Journal of Neuro-Ophthalmology, March 2011, Volume 31, Issue 1

Congenital Myasthenic Syndrome Due to Homozygous CHRNE Mutations- Report of Patients in Arabia

We describe the clinical characteristics of 3 siblings from 1 family with congenital myasthenic syndrome due to homozygous mutations of the gene coding for the epsilon subunit of the acetylcholine receptor (CHRNE). Onset of symptoms occurred in the first few months of life with ptosis, restricted ocular motility, mild proximal weakness, and difficulty swallowing. Multiple hospital admissions were required due to recurrent pulmonary infections. There was no decremental conduction on repetitive nerve stimulation, but jitter was increased on single fiber electromyographic. Since early childhood, our patients have done well without pulmonary or bulbar symptoms and with partial improvement on pyridostigmine therapy. Response of ptosis to diagnostic ice pack test was striking. Although these siblings have a clinical history and examination findings typical of homozygous CHRNE mutations, the clinical presentation of congenital myasthenia subtypes is variable, and accurate genotyping is essential in choosing the appropriate treatment.

Congenital Myasthenic Syndrome Due to Homozygous CHRNE Mutations: Report of Patients in Arabia Mustafa A. Salih, MD, Darren T. Oystreck, MMedSci, OC(C), Yasser H. Al-Faky, MD, FRCS, Mohammed Kabiraj, MD, Mohamed I. A. Omer, FRCP, Elamin M. Subahi, FRCP, David Beeson, PhD, Khaled K. Abu-Amero, PhD, Thomas M. Bosley, MD Abstract: We describe the clinical characteristics of 3 siblings from 1 family with congenital myasthenic syndrome due to homozygous mutations of the gene coding for the epsilon subunit of the acetylcholine receptor (CHRNE). Onset of symptoms occurred in the first few months of life with ptosis, restricted ocular motility, mild proximal weakness, and difficulty swallow-ing. Multiple hospital admissions were required due to recurrent pulmonary infections. There was no decremen-tal conduction on repetitive nerve stimulation, but jitter was increased on single fiber electromyographic. Since early childhood, our patients have done well without pulmonary or bulbar symptoms and with partial improve-ment on pyridostigmine therapy. Response of ptosis to diagnostic ice pack test was striking. Although these siblings have a clinical history and examination findings typical of homozygous CHRNE mutations, the clinical presentation of congenital myasthenia subtypes is vari-able, and accurate genotyping is essential in choosing the appropriate treatment. Journal of Neuro-Ophthalmology 2011;31:42-47 doi: 10.1097/WNO.0b013e3181f50bea 2011 by North American Neuro-Ophthalmology Society The congenital myasthenic syndromes (CMS) are rare and comprise a group of inherited disorders (1) in which the safety margin of the neuromuscular junction transmission is compromised (2,3). CMS may be mis-diagnosed as a congenital muscular dystrophy or myopathy leading to delayed or incorrect treatment (1). The diagnosis of CMS is based on clinical symptom-atology and absence of antiacetylcholine receptor (AChR) antibodies plus at least 1 of the following: electromyo-graphic (EMG) evidence of neuromuscular transmission defect, response to pyridostigmine, and molecular genetic confirmation (3). Depending on the mutated gene, symp-toms and signs may not be present in utero or in the neonatal period (1,4), potentially complicating diagnosis. Molecular diagnosis is critical because incorrect treatment in CMS can be life threatening (4). We describe 3 brothers with CMS from a consanguine-ous family. These patients have homozygous mutations of the gene encoding the epsilon subunit of the AChR (CHRNE) (5,6). They illustrate important features of the history and examination of this CMS variant, including the potential value of the ice pack test reported previously in autoimmune myasthenia. CASE REPORT Three siblings from a consanguineous family with 5 chil-dren were the product of normal pregnancies (with normal in utero movement) and were asymptomatic in the im-mediate postpartum period. However, by the age of 2-3 months, the oldest boy (Patient 1) developed bilateral ptosis, decreased feeding, choking, and failure to thrive. He had several hospital admissions and on 4 occasions was admitted to the neonatal intensive care unit because of respiratory failure. The middle brother (Patient 2) and the younger brother (Patient 3) had less severe