RESUMEN
Deficiency in repair of damaged DNA leads to genomic instability and is closely associated with tumorigenesis. Most DNA double-strand-breaks (DSBs) are repaired by two major mechanisms, homologous-recombination (HR) and non-homologous-end-joining (NHEJ). Although Akt has been reported to suppress HR, its role in NHEJ remains elusive. Here, we report that Akt phosphorylates XLF at Thr181 to trigger its dissociation from the DNA ligase IV/XRCC4 complex, and promotes its interaction with 14-3-3ß leading to XLF cytoplasmic retention, where cytosolic XLF is subsequently degraded by SCF(ß-TRCP) in a CKI-dependent manner. Physiologically, upon DNA damage, XLF-T181E expressing cells display impaired NHEJ and elevated cell death. Whereas a cancer-patient-derived XLF-R178Q mutant, deficient in XLF-T181 phosphorylation, exhibits an elevated tolerance of DNA damage. Together, our results reveal a pivotal role for Akt in suppressing NHEJ and highlight the tight connection between aberrant Akt hyper-activation and deficiency in timely DSB repair, leading to genomic instability and tumorigenesis.
Asunto(s)
Reparación del ADN por Unión de Extremidades/genética , Enzimas Reparadoras del ADN/fisiología , Proteínas de Unión al ADN/fisiología , Proteínas Proto-Oncogénicas c-akt/fisiología , Proteínas 14-3-3/metabolismo , Secuencia de Aminoácidos , Carcinogénesis/genética , Citoplasma/metabolismo , Roturas del ADN de Doble Cadena , ADN Ligasa (ATP) , ADN Ligasas/metabolismo , Enzimas Reparadoras del ADN/química , Enzimas Reparadoras del ADN/genética , Enzimas Reparadoras del ADN/metabolismo , Proteínas de Unión al ADN/química , Proteínas de Unión al ADN/genética , Proteínas de Unión al ADN/metabolismo , Inestabilidad Genómica , Humanos , Datos de Secuencia Molecular , Fosforilación , Proteínas Proto-Oncogénicas c-akt/metabolismo , Proteínas Ligasas SKP Cullina F-box/fisiología , Alineación de SecuenciaRESUMEN
Ig heavy chain (IgH) variable region exons are assembled from V, D, and J gene segments during early B-lymphocyte differentiation. A several megabase region at the "distal" end of the mouse IgH locus (Igh) contains hundreds of V(H)s, separated by an intergenic region from Igh Ds, J(H)s, and constant region exons. Diverse primary Igh repertoires are generated by joining Vs, Ds, and Js in different combinations, with a given B cell productively assembling only one combination. The intergenic control region 1 (IGCR1) in the V(H)-to-D intergenic region regulates Igh V(D)J recombination in the contexts of developmental order, lineage specificity, and feedback from productive rearrangements. IGCR1 also diversifies IgH repertoires by balancing proximal and distal V(H) use. IGCR1 functions in all these regulatory contexts by suppressing predominant rearrangement of D-proximal V(H)s. Such IGCR1 functions were neutralized by simultaneous mutation of two CCCTC-binding factor (CTCF)-binding elements (CBE1 and CBE2) within it. However, it was unknown whether only one CBE mediates IGCR1 functions or whether both function in this context. To address these questions, we generated mice in which either IGCR1 CBE1 or CBE2 was replaced with scrambled sequences that do not bind CTCF. We found that inactivation of CBE1 or CBE2 individually led to only partial impairment of various IGCR1 functions relative to the far greater effects of inactivating both binding elements simultaneously, demonstrating that they function cooperatively to achieve full IGCR1 regulatory activity. Based on these and other findings, we propose an orientation-specific looping model for synergistic CBE1 and CBE2 functions.
Asunto(s)
Cadenas Pesadas de Inmunoglobulina/genética , Modelos Inmunológicos , Receptores de Antígenos de Linfocitos B/genética , Secuencias Reguladoras de Ácidos Nucleicos/genética , Recombinación V(D)J/inmunología , Exones VDJ/genética , Animales , Southern Blotting , Factor de Unión a CCCTC , ADN Intergénico/genética , Citometría de Flujo , Vectores Genéticos/genética , Región de Control de Posición/genética , Ratones , Mutagénesis , Reacción en Cadena de la Polimerasa , Receptores de Antígenos de Linfocitos B/inmunología , Secuencias Reguladoras de Ácidos Nucleicos/inmunología , Proteínas Represoras/metabolismo , Reacción en Cadena de la Polimerasa de Transcriptasa InversaRESUMEN
The classical nonhomologous DNA end-joining (C-NHEJ) double-strand break (DSB) repair pathway in mammalian cells maintains genome stability and is required for V(D)J recombination and lymphocyte development. Mutations in the XLF C-NHEJ factor or ataxia telangiectasia-mutated (ATM) DSB response protein cause radiosensitivity and immunodeficiency in humans. Although potential roles for XLF in C-NHEJ are unknown, ATM activates a general DSB response by phosphorylating substrates, including histone H2AX and 53BP1, which are assembled into chromatin complexes around DSBs. In mice, C-NHEJ, V(D)J recombination, and lymphocyte development are, at most, modestly impaired in the absence of XLF or ATM, but are severely impaired in the absence of both. Redundant functions of XLF and ATM depend on ATM kinase activity; correspondingly, combined XLF and H2AX deficiency severely impairs V(D)J recombination, even though H2AX deficiency alone has little impact on this process. These and other findings suggest that XLF may provide functions that overlap more broadly with assembled DSB response factors on chromatin. As one test of this notion, we generated mice and cells with a combined deficiency for XLF and 53BP1. In this context, 53BP1 deficiency, although leading to genome instability, has only modest effects on V(D)J recombination or lymphocyte development. Strikingly, we find that combined XLF/53BP1 deficiency in mice severely impairs C-NHEJ, V(D)J recombination, and lymphocyte development while also leading to general genomic instability and growth defects. We conclude that XLF is functionally redundant with multiple members of the ATM-dependent DNA damage response in facilitating C-NHEJ and discuss implications of our findings for potential functions of these factors.