Long non‐coding RNA LINC00607 silencing exerts antioncogenic effects on thyroid cancer through the CASP9 Promoter methylation

Thyroid cancer (TC) was the most frequent thyroid malignant tumour, accounting for about 1% of all malignant tumours. Some long non‐coding RNAs (lncRNAs) have been reported to exert essential tumour promotion effects, while caspase‐9 (CASP9) gene could play a promotive role in the cell apoptosis in TC. However, whether they have a specific effect on TC remains unclear. Hence, this study aims to explore the relationship between LINC00607 and CASP9, and its effect in TC. LINC00607 expression in the TC tissues and cell lines was determined. Then, we explored the combination effect between a LINC00607 and a methylation inhibitor 5‐Aza‐dc in doxorubicin‐resistant ARO cells using colony formation assay, flow cytometry, WST‐1 and EdU assay, as well as in vivo tumour growth assay. Besides, the dual‐luciferase reporter gene assay, RIP, ChIP, methylation‐specific PCR and BSP method were employed to detect the relationship between LINC00607 and CASP9 and its methylation. LINC00607 expression was up‐regulated in the doxorubicin‐resistant TC cell lines and tissues and negatively correlated to the poor prognosis of TC patients. Knockdown of LINC00607 suppressed doxorubicin resistance, proliferation and colony formation, and promoted cell apoptosis of TC cells in vitro, as well as suppressed tumour growth in vivo, whereas LINC00607 overexpression was observed to exercise the opposite effects. Notably, it was also revealed that LINC00607 down‐regulated the CASP9 expression by promoting CASP9 promoter methylation. In conclusion, LINC00607 could inhibit the apoptosis and augment the doxorubicin resistance of TC cells by decreasing CASP9 expression, which might provide a novel therapeutic target for TC treatment.