Available online at www.sciencedirect.com
ScienceDirect Neuromuscular Disorders 25 (2015) 253–256 www.elsevier.com/locate/nmd
Case report
Congenital Myasthenic Syndrome caused by mutations in DPAGT Andrea Klein a,*, Stephanie Robb b, Elisabeth Rushing c, Wei-Wei Liu d, Kasiaryna Belaya d, David Beeson d a Department of Paediatric Neurology, University Children’s Hospital Zürich, Zürich, Switzerland Dubowitz Neuromuscular Centre, Institute of Child Health, Great Ormond Street Hospital, London, United Kingdom c Department of Neuropathology, University Hospital Zürich, Zürich, Switzerland d Neurosciences Group, Nuffield Department of Clinical Neurosciences, Weatherall Institute of Molecular Medicine, University of Oxford, Oxford OX3 9DS, United Kingdom Received 25 August 2014; received in revised form 19 November 2014; accepted 20 November 2014 b
Abstract Congenital myasthenic syndromes with prominent limb girdle involvement are an important differential diagnosis for congenital myopathies because of the therapeutic considerations. We present a case where accurate diagnosis was delayed for many years. Fluctuations of weakness were misinterpreted as effects of alternative treatments. Weakness was generalised, most prominently in the arms. Fatigability was more prominent in less affected muscles revealed by a positive Simpson test. Stimulation single fibre electromyography confirmed the suspected neuromuscular transmission defect. The marked response to pyridostigmine and cognitive impairment pointed to a myasthenic syndrome due to impaired glycosylation. Two mutations in trans were found in DPAGT1, the gene coding for dolichyl-phosphate N-acetylglucosaminephosphotransferase, one novel, the other previously reported in a rare form of congenital disorder of glycosylation. Gene expression studies revealed that both mutations reduce DPAGT1 expression. Phenotypic features not previously described for DPAGT1 CMS included restricted ocular abduction and long finger flexor contractures. © 2014 Elsevier B.V. All rights reserved. Keywords: Congenital myasthenia; DPAGT1; Differential diagnosis to congenital myopathy
1. Introduction Congenital myasthenic syndromes (CMS) are rare genetic disorders caused by mutations in genes encoding proteins involved in neuromuscular transmission. To date mutations in at least 20 genes are known to cause CMS [1]. The clinical hallmark of CMS is fatigable weakness. In most cases there is facial weakness and ptosis, and in some forms additional clues of extra-ocular muscle involvement or episodic apnoeas may help direct genetic testing. However, there is considerable phenotypic variation. In subtypes that predominantly involve limb girdle muscles, differentiation from congenital myopathies can be challenging, especially as fatigability is difficult to test in small children and repetitive nerve stimulation may not always show decrement [2,3]. We present the clinical phenotype and diagnostic pathway of a child with mutations in DPAGT1, a
* Corresponding author. Department of Paediatric Neurology, University Children’s Hospital Zürich, Zürich 8032, Switzerland. Tel.: +41 442667439; fax: +41 442667163. E-mail address: [email protected] (A. Klein). http://dx.doi.org/10.1016/j.nmd.2014.11.013 0960-8966/© 2014 Elsevier B.V. All rights reserved.
form of CMS that has only recently been recognised and so far few patients have been reported [4–8]. 2. Case description This now 14 year old boy is the first child of non consanguineous parents with a negative family history. He was born at term after normal pregnancy. As an infant motor development was mildly delayed and he was hypotonic, but walked at age 14 months. There were no breathing or swallowing problems. At age 3 he was first seen by a paediatric neurologist. At that time he was unable to run, or hop and had a positive Gowers sign. Facial expression was reduced, but there was no evidence of ptosis or ophthalmoplegia. At age 5, muscle biopsy revealed myopathic findings with increased variability in fibre size, a predominance of atrophic type 1 fibres, normal immunohistochemistry and no tubular aggregates or other structural abnormalities (Fig. 1A, B). Developmental tests (Snijders-Oomen Nonverbal Intelligence Test) showed learning difficulties with a developmental quotient of 60–70. Over the following years he was re-evaluated at many different hospitals
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Fig. 1. Muscle biopsy at age 5 years: (A) HE-stained muscle section shows variation in myofibre diameter with numerous round atrophic fibres and a single degenerating fibre (arrow) (B) In most cases, the small fibre population is type 1 (slow myosin immunostain). Muscle MRI – T1 weighted axial images: thigh (C) atrophic muscles with mildly increased signal without selective involvement or sparing, lower legs (D) mild changes in all muscles with less involvement of the gastrocnemii. (E) Proximal and distal weakness, with only part antigravity movement of arms.
