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Congenital myasthenic syndrome with novel pathogenic variants in the COLQ gene associated with the presence of antibodies to acetylcholine receptors
Journal of Clinical Neuroscience xxx (xxxx) xxx
Contents lists available at ScienceDirect
Journal of Clinical Neuroscience journal homepage: www.elsevier.com/locate/jocn
Case report
Congenital myasthenic syndrome with novel pathogenic variants in the COLQ gene associated with the presence of antibodies to acetylcholine receptors Chee Geap Tay a, Choong Yi Fong a, Limin Li a, Vigneswari Ganesan b, Chee Ming Teh b, Chin Seng Gan c, Meow-Keong Thong d,⇑ a
Division of Paediatric Neurology, Department of Paediatrics, University of Malaya Medical Centre, Kuala Lumpur, Malaysia Department of Paediatrics, Penang General Hospital, Penang, Malaysia c Division of Paediatric Intensive Care, Department of Paediatrics, University of Malaya Medical Centre, Kuala Lumpur, Malaysia d Genetic Medicine Unit, Department of Paediatrics, University of Malaya Medical Centre, Kuala Lumpur, Malaysia b
a r t i c l e
i n f o
Article history: Received 25 August 2019 Accepted 1 December 2019 Available online xxxx Keywords: Congenital myasthenic syndrome COLQ gene Autoimmune myasthenia gravis Anti-acetylcholine receptor antibody AChR
a b s t r a c t Congenital myasthenic syndrome (CMS) is a heterogeneous group of inherited disorder which does not associate with anti-acetylcholine receptor (AChR) antibody. The presence of AChR autoantibody is pathogenic and highly sensitive and specific for autoimmune myasthenia gravis (MG). We describe 2 children from unrelated families who presented with hypotonia, ptosis and fatigability in early infancy with antiAChR antibodies detected via ELISA on 2 separate occasions in the sera. Both were treated as refractory autoimmune MG due to poor clinical response to acetylcholinesterase inhibitor and immunotherapy. In view of the atypical clinical features, genetic studies of CMS were performed and both were confirmed to have novel pathogenic mutations in the COLQ gene. To the best of our knowledge, the presence of antiAChR antibody in COLQ-related CMS has never been reported in the literature. The clinical presentation of early onset phenotype, and refractoriness to acetylcholinesterase inhibitor and immunotherapy should prompt CMS as a differential diagnosis. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction Congenital myasthenic syndrome (CMS) is a heterogeneous group of inherited diseases characterised by impaired neuromuscular transmission at the neuromuscular junction. Depending on the location of the primary defect, CMS are classified into presynaptic, synaptic, postsynaptic or glycosylation pathway defect. Most CMS are inherited recessively; this implies a positive family history may not be present in the diagnostic evaluation. Hence, the diagnosis is typically considered on the clinical basis of fatigable weakness involving ocular, bulbar and limb muscles at birth to early childhood (though adult onset is increasing recognised), presence of abnormal electrophysiological study suggestive of a neuromuscular disorder and absence of myasthenic autoantibodies in the sera [1]. We describe two children with COLQ-related CMS masquerading as autoimmune myasthenia gravis (MG) with persistent ⇑ Corresponding author at: Department of Paediatrics, Faculty of Medicine, University of Malaya, 50603 Kuala Lumpur, Malaysia. E-mail address: [email protected] (M.-K. Thong).
elevated anti-acetylcholine receptor (AChR) antibody in the sera. To our knowledge there has been no other published case report showing this association. 2. Case report 2.1. Patient 1 AS was delivered at term with a birth weight of 3.6 kg from a non-consanguineous marriage. At birth, he was hypotonic requiring oxygen delivery via nasal prong for 12 days. Blood parameters and chest radiographs did not show evidence of infection. Postdischarge till the age of 4 months, he remained hypotonic, with evidence of muscle weakness, fatigability and slept most of the time. He also had poor suck which resulted in faltering growth and bilateral ptosis which worsened in the evening. He had recurrent respiratory insufficiency requiring supplemental oxygen. At 6 months old, his anti-AChR antibody titre level analysed using enzyme-linked immunosorbent assay (ELISA) (from Euroimmun AG, Germany) was markedly elevated (>8.4 nmol/L; normal range <0.4 nmol/L); conversely, the maternal anti-AChR antibody
https://doi.org/10.1016/j.jocn.2019.12.007 0967-5868/Ó 2019 Elsevier Ltd. All rights reserved.
