Base Editing (BE4max) for MT-ATP6 m.8993T>C in Leigh syndrome with cardiomyopathy

Base Editing (BE4max) via AAV9 delivery is a rationale-driven therapeutic strategy for Leigh syndrome with cardiomyopathy targeting the MT-ATP6 m.8993T>C variant (Pathogenic, transition). The editing system (BE4max (cytosine base editor)) converts the pathogenic C to T (or G to A on the target strand), restoring the wild-type codon. Target tissue: Heart. Therapeutic goal: Correct the m.8993T>G mutation in MT-ATP6 to normalize mitochondrial ATP synthase function and prevent Leigh syndrome with cardiomyopathy. Risk profile: off-target Medium (bystander bases in editing window), delivery complexity Medium, immunogenicity High (AAV pre-existing immunity).

1. Molecular basis: MT-ATP6 NC_012920.1(MT-ATP6):m.8993T>C is classified as Pathogenic (ClinVar variation ID 9642). Molecular consequence: transition. 2. Epidemiology: Leigh syndrome (ORPHA:506) is a rare, early-onset mitochondrial encephalomyelopathy with an estimated birth prevalence around 1 in 36,000–40,000. Onset is typically in infancy (median ~7 months), and over half of patients die within the first few years of life. Cardiac involvement, particularly hype 3. Standard of care: There is no curative therapy for Leigh syndrome. Current management is largely supportive: nutritional and respiratory support, seizure control, management of movement disorders, and treatment of heart failure and arrhythmias when cardiomyopathy is present. Empiric use of vitamin and cofactor supple 4. Pipeline: Small-molecule and cofactor therapies: EPI-743/vatiquinone has completed Phase II and entered Phase III trials in inherited mitochondrial diseases including Leigh syndrome, but no disease-modifying approval exists. Additional agents (idebenone, KH176, rapamycin/mTOR inhibition, hypoxia-based approac 5. CBE clinical validation: BE4max (Koblan et al. 2018) is the gold-standard cytosine base editor. Multiple CBE programs are in clinical development for liver and hematologic targets.

1. No published data specifically correcting MT-ATP6 m.8993T>C with Base Editing (BE4max); strategy is based on general principles and must be validated preclinically.
2. PAM availability and bystander base analysis for the specific genomic context have not been performed. If no canonical NGG PAM positions the target within the editing window, PAM-flexible variants (SpRY) may be needed.
3. Delivery to Heart tissue remains a major translational bottleneck. Current vectors have limited transduction efficiency in these compartments.
4. Long-term durability, off-target genome-wide effects, and immunogenicity in the target patient population require thorough preclinical and clinical evaluation.