Pathscore: a Web Tool for Identifying Altered Pathways in Cancer Data Stephen G
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bioRxiv preprint doi: https://doi.org/10.1101/067090; this version posted August 1, 2016. The copyright holder for this preprint (which was not certified by peer review) is the author/funder. All rights reserved. No reuse allowed without permission. Vol. 00 no. 00 2016 BIOINFORMATICS Pages 1–2 PathScore: a web tool for identifying altered pathways in cancer data Stephen G. Gaffney1* and Jeffrey P. Townsend1,2 1 Department of Biostatistics, Yale School of Public Health, Yale University, New Haven, CT 06511 2 Program in Computational Biology and Bioinformatics, Yale University, New Haven, CT 06511 ABSTRACT is mutated only once in a dataset will increase the enrichment score Summary: PathScore quantifies the level of enrichment of somatic if mutated in a patient with no other pathway alterations. mutations within curated pathways, applying a novel approach Applications of PathScore to lung adenocarcinoma and lung that identifies pathways enriched across patients. The application squamous cell carcinoma data from The Cancer Genome Atlas provides several user-friendly, interactive graphic interfaces for data (Kandoth, 2014) demonstrate its potential to uncover patterns in exploration, including tools for comparing pathway effect sizes, low frequency events (Fig. 1). Numerous pathways known to be frequently affected in these cancers are identified by PathScore. significance, gene-set overlap and enrichment differences between Additional pathways identified include the nicotinic acetylcholine projects. receptor (nAChR) and GABA receptor pathways (21% and 33% Availability and Implementation: Web application available at patient coverage, respectively). Hotnet2 results include their genes, pathscore.publichealth.yale.edu. Site implemented in Python and but scattered in separate low coverage subnetworks (max 10% and MySQL, with all major browsers supported. Source code available 17% coverage). Both pathways are missed by MEMo due to the at github.com/sggaffney/pathscore with a GPLv3 license. low frequencies of their genes. These two related gene families are Contact: [email protected] thought to play a role in lung cancer and have been suggested as Supplementary Information: Additional documentation can be potential therapeutic targets (Schuller, 2009). Our demonstration of found at http://pathscore.publichealth.yale.edu/faq. pathway enrichment supports this hypothesis. 1 INTRODUCTION We present an algorithm and web application, PathScore, for the 2 APPROACH identification of known pathways that are enriched for mutations 2.1 The PathScore algorithm within a multi-patient somatic gene variant dataset. The algorithm belongs to a class of pathway analysis techniques known as over- The PathScore algorithm takes as input a set of patient-gene pairs, that should represent all observed non-silent and potentially function-altering representation analysis (ORA) tools (Khatri et al., 2012). Like mutations. Any listed genes that are not present in the pathways database other ORA tools, this new algorithm uses a hypergeometric test to are discarded. estimate pathway alteration probability, but distinguishes itself in The algorithm interprets mutations as samples from sites in the genome three ways. First, it segregates data by patient, calculating patient- without explicitly modeling differences in nucleotide sequence context or the specific pathway alteration probabilities that account for varied total functional impact of each mutation. It uses the hypergeometric distribution to mutation counts per patient. Second, it accounts for gene transcript calculate the probability that from M samples, corresponding to a patient’s length, incorporating the increased chance of mutation in longer M mutations, at least one occurs at a site belonging to a particular pathway. genes. Third, it uses empirically-derived background mutation rates We define the number of sites occupied by a set of genes Γ as: to account for varied mutation probability across the genome, which P jΓj = k g2Γρgλgk (1) to our knowledge is a unique feature among pathway analysis tools. Our web app implementation of the algorithm uses the collection where ρg is a gene-specific background mutation rate (BMR), in mutations of ‘canonical pathways’ from the Molecular Signatures Database per megabase, and λg is canonical transcript base-length. Scaling by BMR captures the large variation in mutation rate across the genome. As these (MSigDB, Liberzon et al., 2011), which includes pathways from rates are concordant across tissue types, we use by default an average rate the KEGG, Biocarta, Reactome and Nature-NCI databases, among across 91 cancer cell lines (Lawrence et al., 2013, Table S5), but allow others. Unlike tools such as MEMo (Ciriello et al., 2012) and users to substitute custom BMR values (e.g. tissue-specific rates; Polak HotNet2 (Leiserson et al., 2014), that detect significantly altered et al., 2015). For scenarios in which BMR-scaling is inappropriate (e.g. subnetworks, PathScore calculates an enrichment score for every pathway analysis of chromatin marks), ρg can be set to 1. For scenarios in pathway, and all activity in a pathway contributes to the score. which mutation probability doesn’t scale with base-length (e.g. large-scale In contrast to tools that, for tractability, require stringent gene chromosomal events), genes can be assigned equal mutation probability by filtering (MEMo’s filters include a threshold for a minimum patient setting λg to 1. frequency), PathScore has no filtering requirement. Even a gene that The formula for gene set size (Eq. 1) can be used to calculate both the target size of a pathway and the background size (G) for the set of all genes in the pathways database. For each pathway, PathScore calculates likelihood as *to whom correspondence should be addressed the product of patient-specific probabilities, across all n patients. Expressed © Oxford University Press 2016. 1 bioRxiv preprint doi: https://doi.org/10.1101/067090; this version posted August 1, 2016. The copyright holder for this preprint (which was not certified by peer review) is the author/funder. All rights reserved. No reuse allowed without permission. Gaffney & Townsend CHRND Altered in 48 patients (21%) as a function of pathway size, N: CHRNA4 CHRNA1 ( G−N G CHRNA6 n 1 − = if pathway is mutated; CHRNB4 Y Mi Mi CHRNA2 L(N) = Pi where Pi = G−N G CHRNB2 = otherwise: CHRNE i=1 Mi Mi CHRNA3 CHRNB3 (2) CHRNG 5 9 4 4 6 0 0 5 8 8 2 18 15 18 78 62 66 94 76 94 1 12 29 86 46 In the null model, N is given by the actual pathway size (Na). PathScore a 420 0 628 767 145 774 397 424 427 396 0 776 Z 4632 -504 -6211-438 -159 -Z031 -506 4403 -425 -Z027-6774658-439 -4430 Z 5-44325-44 5-3621 4-39 4-57 4-264-26 0-65 4-167-Z022 9-44 5-67 1-68 9-44 7-Z0537-Z062 5-5146 0 0 05-417- 17-Z053 38- 4 50 6 75 05 38-44 4 49-65 55 6 1 17 44-64 50 64-5 67-37705-405- 05-417-Z035 05 05-444-2659 17 44 73- 05 4 55-664 05-405 1 1 17- 7 44-6 then constructs an alternative model by treating N as a free parameter, CHRNG CHRNB3 Nrf2-Keap1 calculating the maximum likelihood estimate of pathway size (N ?), or b c signaling ‘effective pathway size’. L(N ) and L(N ?) are compared using the CHRNA3 D 10 a 0 CHRN 1 likelihood ratio test. The test statistic is assessed by comparison to a chi- CHRN 5 E 1 GABA-A value R.A. square distribution with one degree of freedom, yielding a P value for 1 P CHRNB2 1 5 the pathway—a measure of statistical significance of the disparity between -log 12 nAChRs actual and effective pathway sizes. The ratio of effective pathway size to CHRNA2 1 4 CHRNA4 actual pathway size provides a measure of the magnitude of the effect, and 2 ½1 2 4 is used to rank the relative degree of overburden of pathways with mutation. CHRNB4 Effect size 2 3 P values, Bonferroni-adjusted P values, and effect sizes are calculated for 4 CHRN each pathway in the database. Nrf2-Keap1 3 signaling d CHRNA6 A1 e GABA-A 2.2 The web app 2 R.A. The PathScore app provides both an interactive web interface and a REST nAChRs API. Users can create a project by submitting a dataset of patient–gene pairs (LUSC) size Effect to the ‘Upload’ page. Optional registration entitles users to increased project 1 1 2 3 storage as well as email notification when results are available. Algorithm Effect size (LUAD) options are ‘BMR-scaled gene length’ (with gene-specific ρg and λg), ‘unscaled gene length’ (specifying ρg = 1), and ‘gene count’ (specifying Fig. 1. (a) Matrix plot of patient-gene pairs for Reactome’s Presynaptic ρg = 1 and λg = 1). Analysis can be customized with two user-specified nAChRs pathway in lung adenocarcinoma. (b) Target plot for the same gene lists: genes to suppress in the analysis, or genes to highlight (i.e. filter pathway, with a pathway effect size (overburden of mutations) of 2:1. There out pathways that exclude them). Additional features of the interface include: are 12 genes in the pathway, with 11 mutated in at least one subject, as indicated by the shading of 11/12 of the pie chart. These genes are named Pathway visualization We represent each enriched pathway with two plots. in the tabs whose size and color (via heatmap) are proportional to their A matrix plot (Fig. 1a) conveys patient-gene pairings and reveals patterns respective patient mutation frequency. (c) Volcano plot of altered pathways of mutual exclusivity and co-occurrence. Squares in the matrix will exhibit in lung adenocarcinoma, highlighting the well-known Nrf2-Keap1 signaling user annotations of gene or mutation attributes in each patient. To convey pathway and two pathways with putative links to cancer: GABA-A receptor pathway enrichment attributes, we devised an information-rich, easy to scan activation and Presynaptic nAChRs.