A novel role for the tumor suppressor adenomatous polyposis coli in controlling retinoic acid production in the vertebrate intestine and retina;
The role of the adenomatous polyposis coli (APC) tumor suppressor in regulating intestinal homeostasis has previously been attributed solely to its ability to antagonize canonical WNT signaling via negative regulation of the transcriptional co-activator ?-catenin. Recent studies, however, have suggested the possibility of an additional role for APC in positively regulating retinoic acid production. This dissertation provides powerful genetic evidence that APC promotes epithelial differentiation in the vertebrate intestine and retina by controlling retinoic acid production via positive regulation of retinol dehydrogenases. We have identified two novel retinol dehydrogenases, zRDHA and zRDHB, which are expressed in the zebrafish intestine and, as determined by knockdown experiments, are required for establishment of intestinal cell number and subsequent intestinal differentiation. Similar defects were observed in the intestine of APC mutant zebrafish larvae. Importantly, the defects in APC mutants, and zRDHA and zRDHB morphant intestines were rescued by the addition of exogenous retinoic acid. In addition, APC mutants were rescued by injection of zRDHA thereby establishing a genetic relationship between these two genes. The data herein define hoxc8 as a downstream target of APC and retinoic acid whose overexpression is capable of rescuing intestinal defects in APC and zRDHB morphants. Furthermore, the data presented here firmly distinguish RDHs, rather than RALDHs, as the retinoid biosynthetic enzymes which APC regulates to control retinoic acid production. The findings in this dissertation additionally establish a novel role for APC in regulating RA production in the developing retina. Consistent with defects in the retina of FAP patients, APC mutant zebrafish display molecular and morphological abnormalities in retinal differentiation, particularly in the ventral photoreceptor layer and retinal pigmented epithelium (RPE). These defects were rescued by application of retinoic acid or injection of a unique, RPE-specific retinol dehydrogenase, zRDHH. Knockdown of zRDHH resulted in ventral retinal abnormalities similar to those present in APC mutants and these defects were also rescued by retinoic acid. The data presented in this dissertation illuminate a novel genetic pathway that includes APC regulation of retinol dehydrogenases as a mechanism for promoting epithelial differentiation in the vertebrate intestine and retina.