbejZ9zvtkZ9s9Ifnjj7Q4b2/725s0SeG05jMSRACNCESnfgKplFIyCUTAKRgEmAACNSEE5T+846gAAAABJRU5ErkJggg==","orcid":"","institution":"Queen’s University Belfast","correspondingAuthor":true,"prefix":"","firstName":"Grace","middleName":"","lastName":"McMacken","suffix":""}],"badges":[],"createdAt":"2026-01-14 13:08:14","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-8602044/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-8602044/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1007/s10048-026-00900-8","type":"published","date":"2026-04-18T15:58:04+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":101298018,"identity":"0d5596f0-14f7-481e-8d07-05e8a489fddc","added_by":"auto","created_at":"2026-01-28 09:29:45","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":2101450,"visible":true,"origin":"","legend":"\u003cp\u003eClinical features of \u003cem\u003ePLEC \u003c/em\u003eCMS. (a) Bilateral ptosis (fatiguable); (b, c) Upper limb weakness, wasting of intrinsic hand muscles, and finger and wrist drop (also fatiguable) mimicking slow channel CMS; (d) Late gadolinium enhancement cardiac magnetic resonance imaging demonstrating a non-ischaemic pattern of myocardial fibrosis with ring-like enhancement at basal and mid-ventricular levels.\u003c/p\u003e","description":"","filename":"floatimage149.png","url":"https://assets-eu.researchsquare.com/files/rs-8602044/v1/a1b68e3ad9af799b01d6e8d9.png"},{"id":101273716,"identity":"28d6f30e-1594-4733-a850-fa02de402ab6","added_by":"auto","created_at":"2026-01-28 03:08:17","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":71872,"visible":true,"origin":"","legend":"\u003cp\u003eDeep intronic \u003cem\u003ePLEC\u003c/em\u003e variant causes aberrant splicing. Schematic representation of \u003cem\u003ePLEC \u003c/em\u003eexons 7–9 showing the location of the c.800-20T\u0026gt;G variant within intron 8. Normal splicing generates an in-frame transcript, whereas the c.800-20T\u0026gt;G variant results in intron 8 retention, causing a frameshift and predicted premature termination.\u003c/p\u003e","description":"","filename":"floatimage2.png","url":"https://assets-eu.researchsquare.com/files/rs-8602044/v1/e76f96e4ff9b09ef2917dfef.png"},{"id":107352867,"identity":"86fb7a29-f438-4eaf-b958-4ae5bcfebbe4","added_by":"auto","created_at":"2026-04-20 16:18:12","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":3346277,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8602044/v1/6d522ee2-a0a3-4db0-9ce0-a09194618210.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Adult-Onset PLEC-Related Congenital Myasthenic Syndrome-Myopathy Overlap with Upper Limb Predominant Weakness","fulltext":[{"header":"Introduction","content":"\u003cp\u003eCongenital myasthenic syndromes (CMS) are a heterogeneous group of inherited disorders of neuromuscular transmission caused by pathogenic variants in more than 40 genes.(\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e) Establishing a molecular diagnosis is critical, as prognosis and treatment response vary significantly by genetic subtype. While next-generation sequencing has improved diagnostic yield, pathogenic variants may remain undetected due to technical limitations, particularly when variants lie outside coding regions or require functional validation.\u003c/p\u003e \u003cp\u003ePathogenic variants in \u003cem\u003ePLEC\u003c/em\u003e represent a rare cause of CMS and are frequently associated with epidermolysis bullosa simplex (EBS), reflecting the essential role of plectin in maintaining cytoskeletal integrity at the neuromuscular junction (NMJ) and in mechanically stressed tissues.(\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e) Reported cases of \u003cem\u003ePLEC\u003c/em\u003e-related CMS typically present in infancy or early childhood with dermatological manifestations followed by progressive neuromuscular involvement.(\u003cspan additionalcitationids=\"CR4\" citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e)\u003c/p\u003e \u003cp\u003eHere, we report an unusual adult-onset case of \u003cem\u003ePLEC\u003c/em\u003e-related CMS with upper limb\u0026ndash;predominant weakness, cardiomyopathy and non-diagnostic initial genetic testing. A definitive diagnosis was achieved only through RNA analysis of a deep intronic variant, underscoring the importance of phenotype-driven investigation and functional studies in resolving cryptic Mendelian disease.\u003c/p\u003e"},{"header":"Case Presentation","content":"\u003cp\u003eThe patient first presented at the age of 26 with a 3-year history of progressive upper limb weakness, predominantly affecting shoulder abduction, elbow flexion and finger and wrist extension, with no ocular, bulbar or respiratory symptoms. The patient had a diagnosis of EBS from birth, but normal developmental and motor milestones. There was no family history of neuromuscular problems and no consanguinity.\u003c/p\u003e \u003cp\u003eOn examination there was mild fatiguable ptosis, bilateral facial weakness and proximal predominant upper limb weakness with reduced upper limb deep tendon reflexes (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). Lower limb strength, sensation and axial strength were all normal. Over the following 6 years, he developed mild ophthalmoplegia and progressive facial and upper limb weakness, while lower limb strength remained entirely normal.