Will Bridge RNA Outshine CRISPR in Gene Editing and Therapy?

A new gene-editing technique leveraging “jumping genes,” described in recent papers, shows promise for large-scale genomic modifications without breaking DNA. This method utilizes mobile genetic sequences and a unique RNA molecule called bridge RNA to guide the insertion, deletion, or inversion of DNA segments. Unlike CRISPR–Cas9, which edits small DNA sections by breaking and repairing DNA, this new approach can handle larger changes, potentially revolutionizing gene therapy. While successful in bacteria, its effectiveness in human cells remains uncertain. Researchers emphasize the potential of this technique due to its simplicity and efficiency, though adaptation for use in mammalian cells is still in progress.

Mechanism of Action

CRISPR-Cas9: CRISPR-Cas9 utilizes a guide RNA (gRNA) to direct the Cas9 endonuclease to a specific DNA sequence. The gRNA binds to the complementary DNA sequence, allowing Cas9 to induce a double-strand break (DSB) at the target site. This break is repaired by the cell’s natural mechanisms, either through non-homologous end joining (NHEJ) or homology-directed repair (HDR), leading to targeted mutations or insertions.

Bridge RNA: Bridge RNA operates through a different mechanism involving the IS110 family of mobile genetic elements. It uses a structured non-coding RNA (ncRNA) that binds specifically to a recombinase. This Bridge RNA contains two internal loops that base-pair with the target DNA and donor DNA, facilitating recombination. The modularity of Bridge RNA allows independent reprogramming of the target-binding and donor-binding loops, enabling precise sequence-specific recombination for insertion, excision, and inversion of DNA.

Precision and Efficiency: Bridge RNA vs. CRISPR-Cas9

Both Bridge RNA and CRISPR-Cas9 exhibit high precision and efficiency (Table 1), but they achieve these through different mechanisms. CRISPR-Cas9 is well-established and widely used, with high efficiency in gene editing but potential off-target effects. Bridge RNA offers enhanced precision due to its bispecific binding and flexible targeting without the need for PAM sequences, potentially providing higher specificity with reduced off-target effects. However, its efficiency and applicability may be context-dependent and less established compared to CRISPR-Cas9.

Table 1. Comparison of Precision and Efficiency: Bridge RNA vs. CRISPR-Cas9

Factors Contributing to the Flexibility of Bridge RNA in Genome Editing

The flexibility of Bridge RNA in genome editing is primarily due to its modular binding mechanism, lack of specific sequence constraints, reduced risk of off-target effects, non-disruptive editing approach, programmability, and versatility in performing multiple types of DNA rearrangements within a single system. These features make Bridge RNA a highly adaptable and powerful tool for sophisticated and precise genome engineering tasks (Table 2).

Table 2. Factors Contributing to Flexibility of Bridge RNA

Potential Scenarios Where Bridge RNA Might Outshine CRISPR

1. Complex DNA Rearrangements: When precise and programmable DNA rearrangements like insertions, excisions, and inversions are required, Bridge RNA will be preferred over CRISPR.
2. Avoidance of Double-Strand Breaks: In applications where avoiding DSBs is crucial, such as in sensitive genomic regions or in therapeutic contexts where genomic stability is paramount, Bridge RNA will be preferred.
3. High Specificity Needs: In cases where ultra-high specificity is necessary to minimize off-target effects, such as in therapeutic gene editing or in precise synthetic biology applications, Bridge RNA will be preferred.
4. Single-Step Mechanism: Bridge RNA enables DNA recombination in a single step without creating double-strand breaks, avoiding reliance on error-prone cellular repair mechanisms.
5. Absence of PAM Sequences: In scenarios where the target DNA does not contain suitable PAM sequences required for CRISPR-Cas9 targeting, Bridge RNA does not require PAM sequences, providing more flexibility.
6. Large-Scale DNA Insertions: When inserting large DNA sequences (up to 5,000 base pairs) with high precision is required, Bridge RNA excels at inserting large DNA segments, which is challenging for CRISPR.
7. Scarless Editing: In therapeutic applications where minimizing unintended mutations and maintaining genomic integrity are critical, Bridge RNA offers “scarless” editing without leaving behind additional sequences.
8. Treatment of Repeat Expansion Disorders: Bridge RNA can address diseases caused by expansions of repetitive sequences, such as ALS and Huntington’s disease, by excising or inverting problematic DNA segments.
9. Cell Therapy Engineering: Bridge RNA could facilitate more precise insertion of genes like chimeric antigen receptors (CARs) for cancer treatment, enhancing cell-based therapies.
10. Synthetic Biology Applications: The ability to make large-scale chromosome changes makes Bridge RNA a powerful tool for engineering organisms with novel traits or functions.

Potential Scenarios Where Bridge RNA Might Not Outshine CRISPR

1. Small-Scale Edits and Base Editing: For small genetic modifications and single nucleotide changes, CRISPR will be better than Bridge RNA, especially with advancements like base editors and prime editors.
2. Established Applications with Extensive Validation: CRISPR has a robust body of research and clinical trials supporting its efficacy and safety, unlike Bridge RNA.
3. Applications Requiring Immediate Clinical Translation: CRISPR has already navigated many regulatory and ethical hurdles, making it more immediately applicable.
4. Scenarios Requiring High Efficiency in Mammalian Cells: CRISPR is well-established in these systems, while Bridge RNA’s efficiency in mammalian cells is yet to be demonstrated.
5. Applications with Time Constraints: CRISPR has well-documented procedures, tools, and extensive community support, making it faster to implement.
6. Cost and Resource Constraints: CRISPR is more cost-effective and resource-efficient due to its widespread adoption and established supply chains.
7. High-Throughput Screening: CRISPR is highly efficient and scalable for high-throughput applications, allowing for the rapid screening of large gene libraries.
8. Situations Where Breaking Both DNA Strands is Desired: Some applications may benefit from the double-strand breaks created by CRISPR, which are not induced by Bridge RNA.

Potential for Replacement

Bridge RNA and CRISPR-Cas9 each have unique strengths that make them suitable for different applications. Bridge RNA’s ability to perform programmable DNA rearrangements without DSBs provides a valuable tool for specific genome engineering tasks that CRISPR might not handle as efficiently. Conversely, CRISPR’s simplicity and versatility make it indispensable for a broad range of gene-editing applications.

In conclusion, Bridge RNA is unlikely to completely replace CRISPR-Cas9. Instead, it will serve as a complementary tool, expanding the capabilities of genetic engineering and allowing for more precise and sophisticated genome modifications. Both technologies will likely coexist, each finding use in areas where its particular strengths are most advantageous. As research progresses, the specific scenarios where Bridge RNA may replace or complement CRISPR could evolve, highlighting the dynamic nature of advancements in genetic engineering.

Further Reading

1. Durrant, M.G., Perry, N.T., Pai, J.J., et al. Bridge RNAs direct programmable recombination of target and donor DNA. Nature 630, 984–993 (2024).
2. Hiraizumi, M., Perry, N.T., Durrant, M.G., et al. Structural mechanism of bridge RNA-guided recombination. Nature 630, 994–1002 (2024).
3. Mahillon, J., Léonard, C., Chandler, M. IS elements as constituents of bacterial genomes. Research in Microbiology, 150, 675–687 (1999).
4. Ledford, H. No CRISPR: oddball ‘jumping gene’ enzyme edits genomes without breaking DNA. Nature 631, 470–471 (2024).
5. Kim, H.S., Kweon, J., & Kim, Y. Recent advances in CRISPR-based functional genomics for the study of disease-associated genetic variants. Experimental & Molecular Medicine 56, 861–869 (2024).