Germ line LCP1 mutations cause immunodeficiency with neutropenia, monocytopenia, lymphopenia, and defective cytokinesis.

Severe congenital neutropenia (SCN) is characterized by neutropenia, recurrent infections, and an increased leukemia risk. Multiple genetic defects that underlie SCN have been identified, but a genetic diagnosis is still lacking in a significant proportion of patients. In this study, we report 4 independent pedigrees with heterozygous variants in LCP1. Variants c.740-1G>T and c.740-20_744del produced the same alternatively spliced RNA product, causing an in-frame deletion (p.A247_E254del). Variant c.509C>T in the third pedigree produced p.S170L, and variant c.806T>C in the fourth pedigree produced p.L269P. Affected individuals suffered from neutropenia, poor or complete lack of response to granulocyte colony-stimulating factor (G-CSF) treatment, and variable degrees of lymphopenia, hypogammaglobulinemia, and monocytopenia. Patients with A247_E254del and p.L269P presented with tetraploid cells in the bone marrow, indicative of disturbed cytokinesis. In one of these kindreds, 2 individuals developed acute leukemia. G-CSF nonresponsiveness and defective cell cycling were repaired upon correction of the LCP1 A247_E254del variant in patient-derived induced pluripotent stem cells, supporting the monogenic origin of the disease. Indicative of their gain-of-function effect, both the A247_E254del and S170L variants increased F-actin bundling and the formation of abnormal protrusions. Single-cell transcriptome analysis of A247_E254del bone marrow-derived hematopoietic stem and progenitor cells (HSPCs) showed deregulation of signaling pathways that control mitosis in multilineage and lymphoid-primed HSPC subsets. We concluded that activating LCP1 variants cause a new hematopoietic disorder with autosomal dominant inheritance. Depending on the consequences of the LCP1 variants in terms of protein structure, patients may suffer from G-CSF refractory severe neutropenia, lymphopenia, hypogammaglobulinemia, monocytopenia, and defective cytokinesis.