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A newly discovered gene mutation may account for many cases of immune deficiency, in particular two syndromes known as immunoglobulin A (IgA) deficiency and Common Variable Immunodeficiency (CVID), report researchers in the July issue of Nature Genetics. The discovery may lead to a new diagnostic test for these conditions, which make people highly susceptible to infections and often go unrecognized because of a lack of good tests.

IgA deficiency affects 1 in 600 people; CVID is less common but more severe. Children and adults with either condition suffer relentlessly recurring ear infections, sinus infections, bronchitis, pneumonias and gastrointestinal infections. IgA deficiency and CVID can occur in the same family, and also predispose people to autoimmunity, particularly affecting the thyroid gland and resulting in thyroid hormone insufficiency. Finally, people with CVID are susceptible to B-cell lymphomas.

TACI mutations interfere with two aspects of the immune response that involve maturation of B cells, the white blood cells that make immunoglobulins, which function as antibodies, to fight infections. Normally, TACI triggers B cells to switch from making immunoglobulin M (IgM), an antibody produced early in the body's immune response, to making other immunoglobulins like IgA and IgG. More important, TACI signals B cells to produce antibodies with a high affinity for specific attackers. Because TACI mutations are dominant, people with even one copy of the mutation will be unable to mount a strong antibody response.

Geha's team previously demonstrated that TACI binds to two other proteins made by cells in the respiratory and gastrointestinal lining, known as APRIL and BAFF, to trigger the signal for B cells to mature. Geha believes that additional cases of immune deficiency result from mutations in the APRIL and BAFF genes, and plans to verify this in further studies.