Primary immunodeficiency associated with a SAMD9L mutation

Abstract

BackgroundSAMD9L is a tumor suppressor involved in regulating the proliferation and maturation of cells, particularly those derived from the bone marrow, and appears to play an important role in cerebellar function. It can be activated in hematopoietic stem cells by type I and type II interferons. It has been hypothesized to act as a critical antiviral gatekeeper regulating interferon dependent demand driven hematopoiesis. Gain of function mutations can present with an immunodeficiency due to transient severe cytopenias during viral infection. Case presentationWe report a 3-year-old boy born full term with a history of severe thrombocytopenia requiring transfusions, developmental delay, ataxia, seizure disorder, and recurrent severe respiratory viral infections. His infectious history was significant for respiratory syncytial virus with shock requiring extracorporeal membrane oxygenation complicated by cerebral infarction and a group A streptococcus empyema, osteomyelitis requiring a left below the knee amputation, and infections with rhinovirus, COVID-19, and parainfluenza requiring hospitalizations for respiratory support.Initial immunologic evaluation was done during his hospitalization for parainfluenza. His full T cell subsets was significant for lymphopenia across all cell lines with CD3 934/µL, CD4 653/µL, CD8 227/µL, CD19 76/µL, and CD1656 61/µL. His mitogen stimulation assay to phytohemagglutinin and pokeweed was normal. Immunoglobulin panel showed a mildly decreased IgM of 25 mg/dL, but normal IgA and IgG. Vaccine titers demonstrated protective titers to 12/22 pneumococcus serotypes, varicella, diphtheria, mumps, rubella, and rubeola. Repeat full T cell subsets 6 weeks later revealed marked improvement in lymphocyte counts with CD3 3083/µL, CD4 2101/µL, CD8 839/µL, CD19 225/µL, and CD1656/µL. A primary immunodeficiency genetic panel was ordered and positive for a heterozygous SAMD9L c.1549T>C (p.Trp517Arg) mutation classified as a variant of unknown significance. DiscussionThis patient's history of severe viral infections, ataxia, thrombocytopenia, and severe transient lymphopenia during infection is suggestive of a SAM9DL gain of function mutation. Protein modeling done by the laboratory suggests this missense mutation would affect protein structure. The mutation found has been observed in individuals with thrombocytopenia. This case highlights the importance of immunophenotyping both during acute illness and once recovered.