Loss of heterozygosity at human chromosome 18q, the region containing Deleted in Colorectal Cancer (DCC), has been linked to a number of human cancers including colorectal cancer. Presently no definitive evidence has been detected that pinpoints DCC as being a tumor suppressor. Here we present research on frazzled (fra), the Drosophila homolog of DCC, taking advantage of the powerful genetic tools this model organism presents.
In order to determine whether fra/DCC is a tumor suppressor we generated fra loss of function clones in Drosophila eye discs. While these clones had a low survival rate, we were able to observe an invasive phenotype when we rescued cells from death with a secondary genetic modification (Chapter 2). From this we determined that fra is a tumor suppressor and that second site mutations in addition to loss of fra/DCC, result in highly invasive carcinomas.
Two subsequent microarray studies were completed to determine the global effects of either loss or gain of fra signaling. Many cancer-related genes were revealed by these analyses (Chapter 3). In order to both validate the microarray results and identify loci that genetically interact with fra, we proceeded with a genetic interaction screen. Results of this screen allowed us to observe phenotypes resulting from gene mutations in conjunction with loss of fra. Many interesting genes resulted from this screen including the Hedgehog cascade member cAMP-dependent protein kinase 1 (pka) (Chapter 4). Assays were then designed to study the effects of loss of both fra and pka. From these analyses we were able to determine that loss of both of these genes results in cells that are highly invasive (Chapter 5).
The work presented in this study is the first direct evidence, in any animal model, that fra/DCC is a tumor suppressor. It has also identified a number of genes that when mutated in conjunction with loss of fra/DCC result in increased invasion. The results of this investigation provide new insight into the mechanisms involved in tumorigenesis and metastasis resulting from loss of DCC.