Background

Medical science has made significant strides in treating intractable diseases by utilizing gene-editing technologies such as CRISPR-Cas9 to directly edit DNA. However, permanent gene modification raises concerns about potential off-target effects, where unintended mutations occur in other parts of the genome. Furthermore, once a gene sequence is modified, it cannot be easily reversed. There are also safety concerns regarding the potential for unexpected genotoxicity, as the gene-editing protein may continue to act even after drug administration is stopped, remaining in the body.

To address these concerns about permanent genome modification, there is growing interest in reversible genetic medicines, which temporarily modulate gene expression. These include directly editing messenger RNA (mRNA), the molecule that carries genetic information from DNA to ribosomes, or epigenetically modifying genes without altering the underlying DNA sequence. By precisely controlling the therapeutic effect based on the patient's condition, these approaches offer a balance of safety and controllability.

Key Findings

This review clarifies the concept of reversibility by distinguishing three dimensions: mechanistic, functional, and clinical reversibility. Mechanistic reversibility refers to the transient exposure and subsequent disappearance of the gene-editing agent. Functional reversibility describes the rate at which cells return to their normal physiological state after the editing process is complete. Clinical reversibility refers to the ability of physicians to immediately stop treatment or re-administer the drug based on the patient's response or the occurrence of adverse effects.

Several molecular payloads with different mechanisms of action enable reversible gene therapy. These include fusion proteins consisting of catalytically inactive Cas9 (dCas9) and transcriptional activators or repressors, which regulate gene expression at the transcriptional level. Other examples include complexes that induce histone modifications or DNA methylation, and CRISPRoff-like systems that induce permanent epigenetic repression but can be reversed when needed.

In RNA-based reversible modulation, single-stranded oligonucleotides that target and recruit adenosine deaminase acting on RNA (ADAR) are gaining attention. CRISPR-Cas13 enzymes, which directly degrade or edit target RNA, and chemically modified guide RNAs (gRNAs) with enhanced stability are also promising candidates.

Effective delivery systems are essential for these therapies to overcome biological barriers. Outside the cell, they must protect the therapeutic agent from degradation by nucleases and avoid recognition by the immune system. Tissue selectivity is also important to prevent rapid clearance by the kidneys or excessive accumulation in the liver. Once inside the cell, the therapeutic agent must escape from endosomes, be stably released into the cytoplasm, and ultimately access the nucleus or chromatin structure.

Significance and Future Directions

As reversible modulation technologies mature, gene therapy is expected to expand beyond rare, single-gene disorders to include chronic metabolic diseases and acute conditions that require temporary intervention. This is because the dosage can be precisely adjusted to design personalized treatments. Furthermore, the ability to stop treatment and allow the body to return to its original state in case of adverse effects may reduce the barriers to clinical trials.

However, there are still industrial challenges to overcome. Given the temporary nature of these therapies, the feasibility of repeat dosing must be carefully evaluated. The potential for immune responses or resistance after repeated exposure also needs to be considered. Furthermore, establishing standardized manufacturing processes that ensure consistent product quality and potency assays that can verify the duration of the reversible effect will be critical for market success.