Comprehensive approach to study complement C4 in systemic lupus erythematosus: Gene polymorphisms, protein levels and functional activity

Mol Immunol. 2017 Dec;92:125-131. doi: 10.1016/j.molimm.2017.10.004. Epub 2017 Oct 26.

Abstract

Genetic variation of the genes encoding complement component C4 is strongly associated with systemic lupus erythematosus (SLE), a chronic multi-organ auto-immune disease. This study examined C4 and its isotypes on a genetic, protein, and functional level in 140 SLE patients and 104 healthy controls. Gene copy number (GCN) variation, silencing CT-insertion, and the retroviral HERV-K(C4) insertion) were analyzed with multiplex ligation-dependent probe amplification. Increased susceptibility to SLE was found for low GCN (≪2) of C4A. Serositis was the only clinical manifestation associated with low C4A GCN. One additional novel silencing mutation in the C4A gene was found by Sanger sequencing. This mutation causes a premature stop codon in exon 11. Protein concentrations of C4 isoforms C4A and C4B were determined with ELISA and were significantly lower in SLE patients compared to healthy controls. To study C4 isotypes on a functional level, a new C4 assay was developed, which distinguishes C4A from C4B by its binding capacity to amino or hydroxyl groups, respectively. This assay showed high correlation with ELISA and detected crossing over of Rodgers and Chido antigens in 3.2% (8/244) of individuals. The binding capacity of available C4 to its substrates was unaffected in SLE. Our study provides, for the first time, a complete overview of C4 in SLE from genetic variation to binding capacity using a novel test. As this test detects crossing over of Rodgers and Chido antigens, it will allow for more accurate measurement of C4 in future studies.

Keywords: Clinical manifestations; Complement component C4; Disease activity; Gene polymorphisms; Systemic lupus erythematosus.

Publication types

  • Clinical Trial

MeSH terms

  • Adult
  • Codon, Terminator* / genetics
  • Codon, Terminator* / immunology
  • Complement C4a* / genetics
  • Complement C4a* / immunology
  • Complement C4b* / genetics
  • Complement C4b* / immunology
  • Exons / immunology*
  • Female
  • Humans
  • Lupus Erythematosus, Systemic* / genetics
  • Lupus Erythematosus, Systemic* / immunology
  • Male
  • Middle Aged
  • Polymorphism, Genetic*
  • Protein Isoforms / genetics
  • Protein Isoforms / immunology
  • Retrospective Studies

Substances

  • Codon, Terminator
  • Protein Isoforms
  • Complement C4a
  • Complement C4b