FEBRUARY 2014 AB70 Abstracts S A T U R D A Y 248 Evaluation of a Novel Missense Activation-Induced Deaminase AID Mutation in a Child with Hyper IgM Syndrome: Is it a Pathogenic Mutation? Dr. Ottavia M. Delmonte, MD, Dr. Feilong Meng, PhD, Dr. Frederick Alt, PhD, Dr. Luigi D. Notarangelo, MD, Dr. Jolan E. Walter, MD, PhD; Pediatric Residency Program, Boston Children’s Hospital, Harvard Medical School, Boston, MA, Howard Hughes Medical Institute, Program in Cellular and Molecular Medicine and Immune Disease Institute, Boston Children’s Hospital, Department of Genetcis, Harvard Medical School, Boston, MA, Howard Hughes Medical Institute, Program in Cellular and Molecular Medicine and Immune Disease Institute, Boston Children’s Hospital, Department of Genetics, Harvard Medical School, Boston, MA, Division of Immunology, Boston Children’s Hospital, Harvard Medical School, Boston, MA; The Manton Center for Orphan Disease Research, Boston Children’s Hospital, Boston, MA, Division of Allergy/Immunology, Massachusetts General Hospital for Children, Harvard Medical School, Boston, MA. RATIONALE: Hyper IgM Syndrome (HIGM) is a primary immunodeficiency associated with defects of CD40, CD40L, AID and UNG genes. In the case of a novel mutation, genetic testing requires secondary confirmation such as functional studies to establish a link between the clinical phenotype and the mutation identified. METHODS: Genetic screening of a 4-year-old boy with clinical features of HIGM Syndrome resulted in a novel missense mutation in the AID gene (p.Ala132Pro). The mutation was located away from the core catalytic domain, but in an area highly conserved among species. The effect of the mutation on AID expression or function was unclear. Testing was undertaken to connect the novel mutation to an actual defect in AID, in vivo and in vitro. In vitro studies included compliment class switch recombination (CSR) assay in AICDA-deficient murine B cells andWestern blot to detect protein stability. AID function in vivo was assessed by IgG level and switched memory B cell fraction. RESULTS: Functional studies confirmed that the mouse AID A132P mutant cannot support CSR in AICDA-deficient murine B cells with retroviral-delivered AID mutant proteins. The AID mutant results in a dysfunctional unstable protein which cannot be detected with Western blot, suggesting that this conserved amino acid could be involved in protein folding and stability. In vivo, the very low amount of AID resulted in low IgG level (<7 mg/dL) and a low fraction of class switched memory B cells (<2%). CONCLUSIONS: Link should be established with functional studies for novel mutations, especially in case of missense mutations.