How well do you know Immunoglobulin Genetics?

Understanding Immunoglobulin Genetics

• Immunoglobulin genetics is a complex process that involves multiple genes and segments on different chromosomes that help build antibody molecules.

• The focus is on discovering how an antibody is made and put together, particularly the receptors on B cells.

• B cells are one of the three antigen presenting cells (APCs) and are important for antigen recognition.

• The context is the developing B cell in the bone marrow and how it makes functional B cell receptors, namely IgMs and IgDs.

• The study of immunoglobulin genetics is groundbreaking and won Dr. Tonegawa the Nobel Peace Prize in 1987.

• The idea of one gene, one protein breaks down in immunoglobulin genetics as there are multiple genes that come together to make a functional antibody molecule.

• There is both a germline and somatic part to making a functional antibody molecule.

• Dr. Burnett and Dr. Dreyer proposed the heavy and light chains genes that are encoded in two separate segments in the germline genome.

• The same constant region can be associated with many different variable regions, meaning different specificities for an antibody molecule.

• The same variable region can associate with many constant regions, meaning the same ideotype can be on an IgG and IgM.

• Variable region and constant region genes are coded for separately, and they are essentially cassette genes that make up the parts of an antibody molecule.

• An individual can produce Ig-specific receptors capable of recognizing more than 10^9 different epitopes.Overview of B Cell Receptor Gene Rearrangement

• The diversity of B cell receptors is generated through somatic recombination.

• Immunoglobulin light and heavy chain gene cassettes are located on different chromosomes, with light chains on chromosome 2 and 22, and heavy chains on chromosome 14.

• Allelic exclusion ensures that only one parent's genes are expressed at a time, providing a backup mechanism for functional receptor creation.

• B cell receptors have hypervariable regions where antigens bind, which are formed by the complementary determining regions of the heavy and light chains.

• There are roughly 40 variable region segments for light chains on chromosome 2, and up to 300 variable region segments for heavy chains on chromosome 14.

• Light chains have one constant region gene segment for kappa, and seven for lambda due to the presence of subtypes.

• B cell receptor gene rearrangement occurs through VJ recombination for light chains, and VDJ recombination for heavy chains.

• B cell maturation occurs in stages, starting with stem cells and progressing to mature B cells in the periphery.

• Rag1 and Rag2 enzymes drive the recombination events necessary for B cell receptor creation.

• Promoters and enhancers are necessary for transcriptional activity, and must be in close proximity to each other.

• Light chain gene reorganization occurs in the germ line, and involves bringing together variable, joining, and junctional regions.

• Light chain gene rearrangement occurs in pre-B cell stages, and involves the deletion of non-coding introns and exons.Mechanism of Antibody Gene Rearrangement

• Antibody gene rearrangement occurs in the nucleoplasm of the nucleus.

• The segments between the promoter of V and J will be cut out, leaving V2 J4 in the light chain.

• Promoters are located five prime of the variable segment, and enhancers are located in the introns between the junction of the constant segments.

• Immunoglobulin synthesis is only detected after VJ or VDJ rearrangement.

• Heavy chain genes may undergo further gene rearrangement for immunoglobulin class switching.

• All possible combinations of gene rearrangements take place in the body, resulting in the diversity of our B-cell repertoire.

• The variable region of the antibody molecule is the VJ segment, and the constant region is the C part.

• There are about a thousand variable gene cassettes, 15 diversity segments, and five junctional segments in the heavy chain gene.

• The order of gene expression for heavy chains is DJ rearrangement first, followed by VDJ rearrangement.

• The nucleotide sequences that provide the amino acids for the hypervariable regions on the heavy chain are unique and come from the diversity region.

• The mechanism of antibody gene rearrangement involves recognition of recombination signal sequences by the RAG1 and RAG2 enzyme complexes.

• There are about 10 to 11 steps involved in the mechanism of antibody gene rearrangement, from recognition of the recombination signal sequences to ligation and repair of the gene.Mechanisms of DNA Rearrangement in the Immune System

• Recombination signal sequences (RSS) are signals that tell enzymes where to recombine DNA in the immune system.

• RSS consist of two types of sequences: heptamers and nonamers, with heptamers having seven nuclear type sequences and nonamers having nine.

• The important part of RSS is the stretch of base pairs between the heptamer and nonamer sequences, which tells enzymes when to start and stop cutting.

• The order of the signal sequences is important for the enzymes to know when to cut, with a two-turn signal sequence followed by a one-turn signal sequence telling them to start and stop.

• The 12-23 rule refers to the fact that a one-turn signal sequence can only react with a two-turn signal sequence and vice versa.

• RAG1 and RAG2 enzymes are encoded by the RAG genes and are responsible for the rearrangement of DNA in the immune system.

• The cassettes in the immune system are marked by RSS and are the substrates for recombination, allowing them to be fused together.

• There are additional mechanisms of diversity in the immune system, such as nucleotides being added to junctional flexibility points.

• Hypervariable regions (HVRs) are regions in the immune system with high variability in amino acids.

• CDR1 and CDR2 have moderate variability, while CDR3 has the highest variability.

• CDRs are the amino acids that come from the junctional flexibility points and are responsible for the diversity in the immune system.

• Deficiencies in RAG1 and RAG2 enzymes can lead to combined immunodeficiency disorders, which can be life-threatening.