Liver Cancer Cell of Origin, Molecular Class, and Effects on Patient Prognosis

Primary liver cancer is the second leading cause of cancer-related death worldwide and therefore a major public health challenge. We review hypotheses of the cell of origin of liver tumorigenesis and clarify the classes of liver cancer based on molecular features and how they affect patient prognosis. Primary liver cancer comprises hepatocellular carcinoma (HCC), intrahepatic cholangiocarcinoma (iCCA), and other rare tumors, notably fibrolamellar carcinoma and hepatoblastoma. The molecular and clinical features of HCC versus iCCA are distinct, but these conditions have overlapping risk factors and pathways of oncogenesis. A better understanding of the cell types originating liver cancer can aid in exploring molecular mechanisms of carcinogenesis and therapeutic options. Molecular studies have identified adult hepatocytes as the cell of origin. These cells have been proposed to transform directly into HCC cells (via a sequence of genetic alterations), to dedifferentiate into hepatocyte precursor cells (which then become HCC cells that express progenitor cell markers), or to transdifferentiate into biliary-like cells (which give rise to iCCA). Alternatively, progenitor cells also give rise to HCCs and iCCAs with markers of progenitor cells. Advances in genome profiling and next-generation sequencing have led to the classification of HCCs based on molecular features and assigned them to categories such as proliferation–progenitor, proliferation–transforming growth factor β, and Wnt–catenin β1. iCCAs have been assigned to categories of proliferation and inflammation. Overall, proliferation subclasses are associated with a more aggressive phenotype and poor outcome of patients, although more specific signatures have refined our prognostic abilities. Analyses of genetic alterations have identified those that might be targeted therapeutically, such as fusions in the FGFR2 gene and mutations in genes encoding isocitrate dehydrogenases (in approximately 60% of iCCAs) or amplifications at 11q13 and 6p21 (in approximately 15% of HCCs). Further studies of these alterations are needed before they can be used as biomarkers in clinical decision making. Primary liver cancer is the second leading cause of cancer-related death worldwide and therefore a major public health challenge. We review hypotheses of the cell of origin of liver tumorigenesis and clarify the classes of liver cancer based on molecular features and how they affect patient prognosis. Primary liver cancer comprises hepatocellular carcinoma (HCC), intrahepatic cholangiocarcinoma (iCCA), and other rare tumors, notably fibrolamellar carcinoma and hepatoblastoma. The molecular and clinical features of HCC versus iCCA are distinct, but these conditions have overlapping risk factors and pathways of oncogenesis. A better understanding of the cell types originating liver cancer can aid in exploring molecular mechanisms of carcinogenesis and therapeutic options. Molecular studies have identified adult hepatocytes as the cell of origin. These cells have been proposed to transform directly into HCC cells (via a sequence of genetic alterations), to dedifferentiate into hepatocyte precursor cells (which then become HCC cells that express progenitor cell markers), or to transdifferentiate into biliary-like cells (which give rise to iCCA). Alternatively, progenitor cells also give rise to HCCs and iCCAs with markers of progenitor cells. Advances in genome profiling and next-generation sequencing have led to the classification of HCCs based on molecular features and assigned them to categories such as proliferation–progenitor, proliferation–transforming growth factor β, and