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
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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

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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)

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Step 3 - Class Switching
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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.