symptoms beginning at approximately 4 months of age, although they Departments of Pediatrics (Neurology) (MAS) and Ophthalmology (DTO, YHA-F, KKA-A, TMB), College of Medicine, King Saud University, Riyadh, Saudi Arabia; Division of Neurophysiology, Department of Neuroscience (MK), Armed Forces Hospital, Riyadh, Saudi Arabia; Unit of Child Health, Department of Clinical Sciences (MIAO), Faculty of Medical Sciences, Trinidad & Tobago, West In-dies; Department of Pediatrics (EMS), Maternity and Children Hospital, Buraidah, Saudi Arabia; Weatherall Institute for Molecular Medicine (DB), The John Radcliff, Oxford, United Kingdom; and Neurology Division, Cooper University Hospital (TMB), Camden, New Jersey. The authors report no conflicts of interest. Address correspondence to Darren T. Oystreck, MMedSci OC(C), Department of Ophthalmology, King Abdulaziz University Hospital, PO Box 245, Riyadh 11411, Saudi Arabia; E-mail: darrenoystreck @ymail.com 42 Salih et al: J Neuro-Ophthalmol 2011; 31: 42-47 Original Contribution Copyright © North American Neuro-Ophthalmology Society. Unauthorized reproduction of this article is prohibited. also had hospital admissions for respiratory problems during the first year of life. Pulmonary function testing showed modest reductions of peak flow in all 3 (Patient 1, 497 L/min with predicted value 600 6 50 L/min; Patient 2, 427 L/min, predicted 570 6 50 L/min; and Patient 3, 317 L/min, predicted 475 6 50 L/min). All subjects had complete neurologic, neuro-ophthalmologic, oculoplastic, and ear, nose, and throat examinations. Orthoptic evaluation included near and distance visual acuity, accommodation, ocular alignment, eye movements, and fusional status. All patients had pulmonary function tests using the Wright peak flow meter, EMG with repetitive stimulation, and single fiber EMG (SFEMG). All 3 had the ice pack test to see if ptosis responded to a transient cold environment (7,8). Direct sequencing of CHRNE was performed by bidirectional sequencing of polymerase chain reaction-amplified products containing the 12 exons and the promoter re-gion of CHRNE (9). All 3 siblings had normal developmental milestones. Each began taking pyridostigmine at approximately 5 months of age. Respiratory function and extremity weakness improved, but ptosis improved only minimally. Substantial restriction of ocular motility persisted in all fields of gaze with occasional complaints of diplopia. At times, this oc-curred with near vision as convergence was slow and markedly reduced. An exotropia developed at near fixation distances of 30 cm (Patient 1), 20 cm (Patient 2), and 15 cm (Patient 3). At the last examination, they were aged 21 years (Patient 1), 17 years (Patient 2), and 13 years (Patient 3) each taking 30-60 mg of pyridostigmine 3 times per day. They were all of normal height and weight with intact cognitive function. Mild, diffuse, proximal muscle weakness was present with mild facial involvement but no apparent bulbar weakness. Gait was normal, and Gower sign was absent. Ophthalmologic testing revealed normal visual acuity in each patient with intact pupillary responses and accom-modative range. All 3 had reduced eye movements in all directions of gaze (Fig. 1). Smooth pursuit was deficient in all directions, and saccades were extremely slow and hy-pometric. No patient had a manifest strabismus in the primary position, and all had normal stereopsis (40 seconds of arc). The orbicularis ocular muscle was weak in all 3 patients. Ptosis was present bilaterally and occluded the visual axis in all 3 when the frontalis was at rest. Each had Cogan lid twitch sign. Ice pack test markedly improved ptosis in all 3 individuals (Fig. 2). In all 3 patients, EMG revealed a normal action potential and no significant decrement in amplitude or area (,10%) after repetitive stimulation of the ulnar nerve. SFEMG of the extensor digitorum communis had increased jitter (range: Patient 1, 56-96 ms; Patient 2, 66-95 ms; and Patient 3, 36-52 ms). All 3 patients had homozygous du-plication mutations 123_127dupCTCAC in exon 2 of the CHRNE gene, while their parents were heterozygous for this mutation (Fig. 3). DISCUSSION These siblings have CMS with moderate to severe ptosis and restriction of ocular motility, mild facial and proximal muscle weakness, and partial improvement with FIG. 1. Patient 1. Limited eye movements were more restricted horizontally than vertically. Salih et al: J Neuro-Ophthalmol 