The PBX1 lupus susceptibility gene regulates CD44 expression

Area of Science:

• Immunology
• Molecular Biology
• Genetics

Background:

• The pre-B-cell leukemia homeobox 1 (PBX1) gene has a novel splice isoform, PBX1-d, which lacks DNA-binding and Hox-binding domains and acts as a dominant negative.
• PBX1-d expression in CD4+ T cells is linked to systemic lupus erythematosus (SLE) in both mouse models and human subjects.
• PBX1-d promotes autoreactive T cells, impairs regulatory T (Treg) cells, and expands T follicular helper (Tfh) cells, suggesting a role in SLE pathogenesis.

Purpose of the Study:

• To investigate the molecular mechanisms by which PBX1 isoforms regulate T cell activation markers.
• To determine if PBX1 directly controls CD44 expression, a key marker of T cell activation and memory.
• To compare the DNA-binding and co-factor recruitment properties of the lupus-associated PBX1-d isoform versus the normal PBX1-b isoform.

Main Methods:

• Analysis of PBX1 binding sites in the CD44 promoter region.
• Site-directed mutagenesis to assess the impact of binding site alterations on CD44 expression.
• Quantitative assessment of co-factor recruitment (MEIS, PREP1) by PBX1 isoforms.

Main Results:

• PBX1 directly regulates CD44 expression through two specific binding sites in its promoter.
• The PBX1-d isoform drives higher CD44 expression compared to the PBX1-b isoform.
• Mutations in the binding sites differentially affect PBX1-b and PBX1-d activity, indicating isoform-specific regulation.
• PBX1-d exhibits enhanced recruitment of the co-factor MEIS compared to PBX1-b, while PREP1 recruitment remains similar.

Conclusions:

• The lupus-associated PBX1-d isoform directly transactivates CD44, a critical marker for T cell activation and memory.
• PBX1-d possesses distinct DNA-binding and co-factor recruitment characteristics compared to the canonical PBX1-b isoform.
• These unique molecular functions of PBX1-d provide a mechanistic link between PBX1 dysregulation and the pathogenesis of systemic lupus erythematosus.