Please cite this article as: C. G. Tay, C. Y. Fong, L. Li et al., Congenital myasthenic syndrome with novel pathogenic variants in the COLQ gene associated with the presence of antibodies to acetylcholine receptors, Journal of Clinical Neuroscience, https://doi.org/10.1016/j.jocn.2019.12.007
2
Case report / Journal of Clinical Neuroscience xxx (xxxx) xxx
was negative. Serum creatine kinase was normal. A diagnosis of autoimmune myasthenia gravis (MG) was made. Acetylcholinesterase (AChE) inhibitor and intravenous immunoglobulin (IVIG) were introduced. Since then, he deteriorated and needed ventilatory support during intercurrent infections. Intravenous methylprednisolone and oral prednisolone were subsequently added with no favourable response. He continued to be symptomatic and required home bilevel positive airway pressure (BiPAP) support. His second anti-AChR antibody titre at 13 months remained positive (1.89 nmol/L) via the similar assay. Repeated courses of immunotherapy failed to induce remission. He was planned for thymectomy in view of persistent respiratory symptoms. CT thorax performed at 16 months old revealed a homogenous thymus measuring approximately 2.5 3.0 1.0 cm in size. At 17 months old, his clinical presentation was revisited prior to a planned thymectomy. Repetitive nerve stimulation (RNS) at a rate of 3 Hz showed >20% decremental response of the compound motor action potentials (CMAPs) recording at the right abductor digiti minimi (Fig. 1). Stimulated single fiber electromyography (SFEMG) at the right orbicularis oculi showed abnormal mean jitter >80 ls with blocking. Muscle biopsy showed non-specific features of both type 1 and 2 fiber atrophy. These re-confirmed the presence of a neuromuscular junction disorder and the possibility of CMS was explored in view of unfavourable clinical outcome to AChE inhibitor and early onset phenotype. AChE inhibitor was withheld, and salbutamol was introduced. Following which he showed remarkable improvement. He was successfully weaned off day time BiPAP support within a week and thymectomy was abandoned. CMS genetic testing confirmed presence of two novel compound heterozygous COLQ gene mutations, NM_005677.4(CO LQ):c.220-1G > A and NM_005677.4(COLQ):c.1217G > C (p. Gly406Ala). The c.220–1 G > A variant is present in the gnomAD population database at a very low frequency in keeping with a recessive allele (2/244586), and is predicted to disrupt the invariant last base of the acceptor splice site at the start of exon 3. The skipping of the in-frame exon 3 or creation of a new acceptor splice site 13 bp downstream is likely to result in a truncated or absent protein and therefore considered to be pathogenic (class 5). The variant c.1217G > C is absent in the gnomAD population database, affects a conserved amino acid and is being described in conjunction with a pathogenic variant. Therefore it is considered to be likely pathogenic (class 4) under ACMG Guidelines (2015). (ClinVar accessions SCV000864006 – SCV000864007). During the review at 26 months of age, he had good motor progress and started to walk independently with no recurrent chest infections. 2.2. Patient 2 GR is the second child in the family and both of her parents are first cousins. She was born term with a birth weight of 3.54 kg. She
Fig. 1. Repetitive nerve stimulation at 3 Hz showed 24% decremental response in the CMAPs recorded at the right abductor digiti minimi.
was floppy at birth and required supplemental oxygen for 10 days. At 3 weeks old, she developed respiratory distress and ventilated for 3 days. At the age of 4 month, she was noticed to have ptosis and weak cry. Her ptosis became worse at the age of 9 month with fatigability in nature. She was investigated at the age of 14 month. During the assessment, she was able to sit without support but unable to walk. Her tone was reduced with muscle power of 4/5 and normal reflexes. She had ptosis and weak neck flexor without ophthalmoplegia and bulbar weakness. Her anti-AChR antibody via ELISA (Euroimmun AG, Germany) was positive (>8.4 nmol/L) and she was diagnosed to have autoimmune myasthenia. AChE inhibitor was initiated but she could not tolerate it as she became more lethargic and had diarrhoea. Thus, AChE inhibitor was stopped. A course of immunoglobulin was given with no clinical response. CT thorax showed no thymoma at 22 months old. She walked independently at 24 months and a repeated antiAChR antibody remained positive (0.89 nmol/L). RNS at 3 Hz showed >20% decremental responses of CMAPs recording at her right abductor digiti minimi. Stimulated SFEMG was markedly abnormal. As CMS was suspected, genetic analysis of the COLQ gene showed a novel pathogenic homozygous NM_005677.4 (COLQ):c.588del (p.Gly198fs) variant in exon 9. This variant resulted in a premature translational stop signal (p.Gly198Glufs*26) in the COLQ gene and caused absent or disrupted protein product (ClinVar accession SCV000864008). She was started on oral salbutamol which did not show much benefit in terms of her stamina and muscle strength.
