DNA recombination and cancer Flashcards
The BCR (Ig) constant (C) gene segments each encode
alternative constant regions of the antibody molecule
Different isotypes are generated through a process called
Different isotypes are generated through a process
called Isotype Class Switching (ICS)
• Occurs separately and after the V(D)J recombination
that produced the antigen receptor and after encounter with antigen- occurs in mature B cell after the recognition of the antigen e.g. Sars-Cov-2
• The specificity of the receptor for antigen (depicted in
red) does not change during ICS (depicted in blue)
• The structure of the constant region defines the isotype class
First anti-body secreted
Igm- IgM - secreted as a pentamer (low affinity)
may neutralize the antigen
after this the B cell will receive signal from the T helper cell to change the Isotype
IgM is expressed on immature B cells (the first
BCR). It is later secreted as a pentamer and is
of low affinity lead to complement activation
found in serum
IgG
- secreted as a monomer
- important in neutralizing the virus +toxins
- IgG is expressed as a monomer and has an
important function in neutralizing viruses and
toxins - IgG is found in extracellular fluid and can be found in the fetus
IgE
IgE is expressed as a monomer and has important functions in immunity to helminths and an important role in allergic responses
IgE is found beneath the epithelial surface
• e.g. respiratory tract, GI tract, skin
IgA
IgA is secreted as a monomer and as a dimer
and has important functions in mucosal immunity
• IgA is found in the serum (monomeric) and mucosal
secretions (dimeric)
• e.g. colostrum, lung, gut
• literally pumped into secretions
• 2/3 of all antibody produced is IgA
• ~2g/day (~8 x 1021 molecules)
Question: The BCR (Ig) constant (C) region gene segments encode alternative
constant regions of the antibody, which is important for:
A. Antigen recognition by each antibody specificity
B. Combinatorial diversity in the antibody repertoire
C. Junctional diversity in the antibody repertoire
D. Functional diversity for each antibody specificity
Functional diversity for each antibody specificity
what happens when the mature B cell first encounter an antigen?
No signal from helper T cell
In an IgM-secreting B cell the primary transcript of the rearranged VDJ heavy chain gene is spliced into the m
messenger RNA to produce the m heavy chain and IgM antibody- - complement activation
What signals the B cell to undergo isotypes class switch recombination
T follicular helper (Tfh) cells signal B cells to undergo
isotype class switch recombination (CSR)
Signals from Tfh cells may induce recombination of switch (S) regions
– CD40L
– cytokines
How t-helper cell is activated and direct the b cell to CSR
The cells of the innate system- dendritic cell recognize the antigen and the type of pathogen.e.g. virus bacteria
and present it on its MHC receptor to the t helper cell
the t helper cell with the complementary receptor binds to the dendritic cells and form a complex that activated the t helper cell
the dendritic cell signals the t helper cell what type of pathogen it is and what type of isotype needs to be produced
the t helper cell binds to the b cell through CD40-CD40 ligand
The mechanism of isotype class switch recombination (CSR) differs from V(D)J recombination
IgM producing B cell has rearranged VDJ heavy chain gene adjacent to the constant
region genes (Cmu)
• Signals from Tfh cells (CD40L and cytokines)
induce recombination of switch (S) regions by
driving expression of activation induced
deaminase (AID) in B cells
• AID is an enzyme that changes the nucleotides
in the S region so they can be cleaved and
rejoined to downstream S regions
• Rearranged VDJ is moved close to a C gene
downstream of Cm
When the heavy chain gene is subsequently
transcribed, the VDJ exon is spliced onto the
exons of the downstream C gene
• Importantly, although the C region changes
through this switching, the VDJ region does not
• The specificity of the antibody is thus preserved
Question: Class Switch Recombination (CSR) occurs:
A. In immature B cells during development
B. At the same time as V(D)J recombination
C. In mature B cells during antigen encounter
D. In mature B cells during encounter with T follicular helper (Tfh) cells
D. In mature B cells during encounter with T follicular helper (Tfh) cells
the T follicular helper (Tfh) cells provides key signal to the antigen-experience B cell to promote CSR at the appropriate switch region
Why B cells are more susceptible to error and mutation
because more somatic recombination occurs in B cell as compared to T cell
B cell
- V (D) J Recombination in heavy chain and two light chain locus,
- CSR
T cell
- V(D)J recombination in alpha and beta chain
oth V(D)J and CSR are initiated by
double-strand breaks (DSBs) and repaired by classical NHEJ
• Repair of DSBs occurs through classical non-homologous end joining (C-NHEJ)
• C-NEHJ essential for promoting chromosomal
integrity and suppressing translocations
what leads to alternative end joining in CSR and V(D)J recombination
Disruption of DNA binding and ligation mechanisms has been proposed to contribute to alternative end joining
Without c-NEHJ DSBs can be joined by alternative end-joining (A-EJ) - ku protein not available or problems with ligase
• A-EJ can contribute to translocation
• Misrepair of DSBs introduced during V(D)J or CSR can promote oncogenic translocations
A-EJ during V(D)J or CSR can cause chromosomal translocations
Translocations can occur in both B and T lymphocytes
during V(D)J recombination
• Translocations can also occur in B cells during Class Switch Recombination
• When transcriptional regulatory elements within the lymphocyte receptor loci are translocated to
within close proximity of an oncogene it can lead to
dysregulation of cell cycle and/or differentiation
Question: Somatic recombination in lymphocytes involves V(D)J recombination and
Class Switch Recombination (CSR). This differs in B and T cells because:
A. B cells undergo V(D)J recombination and CSR
B. B cells undergo V(D)J recombination only
C. B cells undergo CSR only
D. T cells undergo CSR only
A. B cells undergo V(D)J recombination and CSR
What are the Three changes occur during tumorigenesis?