2011; 31: 42-47 43 Original Contribution Copyright © North American Neuro-Ophthalmology Society. Unauthorized reproduction of this article is prohibited. pyridostigmine. Symptoms were static since childhood with manifestations slightly more severe in Patient 1 compared to Patients 2 and 3 despite genetic uniformity. Currently, mutations in at least 12 genes are known to cause CMS (Table 1) (4). There are 91 CHRNE mutation entries reported in the human gene mutation database (http://www.hgmd.cf.ac.uk), including 35 missense/non-sense mutations, 14 splicing mutations, 17 small deletions, 18 small insertions, 3 regulatory mutations, 3 large dele-tions, and 1 large insertion. CHRNE mutations have been identified in people from North Africa (10) but to our knowledge, not previously in the Arabian Peninsula. This genetic mutation in our patient has been previously described in heterozygous status (in a compound hetero-zygous patient) (11) and homozygous status (10) and is felt to lead to a frameshift, resulting in the conversion of Leu- Asn-Glu codons to Pro-His-Stop at positions 43-45 fol-lowed by a truncation of the epsilon subunit in its extra-cellular domain with resulting severe AChR deficiency for the adult form of the receptor (9,11). The mutation was not detected in 50 individuals (100 chromosomes) from similar ethnicity. Our patients' clinical course is similar to other patients with mutations of the CHRNE gene (1,5,12-14), including a family history of consanguinity, a brief asymptomatic interval between birth and onset of ptosis, pulmonary symptoms early in life with gradual improvement, prominent early bulbar involvement, and profound oph-thalmoplegia (12). They differ from previous published cases that reported nonconsanguinity (14), decreased movements in utero (11), and worsening during adult life (14). Our patients' clinical history is not diagnostic of CHRNE mutations because similar clinical findings may be due to other genetic defects (Table 2). Patients with symptoms and signs similar to our patients could be mis-taken for chronic progressive external ophthalmoplegia or autoimmune myasthenia gravis if evaluated later in life without careful attention to the clinical history. Diagnostic tools beyond clinical examination are only partially helpful. While repetitive stimulation was normal, SFEMG revealed increased jitter. The ice pack test, which is commonly used in the diagnosis of autoimmune myasthenia gravis, led to dramatic improvement in ptosis of our patients and may prove to be a valuable diagnostic test in patients with CHRNE mutations. Given the limitations of clinical tools and the variable phenotypic characteristics of CMS, a genetic diagnosis is the most accurate method to confirm the CMS subtype and select the most appropriate treatment (3). For example, pyridostigmine is effective in CHRNE mutations and cer-tain other CMS variants. Pyridostigmine is contraindicated in patients with CMS with COLQ or DOK7 mutations or with slow channel defects (3,4), while ephedrine has good long-term effectiveness in patients with COLQ or DOK7 mutations (4,15,16). A molecular approach to diagnosis will likely become more frequent as more genes responsible for CMS are identified and as the ease and availability of genetic testing improves (12). CMS due to mutations in CHRND have been reported in patients from Arabia (17), and this report of an Ara-bian family with CHRNE mutations confirms another CMS genetic variant within this population. In world regions with limited scientific resources, determining the ethnic frequency for CMS genetic variants ensures that the most appropriate candidate genes can be screened first. CHRNE mutations are the most common form of CMS, and a common mutation c.1293insG has been identified in North African populations (10), but further studies will be required to determine if similar founder effects occur in Arabia. FIG. 2. Patient 1. Eyelid position before (A) and after (B) application of an ice pack on the eyelids for 10 minutes. FIG. 3. Electropherograms of sequences from exon 2 of CHRNE for (A) wild type, (B) parental (obligate hetero-zygote), and (C) our patients' DNA illustrating the 5 base pair duplication 123_127dupCTCAC present in this kinship. 