Background/Purpose: Hypocomplementemia is a marked feature of systemic lupus erythematosus (SLE), which may be a result of consumption initiated by immune complexes between self-nuclear antigens and autoantibodies. Lower copy number of C4 genes is cited as a risk factor for SLE. Located in the MHC class III region on chromosome 6, there are two versions of C4 genes: the long gene and the short gene. This variation is triggered by the integration of an endogenous retrovirus HERV-K(C4) into intron 9 of the long genes. The role that long and short C4 genes plays in SLE pathogenesis remains unclear. This is especially important for childhood onset SLE (cSLE), as childhood-onset SLE conveys worse disease outcomes when compared to adult onset SLE. To help further understand the role of C4 genetic variants in the immune pathogenesis of cSLE, we performed a cross-sectional case-control study of 37 patients with cSLE (mean age: 19.08±3.22 yo; 81.1% female) and 40 healthy children (mean age: 19.30±3.75 yo; 75% female).

Methods: EDTA-Blood samples were obtained after informed consent and assent. Blood differentials for all samples were measured using a Sysmex automated hematology analyzer. Plasma protein concentrations of complement C4 and C3 were assayed by radial immunodiffusions (Binding Sites, UK); polymorphisms of C4 proteins for C4A and C4B allotypes were determined by immunofixation; gene copy numbers of total C4, C4A, C4B, long C4 genes and short C4 genes were elucidated by multiple genomic Southern blot analyses.  Binary analyses of categorical data were performed by Chi² analyses. Continuous data between groups were compared by Student’s t-test or analyzed by linear regression.  Because of small sample size, our pilot analysis is focused on Caucasian patients (N=21) and Caucasian healthy controls (N=31).

Results: We observed significantly lower lymphocyte counts (1.81±0.87 x10³ /ml vs 2.51±0.75x x10³ /ml, p=0.0004) and complement C4 plasma protein concentrations (19.8±7.7 mg/dL vs 25.4±7.4 mg/dL; p=0.002), plus lower C4 protein yield per copy of C4 genes (5.63±1.84 mg/dL vs 6.59±1.79 mg/dL; p=0.025) in cSLE patients than in controls. The mean protein levels for complement C3 were similar between patients and controls (126.4 vs 133.0, p=0.25). Genetically, the copy number of long C4 genes was significantly lower among cSLE patients (2.30±1.081) than controls (2.903±0.944; p=0.04).  Subjects with only one or two copies of long C4 genes had an odds ratio of 6.36 (95% CI: 1.83-22.1) on genetic risk of cSLE (p=0.0023). The age of onset or diagnosis among patients with cSLE was inversely correlated with copy number of long C4 genes (R²=0.38, p=0.0036), a phenomenon that remains true when patients with other racial groups were included in the analyses (R²=0.23, p=0.0032). 

Conclusion: Diversities of complement C4 genes and variation in levels of plasma C4 proteins are both a cause and an effect of cSLE.  Intriguingly, the inverse relation of copy number long C4 genes with age of onset was an unexpected finding and its clinical implications, though, is yet to be explored. A larger study sample will help us better understand the potential role that long C4 gene plays in SLE pathogenesis and its implications on lupus disease activity or clinical outcomes.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/low-copy-number-of-long-c4-genes-is-a-genetic-risk-factor-for-childhood-onset-sle-csle-but-is-associated-with-higher-age-of-disease-onset/