Homology directed repair

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Double-strand break repair models that act via homologous recombination

Homology Directed Repair (HDR) is a mechanism of DNA repair that is used by cells to fix double-strand breaks (DSBs) in DNA. This process is crucial for maintaining the integrity of the genome and preventing genetic disorders, cancer, and other diseases. HDR involves the use of a homologous sequence as a template for repair, ensuring that the DNA sequence is restored accurately without the introduction of mutations.

Mechanism

The process of HDR begins when a DSB is recognized by the cell. The broken ends of the DNA are processed to form single-stranded DNA (ssDNA) tails. This is followed by the search for a homologous sequence, often the sister chromatid, which serves as a template for repair. A key protein involved in this search and strand invasion is RAD51, which forms a nucleoprotein filament with the ssDNA. This filament then invades the homologous DNA duplex, leading to the formation of a displacement loop (D-loop). DNA synthesis ensues, using the intact homologous sequence as a template. Finally, the newly synthesized DNA is ligated to the original DNA, completing the repair process.

Importance

HDR is particularly important in the repair of DSBs that occur during the S and G2 phases of the cell cycle, when a sister chromatid is available as a template. This repair pathway is highly accurate because it uses the homologous sequence, minimizing the risk of mutations. HDR plays a critical role in maintaining genomic stability, preventing the accumulation of DNA damage that can lead to cancer and other diseases.

HDR vs. NHEJ

HDR is one of two main pathways for repairing DSBs, the other being Non-Homologous End Joining (NHEJ). Unlike HDR, NHEJ does not require a homologous template and instead directly ligates the broken ends of DNA. While NHEJ is faster and can occur throughout the cell cycle, it is more prone to errors, potentially leading to mutations.

Applications in Gene Editing

The precision of HDR has been harnessed in gene editing technologies, such as CRISPR-Cas9. By introducing a DSB at a specific site in the genome and providing a donor DNA template with the desired sequence, researchers can use HDR to introduce specific mutations or correct genetic defects. This has significant implications for genetic engineering, functional genomics, and the development of therapies for genetic disorders.

Limitations and Challenges

One of the main limitations of using HDR for gene editing is its efficiency, which is significantly lower than NHEJ, especially in non-dividing cells. Additionally, the requirement for a homologous template can be a hurdle in certain applications. Researchers are actively exploring ways to enhance HDR efficiency and specificity to improve its applicability in gene therapy and biotechnology.