Aberrant splicing of the milk fat globule-EGF factor 8 (MFG-E8) gene in human systemic lupus erythematosus

Milk fat globule-EGF factor 8 (MFG-E8) promotes the phagocytosis of apoptotic cells by serving as a bridging molecule between apoptotic cells and phagocytes. Many apoptotic cells are left unengulfed in the germinal centers of the spleen of MFG-E8(-/-) mice, which develop a human systemic lupus erythematosus (SLE)-like autoimmune disease. Here, we analyzed the MFG-E8 gene in human SLE patients, and found in two out of 322 female patients a heterozygous intronic mutation, which caused a cryptic exon from intron 6 to be included in the transcript. The cryptic exon contained a premature termination codon, generating a C-terminally truncated MFG-E8 protein. The mutant MFG-E8 was aberrantly glycosylated and sialylated, but bound to phosphatidylserine and enhanced the phagocytosis of apoptotic cells. When intravenously injected into mice, the mutant MFG-E8 was sustained longer in the blood circulation than wild-type MFG-E8. Repeated administrations of the mutant MFG-E8 protein induced the production of autoantibodies, such as anti-cardiolipin and anti-nuclear antibodies, at a lower dose than that required for the wild-type protein. These results suggested that the intronic mutation in the human MFG-E8 gene can lead to the development of SLE.