Topoisomerases in Immune Cell Development and Function

Abstract

DNA topoisomerases (TOPs) are complex enzymatic machines with extraordinary capacity to maintain DNA topology during torsion-intensive steps of replication and transcription. Recently, TOPs have gained significant attention for their tissue-specific function, and the vital role of TOPs in immune homeostasis and dysfunction is beginning to emerge. TOPs have been implicated in various immunological disorders such as autoimmunity, B cell immunodeficiencies, and sepsis, underscoring their importance in immune regulation. However, much remains unknown about immunological underpinnings of TOPs, and a deeper understanding of the role of TOPs in the immune system will be critical for yielding significant insights into the etiology of immunological disorders. In this review, we first discuss the recent literature highlighting the contribution of TOPs in the development of immune cells, and we further provide an overview of their importance in immune cell responses.

Figure 1. Mammalian topoisomerases and the immune system.

(A) The classification of mammalian DNA topoisomerases is shown. Topoisomerases resolve the torsional strain generated during replication and transcription by cleaving either single or both strands of DNA and forming an intermediate cleavage complex. Based on the number of strands cleaved, they can be classified into type I and type II topoisomerases. Type I topoisomerases (TOP1, TOP1mt, TOP3A and TOP3B) do not require ATP and utilize the energy stored in torsional strain to either pass one strand through the other (type IA) or swivel around a strand (type IB). Type II topoisomerases (TOP2A, TOP2B and SPO11) are ATP-dependent enzymes. They create a transient break in one DNA double helix and pass another DNA double helix through it to relax the supercoiling. (B) The phenotype of mice models carrying genetic deletion of topoisomerases’ is shown. The embryos of Top3a knock-out mice are resorbed before 7.5-dpc. Mice without TOP3B have shorter lifespan and are infertile. Older cohorts of Top3b-null mice possess high level of autoantibodies. Homozygous disruption of Top1 is lethal and the development of Top1 knock-out embryos fails between 4- and 16-cell stages. Unlike Top1 knock-out, mice with germline deletion of Top1mt are viable and fertile. Top2a deletion results in termination of mice embryo development at the 4- or 8-cell stage. Top2b disrupted mice have impaired motor axon growth resulting in perinatal death of pups. Spo11-null mice are infertile due to apoptosis in spermatocytes. Panels (C-E) highlight the role of topoisomerases in T cell development and maturation. (C) Loss-of-function of TOP3A in zebrafish leads to selective defects in T cell development. Two mutations, HI064 and WW20/12, in Top3a impact thymopoiesis in zebrafish, with normal development of T cell progenitors but reduced number of thymocytes. (D) TOP1 and TOP2A are cardinal partners of AIRE with TOP1 localizing at super-enhancers while TOP2A at the promoter of AIRE-induced genes in medullary thymic epithelial cells. Inhibition of either TOP1 or TOP2A in mice leads to manifestation of autoimmunity. (E) While mice with Top3b deletion appear to be healthy, they develop autoimmunity with the progression of age. Panels (F-G) showcase the role of topoisomerases in B cell development and function. (F) The genetic basis of two human primary B cell immunodeficiencies, Hoffman syndrome and BILU, is the mutations in the TOPRIM domain of TOP2B. Hoffman syndrome and BILU are characterised by reduced B cell numbers and antibodies in circulation. On a similar line, deletion of Top2b in mice leads to perturbations in B cell compartment, while T cells remain largely unaffected. (G) In contrast to TOP2B, TOP1 plays a major role in B cell responses. Dynamic regulation of TOP1 is crucial for AID-dependent CSR and SHM. On stimulation of B cells, reduced levels of TOP1 either through haploinsufficiency or siRNA-mediated silencing lead to enhanced CSR and SHM. (H) TOP2B plays a critical role in NK cell development. Hoffman syndrome and BILU patients with mutations in Top2b and mice with heterozygous Top2b allele display reduced frequency of NK cells. TOP2B dysfunction in NK cells leads to lower transcript levels of Nfil3, Ets1, Id2, the key transcription factors for NK cell development. TOP, topoisomerase; dpc, days post coitum; BILU, B cell immunodeficiency, limb anomalies and urogenital malformations; AIRE, autoimmune regulator; AID, activation induced cytidine deaminase; CSR, class switch recombination; SHM, somatic hyper mutation; NK, natural killer.