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    • A complementary study confirms our findings that GATA is nec

    2018-10-24

    A complementary study confirms our findings that GATA6 is necessary for s6 kinase specification as well as the decreased efficiency of β-like cell generation from the GATA6 PSC line (Shi et al., 2017). This other report did have one major discrepancy with our results; they did not see a defect in glucose-stimulated insulin secretion in GATA6 β-like cells. This could be due to genetic background as they examined gene-edited ES cells, not pancreas agenesis patient derived IPSCs, or due to differences in the differentiation protocols. Future studies will be needed to further address the role of GATA6 in β cell function in the PSC system.
    Experimental Procedures
    Acknowledgments We would like to thank Ge Liang, Helen Mac, and Jason Mills at the Stem Cell core at the Children\'s Hospital of Philadelphia for the generation of the CHOP.Panagenesis1 patient IPS+/indel line and cell sorting. We also thank Xin Cheng for initiating the generation of the IPS+/indel EP cell line and Stella Chou for the GATA1 cDNA construct. This work was supported by NIH grant R01 DK092113.
    Introduction Mutations in LDLR (encoding LDL receptor, LDLR), often heterozygous, underlie most cases of familial hypercholesterolemia (FH), which predisposes to premature cardiovascular disease due to marked elevation of plasma levels of lipids, in particular low-density lipoprotein cholesterol (LDL-C) (Brown and Goldstein, 1986). Besides diet control and physical activity, FH patients are treated with statins, a class of drugs that inhibit 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase and hence reduce cholesterol synthesis in the liver (Endo, 1992). Statins also increase LDLR protein levels in hepatocytes and LDL-C clearance from plasma. Because of these properties, statins are used to treat FH patients and also patients with non-familial hypercholesterolemia. However, statins fail to reduce plasma LDL-C adequately in the majority of these patients for prevention of cardiovascular events (Cannon et al., 2015; Reiner, 2015), and a proportion of patients suffers from significant adverse effects (Dormuth et al., 2014; Stroes et al., 2015). Importantly, FH can be caused by mutations in other genes besides LDLR, including gain-of-function mutations in proprotein convertase subtilisin/kexin type 9 (PCSK9) (Sniderman et al., 2014). This observation led to the subsequent identification of PCSK9 as an extracellular protein responsible for the internalization and degradation of surface LDLR in hepatocytes (Maxwell et al., 2005). Interestingly, PCSK9 is induced by statins, and activates a negative feedback loop that controls LDLR expression and consequently restrains the efficacy of statins (Dubuc et al., 2004). The latter may account for the limited efficacy or even loss of efficacy of statins in some patients, and has suggested that PCSK9 modulation could be of therapeutic relevance. In this regard, clinical trials with anti-PCSK9 therapy (in the form of monoclonal antibodies) have shown potent lowering effects on LDL-C and good tolerance, creating a tremendous interest in this approach as an alternative medication for hypercholesterolemia (Robinson et al., 2015; Roth et al., 2012). Besides the potential paradigm shift in FH treatment, these findings imply that interfering with additional components of the LDLR degradation pathway (besides PCSK9) may provide alternative therapeutic avenues for hypercholesterolemia. Despite the exciting developments with PCSK9 antibodies, a relevant caveat for further exploration of new therapies (new drugs or new formulations) for FH (and also other genetic liver diseases) is the lack of easily accessible and bona fide in vitro and in vivo models for preclinical testing. In vitro models for FH are mostly based on patient-derived hepatocytes from liver biopsy (Cayo et al., 2012), which have limited accessibility and cannot be expanded in culture. In vivo models including the hyperlipidemic rabbit, the rhesus macaque, and the Ldlr knockout mouse, have the limitation of not fully recapitulating human hepatocyte function (Bissig-Choisat et al., 2015). Patient-specific induced pluripotent stem cells (iPSCs) can provide an unlimited source of differentiated cell types including hepatocytes (iHeps) that can be used for in vitro and in vivo studies (Grskovic et al., 2011; Takahashi et al., 2007). This approach combined with the transplantation into immunodeficient mice may help overcome existing problems in modeling FH in vitro and in vivo (Carpentier et al., 2014; Chen et al., 2012; Liu et al., 2011). Several groups have generated FH iPSCs that harbor mutations in LDLR (Cayo et al., 2012; Ramakrishnan et al., 2015; Rashid et al., 2010) or PCSK9 (Si-Tayeb et al., 2016) and have tested the ability of the derived iHeps to mimic the disease phenotype and respond to statins in vitro. However, there are no reports so far testing the effect of anti-PCSK9 therapies on FH iPSC-derived iHeps in vitro, or in vivo disease modeling and drug testing with FH iHeps transplanted into appropriate animal models.