Objective: We sought to identify a causative mutation in a previously reported kindred with parental consanguinity and 5 of 10 siblings with adult-onset autoimmune myasthenia gravis.
Methods: We performed genome-wide homozygosity mapping, and sequenced all known genes in the one region of extended homozygosity. Quantitative and allele-specific reverse transcriptase PCR (RT-PCR) were performed on a candidate gene to determine the RNA expression level in affected siblings and controls and the relative abundance of the wild-type and mutant alleles in a heterozygote.
Results: A region of shared homozygosity at chromosome 13q13.3-13q14.11 was found in 4 affected siblings and 1 unaffected sibling. A homozygous single nucleotide variant was found in the 3'-untranslated region of the ecto-NADH oxidase 1 gene (ENOX1). No other variants likely to be pathogenic were found in genes in this region or elsewhere. The ENOX1 sequence variant was not found in 764 controls. Quantitative RT-PCR showed that expression of ENOX1 decreased to about 20% of normal levels in lymphoblastoid cells from individuals homozygous for the variant and to about 50% in 2 unaffected heterozygotes. Allele-specific RT-PCR showed a 55%-60% reduction in the level of the variant transcript in heterozygous cells due to reduced mRNA stability.
Conclusion: These results indicate that this sequence variant in ENOX1 may contribute to the familial autoimmune myasthenia in these patients.