Cumulative evidence has suggested that genetic predisposition contributes to susceptibility to systemic lupus erythematosus (SLE). The search of SLE susceptibility genes has involved mainly testing functionally relevant genes as candidates in population-based association studies. Because positive association may occur as an artifact of population admixture, the addition of linkage analysis offers more compelling evidence for susceptibility genes based on physical locations. We previously reported evidence for linkage of SLE to a 15 cM region on chromosome 1q41-q42. Within this interval, there are several candidate genes including PARP and HLX1 which encode poly(ADP-ribose)polymerase (PARP) and a homeo box protein, respectively. Using a family-based association test, we have recently observed evidence for association of PARP (but not HLX1) alleles with SLE in 124 families containing offspring afflicted with SLE (p= 0.00008 by the multi-allele transmission-disequilibrium test), and thus suggesting a role of PARP in SLE susceptibility. Previously PARP was also implicated in SLE based on its lower than normal range of activity in SLE patients, its intermediate range of activity in unaffected family members of SLE patients, and its decreased mRNA levels in SLE patients. Abnormal DNA repair has been reported in SLE patients. These multiple lines of evidence has led us to hypothesize that PARP is an SLE susceptibility gene. PARP is activated by DNA strand breaks which transfers ADP-ribose residues from NAD to nuclear proteins involved in various chromatin functions including DNA repair. It is specifically cleaved by caspase-3 induced during apoptosis. This proposal tests whether allelic variation of PARP causes functional variation to account for heritable differences in SLE susceptibility primarily using EBV-transformed cell lines from SLE patients and their family members. This PARP polymorphism is a simple sequence repeat (SSR) within the promoter, and thus may affect levels of mRNA and protein expression. It is also likely that this polymorphism represents a marker in linkage disequilibrium with other polymorphism(s) influencing PARP activity. We propose to 1) study PARP expression in individuals homozygous for the SLE-associated allele and individuals with other alleles by comparing levels of protein, mRNA, promoter activity, and post-translational modification, 2) characterize allelic variation of PARP activity and the related DNA repair, 3) study allelic variation of apoptotic cleavage of PARP upon induction, and 4) define the structural basis of any observed variation of PARP. In summary, our proposed study will assess whether allelic variation of PARP causes quantitative variation in expression, enzyme activity, sensitivity as a substrate, and DNA repair which may contribute to SLE susceptibility. Results from this study will support (or refute) a role of PARP in SLE susceptibility.