1. Immortalisation • Failure to die • Ability to divide indefinitely • Remaining sensitive to normal constraints of growth e.g. contact inhibition 2. Transformation • Failure to remain sensitive to normal constraints for growth e.g. contact inhibition fails 3. Metastasis • Ability to invade normal tissues
(Proto)-oncogenes
• Normal genes • When mutated or dysregulated can cause transformation • Can convert into cancer-promoting oncogene • Over 100 identified – common • Protein products have many activities • Cell signaling • Cell cycle control • Transcription factors • Others
Gene rearrangements and oncogenes
- Some oncogenes are dysregulated by gene rearrangements
- Gene rearrangements, such as chromosomal translocations, can cause:
- Sequence alterations
- Different product
- Dysregulation of expression
- Over-produced
- Produced in the wrong cell
- Produced at the wrong time
- Combinations of above
The c-MYC (proto)-oncogene
• Transcription factor that regulates cell proliferation
• Expression is confined to appropriate time in the
cell cycle
• When the expression pattern is altered, it can result in unrestricted growth of cell
• Involved in many tumours, including B cell
lymphomas
Translocations that occur in B cells can cause lymphoma
• The translocation event causes c-MYC to be driven by the BCR gene regulatory sequence (Ig
enhancer)
• Can occur in immature B cells (RAG-1/2 dependent)
• Can occur in mature B cells (AID dependent)
• This causes large increase in MYC expression
• B cell proliferation is thus dysregulated
• Hyper-proliferation of B cells results in B cell lymphoma
Burkitt’s lymphoma caused by a translocation
that juxtaposes c-MYC and BCR gene
enhancer. Can involve BCR (Ig) heavy or light
chain genes
Burkitt’s lymphoma
G-loops found in c-MYC map to translocation breakpoints
Transcription of c-MYC results in formation of G-loops which contain co-transcriptional RNA:DNA hybrids
• Activation Induced Cytidine Deaminase (AID) binds to G-loops
• G-loops in c-MYC map to the regions associated with translocation break points (arrows)
AID binds specifically to G-loops within transcribed c-MYC and S regions
AID thought to initiate destabilisation of genomic integrity at c-MYC • Destabilisation is thought to promote translocation to the switch (S) region of the BCR (Ig) loci • AID binding area corresponds to the region of c-MYC that undergoes translocation in Burkitt’s lymphoma
Different (proto)-oncogenes translocated
to the BCR (Ig) loci cause lymphoma
Translocation to the BCR (Ig) H loci on chromosome 14 by: • BCL-6 on chromosome 3 leads to diffuse large B cell lymphoma • CCND1/Cyclin D1 on chromosome 11 leads to mantle cell lymphoma • BCL-2 on chromosome 18 leads to follicular lymphoma
Question: Translocations between chromosomes 8 and 14 that occur as a consequence of activation induced cytosine deaminase (AID) binding to G-loops in
c-MYC are most likely to occur:
A. In immature B cells during development
B. At the same time as V(D)J recombination
C. In mature B cells during antigen encounter
D. In mature B cells during encounter with T follicular helper (Tfh) cells
D. In mature B cells during encounter with T follicular helper (Tfh) cells
In mature B cells during encounter with T follicular helper cells in CSR is occuring. G-loops in c-MYC and the BCR (Ig) locus switch region both lead to double strand breaks and the potential for alternative end joining chromosomal translocation.
