Funded Grants

Mechanism and Mouse Model for IDHI-Mediated Suppression of Human Secondary Glioblastomas

Gliomas are the most common type of human brain tumors. The most aggressive subtype of glioma, commonly known as glioblastoma multiforme (GBM), has one of worst prognoses among all types of human tumors. Unfortunately, current treatment options are very limited in their efficacy in combating this deadly tumor type. The goal of our research is to better understand the genetic changes that underlie gliomas and to develop a better model system to study tumor development.

Clinically, GBM can be categorized as primary (advanced stage tumors without previous symptoms) or secondary (low-grade tumors that subsequently progress to advanced stages)1. Primary and secondary GBM, although pathologically indistinguishable, exhibit distinct patterns of cancer gene alterations1. Several recent studies, inspired by a major cancer genome sequencing project2, revealed that the isocitrate dehydrogenase-1 (IDH)1 gene is mutated in more than 70% of secondary GBM or low grade gliomas, but infrequently in primary GBM (about 5%)2-6. The IDH1 gene mutations appear to be specific for human brain tumors as no IDH1 gene mutation was found in analyzing over 1000 non-central nervous system tumors. Notably, all of the IDH1 mutations identified in gliomas are heterozygous and produce a single amino acid substitution at arginine 132, indicating that this particular mutation is distinctly important in the tumor progression of this subtype of glioma. Clinically, GBM patients carrying an IDH1 mutation had a significantly longer overall survival (31 months) compared to GBM patients with wild-type IDH1 (15 months). A better understanding of how IDH1 mutation contributes to this subset of gliomas may someday allow more tailored treatments for these patients.

We have recently carried out biochemical, structural and cellular studies of this tumor-derived IDH1 mutant to understand its mechanism and role in tumor development. Normally, two identical IDH1 subunits function as a homodimer to convert isocitrate to α-ketoglutarate (α-KG). We found that tumor-derived IDH1 mutations dominantly inhibit wild-type IDH1 activity because binding of mutant IDH1 with wild-type IDH1 forms inactive heterodimers that are impaired for binding the substrate, isocitrate. Expression of mutant IDH1 in cultured cells reduces the level of the enzyme product, α-KG. A decrease in α-KG, in turn, increases the levels of hypoxia-inducible factor subunit HIF- 1α, a transcription factor that facilitates tumor growth when oxygen is low. The link between IDH1 and HIF-1α highlights the importance of altered metabolic regulation to tumorigenesis. In analyzing human tumors, we found that HIF-1α expression was higher in gliomas harboring the IDH1 mutation than in tumors of similar grade without a mutation. In our cell culture system, the rise in HIF-1α levels was reversible by an α-KG derivative (oct-α-KG). This finding suggests that drugs mimicking α-KG may merit exploration as a therapy for gliomas that harbor an IDH1 mutation. We conclude from our initial studies that IDH1 appears to function as a tumor suppressor that, when mutationally inactivated, contributes to tumor progression through induction of the HIF-1 pathway. These results have recently been accepted for publication in the journal of Science.

In this application, we propose two lines of research aimed at better understanding the molecular mechanisms and developing mouse model for human low grade gliomas and secondary GBM. In the first aim of our proposed research, we will examine how IDH1 genetically and molecularly interacts with p53. The cancer genome study identified that IDH1 mutations frequently occur along with mutations in p53 in the secondary GBM. This suggests that these two genes may collaborate to contribute to tumor development. The tumor suppressor p53 is the most frequently mutated gene in human cancer and normally acts as a brake to uncontrolled cell proliferation. We hypothesize that IDH1 mutation may lead to activation of p53, and that disruption of both IDH1- and p53-mediated pathways is required for the development of low-grade gliomas and secondary GBM.

Our second aim is to develop a mouse model that accurately recapitulates the genetic and clinical features of low-grade gliomas and secondary GBM. We propose to use genetic engineering approach, known as knock-in, to replace one allele of IDH1 with the mutant characterized in tumors. Using this mouse model, we will be able to examine how mutant IDH1 affects glial cell growth and differentiation and the development of tumors. Our hope is that these mice serve as a better model for this particular subclass of glioma, further our understanding of glioma tumor progression and aid in future work to test therapeutic agents for the treatment of glioma patients with mutation in IDH1.