Résumé :
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Communication n° 521 Myasthenia gravis (MG) is an autoimmune disease affecting neuromuscular transmission, where the prime self-antigen, the muscle acetylcholine receptor (AChR), has been unambiguously identified. The AChR alpha-chain, encoded by the CHRNA1 gene, is the target of most pathogenic autoantibodies. We had previously shown an association of CHRNA1 with MG susceptibility. Here, we have exhaustively characterized its polymorphism by re-sequencing 19.9 kb of genomic sequence encompassing the locus in 16 individuals. We have uncovered 37 new polymorphisms and confirmed another 33. Apart from one, a synonymous substitution in exon 7, all are located in non-exonic sequences. The genotyping of 35 additional single nucleotide polymorphisms (SNPs), located up to 50 kb on each side of CHRNA1, allowed us to identify an island of linkage disequilibrium (LD) spanning 40 kb, including CHRNA1 only and excluding neighbour genes. The pattern of allelic associations of common polymorphisms was characterized and their effects on the phenotype of MG patients were investigated. We disclosed a highly significant association of a group of 12 in-phase polymorphisms with an early onset of the disease (<20 years) in 186 French MG patients compared to 154 population-matched controls (OR = 3.5; P<0.00006). Remarkably, two SNPs of this group, located in the upstream region at position -478 and -1353 relative to the translation start site, modify DNA motifs that break the binding of ICSBP (Interferon-Consensus Sequence Binding Protein) and promote the binding of NFAT (Nuclear Factor of Activated T-cells), respectively. This was confirmed by gel shift assays and could affect the transcriptional efficiency of CHRNA1 in immune or muscle cells. Association of these two SNPs with early-onset MG was replicated in the British population, in a sample of 151 unrelated patients and 109 controls (P<0.025). These two SNPs are therefore good candidates to explain the association with early onset MG and to pursue functional studies.
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