Exomes and transcriptomes to reveal actionable findings in patients with negative-targeted panel sequencing

3562 Background: Next generation sequencing targeted panels increasingly inform clinical decisions, but may miss actionable findings detectable by whole exome sequencing (WES) and RNA-seq. There has been no direct comparison of WES plus RNA-seq against targeted panel sequencing to determine its added utility. To address this, we performed WES and RNA-seq analysis in a cohort of 100 patients with no actionable findings on prior panel sequencing. Methods: Ontario-wide Cancer Targeted Nucleic Acid Evaluation (OCTANE; NCT02906943) has sequenced 2,106 patients using a 161-gene Oncomine or 555-gene Hi5 panel. 100 patients (98 Hi5, 2 Oncomine) were chosen for further sequencing. Tumor (100x coverage) and normal (50x) exomes and tumor transcriptomes were sequenced on Illumina HiSeq2500 or NextSeq550. Interpretation included knowledgebase annotation (e.g. OncoKB, CIViC), mutation signatures, homologous recombination deficiency (HRD) scores, gene expression, and pathway analysis. Findings were deemed “actionable” if they could directly inform management or trial eligibility. Results: WES and RNA-seq identified one or more novel actionable findings in 38 patients. Of these, the main actionable finding was tumor mutation burden (TMB), mutation signature, or HRD score in 19 (50%), a copy number variant in 16 (42%), a fusion in 2 (5%), and a point mutation in 1 (2.6%). WES identified a MALAT1-GLI1 fusion in a cancer of unknown primary (CUP) whose transcriptome was consistent with gastric cancer, together suggesting the diagnosis of a rare gastroblastoma. To date, two cases have received exome-supported targeted therapy: (1) a metastatic high grade serous ovarian cancer, HRD-high, treated with olaparib then cisplatin for a combined 15 months, and (2) a metastatic neuroendocrine rectal tumor with RICTOR amplification treated with everolimus starting in Dec 2016 until last follow-up in Sep 2019. Of 62 patients with no actionable finding, expanded sequencing identified one or more known cancer drivers in 25 (40%): 17 CNVs, 3 fusions, and 5 point mutations or indels. In 16 patients, an oncogenic variant found on panel was not captured by WES, and may represent artifacts, germline mutations, or subclonal/localized variants. Conclusions: WES and RNA-seq expanded detection of actionable biomarkers and oncogenic mutations, especially CNV, TMB/signatures, and HRD. Two cases have undergone exome-supported targeted treatment. We performed the first WES of a rare gastroblastoma, originally a CUP but reclassified by expanded fusion detection and RNA-seq.