3. Discussion CMS is a genetic disorder caused by mutations in genes encoding proteins involved in neuromuscular transmission whereas MG is an autoimmune disease caused by generation of antibodies particularly to AChRs attacking the postsynaptic neuromuscular junction. Genetic analyses showed the 6 genes with pathogenic variants that accounted for >1% of the CMS are CHRNE, RAPSN, COLQ, DOK7, CHAT and GFPT1. In addition, another 20 other genes are known to be associated with <1% of CMS [2]. CMS are inherited in an autosomal recessive or an autosomal dominant manner with an unknown prevalence and the awareness of this condition is low amongst healthcare providers, leading to late diagnosis and inappropriate investigations and treatment. This report aims to raise awareness among clinicians to include CMS as an important differential diagnosis when managing a child with possible refractory autoimmune MG even if the AChR autoantibody is positive to avoid unnecessary treatment including thymectomy. AChR autoantibody has been clearly proven to be pathogenic in both in vitro and in vivo approaches for instances, passively transferring this autoantibody to animals produce experimental myasthenia, removal of the antibody allows recovery and immunization of animals with AChR can provoke an autoimmune disease that closely resembles the naturally occurring disease [3]. This autoantibody has been reported highly sensitive and specific for autoimmune MG. However, other non-myasthenic conditions with positive AChR autoantibodies for example, mitochondrial ophthalmoplegia, motor neuron disease, autonomic ganglionopathy and exposure to polyvalent anti-snake venom have been described in adults [4–8]. Occasionally, it is also present in asymptomatic elderly and commercial immunoglobulin preparations [9,10]. There are no comprehensive false positive results available in children in the literature. Here we described two cases of CMS with end-plate AChE deficiency due to mutations in COLQ gene showing varied clinical response to AChE inhibitor treatment, with one case showing a
Please cite this article as: C. G. Tay, C. Y. Fong, L. Li et al., Congenital myasthenic syndrome with novel pathogenic variants in the COLQ gene associated with the presence of antibodies to acetylcholine receptors, Journal of Clinical Neuroscience, https://doi.org/10.1016/j.jocn.2019.12.007
Case report / Journal of Clinical Neuroscience xxx (xxxx) xxx
more severe phenotype with clear deterioration with AChE inhibitor and the other case showing a milder phenotype with poor tolerance to AChE inhibitor. It is possible that missense and truncating mutations may give rise to milder and severe phenotype, respectively. Phenotype-genotype correlation studies are limited at present and do not provide sufficient prognostic information. COLQ encodes the collagen-like tail subunit of asymmetric AChE, which anchors AChE to the synaptic basal lamina. Defect in the gene leads to prolonged exposure of AChR to acetylcholine stimulation and later AChR desensitisation and damage from cholinergic overload resulting in muscle weakness [2]. Both of our cases were initially managed as refractory autoimmune MG as the detection of AChR autoantibodies on 2 separate occasions had masked the clinical suspicion of CMS. However, what prompted a further evaluation of CMS was the early neonatal onset, poor clinical response to immunotherapy and worsening symptoms or lack of response to AChE inhibitor. These cases were subsequently confirmed to have COLQ mutation. In case 1, salbutamol treatment confers a rapid clinical improvement but in case 2, a clear clinical response was not seen. Although clinical response to beta adrenergic agonist in COLQ CMS has not been formally evaluated as in other synaptic CMS such as DOK-7 [11], beta adrenergic agonist treatment is recommended for this COLQ subtype [2]. Our cases suggest that COLQ-related CMS does not only demonstrate heterogeneity in the clinical phenotypes, but also shows variability in treatment responses. We also described the presence of this autoantibody in COLQrelated CMS which has not been reported to date. It is very rare for both genetic and autoimmune neuromuscular junction disorders to occur simultaneously. In addition, the clinical features of these children are more in keeping with CMS than autoimmune MG. Hence, it is possible the detection of AchR autoantibodies could indicate a false positive result as Euroimmune ELISA kit has been reported to be associated with a false positive rate of 5% among the healthy controls as compared to radio immunoprecipitation assay (RIPA) which is the gold standard test for autoimmune MG [12]. However, RIPA is associated with the use of radioactive isotopes and is not available in Malaysia. Interestingly, inflammatory diseases had been described in recent times which lie along a continuum from monogenic autoimmune disorders to monogenic autoinflammatory diseases, with mixed pattern diseases sitting near the boundary of autoimmunity [13]. Thus, it is also possible that concomitant detection of AChR autoantibody in genetic neuromuscular disorders reflect the unusual occasion where breaking of immune tolerance to AChR occurs as a consequence of autoinflammation (innate immunity) secondary to muscle fibre degeneration, rather than through adaptive immune processes in the thymus [14]. The muscle fiber degeneration in genetic neuromuscular disorder may possibly act as a potential source of autoantigens via complex immune mediated responses. This means DNA or RNA particles released from the degenerating muscle tissues may activate the Toll-like receptors signalling pathway which recognises the endogenous antigens released from cell damage and necrosis. These antigens could then prime CD4 cells and B-cells and if sufficient antigenic change is detected, tolerance would be broken and autoantibody production would ensue. This secondary autoimmune response may possibly not exert direct pathogenic effects as the sensitisation originated from the skeletal muscle, and not from the thymus. Some genetic neuromuscular disorders such as dystrophy myotonica and facioscapulohumeral muscular dystrophy have frequently been reported to be associated with positive AChR autoantibodies [14–19]. Sometimes, they may also cause myasthenia in addition to the features of the primary muscle disease [14]. It is postulated that the presence or absence of symptoms of depends on the balance between receptor destruction and regeneration and on other
