Topoisomerase 1 is required for the development and function of thymus

Thymus organogenesis is critical for proper maturation of developing T cells. In this study, we identified Topoisomerase 1 (Top1) as a novel gene involved in thymus development and function. We created a mouse line with deletion of Top1 in thymic epithelial cells (TECs) and our results demonstrate that biallelic loss of Top1 in TECs causes congenital thymic aplasia, precipitating T cell immunodeficiency. Transcriptomic analysis provides insights into the molecular mechanism of Top1 in thymus development as we identify key genes involved in thymus organogenesis as the transcriptional targets of Top1 in TECs. Analysis of peripheral immunological compartments revealed severe loss of ab T cells complemented with a disproportionate accumulation of gd T cells and myeloid cells upon deletion of Top1 in TECs. The residual ab T cells in Top1 knock-out mice were effector and oligoclonal in nature highlighting their possible self-reactivity. These results reveal a previously unknown role of Top1 in thymus development and T cell homeostasis. We propose Top1 as a genetic target for altered thymic development and T cell lymphopenia. We generated Top1-cKO mice having deletion of Top1 in thymic epithelial cells (TECs), to investigate the thymus-intrinsic role of TOP1 in T cell development. Top1-cKO mice had thymic aplasia with no visible thymus present in thoracic cavity at the age of 6-weeks. Since Top1-cKO mice did not have thymic tissue, we performed RNA-seq-based transcriptomic analysis on cTEChi and mTEChi from Top1-WT and Top1-cHet mice (2-weeks old) to understand the molecular mechanism of Top1 in thymus development. Loss of thymus in Top1-cKO mice affected the peripheral T cell populations in the spleen and lymph nodes. We could detect a minor proportion (<5%) of ab T cells in both spleen and MLN of Top1-cKO mice. To understand the impact of loss of TOP1 in TECs on the biology of the residual T cells in the periphery, we performed RNA-seq-based transcriptomic analysis on splenic CD4+ helper T cells from Top1-cKO mice and compared it with control (Top1-WT) littermates.