Quantifying "just-right" APC inactivation for colorectal cancer initiation

TL;DR

This study uses a mathematical model and sequence data from over 2500 colorectal cancers to quantify how partial APC inactivation optimally promotes tumorigenesis, revealing a consistent 'just-right' level of Wnt signaling for cancer development.

Contribution

The paper introduces a novel mathematical framework to quantify the effects of partial APC loss and demonstrates its role in colorectal cancer initiation across various genetic contexts.

Findings

Partial APC loss increases cancer risk 50-fold over complete loss.

Optimal APC inactivation varies with tumor location and mutations.

Wnt hyperactivation presents a potential therapeutic vulnerability.

Abstract

Dysregulation of the tumour suppressor gene Adenomatous Polyposis Coli (APC) is a canonical step in colorectal cancer development. Curiously, most colorectal tumours carry biallelic mutations that result in only partial loss of APC function, suggesting that a "just-right" level of APC inactivation, and hence Wnt signalling, provides the optimal conditions for tumorigenesis. Mutational processes act variably across the APC gene, which could contribute to the bias against complete APC inactivation. Thus the selective consequences of partial APC loss are unclear. Here we propose a mathematical model to quantify the tumorigenic effect of biallelic APC genotypes, controlling for somatic mutational processes. Analysing sequence data from >2500 colorectal cancers, we find that APC genotypes resulting in partial protein function confer about 50 times higher probability of progressing to cancer compared to complete APC inactivation. The optimal inactivation level varies with anatomical location and additional mutations of Wnt pathway regulators. We use this context dependency to assess the regulatory effect of secondary Wnt drivers in combination with APC in vivo, and provide evidence that mutant AMER1 combines with APC genotypes that lead to relatively low Wnt. The fitness landscape of APC inactivation is consistent across microsatellite unstable and POLE-deficient colorectal cancers and tumours in patients with Familial Adenomatous Polyposis, suggesting a general "just-right" optimum, and pointing to Wnt hyperactivation as a potential cancer vulnerability.