congenital myasthenic syndromes ( cmss ) are a group of rare genetic disorders affecting neuromuscular junction transmission .
the subtype of cms depends on whether the defect is presynaptic , synaptic , or postsynaptic . in the past 33 years , at least 23 genes encoding proteins of the neuromuscular junction have been identified containing causative mutations .
cmss are clinically heterogeneous and characterized by fatigable weakness of skeletal muscle that occurs between infancy and adulthood .
ptosis and extraocular muscle , facial , bulbar , and generalized weakness are the common presentations .
subtypes exist with onset later in childhood that exhibit morbid muscle fatigability with difficulty in running or climbing stairs .
clinical manifestation , severity , and course of cms vary , even between patients from the same family .
although myasthenic symptoms may be mild , respiratory insufficiency may occur in patients with cms if respiratory muscles are severely affected .
it has been reported that 75% of cms cases are postsynaptic subtype , and a genetic deficiency of acetylcholine receptor ( achr ) tends to be the most frequent etiological basis .
cmss that show decreased synaptic response to acetylcholine are referred to as fast channel ; conversely , those with an increased acetylcholine response are referred to as slow channel . in patients with slow - channel cms ( sccms ) , electrophysiological examination has revealed that prolonged opening activity of the achr channel causes depolarization block , which contributes to muscle weakness and fatigability .
progressive spinal deformities or scoliosis result from paraspinal muscle weakness and are usually found in adult sccms patients .
patients with cms have been reported worldwide , including in east asia , but there have been only a few published cases of cms in china .
herein , we identified three patients with sccms from a chinese family who presented with muscle weakness during early or late childhood . in these patients
, we found autosomal dominant inheritance of a heterozygous mutation in acetylcholine receptor epsilon - subunit ( chrne ) gene .
the proband was referred to the department of neurology , the first affiliated hospital of chongqing medical university in march 2014 .
detailed medical history was obtained from each family member , and the affected family members underwent laboratory tests and physical and electrophysiological examination , including repetitive nerve stimulation ( rns ) and nerve conduction velocity studies . for low - frequency stimuli , decrements were compared between 1 and the 4 stimuli . for high - frequency stimuli ,
laboratory tests on the proband consisted of assaying for blood biochemicals , endocrine hormones , antibodies against nuclear antigens , and antibodies against achr .
patients included in this study provided written informed consent , and the study was approved by medical ethics committee of the first affiliated hospital of chongqing medical university .
genomic dna was extracted from peripheral leukocytes of fresh blood samples from the proband ( iii-3 ) , elder sister of proband ( iii-2 ) , and mother of proband ( ii-2 ) , using standard methods of proteinase k digestion and phenol - chloroform extraction .
we used next - generation gene sequencing to screen the proband dna for genes associated with neuromuscular diseases , including genes that have been reported to encode factors involved in myasthenic syndromes .
the identity of the mutated chrne gene detected by this screen was verified by sanger sequencing .
all exons of the chrne gene were amplified by polymerase chain reaction ( pcr ) using the geneamp pcr system 9700 thermal cycler ( perkin elmer , shelton , ct , usa ) .
the promoter region and the entire coding sequence of the chrne gene ( genbank accession number af105999/gi4580858 ) were determined as described .
each 25 l pcr reaction contained 50 ng genomic dna , 10 pmol of each forward and reverse primers , 5 mmol dntps , and 2.5 u of taq polymerase in taq reaction buffer ( takara biotechnology , dalian , china ) .
thermal cycling consisted of a denaturation step at 94c for 5 min , and then 35 cycles at 94c for 3040 s , 5763c for 30 s , and 72c for 30 s , followed by a final elongation step at 72c for 10 min .
the pcr products were purified by 1.5% agarose gel electrophoresis and directly sequenced with an abi prism 3730xl dna analyzer ( applied biosystems , foster city , ca , usa ) .
the data were analyzed with chromas 2.22 chromatogram file editor software ( technelysium pty ltd .