3
factors that determine the ‘safety margin’ for neuromuscular transmission [20]. There are also cases of AChR deficiency CMS that were reported to develop autoimmune MG in the adulthood [21,22]. This raises the possibility that patients with CMS may have genetic susceptibility to autoimmune MG as genetic predisposition to autoimmune MG had been described, where microsatellite markers within AChR subunit gene might alter AChR immmunogenicity [23–25]. Since mutation of the AChR gene itself can lead to CMS, it may also change the immunogenicity to cause autoimmune MG. Nonetheless, a clear relationship between CMS and the genetic susceptibility to autoimmune MG remains elusive and has never been studied to date. The association of the presence of AChR autoantibody in children with COLQ-related CMS has not been reported. This unusual and rare observation highlights that CMS should be considered when the onset was early in the neonatal period, especially when the clinical response to immunotherapy was poor or worsened with AChE inhibitors. Meticulous clinical evaluation should be exercised to avoid misdiagnosis of CMS as autoimmune MG. Authors’ contributions Chee Geap Tay investigated the patients, interpreted the data, drafted and revised the manuscript. Choong Yi Fong, Limin Li, Ganesan Vigneswari, Chee Ming Teh, Chin Seng Gan involved in the patients’ clinical examination and review of manuscript. Meow Keong Thong provided genetic input and review of manuscript. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgment We are grateful to the families who provide their informed consent for the publication. References [1] Farmakidis Constantine, Pasnoor Mamatha, Barohn Richard J, Dimachkie Mazen M. Congenital myasthenic syndromes: a clinical and treatment approach. Curr Treat Options Neurol 2018;20(9). https://doi.org/10.1007/ s11940-018-0520-7. [2] Abicht A, Muller JS, Lochmuller H. Congenital Myasthenic Syndromes. In: GeneReviews. Edited by: Adam MP, Ardinger HH, Pagon RA, Wallace SE, Bean LJH, Stephens K et al. Seattle (WA): University of Washington, Seattle.; 19932018. [3] Phillips WD, Vincent A. Pathogenesis of myasthenia gravis: update on disease types, models, and mechanisms. F1000Research. Faculty Rev-513. 2016;5: F1000. [4] Abbott RJ, Holder D, Currie S. False positive anti acetylcholine receptor antibodies in motorneurone disease. Lancet 1986;327:906–7. [5] Behbehani R, Sharfuddin K, Anim JT. Mitochondrial ophthalmoplegia with fatigable weakness and elevated acetylcholine receptor antibody. J Neuroophthalmol 2007;27:41–4. [6] Miglis MG, Racela R, Kaufmann H. Seropositive myasthenia and autoimmune autonomic ganglionopathy: Cross reactivity or subclinical disease?. Auton Neurosci 2011;164:87–8. [7] Rajput R, Sachdev A, Din N, Damato EM, Murray A. False positive acetylcholine receptor antibodies in a case of unilateral chronic progressive external ophthalmoplegia: case report and review of literature. Orbit 2018;37:385–8. [8] Sundar K, Venkatasubramanian S, Shanmugam S, Arthur P, Subbaraya R, Hazeena P. False positive immunoassay for acetyl choline receptor antibody (AChR Ab) in patients exposed to polyvalent antisnake venom. J Neuroimmunol 2017;311:68–70. [9] Vincent A, Clover L, Buckley C, Grimley Evans J, Rothwell PM. Evidence of underdiagnosis of myasthenia gravis in older people. J Neurol Neurosurg Psychiatry 2003;74:1105–8.
Please cite this article as: C. G. Tay, C. Y. Fong, L. Li et al., Congenital myasthenic syndrome with novel pathogenic variants in the COLQ gene associated with the presence of antibodies to acetylcholine receptors, Journal of Clinical Neuroscience, https://doi.org/10.1016/j.jocn.2019.12.007
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Case report / Journal of Clinical Neuroscience xxx (xxxx) xxx
[10] Khan S, Davies L, Cowley D, Wild G, Sewell WA. Anti-acetylcholine receptor antibody reactivity of IgG in commercial immunoglobulin preparations. Clin Neurol Neurosurg 2010;112:835–6. [11] Burke G, Hiscock A, Klein A, Niks EH, Main M, Manzur AY, et al. Salbutamol benefits children with congenital myasthenic syndrome due to DOK7 mutations. Neuromuscul Disord 2013;23:170–5. [12] Oger J, Frykman H. An update on laboratory diagnosis in myasthenia gravis. Clin Chim Acta 2015;444:126–31. [13] McGonagle D, McDermott MF. A proposed classification of the immunological diseases. PLoS Med 2006;3:e297. [14] Lane RJ, Roncaroli F, Charles P, McGonagle DG, Orrell RW. Acetylcholine receptor antibodies in patients with genetic myopathies: clinical and biological significance. Neuromuscul Disord 2012;22:122–8. [15] de los Angeles Avaria M, Kleinsteuber K, Novoa F, Faundez P, Carvallo P. Myotonic dystrophy in a female with myasthenia gravis. Pediatr Neurol 2007;36:421–3. [16] Ekmekci O, Karasoy H, Bademkiran F, Akkus DE, Yuceyar N. Myasthenia gravis and thymoma coexisting with myotonic dystrophy type 1. Neuromuscul Disord 2014;24:40–2. [17] Husain F, Ryan NJ, Hogan GR. Concurrence of limb girdle muscular dystrophy and myasthenia gravis. Arch Neurol 1989;46:101–2. [18] McGonigal A, Thomas AM, Petty RK. Facioscapulohumeral muscular dystrophy and myasthenia gravis co-existing in the same patient: a case report. J Neurol 2002;249:219–20.