and further tests included a myopathic electromyography (EMG). When the boy was next seen by us at the age of 13 he had predominantly proximal, but also axial and distal weakness and had developed a marked scoliosis. The family reported beneficial effects of massages and alternative treatments lasting for weeks or even months and worsening after interventions such as the wearing of insoles. On clinical examination the child had mild facial weakness; ptosis slightly fatigued with gaze elevation and showed mildly restricted abduction of the eyes. Limb girdle weakness was marked, arms (part antigravity only, Fig. 1E) more affected than legs, proximal muscles more than distal. Distal muscles most affected were hand and finger extensors, with flexors spared. Fatigability tests were difficult to judge because of the degree of weakness. He had lax joints except for flexion contractures of the finger joints and tight long finger flexors. A congenital myasthenic syndrome was suspected because of the proximal and distal involvement and the reported
fluctuations. Repetitive stimulation of the facial nerve showed no decrement, however stimulation single fibre EMG, a very sensitive test for disorders of the neuromuscular junction [9] demonstrated increased jitter. For the differential diagnosis of congenital myopathy we performed muscle magnetic resonance imaging (MRI) which did not reveal a selective pattern in the thigh muscles, the muscles in the lower legs showed mild changes with relative sparing of the gastrocnemius. More changes would be expected in a child with such marked weakness caused by a myopathy (Fig. 1C, D). With the clinical suspicion of a congenital myasthenic syndrome with limb girdle weakness and hence the possibility of DOK7 CMS, a trial with Salbutamol was started, resulting in increased muscle force and stamina. After receiving the negative results of the mutation analysis of the DOK7 gene, pyridostigmine was added, resulting in marked additional benefit in function and stamina. Because of the cognitive difficulties, predominant limbgirdle involvement and improvement with pyridostigmine a
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been previously reported in a patient with a mild presentation of a congenital glycolysation disorder with hypotonia, epilepsy and moderate cognitive deficits and aggressive behaviour [10]. The second mutation c.88C > T; p.Pro30Ser has not been reported. Pro30 is highly conserved across species. The substitution p.Pro30Ser exchanges an uncharged polar for a non-polar amino acid. To confirm pathogenicity, cDNAs harbouring these two respective variants of DPAGT1 were generated by in vitro mutagenesis, expressed in HEK293 cells and levels of the DPAGT1 protein compared to wild type on western blots (n = 3). A reduction in the level of DPAGT1 was seen for both mutations with p.Pro30Ser reduced to approximately 10% of wild type levels (Fig. 2). A neuropsychological re-evaluation at age 15 with the Wechsler intelligence scale for children (WISC-IV) confirmed the previous test results, a full scale IQ could not be calculated, because he was not able to perform all tests. Verbal Comprehension Index was 53, Perceptual Reasoning Index 59, Processing Speed Index 50, corresponding to a mild intellectual disability. Brain MRI was normal. 3. Discussion Fig. 2. Expression of mutant DPAGT1 cDNAs in HEK 293 cells. Wild type and mutant DPAGT1 cDNA constructs were transfected into HEK293 cells and after 48 hours whole cell lysates were subjected to western blotting. (A) western blot, DPAGT1 was detected using a rabbit anti-DPAGT1 (AP9967a, Abgent), an HRP-conjugated anti-rabbit secondary antibody (Dako) and ECL (GM Healthcare). Transfection efficiency was normalised through co-expression with EGFP. (B) Relative expression levels of mutant DPAGT1. Expression was quantitated by densitometry using Image J software and given as the ratio of DPAGT1:EGFP (n = 3; Mean ± SD).
