Spastic Paraplegia-50, Autosomal Recessive

Spastic paraplegia 50 (SPG50) is an ultrarare childhood-onset neurological disorder caused by biallelic loss-of-function variants in the AP4M1 gene. SPG50 is characterized by progressive spastic paraplegia, global developmental delay and subsequent intellectual disability, secondary microcephaly, and epilepsy. Preclinical studies evaluated an adeno-associated virus (AAV)/AP4M1 gene therapy for SPG50. In vitro studies demonstrated that transduction of patient-derived fibroblasts with AAV2/AP4M1 resulted in phenotypic rescue. To evaluate efficacy in vivo, Ap4m1 knockout mice were intrathecally (IT) injected with 5E11, 2.5E11, or 1.25E11 vg doses of AAV9/AP4M1 at postnatal day p7-10 (pre-manifesting cohorts) or p90 (early manifesting cohorts). Age- and dose-dependent effects were observed, with early intervention and higher doses achieving the best therapeutic benefits. In parallel, three toxicology studies in wild-type mice, rats, and non-human primates (NHPs) demonstrated that AAV9/AP4M1 had an acceptable safety profile up to a target human dose of 1E15 vg. Of note, similar degrees of minimal to mild dorsal root ganglia (DRG) toxicity were observed in both rats and NHPs, supporting the use of rats to monitor DRG toxicity in future IT AAV studies. These preclinical results identify an acceptably safe and efficacious dose of IT-administered AAV9/AP4M1, supporting an investigational gene transfer clinical trial to treat SPG50.

Bi-allelic variants affecting one of the four genes encoding the AP4 subunits are responsible for the "AP4 deficiency syndrome". Core features include hypotonia that progresses to hypertonia and spastic paraplegia, intellectual disability, postnatal microcephaly, epilepsy, and neuroimaging features. Namely, AP4M1 (SPG50) is involved in autosomal recessive spastic paraplegia 50 (MIM#612936). We report on three patients with core features from three unrelated consanguineous families originating from the Middle East. Exome sequencing identified the same homozygous nonsense variant: NM_004722.4(AP4M1):c.1012C>T p.Arg338* (rs146262009). So far, four patients from three other families carrying this homozygous variant have been reported worldwide. We describe their phenotype and compare it to the phenotype of patients with other variants in AP4M1. We construct a shared single-nucleotide polymorphism (SNP) haplotype around AP4M1 in four families and suggest a probable founder effect of Arg338* AP4M1 variant with a common ancestor most likely of Turkish origin. This article is protected by copyright. All rights reserved.

Biallelic loss-of-function variants in the subunits of the adaptor protein complex 4 lead to childhood-onset hereditary spastic paraplegia (AP-4-HSP): SPG47 (AP4B1), SPG50 (AP4M1), SPG51 (AP4E1), and SPG52 (AP4S1). Here, we describe the generation of induced pluripotent stem cells (iPSCs) from three AP-4-HSP patients with biallelic, loss-of-function variants in AP4M1 and their sex-matched parents (asymptomatic, heterozygous carriers). Following reprogramming using non-integrating Sendai virus, iPSCs were characterized following standard protocols including karyotyping, embryoid body formation, pluripotency marker expression and STR profiling. These first iPSC lines for SPG50 provide a valuable resource for studying this rare disease and related forms of hereditary spastic paraplegia.