S02-01 Modelling cancer in Drosophila: The junctional neoplastic tumour suppressors, Lgl, Dlg and Scrib, in cell proliferation control and tumourigenesis

Abstract

The junctional neoplastic tumour suppressors, Lgl, Dlg and Scrib, function in apical-basal cell polarity and limiting cell proliferation. In homozygous lgl, dlg or scrib mutants over-proliferation and polarity loss occurs in the larval imaginal disc epithelium.However, in lgl mutant mosaic eye epithelium, ectopic Cyclin E expression and cell proliferation occurs without loss of polarity. lgl mutant clones also show upregulation of the inhibitor of apoptosis, DIAP1, and developmental cell death is prevented. These effects on cell proliferation and survival can be explained by the misregulation of several signalling pathways in lgl mutant tissue.However, despite these effects on lgl mutant clonal growth, the overgrowth of the mutant tissue is restrained by differentiation and JNK-mediated apoptosis at the clonal borders.Cancer requires cooperation between tumour suppressors and oncogenes, leading to continued cell proliferation, increased survival, decreased differentiation and increased invasion. Oncogenic alleles of Ras (Ras V12 ) or Notch (Notch intra ) cooperate with mutants in scrib, dlg or lgl to produce invasive tumours. In a genetic screen for novel genes that when overexpressed cooperate with scrib mutants similarly to oncogenic Ras or Notch, we identified abrupt, which encodes a BTB/POZ zinc finger domain transcription factor.We are investigating the mechanism by which this novel oncogene cooperates with scrib mutants to promote tumourigenesis. Melanoma is the most aggressive and lethal form of skin cancer. In melanoma, melanocytes undergo aberrant proliferation, and invade other tissues giving rise to local and distant metastases. Using the GAL4-UAS system, we have developed a zebrafish transgenic line that expresses oncogenic Ras under the kit-a promoter and develops melanomas by 1-3 months of age. Local infiltrations of adjacent tissues (gills, muscles, brain) are present in most cases, whereas distant metastases are mediated by intravasation into newly formed tumor vessels. Transgenic larvae show already at 3 dpf an over-pigmentation phenotype, due to an increased number and size of melanophores, coupled to abnormal migration. We have therefore used the transgenic larvae to screen for pathways and/or mechanisms that revert the abnormal growth and migration phenotype of transformed melanophores. By expressing a dominant negative form of rab5 in melanophores through the UAS system, we observed profound modification of the polarity, size and migration of transformed melanocytes, thus indicating that endocytosis and/or vesicle trafficking is necessary for the transforming activity of oncogenic Ras in these cells. We hypothesized that Ras may rely on endocytosis or vesicular traffic to activate Rac, one of the Ras effectors mediating both transformation and abnormal migration of melanophores. Further studies are in progress to elucidate the role of endocytic processes and the cargos of rab5+ vesicles in Ras induced melanoma. Thus our model offers the advantage of both larval and adult phenotypes and allows...