Splicing abnormalities in congenital myasthenic syndromes.

A total of 173 mutations has been reported to date in eight genes in congenital myasthenic syndromes. Sixteen intronic and five exonic mutations in three genes affect pre-mRNA splicing. Eight of these are of particular interest, and are reviewed in this article. An A-to-G mutation at intron position +3 results in exon skipping only when there are mismatched nucleotides to U1 snRNA at positions +4 to +6. Similarly, a mutation at the last nucleotide of an exon causes exon skipping when a nucleotide at position +6 is not complementary to U1 snRNA. We observe the similar compensation mechanisms for mismatches to U1 snRNA at 179,917 native human splice donor sites. A 7-bp deletion in CHRNE exon 7 causes skipping of the preceding 101-bp exon 6. We found in general that the nonsense-mediated altered splicing of a remote exon (NASRE) is mediated by inherent weak splicing signals flanking the skipped exon and degradation of a normally spliced transcript by the nonsense-mediated mRNA decay (NMD). A 16-bp duplication spanning the CHRNE intron 10/exon 11 boundary generates two copies of 3' splice sites, and the downstream copy is exclusively silenced. Analysis of a series of artificial mutants conforms to the scanning model of recognition of the 3' splice site that predicts that the first 'ag' more than 13 nucleotides downstream of the branch point is selected for splicing. Splicing mutations may be more frequent than suspected, and one must always be aware of possible splicing abnormalities when analyzing human mutations.