[19] Sansone V, Saperstein DS, Barohn RJ, Meola G. Concurrence of facioscapulohumeral muscular dystrophy and myasthenia gravis. Muscle Nerve 2004;30:679–80. [20] Lane R, Wade J, McGonagle D. Myasthenia gravis precipitated by trauma: latent myasthenia and the concept of ’threshold’. Neuromuscul Disord 2009;19:773–5. [21] Croxen R, Vincent A, Newsom-Davis J, Beeson D. Myasthenia gravis in a woman with congenital AChR deficiency due to epsilon-subunit mutations. Neurology 2002;58:1563–5. [22] Santos E, Moreira I, Coutinho E, Goncalves G, Lopes C, Lopes Lima J, et al. Congenital myasthenic syndrome due to mutation in CHRNE gene with clinical worsening and thymic hyperplasia attributed to association with autoimmune-myasthenia gravis. Neuromuscul Disord 2015;25:928–31. [23] Heckmann JM, Morrison KE, Emeryk-Szajewska B, Strugalska H, Bergoffen J, Willcox N, et al. Human muscle acetylcholine receptor alpha-subunit gene (CHRNA1) association with autoimmune myasthenia gravis in black, mixedancestry and Caucasian subjects. J Autoimmun 1996;9:175–80. [24] Garchon HJ, Djabiri F, Viard JP, Gajdos P, Bach JF. Involvement of human muscle acetylcholine receptor alpha-subunit gene (CHRNA) in susceptibility to myasthenia gravis. PNAS 1994;91:4668–72. [25] Giraud M, Eymard B, Tranchant C, Gajdos P, Garchon HJ. Association of the gene encoding the delta-subunit of the muscle acetylcholine receptor (CHRND) with acquired autoimmune myasthenia gravis. Genes Immun 2004;5:80–3.
Please cite this article as: C. G. Tay, C. Y. Fong, L. Li et al., Congenital myasthenic syndrome with novel pathogenic variants in the COLQ gene associated with the presence of antibodies to acetylcholine receptors, Journal of Clinical Neuroscience, https://doi.org/10.1016/j.jocn.2019.12.007
Contents lists available at ScienceDirect
Journal of Clinical Neuroscience journal homepage: www.elsevier.com/locate/jocn
Case report
Congenital myasthenic syndrome with novel pathogenic variants in the COLQ gene associated with the presence of antibodies to acetylcholine receptors Chee Geap Tay a, Choong Yi Fong a, Limin Li a, Vigneswari Ganesan b, Chee Ming Teh b, Chin Seng Gan c, Meow-Keong Thong d,⇑ a
Division of Paediatric Neurology, Department of Paediatrics, University of Malaya Medical Centre, Kuala Lumpur, Malaysia Department of Paediatrics, Penang General Hospital, Penang, Malaysia c Division of Paediatric Intensive Care, Department of Paediatrics, University of Malaya Medical Centre, Kuala Lumpur, Malaysia d Genetic Medicine Unit, Department of Paediatrics, University of Malaya Medical Centre, Kuala Lumpur, Malaysia b
a r t i c l e
i n f o
Article history: Received 25 August 2019 Accepted 1 December 2019 Available online xxxx Keywords: Congenital myasthenic syndrome COLQ gene Autoimmune myasthenia gravis Anti-acetylcholine receptor antibody AChR
a b s t r a c t Congenital myasthenic syndrome (CMS) is a heterogeneous group of inherited disorder which does not associate with anti-acetylcholine receptor (AChR) antibody. The presence of AChR autoantibody is pathogenic and highly sensitive and specific for autoimmune myasthenia gravis (MG). We describe 2 children from unrelated families who presented with hypotonia, ptosis and fatigability in early infancy with antiAChR antibodies detected via ELISA on 2 separate occasions in the sera. Both were treated as refractory autoimmune MG due to poor clinical response to acetylcholinesterase inhibitor and immunotherapy. In view of the atypical clinical features, genetic studies of CMS were performed and both were confirmed to have novel pathogenic mutations in the COLQ gene. To the best of our knowledge, the presence of antiAChR antibody in COLQ-related CMS has never been reported in the literature. The clinical presentation of early onset phenotype, and refractoriness to acetylcholinesterase inhibitor and immunotherapy should prompt CMS as a differential diagnosis. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction Congenital myasthenic syndrome (CMS) is a heterogeneous group of inherited diseases characterised by impaired neuromuscular transmission at the neuromuscular junction. Depending on the location of the primary defect, CMS are classified into presynaptic, synaptic, postsynaptic or glycosylation pathway defect. Most CMS are inherited recessively; this implies a positive family history may not be present in the diagnostic evaluation. Hence, the diagnosis is typically considered on the clinical basis of fatigable weakness involving ocular, bulbar and limb muscles at birth to early childhood (though adult onset is increasing recognised), presence of abnormal electrophysiological study suggestive of a neuromuscular disorder and absence of myasthenic autoantibodies in the sera [1]. We describe two children with COLQ-related CMS masquerading as autoimmune myasthenia gravis (MG) with persistent ⇑ Corresponding author at: Department of Paediatrics, Faculty of Medicine, University of Malaya, 50603 Kuala Lumpur, Malaysia. E-mail address: [email protected] (M.-K. Thong).
