| Research Abstract |
A genome-wide search using fluorescence-based microsatellite markers and affected sib-pair study in 41 Japanese rheumatoid arthritis (RA) families, each with at least 2 affected siblings, assigned 3 principal regions of linkage, D1S253/214, D8S556 and DXS1232, as RA1, RA2 and RA3 for RA disease loci. We report here the mutant forms of death receptor-3 (DR3) gene and Db1 gene as RA1 and RA3 disease genes, respectively.
Death receptor 3 gene as a candidate for RA1 gene : Radiation hybrid mapping showed that position of an apoptosis-inducing receptor DR3 gene was identical to DlS214/253. Disease-specific DNA mutations were found at nt564(A→G) ; Asp159Gly, nt630+622 (del14), nt63l-538 (C→T), nt63l-391 (A→T), nt631-243 (A→G). This mutation was found in 7 out of 297 (2.4%) sporadic patients with RA and 6 out of 60 (10.0%) familial patients with RA Because of this mutation, truncated DR3 molecule without containing intracellular death domains was produced and this truncated DR3 molecule assemb
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led with intact DR3 to inhibit normal apoptotic signal transduction. We speculate that defective apoptosis is the cause of autoimmunity in RA as was the case of mouse model Fas-deficient autoimmune MRL/lpr mice.
Truncated Db1 proto-oncogene as a candidate for RA3 gene : By re-examining the region around DXS1232 using additional microsatellite, a single MLS (maximal lod score) peak was obtained at DXS984, 0.1cM distant from DXS1232. Radiation hybrid mapping showed that the location of Db1 gene was identical to DXS984. There was a 223bp exon skipping without containing 23rd and 24th exons, nt2697(de1223), in the cDNA of patients with RA, which gave rise to a truncated Db1 molecule with 65 amino acid deletion. The mutation was found in 40.4% of sporadic patients with RA and 19.0% of healthy control Exon trapping experiments revealed a base substitution at nt2522+394 (C→T) in a family with RA as the basis of this exon skipping. Db1 is a GEF (GTP exchange factor) for cdc42, Rac and Rho. Rac is also an indispensable component of NADPH oxidase, an enzyme responsible for superoxide generation. We found that superoxide generation as measured by the change of cytochrome C absorbance at 550nm was impaired in the granulocytes of patients having truncated Db1. Less
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