We describe a patient with congenital myasthenic syndrome due to mutations in DPAGT1 that further broadens the clinical spectrum of this disorder where, to date, relatively few cases have been reported [5–8]. Clinical features described so far, also see Table 1, include fatigable weakness that predominantly affects proximal but also distal muscles, spares ocular, and most facial muscles, and is responsive to anticholinesterase medication. Onset ranges from 6 months to childhood, or adolescence. Repetitive nerve stimulation may show decrement in affected muscles, but may not always indicate a neuromuscular transmission disorder if restricted to oculo-facial muscles. Affected individuals demonstrate myopathic features on muscle biopsy, and tubular aggregates are often present. Tubular aggregates associated with CMS suggest causal mutations
CMS caused by one of the genes involved in glycosylation was suspected, although isoelectric focusing of serum transferrin was normal. Two heteroallelic mis-sense variants in DPAGT1, inherited in trans from the asymptomatic parents were detected. Neither has been reported in CMS: c.85A > T; p.Ile29Phe has Table 1 Clinical features of cases reported to date. Cases
Onset (years)
Weakness
Weak finger extensors
Ptosis
Respiratory involvement
Progression
Fluctuation
Scoliosis
Cognition
Response to treatment
B1/F1 B2/F2 B3/F4 B4/F3 B5/F5 Ba1
2.5 7 0.5 2 d, a, mf p, d, a p, d, a p > d, a, mf p
y y
n mild y n n
Mild Mild n n
n Slow Improvement Slow Improvement
y
n Mild Mild n n
na na na Mild learning disability Mild learning disability na
P, S, DAP P P P, DAP P P
Ba2 Ba3 Ba4 S1 S2 S3 K
17 3 13 0.5 1 na 3
p p/d p p > d, a, f p > d, mf Milder than S1 p > d, a, f
y n y
Resp. failure during infections n n n
y y y
y y y y
y
n
y
y
y
y
na na na Intellectual disability Intellectual disability Autistic Intellectual disability
P P P, S
B: reference [5], F: reference [7], Ba: reference [8], S: reference [4], K: this report. Weakness: p: proximal, d: distal, a: axial, f: facial, m: mild. y: yes, present, n: no, na: not mentioned. Treatment: Pyridostigmine, S: Salbutamol, DAP: 3, 4 Diaminopyridine. Empty spaces: information not available.
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affecting early steps of the N-linked glycosylation pathway, but, as in this case, are not always present and in some cases may evolve with age and thus not be evident in early biopsies [7]. Fluctuations of weakness and muscle cramps have been described, similarly seen in patients with mutations in DOK7. Features that differ from DOK7-CMS include learning difficulties and a beneficial response to pyridostigmine treatment [6,8]. Additional phenotypic features in our patient compared with other cases reported to date were restricted abduction of the eyes, upper limb predominance of weakness and finger contractures. DPAGT1 encodes dolichyl-phosphate (UDP-Nacetylglucosamine) N-acetyl-glucosamine-phosphotransferase 1, an enzyme involved in the initial step of the N-linked protein glycosylation pathway. Mutations in DPAGT1 are thought likely to affect the glycosylation of multiple proteins, but assembly and membrane insertion of the muscle AChR is thought to be particularly susceptible to impaired glycosylation and thus loss of endplate AChR explains a beneficial response to anticholinesterase medication [6]. DPAGT1 has also been associated with a very rare subtype of congenital disorder of glycosylation (type Ij) which has a multisystem phenotype with microcephaly, seizures, spasticity, ataxia, learning difficulties and cataracts [10,11]. Why patients with DPAGT1 CMS do not show more multisystemic involvement is