Yandell © 2003JSM Dimension Reduction for Mapping mRNA Abundance as Quantitative Traits Brian S. Yandell University of Wisconsin-Madison [Lan et al Genetics 164(4): August 2003]

Yandell © 2003JSM what is the goal of QTL study? uncover underlying biochemistry –identify how networks function, break down –find useful candidates for (medical) intervention –epistasis may play key role –statistical goal: maximize number of correctly identified QTL basic science/evolution –how is the genome organized? –identify units of natural selection –additive effects may be most important (Wright/Fisher debate) –statistical goal: maximize number of correctly identified QTL select “elite” individuals –predict phenotype (breeding value) using suite of characteristics (phenotypes) translated into a few QTL –statistical goal: mimimize prediction error

Yandell © 2003JSM what is a QTL? QTL = quantitative trait locus (or loci) –trait = phenotype = characteristic of interest –quantitative = measured somehow glucose, insulin, gene expression level Mendelian genetics –allelic effect + environmental variation –locus = location in genome affecting trait gene or collection of tightly linked genes some physical feature of genome

Yandell © 2003JSM typical phenotype assumptions normal "bell-shaped" environmental variation genotypic value G Q is composite of m QTL genetic uncorrelated with environment data histogram

Yandell © 2003JSM why worry about multiple QTL? so many possible genetic architectures! –number and positions of loci –gene action: additive, dominance, epistasis –how to efficiently search the model space? how to select “best” or “better” model(s)? –what criteria to use? where to draw the line? –shades of gray: exploratory vs. confirmatory study –how to balance false positives, false negatives? what are the key “features” of model? –means, variances & covariances, confidence regions –marginal or conditional distributions

Yandell © 2003JSM Pareto diagram of QTL effects major QTL on linkage map major QTL minor QTL polygenes (modifiers)

Yandell © 2003JSM advantages of multiple QTL approach improve statistical power, precision –increase number of QTL detected –better estimates of loci: less bias, smaller intervals improve inference of complex genetic architecture –patterns and individual elements of epistasis –appropriate estimates of means, variances, covariances asymptotically unbiased, efficient –assess relative contributions of different QTL improve estimates of genotypic values –less bias (more accurate) and smaller variance (more precise) –mean squared error = MSE = (bias) 2 + variance

Yandell © 2003JSM epistasis in parallel pathways (Gary Churchill) Z keeps trait value low Neither E 1 nor E 2 is rate limiting Loss of function alleles are segregating from parent A at E 1 and from parent B at E 2 Z X Y E 1 E 2

Yandell © 2003JSM epistasis in a serial pathway (Gary Churchill) ZXY E 1 E 2 Z keeps trait value high Neither E 1 nor E 2 is rate limiting Loss of function alleles are segregating from parent B at E 1 and from parent A at E 2

Yandell © 2003JSM epistasis examples (Doebley Stec Gustus 1995; Zeng pers. comm.) traits 1,4,9 1: dom-dom interaction 4: add-add interaction 9: rec-rec interaction (Fisher-Cockerham effects) 4 91

Yandell © 2003JSM why map gene expression as a quantitative trait? cis- or trans-action? –does gene control its own expression? –evidence for both modes (Brem et al Science) mechanics of gene expression mapping –measure gene expression in intercross (F2) population –map expression as quantitative trait (QTL technology) –adjust for multiple testing via false discovery rate research groups working on expression QTLs –review by Cheung and Spielman (2002 Nat Gen Suppl) –Kruglyak (Brem et al Science) –Doerge et al. (Purdue); Jansen et al. (Waginingen) –Williams et al. (U KY); Lusis et al. (UCLA) –Dumas et al. (2000 J Hypertension)

Yandell © 2003JSM idea of mapping microarrays (Jansen Nap 2001)

Yandell © 2003JSM goal: unravel biochemical pathways (Jansen Nap 2001)

Yandell © 2003JSM central dogma via microarrays (Bochner 2003)

Yandell © 2003JSM coordinated expression in mouse genome (Schadt et al Nature) expression pleiotropy in yeast genome (Brem et al Science)

