HDR vs NHEJ

HDR vs NHEJ refers to the two primary DNA repair pathways that compete to resolve double-strand breaks: homology-directed repair (HDR) for precise editing and non-homologous end joining (NHEJ) for rapid but error-prone repair ¹.

How It Works

When a double-strand break occurs, cells activate both pathways simultaneously. NHEJ directly ligates broken DNA ends without a template, often introducing small insertions or deletions (indels). It is active throughout the cell cycle and is the dominant repair pathway in most mammalian cells. NHEJ is leveraged for gene knockouts, where frameshift mutations disrupt protein coding.

HDR uses a homologous template — either a sister chromatid or an exogenous donor — to faithfully restore the original sequence or introduce specific changes. HDR is restricted to S and G2 phases when sister chromatids are available, making it inherently less efficient than NHEJ in most experimental contexts.

The balance between these pathways determines editing outcomes. Researchers manipulate this balance through cell cycle synchronization, small-molecule inhibitors of NHEJ factors like DNA-PKcs, or by using modified Cas9 variants that nick rather than cut both strands. The choice of pathway is critical: knock-out experiments favor NHEJ, while precise knock-ins require HDR ¹.

Computational Considerations

Predictive models incorporate features such as cell type, local chromatin state, cut-site sequence context, and cell cycle distribution to forecast the HDR-to-NHEJ ratio at a given locus. These models guide experimental design by recommending optimal conditions for the desired editing outcome ².

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