Wnt–catenin β1. iCCAs have been assigned to categories of proliferation and inflammation. Overall, proliferation subclasses are associated with a more aggressive phenotype and poor outcome of patients, although more specific signatures have refined our prognostic abilities. Analyses of genetic alterations have identified those that might be targeted therapeutically, such as fusions in the FGFR2 gene and mutations in genes encoding isocitrate dehydrogenases (in approximately 60% of iCCAs) or amplifications at 11q13 and 6p21 (in approximately 15% of HCCs). Further studies of these alterations are needed before they can be used as biomarkers in clinical decision making. Liver cancer is the second most common cause of cancer-related death worldwide. It is one of the few neoplasms with a steady increasing incidence and mortality1Murray C.J. Vos T. Lozano R. et al.Disability-adjusted life years (DALYs) for 291 diseases and injuries in 21 regions, 1990-2010: a systematic analysis for the Global Burden of Disease Study 2010.Lancet. 2012; 380: 2197-2223Abstract Full Text Full Text PDF PubMed Scopus (3434) Google Scholar, 2Llovet J.M. Zucman-Rossi J. Pikarsky E. et al.Hepatocellular carcinoma.Nat Rev Dis Primers. 2016; 2: 16018Crossref PubMed Google Scholar and is the neoplasm with the greatest increase in mortality in the United States during the past 2 decades (Figure 13Llovet J.M. Villanueva A. Lachenmayer A. et al.Advances in targeted therapies for hepatocellular carcinoma in the genomic era.Nat Rev Clin Oncol. 2015; 12: 436Crossref PubMed Scopus (19) Google Scholar). Liver cancer comprises a heterogeneous group of malignant tumors with different histological features and an unfavorable prognosis that range from hepatocellular carcinoma (HCC) and intrahepatic cholangiocarcinoma (iCCA) to mixed hepatocellular cholangiocarcinoma (HCC-CCA), fibrolamellar HCC (FLC), and the pediatric neoplasm hepatoblastoma.4Lozano R. Naghavi M. Foreman K. et al.Global and regional mortality from 235 causes of death for 20 age groups in 1990 and 2010: a systematic analysis for the Global Burden of Disease Study 2010.Lancet. 2012; 380: 2095-2128Abstract Full Text Full Text PDF PubMed Scopus (4983) Google Scholar, 5World Health Organization WHO classification of tumours of the digestive system. IARC, Lyon, France2010Google Scholar Among these, HCC and iCCA are the most common primary liver cancers; the other neoplasms, including mixed HCC-CCA tumors,5World Health Organization WHO classification of tumours of the digestive system. IARC, Lyon, France2010Google Scholar account for less than 1% of cases. The burden of liver cancer is increasing globally, and there could be 1 million cases by 2030.6Torre L.A. Bray F. Siegel R.L. et al.Global cancer statistics, 2012.CA Cancer J Clin. 2015; 65: 87-108Crossref PubMed Scopus (7370) Google Scholar It is not clear how direct-acting antiviral agents, which can cure hepatitis C virus (HCV) infection, will affect the burden of HCC. It has been estimated that curing more than 90% of cases of HCV infection would eliminate 15% of cases of HCC in the United States.7Singal A.G. El-Serag H.B. Hepatocellular carcinoma from epidemiology to prevention: translating knowledge into practice.Clin Gastroenterol Hepatol. 2015; 13: 2140-2151Abstract Full Text Full Text PDF PubMed Scopus (86) Google Scholar However, there is debate over the effects of direct-acting antiviral agents on progression of HCC.8Llovet J.M. Villanueva A. Effect of HCV clearance with direct-acting antiviral agents on HCC.Nat Rev Gastroenterol Hepatol. 2016; 13: 561-562Crossref PubMed Google Scholar, 9Reig M. Marino Z. Perello C. et al.Unexpected high rate of early tumor recurrence in patients with HCV-related HCC undergoing interferon-free therapy.J Hepatol. 