elevated anti-acetylcholine receptor (AChR) antibody in the sera. To our knowledge there has been no other published case report showing this association. 2. Case report 2.1. Patient 1 AS was delivered at term with a birth weight of 3.6 kg from a non-consanguineous marriage. At birth, he was hypotonic requiring oxygen delivery via nasal prong for 12 days. Blood parameters and chest radiographs did not show evidence of infection. Postdischarge till the age of 4 months, he remained hypotonic, with evidence of muscle weakness, fatigability and slept most of the time. He also had poor suck which resulted in faltering growth and bilateral ptosis which worsened in the evening. He had recurrent respiratory insufficiency requiring supplemental oxygen. At 6 months old, his anti-AChR antibody titre level analysed using enzyme-linked immunosorbent assay (ELISA) (from Euroimmun AG, Germany) was markedly elevated (>8.4 nmol/L; normal range <0.4 nmol/L); conversely, the maternal anti-AChR antibody
https://doi.org/10.1016/j.jocn.2019.12.007 0967-5868/Ó 2019 Elsevier Ltd. All rights reserved.
Please cite this article as: C. G. Tay, C. Y. Fong, L. Li et al., Congenital myasthenic syndrome with novel pathogenic variants in the COLQ gene associated with the presence of antibodies to acetylcholine receptors, Journal of Clinical Neuroscience, https://doi.org/10.1016/j.jocn.2019.12.007
2
Case report / Journal of Clinical Neuroscience xxx (xxxx) xxx
was negative. Serum creatine kinase was normal. A diagnosis of autoimmune myasthenia gravis (MG) was made. Acetylcholinesterase (AChE) inhibitor and intravenous immunoglobulin (IVIG) were introduced. Since then, he deteriorated and needed ventilatory support during intercurrent infections. Intravenous methylprednisolone and oral prednisolone were subsequently added with no favourable response. He continued to be symptomatic and required home bilevel positive airway pressure (BiPAP) support. His second anti-AChR antibody titre at 13 months remained positive (1.89 nmol/L) via the similar assay. Repeated courses of immunotherapy failed to induce remission. He was planned for thymectomy in view of persistent respiratory symptoms. CT thorax performed at 16 months old revealed a homogenous thymus measuring approximately 2.5 3.0 1.0 cm in size. At 17 months old, his clinical presentation was revisited prior to a planned thymectomy. Repetitive nerve stimulation (RNS) at a rate of 3 Hz showed >20% decremental response of the compound motor action potentials (CMAPs) recording at the right abductor digiti minimi (Fig. 1). Stimulated single fiber electromyography (SFEMG) at the right orbicularis oculi showed abnormal mean jitter >80 ls with blocking. Muscle biopsy showed non-specific features of both type 1 and 2 fiber atrophy. These re-confirmed the presence of a neuromuscular junction disorder and the possibility of CMS was explored in view of unfavourable clinical outcome to AChE inhibitor and early onset phenotype. AChE inhibitor was withheld, and salbutamol was introduced. Following which he showed remarkable improvement. He was successfully weaned off day time BiPAP support within a week and thymectomy was abandoned. CMS genetic testing confirmed presence of two novel compound heterozygous COLQ gene mutations, NM_005677.4(CO LQ):c.220-1G > A and NM_005677.4(COLQ):c.1217G > C (p. Gly406Ala). The c.220–1 G > A variant is present in the gnomAD population database at a very low frequency in keeping with a recessive allele (2/244586), and is predicted to disrupt the invariant last base of the acceptor splice site at the start of exon 3. The skipping of the in-frame exon 3 or creation of a new acceptor splice site 13 bp downstream is likely to result in a truncated or absent protein and therefore considered to be pathogenic (class 5). The variant c.1217G > C is absent in the gnomAD population database, affects a conserved amino acid and is being described in conjunction with a pathogenic variant. Therefore it is considered to be likely pathogenic (class 4) under ACMG Guidelines (2015). (ClinVar accessions SCV000864006 – SCV000864007). During the review at 26 months of age, he had good motor progress and started to walk independently with no recurrent chest infections. 2.2. Patient 2 GR is the second child in the family and both of her parents are first cousins. She was born term with a birth weight of 3.54 kg. She
Fig. 1. Repetitive nerve stimulation at 3 Hz showed 24% decremental response in the CMAPs recorded at the right abductor digiti minimi.
