Exome

About: Exome is a research topic. Over the lifetime, 5218 publications have been published within this topic receiving 269398 citations.

Comprehensive molecular portraits of human breast tumours

Abstract: We analysed primary breast cancers by genomic DNA copy number arrays, DNA methylation, exome sequencing, messenger RNA arrays, microRNA sequencing and reverse-phase protein arrays. Our ability to integrate information across platforms provided key insights into previously defined gene expression subtypes and demonstrated the existence of four main breast cancer classes when combining data from five platforms, each of which shows significant molecular heterogeneity. Somatic mutations in only three genes (TP53, PIK3CA and GATA3) occurred at >10% incidence across all breast cancers; however, there were numerous subtype-associated and novel gene mutations including the enrichment of specific mutations in GATA3, PIK3CA and MAP3K1 with the luminal A subtype. We identified two novel protein-expression-defined subgroups, possibly produced by stromal/microenvironmental elements, and integrated analyses identified specific signalling pathways dominant in each molecular subtype including a HER2/phosphorylated HER2/EGFR/phosphorylated EGFR signature within the HER2-enriched expression subtype. Comparison of basal-like breast tumours with high-grade serous ovarian tumours showed many molecular commonalities, indicating a related aetiology and similar therapeutic opportunities. The biological finding of the four main breast cancer subtypes caused by different subsets of genetic and epigenetic abnormalities raises the hypothesis that much of the clinically observable plasticity and heterogeneity occurs within, and not across, these major biological subtypes of breast cancer.

Analysis of protein-coding genetic variation in 60,706 humans

Abstract: Large-scale reference data sets of human genetic variation are critical for the medical and functional interpretation of DNA sequence changes. Here we describe the aggregation and analysis of high-quality exome (protein-coding region) DNA sequence data for 60,706 individuals of diverse ancestries generated as part of the Exome Aggregation Consortium (ExAC). This catalogue of human genetic diversity contains an average of one variant every eight bases of the exome, and provides direct evidence for the presence of widespread mutational recurrence. We have used this catalogue to calculate objective metrics of pathogenicity for sequence variants, and to identify genes subject to strong selection against various classes of mutation; identifying 3,230 genes with near-complete depletion of predicted protein-truncating variants, with 72% of these genes having no currently established human disease phenotype. Finally, we demonstrate that these data can be used for the efficient filtering of candidate disease-causing variants, and for the discovery of human 'knockout' variants in protein-coding genes.

Comprehensive molecular characterization of human colon and rectal cancer

Abstract: To characterize somatic alterations in colorectal carcinoma, we conducted a genome-scale analysis of 276 samples, analysing exome sequence, DNA copy number, promoter methylation and messenger RNA and microRNA expression. A subset of these samples (97) underwent low-depth-of-coverage whole-genome sequencing. In total, 16% of colorectal carcinomas were found to be hypermutated: three-quarters of these had the expected high microsatellite instability, usually with hypermethylation and MLH1 silencing, and one-quarter had somatic mismatch-repair gene and polymerase e (POLE) mutations. Excluding the hypermutated cancers, colon and rectum cancers were found to have considerably similar patterns of genomic alteration. Twenty-four genes were significantly mutated, and in addition to the expected APC, TP53, SMAD4, PIK3CA and KRAS mutations, we found frequent mutations in ARID1A, SOX9 and FAM123B. Recurrent copy-number alterations include potentially drug-targetable amplifications of ERBB2 and newly discovered amplification of IGF2. Recurrent chromosomal translocations include the fusion of NAV2 and WNT pathway member TCF7L1. Integrative analyses suggest new markers for aggressive colorectal carcinoma and an important role for MYC-directed transcriptional activation and repression.

