CD154 variants associated with hyper-IgM syndrome can form oligomers and trigger CD40-mediated signals.
X-linked hyper-IgM syndrome is a rare immunodeficiency disorder resulting from mutations in the gene encoding the CD40 ligand (CD154) molecule. These mutations are very heterogeneous, ranging from a single point mutation to a large deletion in the open reading frame. To investigate the molecular mechanisms that are responsible for the functional defect of these mutants, we examined the biochemical properties of 14 hyper-IgM-related CD154 mutant proteins produced by transient expression in COS7 cells. We show that deletion mutants lacking a significant portion of the tumor necrosis factor homologous domain cannot be stably produced. In contrast, point mutants can be detected as oligomers. Surprisingly, gene products of two point mutants, Thr-211 --> Asp and Met-36 --> Arg, can bind to the receptor, CD40. For Thr-211 --> Asp variant, it is comparable to the wild-type protein in its surface expression level, biochemical structure, and functional activities. Thus, it appears that this mutation is a polymorphism of CD154 gene. For Met-36 --> Arg variant, although it is interactive with CD40, it has a much lower surface expression level than wild-type protein. We propose that Met-36 --> Arg mutant represents a prototype of a defective CD154 family whose low cell surface expression of intrinsically active protein is simply insufficient to trigger productive signals through CD40.
Garber
E
E
Department of Molecular Genetics, Cambridge, Massachusetts 02142, USA.
Su
L
L
Ehrenfels
B
B
Karpusas
M
M
Hsu
Y M
YM
D000818:Animals; D001483:Base Sequence; D001704:Biopolymers; D019013:CD40 Antigens / Q000276:immunology*; D023201:CD40 Ligand; D019556:COS Cells; D002460:Cell Line; D015342:DNA Probes; D006801:Humans; D006942:Hypergammaglobulinemia / Q000276:immunology*; D007075:Immunoglobulin M / Q000097:blood*; D008562:Membrane Glycoproteins / Q000097:blood* / Q000235:genetics; D017354:Point Mutation; D015398:Signal Transduction
D001704:Biopolymers; D019013:CD40 Antigens; D015342:DNA Probes; D007075:Immunoglobulin M; D008562:Membrane Glycoproteins; D023201:CD40 Ligand