Sampling the Herd: When is Enough

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Lecture 12 October 1, 2004 Antibody Gene
Rearrangement
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Overview

• Models for antigenic diversity
• Multigene organization
• DNA rearrangements
• Antibody diversity

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Genetic Basis For Antibody Diversity

• Germ-line theory- All the combinations were
  inherited- millions of different genes
• Somatic-variation theory- Small number of genes-
  diversity generated by mutation or recombination

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Genetic Basis For Antibody Diversity

• Problem- Stability in the Constant region and
  diversity in the Variable region
• Problem- Identical variable regions with both g
  and m heavy chain constant regions

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Genetic Basis For Antibody Diversity

• Two gene model- one gene for variable region and
  one gene for constant region for either heavy or
  light immunoglobulin chain
• Recombination to generate the single gene
  containing variable and constant regions

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Multigene organization

• Exist on three different chromosomes
• Two light chains (k, l) are on two chromosomes
• Heavy chain on a separate chromosome

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Multigene organization

• Four groups of gene segments make up the
  immunoglobulin gene V variable, J joining, C
  constant, D Diversity
• Light chain k and l chains V, J and C
• Heavy chain V, J, D and C

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Genetic Organization of light and heavy chains
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Variable Chain Rearrangements
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Heavy Chain Rearrangements
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Recombination Signal Sequences (RSS)
Two-turn RSS (23 bases two turns of DNA
helix) must join a one-turn RSS (12 bases one
turn of DNA helix)
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B cell Rearrangements- Studs or Duds??

• Studs (Productive)
• Rearrangements are imprecise -to be expressed
  must be in frame
• Duds
• Out of frame
• Pseudogenes
• Less that 1 of 3 in both VL-JL and VH-DHJH are
  productive lt1/9 (11) of pre-B-cells mature
• Second chance-two alleles if one strikes out,
  the second may productively rearrange

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Reciprocal chromosomal translocation
Rearrangements that are Disasters B cell Leukemia
(Myeloma)
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Allelic Exclusion

• Only one of the two chromosomes is expressed
  (either maternal or paternal)

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Rearrangements and Allelic Exclusion

• There is an order first heavy chain
  rearrangement, second kappa and if no kappa than
  lambda

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Generation of Antibody Diversity

• Seven Mechanisms Total possible gt1010 different
  antibody combining sites
• Multiple germ-line segments-human 51 VH ,
  27 D, 6 JH, 40 Vk, 5 Jk, 31 Vl, 4 Jl
• Combinational V-J and V-D-J
• Humans Heavy chain (51 X 27 X 68262)
• Light chain k (40 X 5200)
• Light chain l (31 X 4124)

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Generation of Antibody Diversity

• Seven Mechanisms Total possible gt1010 different
  antibody combining sites
• Junctional Flexibility
• P-region nucleotide addition (P-addition)
• N-region nucleotide addition (N-addition)

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Generation of Antibody Diversity

• Seven Mechanisms Total possible gt1010 different
  antibody combining sites
• Somatic Hypermutation Mutation rate in variable
  regions 10-3 vs 10-8 in other genes
• Combinational Association of Heavy and Light
  Chains
• 8262 heavy X 320 light chain 2,644,240

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