PD01-09: Identifying Novel Mechanisms of Resistance to Lapatinib in ERBB2+ Breast Cancer Cells through Whole Genome Mutational Analysis.
Authors: Jegg A , Ward TM , Iorns E , Gallas M , Aparicio SA , Pegram MD 
Published: June 2012

Abstract
Background: Overexpression of the epidermal growth factor receptor ERBB2 (HER2) is found in 20% of human breast cancers. Therapies targeting ERBB2 including trastuzumab and lapatinib have significantly improved the outlook for women with ERBB2+ breast cancer. However, resistance to these agents occurs frequently and remains a significant clinical problem. In the case of lapatinib resistance, the mechanism(s) of resistance remain poorly understood, since the current proposed rationale thought to limit lapatinib's anti-tumor effects has been difficult to reconcile with clinical data. Therefore, we hypothesize that novel mechanisms of resistance could be identified by mapping genomic variations in ERBB2+ cells with acquired resistance to lapatinib. The identification of such mutations may provide insights into mechanisms of resistance and may indicate therapeutic strategies to overcome lapatinib resistance in ERBB2+ breast cancer.
Material and Methods: SKBR3 breast cancer cells resistant to lapatinib were generated through serial passage by exposure of drug sensitive parental SKBR3 cells to increasing concentrations of lapatinib up to the peak plasma concentration observed in human subjects (2.6 uM (SK-lapR)). Multiple signaling pathways in lapatinib sensitive and resistant cells were interrogated by Reverse Phase Protein Array (RPPA) and western blot analysis. To identify genome wide somatic mutations, the Exome of lapatinib resistant and sensitive SKBR3 cells was sequenced utilizing next generation deep sequencing. Following exclusion of germline variants, the acquired gene mutations in lapatinib resistant SKBR3 cells were confirmed by DNA re-sequencing of PCR amplified DNA segments.
Results and Discussion: Analysis of activated signaling pathways in lapatinib resistant and sensitive SKBR3 cells did not confirm any of the previously proposed mechanisms of resistance. In particular, these cells show no activation of AKT or alternative receptor tyrosine kinases such as IGF-IR, ERBB3 or c-Met. However they exhibit sustained activation of mTORC1 and ERK1/2, as well as phosphorylation of STAT3, STAT5, rpS6 and CREB. Initial sequence analysis of exome and transcriptome reveals the presence of 76 single nucleotide variants/Indels differing between sensitive and resistant cells with 34/76 validated as true mutations present in the genome of lapatinib resistant SKBR3 cells, including mutations in LATS2, MAP3K5, SMAD3 and PDGFRA. This is the first exome sequence analysis to be reported which defines a drug resistant phenotype in ERBB2+ breast cancer. Ongoing work includes investigation of mutations as drug resistance mediators and analysis of copy number variations and gene fusions/translocations to systematically search for molecular alterations, with the goal of providing a rationale for the design of new combination therapies aimed at lapatinib resistance for ERBB2+ breast cancer.
Citation Information: Cancer Res 2011;71(24 Suppl):Abstract nr PD01-09.