Genetic Variations in ABCA7 Can Increase Secreted Levels of Amyloid-β₄₀ and Amyloid-β₄₂ Peptides and ABCA7 Transcription in Cell Culture Models

Article type: Research Article

Authors: Bamji-Mirza, Michelle^{a; b} | Li, Yan^{a; b} | Najem, Dema^{a; b} | Liu, Qing Yan^b | Walker, Douglas^c | Lue, Lih-Fen^c | Stupak, Jacek^b | Chan, Kenneth^b | Li, Jianjun^b | Ghani, Mahdi^d | Yang, Ze^e | Rogaeva, Ekaterina^d | Zhang, Wandong^{a; b; *}

Affiliations: [a] Department of Pathology and Laboratory Medicine, Faculty of Medicine, University of Ottawa, Ottawa, ON, Canada | [b] Human Health Therapeutics Portfolio, Life Sciences Division, National Research Council Canada, Ottawa, ON, Canada | [c] Banner Sun Health Research Institute, Sun City, AZ, USA | [d] Tanz Centre for Research in Neurodegenerative Diseases, University of Toronto, Toronto, ON, Canada | [e] The Key Laboratory of Geriatrics, Beijing Hospital & Beijing Institute of Geriatrics, Chinese Ministry of Health, Beijing, China

Correspondence: [*] Correspondence to: Dr. Wandong Zhang, National Research Council Canada, 1200 Montreal Road, Building M-54, Ottawa, ON, K1A 0R6, Canada. Tel.: +1 613 993 5988; Fax: +1 613 941 4475; E-mails: Wandong.Zhang@nrc.ca; wzhan2@uottawa.ca.

Abstract: Alzheimer’s disease (AD) is characterized by extracellular deposits of amyloid-β (Aβ) in the brain. ABCA7 is highly expressed in the brain and a susceptibility gene for late-onset AD (LOAD). The minor alleles at two ABCA7 single-nucleotide polymorphisms (SNPs), rs3764650 (T>G; intron13) and rs3752246 at a predicted myristoylation site (C>G; exon33; p.Gly1527Ala), are significantly associated with LOAD risk; however, the mechanism of this association is unknown. Functional consequences of both SNPs were examined in HEK293 and CHO cells stably expressing AβPPSwe. Luciferase reporter assays in HEK293 cells suggested that intron13 carrying rs3764650 major T-allele (int13-T) possessed promoter-enhancing capabilities. Co-transfection experiments with hABCA7 and int13-T resulted in significantly increased ABCA7 protein level relative to that with int13-G. Expression of hABCA7 carrying rs3752246 risk allele led to increases in secreted Aβ40 and Aβ42 and β-secretase activity in CHO- and HEK-AβPPSwe cells. Hydroxymyristic acid treatment of cells expressing hABCA7 carrying the rs3752246 major G allele resulted in increased β-secretase activity and levels of Aβ, suggesting that lack of myristoylation contributes to the observed cell-phenotypes. Molecular weight determination, by gel-electrophoresis and mass spectrometry, of hABCA7 peptides spanning position 1527 showed loss of post-translational modification in the risk-allele peptide. These results suggest that decreased expression, or impaired function, of ABCA7 may contribute to AD pathology.

Keywords: ABC transporters, ABCA7 protein, Alzheimer’s disease, amyloid beta-peptides, BACE1 protein, genome-wide association study, myristoylation, SNPs

DOI: 10.3233/JAD-150965

Journal: Journal of Alzheimer's Disease, vol. 53, no. 3, pp. 875-892, 2016