“Patient-specific Alzheimer-like pathology in trisomy 21 cerebral organoids reveals BACE2 as a gene-dose-sensitive AD-suppressor in human brain”

Abstract

A population of >6 million people worldwide at high risk of Alzheimer's disease (AD) are those with Down Syndrome (DS, caused by trisomy 21 (T21)), 70% of whom develop dementia during lifetime, caused by an extra copy of β-amyloid-(Aβ)-precursor-protein gene. We report AD-like pathology in cerebral organoids grown in vitro from non-invasively sampled strands of hair from 71% of DS donors. The pathology consisted of extracellular diffuse and fibrillar Aβ deposits, hyperphosphorylated/pathologically conformed Tau, and premature neuronal loss.Presence/absence of AD-like pathology was donor-specific (reproducible between individual organoids/iPSC lines/experiments). Pathology could be triggered in pathology-negative T21 organoids by CRISPR/Cas9-mediated elimination of the third copy of chromosome-21-gene BACE2, but prevented by combined chemical β and γ-secretase inhibition. We found that T21organoids secrete increased proportions of Aβ-preventing (Aβ1-19) and Aβ-degradation products (Aβ1-20 and Aβ1-34). We show these profiles mirror in cerebrospinal fluid of people with DS.We demonstrate that this protective mechanism is mediated by BACE2-trisomy and crossinhibited by clinically trialled BACE1-inhibitors. Combined, our data prove the physiological role of BACE2 as a dose-sensitive AD-suppressor gene, potentially explaining the dementia delay in ~30% of people with DS. We also show that DS cerebral organoids could be explored as pre-morbid AD-risk population detector and a system for hypothesis-free drug screens as well as identification of natural suppressor genes for neurodegenerative diseases.These studies uncovered that BACE2 can cleave the product of β-secretase (APP β-CTF) between aa19 and aa20, generating a 1-19 fragment [13][14][15] , thereby potentially preventing the formation of amyloidogenic Aβ, and degrading the β-CTF that has been implicated in neuronal toxicity, and impairment of several neuronal functions, such as axonal transport and autophagy 16 .When offered synthetic Aβ40/42 peptides in solution, purified BACE2 protein can rapidly degrade them by cutting after aa20 and aa34, to generate the 1-20 and 1-34 peptide products, but only at very acidic pH (3.5-4). In this reaction, BACE2 is 150-fold more efficient than BACE1, which is also capable of this cleavage, upon conditions of increased enzyme concentration/time 12,14 . Neither of these two putative anti-amyloidogenic actions of BACE2 (the θ-secretase activity, generating aa1-19, or the Aβ-degrading-protease activity (AβDP or Aβ clearance) generating aa1-20 and aa1-34), have yet been demonstrated to be the functional role of BACE2 under physiologically fluctuating gene doses in vivo in the human brain. A naturally occurring form of gene overdose for both APP and BACE2 is DS, caused by the trisomy of human chromosome 21 (T21) that harbours both APP and BACE2 genes. As increased levels of soluble Aβ were observed already in foetal brains in DS 17 , we examined cerebral organoids grown from induced pluripotent stem cell (iPSC) generated by non-integr...

Targeting increased levels of APP in Down syndrome: Posiphen‐mediated reductions in APP and its products reverse endosomal phenotypes in the Ts65Dn mouse model

“…Recent success in using iPSC‐derived cells to create 3D models of AD is encouraging as is the use of organoid technology 133–135 . With respect to AD‐DS, relatively few studies using this technology have been published, but recent advances encourage the view that they will add significantly to our understanding 133,136 . However, one cannot discount that the in vivo environment confers many advantages to the biology of AD‐DS and therefore attempts to translate in vitro human models to a brain‐like environment may prove very important.…”

“…Genes of interest include: DYRK1A , whose multifold actions include effects on tau phosphorylation 144,145 ; and RCAN1 146 and SYNJ1 (the gene encoding Synaptojanin 1), 147 which play a role in membrane trafficking and transmission of neurotrophic signals. Of interest, increased BACE2 (the gene encoding β‐site Aβ precursor protein cleaving enzyme 2 [BACE2]) expression was recently reported to modulate APP processing to reduce the accumulation of Aβ in human organoid culture 133 …”

“…Especially useful may be those in which the 3D environment of AD is recapitulated. Recent success in using iPSC‐derived cells to create 3D models of AD is encouraging as is the use of organoid technology 133–135 . With respect to AD‐DS, relatively few studies using this technology have been published, but recent advances encourage the view that they will add significantly to our understanding 133,136 .…”

Accelerated long-term forgetting: A sensitive paradigm for detecting subtle cognitive impairment and evaluating BACE1 inhibitor efficacy in preclinical Alzheimer's disease

“…Meanwhile, unlike GRL-8234, other BACE1 inhibitor drugs tested to date have poor or no selectivity for BACE1 over its homolog BACE2 (up to ∼3 folds) and their clinical trials at symptomatic AD stages were halted because of futility or adverse effects including cognitive worsening at the highest dosage (Imbimbo and Watling, 2019;McDade et al, 2021;Bazzari and Bazzari, 2022). Given recent evidence for the physiological role of BACE2 as an AD-suppressor gene (Alić et al, 2021;Luo et al, 2022), further investigation is required to address whether cross-inhibition of BACE2 activity by non-selective or partially selective BACE1 inhibitors may diminish the benefit of BACE1 inhibition or contribute to the untoward worsening effect on ALF (if any) in preclinical AD.…”

Personalizing the Care and Treatment of Alzheimer’s Disease: An Overview

“…Simultaneously, emerging technologies, such as induced pluripotent stem cells (iPSC), are highly valuable in the search for genetic determinants and environmental modulators of the disease, as well as in the screening of potential drugs that will, hopefully, revolutionize the treatment of AD patients. 104 Some interventions have been already recognized as significant in prevention and treatment in early stages of AD: physical activity, brain stimulation through music and art, social communication, diet, supplements for energy supply, and drugs targeting neural transmission. For moderate and severe stages of AD, stage-specific intervention strategies in recent years move towards drugs targeting neural inflammation and neural regeneration therapy.…”