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

- 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.