The Broad Institute of MIT and Harvard Differential Analysis The Broad Institute of MIT and Harvard Differential Analysis distinct classes, Given phenotypically distinct classes, find markers that distinguish these classes from one another Tumor Normal Marker selection TumorNormal The Broad Institute of MIT and Harvard Problem Gene Markers Error Example Normal vs. Renal carcinoma I. Tissue or Cell Type ~ ~0% Normal vs. Renal carcinoma Normal vs. Abnormal Leukemia ALL vs. AML II. Morphological ~ ~0-5% Leukemia ALL vs. AML Type ALL B- vs. T-Cell III. Morphological Subtype ~ ~0-15% ALL B- vs. T-Cell Multiclass Classification AML Treatment Outcome IV. Treatment Outcome ~1-20 ~5-50% AML Treatment Outcome Drug Sensitivity Degree of Difficulty adapted from P. Tamayo Hierarchy of difficulty Gene Marker Selection The Broad Institute of MIT and Harvard Gene Marker Selection Compute score for each gene Score Dataset Phenotype/ class labels Compute score: t-test, SNR, etc. Ranked gene list T-test: Signal-to-Noise Ratio (SNR): The Broad Institute of MIT and Harvard Gene Marker Selection Small sample size. Each gene tested is a separate hypothesis likelihood of false positives. Gene interaction not taken into account. Challenges The Broad Institute of MIT and Harvard Gene Markers Selection Generate a 10,000x100 matrix from a Gaussian (mean=0, SD=0.5) Pick n columns (6,14,30,100) Assign sample labels yellow and green Select top 25 markers for yellow, top 25 markers for green Generate a 10,000x100 matrix from a Gaussian (mean=0, SD=0.5) Pick n columns (6,14,30,100) Assign sample labels yellow and green Select top 25 markers for yellow, top 25 markers for green Small Sample Size With small sample size it is easy to find genes correlated with phenotype Yellow Green 6 samples YellowGreen 14 samples Yellow Green 30 samples Yellow Green 100 samples The Broad Institute of MIT and Harvard scores Distribution of permuted scores for given gene P-value calculation If a gene is normally distributed the t-score follows the t-distribution What if they arent normally distributed? Permutation Test: shuffle labels (class membership) compute score for each gene (t-score, SNR,.. ) repeat many times Empirical null distribution Empirical null distribution of scores for each gene Compare observed score to empirical distribution. Observed score of gene No distributional assumptions are made - compute gene-specific p-values The Broad Institute of MIT and Harvard Permutation test and P-value Called Class A Called Class B True classes Permutation 1 Permutation 2 Permutation n To determine how significant a genes statistical score is Known class A samplesKnown class B samplesScore Generates a null distribution of values for this gene Compare with real score for this gene The Broad Institute of MIT and Harvard Marker Selection Process Dataset Phenotype/ class labels Measure of significance Compute score: t-test, SNR, etc. Measure significance: permutation test Correct for multiple hypotheses: FDR, FWER, etc. Markers Score Ranked gene list The Broad Institute of MIT and Harvard Multiple Hypotheses Bonferroni Correction: Most conservative metric Divides the p-value by the number of hypotheses FWER (Family-Wise Error Rate): probability of calling one or more hypotheses significant given that they are all null FDR (False Discovery Rate): probability that the null hypothesis is true given that the result is significant Try to reduce the number of hypotheses tested in the first place (i.e. filtering) What to control The Broad Institute of MIT and Harvard Exercise 1.Choose module: Gene List Selection ComparativeMarkerSelection 2.Choose input file: Next to input file, choose Specify URL View datasets window in Web browser Click and drag all_aml_train.preprocessed.gct 3.Choose class file: Next to cls file, choose Specify URL View datasets window in Web browser Click and drag all_aml_train.cls 4.Click Run ComparativeMarkerSelection Module The Broad Institute of MIT and Harvard Viewing Analysis Results The Broad Institute of MIT and Harvard Reduce number of hypotheses/genes by variation filtering (attempt at reducing false negatives) Choose test statistic (e.g., SNR, t-score,...) If enough samples, compute p-values by permutation test (otherwise, compute asymptotic test using the standard t- distribution). Control for Multiple Hypothesis Testing by using the FDR correction Remember: if you choose FDR 0.05, youre willing to accept 5% of false positives. If number of significant hypotheses/genes too large even for very small threshold values, either: use the maxT correction (possible w/ empirical p-values only). use additional criteria (e.g., min fold-change, min expression value, etc.) Differential Analysis Cookbook The Broad Institute of MIT and Harvard Create expression data set ExpressionFileCreator Reduce number of hypotheses/genes by variation filtering PreprocessDataset Make class file Run Differential Analysis ComparativeMarkerSelection Choose test statistic (say, t-score) View results with ComparativeMarkerSelectionViewer If enough samples, compute p-values by permutation test (otherwise, use asymptotic test). Control for MHT by using the FDR correction Use HeatMapViewer to view results for top genes Use GSEA to find gene sets (or pathways) that are enriched in your dataset. Differential Analysis GenePattern modules The Broad Institute of MIT and Harvard Working with Samples and Features The Broad Institute of MIT and Harvard Overview Extracting a set of samples Computing co-expressed genes Converting probe set ids to gene names Computing overlap between gene sets The Broad Institute of MIT and Harvard Working with Samples and Features 1.From a combined dataset of cancer and normal samples, select the normal samples. 2.Within the normal samples, find the genes coexpressed with LRPPRC (Affymetrix probe M92439_at), a gene with mitochondrial function. 3.Compare these genes and those coexpressed with LRPPRC in another expression dataset to determine the coexpressed genes common to both datasets. SelectFeaturesColumns GeneNeighbors GeneListSignificanceViewer CollapseDataset VennDiagram GCM_Total.r es GCM_Normals.res GCM_Normals.markerdata.g ct GCM_Total_Normals.markerdata.collapsed.row.nam es.txt ExtractRowNames GCM_Normals.markerlist.o df GCM_Total_Normals.markerdata.collapsed. gct The Broad Institute of MIT and Harvard Exercise