\u003c/p\u003e \u003cp\u003eNerve conduction studies showed markedly reduced median and ulnar compound muscular action potential (CMAP) amplitudes motor responses and moderately reduced peroneal responses with preserved velocities and normal sensory studies. Electromyography demonstrated spontaneous activity and small amplitude, long duration polyphasic motor units with early recruitment in bilateral biceps brachii, right vastus lateralis and right tibialis anterior muscles. Repetitive nerve stimulation (RNS) was carried out at 3Hz in abductor pollicis brevis only, due to poor patient tolerance. This revealed a borderline 10\u0026ndash;11% decrement, suggestive of an NMJ defect. There was no post-exercise facilitation. Creatinine kinase was elevated at 1757 U/L. MRI of the brain and spine were normal. Cardiac investigations identified asymptomatic dilated cardiomyopathy with mild LV impairment (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ec).\u003c/p\u003e \u003cp\u003eGiven the combination of myopathy, NMJ dysfunction and epidermolysis bullosa simplex, a Plectinopathy was considered. Targeted CMS gene panel testing was performed and identified a single heterozygous canonical splice-site variant in \u003cem\u003ePLEC\u003c/em\u003e (c.3162\u0026thinsp;+\u0026thinsp;1G\u0026thinsp;\u0026gt;\u0026thinsp;A), initially classified as a variant of uncertain significance (VUS). Chromosomal microarray analysis was normal. Subsequent trio exome sequencing confirmed paternal inheritance of this variant and identified a second, maternally-inherited deep intronic \u003cem\u003ePLEC\u003c/em\u003e variant (c.800-20T\u0026thinsp;\u0026gt;\u0026thinsp;G). Despite its rarity in population databases, this variant was initially classified as a VUS in the absence of functional evidence. Owing to strong clinical suspicion, RNA analysis from PAXgene samples demonstrated intron 8 retention resulting in a frameshift (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). This functional evidence enabled reclassification of both variants as pathogenic under ACMG/ACGS variant interpretation criteria, establishing a definitive molecular diagnosis. Treatment with pyridostigmine was ineffective, while salbutamol was associated with gradual clinical improvement.\u003c/p\u003e"},{"header":"Discussion","content":"\u003cp\u003ePlectin is a cytolinker protein essential for maintaining structural integrity in tissues exposed to mechanical stress, including skeletal and cardiac muscle, skin, and the NMJ.(\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e) Consistent with this multifunctional role, pathogenic variants in \u003cem\u003ePLEC\u003c/em\u003e give rise to a spectrum of overlapping phenotypes encompassing EBS, myopathy, cardiomyopathy and CMS.(\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e) In contrast, the present case demonstrated adult-onset myopathic and myasthenic features with a striking upper limb\u0026ndash;predominant pattern of weakness, slow disease progression and complete sparing of the lower limbs despite prolonged follow-up. Interestingly, one recent case report described adult-onset myopathy with distal upper limb weakness due to \u003cem\u003ePLEC\u003c/em\u003e mutations, although this occurred in the absence of NMJ dysfunction.(\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e) The phenotype observed in our patient therefore appears distinct and broadens the recognised spectrum of \u003cem\u003ePLEC\u003c/em\u003e-related CMS. This case illustrates that \u003cem\u003ePLEC\u003c/em\u003e-related disease cannot be readily categorised as purely myopathic or purely synaptic, but rather represents a disorder in which both components contribute to clinical weakness.\u003c/p\u003e \u003cp\u003eSeveral features indicate a primary defect of neuromuscular transmission in this patient. These include fatiguable weakness and a reproducible decrement on RNS, and improvement with salbutamol. Importantly, prior ultrastructural and electrophysiological studies in \u003cem\u003ePLEC\u003c/em\u003e-related CMS demonstrated reduced miniature endplate potentials and progressive destruction of postsynaptic junctional folds.(\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e) The observed clinical response to salbutamol in our case, further supports a postsynaptic NMJ contribution to disease pathogenesis. While improvement was gradual and incomplete, the preferential response to a β₂-adrenergic agonist mirrors therapeutic responses seen in several CMS subtypes characterised by postsynaptic structural instability. Notably, we and others have previously shown that salbutamol partially rescues postsynaptic architecture, including restoration of junctional folds.(\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e) The response in this patient suggests that β₂-agonists may partially compensate for plectin deficiency at the NMJ by enhancing postsynaptic stability or remodelling, thereby improving neuromuscular transmission despite an underlying structural myopathy.