Low copy numbers of complement C4 and homozygous deficiency of C4A may predispose to severe disease and earlier disease onset in patients with systemic lupus erythematosus

Abstract

ObjectivesLow copy numbers and deletion of complement C4 genes are potent risk factors for systemic lupus erythematosus (SLE). However, it is not known whether this genetic association affects the clinical outcome. We investigated C4 copy number variation and its relationship to clinical and serological features in a Northern European lupus cohort.MethodsWe genotyped the C4 gene locus using polymerase chain reaction (PCR)-based TaqMan assays in 169 patients with SLE classified according to the 1997 revised American College of Rheumatology (ACR) criteria and in 520 matched controls. In the patient group the mean C4 serum protein concentrations nephelometrically measured during a 12-month period prior to genetic analysis were compared to C4 gene copy numbers. Severity of disease was classified according to the intensity of the immunosuppressive regimens applied and compared to C4 gene copy numbers, too. In addition, we performed a TaqMan based analysis of three lupus-associated single-nucleotide polymorphisms (SNPs) located inside the major histocompatibility complex (MHC) to investigate the independence of complement C4 in association with SLE.ResultsHomozygous deficiency of the C4A isotype was identified as the strongest risk factor for SLE (odds ratio (OR) = 5.329; p = 7.7 × 10−3) in the case-control comparison. Moreover, two copies of total C4 were associated with SLE (OR = 3.699; p = 6.8 × 10−3). C4 serum levels were strongly related to C4 gene copy numbers in patients, the mean concentration ranging from 0.110 g/l (two copies) to 0.256 g/l (five to six copies; p = 4.9 × 10−6). Two copies of total C4 and homozygous deletion of C4A were associated with a disease course requiring cyclophosphamide therapy (OR = 4.044; p = 0.040 and OR = 5.798; p = 0.034, respectively). Homozygous deletion of C4A was associated with earlier onset of SLE (median 24 vs. 34 years; p = 0.019) but not significant after correction for multiple testing. SNP analysis revealed a significant association of HLA-DRB1*0301 with SLE (OR = 2.231; p = 1.33 × 10−5).ConclusionsOur findings confirm the important role of complement C4 genes in the development of SLE. Beyond the impact on the susceptibility for lupus, C4 copy numbers may be related to earlier onset and a more severe course of the disease. The association of homozygous deletion of C4A and SLE is accompanied by the presence of HLA-DRB1*0301 without a proven pathophysiological mechanism.

New Horizons in the Genetic Etiology of Systemic Lupus Erythematosus and Lupus-Like Disease: Monogenic Lupus and Beyond

“…C1q AR SLE, AGS [18][19][20][21][22][23][24] C1r/C1s AD [7,25,26] C2 AR [27] C4 AR [28][29][30][31][32] Type…”

“…In contrast, the copy number variation (CNV) of C4 genes (C4A and C4B), which ranges from two to eight copies, is recognized as a crucial factor in prediction of the risk for juvenile-onset SLE [28,29]. Recent population studies consistently showed that the higher the copy number of C4 genes, the lower the risk of non-mendelian SLE, and vice versa [29][30][31][32]. High numbers of auto-reactive B cells, glomerulonephritis and increased levels of autoantibodies were detected in mice with defective C4 genes [116].…”

Complement component 4 genes contribute sex-specific vulnerability in diverse illnesses

“…Rare cases of severe, early-onset SLE can involve complete deficiency of a complement component (C4, C2, or C1Q) 32,33 , and one of the strongest common-variant associations in SLE maps to ITGAM, which encodes a receptor for C3, the downstream 80 effector of C4 21,34 . Though total C4 gene copy number associates with SLE risk [35][36][37] , this association is thought to arise from linkage disequilibrium (LD) with nearby HLA alleles 38 , which have been the focus of fine-mapping analyses [6][7][8][9][10][11]21 .…”

Insights Gained From the Study of Pediatric Systemic Lupus Erythematosus

“…Characteristically, lupus in these patients develops at an early age and many have severe cutaneous involvement ( 16 ). Extrapolating from these observations, it has also been noted that SLE patients as a whole are more likely to have lower copy numbers of C4A and C4B genes as compared to healthy populations, and this is especially striking in earlier onset disease ( 17 , 18 ).…”