Tumourigenic fragments of APC cause dominant defects in directional cell migration in multiple model systems

Abstract

Summary: Nonsense mutations that result in expression of truncated, N-terminal fragments of the APC tumour suppressor protein are found in most sporadic and some hereditary colorectal cancers. These mutations can cause tumourigenesis by eliminating β-catenin binding sites from APC, which leads to upregulation of β-catenin to result in induction of oncogenes such as MYC. Here we show that in three distinct experimental model systems, expression of an N-terminal fragment of APC (N-APC) results in loss of directionality, but not speed, of cell motility independently of changes in β-catenin regulation. We developed a system to culture and fluorescently label live pieces of gut tissue to record high-resolution, three-dimensional time-lapse movies of cells in situ. This revealed an unexpected complexity of cell migration, a key process in gut epithelial maintenance, with cells moving with spatial and temporal discontinuity. Quantitative comparison of wild type and APC heterozygous tissue from APCMin/+ mice demonstrated that cells in precancerous epithelia lack directional preference when moving along the crypt-villus axis. This effect was reproduced in diverse experimental systems: in developing chicken embryos, cells expressing N-APC fail to migrate normally; amoeboid Dictyostelium, which lack endogenous APC, expressing an N-APC fragment maintained cell motility, but failed to perform directional chemotaxis; multicellular Dictyostelium slug aggregates similarly failed to perform phototaxis. We propose that N-terminal fragments of APC represent a gain-of-function mutation that causes cells within tissue to fail to migrate directionally in response to relevant guidance cues. Consistent with this idea, crypts in histologically normal tissues of APCMin/+intestines are overpopulated with cells suggesting that a lack of migration may cause cell accumulation in a pre-cancerous state.