Loss of heterozygosity and DNA ploidy point to a diverging genetic mechanism in the origin of peripheral and central chondrosarcoma.

Chondrosarcomas are malignant cartilaginous tumors arising centrally in bone (central chondrosarcoma), or secondarily within the cartilaginous cap of a hereditary or sporadic exostosis (peripheral chondrosarcoma). Loss of heterozygosity (LOH) was studied by microsatellite analysis at the loci harboring the EXT genes (implicated in hereditary multiple exostoses), the EXT-like genes, and at 9p21, 13q14, 17p13, and chromosome 10. Nineteen of 20 peripheral chondrosarcomas showed LOH at all loci tested, while only 3 of 12 central chondrosarcomas exhibited LOH, restricted to 9p21, 10, 13q14, and 17p13. LOH at 9p21 did not appear to involve the CDKN2A gene, as assessed by SSCP analysis. DNA flow cytometry demonstrated a wide variation in the ploidy status in peripheral chondrosarcomas (DNA indexes, 0.56-2.01), whereas central chondrosarcomas were predominantly peridiploid. Near-haploidy found in peripheral chondrosarcomas could explain part of the high LOH percentages. Ki-67 immunohistochemistry suggested a higher proliferation rate in peripheral chondrosarcomas. Our results indicate that peripheral chondrosarcomas, arising secondarily to an exostosis, may obtain genetic alterations during malignant transformation, with subsequent genetic instability as demonstrated by a high percentage of LOH and a wide variation in ploidy status. In contrast, peridiploidy and a low percentage of LOH in central tumors suggest that a different oncogenic molecular mechanism may be operative.