APC Mutations as a Potential Biomarker for Sensitivity to Tankyrase Inhibitors in Colorectal Cancer

Abstract

In most colorectal cancers, Wnt/β-catenin signaling is activated by loss-of-function mutations in the () gene and plays a critical role in tumorigenesis. Tankyrases poly(ADP-ribosyl)ate and destabilize Axins, a negative regulator of β-catenin, and upregulate β-catenin signaling. Tankyrase inhibitors downregulate β-catenin and are expected to be promising therapeutics for colorectal cancer. However, colorectal cancer cells are not always sensitive to tankyrase inhibitors, and predictive biomarkers for the drug sensitivity remain elusive. Here we demonstrate that the short-form mutations predict the sensitivity of colorectal cancer cells to tankyrase inhibitors. By using well-established colorectal cancer cell lines, we found that tankyrase inhibitors downregulated β-catenin in the drug-sensitive, but not resistant, colorectal cancer cells. The drug-sensitive cells showed higher Tcf/LEF transcriptional activity than the resistant cells and possessed "short" truncated APCs lacking all seven β-catenin-binding 20-amino acid repeats (20-AARs). In contrast, the drug-resistant cells possessed "long" APC retaining two or more 20-AARs. Knockdown of the long APCs with two 20-AARs increased β-catenin, Tcf/LEF transcriptional activity and its target gene expression. Under these conditions, tankyrase inhibitors were able to downregulate β-catenin in the resistant cells. These results indicate that the long APCs are hypomorphic mutants, whereas they exert a dominant-negative effect on Axin-dependent β-catenin degradation caused by tankyrase inhibitors. Finally, we established 16 patient-derived colorectal cancer cells and confirmed that the tankyrase inhibitor-responsive cells harbor the short-form APC mutations. These observations exemplify the predictive importance of mutations, the most common genetic alteration in colorectal cancers, for molecular targeted therapeutics..

Molecular regulation and pharmacological targeting of the β‐catenin destruction complex

“…In line with these findings, a recent study demonstrated that CRC cancer cells with short truncated APC variants lacking all seven 20Rs were dependent on high β‐catenin levels and responded best to TNKS inhibitors. These results suggest that short APC truncations might provide a biomarker for TNKS inhibitor sensitivity (Tanaka et al, ). Despite these promising results, prolonged Wnt stimulation may render cells unresponsive to treatment with TNKS inhibitors and thus potentially put constraints on their use in clinical applications (de la Roche et al, ).…”

Regulation of Wnt/β‐catenin signalling by tankyrase‐dependent poly(ADP‐ribosyl)ation and scaffolding

“…In APC‐truncated CRC cells, an inability to assemble a functional β‐catenin destruction complex underlies the accumulation of transcriptionally active β‐catenin. A range of studies has investigated the responsiveness of model CRC cell lines to tankyrase inhibition (Lau et al, ; de la Roche et al, ; Tanaka et al, ). While it would have been conceivable that the assembly and function of the β‐catenin destruction complex cannot be sufficiently rescued by tankyrase inhibition in the absence of fully functional APC, there clearly are cases in which dysregulated Wnt/β‐catenin signalling can be curbed.…”

“…While it would have been conceivable that the assembly and function of the β‐catenin destruction complex cannot be sufficiently rescued by tankyrase inhibition in the absence of fully functional APC, there clearly are cases in which dysregulated Wnt/β‐catenin signalling can be curbed. In SW403 and COLO‐320DM cells, both of which bear extensive C‐terminal APC truncations (Figure A), tankyrase inhibition (by G007‐LK, IWR‐1 or XAV939) gives rise to AXIN2 stabilization, the formation of β‐catenin degradasomes (in COLO‐320DM cells), a robust reduction in active (non‐phosphorylated) β‐catenin and prominently attenuated β‐catenin‐dependent transcription, both in reporter assays and at the level of endogenous Wnt/β‐catenin target genes (Lau et al, ; de la Roche et al, ; Tanaka et al, ). Importantly, tankyrase inhibition limits the proliferation of these cells in cell culture (for G007‐LK and IWR‐1) and xenograft (for G007‐LK) models (Lau et al, ; Tanaka et al, ).…”

“…In SW403 and COLO‐320DM cells, both of which bear extensive C‐terminal APC truncations (Figure A), tankyrase inhibition (by G007‐LK, IWR‐1 or XAV939) gives rise to AXIN2 stabilization, the formation of β‐catenin degradasomes (in COLO‐320DM cells), a robust reduction in active (non‐phosphorylated) β‐catenin and prominently attenuated β‐catenin‐dependent transcription, both in reporter assays and at the level of endogenous Wnt/β‐catenin target genes (Lau et al, ; de la Roche et al, ; Tanaka et al, ). Importantly, tankyrase inhibition limits the proliferation of these cells in cell culture (for G007‐LK and IWR‐1) and xenograft (for G007‐LK) models (Lau et al, ; Tanaka et al, ). A similar cell response to tankyrase inhibition is observed in DLD‐1 and HCT‐15 cells, although the levels of active β‐catenin are reduced less robustly and the transcriptional effect is more subtle (Lau et al, ; de la Roche et al, ; Tanaka et al, ).…”

“…Importantly, tankyrase inhibition limits the proliferation of these cells in cell culture (for G007‐LK and IWR‐1) and xenograft (for G007‐LK) models (Lau et al, ; Tanaka et al, ). A similar cell response to tankyrase inhibition is observed in DLD‐1 and HCT‐15 cells, although the levels of active β‐catenin are reduced less robustly and the transcriptional effect is more subtle (Lau et al, ; de la Roche et al, ; Tanaka et al, ). Conversely, in SW480 and SW620 cells, although tankyrase inhibition results in the stabilization of AXIN2, degradasome formation and a strong decrease in active β‐catenin levels, it fails to block Wnt/β‐catenin target genes or the TOPFlash reporter (Lau et al, ; de la Roche et al, ).…”