Familial clustering of autoimmune MG has been reported previously. Although no causative mutation has yet been identified, this seems likely in the present family because of the parental consanguinity and the late onset of MG in 5 siblings who had lived apart for up to 50 years. Sequencing of all known genes across the linked region of chromosome 13 showed homozygous single nucleotide variants in 2 genes (ENOX1 and POSTN). Based on RNA and protein analysis, the variation in the POSTN gene is unlikely to be causative in this family. One asymptomatic sibling (II-1; ) was also homozygous for the haplotype shared by the affected individuals at the chromosome 13 locus, possibly because of incomplete penetrance.
Ecto-NADH oxidase (ENOX) proteins are growth-related cell surface enzymes that catalyze hydroquinone oxidation and protein disulfide-thiol interchange. The 2 enzymatic activities oscillate with a periodicity of 24 minutes for constitutively expressed ENOX1
, unlike any other oxidases or protein disulfide isomerases.20
In principle, the ENOX1
mutation might predispose to MG by affecting the autoimmune response or the motor endplate (or both). ENOX1
is highly expressed in fetal brain and at lower levels in skeletal muscle, thymus, and lymph nodes. It plays a role in plasma membrane transport pathways reducing nicotinamide adenine dinucleotide (NAD+) to NADH. NAD and its metabolites are involved in energy metabolism, signal transduction, aging, and cellular injury. They also are involved in immune regulation by inhibiting T-lymphocyte proliferation24
and control of signaling via CD38.21–23
Studies have implicated NAD+ and NADH in the induction of cell death in CD4+ and CD25+ cells,24
which, in cooperation with natural killer T cells, may play a role in the prevention of autoimmune MG.25
Other studies have shown that NAD+ and NADH are involved in the regulation of gene expression.26
The 3′ UTRs of mRNAs play important roles in post-transcriptional regulation of gene expression, by modulating mRNA localization, stability, and translation, in part through association with microRNAs.27
The sequence variant identified here in ENOX1
was predicted to destroy 2 existing microRNA-binding sites and to create a new one. However, we did not detect changes in reporter activity of the wild-type or mutant ENOX1
3′ UTR luciferase reporter in the presence of these specific microRNAs. Nevertheless, it remains possible that this variant affects the binding of other microRNAs that are not predicted by current algorithms. Alternatively, the variant may affect the secondary structure of the transcript, altering its stability.
Based on the homozygosity mapping and sequence data, and also on its reduced expression and what is known of its function, ENOX1 is a candidate gene for the autoimmune MG in the family described here. Although other factors cannot be fully excluded, this finding may provide novel insights into the etiology of MG and other autoimmune diseases.