Understanding the Role of IDH Mutation in Chondrosarcoma

Chondrosarcomas constitute a heterogeneous group of malignant bone tumors that produce hyaline cartilaginous matrix. Chondrosarcomas are generally resistant to conventional chemotherapy and radiotherapy, leaving surgery as the only curative treatment option for most localized tumors. In the absence of effective systemic therapy, metastatic disease is nearly uniformly fatal. Therefore, better therapy for patients with unresectable or metastatic disease is urgently needed. Mutations in isocitrate dehydrogenase 1/2 (IDH1/2) enzymes were recently described in several malignancies including conventional and dedifferentiated chondrosarcomas. These mutations represent a novel genetic abnormality in chondrosarcoma, indicating a potential role for aberrant IDH function in the pathogenesis of this malignancy. IDH mutations lead to the inability of IDH to convert isocitrate into α-ketoglutarate (α-KG). Instead, α-KG is reduced into D-2-hydroxyglutarate (D-2HG), an oncometabolite. IDH mutations have been shown to initiate tumorigenesis by D-2HG acting as a competitive inhibitor of α-KG-dependent dioxygenases involved in DNA and histone demethylation. The resulted hypermethylation status affects gene expression and cellular differentiation, thus promoting tumor formation. However, to date, there is no strong evidence implicating epigenetic changes in chondrosarcoma tumorigenesis. A series of mutant IDH inhibitors have been developed and evaluated in preclinical and clinical studies as single agents and in combination with other anticancer agents. These inhibitors have shown promising clinical benefits in some cancer types. A mutant IDH1 (IDH1^mut) inhibitor, AGI-5198 is one of the earliest developed IDH^mut inhibitors. In gliomas, treatment with this inhibitor prevents IDH1^mut from producing D-2HG, delays tumor growth, and promotes differentiation. We sought to determine whether treatment with AGI-5198 would similarly inhibit tumorigenic activity and D-2HG production in IDH1-mutant chondrosarcoma cells. Our study demonstrates the anti-tumor activity of AGI-5198 in chondrosarcoma cell lines, and suggests that D-2HG is a potential biomarker for IDH mutations in chondrosarcoma. This study provided the first evidence that IDH mutation is associated with chondrosarcoma tumorigenesis, but its mechanistic function in this malignancy is still not clearly defined. To further determine the mechanism of IDH^mut driven tumorigenesis in chondrosarcoma and potentially identify additional therapeutic targets, we developed a CRISPR/Cas9 IDH1^mut knockout model in two chondrosarcoma cell lines. We found IDH1^mut knockout significantly suppressed the tumorigenic properties of chondrosarcoma cells and resulted in marked attenuation of chondrosarcoma formation and elimination of the oncometabolite, D-2HG production. We further demonstrated that loss of IDH1^mut led to widespread downregulation of integrin genes and deregulation of integrin-mediated processes. Our findings suggest that IDH1 mutation promotes chondrosarcoma growth through modulation of integrin, rendering the integrin molecules appealing candidates for combinatorial regimens with IDH1^mut inhibitors for chondrosarcomas that harbor this mutation.