Arthritis is a common manifestation in systemic lupus erythematosus (SLE), appearing in approximately 85% of patients. Often, the polyarthritis at presentation of SLE cannot be distinguished from rheumatoid arthritis (RA) by physical examination or history. Indeed, physicians initially tell many SLE patients that they have RA (one source of "self-reported RA"), only to have SLE established later. In addition, RA aggregates in families with an SLE proband. We predicted that pedigrees multiplex for both SLE and for self-reported RA would better isolate particular genetic effects. If this proved to be true, we would then use the increased genetic homogeneity to more easily reveal genetic linkage.
From a collection of 160 pedigrees multiplex for SLE, we selected 36 pedigrees that also contained >or=2 members with self-reported RA (19 pedigrees were African American, 14 were European American, and 3 were of other ethnic origin). Data from a genome scan of 307 microsatellite markers were evaluated for SLE linkage by contemporary genetic epidemiologic techniques.
The most significant evidence of linkage to SLE was obtained at 5p15.3 in the European American pedigrees by both parametric (logarithm of odds [LOD] score 6.2, P = 9.3 x 10(-8)) and nonparametric (LOD score 6.9, P = 1.7 x 10(-8)) methods. The best-fitting model for this putative SLE gene in this region was a recessive gene with a population frequency of 5% and with 50% penetrance in females and 15% penetrance in males at virtually 100% homogeneity.
For a genetically complex disease phenotype, an unusually powerful linkage has been found with SLE at 5p15.3 in European American pedigrees multiplex for SLE and for self-reported RA. This result predicts the presence of a gene at the top of chromosome 5 in this subset of patients that is important for the pathogenesis of SLE.