Functional aspects of cilia and tumor suppressors

Abstract

Sensing the cellular environment and responding accordingly is pivotal for tissue development and homeostasis. One cellular structure that functions almost exclusively as a sensory organelle is the nearly ubiquitously present primary cilium, that has been implicated in orchestrating cellular responses to kinetic, photonic, morphogenic, olfactory and mitogenic stimuli. Dysfunction of cilia is the etiology for a large diversity of human disease phenotypes that are collectively known as ciliopathies. The severity of these disease manifestations can range from simple tissue cysts, infertility and retinal degeneration to prenatal lethal multi-organ system failure. The expression of cilia is restricted to the quiescent stage of the cell cycle as the centrosome is required for both ciliogenesis and formation of the spindles in cell division, rendering these processes mutually exclusive. Consequently, dysfunctional cilia relieve a physical block for cell cycle progression and for this reason cilia have also been implicated in the field of cancer biology for several years. Intriguingly however, it is becoming clear that cilia appear to be both required and permissive for tumorigenesis depending on the tumor subtype and primary oncogenic lesion. The scope of this thesis is to converge aspects of cilia biology with their role in cancer biology. Microtubules form the core of the cilium and serve as platforms for intracellular transport, without this transport, many cellular processes are greatly disturbed. A large family of molecular motors drive this transport and specifically in cilia, specific motors generate and maintain the axoneme. Few other molecular motors have been implicated in ciliogenesis. We describe an in vitro kinesome screen (targeting molecular motors) using an siRNA library, analyzed cilia frequencies, and identified a number of novel kinesins that perturb ciliary frequency. The kidney is a major site for the development of diseases that are associated with dysfunctional cilia, e.g. renal cysts. Several tumor suppressor proteins were previously associated with both renal cyst and cancer development, and these proteins were shown to function in sustaining ciliogenesis and cilia signaling in renal cysts. We analyzed cilia frequency in a tissue microarray of a variety of renal tumor subtypes, these results suggest that also in, both, familial and sporadic renal tumors, cilia loss is a common event that thus possibly contributes to tumorigenesis. Birt-Hogg-Dubé syndrome is a rare cancer syndrome that also features renal cyst and cancer development, suggesting a putative ciliary function for the mutated gene FLCN. Our observations indeed establish that BHD can be considered a novel atypical ciliopathy as our molecular analysis indicate a role for the FLCN tumor suppressor in ciliogenesis and ciliary signaling. Previously, our lab identified and described mutant lrrc50 zebrafish as a model for the human ciliopathy Kartagener’s syndrome. Unexpectedly, heterozygous zebrafish were found to develop testicular tumors. Characterization of these tumors suggested analogy to the human seminoma and genetic testing for mutations in human LRRC50 revealed loss-of-function mutations in both familial and sporadic samples of seminoma. To understand the role of LRRC50 (in tumorigenesis), we describe molecular aspects and functions of the LRRC50 gene product