Fatty Acid binding protein 7 is a molecular marker in adenoid cystic carcinoma of the salivary glands: implications for clinical significance

Abstract

Adenoid cystic carcinoma (ACC) is an aggressive malignant neoplasm of the salivary glands. Its diagnosis is difficult due to overlapping features with other salivary tumors. Gene expression analysis may complement traditional diagnostic methods. We searched gene expression patterns in the Gene Expression Omnibus (GEO) database and in our tumor and normal samples. The biologic and prognostic potential of the identified genes was analyzed. The GEO data set of primary xenografted ACCs revealed that expression of five genes, engrailed homeobox 1 (EN1), fatty acid binding protein 7 (FABP7), hemoglobin epsilon 1, MYB, and versican (VCAN), was dramatically increased. mRNA expression of EN1, FABP7, MYB, and VCAN distinguished our sporadic ACCs from normal tissues and benign tumors. FABP7 expression appeared to be regulated differently from EN1 and MYB and was crossly correlated with poor prognosis in our ACC cohort. Immunohistochemistry showed that FABP7 protein was predominantly expressed in the nucleus of myoepithelial cells of both tubular and cribriform subtypes. In contrast, in the solid subtype, which is often associated with a lower survival rate, FABP7 protein was uniformly expressed in cancerous cells. One case with cribriform architecture and the highest level of FABP7 mRNA showed strong FABP7 staining in both duct-type epithelial and myoepithelial cells, suggesting that diffuse expression of FABP7 protein might be related to aggressive tumor behavior and poor prognosis. We propose FABP7 as a novel biomarker in ACC. The molecule may be useful in diagnosis and for identifying more effective therapies targeting this protein or upstream molecules that regulate it.

Figure 1

Expression of EN1, FABP7, MYB, and VCAN distinguishes ACCs from normal tissues and benign salivary gland tumors. (A–D) Box plot analysis of relative gene expression. Each box represents the quartile distribution range (25-75%). The median is shown by a black horizontal line. The whole range and individual cases are displayed as a black vertical line and scatter dots. The y-axis indicates fold change relative to the average of normal samples’ gene expression. Statistical comparisons between data sets were performed with two-tailed Student’s t tests. P values < .05 were considered significant.

Figure 2

FABP7 expression is poorly correlated with levels of EN1 and MYB in ACC tumors. (A–F) Scatter plot analysis for the strength of a correlation in relative gene expression between two molecules from EN1, MYB, FABP7, and VCAN. Trend lines are included. Trend line equations and R² values were calculated separately.

Figure 3

FABP7 may predict poor prognosis in salivary ACC patients. (A–H) Kaplan-Meier curves showing overall survival rate of patients with sporadic ACC tumors with a higher or lower mRNA expression of EN1 (A and E), FABP7 (B and F), MYB (C and G), or VCAN gene (D and H). The ACC cohort was divided into two groups according to relative gene expression. Panels A to D compare patients with expression in the top half overall (red line) and the lower half (black line). Panels E to H compare expression in the higher quartile (red line) and lower three quartile (green line) subsets. Circles identify censored data. Statistical comparisons between data sets were made with log rank tests, and each P value is displayed. P < .05 is considered significant.

Figure 4

FABP7 protein expression in ACC tumor specimens. (A–F) Immunohistochemistry using an antibody to FABP7. (A) Normal salivary gland tissue. (B–F) ACC tumor specimens: cribriform (B, C, and E), tubular (D), and solid (F) patterns. (B) ACC tumor specimen from the patient in A. (E) The cribriform case with the highest level of FABP7 mRNA. (B–D) FABP7 protein was predominantly expressed in the nucleus of myoepithelial cells and, to a lesser extent, in the cytoplasm of duct-type epithelial cells (E and F). Diffuse expression of FABP7 protein was associated with aggressive tumor behavior and poor prognosis.