epigenetic control of gene regulation epigenetic vs genetic inheritance genetic inheritance due to...
TRANSCRIPT
Epigenetic control of Gene Regulation
• Epigenetic vs genetic inheritance Genetic inheritance due to differences in DNA
sequence Epigenetic inheritance not due to differences
in DNA sequece
Epigenetic control of Gene Regulation
• DNA methylation is key to epigenetic control of gene regulation Methylated DNA typically associated with inactive
chromatin/Genes Unmethylated DNA associated with transcribed
DNA/Genes
• DNA methylation may play a role as a defense mechanism againts transposable elements but certainly plays a regulatory role in gene regulation Some but not all genes contain very high densities of
CpG methylation sites specifically in promoter regions
Inheritance of Methylation status
-Methylation occurs at CpG motifs in mammals-Cytosine methyltransferases have preference for hemi-methylated DNA and methylate methylated opposite strand
- results in inheritance of methylation status.
Mechanism of transcriptional inactivation by DNA methylation
H3 K9 key regulator in gene silencing
Histone modification
- Histone acetylation - generally associated with promoter activation (histone deacetyleses (HDACs) inhibit transcription
- Neutralizes basic charges on lysines and arginine residues - relaxes nucleosome- Allows direct binding of activating proteins to promoter bound histones
- Histone methylation- Arginine methylation associated with promoter activation- Lysine methylation associated with promoter inactivation
Inheritance of Suppressed Promoters
• Maintains suppressed gene expression as cells divide
• Involved in X inactivation Dosage compensation
• Imprinting occurs in early embryo and is random with respect to Xp or Xm inactivation
Female mammals are therefore mosaics• Calico cat
Gene Regulation Through Somatic Recombination
• Immune Function (Ig and TCR) Generates complexity for recognition of diverse
antigens
B-cells• Heavy Chain (H-chain locus)• Light Chain (lambda and Kappa loci)
T-cells• Alpha and Beta loci• Gamma and Delta loci (expressed on small fraction of T cells
Structure of Ig Heavy Chain Locus
- Differential recombination of individual V, D and J loci generate initial diversity in Heavy chain gene for individual cell.
- Similar recombination occurs in either kappa or lambda light chain loci- Resulting heterodimers of H and L provide wide array of diverse structural motifs for diverse antigen recognition
Step 1 - Variable region Recombination
- Recombination signaling sequences flank each V, D, and J segment which specify recombination- VDJ as well as VJ recombination can occur- Results in unique variable region which splices to M constant region (produces membrane IgM)
(Immature naïve B cell)- Mature naïve B cell expresses heavy chains with M as well as D constant region
- Both of these are membrane bound- Antigen recognition leads to production of secreted form of IgD which provide initial immune response
Step 2 - Somatic Mutation
• Engagement of IgM with antigen causes Conversion to secreted form of IgM Proliferation of immature B cell Somatic mutation of variable regions
• Cells with higher affinity receptors stimulated preferentially by antigen to further proliferate and undergo class switching (step 3)
Step 3 - Class Switching
Step 3 - Class Switching
- Further recombination to G, A, or E constant regions generates secretory antibodies with specificity to same antigen but with different immune functions
- IgG - binds complement and binds Fc receptors on macrophages and neutrophils- IgA - constant region recognized by Fc receptor on secretory epithelial cells for secretionto salive, tears, milk, respiratory and intestinal secretions.- IgE - Bind Fc receptors on mast cells and basophils causing secretion of cytokines and histamine.