In addition, two out of five mice injected with C2del developed a high level of anti-cardiolipin antibodies (Fig. 4C). These results suggested that C2del could be responsible for the development of SLE in the patient. Approximately 70% of human genes have alternatively spliced transcripts 23. While alternative splicing
generally facilitates the synthesis of Z-VAD-FMK nmr a greater variety of proteins, mutations disrupting the splice sites or their regulatory elements can cause hereditary disease through the production of aberrant transcripts 24. In this report, we described SLE patients whose MFG-E8 mRNA carry an insertion of 102 nt that resembles a cryptic exon. A splicing assay using a human MFG-E8 minigene carrying intron 6 revealed that the aberrant splicing of the MFG-E8 gene was caused by an A-to-G mutation in the intron. The inclusion of the cryptic exon in the transcript, as a result of this mutation, may be explained by the generation of a GGG motif, an intronic splicing enhancer 25, 26, which activates an exon choice by interacting with trans factors that regulate splicing 27, 28 The cryptic exon incorporated in C2del had a premature termination codon located in the C2 domain of human MFG-E8. In general, mRNA that contain premature termination codons are eliminated by an mRNAs surveillance
mechanism called nonsense-mediated mRNA decay (NMD) 29. In fact, selleck in a splicing assay with the MFG-E8 minigene, the transcripts containing the cryptic exon increased when the premature termination was blocked by treating selleck screening library the cells with cycloheximide or by removing the termination codon with site-directed mutagenesis (data not shown). On the other hand, a significant proportion of the MFG-E8 transcripts from the patient
carried the cryptic exon. There are two possible explanations for this discrepancy: (i) the efficiency of NMD is different between HEp-2 and human peripheral blood mononuclear cells 30, and (ii) the mutant transcript may be more stable in the white blood cells of the patient. In addition, the NMD efficiency is known to differ among individuals. For example, the same mutation that leads to premature termination in the dystrophin gene can cause a mild (Becher muscular dystrophy) or more severe (Duchene muscular dystrophy) phenotype in different individuals 31. Wild-type human MFG-E8 has an apparent Mr of 46 kDa, carries three N-glycosylation sites, and is glycosylated. C2del retained only one of the glycosylation sites, yet its molecular weight increased to 50 kDa, due to higher glycosylation. This aberrant glycosylation was observed in other cell lines, such as HEK293T cells (data not shown), confirming that it was an intrinsic property of C2del, and is not due to the host cell lines.