Promoter polymorphisms which modulate insulin degrading enzyme expression may increase susceptibility to Alzheimer's disease

Abstract

Cerebral accumulation of amyloid beta protein (Aβ) is believed to play a central role in the pathogenesis of Alzheimer's disease (AD). Insulin degrading enzyme (IDE) is involved in Aβ degradation, therefore the gene encoding for insulin degrading enzyme is one of the candidate genes risky for AD. In Chinese Han populations we found three polymorphisms in IDE promoter: −1002T/G (rs3758505), −179T/C (rs4646953) and −51C/T (rs4646954). The −1002T and −51C alleles were over-represented in 357 sporadic AD (SAD) patients when compared to those in 331 healthy individuals. Furthermore, −1002T/G and −51C/T were in strong linkage disequilibrium and they constructed a relatively risky −1002T/−51C and a relatively protective −1002G/−51T. Luciferase reporter assay indicated −1002T/−51C had lower transcriptional activity than −1002G/−51T. A more marked increase in −1002T/−51C transcriptional activity was seen when under Aβ_25–35 and serum deprivation treatment. The present study provides evidence that IDE promoter polymorphisms that significantly decrease IDE expression levels are associated with development of SAD.

Introduction

The etiology of Alzheimer's disease (AD) is complex and apolipoprotein E (APOE) is the only gene that has been characterized definitely as a risk factor for AD. Identification of other risk factors is therefore of great importance in the elucidation of AD etiology. Recently, several linkage studies show that susceptibility to late-onset AD is linked to a large region on chromosome 10q (Myers and Goate, 2001; and Tanzi and Bertram, 2001). The gene encoding for insulin degrading enzyme (IDE) is one of the candidate genes in this area for AD as it is involved in the catabolism of amyloid beta protein (Aβ) (Bertram et al., 2000). Cerebral accumulation of Aβ is believed to be crucial in AD pathogenesis. The actual amount of neurotoxic Aβ in the brain is determined by Aβ production through amyloid precursor protein (APP) processing and Aβ degradation and clearance. Aβ-degrading proteases could regulate the cerebral levels of the peptide, and IDE is exactly one of these Aβ-degrading proteases (Ling et al., 2003).

IDE is a 110-kDa neutral thiol-metalloendopeptidase that was first identified as the major proteolytic enzyme for insulin (Ogawa et al., 1992). It is ubiquitously expressed, with its highest expression in the liver, testes, muscle and the brain (Kuo et al., 1993). Several short peptides have been shown to serve as the substrates of IDE, including insulin (Kirschner and Goldberg, 1983), insulin-like growth factors I and II (Misbin and Almira, 1989), amylin (Bennett et al., 2000), Aβ (Kurochkin and Goto, 1994, McDermott and Gibson, 1997, Mukherjee et al., 2000; and Qiu et al., 1998) and others, many of which have similar secondary structure and amyloidogenic character (Bennett et al., 2000; and Kurochkin, 2001). Among the substrates of IDE, insulin and amylin are related to the pathogenesis of type 2 diabetes, while Aβ is involved in AD pathology (Mukherjee et al., 2000, Qiu et al., 1997, Vekrellis et al., 2000, Qiu and Folstein, 2006; and Sudoh et al., 2002). These substrates compete with each other to be degraded by IDE in vivo. If the insulin level increases in the brain, it would inhibit IDE to degrade Aβ effectively as a result.

Several association studies using a single nucleotide polymorphism (SNP) approach have investigated the relationship between IDE and AD (Abraham et al., 2001, Bian et al., 2004, Boussaha et al., 2002, Edland et al., 2003, Ertekin-Taner et al., 2004, Nowotny et al., 2005, Prince et al., 2003; and Sakai et al., 2004), and found that some variations were related to late-onset AD in the absence of the ApoEε4 allele. In other case-control studies, genetic variations in the IDE increased both the risk for developing AD and the severity of the disease (Prince et al., 2003). However, the results of these genetic studies remain controversial, with some studies unable to identify an association with AD (Abraham et al., 2001, Boussaha et al., 2002; and Sakai et al., 2004). Variants in these transcriptional regulation sequences may alter the transcriptional activity (Lv et al., 2008, Sander et al., 2005; and Heijmans et al., 2002) specially under certain conditions. To explore the pathogenesis of sporadic AD (SAD), it is reasonable to examine genetic variants in these sequences. In the current study we aimed to systematically screen the proximal promoter of IDE, detect potential variants, and then determine whether these variants are associated with SAD, genetically and functionally.