A new paradigm in human genetics is the presence of frequent inter-individual variations in copy-number (CNV) of specific genomic DNA segments. One such CNV locus encodes an immune effector protein, complement component C4. Located in the major histocompatibility complex (MHC), human C4 is a constituent of the RP-C4-CYP21-TNX module that exhibits a copy-number variation ranging from 2-10 per diploid genome. C4 CNV leads to parallel quantitative and qualitative variations of C4 proteins. The polymorphic C4 protein can be classified into two isotypes: the acidic C4A and the basic C4B. This dissertation focuses on investigating the quantitative and qualitative diversities of C4 in human systemic lupus erythematosus (SLE).
The molecular defects of two North African patients with complete C4 deficiency who developed SLE were determined. These cases underscore the importance of C4 protein in the protection against SLE.
To investigate if C4 CNV contributes to differential SLE risk, we developed specific and sensitive quantitative real-time PCR assays to interrogate the copy-numbers of C4A and C4B. Large cohorts of SLE patients and race-matched controls of European, African and Asian ancestries were studied. The mean total C4 copy-numbers in SLE were consistently lower compared to those in controls. Such decrease was mainly due to homozygous and heterozygous deficiencies of C4A.
We further genotyped C4 CNV and its closely linked DRB1 gene in the MHC in over 1,000 Caucasian subjects. We obtained evidence for that lower C4A copy-number and HLA-DR3 are overlapping but can be independent risk factors for SLE in Caucasians.
The phenotypic serial changes of serum complement proteins C3 and C4, and RBC-bound C4d were characterized in a pilot cohort of SLE patients with defined C4 genotypes. Three distinct C4 protein profiles corresponding to different degrees of disease presentations were observed.
In conclusion, low copy-numbers of C4A is a risk factor for SLE among different races. Elucidating C4 genotypes and classifying C4 protein profiles may be beneficial for diagnosis and management of SLE.