Characterization of a novel tumor suppressor gene involved in osteosarcoma tumorigenesis

Based on previous data and fine mapping of the tumor suppressor locus on chromosome 3q and through expression analysis of ESTs in normal and tumor tissue, we have identified EDR3, a novel member of the Polycomb group (PcG) gene family, as the candidate tumor suppressor gene. ^ In situ hybridization and immunofluorescence analyses indicate that EDR3 is expressed widely expressed during development. We have shown that pEDR3 and E2F6 interact specifically during G0, as a part of a PcG complex, to potentially regulate exit from cell cycle and G 0 arrest. Supershift EMSA analysis on nuclear extracts from normal cells showed that pEDR3 containing complex are capable of binding E2F target sites during specifically during quiescence. pEDR3 also forms distinct foci in nuclei of cells induced to differentiate along the osteoblastic lineage, suggesting a function for the gene during differentiation. This binding is not seen with extracts from serum starved osteosarcoma cell lines, suggesting that the EDR3/E2F6/PcG complex is functionally inactive. ^ Approximately 80% of the cells in adult tissues are in the G0 phase of cell cycle. Functional inactivation of G0 regulatory proteins or protein complexes, combined with genetic events inactivating the G 1/S and G2/M checkpoints, could lead to an exit from quiescence, continued proliferation and hence, tumorigenesis. We have seen loss of expression of pEDR3 in tumors other than osteosarcoma, suggesting that the EDR3/PcG complex defines a novel G0/G1 checkpoint. ^ EDR3 maps to the minimum region of duplication on chromosome 3 (3826.1–826.3), which has been implicated in the etiology of CdLS. We have shown that EDR3 fulfills all three criteria for a candidate gene contributing to a part of the CdLS phenotype, in that, it is consistently duplicated in the genome of affected individuals, it is expressed in all the affected tissues and it is capable of affecting the phenotype in a dose dependent manner. ^ We have thus identified a gene, capable of regulating cell proliferation and differentiation, loss of function and gain of function mutations in which could contribute to tumorigenesis and CdLS phenotypes, respectively.