Autosomal dominant gain-of-function mutations in LCP1 cause a syndromic neutropenia and immunodeficiency.

Actin cytoskeleton defects underlie immuno-actinopathies. We identified three patients with heterozygous LCP1 gain-of-function mutations (L362F, A365D) causing activated LCP1-associated immunodeficiency syndrome, characterized by congenital neutropenia, variable combined immunodeficiency, and allergy. Patients' neutrophils show maturation arrest and excessive apoptosis, while T/B cells are reduced and functionally impaired. Patient-derived iPSCs, CRISPR-edited cells, and LCP1+/L362F mice replicate these defects. Mechanistically, mutant LCP1 hyper-bundles F-actin, inducing VDAC1 oligomerization and mitochondrial apoptosis. This study further establishes LCP1 as a regulator of immune cell fate and suggests targeting actin dynamics as therapy.