RET Mediated Gene Expression and Cell-Migration

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Cockburn, Jessica Grace
Cell-Migration , RET , Gene Expression , Cancer
The RET receptor tyrosine kinase is important during development of neural crest-derived tissues, particularly the enteric and sympathetic nervous systems, and in kidney morphogenesis. Activation of RET requires complex formation between a member of the Glial cell line-Derived Neurotrophic Factor family of ligands and a member of the GDNF-Family Receptor α co-receptors. Upon complex formation, RET becomes phosphorylated and subsequently activates multiple downstream signaling pathways, including those for cell-survival, differentiation, and migration. Mutations in RET can either interfere with or enhance normal RET signaling. Inhibiting RET mutations are associated with development of Hirschsprung disease, which is characterized by a lack of mature ganglia in the gut. Conversely, activating RET mutations are associated with several thyroid cancers. Papillary thyroid carcinoma is frequently associated with sporadic translocations between RET and other genes, known collectively as RET/PTC. A variety of heritable RET missense mutations lead to Multiple Endocrine Neoplasia type 2, which is associated with development of medullary thyroid carcinoma. Two cellular processes disrupted downstream of RET in these diseases are gene-expression and cell-migration. In order to clarify the effects of oncogenic mutations on gene-expression downstream of RET, we analyzed expression microarrays in a model using single mutant and isoform RET expression. We also examined the molecular mechanisms of cell-migration, using both functional cell-based assays and examination of integrins, cell-adhesion molecules important for cell-migration. Finally, we used a large cohort of thyroid tissues to examine RET and integrin expression. We showed that different forms of oncogenic RET do not affect transcription of different target genes, but rather target-gene transcription is proportional to phosphorylatability of mutant RET. We were also able to show that RET leads to activation of at least two integrin subunits (ITGB1 and ITGB3), and that they have unique activation patterns downstream of RET that correlate with cell-adhesion and migration. Finally, we showed that co-expression between RET, ITGB1, and ITGB3 is more frequent in malignant subtypes of thyroid tissues and that their co-expression is correlated to more aggressive thyroid cancer subtypes. Together, we have clarified how RET is able to mediate two important processes, gene-expression and cell-migration.
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