2016; 65: 719-726Abstract Full Text Full Text PDF PubMed Scopus (179) Google Scholar, 10Conti F. Buonfiglioli F. Scuteri A. et al.Early occurrence and recurrence of hepatocellular carcinoma in HCV-related cirrhosis treated with direct-acting antivirals.J Hepatol. 2016; 65: 727-733Abstract Full Text Full Text PDF PubMed Scopus (118) Google Scholar, 11Cheung M.C. Walker A.J. Hudson B.E. et al.Outcomes after successful direct-acting antiviral therapy for patients with chronic hepatitis C and decompensated cirrhosis.J Hepatol. 2016; 65: 741-747Abstract Full Text Full Text PDF PubMed Scopus (71) Google Scholar HCC alone accounts for 90% of all cases of primary liver cancer, with nearly 800,000 new cases annually.2Llovet J.M. Zucman-Rossi J. Pikarsky E. et al.Hepatocellular carcinoma.Nat Rev Dis Primers. 2016; 2: 16018Crossref PubMed Google Scholar The incidence is highest in Asia and Sub-Saharan Africa due to the high prevalence of hepatitis B virus (HBV) infection.6Torre L.A. Bray F. Siegel R.L. et al.Global cancer statistics, 2012.CA Cancer J Clin. 2015; 65: 87-108Crossref PubMed Scopus (7370) Google Scholar Unlike other cancers, the main risk factors associated with HCC are well defined and include viral hepatitis (B and/or C), alcohol abuse, and nonalcoholic fatty liver disease in patients with metabolic syndrome and diabetes. Other cofactors of HCC development, such as aflatoxin B1 and tobacco, increase the incidence of the disease if other common risk factors are present.12Chuang S.C. La Vecchia C. Boffetta P. Liver cancer: descriptive epidemiology and risk factors other than HBV and HCV infection.Cancer Lett. 2009; 286: 9-14Abstract Full Text Full Text PDF PubMed Scopus (158) Google Scholar The second most common liver cancer is iCCA, with the highest incidence in Southeast Asia (30–40 cases/105 inhabitants) and low incidence in Western countries (fewer than 5 cases/105 inhabitants).13Bridgewater J. Galle P.R. Khan S.A. et al.Guidelines for the diagnosis and management of intrahepatic cholangiocarcinoma.J Hepatol. 2014; 60: 1268-1289Abstract Full Text Full Text PDF PubMed Scopus (176) Google Scholar Nevertheless, steady increases in incidence have been reported.13Bridgewater J. Galle P.R. Khan S.A. et al.Guidelines for the diagnosis and management of intrahepatic cholangiocarcinoma.J Hepatol. 2014; 60: 1268-1289Abstract Full Text Full Text PDF PubMed Scopus (176) Google Scholar, 14Moeini A. Sia D. Bardeesy N. et al.Molecular pathogenesis and targeted therapies for intrahepatic cholangiocarcinoma.Clin Cancer Res. 2016; 22: 291-300Crossref PubMed Scopus (126) Google Scholar Risk factors for development of iCCA include primary sclerosing cholangitis (PSC), biliary duct cysts, hepatolithiasis, and parasitic biliary infestation with flukes, which is an etiology prevalent in Asia and linked to a specific molecular fingerprint.13Bridgewater J. Galle P.R. Khan S.A. et al.Guidelines for the diagnosis and management of intrahepatic cholangiocarcinoma.J Hepatol. 2014; 60: 1268-1289Abstract Full Text Full Text PDF PubMed Scopus (176) Google Scholar More recently, shared risk factors with HCC have also been identified, such as HBV and HCV, particularly for iCCAs that develop in cirrhotic liver.15Palmer W.C. Patel T. Are common factors involved in the pathogenesis of primary liver cancers? A meta-analysis of risk factors for intrahepatic cholangiocarcinoma.J Hepatol. 