Molecular Mechanisms of Lupus Susceptibility Allele PBX1D

Lupus susceptibility gene Pbx1 controls the development, stability, and function of regulatory T cells via Rtkn2 expression

The maintenance of regulatory T (Treg) cells critically prevents autoimmunity. Pre-B cell leukemia transcription factor 1 (Pbx1) variants are associated with lupus susceptibility, particularly through the expression of a dominant negative isoform Pbx1-d in CD4+ T cells. Pbx1-d overexpression impaired Treg cell homeostasis and promoted inflammatory CD4+ T cells. Here, we showed a high expression of Pbx1 in human and murine Treg cells, which is decreased in lupus patients and mice. Pbx1 deficiency or Pbx1-d overexpression reduced the number, stability, and suppressive activity of Treg cells, which increased murine responses to immunization and autoimmune induction. Mechanistically, Pbx1 deficiency altered the expression of genes implicated in cell cycle and apoptosis in Treg cells. Intriguingly, Rtkn2, a Rho-GTPase previously associated with Treg homeostasis, was directly transactivated by Pbx1. Our results suggest that the maintenance of Treg cell homeostasis and stability by Pbx1 through cell cycle progression prevent the expansion of inflammatory T cells that otherwise exacerbates lupus progression in the hosts.

PBX1: a TALE of two seasons-key roles during development and in cancer

Pre-B cell leukemia factor 1 (PBX1) is a Three Aminoacid Loop Extension (TALE) homeodomain-containing transcription factor playing crucial roles in organ pattering during embryogenesis, through the formation of nuclear complexes with other TALE class and/or homeobox proteins to regulate target genes. Its contribution to the development of several organs has been elucidated mainly through the study of murine knockout models. A crucial role for human development has been recently highlighted through the discovery of different de novo pathogenic PBX1 variants in children affected by developmental defects. In the adult, PBX1 is expressed in selected tissues such as in the brain, in the gastro-intestinal and urinary systems, or in hematopoietic stem and progenitor cells, while in other organs is barely detectable. When involved in the t(1;19) chromosomal translocation it acts as an oncogene, since the resulting fusion protein drives pre-B cell leukemia, due to the induction of target genes not normally targeted by the native protein. Its aberrant expression has been associated to tumor development, progression, or therapy-resistance as in breast cancer, ovarian cancer or myeloproliferative neoplasm (MPN). On the other hand, in colorectal cancer PBX1 functions as a tumor suppressor, highlighting its context-dependent role. We here discuss differences and analogies of PBX1 roles during embryonic development and in cancer, focusing mainly on the most recent discoveries.

Regulation of the STAT3 pathway by lupus susceptibility gene Pbx1 in T cells

Systemic lupus erythematosus (SLE) is a chronic autoimmune disease in which poorly characterized genetic factors lead to the production of proinflammatory or autoreactive T cells. Pre-B cell leukemia homeobox 1 (PBX1) is a transcription factor whose dominant negative isoform (PBX1-D) is overexpressed in the CD4+ T cells of SLE patients and lupus-prone mice. Pbx1-D overexpression favors the expansion of proinflammatory T cells and impairs regulatory T (Treg) cell development. Here we show that Pbx1 deficiency and Pbx1-D overexpression decreased STAT3 expression and activation in T cells. Accordingly, Pbx1 deficiency in T cells and Pbx1-D overexpression reduced STAT3-dependent TH17 cell polarization in vitro, but it had no effect in vivo at steady state. STAT3-dependent follicular helper T (TFH) cell polarization in vitro and splenic TFH cell frequency were not affected by either Pbx1 deficiency or Pbx1-D overexpression. Pbx1 deficiency also increased the expression of cell cycle arrest and pro-apoptotic genes, with an increased apoptosis in T cells. Our results suggest a complex interplay between PBX1 and STAT3, which may contribute to lupus pathogenesis through dysregulation of the cell cycle and apoptosis.