the base mutations in chrne gene were described and numbered according to criteria provided by the ensembl genome browser ( http://www.ensembl.org ) .
the proband was referred to the department of neurology , the first affiliated hospital of chongqing medical university in march 2014 .
detailed medical history was obtained from each family member , and the affected family members underwent laboratory tests and physical and electrophysiological examination , including repetitive nerve stimulation ( rns ) and nerve conduction velocity studies . for low - frequency stimuli , decrements were compared between 1 and the 4 stimuli . for high - frequency stimuli ,
laboratory tests on the proband consisted of assaying for blood biochemicals , endocrine hormones , antibodies against nuclear antigens , and antibodies against achr .
patients included in this study provided written informed consent , and the study was approved by medical ethics committee of the first affiliated hospital of chongqing medical university .
genomic dna was extracted from peripheral leukocytes of fresh blood samples from the proband ( iii-3 ) , elder sister of proband ( iii-2 ) , and mother of proband ( ii-2 ) , using standard methods of proteinase k digestion and phenol - chloroform extraction .
we used next - generation gene sequencing to screen the proband dna for genes associated with neuromuscular diseases , including genes that have been reported to encode factors involved in myasthenic syndromes .
the identity of the mutated chrne gene detected by this screen was verified by sanger sequencing .
all exons of the chrne gene were amplified by polymerase chain reaction ( pcr ) using the geneamp pcr system 9700 thermal cycler ( perkin elmer , shelton , ct , usa ) .
the promoter region and the entire coding sequence of the chrne gene ( genbank accession number af105999/gi4580858 ) were determined as described .
each 25 l pcr reaction contained 50 ng genomic dna , 10 pmol of each forward and reverse primers , 5 mmol dntps , and 2.5 u of taq polymerase in taq reaction buffer ( takara biotechnology , dalian , china ) .
thermal cycling consisted of a denaturation step at 94c for 5 min , and then 35 cycles at 94c for 3040 s , 5763c for 30 s , and 72c for 30 s , followed by a final elongation step at 72c for 10 min .
the pcr products were purified by 1.5% agarose gel electrophoresis and directly sequenced with an abi prism 3730xl dna analyzer ( applied biosystems , foster city , ca , usa ) .
the data were analyzed with chromas 2.22 chromatogram file editor software ( technelysium pty ltd .
the base mutations in chrne gene were described and numbered according to criteria provided by the ensembl genome browser ( http://www.ensembl.org ) .
the proband ( iii-3 ) , 21-year - old man , had generalized muscle weakness at 4 years of age .
his ability to run and jump was lower compared with his same - age peers . he had experienced fatigue while walking since 4 years old .
symptoms of weakness fluctuated in severity , and exacerbated transiently by physical activity , but slightly relieved during rest breaks .
the symptoms progressed gradually , and the upper limbs became affected at the age of 9 .
symptoms exacerbated several times each year , with each episode lasting a few days to a month , especially during winter .
the symptoms gradually progressed in the last 12 months and he experienced difficulty in climbing stairs ; however , he did not experience diplopia , dysphagia , respiratory insufficiency , or muscle twitching .
bilateral facial weakness was observed , and the bulbar muscles were slightly involved , manifesting as nasal speech ; however , swallowing difficulties were not present , and chewing problems were not marked .
examination showed mild weakness of the neck flexor muscles , bilateral muscles of the tibialis anterior and gastrocnemius , and finger extensor muscles .
the mother of the proband ( ii-2 ) , a 47-year - old woman , had difficulties in grasping and finger extension at 10 years of age . at age 12 , she presented marked muscular weakness and fatigability , especially in the lower limbs .
interestingly , symptoms stopped progressing and she began to recover gradually , though not to normal levels , but she was now able to complete daily housework and perform mildly taxing farm work .
she had bilaterally limited eye movements without ptosis , symmetric bilateral facial weakness , and mild weakness of neck flexor and bilateral muscles of the limbs ( grade 4/5 on the medical research council scale for the proximal muscles and 4/5 for the distal muscles of the limbs ) .