Mutational heterogeneity in cancer and the search for new cancer-associated genes

Abstract: Major international projects are underway that are aimed at creating a comprehensive catalogue of all the genes responsible for the initiation and progression of cancer. These studies involve the sequencing of matched tumour-normal samples followed by mathematical analysis to identify those genes in which mutations occur more frequently than expected by random chance. Here we describe a fundamental problem with cancer genome studies: as the sample size increases, the list of putatively significant genes produced by current analytical methods burgeons into the hundreds. The list includes many implausible genes (such as those encoding olfactory receptors and the muscle protein titin), suggesting extensive false-positive findings that overshadow true driver events. We show that this problem stems largely from mutational heterogeneity and provide a novel analytical methodology, MutSigCV, for resolving the problem. We apply MutSigCV to exome sequences from 3,083 tumour-normal pairs and discover extraordinary variation in mutation frequency and spectrum within cancer types, which sheds light on mutational processes and disease aetiology, and in mutation frequency across the genome, which is strongly correlated with DNA replication timing and also with transcriptional activity. By incorporating mutational heterogeneity into the analyses, MutSigCV is able to eliminate most of the apparent artefactual findings and enable the identification of genes truly associated with cancer.

VarScan 2: Somatic mutation and copy number alteration discovery in cancer by exome sequencing

Abstract: Exome sequencing of tumor samples and matched normal controls has the potential to rapidly identify protein-altering mutations across hundreds of patients, potentially enabling the discovery of recurrent events driving tumor development and growth (International Cancer Genome Consortium 2010; Stratton 2011). Yet the analysis of such data presents significant challenges. Sequencing coverage is nonuniform across targeted regions and from one sample to the next (Ng et al. 2009; Bainbridge et al. 2010; Teer et al. 2010). Many regions achieve high read depth (more than 100×), which can confound variant callers and depth-based filters if not properly addressed (Ku et al. 2011). Repetitive and paralogous sequences can give rise to numerous false positives. The detection of somatic mutations in tumor genomes is even more challenging. The genomes of primary tumors are genetically heterogeneous (Ding et al. 2010), with frequent rearrangements (Campbell et al. 2008) and copy number alterations (CNAs) (Beroukhim et al. 2010). Further, somatic mutations are relatively rare compared with germline variation, often representing <0.1% of variants in a tumor genome (Ley et al. 2008; Mardis et al. 2009). Simply subtracting variants in the matched normal from variants in the tumor (Wei et al. 2011) is poorly suited for the analysis of exome sequence data, because it fails to account for regions that were undersampled in the normal. Accurate mutation detection requires a direct, simultaneous comparison of tumor–normal pairs at every position in the exome, but few algorithms to do so have been described. Numerous algorithms have been developed to assess genome-wide copy number using whole-genome sequencing (WGS) data. Most of these approaches (Campbell et al. 2008; Alkan et al. 2009; Chiang et al. 2009; Yoon et al. 2009; Abyzov et al. 2011) would be confounded by exome data sets, because of the biases introduced by hybridization and the sparse and uneven coverages throughout the genome. However, when both DNA samples in a tumor–normal pair were captured and sequenced under identical hybridization conditions, we reasoned that it might be possible to detect somatic CNAs (SCNAs) as deviations from the log-ratio of sequence coverage depth within a tumor–normal pair, and then quantify the deviations statistically. Such an approach would provide a gene-centric view of copy number in a tumor sample, though it would be limited to the ∼1% of the genome captured by current exome platforms. Previously, we published VarScan (Koboldt et al. 2009), an algorithm for variant detection in next-generation sequencing data. We have since released a new tool, VarScan 2 (http://varscan.sourceforge.net), with several improvements, including the ability to identify somatic mutation, loss of heterozygosity (LOH), and CNA events in tumor–normal pairs. VarScan 2 analyzes sequence data from a tumor sample and its corresponding normal sample simultaneously, applying heuristic methods and a statistical test to detect variants—single nucleotide variants (SNVs) and insertions/deletions (indels)—and classify them by somatic status. By direct comparison of normalized sequence depth, our method also detects SCNAs in the tumor genome. Here, we utilize VarScan 2 for the analysis of exome sequence data from 151 patients with high-grade serous ovarian adenocarcinoma (HGS-OVCa) that were initially characterized within the Cancer Genome Atlas (TCGA) project (Cancer Genome Atlas Research Network 2011). We present a robust pipeline for the detection of both germline (inherited) and somatic (acquired) mutations by exome sequencing and describe filtering approaches for detecting variants with high sensitivity and specificity. To evaluate the performance of our SCNA detection algorithm, we compare our results to copy number data from high-density SNP array and WGS approaches. Our results demonstrate the accuracy of VarScan 2 for somatic mutation and CNA detection and enable a new survey of the genetic landscape in ovarian carcinoma.