2012; 57: 69-76Abstract Full Text Full Text PDF PubMed Scopus (152) Google Scholar HCC and iCCA have been considered to be independent tumors that originate from distinct cell populations. However, more recently, some have been recognized as tumor subtypes of a continuous spectrum of diseases. We review the theories behind the cell(s) of origin of liver cancer, describe emerging molecular classes, link these classes with their etiology and prognosis, and define pathways for future translation. Parenchymal (hepatocytes and cholangiocytes) and nonparenchymal cells (fibroblasts, stellate cells, Kupffer cells, and endothelial cells) form the basic hepatic structure (Figure 2); the existence of stem cells in adult liver has been heavily debated. Hepatocytes constitute 60% to 80% of the total liver mass. Architecturally, these cells are organized in lobules, which can be further divided in functional regions or zones.16Stanger B.Z. Cellular homeostasis and repair in the mammalian liver.Annu Rev Physiol. 2015; 77: 179-200Crossref PubMed Scopus (16) Google Scholar Liver zonation is particularly relevant for hepatocytes, because it affects their function without altering their phenotype. Hepatocytes are frequently polyploid (4N, 8N, and so on); polyploid cells make up 50% of human liver and 90% of mouse liver.17Knouse K.A. Wu J. Whittaker C.A. et al.Single cell sequencing reveals low levels of aneuploidy across mammalian tissues.Proc Natl Acad Sci U S A. 2014; 111: 13409-13414Crossref PubMed Scopus (67) Google Scholar Liver tumors are heterogeneous in morphology, within the same and between different tumors. Some subtypes of HCCs and iCCAs have stem cell features,18Roskams T.A. Libbrecht L. Desmet V.J. Progenitor cells in diseased human liver.Semin Liver Dis. 2003; 23: 385-396Crossref PubMed Scopus (221) Google Scholar, 19Lee J.S. Heo J. Libbrecht L. et al.A novel prognostic subtype of human hepatocellular carcinoma derived from hepatic progenitor cells.Nat Med. 2006; 12: 410-416Crossref PubMed Scopus (591) Google Scholar, 20Wang M. Xiao J. Jiang J. et al.CD133 and ALDH may be the molecular markers of cholangiocarcinoma stem cells.Int J Cancer. 2011; 128: 1996-1997Crossref PubMed Scopus (0) Google Scholar such as HCCs with CK19-positive cells.21Roskams T. Liver stem cells and their implication in hepatocellular and cholangiocarcinoma.Oncogene. 2006; 25: 3818-3822Crossref PubMed Scopus (271) Google Scholar The rare, mixed HCC-CCAs have cells with a phenotype that is intermediate between hepatocytes and cholangiocytes.22Singh S. Chakraborty S. Bonthu N. et al.Combined hepatocellular cholangiocarcinoma: a case report and review of literature.Dig Dis Sci. 2013; 58: 2114-2123Crossref PubMed Scopus (7) Google Scholar These observations have led to several hypotheses about the cell(s) of origin of liver cancer. Hepatic progenitor cells might generate primary liver tumors because, during liver development, hepatocytes and cholangiocytes each arise from a common progenitor (hepatoblasts). However, HCCs and iCCAs might be distinct tumors that originate from mature hepatocytes and cholangiocytes, respectively. Findings from recent studies indicate that adult hepatocytes are the source of HCCs because they can directly degenerate into HCC following sequential genomic insults, dedifferentiate into precursor cells that ultimately can transform into HCC cells with markers of progenitor cells, and transdifferentiate into biliary-like cells able to transform into iCCA23Tanimizu N. Nakamura Y. Ichinohe N. et al.Hepatic biliary epithelial cells acquire epithelial integrity but lose plasticity to differentiate into hepatocytes in vitro during development.J Cell Sci. 2013; 126: 5239-5246Crossref PubMed Scopus (11) Google Scholar, 24Chen Y. Wong P.P. Sjeklocha L. et al.Mature hepatocytes exhibit unexpected plasticity by direct dedifferentiation into liver progenitor cells in culture.Hepatology. 