The role of TCF7L2 alternative splicing in tumor progression and its modulation by splice-switching antisense oligonucleotides (SSOs) for cancer therapy

TCF4 (TCF7L2) is a member of the T-cell Factor/Lymphoid Enhancer Factor (TCF) family of transcription factors, nuclear mediators of the Wnt/β-catenin signaling pathway, a known regulator of cellular functions that may contribute to tumor initiation and progression. A SNP within TCF7L2 is, to date, the most significant genetic marker associated with T2DM risk. TCF7L2 is also known to be involved in liver gluconeogenesis and glucose metabolism. However, its role in cancer initiation and progression has not been studied in depth. The TCF7L2 gene consists of 17 exons, which are known to display a complex pattern of alternative splicing (AS). AS involving exons 13-16 results in TCF4 proteins with differing C-termini. It is not known to what extent the variable C-termini alter the functional properties of the TCF4 isoforms. It has been proposed that the different isoforms can confer differential transcriptional activity.

We have identified TCF7L2 as an important target of the oncogenic splicing factor hnRNP A2/B1, which promotes breast cancer metastasis when up-regulated. TCF7L2 undergoes AS in metastatic breast tumors and knockdown of hnRNP A2/B1 in breast cancer cell lines induced skipping of exons 13-15. We analyzed RNA-seq data from normal brain tissues and glioblastoma tumor samples and found that TCF7L2 alternative splicing is altered in these tumors as well. Glioblastoma tumors show enhanced inclusion of exons 13-15 while normal brain samples show enhanced skipping. We hypothesize that the long TCF7L2 isoform, that includes exons 13-15, possesses oncogenic activity while the skipped isoform is tumor suppressive. Thus, we sought to modulate TCF7L2 splicing as a potential anti-cancer therapy. We performed a screen of splice switching antisense oligonucleotides (SSOs), targeting exon 15 and the adjacent introns, in order to identify efficient SSOs that modulate TCF7L2 splicing. Targeting the intron14/exon15 splicing junction of TCF7L2 in glioblastoma and breast cancer cell lines resulted in skipping of exons 13-15 and inhibition of cell survival and anchorage-independent growth in vitro. We conclude from these results that the AS of TCF7L2 is a crucial step in tumorigenesis, specifically in glioblastomas and metastatic breast cancer, and modulation of it may inhibit tumor progression.

Future studies will use the most efficient oligo to modulate splicing of TCF7L2 in glioblastoma and metastatic breast cancer model systems in vivo in order to examine its effect on tumor development and progression, focusing on proliferation and metastasis.