Reverse Genetics GenotypeDriven

1
Reverse Genetics(Genotype-Driven)

• Bottom ? up approach (i.e., from genotype to
  phenotype) to determine gene effects
• Transgenic - inserting foreign DNA into an
  animal
• Huntington
• Alzheimers
• ALS
• Knock-out (null mutant), Knock-in gene
  expression is disabled (knock-out) or enhanced
  (knock-in)
• Original model was Lesch-Nyhan (hprt)
• There are knock-out models of most genes we have
  discussed -
• Over expression of DRD2 ? reduced alcohol
  consumption
• 5-HTT knockout ? increased alcohol sensitivity
  and reduced alcohol consumption

2
Summary

• Classical methods for establishing heritability
• Inbred strains
• Selective breeding
• Genetic Correlation
• Correlated response to selection
• Inbred strain correlations
• Component Phenotypes
• Preference and Sensitivity are inversely
  associated
• Metabolism assoc with preference but not
  sensitivity
• Withdrawal susceptibility largely a genetically
  independent phenotype

3
Summary

• Forward Genetic Approaches (phenotype-driven)
• QTL Analysis ? Candidate genes candidate
  regions
• Mutation Screens
• Microarray analyses
• Reverse Genetic Approaches (genotype-driven)
• Transgenics
• Knockouts/Knockins

4
Behavioral GeneticsTopic 13

• ALDH

5
Flushing Response

• Dysphoric effects that occur w/i 15 minutes of
  drinking
• Heart palpitation
• Facial reddening
• Nausea, dizziness
• There are large ethnic group differences in rate
  of flushing metabolic not cultural

6
Inherited ALDH2 Deficiency
NAD
NAD
Alcohol
Acetaldehyde
Acetate
ALDH2 Mutation
ADH
ALDH
7
ALDH2 Deficiency

• Mutation (ALDH22 v wild-type ALDH21) is a
  single nucleotide change in exon 12 that results
  in a glu/lys exchange in position 487
• Deficient form of the enzyme carried by approx
  50 of Northern East Asians but only 2 of
  alcoholics from those regions

8
ALDH2 Deficiency Among Japanese Alcoholics
(Higuchi et al. 1994)
Heterozygote Frequency
9
Psychology 5-137

• Topic 14
• Conclusion

10
Methodologies

• Mendelian (single-gene) inheritance
• PKU, Huntingtons, CAH
• Chromosomal anomalies/Structural Variation
• Down Syndrome, Williams Syndrome, VCFS
• Twin/Adoption Studies
• Complementary methodologies
• Biometric Analysis
• Variance component estimation
• Gene Identification in Humans
• Linkage, Association, Linkage Disequilibrium
• Animal Methods
• Selection, Inbred Strains,
• Forward Genetics (QTL, mutagenesis, microarray),
  Reverse Genetic (transgenics, knock-outs)

11
Nature of Genetic Influence

• Heritability
• Virtually all behavioral traits are in part
  heritable
• Common heritable factors may account for
  correlations among disorders
• Heritability estimates are approximations
• Heritability is neither an index of immutability
  nor an explanation of behavior

12
Principled Critiques of BG

• For psychologists, as well as for medical
  researchers, the purpose of identifying
  undesirable predispositions of individuals should
  be to devise more effective health-promoting
  interventions, not to discourage such attempts on
  the supposition that these predispositions are
  genetically based and therefore intractable.
  
• D. Baumrind (1993)

13
Burmeister, M. et al. (2008). Psychiatric
genetics Progress amid controversy. Nature
Reviews Genetics. 9 527-540.
14
Nature of Genetic Influence

• Genes and the Human Genome Project
• 20,000 to 25,000 genes and 11,000,000 SNPs, and
  large amount of structural variation (e.g., CNVs)
  
• Gene identification has been difficult
• Mendelian vs. oligogenic vs. polygenic
• Positional cloning strategy has produced few
  successes
• But there are some leads
• Prospects for the Future
• GWAS under the CDCV model
• Will require very large samples
• Pooling of resources across labs
• CNVs for rare mutations?
• Animal experimental approaches

15
What is the likely benefit of finding genes for
human behavior?

• Better understanding of the nature and origins of
  behavior
• Pharmacological interventions based on genotype
  (the 5-HTT example)
• Targeted early prevention (e.g., treating
  unaffected siblings of schizophrenics?)
• What is the significance of small gene effects?
• Familial hypercholesterolimia affects 0.2 and
  acounts for a small portion of heritability led
  to development of statins

16
Merikangas, K.R. Risch, N. (2003). Genomic
priorities and public health. Science, 302
599-601.
The potent effect of pervasive societal changes
on this behavior will far outweigh any possible
benefits of identification of risk genes ... (p.
601)
17
Nature of Environmental Influence

• Familial resemblance is potentially a function of
  both shared genes and shared environment
• Have we overestimated the impact of parents?
• Shared vs. non-shared environmental effects
• Changing balance across development for some
  traits
• SZ, BP, Depression, ADHD
• Gene-environment interplay
• GxE and the diathesis-stress model
• PKU
• MAO-A and aggression
• 5HTTLPR and depression
• G-E correlation
• Ge et al. adoption study
• Epigenesis

18
Final Admonitions

• Dont believe it unless it is replicable
• Single studies are not definitive, require a
  coherent pattern of results
• Study for the final attend review session on
  Tuesday 16 December 10-12, N227 Elliott
• Arrive for the final on time (130-330PM,
  Wednesday 17 December in N119 Elliott Hall), and