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 .