the affected elder sister ( iii-2 ) was 24 years old and had symptoms similar to the proband .
she began to have difficulty in walking and extending fingers at 11 years of age .
facial weakness , muscle wasting in bilateral forearm and interosseous muscles , and bilateral ophthalmoplegia without ptosis were also found .
muscle strength on the medical research council scale was 3/5 for the neck flexor muscles and 4/5 for the proximal muscles .
she was neither able to extend her fingers nor able to walk using either the toe or heel .
the daughter of the elder sister ( iv-1 ) , a 4-year - old girl , however , had no symptoms or complaints .
the laboratory tests for proband showed that the levels of creatine kinase and thyroid and sex hormones were normal , and antibodies to achr were not present .
a neostigmine test showed no recovery for muscle weakness . with the proband 's consent , we prescribed fluoxetine at a dosage of 20 mg / d as treatment .
after no improvement was observed in his condition for 2 weeks , we increased the dosage to 40 mg daily ; 10 days later , fatigue and weakness were mildly improved .
when the elder sister underwent the same fluoxetine treatment , symptoms also were relieved ; however , after 6 months of therapy , neurological examination showed no changes for either patient .
motor nerve conduction studies revealed a repetitive - compound muscle action potential ( r - cmap ) after a single stimulation in the median , ulnar , and peroneal nerves in affected patients from the pedigree [ figure 2a2c ] , which even included the asymptomatic 4-year - old girl although she did not complain of weakness or fatigue [ figure 2d ] .
however , compared with the recordings from the proband [ iii-3 ; figure 2a ] and his elder sister [ iii-2 ; figure 2b ] , the r - cmap of the peroneal nerve of their mother [ ii-2 ; figure 2c ] was more noticeably affected .
after fluoxetine therapy , r - cmap was still present in the proband ( iii-3 ) and his elder sister ( iii-2 ) .
( a ) iii-3 : two peaks , the second peak of the second wave overlaps the first peak of the first wave .
rns induced cmap decrements in several nerves of the affected patients in response to low- or high - frequency rns , as summarized in table 1 and shown in figure 3 .
for the proband 's mother ( ii-2 ) , no decrements were detected in response to low - frequency rns .
decrements of the proband ( iii-3 ) were more obvious than those of the other family members , despite similar degrees of illness .
decrements in response to high - frequency rns were more significant than those to low - frequency rns for all affected members except the 4-year - old girl ( iv-1 ) , who did not receive high - frequency rns to avoid inflicting unnecessary pain .
for the proband ( iii-3 ) , fluoxetine treatment partially restored the decrease in cmap for the facial and peroneal nerves in response to low - frequency rns [ figure 3a and 3e ] , peroneal nerve ] .
however , significant improvements in the decrements were not observed for any of the tested nerves for the elder sister ( iii-2 ) , especially in response to low - frequency rns [ figure 3b and 3f , ulnar nerve ] . for the 4-year - old girl ( iv-1 ; daughter of iii-2 ) , although she did not have symptoms at the time , we measured a decrease in cmap for the ulnar nerve [ figure 3d , ulnar nerve ] . in the elder sister ( iii-2 ) ,
a significant decrease in the median nerve response at higher frequency rns was not found [ figure 3 g ] ; however , a significant decrease in the ulnar nerve response at higher frequency rns was observed even after fluoxetine treatment [ figure 3h ] .
repetitive nerve stimulation tests in the affected patients of the pedigree ( % amplitude ) na : not available because ii-2 did not receive fluoxetine treatment ; nd : decrement was not detected ; 
proband ( iii-3 ; e ) and his elder sister ( iii-2 ; f ) monitored for the effects of fluoxetine treatment at low frequency .
the proband 's mother ( ii-2 ; g ) and elder sister ( iii-2 ; h ) tested at higher 30-hz stimulation .
next - generation sequencing revealed a mutation in the chrne gene of the proband ( iii-3 ) .