\u003c/p\u003e \u003cp\u003eIn addition, this patient exhibited persistent hyperCKaemia, myopathic EMG changes, and cardiomyopathy, supporting a concomitant structural muscle disease. Cardiac involvement is not a typical feature of most CMS subtypes but has been reported in plectinopathies, particularly in association with EBS\u0026ndash;muscular dystrophy overlap syndromes.(\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e) The identification of asymptomatic cardiomyopathy in this patient underscores the importance of systematic cardiac evaluation in individuals with suspected \u003cem\u003ePLEC\u003c/em\u003e-related disease. Importantly, this cardiac phenotype may remain clinically silent without targeted investigation and genetic clarification.\u003c/p\u003e \u003cp\u003eThis case also highlights the diagnostic limitations of standard genetic testing approaches. Initial CMS panel testing and exome sequencing identified only a single pathogenic \u003cem\u003ePLEC\u003c/em\u003e variant, delaying molecular diagnosis despite strong clinical suspicion. The second pathogenic allele was a deep intronic variant that escaped detection by conventional analyses and required RNA studies to demonstrate aberrant splicing. Functional evidence of intron retention and frameshift was essential for variant reclassification, emphasising the continued importance of transcript-level analysis in resolving cryptic Mendelian disease. Without a phenotype-driven approach and targeted functional studies, this patient would likely have remained genetically unresolved.\u003c/p\u003e \u003cp\u003eEstablishing a precise molecular diagnosis in CMS has direct therapeutic implications. While acetylcholinesterase inhibitors are effective in many CMS subtypes, treatment response varies and may be limited or adverse in specific genetic contexts.(\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e) In this case, pyridostigmine was ineffective, whereas salbutamol was associated with gradual clinical improvement, consistent with previous reports in selected CMS subtypes. These observations further reinforce the importance of accurate genetic classification to guide management and surveillance.\u003c/p\u003e \u003cp\u003eIn summary, this case expands the phenotypic spectrum of PLEC-related CMS to include adult-onset, upper limb\u0026ndash;predominant weakness with cardiomyopathy and demonstrates the diagnostic value of RNA analysis in uncovering pathogenic deep intronic variants. It underscores the need for sustained clinical suspicion and functional investigation when genetic findings do not initially align with a compelling phenotype.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003eWritten informed consent was obtained from the patient for publication of this case report and any accompanying clinical information and images.\u003c/p\u003e\n\u003cp\u003eThe authors declare no competing interests.\u003c/p\u003e\u003ch2\u003eAcknowledgements and Funding\u003c/h2\u003e \u003cp\u003eGMcM receives support from the Neuromuscular Study Group, the Irish Institute of Clinical Neurosciences Brain Research Fellowship, Muscular Dystrophy UK (25GRO-PG12-0865-1) and the Academy of Medical Sciences (SGCL033\\1116). JMcC is supported in his academic work by a generous donation by Mr K McNeany.\u003c/p\u003e\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eA.J. and A.A. led data collection and drafted the manuscript. K.S. and H.L. performed the genetic analyses. C.McK. led the genetic investigations and contributed to interpretation of the genetic results alongside K.S. and H.L. J.McC. performed the clinical assessments and contributed to clinical interpretation. A.J.McK. provided critical review and revision of the manuscript. G.M. contributed to study conception, interpretation of neuromuscular junction mechanisms, and critical revision of the manuscript for intellectual content. All authors reviewed and approved the final manuscript.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eOhno K, Ito M, Ohkawara B (2025 June) Review of 40 genes causing congenital myasthenic syndromes. J Hum Genet 18:1\u0026ndash;10\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eWinter L, Wiche G (2013) The many faces of plectin and plectinopathies: pathology and mechanisms. Acta Neuropathol (Berl) 125(1):77\u0026ndash;93\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eMroczek M, Durmus H, T\u0026ouml;pf A, Parman Y, Straub V Four Individuals with a Homozygous Mutation in Exon 1f of the PLEC Gene and Associated Myasthenic Features. Genes 2020 June 27;11(7):716\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eForrest K, Mellerio JE, Robb S, Dopping-Hepenstal PJC, McGrath JA, Liu L et al (2010) Congenital muscular dystrophy, myasthenic symptoms and epidermolysis bullosa simplex (EBS) associated with mutations in the PLEC1 gene encoding plectin. Neuromuscul Disord NMD 20(11):709\u0026ndash;711\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eMaselli RA, Arredondo J, Cagney O, Mozaffar T, Skinner S, Yousif S et al (2011) Congenital myasthenic syndrome associated with epidermolysis bullosa caused by homozygous mutations in PLEC1 and CHRNE. Clin Genet 80(5):444\u0026ndash;451\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eCasta\u0026ntilde;\u0026oacute;n MJ, Walko G, Winter L, Wiche G (2013 July) Plectin-intermediate filament partnership