2012; 55: 563-574Crossref PubMed Scopus (42) Google Scholar, 25Tarlow B.D. Pelz C. Naugler W.E. et al.Bipotential adult liver progenitors are derived from chronically injured mature hepatocytes.Cell Stem Cell. 2014; 15: 605-618Abstract Full Text Full Text PDF PubMed Scopus (119) Google Scholar (Figure 3). In parallel, progenitor cells may also give rise to HCCs and iCCAs with progenitor-like features, whereas adult cholangiocytes, which lack the plasticity and transforming capacity of hepatocytes, can only give rise to iCCAs.26Guest R.V. Boulter L. Kendall T.J. et al.Cell lineage tracing reveals a biliary origin of intrahepatic cholangiocarcinoma.Cancer Res. 2014; 74: 1005-1010Crossref PubMed Scopus (38) Google Scholar Adult stem cells or somatic stem cells are defined as undifferentiated cells with limitless replicative potential. They can differentiate into all or some specialized cell types, and their primary role is to sustain physiological tissue turnover and direct tissue repair on different types of injury.27Tang D.G. Understanding cancer stem cell heterogeneity and plasticity.Cell Res. 2012; 22: 457-472Crossref PubMed Scopus (195) Google Scholar Stem cell compartments have been successfully identified in several adult tissues with fast turnover, including the gastrointestinal tract, skin, and bone marrow. Because of their potential for self-renewal and long life span, these cells are believed to be more prone to malignant transformation28Pardal R. Clarke M.F. Morrison S.J. Applying the principles of stem-cell biology to cancer.Nat Rev Cancer. 2003; 3: 895-902Crossref PubMed Google Scholar, 29Arwert E.N. Hoste E. Watt F.M. Epithelial stem cells, wound healing and cancer.Nat Rev Cancer. 2012; 12: 170-180Crossref PubMed Scopus (190) Google Scholar and have been identified as the cell of origin of cancer in several organs (eg, skin and intestine).30Barker N. Ridgway R.A. van Es J.H. et al.Crypt stem cells as the cells-of-origin of intestinal cancer.Nature. 2009; 457: 608-611Crossref PubMed Scopus (1091) Google Scholar, 31Zhu L. Gibson P. Currle D.S. et al.Prominin 1 marks intestinal stem cells that are susceptible to neoplastic transformation.Nature. 2009; 457: 603-607Crossref PubMed Scopus (440) Google Scholar, 32Lapouge G. Youssef K.K. Vokaer B. et al.Identifying the cellular origin of squamous skin tumors.Proc Natl Acad Sci U S A. 2011; 108: 7431-7436Crossref PubMed Scopus (0) Google Scholar Unlike these organs, however, cells of the adult liver have relatively slow turnover (a period of several months), with an average hepatocyte life span ranging from 200 to 300 days.33Duncan A.W. Dorrell C. Grompe M. Stem cells and liver regeneration.Gastroenterology. 2009; 137: 466-481Abstract Full Text Full Text PDF PubMed Scopus (301) Google Scholar, 34Fausto N. Campbell J.S. The role of hepatocytes and oval cells in liver regeneration and repopulation.Mech Dev. 2003; 120: 117-130Crossref PubMed Scopus (441) Google Scholar In this context, the contribution of stem cells to liver turnover is unclear (reviewed in Miyajima et al35Miyajima A. Tanaka M. Itoh T. Stem/progenitor cells in liver development, homeostasis, regeneration, and reprogramming.Cell Stem Cell. 2014; 14: 561-574Abstract Full Text Full Text PDF PubMed Scopus (137) Google Scholar). Hepatocytes are quiescent in normal liver, but their turnover in the liver dramatically accelerates on hepatic injury or after a significant reduction of liver mass such as following partial hepatectomy. Under these conditions, adult hepatocytes have enormous proliferative potential (capacity to replicate more than 50 times), a feature that has enabled human liver organ donor transplantation as a standard procedure to treat end-stage liver disease.33Duncan A.W. Dorrell C. Grompe M. Stem cells and liver regeneration.Gastroenterology. 