A novel homozygous PSAP mutation identified by whole exome sequencing in a consanguineous family with metachromatic leukodystrophy: a case report.

Metachromatic leukodystrophy (MLD) is a genetic lysosomal disease. Here, we investigated the role of prosaposin (PSAP) gene mutations in MLD. This current case report describes a female patient who presented with motor development regression at two years and five months of age. The symptoms included difficulty walking, loss of ambulation, increased muscle tension, limb pain, and intentional tremors. Brain magnetic resonance imaging revealed potential white matter lesions, while electromyography indicated neurogenic damage in both lower limbs. Gesell assessment showed severe motor retardation, along with mild retardation in adaptability, speech, and social communication. Whole exome sequencing analysis identified a homozygous mutation in the PSAP gene, specifically c.643A>G, resulting in the amino acid change p.N215D. Immunofluorescence assays of cultured cells indicated no impact on the PSAP protein lysosomal localization, but the mutation was associated with a decreased lysosomal pH and reduced cathepsin D activity. Transmission electron microscopy revealed changes in lysosome morphology and abnormal protein aggregation. These findings suggest that the PSAP c.643A>G (p.N215D) mutation may be a causal factor for MLD in this patient. This discovery may provide new insights into the genetic basis and pathophysiological mechanisms of MLD.