Finding the Molecular Finding the Molecular Basis of Quantitative Basis of Quantitative Genetic Variation Genetic Variation Richard Mott Richard Mott Wellcome Trust Centre for Wellcome Trust Centre for Human Genetics Human Genetics Oxford UK Oxford UK
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Genetic Traits Quantitative (height, weight) Dichotomous (affected/unaffected) Factorial (blood group) Mendelian - controlled by single gene (cystic fibrosis)
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Finding the Molecular Basis Finding the Molecular Basis of Quantitative Genetic of Quantitative Genetic
VariationVariation
Richard MottRichard MottWellcome Trust Centre for Human Wellcome Trust Centre for Human
GeneticsGeneticsOxford UKOxford UK
Genetic Traits
• Quantitative (height, weight)
• Dichotomous (affected/unaffected)
• Factorial (blood group)
• Mendelian - controlled by single gene (cystic fibrosis)
• Complex – controlled by multiple genes*environment (diabetes, asthma)
Molecular Basis of Quantitative Traits
QTL: Quantitative Trait Locus
chromosome
genes
Molecular Basis ofQuantitative Traits
QTL: Quantitative Trait Locus
QTG: Quantitative Trait Gene
chromosome
Molecular Basis ofQuantitative Traits
QTL: Quantitative Trait Locus
QTG: Quantitative Trait Gene
QTN: Quantitative Trait Nucleotide
chromosome
SNP: Single Nucleotide Polymorphism
Association Studies
• Compare unrelated individuals from a population• Phenotypes:
– Cases vs Controls– Quantitative measure
• Genotypes: state of genome at multiple variable locations (Single Nucleotide Polymorphism = SNP) in each individual
• Seek correlation between genotype and phenotype
Problems with Association Studies
• Population stratification
• Linkage Disequilibrium
• Allele Frequencies
• Multiple loci
• Small Effect Sizes
• Very few Successes
Population Stratification
• If the sampling population comprises genetically distinct sub-populations with different disease prevalences
• Then -
• Any variant that distinguishes the sub-populations is likely to show disease association
Admixture Mapping
• Population is homogeneous but each individual’s genome is a mosaic of segments from different populations
• May be used to map disease loci– multiple sclerosis susceptibility– Reich et al 2005, Nature Genetics
Linkage Disequilibrium
Mouse
Effects of Linkage Disequilibrium
• Correlation between nearby SNPs
• SNPs near to QTN will show association– Risk of false positive interpretation– But need only genotype “tagging” SNPs– ~ 1 million tagging SNPs will be in LD with
~50% of common variants in the human genome
The Common-Disease Common-Variant Hypothesis
• Says – disease-predisposing variants will exist at relatively high
frequency (i.e. >1%) in the population.– are ancient alleles occurring on specific haplotypes. – detectable in an case-control study using tagging SNPs.
• Alternative hypothesis says– disease-predisposing alleles are sporadic new mutations,
perhaps around the same genes, on different haplotypes. – families with history of the same disease owe their condition to
different mutations events. – Theoretically detectable with family-based strategies which do
not assume a common origin for the disease alleles, but are harder to detect with case-control studies (Pritchard, 2001).
Power Depends on• Disease-predisposing allele’s
– Effect Size (Odds Ratio)– Allele frequency
• Sample Size: #cases, #controls
• Number of tagging SNPs
• To detect an allele with odds ratio of 1.25 and with allele frequency > 1%, at 5% Bonferroni genome-wide significance and 80% power, we require– ~ 6000 cases, 6000 controls– ~ 0.5 million tagging SNPs, one of which must be in perfect LD with the
causative variant– [Hirschorn and Daly 2005]
WTCCCWellcome Trust Case-Control Consortium
• 2000 cases from each of– Type I Diabetes– Type II Diabetes– rheumatoid arthritis, – susceptibility to TB – bipolar depression– …. and others …
• 3000 common controls• 0.675 million SNPs• ~10 billion genotypes• Data expected mid 2006
Mouse Models
Map inHuman or Animal Models ?
• Disease studied directly• Population and environment
stratification• Very many SNPs (1,000,000?)
required• Hard to detect trait loci – very
large sample sizes required to detect loci of small effect (5,000-10,000)
• Potentially very high mapping resolution – single gene
• Very Expensive
• Animal Model required• Population and environment
controlled• Fewer SNPs required (~100-
10,000)• Easy to detect QTL with ~500
animals
• Poorer mapping resolution – 1Mb (10 genes)
• Relatively inexpensive
QTL Mapping in Mice using Inbred Line Crosses
• Genetically Homozygous – genome is fixed, breed true.
• Standard Inbred Strains available • Haplotype diversity is controlled far more than in
human association studies
• QTL detection is very easy • QTL fine mapping is hard