NHEJ vs HDR: Key Pathways in DNA Repair Mechanisms
Explore the distinct roles and mechanisms of NHEJ and HDR in DNA repair, highlighting their regulatory factors and cell cycle influences.
DNA repair mechanisms are crucial for maintaining genomic stability and preventing mutations that can lead to diseases such as cancer. Among these, Nonhomologous End Joining (NHEJ) and Homology-Directed Repair (HDR) are key pathways tasked with repairing double-strand breaks in DNA. Understanding the differences between NHEJ and HDR is essential due to their roles in cellular health and implications for genetic research and therapy.
Steps In Nonhomologous End Joining
Nonhomologous End Joining (NHEJ) directly ligates broken DNA ends without a homologous template, making it versatile and frequently utilized. The process begins with the recognition of DNA breaks by the Ku protein complex, which consists of Ku70 and Ku80 subunits. This complex binds to DNA ends, protecting them from degradation and recruiting additional repair proteins.
Following Ku complex binding, DNA-dependent protein kinase catalytic subunit (DNA-PKcs) forms the DNA-PK holoenzyme, stabilizing DNA ends and recruiting other repair factors. DNA-PKcs phosphorylates substrates essential for repair progression. The next phase involves processing DNA ends, possibly with limited resection or fill-in synthesis, using enzymes like Artemis. Finally, the XRCC4-Ligase IV complex, often with XLF, seals the DNA break, restoring integrity. While efficient, NHEJ is error-prone, potentially causing mutations through small insertions or deletions.
Steps In Homology-Directed Repair
Homology-Directed Repair (HDR) ensures high-fidelity restoration of double-strand breaks by using a homologous sequence as a template. It is active during the S and G2 phases when sister chromatids are available. HDR begins with the MRN complex (MRE11, RAD50, NBS1) detecting DSBs and recruiting key proteins.
The MRN complex initiates resection of DNA ends to generate single-stranded DNA (ssDNA) overhangs, mediated by nucleases like CtIP and extended by exonucleases such as EXO1. The ssDNA is stabilized by replication protein A (RPA), forming a platform for RAD51 recombinase, facilitated by BRCA2. RAD51 searches for homology and invades a homologous DNA duplex, forming a displacement loop (D-loop) for error-free repair.
DNA synthesis occurs using the intact homologous sequence, facilitated by DNA polymerases like POL δ. The process forms a crossover structure called a Holliday junction, resolved by enzymes such as GEN1 or the SLX1-SLX4 complex, restoring the DNA to its original form.
Key Regulators Of Repair
DNA repair mechanisms rely on regulatory proteins to ensure fidelity and efficiency. The MRN complex is central, detecting DNA damage and recruiting essential repair proteins. Kinases like ATM and ATR phosphorylate substrates, including p53, to mediate cell cycle arrest and facilitate DNA repair. Phosphorylation events modulate repair protein activity, ensuring tight control.
BRCA1 and BRCA2 are critical in HDR regulation, with BRCA2 loading RAD51 onto ssDNA. Mutations in these genes can impair repair processes, increasing cancer susceptibility. The interconnectedness of DNA repair pathways with cellular health underscores the consequences of regulatory failure.
Cell Cycle Influence
The cell cycle influences DNA repair pathway choice between NHEJ and HDR. During G1, cells rely on NHEJ due to the lack of a homologous template, ensuring rapid repair. As cells transition into S and G2 phases, HDR becomes prominent with sister chromatids available for high-fidelity repair. This shift is facilitated by upregulation of proteins like RAD51 and BRCA2 during these phases, ensuring the most appropriate repair mechanism is employed based on the cell’s stage.
Distinguishing Features
Distinguishing NHEJ and HDR involves examining their characteristics and outcomes. NHEJ responds rapidly without sequence homology, making it suitable when speed is crucial, but its error-prone nature can result in genetic sequence alterations. This is relevant in therapeutic contexts like gene editing, where inaccuracies can have unintended consequences.
Conversely, HDR leverages homologous sequences for precise repair, critical in processes requiring exact genetic restoration. HDR’s reliance on homologous sequences restricts its activity to the S and G2 phases, necessitating a regulated environment for efficacy. Its high fidelity is advantageous in therapeutic applications requiring precise genetic corrections, such as in treating genetic disorders through CRISPR-Cas9. The choice between NHEJ and HDR reflects their complementary roles in maintaining genomic integrity.