Roles of EEF1A2 & PTK6 in breast cancer

Abstract

Eukaryotic Translation Elongation Factor 1 Alpha (EEF1A) exists as two forms with different tissue specificities and encoded by separate loci: eEF1A1 on 6q13 and eEF1A2 on 20q13.3. eEF1A1 is ubiquitously expressed whereas eEF1A2 expression is normally limited to the heart, brain and skeletal muscles. eEF1A proteins are GTP-binding proteins that recruit an amino-acylated tRNA to the ribosome during the elongation phase of protein translation. eEF1A2 mRNA and protein are highly expressed in 50–60% of primary human breast tumors and metastases but not in normal breast epithelium. Since it is also overexpressed in 30% of primary human ovarian tumors, transforms rodent fibroblasts and increases their tumorigenicity in nude mice, eEF1A2 is considered to be a potential human oncogene. The mechanism of eEF1A2 expression is yet to be determined. Studies showed no gene mutation and no correlation between locus amplification or methylation and gene expression. Phosphate Tyrosine Kinase-6 (PTK6) is also located on 20q13.3. It is 48kb upstream of EEF1A2. PTK6 is a non-receptor tyrosine-kinase that is normally expressed in epithelial linings, prostate, skin and oral epithelium but it is not detected in the normal human mammary epithelium. PTK6 has been found to be expressed in many breast cancer cell lines and in approximately 60% of primary human breast tumors but it has not been detected in normal human breast tissue nor in fibroadenomas. Like other tyrosine kinases, PTK6 phosphorylates and activates downstream substrates that would be predicted to lead to increased transcriptional activity and therefore mediates proliferation of breast cancer cells. PTK6 is considered a prognostic marker of metastasis-free survival in breast cancer independent of the classical markers of tumor size, lymph node involvement and HER2 status. The aim of this project was to characterize for the first time the genomic region containing the two mentioned breast cancer oncogenes and understand their various roleswhether they act in tandem or independently in breast tumorigenesis. Immunohistochemistry was performed on tissue microarrays from 300 breast cancer patients to detect the expression levels of eEF1A2 and PTK6. Tumors that showed a high co-expression were analyzed for the genes’ copy number. An increased copy number of PTK6 was detected but not of eEF1A2 nor of adjacent genes on the 20q13.3 amplicon. To understand the effect of EEF1A2 expression on other genes, microarray analysis was performed on NIH-3T3 cells stably transfected with EEF1A2. Many upregulated genes were associated with different types of cancers. This was further confirmed by real-time PCR. However, when the NIH-3T3 cells were transiently transfected with EEF1A2, the genes that were upregulated in the microarray study showed no change in expression. In conclusion, EEF1A2 and PTK6 act independently and each acts through a different mechanism in breast tumorigenesis.