was floppy at birth and required supplemental oxygen for 10 days. At 3 weeks old, she developed respiratory distress and ventilated for 3 days. At the age of 4 month, she was noticed to have ptosis and weak cry. Her ptosis became worse at the age of 9 month with fatigability in nature. She was investigated at the age of 14 month. During the assessment, she was able to sit without support but unable to walk. Her tone was reduced with muscle power of 4/5 and normal reflexes. She had ptosis and weak neck flexor without ophthalmoplegia and bulbar weakness. Her anti-AChR antibody via ELISA (Euroimmun AG, Germany) was positive (>8.4 nmol/L) and she was diagnosed to have autoimmune myasthenia. AChE inhibitor was initiated but she could not tolerate it as she became more lethargic and had diarrhoea. Thus, AChE inhibitor was stopped. A course of immunoglobulin was given with no clinical response. CT thorax showed no thymoma at 22 months old. She walked independently at 24 months and a repeated antiAChR antibody remained positive (0.89 nmol/L). RNS at 3 Hz showed >20% decremental responses of CMAPs recording at her right abductor digiti minimi. Stimulated SFEMG was markedly abnormal. As CMS was suspected, genetic analysis of the COLQ gene showed a novel pathogenic homozygous NM_005677.4 (COLQ):c.588del (p.Gly198fs) variant in exon 9. This variant resulted in a premature translational stop signal (p.Gly198Glufs*26) in the COLQ gene and caused absent or disrupted protein product (ClinVar accession SCV000864008). She was started on oral salbutamol which did not show much benefit in terms of her stamina and muscle strength.
3. Discussion CMS is a genetic disorder caused by mutations in genes encoding proteins involved in neuromuscular transmission whereas MG is an autoimmune disease caused by generation of antibodies particularly to AChRs attacking the postsynaptic neuromuscular junction. Genetic analyses showed the 6 genes with pathogenic variants that accounted for >1% of the CMS are CHRNE, RAPSN, COLQ, DOK7, CHAT and GFPT1. In addition, another 20 other genes are known to be associated with <1% of CMS [2]. CMS are inherited in an autosomal recessive or an autosomal dominant manner with an unknown prevalence and the awareness of this condition is low amongst healthcare providers, leading to late diagnosis and inappropriate investigations and treatment. This report aims to raise awareness among clinicians to include CMS as an important differential diagnosis when managing a child with possible refractory autoimmune MG even if the AChR autoantibody is positive to avoid unnecessary treatment including thymectomy. AChR autoantibody has been clearly proven to be pathogenic in both in vitro and in vivo approaches for instances, passively transferring this autoantibody to animals produce experimental myasthenia, removal of the antibody allows recovery and immunization of animals with AChR can provoke an autoimmune disease that closely resembles the naturally occurring disease [3]. This autoantibody has been reported highly sensitive and specific for autoimmune MG. However, other non-myasthenic conditions with positive AChR autoantibodies for example, mitochondrial ophthalmoplegia, motor neuron disease, autonomic ganglionopathy and exposure to polyvalent anti-snake venom have been described in adults [4–8]. Occasionally, it is also present in asymptomatic elderly and commercial immunoglobulin preparations [9,10]. There are no comprehensive false positive results available in children in the literature. Here we described two cases of CMS with end-plate AChE deficiency due to mutations in COLQ gene showing varied clinical response to AChE inhibitor treatment, with one case showing a
Please cite this article as: C. G. Tay, C. Y. Fong, L. Li et al., Congenital myasthenic syndrome with novel pathogenic variants in the COLQ gene associated with the presence of antibodies to acetylcholine receptors, Journal of Clinical Neuroscience, https://doi.org/10.1016/j.jocn.2019.12.007
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more severe phenotype with clear deterioration with AChE inhibitor and the other case showing a milder phenotype with poor tolerance to AChE inhibitor. It is possible that missense and truncating mutations may give rise to milder and severe phenotype, respectively. Phenotype-genotype correlation studies are limited at present and do not provide sufficient prognostic information. COLQ encodes the collagen-like tail subunit of asymmetric AChE, which anchors AChE to the synaptic basal lamina. Defect in the gene leads to prolonged exposure of AChR to acetylcholine stimulation and later AChR desensitisation and damage from cholinergic overload resulting in muscle weakness [2]. Both of our cases were initially managed as refractory autoimmune MG as the detection of AChR autoantibodies on 2 separate occasions had masked the clinical suspicion of CMS. However, what prompted a further evaluation of CMS was the early neonatal onset, poor clinical response to immunotherapy and worsening symptoms or lack of response to AChE inhibitor. These cases were subsequently confirmed to have COLQ mutation. In case 1, salbutamol treatment confers a rapid clinical improvement but in case 2, a clear clinical response was not seen. Although clinical response to beta adrenergic agonist in COLQ CMS has not been formally evaluated as in other synaptic CMS such as DOK-7 [11], beta adrenergic agonist treatment is recommended for this COLQ subtype [2]. Our cases suggest that COLQ-related CMS does not only demonstrate heterogeneity in the clinical phenotypes, but also shows variability in treatment responses. We also described the presence of this autoantibody in COLQrelated CMS which has not been reported to date. It is very rare for both genetic and autoimmune neuromuscular junction disorders to occur simultaneously. In addition, the clinical features of these children are more in keeping with CMS than