Regulation of Cartilage Tumors by Mutations in Isocitrate Dehydrogenases

Enchondroma and chondrosarcoma are common benign and malignant cartilaginous neoplasms. Mutations in isocitrate dehydrogenase 1 and 2 (IDH1/2) are present in the majority of these tumors. Mutant IDH enzymes gain a neomorphic function of producing D-2-hydroxyglutarate (D-2HG) from ?-ketoglutarate. Expression of a mutant Idh1 gene is sufficient for enchondroma initiation but inhibiting mutant IDH enzymes did not cause a consistent change in the tumorigenic properties of chondrosarcomas. It is unclear how mutations of isocitrate dehydrogenases regulate cartilage tumors from initiation to cancer progression and maintenance. I hypothesize that mutations in IDH enzymes could regulate cartilage tumors through changes in gene expression and cellular metabolism. To address these questions, I examined the transcriptional regulation and metabolic regulations of mutant isocitrate dehydrogenases in chondrocytes and chondrosarcomas and identified cholesterol biosynthesis and glutamine metabolism as two key pathways dictating tumor behavior.

To understand the transcriptional regulation of IDH1 mutation in cartilage tumors, I performed RNA-sequencing analysis in chondrocytes from Col2a1Cre;Idh1LSL/+ mutant animals and their littermate wildtype controls and identified that cholesterol biosynthesis pathway was upregulated. Genetic inhibition of cholesterol biosynthesis in an enchondroma mouse model and pharmacological inhibition of cholesterol biosynthesis in human patient chondrosarcoma samples suppressed tumor development in vivo. Taken together, these data suggest that intracellular cholesterol synthesis is a potential therapeutic target for enchondromas and chondrosarcomas.

From a metabolic perspective, I found that chondrocytes and chondrosarcomas with mutations in IDH1/2 genes had enhanced glutamine utilization for downstream metabolism. Using genetic and pharmacological approaches, I demonstrated that glutaminase-mediated glutamine metabolism played distinct roles in enchondromas and chondrosarcomas with IDH1/2 mutations. Genetic ablation of glutaminase in chondrocytes with Idh1 mutation increased the number and size of enchondroma-like lesions. Pharmacological inhibition of glutaminase led to decreased tumor weight of chondrosarcoma xenograft. During enchondroma development, glutamine-derived -ketoglutarate plays important roles in regulating chondrocyte differentiation and proliferation. In chondrosarcoma, glutamine-derived non-essential amino acids are important in preventing cell apoptosis.

In summary, findings in this dissertation described transcriptional and metabolic regulations by mutations in isocitrate dehydrogenases in cartilage tumors enchondroma and chondrosarcoma and provide novel insights for developing therapies for these diseases.