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BACKGROUND Left ventricular non-compaction cardiomyopathy (LVNC) is a congenital cardiomyopathy, characterized by excessive ventricular trabeculation, with a ‘spongy’ appearance of the myocardium, deep intertrabecular recesses, and a thin compacted myocardial layer. LVNC has recently gained more attention because it has been increasingly recognized as a cause of heart failure, thromboembolism, or ventricular arrhythmia, which can all lead to sudden cardiac death. However, due to its heterogeneous anatomical and clinical features, LVNC is still a poorly understood condition. In addition, LVNC is suggested to be due to a premature arrest in the myocardial compaction process, but its genetic basis and pathologic mechanism remain largely unknown. METHODS AND RESULTS Here we introduce novel genetic factors implicated in ventricular compaction. Specifically, combinatorial loss of two Iroquois homeobox (Irx) transcription factors, Irx3, critical for development and function of the ventricular conduction system (e.g., His-bundle, bundle branches and Purkinje fibers) and Irx4, essential for establishing ventricular identity, recapitulates the clinical hallmarks of LVNC in mice. While mice lacking either Irx3 or Irx4 do not exhibit gross congenital cardiac malformations, mice lacking both Irx3 and Irx4 (Irx3;Irx4DKO) show postnatal lethality with abnormal muscular growths that resemble the polypoid pattern of LVNC, characterized by multiple muscular nodes in the left ventricle. Optical projection tomography in postnatal day 14 (P14) mutant hearts unveiled increased trabeculation, abnormal bridging below the papillary muscles, and disorganized muscle structures (Fig. 1). During embryogenesis, Irx3 and Irx4 exhibit overlapping expression in the developing ventricles. Consistent with the hypothesis that LVNC can be caused by a defect in embryonic heart growth, Irx3;Irx4DKO embryonic hearts showed abnormal trabeculation with thinner ventricular walls as early as E14.5. RNA sequencing on E14.5 Irx3;Irx4DKO ventricles revealed altered ventricular identity with atrial gene activation and enriched Bmp2/Tbx2 pathway that is critical for endocardial cushion development by suppressing cardiomyocyte differentiation. Moreover, co-immunoprecipitation discovered that Irx4 physically interacts with Irx3 and Gata4, and promoter assay further showed that Irx3, Irx4, and Gata4 synergistically suppress Tbx2 promoter activity, suggesting the repressive role of this transcriptional regulatory complex in the developing myocardium. CONCLUSION Our study demonstrates a novel mechanistic interaction of Irx3 and Irx4, in cooperation with Gata4, during the ventricular compaction process, thereby ensuring proper ventricular development. This finding can promote improvement of genetic testing for LVNC and prediction of individual outcomes with LVNC.

SummaryLimb skeletal pattern relies heavily on graded Sonic hedgehog (Shh) signaling. As a morphogen and growth cue, Shh regulates identities of posterior limb elements, including the ulna/fibula and digits 2 through 5. In contrast, proximal and anterior structures, including the humerus/femur, radius/tibia, and digit 1, are regarded as Shh independent, and mechanisms governing their specification are unclear. Here, we show that patterning of the proximal and anterior limb skeleton involves two phases. Irx3 and Irx5 (Irx3/5) are essential in the initiating limb bud to specify progenitors of the femur, tibia, and digit 1. However, these skeletal elements can be restored in Irx3/5 null mice when Shh signaling is diminished, indicating that Shh negatively regulates their formation after initiation. Our data provide genetic evidence supporting the concept of early specification and progressive determination of anterior limb pattern.

SUMMARY Cellular heterogeneity is frequently observed in cancer, but the biological significance of heterogeneous tumor clones is not well defined. Using multicolor reporters and CRISPR-Cas9 barcoding, we trace clonal dynamics in a mouse model of sarcoma. We show that primary tumor growth is associated with a reduction in clonal heterogeneity. Local recurrence of tumors following surgery or radiation therapy is driven by multiple clones. In contrast, advanced metastasis to the lungs is driven by clonal selection of a single metastatic clone (MC). Using RNA sequencing (RNA-seq) and in vivo assays, we identify candidate suppressors of metastasis, namely, Rasd1, Reck, and Aldh1a2. These genes are downregulated in MCs of the primary tumors prior to the formation of metastases. Overexpression of these suppressors of metastasis impair the ability of sarcoma cells to colonize the lungs. Overall, this study reveals clonal dynamics during each step of tumor progression, from initiation to growth, recurrence, and distant metastasis., In Brief Tang et al. demonstrate that distinct clonal architecture underlies different stages of tumor progression, from tumor initiation, local recurrence after therapy, to advanced metastasis., Graphical Abstract

Over 1 billion people are estimated to be overweight, placing them at risk for diabetes, cardiovascular disease, and cancer. We performed a systems-level genetic dissection of adiposity regulation using genome-wide RNAi screening in adult Drosophila. As a follow-up, the resulting approximately 500 candidate obesity genes were functionally classified using muscle-, oenocyte-, fat-body-, and neuronal-specific knockdown in vivo and revealed hedgehog signaling as the top-scoring fat-body-specific pathway. To extrapolate these findings into mammals, we generated fat-specific hedgehog-activation mutant mice. Intriguingly, these mice displayed near total loss of white, but not brown, fat compartments. Mechanistically, activation of hedgehog signaling irreversibly blocked differentiation of white adipocytes through direct, coordinate modulation of early adipogenic factors. These findings identify a role for hedgehog signaling in white/brown adipocyte determination and link in vivo RNAi-based scanning of the Drosophila genome to regulation of adipocyte cell fate in mammals.