For the heavy-chain, there is one additional complication. The heavy-chain V region (VH) is encoded in three gene segments, rather than two. In addition to the V and J gene segments (denoted VH and JH to distinguish them from the light-chain VL and JL), the heavy chain uses a third gene segment called the diversity or DH gene segment, which lies between the VH and JH gene segments. The recombination process that generates a complete heavy-chain V region is shown in Fig. 5.3 (right panel), and occurs in two separate stages. First, a DH gene segment is joined to a JH gene segment; then a VH gene segment rearranges to DJH to make a complete VH-region exon. As with the light-chain genes, RNA splicing joins the assembled V-region sequence to the neighboring C-region gene.
Fig. 5.3 V-region genes are constructed from gene segments. Heavy-chain V regions are constructed from three gene segments Light-chain V-region genes are constructed from two segments (right panel). First, the diversity (D) and J gene segments join, and (center panel). A variable (V) and a joining (J) gene segment in the then the V gene segment joins to the combined DJ sequence, genomic DNA are joined to form a complete light-chain V-region forming a complete VH exon. A heavy-chain C-region gene is exon. Immunoglobulin chains are extracellular proteins, and the encoded by several exons. The C-region exons, together with the V gene segment is preceded by an exon encoding a leader peptide leader sequence, are spliced to the V-domain sequence during (L), which directs the protein into the cell’s secretory pathways processing of the heavy-chain RNA transcript. The leader sequence and is then cleaved. The light-chain C region is encoded in a is removed after translation, and the disulfide bonds that link the separate exon and is joined to the V-region exon by splicing of polypeptide chains are formed. The hinge region is shown in purple. the light-chain RNA to remove the L-to-V and the J-to-C introns.
5-3 Multiple contiguous V gene segments are present at each immunoglobulin locus.
For simplicity we have discussed the formation of a complete V-region sequence as though there were only a single copy of each gene segment. In fact, there are multiple copies of the V, D, and J gene segments in germline DNA. It is the random selection of just one gene segment of each type that produces the great diversity of V regions among immunoglobulins. The numbers of functional gene segments of each type in the human genome, as determined by gene cloning and sequencing, are shown in Fig. 5.4. Not all the gene segments discovered are functional, as some have accumulated mutations that prevent them from encoding a functional protein. Such genes are termed ‘pseudogenes.’ Because there are many V, D, and J gene segments in germline DNA, no single gene segment is essential, resulting in a relatively large number of pseudogenes. Since some of these can undergo rearrangement just like a functional gene segment, a significant proportion of rearrangements incorporate a pseudogene and will thus be nonfunctional.
We saw in Section 4-1 that there are three sets of immunoglobulin chains— the heavy chain, and two equivalent types of light chains, the κ and λ chains. The immunoglobulin gene segments that encode these chains are organized into three clusters or genetic loci—the κ, λ, and heavy-chain loci—each of which can assemble a complete V-region sequence. Each locus is on a different chromosome and is organized slightly differently, as shown for the human loci in Fig. 5.5. At the λ light-chain locus, located on human chromosome 22, a cluster of Vλ gene segments is followed by four (or in some individuals five) sets of Jλ gene segments each linked to a single Cλ gene. In the κ light-chain locus, on chromosome 2, the cluster of Vκ gene segments is followed by a cluster of Jκ gene segments, and then by a single Cκ gene.