Squalene epoxidase expression is associated with breast tumor progression and with a poor prognosis in breast cancer

Abstract

Differentially expressed genes (DEGs) have been previously identified using massive parallel RNA sequencing in matched normal, breast cancer (BC) and nodal metastatic tissues. Squalene epoxidase (SQLE), one of these DEGs, is a key enzyme in cholesterol synthesis. The aim of the present study was to investigate the potential involvement of SQLE in the tumorigenic process of BC and to determine its association with the clinical outcome of BC. SQLE mRNA expression was measured using reverse transcription-quantitative PCR in 10 pairs of ductal carcinoma in situ (DCIS) and BC tissues and their adjacent normal tissues. Immunohistochemical staining of SQLE on tissue microarray was performed in 26 normal breast, 79 DCIS and 198 BC samples. The role of SQLE as a prognostic biomarker in patients with BC has been verified using BreastMark. SQLE mRNA expression was significantly increased in DCIS and BC tissues compared with that in their adjacent normal tissues. High SQLE expression was detected in 0, 48.1 and 40.4% of normal breast, DCIS and BC tissues, respectively. SQLE expression in DCIS and BC tissues was significantly higher than that in normal breast tissues. High SQLE expression was observed in DCIS with higher nuclear grade, comedo-type necrosis and HER2 positivity. High SQLE expression in BC was associated with larger tumor size, nodal metastases, higher stage, HER2 subtype and distant metastatic relapse. High SQLE expression was associated with poor disease-free and overall survival, and independently predicted poor disease-free survival in patients with BC. Following BreastMark analysis, high SQLE mRNA expression in BC was significantly associated with a poor prognosis in the 'all', lymph node negative, lymph node positive, luminal A subtype and luminal B subtype groups. Therefore, SQLE expression may be upregulated during the tumorigenic process of BC, and high SQLE expression may be a useful biomarker for predicting a poor prognosis in patients with BC.

Keywords: breast; immunohistochemistry; prognosis; squalene epoxidase; tumorigenesis.

Figure 1.

SQLE expression assessed via (A) RNA-Sequencing and (B) reverse transcription-quantitative PCR in matched N, C and LN tissues in seven patients with breast cancer. FPKM, fragments per kilobase of transcript per million mapped reads; N, normal; C, cancer; LN, lymph node metastatic; SQLE, squalene epoxidase.

Figure 2.

SQLE mRNA expression assessed via reverse transcription-quantitative PCR in (A) DCIS and (B) BC tissues and their adjacent N breast tissues. N, normal; DCIS, ductal carcinoma in situ; BC, breast cancer; SQLE, squalene epoxidase.

Figure 3.

SQLE expression in normal breast, DCIS and BC tissues. (A and B) Normal breast tissue exhibited weak cytoplasmic SQLE expression (A: magnification, ×4; scale bar, 500 µm; B: magnification, ×400; scale bar, 60 µm). (C) High and (D) low SQLE expression in the cytoplasm of DCIS tissues. (E) High and (F) low SQLE expression in the cytoplasm of BC tissues. Magnification, ×4; scale bar, 500 µm (inlet magnification, ×400). DCIS, ductal carcinoma in situ; BC, breast cancer; SQLE, squalene epoxidase.

Figure 4.

Survival of patients grouped according to SQLE expression. High SQLE expression predicted a significantly poor disease-free survival and overall survival (P=0.001 and P=0.001, respectively). SQLE, squalene epoxidase.

Figure 5.

Prognostic role of squalene epoxidase expression in breast cancer assessed using BreastMark.