Our approach leads to a better comprehension of the physiological and molecular mechanisms of CMSs and allows the diagnosis to be improved. This should lead to better genetic counselling and an adapted therapy. More specifically, last year saw the following advances:

 

 

In a Portuguese patient with a mild form of recessive CMS, CHRNE sequencing identified an unknown homozygous transition. This variation affects the third nucleotide of Glycine 285 and leads to a synonymous variant. Analysis of transcripts demonstrated that this single change creates a new splice donor site located 4 nucleotides upstream to the normal site, leading to a 4 nucleotide deletion at the end of exon 8, which generates a frameshift in exon 9 and a premature termination codon.
This paper relates the discovery of a synonymous mutation in CHRNE that creates a new splice donor site leading to an aberrant splicing of pre-mRNA. This is the first known silent mutation in CHRNE that generates a cryptic splice site, thus strongly suggesting it to be the disease causing mutation (Richard et al., 2007).

 

• Identification of a Maghrebian mutation of the acetylcholine receptor ε-subunit and demonstration of its founder effect
 

 

This work assembled clinical and molecular data of patients originating from North Africa. This data comes from different groups in our network in France, Algeria and Tunisia. The CHRNE 1293insG mutation was found in 20% of CMS patients of North African origin (particularly Algeria, Morocco and Tunisia) followed up in Paris, Marseille, Nice and Tunis. The study of microsatellite markers and intragenic polymorphisms in these families has shown a large linkage imbalance, suggesting the existence of a founder effect (submitted article).
 

• Demonstration that mutations of the MuSK gene cause CMS
 

MuSK is a post-synaptic muscle receptor of the tyrosine-kinase type. Its activation by the neural agrin liberated by the motor neuron determines the aggregation of acetylcholine receptors under the nerve ending and their stabilisation by rapsyn. Analyses of the neuromuscular junction on patient biopsies have allowed the research to be oriented towards as yet unidentified genes. The changes of MuSK expression in a patient suffering from congenital myasthenic syndrome have led to first-time identification of two mutations in the MuSK gene. Genetic analysis has allowed the identification of two heteroallelic mutations (c.220insC and V790M). In vivo and in vitro experiments were carried out using MuSK mutants reproducing human mutations. Their results indicate that these mutations are responsible for the very considerable synaptic modifications observed in the patient (Chevessier et al., 2004). The same kind of approaches is being used for the characterization of MUSK mutations in 2 other patients (poster presentation at Myologie 2008, Marseille).
The MuSK mutations seem to induce the liberation of muscle factors capable of modifying motor innervations, one of them being the perlecan or one of its fragment (poster presentation at Myologie 2008, Marseille). Mice carrying the human identified human mutations have been generated (with cognitive and perhaps therapeutic aims) at EMBL (Veit Witzemann, Heidelberg).