2009; 137: 466-481Abstract Full Text Full Text PDF PubMed Scopus (301) Google Scholar, 34Fausto N. Campbell J.S. The role of hepatocytes and oval cells in liver regeneration and repopulation.Mech Dev. 2003; 120: 117-130Crossref PubMed Scopus (441) Google Scholar, 35Miyajima A. Tanaka M. Itoh T. Stem/progenitor cells in liver development, homeostasis, regeneration, and reprogramming.Cell Stem Cell. 2014; 14: 561-574Abstract Full Text Full Text PDF PubMed Scopus (137) Google Scholar There is evidence that regeneration after partial hepatectomy relies predominantly on the replication of hepatocytes rather than stem cells. However, when the replicative capacity of hepatocytes is severely impaired, such as in patients with acute liver failure or chronic hepatitis, epithelial cells with intermediate hepatocyte-cholangiocyte phenotypes emerge and expand.36Fausto N. Liver regeneration and repair: hepatocytes, progenitor cells, and stem cells.Hepatology. 2004; 39: 1477-1487Crossref PubMed Scopus (516) Google Scholar, 37Roskams T.A. Theise N.D. Balabaud C. et al.Nomenclature of the finer branches of the biliary tree: canals, ductules, and ductular reactions in human livers.Hepatology. 2004; 39: 1739-1745Crossref PubMed Scopus (404) Google Scholar, 38Turanyi E. Dezso K. Csomor J. et al.Immunohistochemical classification of ductular reactions in human liver.Histopathology. 2010; 57: 607-614Crossref PubMed Scopus (0) Google Scholar These cells, also known as oval cells, are believed to localize to the periportal area in the canals of Hering and are considered to be bipotential progenitors35Miyajima A. Tanaka M. Itoh T. Stem/progenitor cells in liver development, homeostasis, regeneration, and reprogramming.Cell Stem Cell. 2014; 14: 561-574Abstract Full Text Full Text PDF PubMed Scopus (137) Google Scholar, 39Peng N. Li L. Cai X. et al.Liver stem/progenitor cells in the canals of Hering: cellular origin of hepatocellular carcinoma with bile duct tumor thrombi?.Stem Cell Rev. 2010; 6: 579-584Crossref PubMed Scopus (9) Google Scholar (Figure 2). The capacity of these cells to repopulate the damaged liver has been shown in mice with massive liver damage, restoring more than 80% of hepatocytes.40Lu W.Y. Bird T.G. Boulter L. et al.Hepatic progenitor cells of biliary origin with liver repopulation capacity.Nat Cell Biol. 2015; 17: 971-983Crossref PubMed Scopus (98) Google Scholar Other studies from liver injury models have supported the liver stem cell hypothesis, showing proliferation of duct-like cells, frequently termed the ductular reaction, in the portal zone of the hepatic lobule.41Furuyama K. Kawaguchi Y. Akiyama H. et al.Continuous cell supply from a Sox9-expressing progenitor zone in adult liver, exocrine pancreas and intestine.Nat Genet. 2011; 43: 34-41Crossref PubMed Scopus (429) Google Scholar, 42Espanol-Suner R. Carpentier R. Van Hul N. et al.Liver progenitor cells yield functional hepatocytes in response to chronic liver injury in mice.Gastroenterology. 2012; 143: 1564-1575 e7Abstract Full Text Full Text PDF PubMed Scopus (148) Google Scholar, 43Petersen B.E. Grossbard B. Hatch H. et al.Mouse A6-positive hepatic oval cells also express several hematopoietic stem cell markers.Hepatology. 