Alzheimer's disease (AD) is the most common form of dementia affecting millions of people worldwide [1]. The decline in cognitive ability is accompanied by characteristic pathological changes in the brain which include deposition of extracellular amyloid plaques composed of β-amyloid (Aβ) [2] and the accumulation of intracellular neurofibrillary tangles (hyperphosphorylated tau) [3]. Mutations in three genes, β-amyloid precursor protein (APP), presenilin-1 (PSEN1) and presenilin-2 (PSEN2), have been found in early-onset autosomal dominant AD [4–6]. In contrast, the majority of AD cases have a later age of onset (>65 years) [1] and although heritability is high, there are multiple environmental and genetic risk factors with no clear Mendelian pattern of inheritance [7]. To date, the main genetic risk factor for late onset Alzheimer's disease (LOAD) is the apolipoprotein E gene (APOE) where the ε4 allele is over-represented in affected individuals (50%) compared to controls [8]. The partial penetrance for ε4 suggests that other genes are also likely to be important in the development of LOAD [9].

Linkage studies, and more recently genome-wide association studies (GWAS), have provided evidence to support a role for multiple genes in the aetiology of LOAD [9, 10]. Difficulties in replicating findings across multiple studies still leaves considerable uncertainty in defining the genetic risk factors. Significant evidence exists for a susceptibility locus on chromosome 10q [11–14], however, there are conflicting reports on its precise location and to date no clear association to an individual gene has been demonstrated. Insulin degrading enzyme (IDE) is encoded by a gene that maps to Chr 10q and has been implicated in the breakdown and clearance of extracellular Aβ [15]. Although there is contradictory evidence for an allelic association between IDE and AD [16–18] a linkage study carried out in 435 multiplex AD families found significant linkage to markers adjacent to IDE (D10S583 and D10S1671) [11, 19].

HECTD2 encodes an E3-ubiquitin ligase protein and maps within 1 Mb of D10S583 and IDE. HECTD2 has been linked to prion disease incubation time in mice and a susceptibility haplotype has been associated with human prion disease [20]. Prion diseases, like AD, are neurodegenerative disorders with a complex aetiology. Mutations in the prion protein gene (PRNP) cause the inherited prion diseases [21, 22] and coding polymorphisms in the prion protein (PrP) are associated with susceptibility to acquired and sporadic Creutzfeldt-Jakob disease (CJD), kuru and influence age of onset in some inherited prion diseases [23–25]. Similarly to LOAD, acquired and sporadic CJD have a complex non-Mendelian genetic component where there is evidence for the role of multiple genes [26–29]. Prion diseases also share some of the neuropathologic hallmarks of AD and are considered to be diseases of protein misfolding where abnormal forms of PrP accumulate in the brain often in the form of amyloid plaques [21, 30].

The similarities between AD, prion diseases and other neurodegenerative disorders suggest that common molecular mechanisms and pathways may be involved. The ubiquitin-proteosome system is one such example that has been implicated in the pathogenesis of several neurodegenerative diseases which show an accumulation of an abnormally folded protein including AD and prion disease [31–33]. The substrate and exact function of HECTD2 are unknown but by homology to other family members it is thought to function as an E3 ubiquitin ligase, catalysing the transfer of ubiquitin to specific proteins, thus targeting them for degradation by the proteosome. Mutations in an E3 ubiquitin ligase, parkin, are associated with forms of inherited Parkinson's disease [34].

The location of HECTD2 on Chr 10q, its association with another neurodegenerative proteinopathy and its involvement in the ubiquitin-proteosome pathway suggest that HECTD2 is a suitable candidate gene for LOAD susceptibility. The aim of this study was therefore to test whether the HECTD2 risk allele seen in CJD is also a susceptibility factor for non-familial AD.

We have previously shown that strong linkage disequilibrium (LD) extends across the whole of the HECTD2 gene and interrogation of HapMap data suggests that this block of LD does not extend into neighbouring genes [20]. It is therefore possible to tag the haplotype associated with susceptibility to prion disease with a single nucleotide polymorphism (SNP), rs12249854 (A/T) that occurs within intron 1 of HECTD2. rs12249854 was genotyped in a panel of 320 samples from patients with a definite or probable diagnosis of AD and no known mutations in APP, PSEN1 or PSEN2. APOE genotypes were available for 316 individuals and showed enrichment for the ε4 allele (55.1% of individual have one or more ε4 allele). Genotypes were compared with those of a previously genotyped UK control group (n = 601) that were ethnically matched [20].

LOAD is a major cause of dementia and is becoming increasingly important in ageing populations. Identifying the genetic susceptibility factors may help to identify those at risk of developing the disease and the effective targeting of future preventative treatments. With the exception of APOE, no genes have been conclusively shown to be susceptibility factors for LOAD. In this study, we genotyped a panel of AD patients and looked for an association with a HECTD2 SNP previously shown to be associated with prion disease [20]. Although the minor allele frequency was higher in the AD samples than the controls this was not significant. This result suggests that in spite of being a promising candidate, HECTD2 is not implicated in AD. Although a negative study, our data do not totally exclude HECTD2. Our study assumes that, as for prion disease, a common variant of HECTD2 is the susceptibility allele. However, multiple rare variants would not be detectable by our tagging SNP. Re-sequencing of HECTD2 in a panel of AD samples would be required to exclude this possibility.

LOAD has a complex aetiology and collections of patient samples may be highly heterogeneous, therefore, very large sample sizes may be required to detect small effects. The heterogeneity of the disease phenotype and the varying inclusion criteria used in different studies may contribute to the difficulties in replicating promising findings from different centres.

Prevalence figures suggest that LOAD affects 20% of people aged 75-84 years and rising in older age groups [1]. No detailed age information was available for our control group, however, samples were taken from current blood donors which suggests that although currently unaffected by dementia it is likely that a significant proportion will develop AD later in life and are therefore likely to carry susceptibility alleles. This may confound our findings. Large cohorts of non-demented elderly individuals are difficult to obtain, however, their use would increase the power to detect genetic susceptibility factors.

Several studies have reported evidence for a LOAD susceptibility locus on Chr 10q, however, the exact locations vary and the regions are frequently broad with added uncertainty about the number of genes involved. It is hoped that results from large, well controlled and replicated GWAS studies will successfully identify the susceptibility alleles on Chr 10q and at other loci across the genome.

Based on linkage data, association with another neurodegenerative disease and association with the ubiquitin-proteosome system, HECTD2 is a promising candidate susceptibility factor for LOAD. In this study we have genotyped a panel of AD patients for the HECTD2 SNP rs12249854 (A/T) and compared this data to a previously genotyped control population. The minor allele (A) frequency was increased (5.8%) compared to controls (3.9%), however, this was not statistically significant. We were unable to exclude weak or moderate effects on AD risk. No differences were seen when the population was further stratified by APOE ε4 genotype. Based on this data we conclude that the common variants of HECTD2, as detected by our tagging SNP, are unlikely to be strong susceptibility factors for LOAD in the UK population.