Lysyl Oxidase Is a Key Player in BRAF/MAPK Pathway-Driven Thyroid Cancer Aggressiveness

The BRAF^V600E mutation is the most common somatic mutation in thyroid cancer. The mechanism associated with BRAF-mutant tumor aggressiveness remains unclear. Lysyl oxidase (LOX) is highly expressed in aggressive thyroid cancers, and involved in cancer metastasis. The objective was to determine whether LOX mediates the effect of the activated MAPK pathway in thyroid cancer.

The prognostic value of LOX and its association with mutated BRAF was analyzed in The Cancer Genome Atlas and an independent cohort. Inhibition of mutant BRAF and the MAPK pathway, and overexpression of mutant BRAF and mouse models of BRAF^V600E were used to test the effect on LOX expression.

In The Cancer Genome Atlas cohort, LOX expression was higher in BRAF-mutant tumors compared to wild-type tumors (p < 0.0001). Patients with BRAF-mutant tumors with high LOX expression had a shorter disease-free survival (p = 0.03) compared to patients with a BRAF mutation and the low LOX group. In the independent cohort, a significant positive correlation between LOX and percentage of BRAF mutated cells was found. The independent cohort confirmed high LOX expression to be associated with a shorter disease-free survival (p = 0.01). Inhibition of BRAF^V600E and MEK decreased LOX expression. Conversely, overexpression of mutant BRAF increased LOX expression. The mice with thyroid-specific expression of BRAF^V600E showed strong LOX and p-ERK expression in tumor tissue. Inhibition of BRAF^V600E in transgenic and orthotopic mouse models significantly reduced the tumor burden as well as LOX and p-ERK expression.

The data suggest that BRAF^V600E tumors with high LOX expression are associated with more aggressive disease. The biological underpinnings of the clinical findings were confirmed by showing that BRAF and the MAPK pathway regulate LOX expression.