MKL1 deficiency results in a severe neutrophil motility defect due to impaired actin polymerization

Abstract

Megakaryoblastic leukemia 1 (MKL1) promotes the regulation of essential cell processes, including actin cytoskeletal dynamics, by coactivating serum response factor. Recently, the first human with MKL1 deficiency, leading to a novel primary immunodeficiency, was identified. We report a second family with 2 siblings with a homozygous frameshift mutation in MKL1. The index case died as an infant from progressive and severe pneumonia caused by Pseudomonas aeruginosa and poor wound healing. The younger sibling was preemptively transplanted shortly after birth. The immunodeficiency was marked by a pronounced actin polymerization defect and a strongly reduced motility and chemotactic response by MKL1-deficient neutrophils. In addition to the lack of MKL1, subsequent proteomic and transcriptomic analyses of patient neutrophils revealed actin and several actin-related proteins to be downregulated, confirming a role for MKL1 as a transcriptional coregulator. Degranulation was enhanced upon suboptimal neutrophil activation, whereas production of reactive oxygen species was normal. Neutrophil adhesion was intact but without proper spreading. The latter could explain the observed failure in firm adherence and transendothelial migration under flow conditions. No apparent defect in phagocytosis or bacterial killing was found. Also, monocyte-derived macrophages showed intact phagocytosis, and lymphocyte counts and proliferative capacity were normal. Nonhematopoietic primary fibroblasts demonstrated defective differentiation into myofibroblasts but normal migration and F-actin content, most likely as a result of compensatory mechanisms of MKL2, which is not expressed in neutrophils. Our findings extend current insight into the severe immune dysfunction in MKL1 deficiency, with cytoskeletal dysfunction and defective extravasation of neutrophils as the most prominent features.

When Actin is Not Actin’ Like It Should: A New Category of Distinct Primary Immunodeficiency Disorders

“…Loss-of-function mutations in MKL1 result in a PID, first described by Record et al [27]. More recently, a second family with 2 siblings with a homozygous frameshift mutation in MKL1 was identified [28]. The first identified MKL1-deficient patient suffered from severe bacterial infections, and the second MKL1-deficient patient deceased as an infant from progressive and severe pneumonia by Pseudomonas aeruginosa .…”

“…Furthermore, proteomic and transcriptomic analyses of MKL1-deficient primary neutrophils and an HL-60 myeloid leukemia MKL1 knockdown cell line revealed several actin-related proteins and genes to be downregulated, confirming a role of MKL1 as transcriptional co-regulator. No difference in the capacity of production of ROS was observed in MKL1-deficient neutrophils, while degranulation was shown to be enhanced [27, 28]. In contrast to the first report [27], no phagocytosis defect and an intact killing of various microbial pathogens was found in neutrophils of the MKL1-deficient patient described more recently [28].…”

“…No difference in the capacity of production of ROS was observed in MKL1-deficient neutrophils, while degranulation was shown to be enhanced [27, 28]. In contrast to the first report [27], no phagocytosis defect and an intact killing of various microbial pathogens was found in neutrophils of the MKL1-deficient patient described more recently [28]. The susceptibility to (bacterial) infections observed in MKL1 deficiency could thus be explained by the inability of neutrophils to migrate towards the site of infection as opposed to a killing defect.…”

“…Their motility has not been formally tested. Non-hematopoietic MKL1-deficient fibroblasts were not affected in their morphology, F-actin content, and migratory capacity, which is most likely due to compensatory mechanisms of MKL2, also member of the myocardin-related transcription factor family [28]. It is reported that MKL1 and MKL2 could have redundant roles.…”

“…It is reported that MKL1 and MKL2 could have redundant roles. However, in contrast to fibroblasts, MKL2 is not expressed in neutrophils [28].…”