Mechanisms of change in the gene expression landscape during development and cancer in Drosophila melanogaster

How multicellular organisms establish and maintain a diversity of incredibly complex body plans is a fundamental question in development. Complex body plans require patterning of limbs, organs, and tissues along axes, such as the segmentation seen along the anteroposterior axis in insects and the vertebrate central nervous system. This diversity of forms is regulated by an intricate network of genetic interactions, and allows for a diversity of functions in the living world. To reproducibly and correctly create complex systems, the genes that control patterning, known as selector genes, must be tightly regulated so that they are only expressed in their correct locations and at the correct time. Normally these patterns of gene expression are stable and invariant. How this regulation occurs continues to be a major area of study in developmental biology./nKey genes that confer patterning were first discovered in the fruit fly Drosophila melanogaster, and the utility of these selector genes was later found to be conserved throughout metazoans. Flies remain an incredibly valuable tool to study the principles of how patterns form. In Chapter 1, I discuss some basic principles of the genetics of pattern formation by selector genes and how this is regulated at the epigenetic level, with a focus on patterning in Drosophila. I also cover how cancer coopts these processes and what the relationship between tumorous growth and developmental patterning means for the treatment of cancer. I focus on two pathways, the Hippo pathway and the Hedgehog (Hh) pathway, which have been extensively studied for their conserved roles in growth and patterning, and which are the focus of my dissertation research./nThe Hippo pathway has been implicated as a driver of tissue growth in multiple different types of cancer. Over the last fifteen years, the core pathway as well as several targets of the Hippo pathway have been identified, but it remains unclear exactly how the pathway acts through these targets to control growth, as well as what other effects the pathway has on processes including patterning, cell-cell communication, and regulation of chromatin modification factors. Some aspects of Hippo pathway interactions with signaling pathways that control patterning have been studied, including between the Hippo pathway and Decapentaplegic (Dpp), which is the ortholog of vertebrate BMP2/4. However, interactions with other pathways aren’t fully understood. There has been some work done on the relationship between the Hippo pathway and the Hh pathway, but this mostly relates to Hh’s role upstream of the Hippo pathway and not on whether the Hippo pathway works through the Hh pathway. The Hh pathway has also been implicated in various types of cancer, but it is still not fully clear what regulates the inappropriate activation of the Hh pathway in cancer, nor what its role might be in tumor progression. My work identifies a role for Yorkie (Yki), the transcriptional co-activator of the Hippo pathway, in activating inappropriate Hh pathway signaling in tumorous tissue, which leads to downstream effects within the tumor and in surrounding cells./nChapter 2 describes my work investigating the relationship between these two pathways and how it relates to growth and heterotypic interactions between tumor tissue and surrounding cells. I characterized the intriguing phenomenon of Yki inappropriately activating selector gene expression in tumor tissue, and found that this phenomenon relies on Yki’s activity through two targets, the microRNA bantam and the chromatin regulator taranis. I identified evidence that Yki-mediated activation of these targets leads to changes in chromatin modifiers that facilitate inappropriate patterning gene expression, and I also found evidence that inappropriate developmental signaling from Yki-expressing tumors has an effect on the growth of surrounding wild type cells. Chapter 3 discusses the significance of this work in the budding study of how tumor cells control and communicate with their environment, as well as how they cause epigenetic changes that broadly alter gene expression./nMy work provides key information about how oncogenes can hijack normal developmental and genetic processes to facilitate the formation of cancer in otherwise healthy tissue. It contributes to our knowledge of how cancer cells behave, which can be used in future study on the biology of tumor tissue, as well as how to treat, categorize, and screen for it.

Advisor: Hariharan, Iswar K