Common founder effect of rapsyn N88K studied using intragenic markers.

Mutations in the human gene encoding rapsyn have been linked to a recessive form of postsynaptic congenital myasthenic syndrome due to deficient clustering of acetylcholine receptors at the endplate. All patients reported to date carry the N88K mutation, suggesting a possible common founder effect. To decrease the likelihood of a recombination event occurring within the span of neighboring microsatellite markers, we used seven intragenic single nucleotide polymorphisms (SNPs) spanning 8 kb to characterize the haplotype associated with N88K. In three affected N88K homozygous individuals, we identified a common haplotype present in all heterozygous carriers of N88K. Of note, in two asymptomatic N88K homozygous individuals, a second haplotype was present that differed at three SNP sites downstream from the N88K mutation. Our findings of a common haplotype associated with the N88K mutation support a founder effect. The discordant haplotype found in homozygous individuals suggests that recombination events may have occurred within the rapsyn gene and that this may have implications in the phenotypic expression of the disease.