Class-Switch Recombination Defects

Abstract

Class-switch recombination (CSR), also known as isotype switching, is the biological mechanism that changes the isotype of an antibody (immunoglobulin) from one type to the other (i.e., from IgM to IgG, IgA or IgE). This confers to the antibodies specific effector function and tissue distribution. Class-switch recombination deficiencies are a heterogeneous group of primary immunodeficiencies characterized by normal or increased levels of serum IgM in combination with reduced or absence of serum IgG, IgA, or IgE. The former name of CSR deficiency was hyper-IgM syndrome. The estimated frequency of CSR defects is around 1:500,000 newborns. There are different underlying genetic causes of CSR deficiencies, and they can be divided into groups with genetic defects hampering the cognate T-B interaction (CD40L, CD40, and NEMO), a group with intrinsic B-cell defects (AID and UNG), and finally a group with DNA repair defects involving the non-homologous end-joining (NHEJ) pathway or the mismatch repair (MMR) pathway. The different CSR deficiencies have their specific immunological and clinical characteristics, which also require different treatment strategies.