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