Methodological Issues in Molecular Genetic Studies of Mental Disorders (Bearden et al., 2009)

1. Methodological Issues inMolecular Genetic Studiesof Mental Disorders (Bearden et al., 2009)

2. Genomewide investigations: When it’s Easy • Genomewide investigations have found > 100 loci associated with common disorders • Inflammatory bowel disease • Type 2 diabetes • Rheumatoid arthritis • Features of mapped disorders • Objective diagnostic • Example = Type 2 diabetes • Blood glucose levels • Low symptom variability • Clear biological basis

3. Why isn’t that easy for us? • Mental Disorders • Lack biological assays • Phenotypic features assessed by subjective ratings • Diagnosis based on symptom report • Considerable symptom variability across individuals • Why are current diagnostic methods and symptom variability a problem? • Will a dimensional approach be helpful in respect to the symptom variability problem?

4. Genes and Mendel • Mendelian Disorders • Simple dominant / recessive patterns • Punnett Squares • Examples • Sickle-cell anemia, Tay Sachs disease, Cystic fibrosis, Huntington’s • Laws • Segregation • Independent Assortment • Linked genes http://www.youtube.com/watch?v=D1_-mQS_FZ0&feature=related

5. Mental Disorders • More complex • How do we study heritability? • Twin studies • Monozygotic (MZ) vs. Dizygotic (DZ) • Example = Bipolar Disorder • MZ = 60-80% • DZ = < 10%

6. Endophenotype Approach • Involves conducting genetic mapping studies on “intermediate phenotypes,” • Quantifiable characteristics such as brain structure or neurocognitive performance that are hypothesized to be closer to the biology represented by the actions of risk genes than the observable manifestations of psychopathology, i.e., psychiatric symptoms • What does this mean?

7. Endophenotype Approach Advantages • Relatives are presumed carriers but do not meet criteria for the disorder • We currently have no means to identify carriers • Help to clarify carrier status of family members of individuals affected with psychiatric disorders

8. Arguments Against Endophenotype Approach for Mental Disorders • No evidence for Schizophenia • Flint & Munafo (2007) • Problems?

9. Candidate gene studies • Candidate gene studies • Must have hypotheses about trait genes relationship associated with disease • Disadvantages • Depend on validity of hypotheses

10. Endophenotype Friendly DSM-V? • New classification system for psychiatric disorders based on pathophysiologicand etiologic processes rather than on overt symptom clusters (Charney& Babich2002, Hyman 2007, Phillips 2007) • Problems? • How does this relate to the dimensional approach? • If specific genetic markers are found for many psychological disorders, how would psychology change? • What if some disorders could be treated with gene therapy?

11. Mediators and Moderators • In addition to genes, what else contributes to mental disorders? • Diathesis Stress Model • Genes + Stress = Disorder

12. Environmental Factors • Determining which environmental factors to investigate remains daunting • Large sample sizes needed • Schizophrenia = n =1,000,000 • Relates to Power • How can this be reduced?

13. METHODOLOGIES FORIDENTIFYING DISEASESUSCEPTIBILITY VARIANTS • Variants of small effect • Variations in genes that will have a small effect on traits • Variants of large effect • Infrequent variants of relatively large effect may segregate with disease in families; even if non-Mendelian • Studied in small groups • Amish

14. Linkage Studies • Technique first used • Variants of large effect • Identify genetic loci transmitted with a disease phenotype more often than expected by chance or that are shared identically by sets of affected relatives (e.g., siblings) more often than expected by chance • Linkage methodology

15. Linkage studies and Mental Disorders • Unreplicated studies • Egeland et al. 1987 • bipolar disorder localized to chromosome 11p15 among the old order Amish • Baron et al. 1987 • gene to chromosome Xq27-28 among non-Ashkenazi Jews

16. Endophenotype Approach and Linkage Studies • Advantages • Does not matter if affected individuals share specified genes with unaffected individuals • May still exhibit common traits linked to shared genes • Disadvantages • Power still an issue

17. Linkage Studies Findings • Dysbindin at 6p22.3, and a region on chromosome 1 containing the genes DISC1 and DISC2 (disrupted in schizophrenia 1 and 2), located on chromosome1q42.1-1q42.2 (Sklar 2002) • Dysbindin = implicated in synaptic structure and signaling • DISC 1= mutations show impairments in a wide variety of tests, including learning, memory, and sociability • Dysbindin and Schizophrenia

18. Association Studies • Are given genetic variants more frequent in affected individuals than in controls • More power than linkage • Do not depend on detection or transmission of genetic variants with a phenotype in a family • Association Study Methodology

19. Genomewide association studies (GWAS) • Unbiased method for the identification of multiple susceptibility genes for complex diseases • Advantages • Can scan for common variations across entire genome • Single-nucleotide Polymorphism (SNP) • Power is relative • It increases with the effect size of the causative allele and sample size • Differs between ethnicities • Need 2x as many for African population

20. Genomewide association studies (GWAS) • Disadvantages • Massive number of statistical tests • alpha build up = > false-positive results • Typical p values = .0000001 • Replication required • Countered with multi-stage experiment • Why would this help? • Reduce amount of comparisons

21. GWAS and Medical Conditions • In past 2 year100 loci for approximately 40 common diseases have been identified and replicated. • Including complex disorders • Type II diabetes and obesity

22. GWAS and Mental Disorder • No unequivocally significant or replicated results • Even with the large sample sizes • >15,000 individuals • Effect sizes too small • What could be contributing to the hypothesized weak relationships?

23. Copy Number Variants (CNV) • Genomic variants involving large DNA segments • Comprise12% of genome • May have impact on complex diseases • Most studies are non-specific • Look at # of CNV, not specific CNV

24. CNV and Mental Disorders • (Walsh et al., 2008) • CNVs associated with schizophrenia in up to 10% of nonfamilial cases • Arose spontaneously • Other studies • Novel deletions and duplications in patients with schizophrenia and other neuropsychiatric disorders • Autism

25. CNV Findings Continued • Regions for which CNVs have been implicated in one psychiatric disorder (e.g.,schizophrenia) may also be involved in other disorders (e.g., autism) • 16p11 chromosomal • How does this relate to the Wan et. al (2008) article? • Multiple phenotypes from a single mutation • What are the implications for the DSM classification system?

26. Future Directions • Using transcription levels as phenotypes • Expression quantitative trait loci (eQTL) analysis: • Genetic mapping techniques used to identify specific genes affecting a quantitative trait(e.g., neuroticism) • loci regulating mRNA levels • Help locate candidate genes or association regions • Statistical Approaches for rare variant • Collapse genotypes across variants and applying a univariate test • Less comparisons = more power

27. Animals • Rodents • Large databases in place • Easy to create specific genotypes and phenotypes through breeding • Non-human primates • More similar genetically • Can create colonies that model human diseases • More invasive tests can be conducted • Canines • Breeds already contain unique combinations of genetic material

28. Conclusion • Locating genes associated with mental disorders is much more complicated than locating genes for diseases • Even those with complex inheritance patterns • Why is this? • If specific gene combinations are never found for mental disorders, what will that mean, was the process worth it? • Implications for treatment • Methodology is improving and our conceptualization of mental disorders are evolving

29. Schizophrenia • http://www.youtube.com/watch?v=O4bHLkFajhQ • http://www.youtube.com/watch?v=dQnlrSRESJ8 • http://www.youtube.com/watch?v=wgsBxoJ9z3Q