Yandell © 2003JSM glucoseinsulin (courtesy AD Attie)

Yandell © 2003JSM Insulin Requirement from Unger & Orci FASEB J. (2001) 15,312 decompensation

Yandell © 2003JSM Type 2 Diabetes Mellitus from Unger & Orci FASEB J. (2001) 15,312

Yandell © 2003JSM studying diabetes in an F2 segregating cross of inbred lines –B6.ob x BTBR.ob  F1  F2 –selected mice with ob/ob alleles at leptin gene (chr 6) –measured and mapped body weight, insulin, glucose at various ages (Stoehr et al Diabetes) –sacrificed at 14 weeks, tissues preserved gene expression data –Affymetrix microarrays on parental strains, F1 key tissues: adipose, liver, muscle,  -cells novel discoveries of differential expression (Nadler et al PNAS; Lan et al in review; Ntambi et al PNAS) –RT-PCR on 108 F2 mice liver tissues 15 genes, selected as important in diabetes pathways SCD1, PEPCK, ACO, FAS, GPAT, PPARgamma, PPARalpha, G6Pase, PDI,…

Yandell © 2003JSM LOD map for PDI: cis-regulation Lan et al. (2003 submitted)

Yandell © 2003JSM Multiple Interval Mapping SCD1: multiple QTL plus epistasis!

Yandell © 2003JSM D scan: assumes only 2 QTL! epistasis LOD joint LOD

Yandell © 2003JSM trans-acting QTL for SCD1 (no epistasis yet: see Yi, Xu, Allison 2003) dominance?

Yandell © 2003JSM Bayesian model assessment: chromosome QTL pattern for SCD1

Yandell © 2003JSM high throughput dilemma want to focus on gene expression network –hundreds or thousands of genes/proteins to monitor –ideally capture networks in a few dimensions multivariate summaries of multiple traits –elicit biochemical pathways (Henderson et al. Hoeschele 2001; Ong Page 2002) may have multiple controlling loci –allow for complicated genetic architecture –could affect many genes in coordinated fashion –could show evidence of epistasis –quick assessment via interval mapping may be misleading

Yandell © 2003JSM why study multiple traits together? environmental correlation –non-genetic, controllable by design –historical correlation (learned behavior) –physiological correlation (same body) genetic correlation –pleiotropy one gene, many functions common biochemical pathway, splicing variants –close linkage two tightly linked genes genotypes Q are collinear

Yandell © 2003JSM high throughput: which genes are the key players? one approach: clustering of expression seed by insulin, glucose advantage: subset relevant to trait disadvantage: still many genes to study

Yandell © 2003JSM PC simply rotates & rescales to find major axes of variation

Yandell © 2003JSM multivariate screen for gene expressing mapping principal components PC1(red) and SCD(black) PC2 (22%) PC1 (42%)

Yandell © 2003JSM mapping first diabetes PC as a trait

Yandell © 2003JSM pFDR for PC1 analysis prior probability fraction of posterior found in tails

Yandell © 2003JSM false detection rates and thresholds multiple comparisons: test QTL across genome –size = pr( LOD( ) > threshold | no QTL at ) –threshold guards against a single false detection very conservative on genome-wide basis –difficult to extend to multiple QTL positive false discovery rate (Storey 2001) –pFDR = pr( no QTL at | LOD( ) > threshold ) –Bayesian posterior HPD region based on threshold  = { | LOD( ) > threshold }  { | pr( | Y,X,m ) large } –extends naturally to multiple QTL

Yandell © 2003JSM pFDR and QTL posterior positive false detection rate –pFDR = pr( no QTL at | Y,X, in  ) –pFDR = pr(H=0)*size pr(m=0)*size+pr(m>0)*power –power = posterior = pr(QTL in  | Y,X, m>0 ) –size = (length of  ) / (length of genome) extends to other model comparisons –m = 1 vs. m = 2 or more QTL –pattern = ch1,ch2,ch3 vs. pattern > 2*ch1,ch2,ch3