Role in Tumor Growth of a Glycogen Debranching Enzyme Lost in Glycogen Storage Disease

RNA interference (RNAi) is a cellular mechanism that negatively regulates gene expression, and has been utilized in loss-of-function studies (1). There are two primary methods of performing RNAi in animal cells, small interfering RNAs (siRNAs) and plasmid-encoded short-hairpin RNAs (shRNAs) (2); however, only the latter can be used to study complex in vivo xenograft phenotypes where long-term target depletion is required. Here, we used a genome-wide lentiviral shRNA library coupled with next-generation sequencing (NGS) to identify new genes that are important in tumor growth, using human xenograft models of bladder cancer, a common malignancy affecting the urinary system with an estimated 72570 new cases and 15210 deaths in 2013 in the United States (3). Our genome-wide screen identified the glycogen debranching enzyme amylo-α-1, 6-glucosidase, 4-α-glucanotransferase (AGL) as a regulator of in vitro and in vivo human cancer cell growth and prognostic marker in patients. Germline mutation of AGL causes glycogen storage disease III (GSD III, Cori disease) via abnormal glycogen breakdown. However, our studies demonstrate that AGL reduces tumor growth independent of its enzymatic activity, and loss of AGL in cancer cells promotes tumor growth in vitro and in vivo through increased glycine synthesis via induction of the glycine synthesizing enzyme serine hydroxymethyltransferase 2 (SHMT2). In summary, our discovery approach appears to be valuable in uncovering new functional pathways driving tumor growth, complementing current approaches in human tumors, and paving the way for novel therapeutic strategies and biomarkers for selecting patients for personalized approaches.