Cancer is as unique as the person fighting it. With the exception of a few biomarker-driven therapies, patients go through rounds of trial-and-error approaches to find the best treatment. Using patient-derived cell lines, we show that zebrafish larvae xenotransplants constitute a fast and highly sensitive in vivo model for differential therapy response, with resolution to reveal intratumor functional cancer heterogeneity. We screened international colorectal cancer therapeutic guidelines and determined distinct functional tumor behaviors (proliferation, metastasis, and angiogenesis) and differential sensitivities to standard therapy. We observed a general higher sensitivity to FOLFIRI [5-fluorouracil(FU)+irinotecan+folinic acid] than to FOLFOX (5-FU+oxaliplatin+folinic acid), not only between isogenic tumors but also within the same tumor. We directly compared zebrafish xenografts with mouse xenografts and show that relative sensitivities obtained in zebrafish are maintained in the rodent model. Our data also illustrate how mutations can provide proliferation advantages in relation to KRASWT and how chemotherapy can unbalance this advantage, selecting for a minor clone resistant to chemotherapy. Zebrafish xenografts provide remarkable resolution to measure Cetuximab sensitivity. Finally, we demonstrate the feasibility of using primary patient samples to generate zebrafish patient-derived xenografts (zPDX) and provide proof-of-concept experiments that compare response to chemotherapy and biological therapies between patients and zPDX. Altogether, our results suggest that zebrafish larvae xenografts constitute a promising fast assay for precision medicine, bridging the gap between genotype and phenotype in an in vivo setting.

HES transcriptional repressors are important components of the Notch pathway that regulates neurogenesis from Drosophila to vertebrates. These proteins are normally induced by Notch activity and inhibit neural commitment by antagonizing the activity of proneural genes. We describe here four chick hes genes that are expressed during neurogenesis: three hes5-like genes (hes5-1, hes5-2 and hes5-3) and one hes6-like (hes6-2). We show that hes6-2 represses transcription of the hes5 genes, thus functioning as a negative regulator of Notch signaling. Conversely, hes6-2 may be repressed by hes5 activity. In cells committing to differentiation, we find that hes6-2 is up-regulated by proneural genes and contributes to the proneural program of neuronal commitment by preventing Notch activity in these cells. In neural progenitors, Notch signaling produces an initial burst of hes5 activity, which represses hes6-2. However, as hes5 transcription declines due to negative auto-regulation, hes6-2 may become active and inhibit the remaining hes5 activity to end Notch signaling. These cells can then enter a new cycle of fate decisions and will be kept as progenitors if a new pulse of Notch activity occurs. Maintenance of progenitors during vertebrate neurogenesis therefore requires that these cells go through successive cycles of Notch activity. We propose that the hes5/hes6 circuitry of negative cross-regulations is a conserved feature of the Notch pathway that underlies these cycles in neural progenitors.

SUMMARYSomites are formed from the presomitic mesoderm (PSM) and give rise to the axial skeleton and skeletal muscles. The PSM is dynamic; somites are generated at the anterior end, while the posterior end is continually renewed with new cells entering from the tailbud progenitor region. Which genes control the conversion of tailbud progenitors into PSM and how is this process coordinated with cell movement? Using loss-and gain-of-function experiments and heat-shock transgenics we show in zebrafish that the transcription factor Mesogenin 1 (Msgn1), acting with Spadetail (Spt), has a central role. Msgn1 allows progression of the PSM differentiation program by switching off the progenitor maintenance genes ntl, wnt3a, wnt8 and fgf8 in the future PSM cells as they exit from the tailbud, and subsequently induces expression of PSM markers such as tbx24. msgn1 is itself positively regulated by Ntl/Wnt/Fgf, creating a negative-feedback loop that might be crucial to regulate homeostasis of the progenitor population until somitogenesis ends. Msgn1 drives not only the changes in gene expression in the nascent PSM cells but also the movements by which they stream out of the tailbud into the PSM. Loss of Msgn1 reduces the flux of cells out of the tailbud, producing smaller somites and an enlarged tailbud, and, by delaying exhaustion of the progenitor population, results in supernumerary tail somites. Through its combined effects on gene expression and cell movement, Msgn1 (with Spt) plays a key role both in genesis of the paraxial mesoderm and in maintenance of the progenitor population from which it derives.