2003; 37: 632-640Crossref PubMed Scopus (203) Google Scholar However, this feature is controversial.44Yanger K. Knigin D. Zong Y. et al.Adult hepatocytes are generated by self-duplication rather than stem cell differentiation.Cell Stem Cell. 2014; 15: 340-349Abstract Full Text Full Text PDF PubMed Scopus (140) Google Scholar, 45Schaub J.R. Malato Y. Gormond C. et al.Evidence against a stem cell origin of new hepatocytes in a common mouse model of chronic liver injury.Cell Rep. 2014; 8: 933-939Abstract Full Text Full Text PDF PubMed Scopus (101) Google Scholar Other cells have been implicated in regeneration, repair, and eventually carcinogenesis. HCV-related chronic injury can induce ductular metaplasia and proliferation, with mature hepatocytes transdifferentiating to bipotential oval cells via epigenetic reprograming.25Tarlow B.D. Pelz C. Naugler W.E. et al.Bipotential adult liver progenitors are derived from chronically injured mature hepatocytes.Cell Stem Cell. 2014; 15: 605-618Abstract Full Text Full Text PDF PubMed Scopus (119) Google Scholar These cells are different from the classic oval cells of the canals of Hering and may have a more relevant role in hepatocarcinogenesis (Figure 2). However, other sources of precursors have been identified. These include specialized pericentral liver cells, capable of self-renewal in the uninjured liver under the influence of endothelial Wnt signaling.46Wang B. Zhao L. Fish M. et al.Self-renewing diploid Axin2(+) cells fuel homeostatic renewal of the liver.Nature. 2015; 524: 180-185Crossref PubMed Scopus (134) Google Scholar These pericentral cells express the early liver progenitor marker TBX347Suzuki A. Sekiya S. Buscher D. et al.Tbx3 controls the fate of hepatic progenitor cells in liver development by suppressing p19ARF expression.Development. 2008; 135: 1589-1595Crossref PubMed Scopus (0) Google Scholar and are diploid. Hybrid periportal cells express hepatocyte markers, along with low levels of SOX9 and several bile duct genes, and are able to repopulate the healthy and diseased liver mass.48Font-Burgada J. Shalapour S. Ramaswamy S. et al.Hybrid periportal hepatocytes regenerate the injured liver without giving rise to cancer.Cell. 2015; 162: 766-779Abstract Full Text Full Text PDF PubMed Google Scholar It is not clear whether these cell populations are bona fide stem or progenitor cells or are distinct hepatocyte subpopulations and whether they contribute to tumorigenesis.49Zaret K.S. Regenerative biology: maintaining liver mass.Nature. 2015; 524: 165-166Crossref PubMed Scopus (2) Google Scholar Extensive experimental studies support the hypothesis that progenitor cells might originate liver cancer50Kitade M. Factor V.M. Andersen J.B. et al.Specific fate decisions in adult hepatic progenitor cells driven by MET and EGFR signaling.Genes Dev. 2013; 27: 1706-1717Crossref PubMed Scopus (27) Google Scholar, 51Chiba T. Zheng Y.W. Kita K. et al.Enhanced self-renewal capability in hepatic stem/progenitor cells drives cancer initiation.Gastroenterology. 2007; 133: 937-950Abstract Full Text Full Text PDF PubMed Scopus (156) Google Scholar, 52Tschaharganeh D.F. Xue W. Calvisi D.F. et al.p53-dependent Nestin regulation links tumor suppression to cellular plasticity in liver cancer.Cell. 2014; 158: 579-592Abstract Full Text Full Text PDF PubMed Google Scholar (Table 1). For example, mice with genetic alterations that affect the Hippo pathway within the liver expand progenitor-like cells and subsequently develop HCC, iCCA, and mixed HCC-CCA.53Fitamant J. Kottakis F. Benhamouche S. et al.YAP inhibition restores hepatocyte differentiation in advanced HCC, leading to tumor regression.Cell Rep. 2015 Mar 10; ([Epub ahead of print])PubMed Google Scholar, 54Lu L. Li Y. Kim S.M. et al.Hippo signaling is a potent in vivo growth and tumor suppressor pathway in the mammalian liver.Proc Natl Acad Sci U S A. 2010; 107: 1437-1442Crossref PubMed Scopus (340) Google Scholar, 55Lee K.P. Lee J.H. Kim T.S. et al.The Hippo-Salvador pathway restrains hepatic oval cell proliferation, liver size, and liver tumorigenesis.Proc Natl Acad Sci U S A. 2010; 107: 8248-8253Crossref PubMed Scopus (218) Google Scholar Similarly, liver-specific deletion of the neurofibromatosis type 2 (NF2) tumor suppressor gene