Epigenetic control of Gene Regulation

1. 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

2. 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

3. 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.

4. Mechanism of transcriptional inactivation by DNA methylation H3 K9 key regulator in gene silencing

5. 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

6. 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

7. 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

8. 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

9. 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

10. 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)

11. Step 3 - Class Switching

12. 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 secretion to salive, tears, milk, respiratory and intestinal secretions. - IgE - Bind Fc receptors on mast cells and basophils causing secretion of cytokines and histamine.