autoimmune MG. Hence, it is possible the detection of AchR autoantibodies could indicate a false positive result as Euroimmune ELISA kit has been reported to be associated with a false positive rate of 5% among the healthy controls as compared to radio immunoprecipitation assay (RIPA) which is the gold standard test for autoimmune MG [12]. However, RIPA is associated with the use of radioactive isotopes and is not available in Malaysia. Interestingly, inflammatory diseases had been described in recent times which lie along a continuum from monogenic autoimmune disorders to monogenic autoinflammatory diseases, with mixed pattern diseases sitting near the boundary of autoimmunity [13]. Thus, it is also possible that concomitant detection of AChR autoantibody in genetic neuromuscular disorders reflect the unusual occasion where breaking of immune tolerance to AChR occurs as a consequence of autoinflammation (innate immunity) secondary to muscle fibre degeneration, rather than through adaptive immune processes in the thymus [14]. The muscle fiber degeneration in genetic neuromuscular disorder may possibly act as a potential source of autoantigens via complex immune mediated responses. This means DNA or RNA particles released from the degenerating muscle tissues may activate the Toll-like receptors signalling pathway which recognises the endogenous antigens released from cell damage and necrosis. These antigens could then prime CD4 cells and B-cells and if sufficient antigenic change is detected, tolerance would be broken and autoantibody production would ensue. This secondary autoimmune response may possibly not exert direct pathogenic effects as the sensitisation originated from the skeletal muscle, and not from the thymus. Some genetic neuromuscular disorders such as dystrophy myotonica and facioscapulohumeral muscular dystrophy have frequently been reported to be associated with positive AChR autoantibodies [14–19]. Sometimes, they may also cause myasthenia in addition to the features of the primary muscle disease [14]. It is postulated that the presence or absence of symptoms of depends on the balance between receptor destruction and regeneration and on other
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factors that determine the ‘safety margin’ for neuromuscular transmission [20]. There are also cases of AChR deficiency CMS that were reported to develop autoimmune MG in the adulthood [21,22]. This raises the possibility that patients with CMS may have genetic susceptibility to autoimmune MG as genetic predisposition to autoimmune MG had been described, where microsatellite markers within AChR subunit gene might alter AChR immmunogenicity [23–25]. Since mutation of the AChR gene itself can lead to CMS, it may also change the immunogenicity to cause autoimmune MG. Nonetheless, a clear relationship between CMS and the genetic susceptibility to autoimmune MG remains elusive and has never been studied to date. The association of the presence of AChR autoantibody in children with COLQ-related CMS has not been reported. This unusual and rare observation highlights that CMS should be considered when the onset was early in the neonatal period, especially when the clinical response to immunotherapy was poor or worsened with AChE inhibitors. Meticulous clinical evaluation should be exercised to avoid misdiagnosis of CMS as autoimmune MG. Authors’ contributions Chee Geap Tay investigated the patients, interpreted the data, drafted and revised the manuscript. Choong Yi Fong, Limin Li, Ganesan Vigneswari, Chee Ming Teh, Chin Seng Gan involved in the patients’ clinical examination and review of manuscript. Meow Keong Thong provided genetic input and review of manuscript. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgment We are grateful to the families who provide their informed consent for the publication. References [1] Farmakidis Constantine, Pasnoor Mamatha, Barohn Richard J, Dimachkie Mazen M. Congenital myasthenic syndromes: a clinical and treatment approach. Curr Treat Options Neurol 2018;20(9). https://doi.org/10.1007/ s11940-018-0520-7. [2] Abicht A, Muller JS, Lochmuller H. Congenital Myasthenic Syndromes. In: GeneReviews. Edited by: Adam MP, Ardinger HH, Pagon RA, Wallace SE, Bean LJH, Stephens K et al. Seattle (WA): University of Washington, Seattle.; 19932018. [3] Phillips WD, Vincent A. Pathogenesis of myasthenia gravis: update on disease types, models, and mechanisms. F1000Research. Faculty Rev-513. 2016;5: F1000. [4] Abbott RJ, Holder D, Currie S. False positive anti acetylcholine receptor antibodies in motorneurone disease. Lancet 1986;327:906–7. [5] Behbehani R, Sharfuddin K, Anim JT. Mitochondrial ophthalmoplegia with fatigable weakness and elevated acetylcholine receptor antibody. J Neuroophthalmol 2007;27:41–4. [6] Miglis MG, Racela R, Kaufmann H. Seropositive myasthenia and autoimmune autonomic ganglionopathy: Cross reactivity or subclinical disease?. Auton Neurosci 2011;164:87–8. [7] Rajput R, Sachdev A, Din N, Damato EM, Murray A. False positive acetylcholine receptor antibodies in a case of unilateral chronic progressive external ophthalmoplegia: case report and review of literature. Orbit 2018;37:385–8. [8] Sundar K, Venkatasubramanian S, Shanmugam S, Arthur P, Subbaraya R, Hazeena P. False positive immunoassay for acetyl choline receptor antibody (AChR Ab) in patients exposed to polyvalent antisnake venom. J Neuroimmunol 2017;311:68–70. [9] Vincent A, Clover L, Buckley C, Grimley Evans J, Rothwell PM. Evidence of underdiagnosis of myasthenia gravis in older people. J Neurol Neurosurg Psychiatry 2003;74:1105–8.
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Please cite this article as: C. G. Tay, C. Y. Fong, L. Li et al., Congenital myasthenic syndrome with novel pathogenic variants in the COLQ gene associated with the presence of antibodies to acetylcholine receptors, Journal of Clinical Neuroscience, https://doi.org/10.1016/j.jocn.2019.12.007