What Is the Bilateral Transfer Effect in LHON Gene Therapy?

The term bilateral transfer effect is often used to describe an unexpected observation in several major gene therapy clinical trials for Leber hereditary optic neuropathy (LHON). When researchers administered the investigational gene therapy into only one eye, patients frequently showed measurable improvements in visual acuity chart scores in both the treated eye and the untreated (or sham-injected) fellow eye ^[1][2].

While this observed bilateral improvement is encouraging, it does not definitively prove that the therapy physically transferred between the eyes or that the injection alone caused the untreated eye to improve ^[3][2]. Scientists are still actively researching whether this is a true biological effect of the drug or a result of other factors ^[3].

Who Was Studied?

The trials demonstrating these effects specifically focused on patients with LHON caused by pathogenic variants in mitochondrial DNA, primarily the MT-ND4 mutation (m.11778G>A) ^[4][5]. The gene therapy tested, lenadogene nolparvovec (Lumevoq), is designed to deliver a functional ND4 gene product to the cells, rather than editing the patient’s existing mitochondrial DNA ^[4].

These trials enrolled specific groups:

• RESCUE: Patients who had experienced vision loss for 6 months or less ^[1].
• REVERSE: Patients who had experienced vision loss for 6 to 12 months ^[2].

Because outcomes can vary based on the specific genetic mutation and the duration of vision loss, these results may not apply to patients with different LHON genotypes or long-standing disease.

The Clinical Trial Evidence

The observation stems from a series of clinical trials:

• RESCUE and REVERSE Trials: Patients received the gene therapy injection in one eye and a “sham” (simulated) procedure in the fellow eye. Researchers originally expected only the treated eye to improve. Instead, both eyes showed similar visual gains on eye charts ^[1][2]. In the REVERSE trial at week 96, 78% of participants showed improvement in visual acuity scores in both eyes ^[2]. Because the sham-injected fellow eyes also improved, it was difficult to prove the therapy was statistically superior to the sham procedure based solely on comparing one eye to the other ^[1][2].
• REFLECT Trial: This trial treated the first-affected eye with gene therapy and randomly assigned the second eye to receive either gene therapy or a placebo injection. Again, placebo-treated fellow eyes showed measurable visual improvements from their lowest recorded vision scores ^[6].
• Long-Term Follow-Up: A follow-up study of RESCUE and REVERSE participants reported that bilateral visual improvements were sustained for up to five years ^[4]. However, this long-term data involved 55 of the original 76 participants and lacked a concurrent, entirely untreated control group for comparison ^[4].

Safety and Regulatory Status

It is important to note that lenadogene nolparvovec (Lumevoq) is an investigational treatment and is not routinely approved or available in all jurisdictions. Access may be limited to clinical trials or specialized compassionate use programs.

Furthermore, gene therapy is not risk-free. While the viral vector is modified to be non-replicating, the injection procedure and the vector itself can cause ocular inflammation (uveitis), increased intraocular pressure, and immune reactions ^[7][8]. Any decision to pursue treatment requires a thorough discussion of these risks with a neuro-ophthalmologist.

How Might the Untreated Eye Improve?

Researchers have proposed several hypotheses for why an untreated eye might improve, ranging from biological mechanisms to natural disease processes:

1. Viral Vector Migration

One theory is that a small amount of the gene therapy vector physically travels from the injected eye to the other eye ^[5][9]. Preclinical animal studies and limited post-mortem tissue samples from two human patients have detected traces of the gene therapy DNA in the untreated eye and visual pathways ^[2][9]. However, detecting DNA does not prove that functional proteins were produced or that this biological transfer caused the visual improvement ^[9].

2. The Neural Pathway Route

The optic nerves from both eyes cross over at the optic chiasm in the brain. It is biologically plausible that the vector or its protective effects travel along the optic nerve and cross the chiasm to reach the opposite side ^[5]. This remains an unproven hypothesis in humans.

3. Mitochondrial Transfer

Preclinical laboratory studies suggest that healthy mitochondria might be capable of traveling between cells to support damaged areas ^[9]. While observed in experimental settings, there is no proof yet that this mechanism explains the clinical trial findings in human eyes receiving LHON gene therapy ^[9].

4. Alternative Explanations

Not all researchers believe a physical transfer occurred. Other factors that could explain the bilateral improvements include:

• Spontaneous Visual Recovery: The natural history of LHON shows that some patients experience spontaneous, partial visual recovery over time without any treatment ^[3][1].
• Learning Effects: As patients repeatedly take vision tests over multi-year trials, they may simply get better at taking the test, which could inflate the scores of the untreated eye ^[3].

What This Means for Patients

The observed bilateral improvements are a scientifically important finding, but they do not guarantee that a single injection will benefit an individual’s untreated eye. Improving a score on a visual acuity chart also may not equate to fully restored central vision or daily independence ^[3][10]. Patients should discuss their specific mutation, disease timeline, and functional goals with their care team to understand what outcomes are realistic.