Pancreatic intraepithelial neoplasia (PanIN) is a premalignant lesion that can progress to pancreatic ductal adenocarcinoma, a highly lethal malignancy marked by its late stage at clinical presentation and profound drug resistance1. The genomic alterations that commonly occur in pancreatic cancer include activation of KRAS2 and inactivation of p53, and SMAD42-4. To date, however, it has been challenging to target these pathways therapeutically; thus the search for other key mediators of pancreatic cancer growth remains an important endeavor. Here we show that the stem cell determinant Musashi (Msi) is a critical element of pancreatic cancer progression in both genetic models and patient derived xenografts. Specifically, we developed Msi reporter mice that allowed image based tracking of stem cell signals within cancers, revealing that Msi expression rises as PanIN progresses to adenocarcinoma, and that Msi-expressing cells are key drivers of pancreatic cancer: they preferentially harbor the capacity to propagate adenocarcinoma, are enriched in circulating tumor cells, and are markedly drug resistant. This population could be effectively targeted by deletion of either Msi1 or Msi2, which led to a striking defect in PanIN progression to adenocarcinoma and an improvement in overall survival. Msi inhibition also blocked the growth of primary patient-derived tumors, suggesting that this signal is required for human disease. To define the translational potential of this work we developed antisense oligonucleotides against Msi; these showed reliable tumor penetration, uptake and target inhibition, and effectively blocked pancreatic cancer growth. Collectively, these studies highlight Msi reporters as a unique tool to identify therapy resistance, and define Msi signaling as a central regulator of pancreatic cancer.

Cell fate can be controlled through asymmetric division and segregation of protein determinants. But the regulation of this process in the hematopoietic system is poorly understood. Here we show that the dynein binding protein Lis1 (Pafah1b1) is critically required for blood formation and hematopoietic stem cell function. Conditional deletion of Lis1 in the hematopoietic system led to a severe bloodless phenotype, depletion of the stem cell pool and embryonic lethality. Further, the loss of Lis1 accelerated cell differentiation, in part through defects in spindle positioning and inheritance of cell fate determinants. Finally, deletion of Lis1 blocked propagation of myeloid leukemia and led to a marked improvement in animal survival, suggesting that Lis1 is also required for oncogenic growth. These data identify a key role for Lis1 in hematopoietic stem cells, and mark the directed control of asymmetric division as a critical regulator of normal and malignant hematopoietic development.

SUMMARY Acute myelogenous leukemia (AML) is an aggressive disease associated with drug resistance and relapse. To improve therapeutic strategies, it is critical to better understand the mechanisms that underlie AML progression. Here we show that the integrin binding glycoprotein CD98 plays a central role in AML. CD98 promotes AML propagation and lethality by driving engagement of leukemia cells with their microenvironment and maintaining leukemic stem cells. Further, delivery of a humanized anti-CD98 antibody blocks growth of patient-derived AML, highlighting the importance of this pathway in human disease. These findings indicate that microenvironmental interactions are key regulators of AML and that disrupting these signals with inhibitors such as CD98-antibodies may be a valuable therapeutic approach for adults and children with this disease.