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Investigation of the Causal Etiology in a Patient with T-B+NK+ Immunodeficiency

We describe a male proband with normal B lymphocyte and natural killer (NK) cell numbers, but who had T lymphopenia beginning at 1 week of age. His peripheral blood contained very few T cell receptor excision circles (TREC), suggesting impaired thymic T cell development. He received an allogeneic hematopoietic stem cell marrow transplant, but despite engraftment, it did not fully correct the T lymphopenia. Sequencing excluded known immunodeficiency genes; however, whole exome sequencing revealed X-linked inheritance by the male proband of a missense mutation in MED14 (MED14V763A), a component of the mediator complex. Functional screening of the MED14V763A variant in zebrafish suggested that MED14 is essential for normal development, since morpholino (MO)-mediated loss of MED14 function attenuated T cell development, but it was rescued by ectopic expression of the wild type human MED14 but not by the MED14V763A variant. This defect in T cell repopulation is consistent with the incomplete T cell reconstitution observed upon transplantation of the proband with bone marrow from his healthy male sibling, who was found to have the same MED14V763A variant. However, the brother was normal, indicating that the altered MED14 could not by itself explain the disease. WES also revealed a mutation in the L1 cell adhesion molecule (L1CAMQ498H); however, introducing that mutation in vivo in mice did not disrupt T cell development. Consequently, the immunodeficiency in the patient is not fully elucidated. It is possible that MED14 contributes, but it is also possible that another gene is fully responsible for the disease.