Mutant IDH regulates glycogen metabolism from early cartilage development to malignant chondrosarcoma formation.

Chondrosarcomas are the most common malignancy of cartilage and are associated with somatic mutations in isocitrate dehydrogenase 1 (IDH1) and IDH2 genes. Somatic IDH mutations are also found in its benign precursor lesion, enchondromas, suggesting that IDH mutations are early events in malignant transformation. Human mutant IDH chondrosarcomas and mutant Idh mice that develop enchondromas investigated in our studies display glycogen deposition exclusively in mutant cells from IDH mutant chondrosarcomas and Idh1 mutant murine growth plates. Pharmacologic blockade of glycogen utilization induces changes in tumor cell behavior, downstream energetic pathways, and tumor burden in vitro and in vivo. Mutant IDH1 interacts with hypoxia-inducible factor 1α (HIF1α) to regulate expression of key enzymes in glycogen metabolism. Here, we show a critical role for glycogen in enchondromas and chondrosarcomas, which is likely mediated through an interaction with mutant IDH1 and HIF1α.

Skeletal muscle hemojuvelin is dispensable for systemic iron homeostasis.

Hepcidin, a hormone produced mainly by the liver, has been shown to inhibit both intestinal iron absorption and iron release from macrophages. Hemojuvelin, a glycophosphatidyl inositol-linked membrane protein, acts as a bone morphogenetic protein coreceptor to activate hepcidin expression through a SMAD signaling pathway in hepatocytes. In the present study, we show in mice that loss of hemojuvelin specifically in the liver leads to decreased liver hepcidin production and increased tissue and serum iron levels. Although it does not have any known function outside of the liver, hemojuvelin is expressed at very high levels in cardiac and skeletal muscle. To explore possible roles for hemojuvelin in skeletal muscle, we analyzed conditional knockout mice that lack muscle hemojuvelin. The mutant animals had no apparent phenotypic abnormalities. We found that systemic iron homeostasis and liver hepcidin expression were not affected by loss of hemojuvelin in skeletal muscle regardless of dietary iron content. We conclude that, in spite of its expression pattern, hemojuvelin is primarily important in the liver.