not understood, though it could be associated with the degree to which mutations reduce DPAGT1 protein levels and enzymatic function. Altered enzymatic activity was not investigated for the p.Ile29Phe or p.Pro30Ser mutations, but both gave reduced levels of DPAGT1 following expression in HEK293 cells, p.Pro30Ser more marked than p.Ile29Phe. Given that p.Ile29Phe has been previously identified as pathogenic it is highly probable that p.Pro30Ser is also. The mild cognitive impairment that may also be seen in DPAGT1-CMS is likely to be due to impaired glycosylation affecting components of the central nervous system. In conclusion this case extends the phenotype of CMS due to mutations in DPAGT1 and provides further clues for distinguishing CMS with predominant limb girdle muscle involvement from myopathies, especially in small children. Repetitive nerve stimulation may be normal. The additional distal involvement and fluctuating weakness raised the suspicion of a CMS. After exclusion of mutations in DOK7, a subtype of CMS that can dramatically worsen with
cholinesterase inhibitors [12], the addition of pyridostigmine resulted in a marked improvement in our patient, providing a pointer to the genetic investigation of DPAGT1. We note that both mild facial weakness, mildly restricted abduction of the eyes, and long finger flexor contractures can also be apparent in DPAGT1-CMS. Acknowledgements We thank the Myasthenia Gravis Association (Myaware) for funding. KB is a Wellcome Trust Oxion Training Fellow. References [1] Cruz PM, Palace J, Beeson D. Congenital myasthenic syndromes and the neuromuscular junction. Curr Opin Neurol 2014;27:566–75. [2] Abicht A, Dusl M, Gallenmuller C, et al. Congenital myasthenic syndromes: achievements and limitations of phenotype-guided gene-after-gene sequencing in diagnostic practice: a study of 680 patients. Hum Mutat 2012;33:1474–84. [3] Klein A, Pitt MC, McHugh JC, et al. DOK7 congenital myasthenic syndrome in childhood: early diagnostic clues in 23 children. Neuromuscul Disord 2013;23:883–91. [4] Selcen D, Shen XM, Brengman J, et al. DPAGT1 myasthenia and myopathy: genetic, phenotypic, and expression studies. Neurology 2014; 82:1822–30. [5] Belaya K, Finlayson S, Cossins J, et al. Identification of DPAGT1 as a new gene in which mutations cause a congenital myasthenic syndrome. Ann N Y Acad Sci 2012;1275:29–35. [6] Belaya K, Finlayson S, Slater CR, et al. Mutations in DPAGT1 cause a limb-girdle congenital myasthenic syndrome with tubular aggregates. Am J Hum Genet 2012;91:193–201. [7] Finlayson S, Palace J, Belaya K, et al. Clinical features of congenital myasthenic syndrome due to mutations in DPAGT1. J Neurol Neurosurg Psychiatry 2013;84:1119–25. [8] Basiri K, Belaya K, Liu WW, et al. Clinical features in a large Iranian family with a limb-girdle congenital myasthenic syndrome due to a mutation in DPAGT1. Neuromuscul Disord 2013;23:469–72. [9] Pitt M. Neurophysiological strategies for the diagnosis of disorders of the neuromuscular junction in children. Dev Med Child Neurol 2008; 50:328–33. [10] Iqbal Z, Shahzad M, Vissers LE, et al. A compound heterozygous mutation in DPAGT1 results in a congenital disorder of glycosylation with a relatively mild phenotype. Eur J Hum Genet 2013;21: 844–9. [11] Wurde AE, Reunert J, Rust S, et al. Congenital disorder of glycosylation type Ij (CDG-Ij, DPAGT1-CDG): extending the clinical and molecular spectrum of a rare disease. Mol Genet Metab 2012;105:634–41. [12] Ben Ammar A, Petit F, Alexandri N, et al. Phenotype genotype analysis in 15 patients presenting a congenital myasthenic syndrome due to mutations in DOK7. J Neurol 2010;257:754–66.
