Humanized APP is the primary determinant of regional brain volume in humanized APOE knock-in mice: a cross-sectional ex vivo MRI study

Abstract

INTRODUCTION: Age, Apolipoprotein E4 (APOE4) genotype, and biological sex are major risk factors for late-onset Alzheimer's disease (LOAD), and preclinical mouse models enable controlled investigation of these factors. To date, humanized APOE4 has not recapitulated LOAD-relevant brain volume phenotypes. Given the central role of amyloid precursor protein (APP) in LOAD pathogenesis, incorporating humanized APP (hAPP) alongside humanized APOE (hAPOE) may therefore improve translational modeling of structural brain changes characterized by neuroimaging. METHODS: Aged mice (mean age = 23.25 months) carrying murine (m) or humanized (h) APP and either murine Apoe or hAPOE3/3 (hAPOE3-HOM), hAPOE3/4 (hAPOE4-HET), or hAPOE4/4 (hAPOE4-HOM) underwent in-skull ex vivo volumetric MRI. Regional volumes were quantified in absolute terms and relative to total brain volume (TBV). Linear models included APP type, APOE genotype, and sex, with FDR correction applied within contrasts. RESULTS: Brain volumes were primarily determined by APP background, with hAPP globally reducing total and regional volumes relative to mAPP mice. Across hAPP models, hAPOE4-HOM exhibited the greatest brain-wide reductions, which was mitigated by a single hAPOE3 allele. In contrast, mouse APP exerted modest effect in hAPOE, with hAPOE4 carriers exhibiting greater total volume without regional specificity. After TBV adjustment, hAPP mice exhibited subcortical vulnerability with relative cortical preservation. Females exhibited larger brain volumes than males, independent of APP or APOE genotype. DISCUSSION: hAPP induces distinctly smaller brain volumes in this humanized APOE knock-in model, and hAPOE4 homozygosity amplifies that effect, indicating genotype-dependent susceptibility. Because this study is cross-sectional and lacks histopathological confirmation, volume differences may reflect developmental or constitutive effects rather than neurodegeneration. Humanized APP and APOE are therefore necessary but not sufficient to recapitulate the complete volumetric signature of established LOAD. Longitudinal and histological studies are required to determine whether these differences reflect a prodromal trajectory or a developmental effect.