RESUMEN
Ig class-switch recombination (Ig-CSR) deficiencies are rare primary immunodeficiencies characterized by defective switched isotype (IgG/IgA/IgE) production. Depending on the molecular defect, defective Ig-CSR may also be associated with impaired somatic hypermutation (SHM) of the Ig V regions. Although the mechanisms underlying Ig-CSR and SHM in humans have been revealed (at least in part) by studying natural mutants, the role of mismatch repair in this process has not been fully elucidated. We studied in vivo and in vitro Ab maturation in eight MSH6-deficient patients. The skewed SHM pattern strongly suggests that MSH6 is involved in the human SHM process. Ig-CSR was found to be partially defective in vivo and markedly impaired in vitro. The resolution of γH2AX foci following irradiation of MSH6-deficient B cell lines was also found to be impaired. These data suggest that in human CSR, MSH6 is involved in both the induction and repair of DNA double-strand breaks in switch regions.
Asunto(s)
Proteínas de Unión al ADN/deficiencia , Cambio de Clase de Inmunoglobulina , Síndromes de Inmunodeficiencia/genética , Adolescente , Linfocitos B , Secuencia de Bases , Proliferación Celular , Células Cultivadas , Niño , Preescolar , Roturas del ADN de Doble Cadena , Reparación del ADN , Femenino , Histonas/genética , Humanos , Deficiencia de IgG/genética , Inmunoglobulina G/sangre , Región Variable de Inmunoglobulina/genética , Recuento de Linfocitos , Masculino , Persona de Mediana Edad , Análisis de Secuencia de ADN , Hipermutación Somática de Inmunoglobulina , Adulto JovenRESUMEN
Ig class-switch recombination (CSR) is a region-specific process that exchanges the constant Ig heavy-chain region and thus modifies an antibody's effector function. DNA lesions in switch (S) regions are induced by activation-induced cytidine deaminase (AID) and uracil-DNA glycosylase 2 (UNG2), subsequently processed to DNA breaks, and resolved by either the classical nonhomologous end-joining pathway or the alternative end-joining pathway (XRCC4/DNA ligase 4- and/or Ku70/Ku80-independent and prone to increased microhomology usage). We examined whether the induction of DNA lesions influences DNA end-joining during CSR by analyzing Sµ-Sα recombination junctions in various human Ig CSR defects of DNA lesion induction. We observed a progressive trend toward the usage of microhomology in Sµ-Sα recombination junctions from AID-heterozygous to AID-autosomal dominant to UNG2-deficient B lymphocytes. We thus hypothesize that impaired induction of DNA lesions in S regions during CSR leads to unusual end-processing of the DNA breaks, resulting in microhomology-mediated end-joining, which could be an indication for preferential processing by alternative end-joining rather than by classical nonhomologous end-joining.
Asunto(s)
Roturas del ADN , Reparación del ADN/fisiología , ADN/metabolismo , Cambio de Clase de Inmunoglobulina/fisiología , Recombinación Genética/fisiología , Antígenos Nucleares/genética , Antígenos Nucleares/inmunología , Antígenos Nucleares/metabolismo , Citidina Desaminasa/genética , Citidina Desaminasa/inmunología , Citidina Desaminasa/metabolismo , ADN/genética , ADN/inmunología , ADN Glicosilasas/genética , ADN Glicosilasas/inmunología , ADN Glicosilasas/metabolismo , Proteínas de Unión al ADN/genética , Proteínas de Unión al ADN/inmunología , Proteínas de Unión al ADN/metabolismo , Femenino , Humanos , Autoantígeno Ku , MasculinoRESUMEN
The investigation of an inherited primary immunodeficiency, the immunoglobulin class switch recombination deficiency, has allowed the delineation of complex molecular events that underlie antibody maturation in humans. The Activation-induced cytidine deaminase (AID)-deficiency, characterized by a defect in Class Switch Recombination (CSR) and somatic hypermutation, has revealed the master role of this molecule in the induction of DNA damage, the first step required for these two processes. The description that mutations in the gene encoding the Uracil-DNA glycosylase (UNG) lead to defective CSR has been essential for defining the DNA-editing activity of AID. Analysis of post meiotic segregation 2 (PMS2)-deficient patients gave evidence for the role of this mismatch repair enzyme in the generation of the DNA breaks that are required for CSR. Novel findings are awaited from the study ofyet-genetically undefined CSR-deficiencies, probably leading to the identification of AID cofactor(s) and/or proteins involved in CSR-induced DNA repair.
Asunto(s)
Trastornos por Deficiencias en la Reparación del ADN , Enfermedades Genéticas Congénitas , Cambio de Clase de Inmunoglobulina , Síndromes de Inmunodeficiencia , Recombinación Genética , Hipermutación Somática de Inmunoglobulina , Adenosina Trifosfatasas/genética , Adenosina Trifosfatasas/metabolismo , Animales , Citidina Desaminasa/genética , Citidina Desaminasa/metabolismo , Roturas del ADN , Daño del ADN/genética , Reparación del ADN/genética , Enzimas Reparadoras del ADN/genética , Enzimas Reparadoras del ADN/metabolismo , Trastornos por Deficiencias en la Reparación del ADN/enzimología , Trastornos por Deficiencias en la Reparación del ADN/genética , Proteínas de Unión al ADN/genética , Proteínas de Unión al ADN/metabolismo , Enfermedades Genéticas Congénitas/enzimología , Enfermedades Genéticas Congénitas/genética , Humanos , Síndromes de Inmunodeficiencia/enzimología , Síndromes de Inmunodeficiencia/genética , Endonucleasa PMS2 de Reparación del Emparejamiento Incorrecto , Uracil-ADN Glicosidasa/genética , Uracil-ADN Glicosidasa/metabolismoRESUMEN
Immunoglobulin (Ig) class switch recombination (CSR) deficiencies are rare primary immunodeficiencies characterized by the lack of switched isotype (IgG/IgA/IgE) production. In some cases, CSR deficiencies can be associated with abnormal somatic hypermutation. Analysis of CSR deficiencies has helped reveal the key functions of CSR-triggering molecules, i.e., CD40L, CD40, and effector molecules such as activation-induced cytidine deaminase and uracil N-glycosylase. We report a new form of B cell-intrinsic CSR deficiency found in three patients with deleterious, homozygous mutations in the gene encoding the PMS2 component of the mismatch repair machinery. CSR was found partially defective in vivo and markedly impaired in vitro. It is characterized by the defective occurrence of double-strand DNA breaks (DSBs) in switch regions and abnormal formation of switch junctions. This observation strongly suggests a role for PMS2 in CSR-induced DSB generation.