MUTATIONS OF THE CD40 LIGAND GENE AND ITS EFFECT ON CD40 LIGAND EXPRESSION IN PATIENTS WITH X-LINKED HYPER IGM SYNDROME

X-linked hyper IgM syndrome (XHIM) is a primary immunodeficiency disor der caused by mutations of the gene encoding CD40 ligand (cD40L). We c orrelated mutations of the CD40L gene, CD40L expression, and the clini cal manifestations observed in XHIM patients from 30 families. The 28 unique mutations identified included 9 missense, 5 nonsense, 9 splice site mutations, and 5 deletions/insertions. In 4 of 9 splice site muta tions, normally spliced and mutated mRNA transcripts were simultaneous ly expressed. RNase protection assay demonstrated that 5 of 17 mutatio ns tested resulted in decreased levels of transcript. The effect of th e mutations on CD40L expression by activated peripheral blood mononucl ear cells (PBMC) and T-cell lines or clones was assessed using one pol yclonal and four monoclonal antibodies and a CD40-Ig fusion protein. I n most patients, the binding of at least one antibody but not of CD40- Ig was observed, suggesting nonfunctional CD40L. However, activated PB MC from three patients and activated T-cell lines from two additional patients, each with different genotype, bound CD40-Ig at low intensity , suggesting functional CD40L, Thus, failure of activated PBMC to bind CD40-Ig is not an absolute diagnostic hallmark of XHIM and molecular analysis of the CD40L gene may be required for the correct diagnosis. Patients with genotypes resulting in diminished expression of wild-typ e CD40L or mutant CD40L that can still bind CD40-Ig appear to have mil der clinical consequences. (C) 1998 by The American Society of Hematol ogy.