Molecular Etiology of Enchondromatosis

Abstract
More than 3% of the population develops an enchondroma (ECA), a benign tumor in bone composed of cells
derived from the growth plate that can cause pain, deformity, and can be responsible for pathologic fractures.
Enchondromas can progress to malignant chondrosarcoma (CSA). Mutations in genes encoding isocitrate
dehydrogenase (IDH1 and 2) were identified in a large proportion of ECAs and CSAs. In our prior work, we
found that IDH mutations inhibit growth plate chondrocyte differentiation, and chondrocyte-specific conditional
Idh1 mutant mice develop ECAs. Mutant IDH uniquely produces the metabolite 2-hydroxyglutarate (2-HG), but
we and others found that blocking the production of 2-HG pharmacologically does not alter CSA cell viability.
While 2-HG has epigenetic effects that are likely important in tumor initiation, tumor maintenance must rely on
other factors. Since IDH plays an important role in in metabolism, associated metabolic changes could drive
the observed phenotype. We found high levels of glycogen in cells expressing a mutant IDH. Glycogen is also
found in proliferating and pre-hypertrophic cells of the growth plate. In our previous work, we found that
intracellular cholesterol biosynthesis was activated in IDH mutant chondrocytes and that it is also regulated in
the growth plate, and its activity corelates with glycogen levels. This raises the possibility that intracellular
cholesterol biosynthesis, which is activated by Sterol regulatory-element binding proteins (SREBP)
transcription, also regulates glycogen. Our premise is that glycogen is an important energy source for pre-
hypertrophic and hypertrophic growth plate chondrocytes and that glycogen stores are required to maintain the
neoplastic phenotype in ECA and CSA. We also propose that glycogen depletion can suppress the neoplastic
phenotype. In this proposal we will study what regulates glycogen in the growth plate, ECA and CSA, and
determine the function of glycogen in these growth plate and neoplastic chondrocytes.
To determine what regulates glycogen in the growth plate, ECA, and CSA, we prioritized genes known to
regulate glycogen that were differentially regulated in the growth plate and by IDH mutations. Protein
phosphatase 1 regulatory subunit 3C (PPP1R3C) is one such gene which is differentially and interestingly,
contains SREBP binding sites in its promoter region. Our preliminary data suggest that SREBP regulates
PPP1R3C which then regulates glycogen. Our studies will use cell lines from human tumors and genetically
modified mice that develop enchondromas to define the function of glycogen and PPP1R3C in the growth
plate, ECA, and CSA. In addition, we will study how SREBP regulates PPP1R3C and glycogen. Glycogen
synthase will be deleted genetically, or we will cells with drugs that inhibit glycogen synthesis and breakdown.
This data will provide pre-clinical information on which to base novel therapies for ECA and CSA.

Grant Number: 5R01AR066765-10
NIH Institute/Center: NIH
Principal Investigator: Benjamin Alman