Evolution generates a remarkable breadth of living forms, but many traits evolve repeatedly, by still poorly understood mechanisms. A classic example of repeated evolution is the loss of pelvic hindfins in stickleback fish (Gasterosteus aculeatus). Repeated pelvic loss maps to recurrent deletions of a pelvic enhancer of the Pitx1 gene. Here, we identify molecular features contributing to these recurrent deletions. Pitx1 enhancer sequences form alternative DNA structures in vitro, and increase double-strand breaks and deletions in vivo. Enhancer mutability depends on DNA replication direction and is caused by (TG)-dinucleotide repeats. Modeling shows that elevated mutation rates can influence evolution under demographic conditions relevant for sticklebacks and humans. DNA fragility may thus help explain why the same loci are often used repeatedly during parallel adaptive evolution.

The Hippo pathway is an important regulator of organ size and tumorigenesis. It is unclear, however, how Hippo signaling provides the cellular building blocks required for rapid growth. Here, we demonstrate that transgenic zebrafish expressing an activated form of the Hippo pathway effector Yap1 (also known as YAP) develop enlarged livers and are prone to liver tumor formation. Transcriptomic and metabolomic profiling identify that Yap1 reprograms glutamine metabolism. Yap1 directly enhances glutamine synthetase (glul) expression and activity, elevating steady-state levels of glutamine and enhancing the relative isotopic enrichment of nitrogen during de novo purine and pyrimidine biosynthesis. Genetic or pharmacological inhibition of GLUL diminishes the isotopic enrichment of nitrogen into nucleotides, suppresses hepatomegaly and the growth of liver cancer cells. Consequently, Yap-driven liver growth is susceptible to nucleotide inhibition. Together, our findings demonstrate that Yap1 integrates the anabolic demands of tissue growth during development and tumorigenesis by reprogramming nitrogen metabolism to stimulate nucleotide biosynthesis.

SUMMARY The liver and pancreas arise from common endodermal progenitors. How these distinct cell fates are specified is poorly understood. Here, we describe prostaglandin E2 (PGE2) as a regulator of endodermal fate specification during development. Modulating PGE2 activity has opposing effects on liver-versus-pancreas specification in zebrafish embryos as well as mouse endodermal progenitors. The PGE2 synthetic enzyme cox2a and receptor ep2a are patterned such that cells closest to PGE2 synthesis acquire a liver fate whereas more distant cells acquire a pancreas fate. PGE2 interacts with the bmp2b pathway to regulate fate specification. At later stages of development, PGE2 acting via the ep4a receptor promotes outgrowth of both the liver and pancreas. PGE2 remains important for adult organ growth, as it modulates liver regeneration. This work provides in vivo evidence that PGE2 may act as a morphogen to regulate cell fate decisions and outgrowth of the embryonic endodermal anlagen.

Author contributions S.N. and W.G. conceived and designed the overall project. S.S. and C.I.U. assisted with selecting the family, gathering the clinical histories and collecting DNA samples under human subject IRB-approved protocols. S.N., W.G. and I.L. designed the WGS analysis. I.L. performed the WGS analysis and candidate variant filtering. S.N., J.W., A.J.K., J.E.H., A.G.C. and J.H. designed and generated the zebrafish rabl3 mutant lines and performed the cancer studies. J.R.H. and S.N. performed zebrafish histology preparation and analysis. J.D.M. performed and analyzed the AP-MS experiments and CompPASS suite protein interactomics. S.N., W.G. and C.W. conceived and designed the in vitro immunoprecipitation, prenylation assays and HEK293T cell proliferation assays, and P.G., A.B., E.L. and B.U. performed these experiments. S.N. and O.M. designed and performed RASless MEF experiments. J.W.P. performed protein structural modeling. B.C.J. and C.A.F. designed and performed purification of recombinant protein. J.A.P., S.G. and J.D.M. assisted with mass spectrometry analysis. Y.H. assisted with RNA-seq data analysis. M.B.G. performed the zebrafish μCT and bone histomorphometric analysis. O.M., X.W. and J.D.M. provided assistance with tissue culture experiments. C.A.C. and J.A.R. provided analysis of clinical exome sequencing data. C.A.C. and I.L. provided analysis of variants in the Exome Aggregation Consortium.