VDJ recombination is a mechanism of DNA recombination used by humans and other vertebrates for protection against attacks by bacterial, viral, and parasitic invaders. It is a site-specific recombination reaction that generates diverse T-cell receptor (TCR) and immunoglobulin (Ig) molecules that are necessary for the recognition of the greatly diverse foreign antigens. The T cell receptor or TCR is responsible for recognizing antigen bound to Major histocompatibility complex (MHC). ... Schematic of antibody binding to an antigen An antibody is a protein complex used by the immune system to identify and neutralize foreign objects like bacteria and viruses. ...
Background
Antibody molecules are encoded by three types of genes.
Light chain kappa (κ)gene – located on chromosome 2
Light chain lambda (λ)gene – located chromosome 22
The genes for the variable regions contain three distinct types of segments (Heavy-chain): Schematic of antibody binding to an antigen An antibody is a protein used by the immune system to identify and neutralize foreign objects like bacteria and viruses. ...
Variable (V) – 200 region genes
Diversity (D) – 12 region genes
Joining (J) – 4 region genes
By the mechanism of DNA rearrangement of these regional genes it is possible to generate a antibody repertoire >107 possible combinations. (200 x 12 x 4 = 9600, with addition to the 1000 possible light chain combinations)
Mechanism
Recognition of the these regional genes (V,D,J) is possible by DNA segment substrates flanking these regional genes called Recombination Signal Sequences (RSSs). RSSs are composed of a heptamer (7 base pairs) and a nonamer (9 base pairs). The heptamer is next to the coding region, which is then followed by the nonamer. The RSSs are present on the 3’ side of a V region and the 5’ side of the J region. These are the sides that will be involved in the joining
The initial steps of VDJ recombination are carried out by critical enzymes, (lymphocyte specific) called RAG1 and RAG2 (recombination activation genes). These enzymes are needed as they associate with each other and recognize the initial RSS sequences and inducing cleavage at the RSS sequences. This cleavage only takes place on one strand on DNA, which leads to a nucleotide attack and creation of a hairpin loop.
First, the joining a specific D segment to a specific J segment.
This removes the intervening DNA between a particular D segment and a particular J segment
Second, the joining of a specific V segment to the newly created DJ combination.
Those removes the intervening DNA between the particular V segment and the new DJ combination, and brings the V, D, J gene segments together in translations reading frame at the DNA level.
Lastly, One V to J rearrangement occurs in either the κ or λ light-chain gene.
The V and J segments of the light chains are combined with a few of the 1000 base pairs separating the J and the C regions which upon transcription, a poly-A tail is added.
References
Leland H Hartwell, Leroy Hood, Michael L. Goldberg, Ann E. Reynolds, Lee M. Silver, Ruth C. Veres (copyright 2000). Chapter 24, Evolution at the molecular level; pages 805-807 ISBN 0-07-540923-2
The VDJrecombination process is believed to involve multiple activities, including recognition of the RS, endonucleolytic activity that site-specifically cleaves at the border of RS and adjacent gene segments, potential exonucleolytic and nucleotide-addition activities, polymerase activities, and ligase activity to join the free ends (reviewed by Blackwell and Alt, 1989 and Lewis and Gellert, 1989).
The VDJrecombinations in the lymphocytes of the RAG-2 deficient animal are deficient, and the lymphocytes of the said animal cannot reach maturity, resulting in the SCID phenotype.
The level of VDJrecombination activity measured by this semiquantitative assay (Lieber et al., 1987) in homozygous mutant lines co-transfected with RAG-2 was similar to that observed in the control line (Line 64-3) assayed alone or with co-transfected expression constructs (Table 1).
These enzymes are involved in the first stage of VDJrecombination, the process by which segements of a B cell or T cell's DNA are rearranged to create a new T cell receptor or B cell receptor (and, in the B cell's case, the template for antibodies).
Certain mutations of the RAG-1 or RAG-2 genes prevent VDJrecombination, causing SCID.
Another well-known form of SCID is caused by a defective enzyme, adenosine deaminase (ADA), necessary for the breakdown of purines.
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