Surgery (RMH) - Theses
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ItemCharacterisation of the nuclear pore abnormalities in the intestinal zebrafish mutant, flotte lotte (flo)( 2012)The evolution of eukaryotic cells is defined by the compartmentalisation of the genetic material inside the nucleus, segregated from cytoplasm by a nuclear envelope. This barrier is punctuated by approximately 3000 large multi-protein structures known as nuclear pore complexes, which permit the bidirectional transport of protein and RNA molecules between the nucleus and cytoplasm. This study provided the opportunity to investigate the importance of the nuclear pore protein Elys during vertebrate development. Zebrafish mutants generated by ethylnitrosourea (ENU) mutagenesis provide a powerful tool for dissecting the genetic regulation of developmental processes. Our interest has focused on a panel of ENU generated mutants exhibiting a variety of defects in the formation and differentiation of the intestinal epithelium. One of these mutants, flotte lotte (flo), harbours a premature stop codon in the coding sequence of the nuclear pore component elys (embryonic large molecule derived from yolk sac). Elys is an essential component of the nuclear pore complex, yet surprisingly, its mutation in the flo mutant does not result in a global dysfunction in nuclear pore formation throughout the developing zebrafish embryo. Instead, flo mutants exhibit tissue-specific abnormalities in the development of the intestinal epithelium, liver, pancreas and eye; organs that are highly proliferative from 48hpf. We show that this time-point coincides with the exhaustion of maternally-deposited stocks of elys mRNA from flo embryos. Not surprisingly, we found that the ensuing inability to create new nuclear pore complexes appears to impact most severely on these rapidly proliferating tissues. Using multi-photon microscopy we reconstructed three-dimensional renditions of the endodermal organs in wild-type and flo larvae. Compared to the highly elaborated and polarized intestinal epithelium of wild-type zebrafish, the intestinal epithelium in flo is thin, unfolded and poorly polarised. Moreover, nuclear pore complexes in flo intestinal cells are not embedded in the nuclear envelope but are found in profuse cytoplasmic aggregates. Catastrophic levels of apoptosis accompany the loss of a functional nuclear envelope in intestinal epithelial cells. Thus, flo mutants provide an opportunity to identify signals that commit nuclear pore-deficient cells to an apoptotic fate. We found that the apoptotic response observed in the flo intestine is mediated via a Tp53-independent mechanism. Since Elys function is critical for the integrity of proliferative cells in zebrafish, we investigated whether ELYS is also critical for the proliferation of human cancer cells. We discovered a strong up-regulation of ELYS expression in many cancers when compared to their respective normal tissues. We observed ELYS to be ranked in the top 1% of all up-regulated genes investigated in gene expression studies of colorectal, kidney, liver and breast cancers available in the Oncomine database. The discovery that ELYS is frequently over-expressed in human colorectal cancer suggests that our functional genomics approach to novel cancer gene discovery using zebrafish mutants is valid. Moreover, we propose that targeted approaches to disabling ELYS synthesis or function may activate apoptosis in colorectal cancer cells and provide a useful therapeutic approach in the future.