Protein tyrosine kinase 6 (PTK6, BRK) amplification in HER2+ breast cancer as a mechanism of HER2 resistance.

Abstract

11021 Background: PTK6 gene on chromosome 20q13 encodes the intracellular non-receptor tyrosine kinase. Studies in vivo and in vitro revealed a role for PTK6 in cell proliferation and survival, particularly in HER2+ breast cancer cells suggesting that PTK6 may associate with the HER2 pathway and confer resistance to HER2-targeted therapy. PTK6 protein is frequently overexpressed in breast cancer, however, the mechanism(s) underlying PTK6 overexpression and its role in cancer remains unclear. To address this problem, we analyzed the frequency of PTK6 gene copy number variation (CNV) and expression in association with breast cancer subtypes. Methods: Retrospectiveparaffinsamples of invasive tumor and normal epithelium, and matching DCIS and metastases were mounted on TMA. PTK6 CNV was determined using PTK6:CEP20 FISH assay. Tumor subtypes were defined using the five-marker IHC classifier. The correlation between PTK6 CNV and mRNA expression and association of both with the intrinsic PAM50 tumor subtype were studied using TCGA database (547 cases) and publicly available seven breast cancer data sets (1005 cases). Data were normalized, gene median centered and standardized for the purpose of the study. Results: By FISH, 20% of 41 invasive tumors carried PTK6 CNV: amplification (10%) and gene polysomy (10%). The proportion of PTK6 amplified cases differed by subtype, with the largest proportion in HER2-enriched (17%) and LumB (14%). Strikingly, amplified invasive cases also showed amplification in matching DCIS and metastases. Analysis of the public datasets confirmed the frequent PTK6 amplification in breast cancer. Both low and high levels of amplification were detected with the largest proportion in HER2+ tumors (HER2-enriched and LumB; p=2.05e-26). None of the basal-like tumors showed high levels of PTK6 amplification. A high correlation between PTK6 gene copies and mRNA expression was observed (p=1.13e-08). Conclusions: PTK6 gene is amplified early in breast cancer progression, particularly in HER2+ tumors. Further studies on PTK6 biology may help clinicians to understand its potential role in HER2 resistance. Supported by BREAST CANCER SPORE, NCI K12CA139160 and CTSA-ITM CS UL1 RR024999.