Low-Grade Fibro-Osseous Lesions With Isolated Chromosome 12 Chromothripsis: A Distinct Entity Or a Low-Grade Central Osteosarcoma With a Novel Driver?

Chromothripsis, a catastrophic genomic event causing extensive chromosomal fragmentation and rearrangement, has been identified in conventional osteosarcoma, contributing to karyotypic heterogeneity. Chromothripsis was also detected in a subset of parosteal osteosarcomas, primarily involving chromosome 12, but not well documented in low-grade central osteosarcoma (LGCOS). Here, we report 2 cases of low-grade fibro-osseous lesions with isolated chromosome 12 chromothripsis, aiming to characterize them by comparing their genomic and epigenetic profiles to those of other bone neoplasms. Case 1 was a 12-year-old male with a destructive lesion involving the proximal left tibia, and case 2 was a 13-year-old female with an expansile lucent bone lesion involving the distal tibia with cortical breakthrough. Both cases consisted of hypocellular bland fibroblastic proliferation. Areas of thin trabeculae of woven bone surrounded by osteoblasts were present. In addition, case 1 showed areas of atypical cartilaginous islands and areas reminiscent of LGCOS. By single-nucleotide polymorphism array, both cases showed extremely complex and numerous genomic alterations involving chromosome 12. Whole-genome sequencing and optical genome sequencing confirmed the chromothriptic event involving chromosome 12. Using the Heidelberg Epignostix sarcoma methylation classifier (v12.3), both cases matched to fibrous dysplasia methylation class. Dimensionality reduction using Uniform Manifold Approximation and Projection demonstrated that the 2 index cases clustered primarily with low-grade osteosarcomas lacking MDM2 amplification and with fibrous dysplasia, whereas remaining separate from parosteal osteosarcomas and MDM2-amplified low-grade osteosarcomas. Our findings suggest that chromothripsis of chromosome 12 may represent an early and/or an alternate mechanism in fibro-osseous lesions progressing to LGCOS instead of or before the development of the well-recognized CDK4/MDM2 amplification. Larger cohorts with long-term follow-ups and integrative molecular studies are needed to clarify the biological significance and clinical implications, including recurrence, dedifferentiation, and survival.