IDH mutations in liver cell plasticity and biliary cancer

Abstract

Intrahepatic cholangiocarcinoma (ICC) is an aggressive cancer associated with the bile ducts within the liver. These tumors are characterized by frequent gain-of-function mutations in the isocitrate dehydrogenase 1 and 2 (IDH1 and IDH2) genes-that are also common in subsets of neural, haematopoietic and bone tumors, but rare or absent in the other types of gastrointestinal malignancy. Mutant IDH acts through a novel mechanism of oncogenesis, producing high levels of the metabolite 2-hydroxyglutarate, which interferes with the function of α-ketoglutarate-dependent enzymes that regulate diverse cellular processes including histone demethylation and DNA modification. Recently, we used in vitro stem cell systems and genetically engineered mouse models (GEMMs) to demonstrate that mutant IDH promotes ICC formation by blocking hepatocyte differentiation and increasing pools of hepatic progenitors that are susceptible to additional oncogenic hits leading to ICC. We found that silencing of HNF4A-encoding a master transcriptional regulator of hepatocyte identity and quiescence-was critical to mutant IDH-mediated inhibition of liver differentiation. In line with these findings, human ICC with IDH mutations are characterized by a hepatic progenitor cell transcriptional signature suggesting that they are a distinct ICC subtype as compared to IDH wild type tumors. The role of mutant IDH in controlling hepatic differentiation state suggests the potential of newly developed inhibitors of the mutant enzyme as a form of differentiation therapy in a solid tumor.

Keywords: IDH1; IDH2; Isocitrate Dehydrogenase; cholangiocarcinoma; mouse models.

Figure 1.

ICC is unique among gastrointestinal (GI) malignancies in harboring a high frequency of IDH mutations. The chart shows the frequency of each tumor type exhibiting a gain-of-function hotspot mutation in either IDH1 or IDH2. (http://www.sanger.ac.uk/cosmic) Major GI tumor types are shown as are other tissues where tumors with IDH mutations are common.

Figure 2.

Cellular plasticity in the liver. (A) Under normal conditions, the liver is largely quiescent and its low rate of cell turnover is maintained by proliferation of differentiated bile duct cells (cholangiocytes) and hepatocytes. The liver also exhibits extensive capacity to regenerate following damage. Depending on the nature of the injury, cholangiocytes or hepatocytes can be replaced by their neighboring non-injured counterparts, or by oval cells—existing as rare endogenous progenitors or generated by de-differentiation of either cholangioctyes or hepatocytes. (B) The major types of adult liver cancer are ICC and HCC, which show histologic and immunophenotypic resemblance to the normal cholangiocytes and hepatocytes, respectively. Experimental studies indicate that differentiated liver cells can give rise to both tumor types directly or though an oval cell intermediate. Mixed HCC/ICC tumors with histopathologic features of both tumors may be associated with oval cell expansion. (C) left panel, Impact of mutant IDH on differentiation of bipotential hepatoblast (HB) cells in vitro. Expression of mutant IDH in HBs leads to production of 2HG which blocks hepatocyte differentiation through suppression of HNF4α via an unknown mechanism. Right panel, IDH acts in the adult liver to block the differentiation of hepatic progenitors. These liver progenitors are sensitized to transformation by additional oncogenic hits, and can progress through graded premalignant biliary lesions leading to ICC.

Figure 3.

Mutant IDH allele frequency varies widely across different cancers. The relative frequency of the different mutant IDH1 or IDH2 alleles in the indicated cancer subtypes are shown. The most common alleles for each tumor type are labeled on the individual pie charts. Less common variants of IDH1 and IDH2 are grouped together.

Figure 4.

Functional significance of different IDH mutant alleles. (A) The R132H mutation appears to occur as a result of a spontaneous deamination of a CpG dinucleotide of the reverse (bottom) strand, yielding a TpG dinucleotide. Similarly, the R132C mutation appears to occur from the same spontaneous deamination event of a CpG to a TpG dinucleotide on the forward strand in the same codon. (B) HB cells were engineered to express different IDH alleles under a doxycycline-inducible promoter. At all levels of expression, the R132C-expressing HBs produced ∼5 fold higher concentrations of 2HG compared with the R132C-expressing HB cells. (C) Table describing the phenotypes of HB cells expressing the indicated alleles.