Mechanisms Of Stromal Lkb1 Loss Induced Tumorigenesis In Mouse Models Of Peutz-Jeghers Syndrome

Germline mutations in tumor suppressor kinase Lkb1 predispose to Peutz-Jeghers syndrome (PJS), with highly penetrant gastrointestinal polyposis and increased cancer risk. PJS polyps display abnormal growth of both stromal and epithelial cells. We have identified clonally expanding fibroblasts as the drivers of tumorigenesis in PJS mouse models by using Fsp1-Cre and Twist2-Cre mice to restrict Lkb1 deletion to stromal cells (unpublished), highlighting the importance of stromal-epithelial signaling in PJS tumorigenesis. Here, we address the molecular mechanisms of the pathogenesis linked to stromal Lkb1 loss in PJS mouse models. Lkb1 is involved in tissue size control by inhibiting of mTORC1 pathway via AMPK (Shaw et al, Cancer Cell 2004) and by regulating Hippo pathway (Mohseni et al, Nat Cell Biol 2014). First, we investigated the involvement of Lkb1-AMPK-mTORC1 pathway in polyposis. We conditionally deleted both catalytic AMPKa subunits (a1 and a2) in mice using Fsp1-Cre. Surprisingly, Fsp1-Cre-AMPK mice did not develop any tumors by 17 months of age indicating that AMPK is not the critical mediator of Lkb1 tumor suppression in PJS. We next studied the Hippo pathway in polyps of PJS mouse models. Levels of Yap and Taz, the main downstream effectors of Hippo, were elevated in polyps as shown by Western blotting. Immunohistochemical staining revealed profound nuclear Yap/Taz localization indicating transcriptional activity in the stromal compartment throughout the polyps. In contrast, nuclear Yap/Taz staining in epithelial cells was only noted in a restricted polyp base stem cell zone. In addition, we performed RNA-seq analysis of the of Fsp1-Cre;Lkb1flox mouse polyps and noted shared gene expression changes with PJS patient polyps. RNA-seq also revealed significant enrichment of Yap signature, which was validated by qPCR. Finally, we used small intestinal epithelial organoid culture to study the potential of Yap/Taz induced secreted factors to stimulate growth of epithelial cells. We observed that Wnt5a and Epiregulin enhanced epithelial organoid growth, indicating them as candidates mediating the stromal-epithelial signaling and promoting tumorigenesis in PJS polyps. In conclusion, we show that stromal Lkb1 mutations lead to phenotypes identical to germline Lkb1 heterozygosity in mice and propose increased stromal Yap/Taz activity as a potential mechanism to drive PJS tumorigenes. Citation Format: Saara Ollila, Kari Vaahtomeri, Iris Wong, Kaisa Laajanen, Tomi P. Makela. Mechanisms of stromal Lkb1 loss induced tumorigenesis in mouse models of Peutz-Jeghers syndrome. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 712.