AXL is a receptor tyrosine kinase (RTK) activated by growth arrest-specific gene 6 (GAS6) which has been shown to play a key role in epithelial-mesenchymal transition (EMT). EMT leads to increased tumor invasion, migration, and drug resistance. AXL activation and subsequent EMT has been associated with poor prognosis in various solid and hematologic malignancies, in both de novo and relapse settings. GAS6 activation of AXL leads to upregulation of PI3K/AKT, NFKB, RAS and MAPK pathways.
Anaplastic thyroid cancer (ATC) accounts for 2% of thyroid cancers and can arise de novo from follicular thyroid cells or in association with papillary thyroid carcinoma (PTC). ATCs are aggressive tumors associated with increased invasion, migration and drug resistance. The molecular differences among different subtypes of ATC have not been well-characterized. We hypothesize that AXL activation plays a role in the development of classic and mixed variants of ATC.
First, we evaluated the TCGA thyroid cancer dataset for AXL gene expression (N=501). We investigated survival differences among patients with high (above 7.89 FPKM) and low (below 7.89 FPKM) expression of AXL. Additionally, we identified 12 patients with known classic ATC and mixed ATC/PTC at our institution. Formalin-fixed paraffin-embedded (FFPE) tissue from these patients were reviewed by a pathologist and 8 cases exhibited mixed PTC/ATC phenotype while 4 were classic ATC subtypes. The NanoString PanCancer Pathway panel was performed on the extracted tumor RNA to evaluate the gene profile of classic and mixed ATC subtypes.
Of the 501 samples in the TCGA dataset, 206 samples (41%) showed low expression and 295 samples (59%) showed high expression of AXL. Patients with high expression of AXL showed a decrease in survival probability. The 5-year survival rates were 98% and 91% for patients with low expression and high expression of AXL, respectively. In our cohort, most patients (N=11, 92%) developed metastatic disease with a median overall survival of 411 for the classic ATC cohort and 543 days for the mixed ATC/PTC cohort. Nanostring PanCancer Pathway analysis identified differential expression of AXL-associated genes and cancer pathways between the mixed ATC/PTC and the classic ATC cohort including MAPK, cell cycle-apoptosis, driver genes, and RAS. These were significantly upregulated with at least two-fold change in the mixed ATC/PTC histology group compared to the classic ATC histology group (p<0.01). Three genes associated with AXL and thus, EMT—MMP3, TNF, and PIK3CG—were upregulated in the mixed ATC/PTC cohort in comparison to the classic ATC cohort (p<0.005). With a 41.4 linear fold change (p=0.001), MMP3, a gene involved in degrading the extracellular matrix and tumor formation, was the most expressed gene in the mixed ATC/PTC cohort.
Our study suggests increased activation of AXL-pathway may lead to EMT in ATC. There is also a significant difference in the expression of genes, including 3 AXL-linked genes which were upregulated in the mixed ATC/PTC versus classic ATC pathological subtypes. Our findings suggest small molecule inhibitors targeting the AXL-pathway could potentially have therapeutic benefit in patients with mixed ATC/PTC histologies by preventing EMT.