Abstract 2705: Visualization of KRAS point mutations in non-small cell lung cancer tumors with morphological context using the BaseScope in situ hybridization assay
Authors: Dikshit Anushka , Jarnagin Helen , Doolittle Emerald , Anderson Courtney , Zhang Bingqing , Ma Xiao-Jun 
Published: September 2020

About 25% of non-small cell lung cancer (NSCLC) patients bear one or more KRAS mutations in their tumors, which is correlated with poor prognosis. The precise identification of somatic mutations in tumors is becoming increasingly important for studying tumor progression and developing targeted therapies. While sequencing technologies allow for mutation-profiling, they do not permit direct visualization and association of genetic alterations with cellular morphology. In addition, DNA mutational status does not predict expression of the mutant allele which may provide information connecting genotype to phenotype. Therefore, a technology for mutation detection at the transcript level directly in the tumor context is desirable. To address this need we developed a specialized RNA in situ hybridization (ISH) method known as BaseScope. The BaseScope assay has a unique signal amplification system that allows for highly sensitive and specific detection of single nucleotide point mutations in tissues. BaseScope probes specific for KRAS G12C, G12A, G12V, G12S and wild type KRAS were designed and expression of each point mutation was assessed in a NSCLC tumor microarray with 48 tumor cores with known KRAS mutation status as determined by DNA sequencing. RNA quality and background signal threshold for each tumor core were determined using PPIB (positive) and dapB (negative) control probes. Using the sequencing data as the gold standard, the BaseScope assay demonstrated 83-100% sensitivity and 97-100% specificity for various KRAS mutations [Table 1]. For KRAS G12C, the assay correctly identified all 6 sequencing-positive cores and identified the rest as negatives. For KRAS G12V, the assay detected 5 of 6 mutated cores with 100% specificity. Interestingly, for KRAS G12S and KRAS G12A mutations, the BaseScope assay demonstrated 100% sensitivity and 97% specificity. Furthermore, it was observed that 100% of the KRAS-mutated tumors showed expression for both wild type and mutant KRAS alleles within these NSCLC tumors. In summary, we demonstrate the development of an RNA ISH assay for point mutations detection with morphological context in FFPE tissues. Unlike current sequencing methods that lack spatial information this assay has the unique ability to identify very small subclones whose frequency within the tumor might fall below the detection limit of sequencing.

Performance characteristics of BaseScope KRAS assaysKRAS POINT MUTATIONSNo. of cores with specified mutationsNo. of cores without specified mutationsBaseScope sensitivityBaseScope specificityG12C633100% (6/6)100% (33/33)G12A336100% (3/3)97.2% (35/36)G12V63383% (5/6)100% (33/33)G12S138100% (1/1)97.3% (37/38)

Citation Format: Anushka Dikshit, Helen Jarnagin, Emerald Doolittle, Courtney Anderson, Bingqing Zhang, Xiao-Jun Ma. Visualization of KRAS point mutations in non-small cell lung cancer tumors with morphological context using the BaseScope in situ hybridization assay [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 2705.