proband 's dna was amplified and sequenced . a heterozygous missense mutation c.865c > t in exon 8 was found [ figure 4 ] , resulting in a substitution from leucine to phenylalanine at position 289 ( l289f ) .
the mother ( ii-2 ) and elder sister ( iii-2 ) [ figure 4 ] carried the same heterozygous mutation .
dna of the 4-year - old girl ( iv-1 ) was unavailable because her mother refused it .
the mutation c.865c > t appears to have been initially reported as c.805c > t , leading to the changed l269f ( i.e. , a different residue number ) .
this variant is located in chromosome 17 ( focus : 4804140 g > a ) . the ensembl transcript i d is enst00000293780 , leading to a cdna.865c > t change . according to the latest mrna sequence of genbank ( mrna sequence nm_000080 ) ,
these two residue positions are in fact identical , and the current accepted numbering is l289 ; accordingly , the mutation site has been updated as c.865c > t . in the human gene mutation database ( hgmd ) , this mutation c.865c > t has been reported as a known disease mutation ( hgmd i d cm960300 ) .
the heterozygous missense mutation ( arrows ) , c.865c > t , was identified in the proband ( iii-3 ) , his elder sister ( iii-2 ) , and his mother ( ii-2 ) .
the potential functional impacts of mutations within chrne gene were predicted using polymorphism phenotyping 2 ( polyphen-2 ) software ( http://geneics.bwh.harvard.edu/pph2/ ) , sorting intolerant from tolerant ( sift ) software ( http://sift.jcvi.org/ ) and mutationtaster ( http://www.mutationtaster.org/ ) .
the p. leu289phe substitution was predicted by the polyphen-2 software to be  probably damaging  and disruptive of the function of chrne .
mutationtaster and sift predicted that the p. leu289phe mutation is functionally  disease causing  and  damaging  , respectively .
with mutationtaster software , this mutation is predicted to result in splice site changes and that protein features might be affected .
it has been reported that the mutation can lead to an unusually high rate of spontaneous achr channel openings and a 9-fold increase in affinity for acetylcholine .
this mutation has been found to be absent in the 100 healthy controls ( 50% male ) .
the proband ( iii-3 ) , 21-year - old man , had generalized muscle weakness at 4 years of age .
his ability to run and jump was lower compared with his same - age peers . he had experienced fatigue while walking since 4 years old .
symptoms of weakness fluctuated in severity , and exacerbated transiently by physical activity , but slightly relieved during rest breaks .
the symptoms progressed gradually , and the upper limbs became affected at the age of 9 .
symptoms exacerbated several times each year , with each episode lasting a few days to a month , especially during winter .
the symptoms gradually progressed in the last 12 months and he experienced difficulty in climbing stairs ; however , he did not experience diplopia , dysphagia , respiratory insufficiency , or muscle twitching .
bilateral facial weakness was observed , and the bulbar muscles were slightly involved , manifesting as nasal speech ; however , swallowing difficulties were not present , and chewing problems were not marked .
examination showed mild weakness of the neck flexor muscles , bilateral muscles of the tibialis anterior and gastrocnemius , and finger extensor muscles .
the mother of the proband ( ii-2 ) , a 47-year - old woman , had difficulties in grasping and finger extension at 10 years of age . at age 12 , she presented marked muscular weakness and fatigability , especially in the lower limbs .
interestingly , symptoms stopped progressing and she began to recover gradually , though not to normal levels , but she was now able to complete daily housework and perform mildly taxing farm work .
she had bilaterally limited eye movements without ptosis , symmetric bilateral facial weakness , and mild weakness of neck flexor and bilateral muscles of the limbs ( grade 4/5 on the medical research council scale for the proximal muscles and 4/5 for the distal muscles of the limbs ) .
the affected elder sister ( iii-2 ) was 24 years old and had symptoms similar to the proband .
she began to have difficulty in walking and extending fingers at 11 years of age .
