Modifier-Sensitive Phenotypic Divergence in XMEN Disease (MAGT1 Deficiency): Neurodegenerative and Immuno-Hematologic Trajectories

<b>Background:</b> X-linked immunodeficiency with magnesium defect, Epstein-Barr virus (EBV) infection, and neoplasia (XMEN) disease is a rare inborn error of immunity caused by loss-of-function mutations in <i>MAGT1</i>, leading to impaired N-linked glycosylation. Although chronic EBV viremia is a hallmark of XMEN disease, the mechanisms underlying its marked clinical heterogeneity remain poorly understood. <b>Methods:</b> We performed an in-depth clinical, immunological, and genetic characterization of two siblings carrying a pathogenic <i>MAGT1</i> variant (c.369_370insCC; p.Gly124fs), validated and deposited in ClinVar (SCV007293792). Assessments included whole-exome sequencing, multiparametric flow cytometry focusing on NKG2D expression, and longitudinal clinical follow-up. <b>Results:</b> Despite shared absence of NKG2D expression, the siblings exhibited strikingly divergent phenotypes. One sibling developed progressive neurodegeneration with central nervous system atrophy. The other presented with a complex immuno-hematologic phenotype, including EBV-positive Hodgkin lymphoma, recurrent autoimmune cytopenias, and lymphoma-associated thrombotic microangiopathy, representing a novel clinical association in XMEN disease. Comparative immunophenotyping revealed shared defects in B-cell maturation but distinct T-cell differentiation patterns. To contextualize neurological variability, we propose a descriptive, hypothesis-generating three-category conceptual classification comprising early-onset neurodevelopmental forms, adult-onset neurodegenerative manifestations, and secondary immune-mediated or vascular involvement of the nervous system. <b>Conclusions:</b> These findings demonstrate profound intrafamilial heterogeneity in XMEN disease and suggest a model in which modifier-sensitive factors influence organ-specific disease expression. The observation of lymphoma-associated thrombotic microangiopathy and the proposed descriptive neurological classification provide a conceptual framework that may help guide tailored, multidisciplinary surveillance beyond the primary genetic defect.