Homozygous Loss of Function PIK3CD Mutation in Multiple Siblings Leading To B Cell Dysregulation and Autoimmunity

Phosphatidylinositol 3-kinases (PI3Ks) are heterodimeric lipid kinases that are involved in a diverse array of cellular functions such as growth, metabolism, and migration. Mutations in PIK3CD, which encodes an immune-specific catalytic subunit of PI3K, cause both dominant (activating) and recessive (loss of function) immune deficiencies in humans. Here we report a family with three affected children carrying a novel bi-allelic, truncating mutation in PIK3CD. All three patients exhibited chronic diarrhea and recurrent sinopulmonary infections. Immunoblot confirmed loss of protein along with reduced expression of the associated p85α regulatory subunit. Immune phenotyping showed B cell dysregulation with abnormally high levels of naïve cells. In vitro functional testing of CD19 + and enriched naïve B cells revealed impaired proliferation, and reduction in class-switch recombination upon CD40L and IL-21 stimulation. Our data raise the possibility that PI3K-related dysregulation in human B cells may be broader than in mouse models, where class-switch recombination can still occur with external T cell help. Our study substantially increases the limited number of patients known to have immune deficiency due to loss of PIK3CD.