Diversities of Complement Components C4A and C4B in Human Populations

The human complement component C4 represents an extraordinary paradigm of innate immune diversity (Yu et al., 2003). There are two classes of proteins: the acidic C4A (hence the "A" designation) and the basic C4B (hence the "B" designation). In each class there are multiple polymorphic variants with different frequencies among human populations. The plasma or serum protein levels of total C4 vary widely (between about 100 and 800 mg/L) among different individuals (Porter, 1983).

The genetics of human C4 is complex. The C4 gene is located at the class III region of the major histocompatibility complex (MHC). There may be one, two, three, or four copies of C4 genes in an MHC and two to seven copies of C4 genes with different combinations of long and short C4A and C4B genes have been found in diploid genomes from different subjects (Yang et al., 1999; Blanchong et al., 2000; Chung et al., 2002a). In about half of the Caucasian populations, there are four copies of C4 genes in an individual's diploid genome (gene dosage). Approximately 40% of normal Caucasians have a heterozygous deficiency of either C4A or C4B. In contrast, about one third of Caucasians have five or six C4 genes. This increase in gene dosage results in increased plasma protein levels of C4A, C4B, or both (Yang et al., 2003). Such genetic diversity of human C4A and C4B genes is one of the major determinants for the quantitative and qualitative variations of the C4A and C4B proteins that may be the result of the selection pressure imposed by the variety of microbes and parasites. A deficiency of C4A or C4B increases the likelihood or severity of viral and bacterial infections, and the susceptibility to autoimmune diseases.

2.1. Dichotomy in Gene Sizes, Polygenes, and RCCX Module Variants

There are two forms of C4 genes: the long gene is 20.6 kb and the short gene is 14.2 kb in size. In Caucasians, 76-77% of the C4 genes are long and 23-24% are short (Blanchong et al., 2000; Yang et al., 2003). Such gene size variation is attributable to the integration of a 6.36-kb endogenous retrovirus, HERV-K(C4), into intron 9 of the long gene (Dangel et al, 1994; Mack et al, 2004). Multiple mutations in HERV-K(C4) probably had knocked out most of its retroviral activities. The configuration of HERV-K(C4) is opposite to that of C4. Therefore, an antisense HERV-K(C4) sequence will be produced whenever a C4 transcript is synthesized from a long gene. The selection advantage for the coexistence of both long and short C4 genes in human populations is uncertain. However, the long gene has become the predominant form among different human races.

The duplication of C4 genes in the MHC is discretely modular, which includes Ser/Thr nuclear protein kinase gene RP1 or RP2 at the 5' end, steroid

21-hydroxylase gene CYP21A or CYP21B, and extracellular matrix protein tenascin-X gene TNXA or TNXB at the 3' end. A duplicated RCCX module is either 32.7 kb or 26.2 kb in size, and usually contains a nonfunctional CYP21A with multiple point mutations, followed by gene fragments TNXA and RP2, and a functional C4A or C4B gene that is either long or short (Figure 7.1, panel A) (Shen et al, 1994). The multiplication of RCCX modules can be clearly depicted by Southern blot analysis of PmeI-digested genomic DNA resolved by pulsed field gel electrophoresis, or by TaqI RFLP (Chung et al., 2002b).

2.2. Diversity of Human C4A and C4B Proteins

About 40 polymorphic protein variants of human C4 have been detected, based on gross differences in electric charge and serologic variations. The most widely used method for C4 phenotyping is immunofixation of EDTA-plasma proteins resolved by high-voltage agarose gel electrophoresis (Figure 7.1C). The most common C4A and C4B allotypes are C4A3 and C4B1, respectively. Other common allotypes for C4A include A2, A4, and A6, and for C4B, B2, B3, and B5. These allotypes exhibit different frequencies among different races or ethnic groups. For example, in the Ohio population, C4B2 has a frequency of 9.4% in Caucasians, but 33.6% in Asian Chinese (Yang et al., 2003; Yang, 2004).

The C4A and C4B isotypes are mainly defined by four specific amino acid residues at positions 1101, 1102, 1105 and 1106, located at the C4d region (Yu et al., 1988). The C4B isotypic residues LSPVIH catalyze the formation of a covalent ester bond between the thioester carbonyl group of activated C4B (C4Bb) and a hydroxyl group from substrates. This transesterification reaction is rapid. However, due to hydrolysis, the half-life of the C4Bb thioester bond is relatively short, less than 1 s (Isenman and Young, 1986; Dodds et al., 1996;). Hence, C4B is important for the propagation of the classical and the mannose-binding lectin (MBL) complement activation pathways, culminating in the rapid and focal formation of the membrane attack complex against microbes.

The C4A isotypic residues PCPVLD probably modulate the reactivity of the thioester bond from the activated C4A (C4Ab) molecule to efficiently form a covalent amide bond with substrates. While the reaction rate of activated C4A toward its targets is about four times slower than that of activated C4B, the iso-typic residues also confer on C4Ab a relatively longer half-life against hydrolysis (~10 s) (Sepp et al., 1993) and a higher affinity for complement receptor CR1 (Gatenby et al, 1990; Gibb et al, 1993; Reilly and Mold, 1997). Therefore, it is thought that C4A is important in the solubilization of immune aggregates, immunoclearance, and opsonization.

The 3-dimensional structure of a human C4d polypeptide from C4A has been solved by X-ray crystallography (van den Elsen et al., 2002). The isotypic residues of C4A are found facing the thioester residues located on the convex side of a barrel-shaped structure. The major determinants of the Rodgers or Chido blood group antigens, which are VDLL for Rg1 and ADLR for Ch1, respectively, at positions 1188-1191 (Yu etal., 1988), are located at the concave surface on the opposite side of the isotype thioester residues (van den Elsen et al., 2002).

A. The MHC complement gene cluster and RCCX length variants

<- Telomeric

Centromeric -> ■ CREB-BP---Notch4---DRB---HLA class II

0 0

Post a comment