Background
The human 4-hydroxyphenylpyruvate dioxygenase-like protein (HPDL) has been linked to hereditary spastic paraplegia (HSP) with potential roles in neurogenesis and energy metabolism. However, the prevalence of HPDL variants in childhood-onset motor impairments remains unclear.
Objective
This study set out to characterize new patients with biallelic HPDL variants, and to explore the role of this protein both in vitro and in vivo.
Methods
Exome sequencing was performed on 210 patients with HSP, diplegic cerebral palsy (CP), or moderate/severe neurodevelopmental disorders. To understand the role of HPDL, we performed functional studies in patient-derived fibroblasts and crispant (hpdl-F0) zebrafish larvae.
Results
We identified 14 patients who exhibited reduced HPDL protein expression in cultured skin fibroblasts. Children with HPDL variants had elevated plasma glial fibrillary acidic protein levels, and serum metabolomics revealed reduced levels of 4-hydroxybenzeneacetic acid, a precursor of 4-hydroxymandelic acid (4HMA) and 4-hydroxybenzoic acid (4HB), with disruptions in metabolic pathways, including the Krebs cycle. The involvement of HPDL in energy metabolism was supported by altered mitochondrial respiration and increased cytosolic reactive oxygen species in human cells. Investigations into hpdl-F0 revealed neurodevelopmental abnormalities and epilepsy-like behavior, likely due to mitochondrial dysfunction, mirroring the phenotypes observed in children. Bypass therapy with 4HMA rescued the disease phenotypes in cells and hpdl-F0 crispant, while 4HB did so only partially in fish.
Conclusions
HPDL variants are responsible for CP-like spastic paraparesis in children. Loss of HPDL function disrupts cellular oxidative metabolism, highlighting its role in neurodevelopment and energy homeostasis, both in vitro and in vivo.
