Haploinsufficiency of lysosomal enzyme genes in Alzheimer's disease

There is growing evidence suggesting that the lysosome or lysosome dysfunction is associated with Alzheimer's disease (AD). Pathway analysis of post mortem brain-derived proteomic data from AD patients shows that the lysosomal system is perturbed relative to similarly aged unaffected controls. However, it is unclear if these changes contributed to the pathogenesis or are a response to the disease. Consistent with the hypothesis that lysosome dysfunction contributes to AD pathogenesis, whole genome sequencing data indicate that heterozygous pathogenic mutations and predicted protein-damaging variants in multiple lysosomal enzyme genes are enriched in AD patients compared to matched controls. Heterozygous loss-of-function mutations in the palmitoyl protein thioesterase-1 (PPT1), α-L-iduronidase (IDUA), β-glucuronidase (GUSB), N-acetylglucosaminidase (NAGLU), and galactocerebrosidase (GALC) genes have a gene-dosage effect on Aβ₄₀ levels in brain interstitial fluid in C57BL/6 mice and significantly increase Aβ plaque formation in the 5xFAD mouse model of AD, thus providing in vivo validation of the human genetic data. A more detailed analysis of PPT1 heterozygosity in 18-month-old mice revealed changes in α-, β-, and γ-secretases that favor an amyloidogenic pathway. Proteomic changes in brain tissue from aged PPT1 heterozygous sheep are consistent with both the mouse data and the potential activation of AD pathways. Finally, CNS-directed, AAV-mediated gene therapy significantly decreased Aβ plaques, increased life span, and improved behavioral performance in 5xFAD/PPT1+/- mice. Collectively, these data strongly suggest that heterozygosity of multiple lysosomal enzyme genes represent risk factors for AD and may identify precise therapeutic targets for a subset of genetically-defined AD patients.