Rearrangement The normal process by which antibodies and T cell receptors are made

So What Is a Rearrangement? Rearrangement is a term used to describe a recombination of DNA within a single chromosome.

Rearrangement Remember that every immunoglobulin molecule and every TCR contains both a variable region and a constant region and that the variable region is the part of the molecule that interacts with the antigen.

TCR = T cell receptor

Immunoglobulin = Antibody = B cell receptor (BCR)

What is meant by the term rearrangement in relation to DNA? Antigen independent maturation of B and T cells to immunocompetent B and T cells that express receptors for antigen recognition involves a rearrangement of the cell’s germline DNA.  Germline DNA = DNA that is an exact sequence replicate of the genome present at fertilization of egg by sperm. It has not been rearranged.  Most cells in the body maintain copies of germline DNA.

Rearrangement continued The genes that code for the different chains of the B and T cell receptors include a large number of exons. The exons that code for the variable regions of the immunoglobulin molecules and TCRs are called the variable (V) segments, diversity (D) segments, and junctional (J) segments, and those that code for the constant regions are called the C segments.

Shematic of germline genes for TCR alpha and beta and immunoglobulin heavy and kappa light chains Note how large a number “n” is!

Rearrangement continued For the immunoglobulin heavy chain, there are different V regions, 30 different D regions, 6 different J regions, and 9 different C regions. For the TCR beta chain, there are different V regions, 2 different D regions, 13 different J regions, and 2 different C regions. Note: these numbers are more current than those on the preceding schematic.

Change V1 to Vn in the right hand column.

Rearrangement continued The first step in the rearrangement process involves the the apposition of one D segment of the gene with one J segment by deletion of all intervening sequences, both coding and non-coding (a DJ rearrangement). In a similar process a V segment becomes apposed to the DJ rearrangement to form a VDJ rearrangement.

Rearrangement continued After rearrangement of the DNA, transcription occurs. Subsequent splicing of the primary transcript with deletion of noncoding sequences results in apposition of VDJ to C coding regions to form a mature VDJC mRNA which can be translated into an immunoglobulin heavy chain or TCR.

Gene Rearrangements to make an antibody Remember: this rearrangement is happening in the gene – not during RNA processing. Can you list multiple inaccuracies in this animation?

Rearrangement continued Once the productive rearrangement occurs on one chromosome, an active protein chain is synthesized and it provides a trigger to prevent rearrangement of the other allele (allelic exclusion). Thus each B or T cell expresses only one immunoglobulin or TCR, respectively.

Rearrangement continued This complex process of rearrangement allows for tremendous diversity of both the cell-mediated and humoral arms of the immune system, and the ability to respond to a wide array of antigens.

Review: How do translocation and rearrangement differ? Translocation is the recombination of DNA between two different chromosomes. Rearrangement is the recombination of DNA within a single chromosome (and within a single gene). Translocations are abnormal. Rearrangements of genes for immunoglobulins and TCRs are normal.