ScienceDirect Neuromuscular Disorders 25 (2015) 253–256 www.elsevier.com/locate/nmd
Case report
Congenital Myasthenic Syndrome caused by mutations in DPAGT Andrea Klein a,*, Stephanie Robb b, Elisabeth Rushing c, Wei-Wei Liu d, Kasiaryna Belaya d, David Beeson d a Department of Paediatric Neurology, University Children’s Hospital Zürich, Zürich, Switzerland Dubowitz Neuromuscular Centre, Institute of Child Health, Great Ormond Street Hospital, London, United Kingdom c Department of Neuropathology, University Hospital Zürich, Zürich, Switzerland d Neurosciences Group, Nuffield Department of Clinical Neurosciences, Weatherall Institute of Molecular Medicine, University of Oxford, Oxford OX3 9DS, United Kingdom Received 25 August 2014; received in revised form 19 November 2014; accepted 20 November 2014 b
Abstract Congenital myasthenic syndromes with prominent limb girdle involvement are an important differential diagnosis for congenital myopathies because of the therapeutic considerations. We present a case where accurate diagnosis was delayed for many years. Fluctuations of weakness were misinterpreted as effects of alternative treatments. Weakness was generalised, most prominently in the arms. Fatigability was more prominent in less affected muscles revealed by a positive Simpson test. Stimulation single fibre electromyography confirmed the suspected neuromuscular transmission defect. The marked response to pyridostigmine and cognitive impairment pointed to a myasthenic syndrome due to impaired glycosylation. Two mutations in trans were found in DPAGT1, the gene coding for dolichyl-phosphate N-acetylglucosaminephosphotransferase, one novel, the other previously reported in a rare form of congenital disorder of glycosylation. Gene expression studies revealed that both mutations reduce DPAGT1 expression. Phenotypic features not previously described for DPAGT1 CMS included restricted ocular abduction and long finger flexor contractures. © 2014 Elsevier B.V. All rights reserved. Keywords: Congenital myasthenia; DPAGT1; Differential diagnosis to congenital myopathy
1. Introduction Congenital myasthenic syndromes (CMS) are rare genetic disorders caused by mutations in genes encoding proteins involved in neuromuscular transmission. To date mutations in at least 20 genes are known to cause CMS [1]. The clinical hallmark of CMS is fatigable weakness. In most cases there is facial weakness and ptosis, and in some forms additional clues of extra-ocular muscle involvement or episodic apnoeas may help direct genetic testing. However, there is considerable phenotypic variation. In subtypes that predominantly involve limb girdle muscles, differentiation from congenital myopathies can be challenging, especially as fatigability is difficult to test in small children and repetitive nerve stimulation may not always show decrement [2,3]. We present the clinical phenotype and diagnostic pathway of a child with mutations in DPAGT1, a
* Corresponding author. Department of Paediatric Neurology, University Children’s Hospital Zürich, Zürich 8032, Switzerland. Tel.: +41 442667439; fax: +41 442667163. E-mail address: [email protected] (A. Klein). http://dx.doi.org/10.1016/j.nmd.2014.11.013 0960-8966/© 2014 Elsevier B.V. All rights reserved.
form of CMS that has only recently been recognised and so far few patients have been reported [4–8]. 2. Case description This now 14 year old boy is the first child of non consanguineous parents with a negative family history. He was born at term after normal pregnancy. As an infant motor development was mildly delayed and he was hypotonic, but walked at age 14 months. There were no breathing or swallowing problems. At age 3 he was first seen by a paediatric neurologist. At that time he was unable to run, or hop and had a positive Gowers sign. Facial expression was reduced, but there was no evidence of ptosis or ophthalmoplegia. At age 5, muscle biopsy revealed myopathic findings with increased variability in fibre size, a predominance of atrophic type 1 fibres, normal immunohistochemistry and no tubular aggregates or other structural abnormalities (Fig. 1A, B). Developmental tests (Snijders-Oomen Nonverbal Intelligence Test) showed learning difficulties with a developmental quotient of 60–70. Over the following years he was re-evaluated at many different hospitals
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A. Klein et al. / Neuromuscular Disorders 25 (2015) 253–256
Fig. 1. Muscle biopsy at age 5 years: (A) HE-stained muscle section shows variation in myofibre diameter with numerous round atrophic fibres and a single degenerating fibre (arrow) (B) In most cases, the small fibre population is type 1 (slow myosin immunostain). Muscle MRI – T1 weighted axial images: thigh (C) atrophic muscles with mildly increased signal without selective involvement or sparing, lower legs (D) mild changes in all muscles with less involvement of the gastrocnemii. (E) Proximal and distal weakness, with only part antigravity movement of arms.
