Abstract 2055: Evaluation of DNA damage repair gene alterations, microsatellite instability status, and tumor mutational burden as predictive biomarkers of olaparib sensitivity in gastric cancer
Authors: Hwang Jihyun , Kwon Woo sun , Park Juin , Bae Hyun Joo , Jeong Inhye , Kang Sun Kyoung , Kim Tae Soo , Che Jingmin , Chung Hyun Cheol , Rha Sun Young 
Published: July 2021

Abstract
One of the many causes of cancer is a dysfunction of the DNA damage repair (DDR) pathway. Cancers with alterations in homologous recombination (HR) genes are known to be increased the sensitivity of PARP inhibitors and genomic instability. Besides, cancers with alterations in other DDR genes like mismatch repair (MMR), have been recently reported to have similar features. Furthermore, some studies suggested that hypermutated cancers, such as microsatellite instability-high (MSI-H) or tumor mutational burden (TMB) high, have DDR gene alterations. Nevertheless, many studies still have focused on HR-related gene alteration as a predictor of olaparib. Here, we aim to determine if the DDR gene alteration and genomic instability markers can predict olaparib efficacy in 49 gastric cancer cell lines.
First, we profiled the genomic status of 48 DDR genes, MSI status, and TMB using targeted sequencing (524 genes). BRCA1, RAD51C, and MLH1 methylation were detected by bisulfite sequencing. If cell lines had the deleterious mutation, homozygous deletion, or methylation in DDR genes, it was categorized as an alteration group. To determine the efficacy of olaparib, cell lines were treated with olaparib for 5 days and assessed by CCK-8 assay. IC50 values were determined by the CalcuSyn software and cell lines were classified as a sensitive group when the IC50 was less than 10µM. MMR-related and PARP1 protein expression was determined by western blot.
As a result, 17 cell lines had the alteration in one or more of the 18 DDR-related genes. In our cell line panel, the expression of PARP1 protein was varied, and 12 cell lines were sensitive to olaparib. DDR alteration group was more sensitive to olaparib than the DDR wild type group (median of IC50 = 14.08µM vs &gt; 20µM, p = 0.034). In detail, cell lines with alterations in the HR and MMR were significantly sensitive among the subtype of DDR pathways (p = 0.011, p = 0.009). TMB was widely distributed among the GC cell lines (range = 4.36 to 21.80, median = 10.90), and the DDR alteration group had higher TMB than DDR wild type group (median = 11.63 vs 10.90, p = 0.029). Interestingly, the olaparib sensitive group had higher TMB than the resistant group (median = 15.3 vs 10.9, p &lt; 0.0001). Four cell lines (SNU-1, SNU-638, IM95m, and NUGC-3) were MSI-H, and all of them were no MLH1 protein expression. MSI-H cell lines had significantly higher TMB than other cell lines (median=19.3, p &lt; 0.0001). Three MSI-H cell lines were sensitive to olaparib, but NUGC-3 with the lowest TMB (13.8) among MSI-H cell lines was resistant.
In our result, the olaparib sensitive group had the alteration in DDR genes and high TMB. MSI-H cell lines were sensitive to olaparib when it had high TMB. MSI status and TMB could predict olaparib sensitivity in gastric cancer as well as DDR gene alterations.
Citation Format: Jihyun Hwang, Woo sun Kwon, Juin Park, Hyun Joo Bae, Inhye Jeong, Sun Kyoung Kang, Tae Soo Kim, Jingmin Che, Hyun Cheol Chung, Sun Young Rha. Evaluation of DNA damage repair gene alterations, microsatellite instability status, and tumor mutational burden as predictive biomarkers of olaparib sensitivity in gastric cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 2055.