Melpida: Recombinant Adeno-associated Virus (serotype 9) Encoding a Codon Optimized Human AP4M1 Transgene (hAP4M1opt)

This study tests Melpida, a gene therapy delivered via an injection, in children with SPG50 — a very rare genetic disease caused by mutations in the AP4M1 gene that causes progressive muscle stiffness, developmental delay, and difficulty walking. The goal is to replace the faulty gene and slow or reverse disease progression. You may be eligible if... - Your child is between 4 months and 10 years old - Your child has a confirmed SPG50 diagnosis (two abnormal AP4M1 gene mutations) - Your child can stand for more than 5 seconds, take 5 steps independently or with a walker, or has a Modified Ashworth Scale score of 2 or below at the ankles You may NOT be eligible if... - Your child has a medical condition preventing lumbar puncture or anesthesia - Your child has a bleeding disorder or active infection - Your child has a significant non-SPG50 brain condition - Your child is already enrolled in another interventional clinical trial - Your child received an investigational drug in the past 30 days - Your child has abnormal liver or kidney lab values Talk to your doctor to see if this trial is right for you.