and further tests included a myopathic electromyography (EMG). When the boy was next seen by us at the age of 13 he had predominantly proximal, but also axial and distal weakness and had developed a marked scoliosis. The family reported beneficial effects of massages and alternative treatments lasting for weeks or even months and worsening after interventions such as the wearing of insoles. On clinical examination the child had mild facial weakness; ptosis slightly fatigued with gaze elevation and showed mildly restricted abduction of the eyes. Limb girdle weakness was marked, arms (part antigravity only, Fig. 1E) more affected than legs, proximal muscles more than distal. Distal muscles most affected were hand and finger extensors, with flexors spared. Fatigability tests were difficult to judge because of the degree of weakness. He had lax joints except for flexion contractures of the finger joints and tight long finger flexors. A congenital myasthenic syndrome was suspected because of the proximal and distal involvement and the reported
fluctuations. Repetitive stimulation of the facial nerve showed no decrement, however stimulation single fibre EMG, a very sensitive test for disorders of the neuromuscular junction [9] demonstrated increased jitter. For the differential diagnosis of congenital myopathy we performed muscle magnetic resonance imaging (MRI) which did not reveal a selective pattern in the thigh muscles, the muscles in the lower legs showed mild changes with relative sparing of the gastrocnemius. More changes would be expected in a child with such marked weakness caused by a myopathy (Fig. 1C, D). With the clinical suspicion of a congenital myasthenic syndrome with limb girdle weakness and hence the possibility of DOK7 CMS, a trial with Salbutamol was started, resulting in increased muscle force and stamina. After receiving the negative results of the mutation analysis of the DOK7 gene, pyridostigmine was added, resulting in marked additional benefit in function and stamina. Because of the cognitive difficulties, predominant limbgirdle involvement and improvement with pyridostigmine a
A. Klein et al. / Neuromuscular Disorders 25 (2015) 253–256
255
been previously reported in a patient with a mild presentation of a congenital glycolysation disorder with hypotonia, epilepsy and moderate cognitive deficits and aggressive behaviour [10]. The second mutation c.88C > T; p.Pro30Ser has not been reported. Pro30 is highly conserved across species. The substitution p.Pro30Ser exchanges an uncharged polar for a non-polar amino acid. To confirm pathogenicity, cDNAs harbouring these two respective variants of DPAGT1 were generated by in vitro mutagenesis, expressed in HEK293 cells and levels of the DPAGT1 protein compared to wild type on western blots (n = 3). A reduction in the level of DPAGT1 was seen for both mutations with p.Pro30Ser reduced to approximately 10% of wild type levels (Fig. 2). A neuropsychological re-evaluation at age 15 with the Wechsler intelligence scale for children (WISC-IV) confirmed the previous test results, a full scale IQ could not be calculated, because he was not able to perform all tests. Verbal Comprehension Index was 53, Perceptual Reasoning Index 59, Processing Speed Index 50, corresponding to a mild intellectual disability. Brain MRI was normal. 3. Discussion Fig. 2. Expression of mutant DPAGT1 cDNAs in HEK 293 cells. Wild type and mutant DPAGT1 cDNA constructs were transfected into HEK293 cells and after 48 hours whole cell lysates were subjected to western blotting. (A) western blot, DPAGT1 was detected using a rabbit anti-DPAGT1 (AP9967a, Abgent), an HRP-conjugated anti-rabbit secondary antibody (Dako) and ECL (GM Healthcare). Transfection efficiency was normalised through co-expression with EGFP. (B) Relative expression levels of mutant DPAGT1. Expression was quantitated by densitometry using Image J software and given as the ratio of DPAGT1:EGFP (n = 3; Mean ± SD).
