In this article we will discuss about the gene rearrangements in light and heavy chain DNAs.
Rearranged k and λ genes of light chain contain gene segments in the order from 5′ to 3′ end- a short leader (L) gene segment, an intron (noncoding sequence), a joined VJ-gene segment, a second intron, and a C-gene segment (Fig. 22.18).
The rearranged light chain sequence is transcribed by RNA polymerase and yields a light chain primary RNA transcript. Intron is removed by RNA processing enzyme. The mRNA upon translation produces the light chain protein.
Two separate rearrangement events occur within the variable region before the generation of a functional immunoglobulin heavy chain gene. A D-gene segment first joins to a J-segment and form a DJ-segment. Therefore, the DJ-segment moves and joins a V-segment. Thus, a VDJ unit is formed which encodes complete variable region (Fig. 22.19).
Thus, the rearranged heavy chain contains the gene subunits in the following sequence starting from end -a short L segment, an intron, VDJ-segment, second intron, and a series of C-gene segments. A short distance upstream from each heavy chain, a leader sequence at promoter gene is located. After gene rearrangement RNA polymerase binds to promotor sequence and transcribes the entire heavy chain gene.
By differential processing immature B-cells express IgM alone, and mature cells co-express IgM and IgD. When an antigen stimulates the mature B-cells, additional rearrangement of their heavy chain C-gene segment can occur resulting in expression of different isotypes of antibodies (IgG, IgA and IgE) by some antigens.
Antibody Diversity:
Tonegawa (1983) published an excellent paper on somatic generation of antibody diversity. Various mechanisms are involved in generating antibody diversity. These mechanisms can generate to about 108 possible combinations of antibodies. This results from random joining of multiple V, J and D germ-line gene segments and random association of a given heavy chain and light chain in a given cell.
For example, any of the 300 – 1,000 V-gene segments of heavy chain can combine with any of the 13 D segments and any of 4 J segments. The possibility of generating the enormous amount of diversity with these combinations would be as 1.6 x 104 (300 × 13 × 4 = 1.6 × 104) (Table 22.4).
Similarly 300 V-gene segments of K-light chain randomly can combine with 4 J segments of the same, and can generate 1.2 × 103 possible combinations (300 × 4 = 1.2 × 103). The combinational diversity of the δ-light chain DNA is much less (2 × 3 = 6) as compared to K-light chain DNA.
Table 22.4 : Antibody diversity in mouse.
In the beginning both Cµ and Cδ gene segments are transcribed. Polyadenylation and RNA splicing delete the introns, and process the primary RNA transcript into mRNA which encodes either Cµ or Cδ. The two molecules of mRNA (one for Cµ and other for Cδ) are translated to yield leader polypeptide. Later on leader polypeptide of the nascent polypeptide is cleared and the complete µ and δ chains are produced.
Leader (1982) has postulated the following stages in antibody formation:
(i) Firstly, synthesis of A. heavy chain in the pre-B- cells, thereafter its joining with specific variable region,
(ii) Association of light chains and 8-chains, and production of complete antibodies of the IgM and IgD classes,
(iii) Concurrent appearance of both the IgM and IgD antibodies on the surface of B-cell,
(iv) Attachment of the antigen to a receptor on the surface of B-cells,
(v) Cell proliferation (with the selected Ig molecules) and production of a clone of B-cells producing specific antibodies, and
(vi) Disappearance of the IgD and IgM from the cell surface and the secretion by the cell of IgM, IgG, IgE or IgA molecules.
Differential RNA processing of Ig heavy chain primary transcript produces either membrane bound antibody or secreted antibody.