in developing or adult mice leads to expansion of progenitor cells without affecting differentiated hepatocytes; these mice develop HCC and iCCA.56Benhamouche S. Curto M. Saotome I. et al.Nf2/Merlin controls progenitor homeostasis and tumorigenesis in the liver.Genes Dev. 2010; 24: 1718-1730Crossref PubMed Scopus (138) Google Scholar Furthermore, any mouse hepatic cells, including hepatic progenitors, hepatoblasts, and hepatocytes, that express activated oncogenes (such as H-RAS or SV40LT) can undergo transformation to develop into iCCA or HCC57Holczbauer A. Factor V.M. Andersen J.B. et al.Modeling pathogenesis of primary liver cancer in lineage-specific mouse cell types.Gastroenterology. 2013; 145: 221-231Abstract Full Text Full Text PDF PubMed Scopus (69) Google Scholar (Table 1). There could also be subpopulations of stem or progenitor cells in peribiliary glands, located throughout the biliary tree,58Cardinale V. Wang Y. Carpino G. et al.The biliary tree—a reservoir of multipotent stem cells.Nat Rev Gastroenterol Hepatol. 2012; 9: 231-240Crossref PubMed Scopus (97) Google Scholar that give rise to iCCA59Cardinale V. Wang Y. Carpino G. et al.Multipotent stem/progenitor cells in human biliary tree give rise to hepatocytes, cholangiocytes, and pancreatic islets.Hepatology. 2011; 54: 2159-2172Crossref PubMed Scopus (152) Google Scholar or FLC.60Torbenson M. Fibrolamellar carcinoma: 2012 update.Scientifica (Cairo). 2012; 2012: 743790PubMed Google Scholar Proteins involved in stem cell renewal and cell fate decisions include MET,50Kitade M. Factor V.M. Andersen J.B. et al.Specific fate decisions in adult hepatic progenitor cells driven by MET and EGFR signaling.Genes Dev. 2013; 27: 1706-1717Crossref PubMed Scopus (27) Google Scholar which primarily induces hepatocyte differentiation, and EGFR and NOTCH, which promote cholangiocyte specification.50Kitade M. Factor V.M. Andersen J.B. et al.Specific fate decisions in adult hepatic progenitor cells driven by MET and EGFR signaling.Genes Dev. 2013; 27: 1706-1717Crossref PubMed Scopus (27) Google Scholar, 51Chiba T. Zheng Y.W. Kita K. et al.Enhanced self-renewal capability in hepatic stem/progenitor cells drives cancer initiation.Gastroenterology. 2007; 133: 937-950Abstract Full Text Full Text PDF PubMed Scopus (156) Google Scholar Activation of NOTCH in mice leads to development of HCC61Villanueva A. Alsinet C. Yanger K. et al.Notch signaling is activated in human hepatocellular carcinoma and induces tumor formation in mice.Gastroenterology. 2012; 143: 1660-1669 e7Abstract Full Text Full Text PDF PubMed Scopus (133) Google Scholar and iCCA.62Zender S. Nickeleit I. Wuestefeld T. et al.A critical role for notch signaling in the formation of cholangiocellular carcinomas.Cancer Cell. 2013; 23: 784-795Abstract Full Text Full Text PDF PubMed Scopus (0) Google Scholar In addition, mutations in genes encoding isocitrate dehydrogenases, frequently detected in iCCAs14Moeini A. Sia D. Bardeesy N. et al.Molecular pathogenesis and targeted therapies for intrahepatic cholangiocarcinoma.Clin Cancer Res. 2016; 22: 291-300Crossref PubMed Scopus (126) Google Scholar but rarely in HCCs, inhibit hepatocyte differentiation and promote oval cell proliferation and biliary transformation following liver injury, in cooperation with KRAS mutations63Saha S.K. Parachoniak C.A. Ghanta K.S. et al.Mutant IDH inhibits HNF-4alpha to block hepatocyte differentiation and promote biliary cancer.Nature. 2014; 513: 110-114Crossref PubMed Scopus (105) Google Scholar (Table 1). Similarly, KRAS mutations and homozygous PTEN deletion in embryonic bipotential progenitor cells cooperate to induce onset of iCCA.64Ikenoue T. Terakado Y. Nakagawa H. et al.A novel mouse model of intrahepati