44 Salih et al: J Neuro-Ophthalmol 2011; 31: 42-47 Original Contribution Copyright © North American Neuro-Ophthalmology Society. Unauthorized reproduction of this article is prohibited. TABLE 1. Genetically defined CMS variants Gene Symbol Gene Location Protein Name Function Defect Site Neuromuscular Transmission Defect CHAT 10q11.2 Choline-O-acetyltransferase Catalyzes reversible synthesis of ACh from acetyl CoA and choline Presyn Choline acetyltransferase deficiency/reduced catalytic activity COLQ 3p25 AChE collagenic tail peptide Anchors catalytic subunits of asymmetric AChE to synaptic basal lamina Synaptic Synaptic AChE deficiency CHRNA1 2q24-q32 AChR receptor subunit alpha Opening of ion-conducting channel Postsyn AChR deficiency, FCS and SCS:kinetic abnormalities of AChR CHRNB1 17p13.1 AChR receptor subunit beta Opening of ion-conducting channel Postsyn AChR deficiency, SCS:kinetic abnormalities of AChR CHRND 2q33-q34 AChR receptor subunit delta Opening of ion-conducting channel Postsyn AChR deficiency, FCS and SCS:kinetic abnormalities of AChR CHRNE 17p13-p12 AChR receptor subunit epsilon Opening of ion-conducting channel Postsyn AChR deficiency, FCS and SCS:kinetic abnormalities of AChR CHRNG 2q33-q34 AChR receptor subunit Opening of ion-conducting channel (fetal form) Postsyn AChR deficiency RAPSN 11p11.2-p11.1 43 kDa receptor-associated protein of the synapse Anchors AChR to postsynaptic cytoskeleton Postsyn Affects clustering of AChR at the end plate MUSK 9q31.3-q32 Muscle, skeletal receptor tyrosine protein kinase NMJ organization Postsyn Decreased agrin-dependent AChR aggregation leading to defect of NMJ formation AGRN 1p36.33 Proteoglycan agrin NMJ development and AChR clustering Presynaptic/ postsynaptic Unstable and constant reforming of NMJ DOK7 4p16.2 Protein Dok-7 Encodes MuSK-interacting cytoplasmic protein Dok-7 Postsyn Small, simplified NMJ; normal AChR and AChE function SCN4A 17p13-p12 Sodium channel protein type 4 subunit alpha Mediates voltage-dependent sodium ion permeability Postsyn Decreased safety margin of NMT by increasing the size of the endplate potential to reach threshold for MAP generation ACh, acetylcholine; AChE, acetylcholinesterase; FCS, fast channel syndrome, resulting in fewer and/or shorter AChR channel opening episodes; MAP, muscle action potential; NMJ, neuromuscular junction; NMT, neuromuscular transmission; postsyn, postsynaptic defect; presyn, presynaptic defect; SCS, slow channel syndrome, resulting in prolonged opening of AChR channels; synaptic, synaptic defect (basal lamina). Adapted from the Protein Knowledgebase (UniProtKB; http://www.uniprot.org) and Ohno et al (11) and Harper (17). Salih et al: J Neuro-Ophthalmol 2011; 31: 42-47 45 Original Contribution Copyright © North American Neuro-Ophthalmology Society. Unauthorized reproduction of this article is prohibited. TABLE 2. Clinical features of CMS variants Syndrome/ Gene Symptom Onset Respiratory Problems Neuro-ophthalmological Features Other Salient Features ChAT deficiency/CHAT First year of life Respiratory crisis common Generally full eye movements Severe respiratory crises in early life tend to resolve with age EAD/COLQ Birth:first year; occasionally childhood - Common: ophthalmoparesis, ptosis; rare: slow pupil response to light; full eye movements Muscle weakness, poor head control, ptosis, poor cry and suck, respiratory insufficiency FCS/CHRNA, CH;RND, CHRNE Birth Respiratory crises common Common: ophthalmoparesis, ptosis Homozygous nonsense mutations cause stillbirth and fetal akinesia sequence SCS/CHRNA, CHRNB, CHRND, CHRNE Variable: birth to adulthood - Variable Only CMS that shows dominant inheritance AChR deficiency/CHRNA, CHRNB, CHRND, CHRNE Birth; occassionally in childhood - Common: ophthalmoparesis and ptosis CHRNE mutations common (compensation by expression for fetal AChR g subunit); CHRNA, CHRNB, CHRND: rare and tend to be more severe FADS/CHRNG Fetus N/A N/A Reduced fetal movement, arthrogryposis, multiple pterygium AChR deficiency/RAPSN Birth:first year of life; rare late onset reported Respiratory crisis common Usually normal eye movements; strabismus common Muscle weakness, contractures, and arthrogryposis common; weakness and respiratory crises tends to improve with age DOK7 Variable from birth to adulthood but usually second year of life - Usually normal eye movements Proximal muscle weakness predominates; stridor ChAT, choline acetyl transferase; EAD, endplate acetylcholinesterase deficiency; FADS, fetal akinesia deformation sequence; FCS, fast channel syndrome; NA, not applicable; SCS, slow channel syndrome. 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