Different molecular behavior of CD40 mutants causing hyper-IgM syndrome.
CD40/CD40 ligand (CD40L) cross-talk plays a key role in B-cell terminal maturation in the germinal centers. Genetic defects affecting CD40 cause a rare form of hyper-immunoglobulin M (IgM) syndrome, a disorder characterized by low or absent serum IgG and IgA, associated with recurrent infections. We previously reported on a few patients with homozygous CD40 mutations resulting in lack or severe reduction of CD40 cell surface expression. Here we characterize the 3 CD40 mutants due to missense mutations or small in-frame deletions, and show that the mutated proteins are synthesized but retained in the endoplasmic reticulum (ER), likely due to protein misfolding. Interestingly, the intracellular behavior and fate differ significantly among the mutants: progressive accumulation of the P2 mutant causes endoplasmic reticulum stress and the activation of an unfolded protein response; the mutant P4 is rather efficiently disposed by the ER-associated degradation pathway, while the P5 mutant partially negotiates transport to the plasma membrane, and is competent for CD40L binding. Interestingly, this latter mutant activates downstream signaling elements when overexpressed in transfected cells. These results give new important insights into the molecular pathogenesis of HIGM disease, and suggest that CD40 deficiency can also be regarded as an ER-storage disease.
Lanzi
Gaetana
G
A. Nocivelli Institute for Molecular Medicine and Pediatric Clinic, University of Brescia, Brescia, Italy.
Ferrari
Simona
S
Vihinen
Mauno
M
Caraffi
Stefano
S
Kutukculer
Necil
N
Schiaffonati
Luisa
L
Plebani
Alessandro
A
Notarangelo
Luigi Daniele
LD
Fra
Anna Maria
AM
Giliani
Silvia
S
D000595:Amino Acid Sequence; D001402:B-Lymphocytes / Q000378:metabolism; D015153:Blotting, Western; D019013:CD40 Antigens / Q000737:chemistry / Q000235:genetics* / Q000378:metabolism*; D002462:Cell Membrane / Q000378:metabolism; D002478:Cells, Cultured; D002648:Child; D002675:Child, Preschool; D004721:Endoplasmic Reticulum / Q000378:metabolism; D005260:Female; D005434:Flow Cytometry; D016368:Frameshift Mutation / Q000235:genetics*; D006031:Glycosylation; D006801:Humans; D053306:Hyper-IgM Immunodeficiency Syndrome / Q000235:genetics* / Q000276:immunology; D007223:Infant; D007668:Kidney / Q000166:cytology / Q000378:metabolism; D008297:Male; D008969:Molecular Sequence Data; D050505:Mutant Proteins / Q000737:chemistry / Q000235:genetics* / Q000378:metabolism*; D020125:Mutation, Missense / Q000235:genetics*; D010375:Pedigree; D011487:Protein Conformation; D017510:Protein Folding; D012333:RNA, Messenger / Q000235:genetics; D020133:Reverse Transcriptase Polymerase Chain Reaction; D017386:Sequence Homology, Amino Acid
D019013:CD40 Antigens; D050505:Mutant Proteins; D012333:RNA, Messenger