PATH-22. SINGLE-CELL RNA (SCRNA) SEQUENCING OF PAIRED PRIMARY AND RECURRENT ATYPICAL TERATOID/RHABDOID TUMORS (AT/RT) REVEALED MOLECULAR DIFFERENCES AND THERAPY-RESISTANT PRIMARY TUMOR CELLS

Abstract Atypical teratoid/rhabdoid tumors (AT/RT) are the most common malignant brain tumors during infancy. They split into four molecular types. The major three (AT/RT-SHH, AT/RT-TYR, and AT/RT-MYC) all carry mutations in SMARCB1, the fourth smaller type is characterized by SMARCA4 mutations (AT/RT-SMARCA4). AT/RT are associated with a dismal outcome, and 50% of the patients suffer a progress or recurrence of the tumor. Molecular mechanisms of disease recurrence are currently unclear. Here, we identified molecular characteristics of AT/RT primaries and recurrences, which could help to improve the therapy for relapsed patients. Previously, we investigated tumor tissue from 26 patients revealing signatures of recurrences in comparison to matched primary tumor samples using histology, DNA methylation analysis, and bulk RNA sequencing. To further identify mechanisms regarding tumor progression and recurrence, we performed single-cell RNA (scRNA) sequencing for five of these primary-recurrence pairs so far using formalin-fixed paraffin-embedded (FFPE) tissue. Using Uniform Manifold Approximation and Projection (UMAP) for dimensionality reduction, we observed a distinct separation in the gene expression profiles of primary and recurrent tumor cells. In addition, we aimed to identify primary tumor cells which potentially persisted after therapy and developed to recurrences. Therefore, we identified primary tumor cells showing highest similarity to recurrent tumor cells using CIBERSORT and trajectory analysis. Differentially expressed genes of these specific primary tumor cells were investigated. They are involved in cell migration, -adhesion, -proliferation, and -stability, among others. Overall, molecular characteristics that occur in the course of the disease were identified, which may help to define new therapeutic targets for AT/RT recurrences.