Thesis (Ph.D.)--Chulalongkorn University, 2011

SLE is a multifactorial disorder. Although several of its underlying genes have been identified, many more remain to be discovered. The fact that DcR3 transgenic mice had phenotypes similar to human SLE and SLE patients had an increased level of serum DcR3 has led us to hypothesize that a proportion of SLE patients might have a DcR3 gain-of-function mutation. Here, we performed PCR-sequencing of the entire coding regions of DcR3 in 200 unrelated SLE patients and found two heterozygous missense mutations (p.T56I and p.H122Y) in two patients. They inherited these from their unaffected mothers. Many lines of evidence suggest their etiologic role. Of the 500 controls, the p.H122Y was present in one control, while the p.T56I was absent. Both mutations changed polarity of the amino acid residues and resided in the Fas ligand binding domain. The amino acid residue 56th was conserved in 14 vertebrate species. Using surface plasmon resonance assay, we found that the p.T56I mutant had a 2.4 fold greater, whereas the p.H122Y had 0.7 fold lower affinity to FasL. AnnexinV/PI staining assays showed no differences of the ability to suppress FasL-induced Jurkat apoptosis and activation-induced cell death in lymphocytes between the DcR3 mutants and the wild-type. Interestingly, using [3H] thymidine incorporation assay, we found that both DcR3 mutants statistically significantly increased lymphocyte proliferation more than that of the wild-type. In summary, for the first time, we identified two DcR3 mutations that might play an etiologic role in human SLE pathogenesis.