Proteolytic Cleavage of Apolipoprotein E in the Down Syndrome Brain

Down syndrome (DS) is a chromosomal disorder (Trisomy 21) that is caused by nondisjunction resulting in the triplication of the chromosome 21 in the large majority of cases and is the most commonly identified genetic cause of intellectual disability in the United States [1, 2]. In addition to several phenotypic features of DS [3-7], people with DS also exhibit mild to moderate cognitive dysfunction [8] delayed verbal short-term memory and neurobehavioral problems [9]. In addition to the phenotypic and cognitive impairments associated with this disorder, an increased risk of Alzheimer’s disease (AD) in patients with DS is also well established. In DS, the postmortem findings of senile plaques and neurofibrillary plaques (NFTs) is assumed to be caused by the overexpression of the amyloid precursor protein (APP) following triplication of chromosome 21 and accumulation of beta-amyloid. In DS, nearly all adults over 35 to 40 years of age exhibit key neuropathological changes characteristic of AD including the formation of extracellular plaques of beta-amyloid and NFTs [10-12]. Previous studies have shown a substantial increase in the cumulative incidence of dementia in adults with DS between ages 50 and 72 [13]. Risk factors that affect the age of onset of dementia in DS include harboring the APOE4 allele as well as high levels of plasma beta-amyloid 1-42 [14]. In contrast, there is a reduced risk for the onset of dementia associated with APOE2 allele and atypical karyotypes in DS [14]. The human APOE gene is polymorphic resulting in three major isoforms, apoE2, apoE3, and apoE4, which differ by single amino acid substitutions involving cysteine-arginine replacements at positions 112 and 158 [15]. Inheritance of one copy of the APOE4 allele increases the disease risk of AD four-fold, while two copies enhances disease risk approximately ten-fold [16]. Thus, harboring the APOE4 allele represents the most significant late-onset genetic risk factor. A recent study highlighted this risk by demonstrating that the lifetime risk of AD at the age of 85 without reference to the APOE genotype was 11% in males and 14% in females [17]. At the same age, this risk ranged from 51% for APOE 4/4 male carriers to 60% for APOE 4/4 female carries, consistent with a semi-dominant inheritance pattern [17]. The preponderance of evidence suggests that harboring the APOE4 allele in DS also increases AD disease risk, although to a lower extent to what has been found in AD [18]. Additionally, studies suggest harboring the APOE4 allele leads to earlier mortality in the DS population that is independent of the risk of dementia [19, 20]. How apoE4 increases the risk for AD is unknown, however, evidence suggests that the enhanced susceptibility of apoE4 to proteolysis as compared to E2 and E3 may play a critical role leading to loss of function including impaired cholesterol transport and beta-amyloid clearance [21]. The purpose of the current study was to investigate whether apoE proteolysis is prevalent in postmortem DS human brain sections utilizing an antibody that detects the amino-terminal fragment of apoE (herein termed, nApoECF antibody) [22]. Previous studies carried out with the nApoECF antibody demonstrated that it consistently labeled NFT’s in sporadic AD, Picks disease and vascular dementia in addition to the labeling of blood vessels and reactive astrocytes [22-24]. Our findings using the nApoECF antibody in the present study support a role for the proteolytic cleave of apoE with aging and AD in DS and suggest that apoE fragmentation is closely associated with mature NFTs.