Physical linkage and orientation of the human complement C8α and C8β genes on chromosome 1p32

Human C8 is one of five components (C5b, C6, C7, C8, C9) of the cytolytic C5b-9 complex of complement. It consists of three nonidentical subunits (C8α, C8β, C8γ), which are encoded in separate genes. Genetic linkage and chromosomal localization studies previously established that C8α and C8β are closely linked on chromosome 1p32. In this study, clones with inserts containing genes for both C8α and C8β were isolated from a yeast artificial chromosome (YAC) human genomic DNA library and characterized in an effort to determine intergenic distance and orientation. One clone with a ∼330-kb insert yielded restriction digest patterns for C8α and C8β that agreed with those obtained previously from digests of human genomic DNA, thereby confirming the presence of intact copies of both genes. A second clone with a ∼280-kb insert yielded similar results; however, it was truncated at the 5′ end of the C8α gene. Restriction digests of both clones were subjected to PFGE and Southern blot analysis using probes specific for the terminal exons of C8α (exons 1 and 11) and C8β (exons 1 and 12). Results indicate the genes are physically linked (< 23 kb) and in a 3′– 3′ orientation. This is the same orientation as the ancestrally related C6 and C7 genes, which are also physically linked on chromosome 5p13.