Improved AAV9-based gene therapy design for SURF1-related Leigh syndrome with minimal toxicity

Surfeit locus protein 1 (SURF1)-related Leigh syndrome is an early-onset neurodegenerative disorder characterized by a reduction in complex IV activity that disrupts mitochondrial function. Currently, there are no disease-modifying treatments available. Previously, we reported that a gene replacement therapy for <i>SURF1</i>-related Leigh syndrome was developed, which showed improved complex IV activity and restored exercise-induced lactate acidosis, as well as a high safety profile in wild-type (WT) mice. However, further investigations of this original SURF1 vector design uncovered cytotoxicity in multiple tissues of WT rats despite having minimal immune responses. We hypothesized that this cytotoxicity was elicited by SURF1 protein overexpression driven by a strong ubiquitous promoter, CBh. Here, we report the development of an improved gene therapy for SURF1 Leigh syndrome by utilizing a different promoter and polyadenylation sequence. Our data showed that, with lower SURF1 protein expression, the new design conferred a similar level of efficacy, but with minimal cytotoxicity in mice or rats. We propose this new vector design as a promising therapeutic candidate for <i>SURF1</i>-related Leigh syndrome, warranting further translational studies.