Unravelling the role of BACE2 in Alzheimer's disease using a cerebral organoid model

Abstract

BACE2 is a protease intricately involved in the amyloid processing pathway, crucial to the pathogenesis of Alzheimer’s Disease (AD), yet its role remains contentious due to conflicting evidence suggesting it may be neuroprotective or disease-driving. This thesis hypothesizes that BACE2 deficiency contributes to AD pathogenesis and aims to address this by investigating its role in AD using 3D cerebral organoids. To develop this disease model, fibroblasts from an early-onset AD patient with a single allele intronic deletion in BACE2 were reprogrammed to generate induced pluripotent stem cells (iPSCs) that were differentiated into 3D cerebral organoids. Leveraging on this model, this thesis aims to locate and pinpoint the impact of the intronic deletion to AD, explore the role of BACE2 using genetic and pharmacological approaches, and identify key genetic players and biological pathways. This thesis revealed that the intronic deletion spans Chr21: 41,207,640 - 41,219,853 and harbours enhancers that might potentially regulate BACE2 expression and single nucleotide polymorphisms (SNPs) linked to age of AD onset. These findings might explain why cerebral organoids generated from the patient exhibited early amyloid pathology followed by tau pathology, neuronal death, and astrogliosis compared to organoids generated from his non-AD father. Furthermore, pharmacological inhibition of BACE2 in organoids generated from individuals without AD induced early amyloid pathology while the treatment of patient organoids with β- and γ-secretase inhibitors partially mitigated amyloid pathology and neuronal death. Interestingly, CRISPR/Cas9-mediated biallelic BACE2 knockout in organoids revealed early neurodevelopmental abnormalities, suggesting an unexplored role of BACE2. RNA sequencing further demonstrated altered synaptic, immune, and neurodevelopmental response pathways. Our findings emphasized the gene dose sensitivity of BACE2, where its heterozygous intronic deletion contributes to a higher burden of AD-associated pathologies while a biallelic knockout affects neurodevelopment. Consequently, this thesis advocates for BACE2 as a therapeutic target to pave the way for BACE2 boosting strategies.