Different molecular behavior of CD40 mutants causing hyper-IgM syndrome.

Area of Science:

• Immunology
• Molecular Biology
• Cell Biology

Background:

• CD40/CD40 ligand (CD40L) interactions are crucial for B-cell maturation.
• Genetic defects in CD40 cause hyper-immunoglobulin M (HIGM) syndrome, characterized by recurrent infections and low IgG/IgA.
• Previous studies identified patients with homozygous CD40 mutations leading to absent or reduced cell surface CD40.

Purpose of the Study:

• To characterize three specific CD40 mutants resulting from missense mutations or small in-frame deletions.
• To investigate the intracellular behavior and fate of these mutated CD40 proteins.
• To gain insights into the molecular pathogenesis of HIGM disease.

Main Methods:

• Characterization of three CD40 mutants (P2, P4, P5) with missense mutations or in-frame deletions.
• Analysis of protein synthesis, intracellular retention in the endoplasmic reticulum (ER), and protein misfolding.
• Assessment of ER stress, unfolded protein response (UPR) activation, ER-associated degradation (ERAD) pathway, and plasma membrane transport.
• Evaluation of CD40L binding and downstream signaling activation in transfected cells.

Main Results:

• Mutated CD40 proteins are synthesized but retained in the ER due to misfolding.
• Mutant P2 causes ER stress and UPR activation due to progressive accumulation.
• Mutant P4 is degraded via the ERAD pathway, while mutant P5 partially reaches the plasma membrane and binds CD40L.
• Overexpression of P5 activates downstream signaling pathways.

Conclusions:

• CD40 deficiency in HIGM disease can be viewed as an ER-storage disease.
• Different CD40 mutants exhibit distinct intracellular fates and consequences.
• These findings provide crucial insights into the molecular mechanisms underlying HIGM syndrome.