Transcriptome sequencing of malignant pleural mesothelioma tumors

Abstract

Cancers arise by the gradual accumulation of mutations in multiple genes. We now use shotgun pyrosequencing to characterize RNA mutations and expression levels unique to malignant pleural mesotheliomas (MPMs) and not present in control tissues. On average, 266 Mb of cDNA were sequenced from each of four MPMs, from a control pulmonary adenocarcinoma (ADCA), and from normal lung tissue. Previously observed differences in MPM RNA expression levels were confirmed. Point mutations were identified by using criteria that require the presence of the mutation in at least four reads and in both cDNA strands and the absence of the mutation from sequence databases, normal adjacent tissues, and other controls. In the four MPMs, 15 nonsynonymous mutations were discovered: 7 were point mutations, 3 were deletions, 4 were exclusively expressed as a consequence of imputed epigenetic silencing, and 1 was putatively expressed as a consequence of RNA editing. Notably, each MPM had a different mutation profile, and no mutated gene was previously implicated in MPM. Of the seven point mutations, three were observed in at least one tumor from 49 other MPM patients. The mutations were in genes that could be causally related to cancer and included XRCC6, PDZK1IP1, ACTR1A, and AVEN.DNA sequencing ͉ tumor mutations ͉ lung cancer ͉ bioinformatics ͉ loss of heterozygosity B ecause cancer arises as a consequence of multiple mutations, human cancer genomes are being sequenced to identify the mechanisms of tumorigenesis. Pilot sequencing studies include recent exon resequencing of tumors and cell lines that revealed somatic mutations in hundreds of genes not previously implicated in oncogenesis. These studies generally focused on a single class of mutations such as point mutations in coding regions of preselected candidate genes, and the results so far indicate that even within similar histological classes, tumors possess unique mutational profiles (1-3). However, there has rarely been an analysis of whether a mutated gene is actually expressed in the tumor cell nor has there been an attempt to use sequencing to identify other types of mutations such as chromosomal deletions or translocation (4, 5) or loss of heterozygosity related to epigenetic silencing (6, 7). Moreover, no unbiased deep sequencing analysis of all expressed genes in cancer tissues has been reported to date.Malignant pleural mesothelioma (MPM) is an asbestosrelated, rapidly fatal cancer. Its genetic basis is unknown but appears to involve multiple types of chromosomal abnormalities (5,(8)(9)(10)(11)(12)(13)(14). Central mechanisms underlying MPM are unclear, although MPM tumors evoke a strong inflammatory response thought to contribute to tumorigenesis (15). In addition, tumor cell survival promoted by TNF-␣ responsive antiapoptotic proteins such as Inhibitor of Apoptosis-1 (IAP-1) facilitates the resistance of MPM to most cytotoxic chemotherapeutic drugs (16). Expression profiling with microarrays has supported the general role of inflammation in MPM etiology and has provided...

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