RESUMO
Mutations in the tumour suppressor gene BRCA1 lead to breast and/or ovarian cancer. Here we show that loss of Brca1 in mice results in transcriptional de-repression of the tandemly repeated satellite DNA. Brca1 deficiency is accompanied by a reduction of condensed DNA regions in the genome and loss of ubiquitylation of histone H2A at satellite repeats. BRCA1 binds to satellite DNA regions and ubiquitylates H2A in vivo. Ectopic expression of H2A fused to ubiquitin reverses the effects of BRCA1 loss, indicating that BRCA1 maintains heterochromatin structure via ubiquitylation of histone H2A. Satellite DNA de-repression was also observed in mouse and human BRCA1-deficient breast cancers. Ectopic expression of satellite DNA can phenocopy BRCA1 loss in centrosome amplification, cell-cycle checkpoint defects, DNA damage and genomic instability. We propose that the role of BRCA1 in maintaining global heterochromatin integrity accounts for many of its tumour suppressor functions.
Assuntos
Proteína BRCA1/metabolismo , Neoplasias da Mama/genética , Inativação Gênica , Genes BRCA1/fisiologia , Heterocromatina/genética , Heterocromatina/metabolismo , Animais , Proteína BRCA1/deficiência , Proteína BRCA1/genética , Mama/citologia , Neoplasias da Mama/patologia , Linhagem Celular Tumoral , Células Cultivadas , DNA Satélite/genética , Células Epiteliais/metabolismo , Feminino , Regulação Neoplásica da Expressão Gênica , Instabilidade Genômica/genética , Células HeLa , Histonas/metabolismo , Humanos , Camundongos , Neoplasias Ovarianas/genética , RNA Mensageiro/genética , Transcrição Gênica/genética , Ubiquitina-Proteína Ligases/metabolismo , Proteínas Ubiquitinadas/metabolismo , UbiquitinaçãoRESUMO
Proteases that degrade the amyloid-ß peptide (Aß) are important in protecting against Alzheimer's disease (AD), and understanding these proteases is critical to understanding AD pathology. Endopeptidases sensitive to inhibition by thiorphan and phosphoramidon are especially important, because these inhibitors induce dramatic Aß accumulation (â¼30- to 50-fold) and pathological deposition in rodents. The Aß-degrading enzyme neprilysin (NEP) is the best known target of these inhibitors. However, genetic ablation of NEP results in only modest increases (â¼1.5- to 2-fold) in Aß, indicating that other thiorphan/phosphoramidon-sensitive endopeptidases are at work. Of particular interest is the NEP homolog neprilysin 2 (NEP2), which is thiorphan/phosphoramidon-sensitive and degrades Aß. We investigated the role of NEP2 in Aß degradation in vivo through the use of gene knockout and transgenic mice. Mice deficient for the NEP2 gene showed significant elevations in total Aß species in the hippocampus and brainstem/diencephalon (â¼1.5-fold). Increases in Aß accumulation were more dramatic in NEP2 knockout mice crossbred with APP transgenic mice. In NEP/NEP2 double-knockout mice, Aß levels were marginally increased (â¼1.5- to 2-fold), compared with NEP(-/-)/NEP2(+/+) controls. Treatment of these double-knockout mice with phosphoramidon resulted in elevations of Aß, suggesting that yet other NEP-like Aß-degrading endopeptidases are contributing to Aß catabolism.
