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Comparative analysis of somatic variant calling on matched FF and FFPE WGS samples

ABSTRACT: This study aims at assessing whether FFPE material can be used for cohort analysis. To this end, we performed an in-depth comparison of somatic SNVs called on matching FF and FFPE whole genome sequences samples extracted from the same metastatic prostate tumor. Four variant callers were used to call somatic variants on both the FF and the FFPE samples. An heuristic was developed to maximize the overlap between the FF and its FFPE counterparts. Then, the extent to which intra-tumor heterogeneity explains discrepancies in variants called between the FF and the FFPE samples was assessed by comparing the clonal and subclonal somatic variants. This study illustrates that when using the correct variant calling strategy, the majority of clonal SNVs can be recovered in an FFPE sample with high precision and sensitivity. These results suggest that somatic variants derived from WGS of FFPE material can be used in cohort studies.

PROVIDER: EGAS00001004456 | EGA |

REPOSITORIES: EGA

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Project description:This study was performed to evaluated RNA extraction and gene expression analysis of FFPE specimen stored for more than 20 years. Using long time stored FFPE material; large retrospective studies correlating molecular features with therapeutic response and clinical outcome, can be performed. Quantitative PCR (qPCR) was used to evaluate RNA extraction methods and to compare gene expression profiles of FFPE and fresh frozen (FF) tissue. Extracted RNA was subsequently subjected to microarray analysis and compared to qPCR data. The Ambion RecoverAll kit appears to be particularly suited for RNA extraction of long time stored FFPE tissues. Gene expression analysis using Affymetrix platform displayed a high degree of correlation for endogenous control genes comparing FF and FFPE tissues. We conclude that high quality gene expression signatures can be recognized using Affymetrix gene expression platform on FFPE tissue stored for more than 20 years. However, a general interpretation must be done with caution as different FFPE procedures have varying effects on RNA quality. Three RNA extraction methods from Roche (Basel, Switzerland), Ambion (Austin, TX, USA) and Qiagen (Hilden, Germany), designed for FFPE material was used. The methods were compared using qPCR. For the qPCR analysis, two different concentrations of input cDNA were used (20ng and 100ng) and 32 human endogenous control genes were examined. RNA from the Ambion FFPE kit was further analyzed by using microarray. Amplification of RNA prior to microarray analysis was performed using Nugen technologies (San Carlos, CA, USA). Nugen has developed amplification kits both for FFPE and FF materials: WT- ovation FFPE RNA amplification System V2 (Nugen FFPE), Ovation FF RNA Amplification System V2 (Nugen V2 FF) and Ovation FF Pico RNA Amplification System (Nugen PICO FF). The Nugen FFPE kit, designed for FFPE material was only used for FFPE tissue. The Nugen Pico FF kit, designed to target small amounts of FF RNA (>500 pg) and the Nugen V2 FF kit designed to target total FF RNA, were only used for FF tissue. Affymetrix standard amplification protocol (Affy FF) designed for FF RNA was also included as the standard method for amplification.

Project description:This experiment contains the RNA-Seq samples only. Formalin-fixed, paraffin-embedded (FFPE) tissues are an invaluable resource for clinical research. However, nucleic acids extracted from FFPE tissues are fragmented and chemically modified making them challenging to use in molecular studies. We analysed 23 fresh-frozen (FF), 35 FFPE and 38 paired FF/FFPE specimens, representing six different human tissue types (bladder, prostate and colon carcinoma; liver and colon normal tissue; reactive tonsil) in order to examine the potential use of FFPE samples in next-generation sequencing (NGS) based retrospective and prospective clinical studies. Two methods for DNA and three methods for RNA extraction from FFPE tissues were compared and were found to affect nucleic acid quantity and quality. DNA and RNA from selected FFPE and paired FF/FFPE specimens were used for exome and transcriptome analysis. Preparations of DNA Exome-Seq libraries was more challenging (29.5 % success) than that of RNA-Seq libraries, presumably because of modifications to FFPE tissue-derived DNA. Libraries could still be prepared from RNA isolated from two-decade old FFPE tissues. Data were analysed using the CLC Bio Genomics Workbench and revealed systematic differences between FF and FFPE tissue-derived nucleic acid libraries. In spite of this, pairwise analysis of DNA Exome-Seq data showed concordance for 70-80 % of variants in FF and FFPE samples stored for fewer than three years. RNA-Seq data showed high correlation of expression profiles in FF/FFPE pairs (Pearson Correlations of 0.90 +/- 0.05), irrespective of storage time (up to 244 months) and tissue type. A common set of 1,494 genes was identified with expression profiles that were significantly different between paired FF and FFPE samples irrespective of tissue type. Our results are promising and suggest that NGS can be used to study FFPE specimens in both prospective and retrospective archive-based studies in which FF specimens are not available.

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Comparative analysis of somatic variant calling on matched FF and FFPE WGS samples

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