Copy number variation analysis of C4A/C4B [2 genes]

The C4 gene is mainly synthesized in the liver as a 200-kDa β1-globulin and is intracellularly processed into three polypeptide chains: α, β, and γ, joined together by disulfide bonds. This gene exists as two highly homologous isotypes (C4A, “acidic”, and C4B, “basic”) encoded by two genes arranged in tandem in chromosome 6, which are a part of a larger genetic module known as RCCX, which includes genes STK19 (RP1), C4A, C4B, CYP21A2, and TNXB. Segment duplications of the RCCX module lead to significant variations in the number of C4 genes. In the general population, the total number of copies of the C4A and C4B genes ranges from 0 to 8.

C4A and C4B isotypes have 90% homology. The structure of the C4 molecule is composed of 14 functional domains: 8 macroglobulin-type domains (MG, MG1-MG8), one link domain, one anaphylatoxin domain (ANA, C4a), the thioester-containing domain (TED), the CUB domain, the anchor domain, and the C345C domain (Isenman, 2018). The C4A domain more efficiently forms a covalent amide bond with antigens containing amine groups, while C4B is prone to forming covalent ester bonds with substrates containing hydroxyl groups. C4A acts as an anaphylatoxin involved in complement control through the clearance of immune complexes.

Generally speaking, complete C4A deficiency is associated with the development of autoimmune diseases, particularly systemic lupus erythematosus (SLE), Sjögren syndrome (SS), and myositis, while C4B deficiency is associated with increased susceptibility to viral and bacterial infections. Complete C4 deficiency is uncommon and is strongly associated with SLE, with approximately 80% of individuals affected with lupus-like diseases.

It is normally common to naturally have two C4A and two C4B copies available, one of each per allele. However, the high predisposition to genetic events causing duplications/deletions can lead to genotypes showing 0 to 8 copies. It is estimated that less than 1% and approximately 2% of the population show complete deficiency of C4A and C4B, respectively. This percentage is four times higher in SLE/SS/myositis patients with 0 copies of C4A and almost twice as high in SLE/SS/myositis patients with one copy of C4A.

The identification of a partial or total C4A deficiency in patients with autoimmune diseases allows determining whether a genetic disorder is involved in the pathology and stratifying patients with this deficiency.

This study is recommended in patients with autoimmune diseases such as SLE, SS, or myositis who show constitutively decreased C4 serum levels with the purpose of ruling out the involvement of C4A in their disease.