facial weakness , muscle wasting in bilateral forearm and interosseous muscles , and bilateral ophthalmoplegia without ptosis were also found .
muscle strength on the medical research council scale was 3/5 for the neck flexor muscles and 4/5 for the proximal muscles .
she was neither able to extend her fingers nor able to walk using either the toe or heel .
the daughter of the elder sister ( iv-1 ) , a 4-year - old girl , however , had no symptoms or complaints .
the laboratory tests for proband showed that the levels of creatine kinase and thyroid and sex hormones were normal , and antibodies to achr were not present .
a neostigmine test showed no recovery for muscle weakness . with the proband 's consent , we prescribed fluoxetine at a dosage of 20 mg / d as treatment .
after no improvement was observed in his condition for 2 weeks , we increased the dosage to 40 mg daily ; 10 days later , fatigue and weakness were mildly improved .
when the elder sister underwent the same fluoxetine treatment , symptoms also were relieved ; however , after 6 months of therapy , neurological examination showed no changes for either patient .
motor nerve conduction studies revealed a repetitive - compound muscle action potential ( r - cmap ) after a single stimulation in the median , ulnar , and peroneal nerves in affected patients from the pedigree [ figure 2a2c ] , which even included the asymptomatic 4-year - old girl although she did not complain of weakness or fatigue [ figure 2d ] .
however , compared with the recordings from the proband [ iii-3 ; figure 2a ] and his elder sister [ iii-2 ; figure 2b ] , the r - cmap of the peroneal nerve of their mother [ ii-2 ; figure 2c ] was more noticeably affected .
after fluoxetine therapy , r - cmap was still present in the proband ( iii-3 ) and his elder sister ( iii-2 ) .
( a ) iii-3 : two peaks , the second peak of the second wave overlaps the first peak of the first wave .
rns induced cmap decrements in several nerves of the affected patients in response to low- or high - frequency rns , as summarized in table 1 and shown in figure 3 .
for the proband 's mother ( ii-2 ) , no decrements were detected in response to low - frequency rns .
decrements of the proband ( iii-3 ) were more obvious than those of the other family members , despite similar degrees of illness .
decrements in response to high - frequency rns were more significant than those to low - frequency rns for all affected members except the 4-year - old girl ( iv-1 ) , who did not receive high - frequency rns to avoid inflicting unnecessary pain .
for the proband ( iii-3 ) , fluoxetine treatment partially restored the decrease in cmap for the facial and peroneal nerves in response to low - frequency rns [ figure 3a and 3e ] , peroneal nerve ] .
however , significant improvements in the decrements were not observed for any of the tested nerves for the elder sister ( iii-2 ) , especially in response to low - frequency rns [ figure 3b and 3f , ulnar nerve ] .
for the 4-year - old girl ( iv-1 ; daughter of iii-2 ) , although she did not have symptoms at the time , we measured a decrease in cmap for the ulnar nerve [ figure 3d , ulnar nerve ] . in the elder sister ( iii-2 ) ,
a significant decrease in the median nerve response at higher frequency rns was not found [ figure 3 g ] ; however , a significant decrease in the ulnar nerve response at higher frequency rns was observed even after fluoxetine treatment [ figure 3h ] .
repetitive nerve stimulation tests in the affected patients of the pedigree ( % amplitude ) na : not available because ii-2 did not receive fluoxetine treatment ; nd : decrement was not detected ;  : signal not detected .
proband ( iii-3 ; e ) and his elder sister ( iii-2 ; f ) monitored for the effects of fluoxetine treatment at low frequency .
the proband 's mother ( ii-2 ; g ) and elder sister ( iii-2 ; h ) tested at higher 30-hz stimulation .
next - generation sequencing revealed a mutation in the chrne gene of the proband ( iii-3 ) .
a heterozygous missense mutation c.865c > t in exon 8 was found [ figure 4 ] , resulting in a substitution from leucine to phenylalanine at position 289 ( l289f ) .