We describe a patient with congenital myasthenic syndrome due to mutations in DPAGT1 that further broadens the clinical spectrum of this disorder where, to date, relatively few cases have been reported [5–8]. Clinical features described so far, also see Table 1, include fatigable weakness that predominantly affects proximal but also distal muscles, spares ocular, and most facial muscles, and is responsive to anticholinesterase medication. Onset ranges from 6 months to childhood, or adolescence. Repetitive nerve stimulation may show decrement in affected muscles, but may not always indicate a neuromuscular transmission disorder if restricted to oculo-facial muscles. Affected individuals demonstrate myopathic features on muscle biopsy, and tubular aggregates are often present. Tubular aggregates associated with CMS suggest causal mutations
CMS caused by one of the genes involved in glycosylation was suspected, although isoelectric focusing of serum transferrin was normal. Two heteroallelic mis-sense variants in DPAGT1, inherited in trans from the asymptomatic parents were detected. Neither has been reported in CMS: c.85A > T; p.Ile29Phe has Table 1 Clinical features of cases reported to date. Cases
Onset (years)
Weakness
Weak finger extensors
Ptosis
Respiratory involvement
Progression
Fluctuation
Scoliosis
Cognition
Response to treatment
B1/F1 B2/F2 B3/F4 B4/F3 B5/F5 Ba1
2.5 7 0.5 2 d, a, mf p, d, a p, d, a p > d, a, mf p
y y
n mild y n n
Mild Mild n n
n Slow Improvement Slow Improvement
y
n Mild Mild n n
na na na Mild learning disability Mild learning disability na
P, S, DAP P P P, DAP P P
Ba2 Ba3 Ba4 S1 S2 S3 K
17 3 13 0.5 1 na 3
p p/d p p > d, a, f p > d, mf Milder than S1 p > d, a, f
y n y
Resp. failure during infections n n n
y y y
y y y y
y
n
y
y
y
y
na na na Intellectual disability Intellectual disability Autistic Intellectual disability
P P P, S
B: reference [5], F: reference [7], Ba: reference [8], S: reference [4], K: this report. Weakness: p: proximal, d: distal, a: axial, f: facial, m: mild. y: yes, present, n: no, na: not mentioned. Treatment: Pyridostigmine, S: Salbutamol, DAP: 3, 4 Diaminopyridine. Empty spaces: information not available.
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A. Klein et al. / Neuromuscular Disorders 25 (2015) 253–256
affecting early steps of the N-linked glycosylation pathway, but, as in this case, are not always present and in some cases may evolve with age and thus not be evident in early biopsies [7]. Fluctuations of weakness and muscle cramps have been described, similarly seen in patients with mutations in DOK7. Features that differ from DOK7-CMS include learning difficulties and a beneficial response to pyridostigmine treatment [6,8]. Additional phenotypic features in our patient compared with other cases reported to date were restricted abduction of the eyes, upper limb predominance of weakness and finger contractures. DPAGT1 encodes dolichyl-phosphate (UDP-Nacetylglucosamine) N-acetyl-glucosamine-phosphotransferase 1, an enzyme involved in the initial step of the N-linked protein glycosylation pathway. Mutations in DPAGT1 are thought likely to affect the glycosylation of multiple proteins, but assembly and membrane insertion of the muscle AChR is thought to be particularly susceptible to impaired glycosylation and thus loss of endplate AChR explains a beneficial response to anticholinesterase medication [6]. DPAGT1 has also been associated with a very rare subtype of congenital disorder of glycosylation (type Ij) which has a multisystem phenotype with microcephaly, seizures, spasticity, ataxia, learning difficulties and cataracts [10,11]. Why patients with DPAGT1 CMS do not show more