Polymorphisms of the ITGAM Gene Confer Higher Risk of Discoid Cutaneous Than of Systemic Lupus Erythematosus

BACKGROUND Lupus erythematosus (LE) is a heterogeneous disease ranging from mainly skin-restricted manifestations (discoid LE [DLE] and subacute cutaneous LE) to a progressive multisystem disease (systemic LE [SLE]). Genetic association studies have recently identified several strong susceptibility genes for SLE, including integrin alpha M (ITGAM), also known as CD11b, whereas the genetic background of DLE is less clear. PRINCIPAL FINDINGS To specifically investigate whether ITGAM is a susceptibility gene not only for SLE, but also for cutaneous DLE, we genotyped 177 patients with DLE, 85 patients with sporadic SLE, 190 index cases from SLE families and 395 population control individuals from Finland for nine genetic markers at the ITGAM locus. SLE patients were further subdivided by the presence or absence of discoid rash and renal involvement. In addition, 235 Finnish and Swedish patients positive for Ro/SSA-autoantibodies were included in a subphenotype analysis. Analysis of the ITGAM coding variant rs1143679 showed highly significant association to DLE in patients without signs of systemic disease (P-value  = 4.73×10(-11), OR  = 3.20, 95% CI  = 2.23-4.57). Significant association was also detected to SLE patients (P-value  = 8.29×10(-6), OR  = 2.14, 95% CI  = 1.52-3.00), and even stronger association was found when stratifying SLE patients by presence of discoid rash (P-value  = 3.59×10(-8), OR  = 3.76, 95% CI  = 2.29-6.18). SIGNIFICANCE We propose ITGAM as a novel susceptibility gene for cutaneous DLE. The risk effect is independent of systemic involvement and has an even stronger genetic influence on the risk of DLE than of SLE.