Dual novel variants in CD40 leading to hyper IgM syndrome: A case report of a school-aged female with new-onset recurrent pneumonia.

<b>Introduction: </b> Hyper IgM (HIGM) syndrome is an inborn error of immunity (IEI) that occurs due to defects in immunoglobulin class switch recombination (Ig-CSR). HIGM syndrome typically presents with recurrent infections in early childhood, and is often characterized on investigation with decreased IgG, IgA, and IgE titres, alongside normal or elevated IgM. A common cause of HIGM syndrome is a disruption to the CD40-CD40L interaction that triggers Ig-CSR, of which variants in CD40 are much rarer than in CD40L. We present a case of an 11-year-old female with HIGM syndrome caused by two novel variants of in <i>CD40</i>. <b>Aim:</b> To describe a case report of an eleven-year-old female with HIGM syndrome presenting with recurrent pneumonia. <b>Methods:</b> Data was collected retrospectively from the patient’s medical records. Laboratory investigations included quantitative immunoglobulins, quantitative B and T cell subsets, genetic testing using a primary immunodeficiency panel, and a functional assay for CD40 expression. <b>Results: </b>The proband is an 11-year-old female, who presented with recurrent pneumonia, otitis and septic arthritis. Investigations revealed neutropenia, low IgA, elevated IgM and normal IgG, along with absent vaccine responses. She was identified to harbour two novel variants in CD40: an intronic variant c.52-13A>G p.(?) and a missense variant c.466T>C p.(Ser156Pro). Functional assay indicated low expression of CD40 compared to healthy control, confirming the diagnosis of CD40 deficiency. <b>Conclusion:</b> Class switch defects, such as CD40 deficiency, are rare but significant diagnoses within the spectrum of IEI. This case demonstrates that despite the absence of some clinical red flags for immunodeficiency in infancy, IEIs remain an important consideration in pediatric patients regardless of age. Increasing clinical awareness of IEI will lead to earlier diagnoses, initiation of appropriate treatment, and prevention of potential complications. <b>Statement of Novelty:</b> We describe a patient with a late presentation of hyper IgM syndrome due to two novel variants in the CD40 gene, thus expanding the spectrum of CD40 gene variants.