Lung histopathology evaluation of an X-MAID patient with a novel mutation in MSN

The Ezrin-radixin-moesin (ERM) family of proteins are responsible for developing networks between transmembrane proteins and the actin cytoskeleton (Fehon et al. 2010). They are essential for normal membrane organization and function, preserving cell shape, microvilli, pseudopod/uropod and immune synapse formation, phagocytosis, and apoptosis, as well as signal transduction pathway control (Niggli and Rossy 2008).

All ERM proteins are characterized by a plasma membrane-associated N-terminal 4.1 ezrin, radixin, moesin (FERM) domain, followed by a long region with a high α-helical propensity, and a C-terminal domain (C-ERMAD: C-terminal ERM-association domain) which binds to the FERM domain, creating a head to tail folding structure (Fehon et al. 2010; Niggli and Rossy 2008) (Figure 1A). The FERM domain contains 3 subdomains: the F1 ubiquitin-like domain, F2 consisting of 4 α-helices, and F3 composed of a pleckstrin homology domain (Figure 1B). A preserved FERM domain is critical for the overall ERM protein function (Michie et al. 2019). Phosphorylation and subsequent activation of ERM proteins enable interactions necessary for shape regulation, including those of actin filaments, transmembrane proteins, and scaffolding proteins. The participation of phosphatidylinositol 4,5 biphosphate (PIP2) is required for ERM protein activation, by binding to the FERM domain and phosphorylating the C-ERMAD domain. (García-Ortiz and Serrador 2020).

The MSN gene, located on the X-chromosome, encodes moesin which is ubiquitously expressed in lungs, spleen, kidney, endothelial cells of vessels, and is the predominant ERM protein in lymphocytes and neutrophils (Liu et al. 2015). Moesin is the abundant in leukocytes, while ezrin and radixin are near absent in leukocytes (García-Ortiz and Serrador 2020). Murine studies have previously demonstrated that moesin and radixin knockout mice appear clinically less severe. Mice reached adulthood despite abnormalities in the liver, lymphocytes and neutrophils, whereas knockout of ezrin resulted in embryonic lethality (Kawaguchi et al. 2017; Lagresle-Peyrou et al. 2016; Liu et al. 2015). Notably, loss-of-function of any one ERM protein is not compensated by other protein family members (Kawaguchi et al. 2017).

In recent years, a number of primary immunodeficiencies have been identified that result from aberrations in genes essential for actin cytoskeleton regulation. These fall under the scope of actinopathies, and affect cell motility, signaling, and antimicrobial function (Sprenkeler et al. 2021). X-linked moesin-associated immunodeficiency (X-MAID), first described in 2016, is part of a group of immunodeficiencies caused by defects in cytoskeleton. X-MAID is characterized by severe leukopenia affecting T cells, B cells, and neutrophils (Lagresle-Peyrou et al. 2016). To date, the clinical presentation of patients with X-MAID is variable, from those requiring minimal intervention, immunoglobulin replacement therapy and (or) antibiotic prophylaxis (Bradshaw et al. 2018; Henrickson et al. 2019; Lagresle-Peyrou et al. 2016), to patients needing life-saving hematopoietic stem cell transplant based on severe lymphopenia with significant clinical manifestations (Henrickson et al. 2019).

Here, we describe the presentation, immune-workup, and histopathology findings of a young male patient with X-MAID and multi-organ involvement, whose severe pulmonary vein stenosis necessitated a double lung transplant.

The patient was enrolled in the Primary Immunodeficiency Registry and Tissue Bank (REB protocol number 1000005598) at The Hospital for Sick Children. His chart was reviewed retrospectively.

Moesin deficiency, also known as X-MAID has only recently been described. Severe forms of X-MAID may present during infancy or childhood with combined immunodeficiency (including lymphopenia and neutropenia), while adults appear to have a milder clinical picture (Lagresle-Peyrou et al. 2016). Other manifestations, including hypogammaglobulinemia and poor vaccine responses, have also been reported. With the recent introduction of newborn screening for severe combined immunodeficiency, cases of X-MAID have also been identified due to low levels of T cell receptor excision circles, demonstrating the importance of this screening tool (Delmonte et al. 2017; Dvorak et al. 2019).

The majority of previously reported MSN mutations affect the FERM domain, particularly the F2 subdomain (Bradshaw et al. 2018; Delmonte et al. 2017; Henrickson et al. 2019; Lagresle-Peyrou et al. 2016). Our patient’s de novo mutation similarly affected the FERM domain, but in the F3 subdomain (pleckstrin homology domain), essential for binding to transmembrane adhesion molecules. Aberrations this region, from amino acid 202 to 298, have not been extensively described. A recent case report of a de-novo MSN mutation affecting amino acid 217, was clinically described with some overlapping respiratory features like most X-MAID cases, but with marked features of autoimmunity and immune dysregulation (Kovács et al. 2022). Our patient also presented with autoimmune cytopenia and alopecia areata, which may represent a distinctive feature within these group of disorders. Moesin protein expression in lymphocytes was abnormal in our patient, similar to other case reports (Bradshaw et al. 2018; Lagresle-Peyrou et al. 2016).

Our histopathology evaluation of the lungs represent the first documented for a patient with X-MAID who suffered severe pulmonary vein stenosis, requiring a double lung transplant. The muscularization of the walls of the small pulmonary veins (so-called ‘arteriolization change’) results in sclerosis and loss of elasticity of the veins, causing functional obstruction of blood flow and hence hypertension. Moesin is expressed in high levels in the lung, indicating a critical role in the maintenance of alveolar structure as well as regulation of inflammation and repair. The absence of moesin expression likely explains the recurrent pulmonary disease since childhood in X-MAID patients. In line with our findings, the lungs of moesin-deficient mice demonstrate abnormal alveolar architecture with emphysematous changes, similar to what is seen in our patient. Further, in response to lung injury, moesin-deficient mice show greater lung damage and fibrosis compared to controls, indicating dysregulated repair mechanisms (Hashimoto et al. 2008). This is consistent with the high expression of moesin (compared to ezrin and radixin) in the lungs and its role in alveolar preservation and lung homeostasis (Hashimoto et al. 2008).

Taken together, the clinical presentation of our X-MAID patient described here adds to the known spectrum of disease and highlights the non-redundant functions of moesin, particularly in the lungs.