the mother ( ii-2 ) and elder sister ( iii-2 ) [ figure 4 ] carried the same heterozygous mutation .
dna of the 4-year - old girl ( iv-1 ) was unavailable because her mother refused it .
the mutation c.865c > t appears to have been initially reported as c.805c > t , leading to the changed l269f ( i.e. , a different residue number ) .
this variant is located in chromosome 17 ( focus : 4804140 g > a ) .
the ensembl transcript i d is enst00000293780 , leading to a cdna.865c > t change . according to the latest mrna sequence of genbank ( mrna sequence nm_000080 ) , these two residue positions are in fact identical , and the current accepted numbering is l289 ; accordingly , the mutation site has been updated as c.865c > t . in the human gene mutation database ( hgmd ) , this mutation c.865c > t has been reported as a known disease mutation ( hgmd i d cm960300 ) .
the heterozygous missense mutation ( arrows ) , c.865c > t , was identified in the proband ( iii-3 ) , his elder sister ( iii-2 ) , and his mother ( ii-2 ) .
the potential functional impacts of mutations within chrne gene were predicted using polymorphism phenotyping 2 ( polyphen-2 ) software ( http://geneics.bwh.harvard.edu/pph2/ ) , sorting intolerant from tolerant ( sift ) software ( http://sift.jcvi.org/ ) and mutationtaster ( http://www.mutationtaster.org/ ) .
the p. leu289phe substitution was predicted by the polyphen-2 software to be  probably damaging  and disruptive of the function of chrne .
mutationtaster and sift predicted that the p. leu289phe mutation is functionally  disease causing  and  damaging  , respectively .
with mutationtaster software , this mutation is predicted to result in splice site changes and that protein features might be affected .
it has been reported that the mutation can lead to an unusually high rate of spontaneous achr channel openings and a 9-fold increase in affinity for acetylcholine .
this mutation has been found to be absent in the 100 healthy controls ( 50% male ) .
sccms is a progressive disorder and may present with ophthalmoplegia , ptosis , facial paralysis , weakness , fatigue of trunk and limb muscles , and spinal deformity such as scoliosis .
a pathological gain of function of achr located in the postsynaptic membrane has been observed in sccms , which was caused by mutations in the achr alpha - subunit ( chrna ) gene and chrne gene .
however , mutations of the chrne gene can also lead to the fast - channel variation of csm .
the clinical phenotype can range from mild to severe and vary among sccms patients with different mutations and even among patients with the same mutation .
the mutation c.865c > t / l289f of chrne gene identified in our study was first identified in 1995 .
the mutation identified in 1995 came from a family that was initially described in 1982 .
at least seven patients with this mutation in chrne gene have been reported ; however , no patient with this mutation has been found in asia until now . compared with other patients
reported to have the same mutation , certain characteristics were different in our chinese family .
for example , all members in the family experienced childhood onset and none presented ptosis . respiratory failure did not occur , hence ventilator - assisted breathing was not used . however , severe symptoms have been reported in other patients carrying this identical mutation , including onset in infancy and severe effects on respiratory muscles that caused respiratory failure requiring mechanical ventilation in two cases .
interestingly , the proband in our family experienced episodes of weakness and deterioration in winter for unknown reasons , the influence of climate on the disease has not been reported previously .
we also discovered that , in the mother of the proband , the weakness improved gradually with an increase in age , which suggested that even without medications , this disease might gradually improve with age during adulthood .
the role of the mutation c.865c > t in the pathogenesis of cms has been investigated in several studies .
this mutation was definitively demonstrated to be a causative factor in the development of sccms and was localized to chrne and more precisely to the pore , within the m2 domain , which forms a part of the achr channel .
mutations in this m2 domain have more severe phenotypic consequences than those in the extracellular domain .
indeed , patch clamp studies revealed an unusually high rate of spontaneous achr channel openings and a 9-fold increase in affinity for acetylcholine resulting from this mutation , leading to pathological gain of function .