multisystemic involvement is not understood, though it could be associated with the degree to which mutations reduce DPAGT1 protein levels and enzymatic function. Altered enzymatic activity was not investigated for the p.Ile29Phe or p.Pro30Ser mutations, but both gave reduced levels of DPAGT1 following expression in HEK293 cells, p.Pro30Ser more marked than p.Ile29Phe. Given that p.Ile29Phe has been previously identified as pathogenic it is highly probable that p.Pro30Ser is also. The mild cognitive impairment that may also be seen in DPAGT1-CMS is likely to be due to impaired glycosylation affecting components of the central nervous system. In conclusion this case extends the phenotype of CMS due to mutations in DPAGT1 and provides further clues for distinguishing CMS with predominant limb girdle muscle involvement from myopathies, especially in small children. Repetitive nerve stimulation may be normal. The additional distal involvement and fluctuating weakness raised the suspicion of a CMS. After exclusion of mutations in DOK7, a subtype of CMS that can dramatically worsen with
cholinesterase inhibitors [12], the addition of pyridostigmine resulted in a marked improvement in our patient, providing a pointer to the genetic investigation of DPAGT1. We note that both mild facial weakness, mildly restricted abduction of the eyes, and long finger flexor contractures can also be apparent in DPAGT1-CMS. Acknowledgements We thank the Myasthenia Gravis Association (Myaware) for funding. KB is a Wellcome Trust Oxion Training Fellow. References [1] Cruz PM, Palace J, Beeson D. Congenital myasthenic syndromes and the neuromuscular junction. Curr Opin Neurol 2014;27:566–75. [2] Abicht A, Dusl M, Gallenmuller C, et al. Congenital myasthenic syndromes: achievements and limitations of phenotype-guided gene-after-gene sequencing in diagnostic practice: a study of 680 patients. Hum Mutat 2012;33:1474–84. [3] Klein A, Pitt MC, McHugh JC, et al. DOK7 congenital myasthenic syndrome in childhood: early diagnostic clues in 23 children. Neuromuscul Disord 2013;23:883–91. [4] Selcen D, Shen XM, Brengman J, et al. DPAGT1 myasthenia and myopathy: genetic, phenotypic, and expression studies. Neurology 2014; 82:1822–30. [5] Belaya K, Finlayson S, Cossins J, et al. Identification of DPAGT1 as a new gene in which mutations cause a congenital myasthenic syndrome. Ann N Y Acad Sci 2012;1275:29–35. [6] Belaya K, Finlayson S, Slater CR, et al. Mutations in DPAGT1 cause a limb-girdle congenital myasthenic syndrome with tubular aggregates. Am J Hum Genet 2012;91:193–201. [7] Finlayson S, Palace J, Belaya K, et al. Clinical features of congenital myasthenic syndrome due to mutations in DPAGT1. J Neurol Neurosurg Psychiatry 2013;84:1119–25. [8] Basiri K, Belaya K, Liu WW, et al. Clinical features in a large Iranian family with a limb-girdle congenital myasthenic syndrome due to a mutation in DPAGT1. Neuromuscul Disord 2013;23:469–72. [9] Pitt M. Neurophysiological strategies for the diagnosis of disorders of the neuromuscular junction in children. Dev Med Child Neurol 2008; 50:328–33. [10] Iqbal Z, Shahzad M, Vissers LE, et al. A compound heterozygous mutation in DPAGT1 results in a congenital disorder of glycosylation with a relatively mild phenotype. Eur J Hum Genet 2013;21: 844–9. [11] Wurde AE, Reunert J, Rust S, et al. Congenital disorder of glycosylation type Ij (CDG-Ij, DPAGT1-CDG): extending the clinical and molecular spectrum of a rare disease. Mol Genet Metab 2012;105:634–41. [12] Ben Ammar A, Petit F, Alexandri N, et al. Phenotype genotype analysis in 15 patients presenting a congenital myasthenic syndrome due to mutations in DOK7. J Neurol 2010;257:754–66.
