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

Area of Science:

• Immunogenetics
• Rheumatology
• Molecular Biology

Background:

• Complement component C4 (C4) genetic variations are strongly associated with systemic lupus erythematosus (SLE).
• Understanding C4's role requires examining its genetic, protein, and functional aspects in SLE patients.

Purpose of the Study:

• To comprehensively analyze C4 gene copy number variation, mutations, protein levels, and functional activity in SLE patients.
• To investigate the association between C4 genetic factors and clinical manifestations of SLE.
• To develop and validate a novel functional assay for C4 isotypes.

Main Methods:

• Multiplex ligation-dependent probe amplification (MLPA) for C4 gene copy number (GCN) variation and insertions.
• Sanger sequencing to identify novel C4 gene mutations.
• Enzyme-linked immunosorbent assay (ELISA) for C4A and C4B protein quantification.
• A newly developed functional C4 assay distinguishing C4A and C4B binding capacities.

Main Results:

• Low C4A GCN (≪2) was associated with increased SLE susceptibility and serositis.
• A novel silencing mutation in C4A leading to a premature stop codon was identified.
• SLE patients exhibited significantly lower C4A and C4B protein concentrations compared to controls.
• The novel functional C4 assay correlated well with ELISA and detected antigen crossing over in 3.2% of individuals; C4 binding capacity was unaffected in SLE.

Conclusions:

• This study provides a comprehensive analysis of C4 in SLE, linking genetic variations to protein levels and function.
• Low C4A GCN is a significant genetic risk factor for SLE, particularly associated with serositis.
• A novel functional C4 assay offers improved accuracy for measuring C4, aiding future SLE research.