Using the NAFX to Measure the Effectiveness Over Time of Gene Therapy in Canine LCA

Abstract

Abstract: : Purpose: To use ocular motility recordings to determine the changes over time of infantile nystagmus syndrome (INS) in RPE65–deficient Leber Congenital Ameurosis (LCA) canines and assess the dosage–effectiveness of gene therapy. Methods: Nine dogs were treated bilaterally with AAV–RPE65 (2 at standard dosage of 10^12 particles in 150µl, 2 controls, and the rest at 0.01, 0.001, or 0.0001 of standard dosage). The dogs were suspended in a sling and encouraged to fixate on distant (57 inches) targets at gaze angles varying between ±15°, horizontally, and ±10° vertically. Their fixation eye movements were recorded prior to treatment, and at four–week intervals for three months, using high–speed (500 Hz) digital videography. The records for each eye were examined for qualitative changes in waveform and for quantitative changes in foveation using the eXpanded Nystagmus Acuity Function (NAFX) and compared with ERG results for restoration of receptor function. Results: Prior to treatment, four of the dogs had clinically apparent INS with jerk, pendular, or both waveforms and with peak–to–peak amplitudes as great as 15 degrees. One dog had intermittent nystagmus. At the one– and two–month examinations, no change in nystagmus waveform or NAFX was observed in any of the dogs. At 10 weeks, one dog treated bilaterally with the standard dosage showed reduced nystagmus in only one eye. The other eye did not respond to treatment, as confirmed by ERG. This result was unexpected since it was previously documented that unilateral treatment led to bilateral reduction of INS. The other dog treated with the standard dosage showed no reduction of its small–amplitude, high–frequency pendular nystagmus despite positive ERG responses. Conclusions: NAFX measurements of RPE65 gene therapy effectiveness become apparent no sooner than three months post–treatment. Longer term, dose–related studies are needed to determine: the minimum necessary degree of restored receptor functionality; the duration following rescue for recalibration of the OMS; and the conditions under which recalibration information can successfully affect the contralateral eye.

Keywords: nystagmus • gene transfer/gene therapy • eye movements