CpG island hypermethylation frequently silences FILIP1L isoform 2 expression in prostate cancer

Purpose: Senescence related regulatory pathways serve as barriers to cancer immortalization and progression but they are currently not well defined. FILIP1L is a growth inhibitory gene with multiple isoforms whose expression is increased in senescent prostate and prostate cancer cells, and decreased in many cancers. We investigated whether DNA methylation regulates FILIP1L in senescence and in prostate cancer development.

Materials and methods: FILIP1L mRNA expression was assessed in prostate cancer and associated normal prostate tissues using quantitative polymerase chain reaction. A tissue microarray was constructed using 95 prostate cancer specimens and 45 benign prostate specimens. Vectra™ imaging was used to quantitate nuclear and cytoplasmic FILIP1L protein expression. Bisulfite sequencing and Pyrosequencing® were used to assess methylation. Prostate cancer cell lines were treated with 2'-deoxy-5-azacytidine and mRNA expression was assessed.

Results: FILIP1L isoform 2 mRNA was increased in replicatively senescent human prostate epithelial cells and decreased in prostate cancer specimens. We verified a reduction in nuclear FILIP1L protein in prostate cancer using tissue microarrays (p = 0.006). A CpG island 5' of the isoform 2 translational start site was identified that showed hypermethylation in prostate cancer cell lines and tumors compared to normal prostate cells and tissues. Pyrosequencing confirmed FILIP1L hypermethylation in all 14 tumors compared to paired normal tissues (p <0.0001). Isoform 2 expression was induced in prostate cancer cell lines using 2'-deoxy-5-azacytidine.

Conclusions: FILIP1L isoform 2 is one of the most commonly hypermethylated genes in prostate cancer. It may serve as an important marker of prostate cancer. Isoform 2 expression is associated with senescence and its down-regulation may represent an early important biological event in prostate cancer development.