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Novel mutation in PIK3CD affecting the Ras-binding domain

Introduction: The PI3K pathway plays critical roles in diverse cellular processes, including differentiation, proliferation, motility, survival, and growth. PI3Kδ, comprised of the catalytic subunit p110δ and regulatory subunit p85α, is essential for normal lymphocyte and myeloid development and function. Gain-of-function mutations in PIK3CD (encoding p110δ) cause a combined immunodeficiency known as activated PI3Kδ syndrome (APDS), in which patients frequently present with recurrent respiratory infections and associated lung damage, severe recurrent (or persistent) infections with herpes family viruses, and lymphadenopathy. Aim: To describe the clinical presentation, immune evaluation, and genetic work-up of 2 patients (daughter and mother) with recurrent sinopulmonary, soft tissue, and skin infections. Results: Both daughter and mother presented with recurrent sinopulmonary, soft tissue and skin infections caused by atypical bacteria. Immune evaluation of the daughter revealed intermittent hypogammaglobulinemia and abnormal specific vaccine responses, while immune parameters of her mother were normal. Whole exome sequencing identified a novel mutation in PIK3CD (NM_005026), c.C719T, resulting in p.T240M. Western blot analysis of downstream AKT levels revealed increased basal phosphorylation, in line with gain-of-function mutations of PIK3CD. Conclusion: The novel missense mutation in PIK3CD occurs in the region encoding the Ras-binding domain (RBD) of p110δ, and likely alters the structural configuration of the domain. To date, pathogenic mutations targeting the RBD of p110δ have not yet been described. Our results expand on the known genotype-phenotype of APDS.