Nucleolar Stress and Disease Group, Danish Cancer Institute, 2100, Copenhagen, Denmark.
Novo Nordisk Foundation Center for Protein Research, University of Copenhagen, Blegdamsvej 3B, DK-2200, Copenhagen, Denmark.
Nat Commun. 2024 Sep 6;15(1):7797. doi: 10.1038/s41467-024-52189-6.
Ribosomal DNA (rDNA) encodes the ribosomal RNA genes and represents an intrinsically unstable genomic region. However, the underlying mechanisms and implications for genome integrity remain elusive. Here, we use Bloom syndrome (BS), a rare genetic disease characterized by DNA repair defects and hyper-unstable rDNA, as a model to investigate the mechanisms leading to rDNA instability. We find that in Bloom helicase (BLM) proficient cells, the homologous recombination (HR) pathway in rDNA resembles that in nuclear chromatin; it is initiated by resection, replication protein A (RPA) loading and BRCA2-dependent RAD51 filament formation. However, BLM deficiency compromises RPA-loading and BRCA1/2 recruitment to rDNA, but not RAD51 accumulation. RAD51 accumulates at rDNA despite depletion of long-range resection nucleases and rDNA damage results in micronuclei when BLM is absent. In summary, our findings indicate that rDNA is permissive to RAD51 accumulation in the absence of BLM, leading to micronucleation and potentially global genomic instability.
核糖体 DNA（rDNA）编码核糖体 RNA 基因，代表一个固有不稳定的基因组区域。然而，其对基因组完整性的潜在影响和相关机制仍不明确。在这里，我们使用布卢姆综合征（BS）作为模型，该疾病是一种罕见的遗传疾病，其特征为 DNA 修复缺陷和超不稳定 rDNA，以研究导致 rDNA 不稳定的机制。我们发现，在布卢姆解旋酶（BLM）功能正常的细胞中，rDNA 中的同源重组（HR）途径与核染色质中的途径相似；它由核酸酶切除、复制蛋白 A（RPA）加载和 BRCA2 依赖性 RAD51 丝形成启动。然而，BLM 缺陷会损害 RPA 加载和 BRCA1/2 向 rDNA 的募集，但不会损害 RAD51 的积累。RAD51 会在 rDNA 上积累，尽管长距离核酸酶切除被耗尽，并且当 BLM 不存在时，rDNA 损伤会导致微核。总之，我们的研究结果表明，在没有 BLM 的情况下，rDNA 允许 RAD51 积累，从而导致微核形成和潜在的全球基因组不稳定性。