in skin, skeletal muscle, and peripheral nerve. Histochem Cell Biol 140(1):33\u0026ndash;53\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eLlans\u0026oacute; L, Reyes-Leiva D, Segarra-Casas A, Xucl\u0026agrave;-Ferrarons T, Gallardo E, Blanco R et al (2025) Adult-onset distal myopathy with predominant hand involvement as a rare phenotype of plectinopathy. J Neuromuscul Dis 12(6):865\u0026ndash;869\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eSelcen D, Juel VC, Hobson-Webb LD, Smith EC, Stickler DE, Bite AV et al (2011) Myasthenic syndrome caused by plectinopathy. Neurology 76(4):327\u0026ndash;336\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eMcMacken GM, Spendiff S, Whittaker RG, O\u0026rsquo;Connor E, Howarth RM, Boczonadi V et al Salbutamol modifies the neuromuscular junction in a mouse model of ColQ myasthenic syndrome. Hum Mol Genet 2019 July 15;28(14):2339\u0026ndash;2351\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eMcMacken G, Abicht A, Evangelista T, Spendiff S, Lochm\u0026uuml;ller H (2017) The Increasing Genetic and Phenotypical Diversity of Congenital Myasthenic Syndromes. Neuropediatrics 48(4):294\u0026ndash;308\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"neurogenetics","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"nege","sideBox":"Learn more about [neurogenetics](https://link.springer.com/journal/10048)","snPcode":"10048","submissionUrl":"https://submission.nature.com/new-submission/10048/3","title":"neurogenetics","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"PLEC, Congenital myasthenic syndrome, Deep intronic variant, RNA analysis, Genetic diagnosis","lastPublishedDoi":"10.21203/rs.3.rs-8602044/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-8602044/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eCongenital myasthenic syndromes (CMS) are genetically heterogeneous disorders in which precise molecular diagnosis is essential for management. Pathogenic variants in PLEC are a rare cause of CMS and are frequently associated with epidermolysis bullosa simplex. We report an adult-onset case with upper limb\u0026ndash;predominant weakness, combined myopathic and myasthenic features, and cardiomyopathy. Standard CMS gene panel testing and exome sequencing were initially non-diagnostic. Trio exome sequencing identified a second deep intronic PLEC variant, and RNA analysis demonstrated intron retention with frameshift, enabling variant reclassification and definitive diagnosis. This case expands the phenotypic spectrum of PLEC-related disease and highlights the diagnostic value of transcript-level analysis.\u003c/p\u003e","manuscriptTitle":"Adult-Onset PLEC-Related Congenital Myasthenic Syndrome-Myopathy Overlap with Upper Limb Predominant Weakness","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-01-28 03:08:03","doi":"10.21203/rs.3.rs-8602044/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2026-02-09T07:14:05+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-01-26T03:41:59+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"99453819990166351409093325444202241229","date":"2026-01-25T00:09:40+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-01-23T16:27:06+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"308563184412073231001695665005902919998","date":"2026-01-23T15:19:45+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-01-23T14:04:09+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"26264494089547269233093201707322276786","date":"2026-01-22T16:37:17+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"213801881017108826483243868974073719531","date":"2026-01-22T12:57:35+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"307277146530911071138038253416256779837","date":"2026-01-22T12:30:53+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2026-01-22T12:10:38+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2026-01-22T09:32:20+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2026-01-22T09:29:32+00:00","index":"","fulltext":""},{"type":"submitted","content":"neurogenetics","date":"2026-01-14T12:53:41+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"neurogenetics","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"nege","sideBox":"Learn more about [neurogenetics](https://link.springer.com/journal/10048)","snPcode":"10048","submissionUrl":"https://submission.nature.com/new-submission/10048/3","title":"neurogenetics","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"266bab55-1b1b-4f91-a302-5e49c1eb98b7","owner":[],"postedDate":"January 28th, 2026","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[],"tags":[],"updatedAt":"2026-04-20T16:18:06+00:00","versionOfRecord":{"articleIdentity":"rs-8602044","link":"https://doi.org/10.1007/s10048-026-00900-8","journal":{"identity":"neurogenetics","isVorOnly":false,"title":"neurogenetics"},"publishedOn":"2026-04-18 15:58:04","publishedOnDateReadable":"April 18th, 2026"},"versionCreatedAt":"2026-01-28 03:08:03","video":"","vorDoi":"10.1007/s10048-026-00900-8","vorDoiUrl":"https://doi.org/10.1007/s10048-026-00900-8","workflowStages":[]},"version":"v1","identity":"rs-8602044","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8602044","identity":"rs-8602044","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}