Using the NAFX to measure the effectiveness over time of gene therapy in canine LCA

Abstract

Purpose: To use ocular motility recordings to determine the changes over time of infantile nystagmus syndrome (INS) in RPE65-deficient canines with Leber Congenital Amaurosis (LCA) and assess the time course of the recalibration of the ocular motor system (OMS).

Methods: Nine dogs were treated bilaterally with AAV-RPE65. A second cohort of four dogs was treated with AAV2.RPE65, an optimized vector. Their fixation eye movements were recorded before treatment and at 4-week intervals for 3 months, by using high-speed (500 Hz) digital videography. The dogs were suspended in a sling and encouraged to fixate on distant (57 inches) targets at gaze angles varying between +/-15 degrees horizontally and +/-10 degrees vertically. The records for each eye were examined for qualitative changes in waveform and for quantitative changes in centralisation with the expanded nystagmus acuity function (NAFX) and compared with ERG results for restoration of receptor function.

Results: First group: Before treatment, five 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 1- and 2-month examinations, no change in nystagmus waveform or NAFX was observed in any of the initial dogs, while 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 leads 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. Second group: Only one dog of the four had clinically detectable INS, similar in characteristics to that seen in the affected dogs of the first group. Unlike any previous dog studied, this one showed a damping of the nystagmus within the first 4 weeks after treatment.

Conclusions: In all but one of the cases in which OMS recalibration occurred, as measured by the clinical appearance of nystagmus and by quantitative measurement using the NAFX, the improvement was apparent no sooner than 10 weeks after treatment. Longer term, dose-related studies are needed to determine the minimum necessary degree of restored receptor functionality, the duration after rescue for recalibration of the OMS, and the conditions under which recalibration information can successfully affect the contralateral eye.

Conflict of interest statement

Disclosure: J.B. Jacobs, None; L.F. Dell'Osso, None; Z.I. Wang, None; G.M. Acland, None; and J. Bennett, None

Figures

Figure 1

Representative pretreatment waveforms from four of the dogs. Note the presence of large drifts of eye position, in addition to the oscillations due to the nystagmus and the dual-jerk waveforms of EMB28 (bottom right).

Figure 2

Posttreatment waveforms from BR235. The underlying pendular waveform was not diminished in amplitude, nor did any centralising saccades develop. REH, right eye horizontal; LEH, left eye horizontal.

Figure 3

BR248, 10 weeks after treatment. Three successive segments of data demonstrate the variability in nystagmus improvement. Top: most commonly, the right eye showed improvement in damping, whereas the left eye did not. Middle: although considerably less common than before treatment, the nystagmus could appear at its pretreatment levels. Bottom: damping was apparent in both eyes. Dashed-dotted lines: area centralis. Abbreviations as in Figure 2.

Figure 4

Detailed examination of the posttreatment waveforms for BR248. Note the appearance of distinct centralisation periods, marked in bold. The presence of these periods almost every cycle has a direct correlation to increased visual acuity. Abbreviations as in Figure 2.

Figure 5

BR334's horizontal nystagmus before and after treatment. Waveforms are plotted on the same scale to facilitate comparison. In addition to the amplitude damping, ocular motor control appears to be improved as indicated by reduction in drifts off target. Abbreviations as in Figure 2.

Figure 6

BR334, after treatment. Top: the right eye, which did not respond to treatment, still demonstrated brief periods when the nystagmus was damped. Bottom: comparison of damping in the left eye, which did respond to treatment, and the right eye. Note the difference in peak-to-peak amplitudes between the two eyes. Abbreviations as in Figure 2.

Figure 7

Pre- and posttreatment NAFX values in both eyes of BR248 and BR334.