in addition , ultrastructural studies showed that endplate myopathy occurs in the postsynaptic muscle fiber .
after fluoxetine treatment , our patients reported mild improvement of muscular weakness although neurological signs did not change .
electrophysiological studies showed that r - cmap remained after fluoxetine treatment in both the proband ( iii-3 ) and his elder sister ( iii-2 ) , which were consistent with a previous study .
varying degrees of decrements in response to rns were found in the affected family members in our study , especially in response to high - frequency stimulation , which indicated that high - frequency rns is a more sensitive indicator than low - frequency rns for this disease .
in contrast , decrements were not observed in the mother ( ii-2 ) in response to low - frequency rns .
heterogeneities in the family 's electrophysiological data were also found , but we could not fully address the issue , at least not yet , as to whether the magnitude of the cmap decrease observed in the rns test positively correlated with the severity of illness .
the 4-year - old girl ( iv-1 ) did not have any symptoms at present ; however , decrements were observed when she received low - frequency rns , especially with 3 hz , which indicated that she would likely suffer from this disorder in the future .
furthermore , decrements in response to low - frequency rns of the ulnar nerve of this girl ( iv-1 ) were more pronounced than those in her mother ( iii-2 ) .
in addition , although the decrease in rns was greater in the proband ( iii-3 ) than the elder sister ( iii-2 ) , the severity of the clinical symptoms was nearly same . therefore , we hypothesized that the extents of decrements in response to rns did not necessarily correlate with the severity of illness .
after fluoxetine treatment , decrements in response to rns were observed to improve in a subset of the nerves of one patient but not in the others receiving the same treatment , although they both reported clinical relief of symptoms .
electrophysiological heterogeneities in cms due to chrne gene mutations have been reported . a patient with c.855c >
t mutation in chrne gene showed a mild but significant decrement in all the muscles except for the tibialis anterior .
the greatest decrement was observed in the anconeus muscle ( 15% in amplitude ) . a mild decrement ( 10% )
was found in a patient due to the mutation ( epsilon1369delg ) of chrne gene , but another patient with the same mutation had no decrement .
patients due to duplication mutations 123_127dupctcac in exon 2 of the chrne gene showed decrements .
for sccms patients whose illness can be attributed to the same mutation as in this study , a decremented electromyographic response was observed on 2 to 3 hz of stimulation ; however , r - cmap was not found .
rns test revealed 30%70% decrements in different muscles of a spanish boy with the same mutation l269f
cms may be misdiagnosed as myasthenia gravis or congenital muscular dystrophy or myopathy , such as ulrich congenital muscular dystrophy , which could lead to incorrect or delayed treatment .
for example , pyridostigmine treatment for sccms patients can induce endplate myopathy , and it is thus contraindicated .
the presence of r - cmap indicates a diagnosis of either sccms or acetylcholine esterase deficiency , which is caused by mutations in the acetylcholinesterase - associated collagen ( colq ) gene ; hence , the identification of gene mutations underlying sccms will be necessary for further differential diagnosis .
 background : congenital myasthenic syndromes are a group of rare disorders that are clinically and genetically heterogeneous and caused by mutations in the genes encoding proteins of the neuromuscular junction . here 
 , we described a chinese family that presented with phenotypes of classic slow - channel congenital myasthenic syndrome ( sccms).methods : clinical characteristics and electrophysiological features of three patients from a chinese family were examined , and next - generation sequencing followed by direct sequencing was carried out.results:the patients revealed variability in clinical and electrophysiological features . 
 however , weakness , scoliosis , and repetitive - compound muscle action potential were found in all affected members in the family . 
 a heterozygous c > t missense mutation at nucleotide 865 in acetylcholine receptor epsilon - subunit ( chrne ) gene that causes a leucine - to - phenylalanine substitution at position 289 ( l289f ) was found.conclusions:we reported a sccms family of chinese origin . in the family , classical clinical phenotype with phenotypic variability among different members 
 was found . 
 genetic testing could help diagnose this rare disease . 