N-terminal truncation mutations of adenomatous polyposis coli are associated with primary cilia defects

Adenomatous polyposis coli (APC) gene is a tumor suppressor gene and its truncated mutations cause a few cilia-related diseases such as Gardner's syndrome. However, little is known about the mechanism that links APC mutations and cilia disorder. APC mutations lead to the expression of N-terminal fragments, which have dominant effects in tumors owing to loss of the C-terminal region or a gain of function. The present study investigated the impact of tumor-associated N-terminal APC fragments on primary cilia assembly and the possible molecular mechanism involved. We discovered that expression of tumor-associated N-terminal APC fragments (APC-N, APC-N1, APC-N2, and APC-N3, which contain amino acids 1-1018, 1-448, 449-781, and 782-1018 respectively), resulted in primary cilia defects. We found that a β-catenin/PI3K/AKT/GSK-3β feedback signal cascade is responsible for causing N-terminal APC fragment-induced cilia defects. In this cascade, dysfunctions of both β-catenin and GSK-3β were involved in the up-regulation of HDAC6 and subsequent decreased acetylated tubulin levels, which thereby led to cilia defects. These data suggest a mechanism for linking N-terminal APC fragments and cilia loss, thus accelerating our understanding of human cilia-related diseases such as Gardner's syndrome and their cause due to APC mutations.