A Novel Deletion in FERMT3 Causes LAD-III in a Turkish Family

Leukocyte adhesion deficiency-III (LAD-III) is an extremely rare autosomal recessive syndrome caused by mutations in FERMT3, the gene encoding kindlin-3. The genetic alterations in this gene lead to abnormal expression or activity of kindlin-3 in leukocytes and platelets. Kindlin-3 acts as an important regulator of integrin activation. LAD-III has features of the bleeding syndrome of Glanzmann and also of leukocyte adhesion deficiency. In this study, we report on two families, one of Turkish and one of Syrian origin, with clinical features of LAD-III, loss of kindlin-3 protein expression, and a functional leukocyte defect. A novel, homozygous deletion in FERMT3 (c.921delC, p.Ser307Argfs*21) was found in the Turkish patient. The parents were carriers of the mutation, consistent with an autosomal recessive inheritance. A common c.1525C > T (p.Arg509*) mutation was found in the Syrian patient. In conclusion, beside the variant c.1525C > T in the FERMT3 gene, which was previously found in more than 15 patients in Anatolia, our study is the first to identify the novel homozygous variant c.921delC in the FERMT3 gene.

Hematologically important mutations: The autosomal forms of chronic granulomatous disease (third update)

Chronic granulomatous disease (CGD) is an immunodeficiency disorder affecting about 1 in 250,000 individuals. CGD patients suffer from severe, recurrent bacterial and fungal infections. The disease is caused by mutations in the genes encoding the components of the leukocyte NADPH oxidase. This enzyme produces superoxide, which is subsequently metabolized to hydrogen peroxide and other reactive oxygen species (ROS). These products are essential for intracellular killing of pathogens by phagocytic leukocytes (neutrophils, eosinophils, monocytes and macrophages). The leukocyte NADPH oxidase is composed of five subunits, four of which are encoded by autosomal genes. These are CYBA, encoding p22^phox, NCF1, encoding p47^phox, NCF2, encoding p67^phox and NCF4, encoding p40^phox. This article lists all mutations identified in these genes in CGD patients. In addition, cytochrome b₅₅₈ chaperone-1 (CYBC1), recently recognized as an essential chaperone protein for the expression of the X-linked NADPH oxidase component gp91^phox (also called Nox2), is encoded by the autosomal gene CYBC1. Mutations in this gene also lead to CGD. Finally, RAC2, a small GTPase of the Rho family, is needed for activation of the NADPH oxidase, and mutations in the RAC2 gene therefore also induce CGD-like symptoms. Mutations in these last two genes are also listed in this article.

Kindlin3-Dependent CD11b/CD18-Integrin Activation Is Required for Potentiation of Neutrophil Cytotoxicity by CD47-SIRPα Checkpoint Disruption

The CD47-signal regulatory protein-alpha (SIRPα) immune checkpoint constitutes a therapeutic target in cancer, and initial clinical studies using inhibitors of CD47-SIRPα interactions in combination with tumor-targeting antibodies show promising results. Blockade of CD47-SIRPα interaction can promote neutrophil antibody-dependent cellular cytotoxicity (ADCC) toward antibody-opsonized targets. Neutrophils induce killing of antibody-opsonized tumor cells by a process identified as trogoptosis, a necrotic/lytic type of cancer cell death that involves trogocytosis, the antibody-mediated endocytic acquisition of cancer membrane fragments by neutrophils. Both trogocytosis and killing strictly depend on CD11b/CD18-(Mac-1)-mediated neutrophil-cancer cell conjugate formation, but the mechanism by which CD47-SIRPα checkpoint disruption promotes cytotoxicity has remained elusive. Here, by using neutrophils from patients with leukocyte adhesion deficiency type III carrying FERMT3 gene mutations, hence lacking the integrin-associated protein kindlin3, we demonstrated that CD47-SIRPα signaling controlled the inside-out activation of the neutrophil CD11b/CD18-integrin and cytotoxic synapse formation in a kindlin3-dependent fashion. Our findings also revealed a role for kindlin3 in trogocytosis and an absolute requirement in the killing process, which involved direct interactions between kindlin3 and CD18 integrin. Collectively, these results identified a dual role for kindlin3 in neutrophil ADCC and provide mechanistic insights into the way neutrophil cytotoxicity is governed by CD47-SIRPα interactions.