OP08. MUTATIONS IN THE IDH GENE (R132H) ARE NOT SUFFICIENT FOR EXPERIMENTAL GLIOMAGENESIS
Authors: Dr Li Ningning, Mr Kastytis Sidlauskas, Ms Ying Zhang, Ms Andreia Ferreira, Ms Joanne Lau, Dr Stefan Pusch, Prof Andreas Von Deimling, Prof Sebastian Brandner
Journal: Neuro-Oncology (2017)

INTRODUCTION: Mutations in isocitrate dehydrogenase (IDH), primarily IDH1R132H, are associated with favourable prognosis and are defining features of oligodendrogliomas and astrocytomas. They produce the oncometabolite 2-hydroxyglutarate which interferes with the TET family of 5’-methylcytosine hydroxylases, and results in a CpG island methylator phenotype. IDH1 mutations are thought to be an early event in the pathogenesis of gliomas, followed by other genetic alterations. It remains to be established how mutant IDH1 contributes to the pathogenesis of human glioma. We previously demonstrated that activation of oncogenic signals in the neurogenic cell population of the subventricular zone (SVZ) in the brain causes tumours and here we use these models to understand the role of mutant IDH1 in brain tumour pathogenesis. MATERIAL AND METHODS: Mice carrying inducible mutations (p53lox/lox, Ptenlox/lox, and IDH1lox-R132H/wt,) were used. Cre mediated recombination of these genes in the neurogenic population in the subventricular zone (SVZ) was achieved by (i) activating the tamoxifen-inducible Cre-ERT2 fusion protein under the control of the promoter/enhancer of GLAST, GFAP, or Nestin, or (ii) by intracranial injection of Adeno virus expressing Cre (Adeno-cre). Furthermore, neural stem cells, isolated from the SVZ, were recombined by Adeno-Cre in vitro and subsequently orthotopically transplanted into the striatum of NOD/SCID mice for in vivo tumour analysis. Finally, human primary glioma cultures of wild-type or mutant IDH1 glioblastomas were xenografted. RESULTS: Whilst the introduction of a mutation of the Pten and p53 genes into the SVZ population results in gliomas, the heterozygous IDH1R132H mutation alone in the same population is not tumorigenic. Similarly, IDH mutant neural stem cells, allografted into the striatum of recipient mice did not generate tumours. Instead, adding the IDH mutation to Pten and p53 mutations resulted in reduced tumour incidence and increased survival. Further, IDH1 mutation reduced the proliferative, migratory, and invasive potential of human or murine glioma-initiating cells in vitro. RNA-seq analysis of murine cells revealed a marked change of global transcriptional expression in IDH1 mutant cells, suggesting an increase in global DNA methylation. Concomitant microRNA-seq analysis identified a miR-335/transmembrane and tetratricopeptide repeat containing 1 (Tmtc1) regulatory axis. miR-335 and its host gene, mesoderm specific transcript (MEST), were significantly down-regulated in IDH1 mutant cells where epigenetic silencing dominated. Tmtc1, a putative target of miR-335, is regulated by DNA methyltransferases (DNMT). We found that IDH1R132H/WT;Pten-/-;p53-/- and IDH1R132H/WT cells had significantly higher Tmtc1 expression, compared to their controls, Pten-/-;p53-/- and non-mutation cells that shared identically low levels of Tmtc1. However, IDH1R132H/WT;Pten-/-;p53-/- cells showed a comparably lower Tmtc1 expression than that in IDH1R132H/WT cells. CONCLUSIONS: Our study demonstrates that IDH1R132H alone is not sufficient to induce tumour formation in vivo. This finding is complemented by the observation that the IDH1 mutation reduces cell proliferation and migration in vitro. Impairment of Tmtc1 expression correlates with the role of IDH1R132H to modulate DNA methylation, which is complementary to its classic commitment to inhibition of DNA demethylases.