AIRE-TOP2A · The Role of Topoisomerase 2a in Aire-mediated Induction of Immunologic Tolerance

The Role of Topoisomerase 2a in Aire-mediated Induction of Immunologic Tolerance

In a functional immune system, T cells serve as potent immunologic weapons, attacking foreign invaders (bacteria, viruses, etc.), while tolerating the body’s own components. Unfortunately, these beneficial weapons can occasionally turn against self and attack the body’s own organs, resulting in severe autoimmune diseases such as type-1 diabetes. This is often due to a failure in a process called negative selection, which takes place in the thymus, where potentially self-reactive T cells (i.e. cells that can potentially recognize, attack and destroy body’s own organs and cause autoimmunity) are eliminated. A long-standing tenet of immunological tolerance was that the negative selection process can eliminate only T cells that are reactive to either ubiquitous or blood-circulating self-antigens, while tolerance to antigens expressed exclusively in parenchymal organs (liver, brain, pancreas etc) is imposed by “peripheral” mechanisms. However, it is now known that a rare cell population of thymic stroma, called medullary thymic epithelial cells (mTECs), transcribes thousands of genes whose expression was thought to be restricted to parenchymal organs. Hence, transcription of tissue-restricted-antigen (TRA) genes in the thymus “foreshadows” the self-antigens that T cells would encounter once they reach maturity and are released into the body. Recently, many of these ectopic transcripts were found to be regulated by the product of a single gene, the Autoimmune regulator (Aire), as mice with a mutation at this locus express only a fraction of the TRA repertoire. As a result, these animals develop antibodies and immune infiltrates directed at multiple peripheral tissues resembling a multiorgan autoimmune disorder characteristic of humans with a mutated AIRE gene, the autoimmune polyendocrine syndrome type 1 (APS-1). In order to better understand the molecular mechanisms by which Aire induces ectopic expression of TRAs in mTECs, we sought to identify the interacting partners of Aire. Results from the interaction and functional analyses converged to suggest that among the most important candidates involved in the regulation of Aire-mediated gene expression is a multi-molecular complex, consisting of a number of transcriptional control elements (e.g. DNA-PK, PARP-1, TOP2a, FACT) (Abramson J, et al Cell 2010) as well as a protein deacetylase Sirt1, which is highly expressed in Aire+ mTECs (Chuprin A, et al, Nature Immunology 2015). In a follow up studies were able to demonstrate that the capacity of Aire can be mimicked and rescued by a known anti-cancer drug – etoposide, which operates as a Topoisomerase 2 (TOP2) poison. We further demonstrate that Aire-mediated promiscuous gene expression involves additional components of DNA damage response which physically associate with Aire and are critical for its capacity to induce promiscuous gene expression in vivi (Chuprin A & Abramson J; manuscript in preparation).

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