Molecular Approaches to Diagnosis in Ewing Sarcoma: Targeted RNA Sequencing

Optical Genome Mapping for Comprehensive Cytogenetic Analysis of Soft-Tissue and Bone Tumors for Diagnostic Purposes

Soft-tissue and bone tumors represent a heterogeneous group of tumors encompassing more than 100 histologic subtypes today. Identifying genetic aberrations increasingly is important in these tumors for accurate diagnosis. Although gene mutations typically are detected by second-generation sequencing, the identification of structural variants (SVs) and copy number alterations (CNAs) remains challenging and requires various cytogenetic techniques including karyotyping, fluorescence in situ hybridization, and arrays, each with important limitations. Optical Genome Mapping (OGM), a non-sequencing-based technique for high-resolution detection of SVs and CNAs, was applied in a retrospective series of diagnostic soft-tissue and bone tumor samples. Sample preparation was successful in 38 of 53 cases, with the highest success rate in nonadipocytic soft-tissue tumors (24 of 27 cases; 89%). In 32 of 35 cases carrying a diagnostic SV or CNA, OGM identified the aberration (91%), including a POU2AF3::EWSR1 fusion in a round cell sarcoma and a translocation t(1;5)(p22;p15) in a myxoinflammatory fibroblastic sarcoma. Interestingly, OGM shed light on the genomic complexity underlying the various aberrations. In five samples, OGM showed that chains of rearrangements generated the diagnostic fusion, three of which involved chromoplexy. In addition, in nine samples, chromothripsis was causal to the formation of giant marker/ring/double-minute chromosomes. Finally, compared with standard-of-care cytogenetics, OGM revealed additional aberrations, requiring further investigation of their potential clinical relevance.