The role of heterozygous TACI mutations in CVID

Common variable immunodeficiency (CVID) is the most common primary immunodeficiency that comes to medical attention. CVID is characterized by low serum levels of IgG, IgA and, frequently, IgM, impaired ability to produce specific antibodies, recurrent sinopulmonary and gastrointestinal infections and an increased incidence of autoimmune disorders and of lymphoid malignancies. The TNF receptor (TNFR) family member TACI (transmembrane activator and calcium-modulator and cyclophilin ligand (CAML) interactor) is mutated in 10% of CVID patients. In the majority of cases, only one of the two alleles is mutated. C104R and A181E are the two most common TACI mutations significantly present in CVID and they are highly conserved, together with their surrounding sequence. C104, located in the cysteine rich domain 2 (CRD2), and A181 in the transmembrane domain of TACI. The C104R mutation abolishes binding to the TACI ligands BAFF and APRIL and impairs the ability of B cells from heterozygous patients to produce Igs in response to APRIL and IL-4. The A181E mutant, which substitutes a negatively charged amino acid (a.a.) for a neutral one in the TM domain of TACI, binds BAFF normally. Two explanations have been proposed for the potential deleterious effect of the heterozygous TACI mutations on B cell function in CVID. One invokes haploinsufficiency because one normal copy of TACI may not be sufficient for proper function. However TACI +/- mice with a single copy of the TACI gene have a normal phenotype in vivo and in vitro, strongly arguing that TACI haploinsufficiency may not cause B cell dysfunction. Alternatively, the mutant protein may act as a dominant negative and interfere with the function of the normal allele. The overall hypothesis of this application is to investigate if the heterozygous TACI mutations in CVID may function as dominant negative one. To test the hypothesis we propose to examine the effect of the two TACI mutants most commonly found in CVID patients, C104R, and A181E, on receptor assembly and ligand binding in the human system.