APOE ε4 Increases Risk for Dementia in Pure Synucleinopathies

METHODS

RESULTS

The demographic and clinical characteristics of the study population are shown in Table 1. Control subjects were slightly older at death than those in all 4 dementia groups, and a substantially greater proportion of subjects with PDD and pDLB were male (P<1×10⁻⁴ for all). There was no significant difference in age at onset of dementia among the case groups.

The APOE genotype and allele frequencies observed in the sample are presented in Table 2. There was a highly significant over representation of the ε4 allele in all case groups, including PDD (${\chi }_1^2=18.25$; P = 1.94 × 10⁻⁵) and pDLB (${\chi }_1^2=68.61$; P = 1.2 × 10⁻¹⁶). In a dominant genetic model adjusting for sex and age at death with control subjects as the reference, the ORs were 9.9 (95% CI, 6.4–15.3) for AD, 12.6 (95% CI, 8.1–19.8) for LBD-AD, 6.1 (95% CI, 3.5–10.5) for pDLB, and 3.1 (95% CI, 1.7–5.6) for PDD (Table 3).

Within the groups with synucleinopathy, the ε4 allele frequency was significantly lower in subjects with PDD (19.1%) than those with pDLB (31.9%; ${\chi }_1^2=6.60$; P = .01) or LBD-AD (40.6%; ${\chi }_1^2=23.24$; P = 1.43 × 10⁻⁶).

Inspection of the ORs suggested that for individuals with PDD and pDLB, the ε4 allele might convey risks intermediate between the control group and the LBD-AD and AD groups. We assessed this hypothesis using a trend test, coding control subjects as 0, PDD as 1, pDLB as 2, and AD and LBD-AD as 3. The results indicated an increased likelihood of carrying the ε4 allele across groups (control<PDD<pDLB<LBD-AD/AD [${\chi }_1^2=166.47$; P = 4.37 × 10⁻³⁸]).

COMMENT

We observed a strong overall association between the APOE ε4 allele and AD, LBD-AD, pDLB, and PDD. While the ε4 allele is a well-established risk factor for AD^3,4 and several studies have reported an association between APOE and LBD-AD,^5–7 the finding that ε4 was over represented in pDLB and PDD was unexpected. By far, the most widely held view for the mechanism by which APOE ε4 influences AD risk is that apoE isoforms have direct or indirect effects on amyloid-β (Aβ) peptide metabolism. There is a large body of evidence to support this hypothesis, including experiments in mice over expressing human apoE isoforms in which the ε4 allele results in lower Aβ clearance and increased deposition of insoluble Aβ in the brain than the ε2 and ε3 alleles.^23,24 Thus, in humans, ε4 is predicted to accelerate the accumulation of neurotoxic Aβ, which ultimately leads to neuritic plaque formation and neurodegeneration. However, our observation of an elevated ε4 frequency in the pDLB and PDD groups in which the overall brain neuritic plaque burden is low suggests the possibility that apoE isoforms also might modulate neurodegeneration by nonamyloidogenic mechanisms. A caveat is that we were not able to account for the potential influence of ε4 on soluble oligomeric Aβ levels because soluble oligomeric Aβ might not be well represented by measures of histologically detectable Aβ plaques, and soluble oligomeric Aβ could adversely affect cognition. Nonetheless, evidence that Aβ-independent pathways might exist is beginning to emerge from work in model systems. For example, C-terminal–truncated fragments of apoE4 are neurotoxic in vivo and in vitro, possibly through impairment of mitochondrial function or disruption of the cytoskeleton.^25,26 In the presence of lipids, apoE4 impairs neuronal plasticity in vitro.²⁵ Also, greater microglia-mediated neurotoxicity has been observed in mice expressing human apoE4 than other apoE isoforms.²⁷

Interpretation of previous studies of APOE in DLB has been challenging given the wide difference in methods and diagnostic criteria used between studies. Few neuropathologically verified studies have reported separate APOE frequencies in LBD-AD and pDLB,^5,6,28 and some did not collect sufficient information to exclude subjects with PDD.²⁸ Furthermore, the sample size of the pDLB group in these previous studies ranged from 6 to 18 subjects, and the ε4 allele frequency varied from 6% to 22%, precluding firm conclusions about the association of APOE with pDLB. On the other hand, a number of studies have reported a significant over representation of APOE ε4 in subjects with LBD-AD, with ε4 allele frequencies ranging from 29% to 47%.^5–7,28,29 Finally, several studies have examined APOE in DLB without making a distinction between LBD-AD and pDLB, often because the diagnosis was solely based on clinical criteria.^8–10

Data from genome wide association studies indicate that APOE is not a susceptibility gene for PD.^30–32 While PD is clinically defined by motor symptoms, more than 50% of patients develop dementia within 10 years of diagnosis.^33,34 Whether APOE acts as a modifier gene by influencing the manifestation of cognitive dysfunction in PD is still a matter of debate. There are several possible approaches to address this question, including assessing whether APOE genotypes (1) differ in frequency between patients with PDD and control subjects or cognitively intact patients with PD, (2) influence the rate of progression to dementia in PD cohorts, or (3) associate with performance on cognitive testing in cross-sectional or longitudinal studies of patients with PD. In a meta-analysis of 17 studies, Williams-Gray and colleagues³⁵ reported a significantly higher APOE ε4 frequency in patients with PDD (n = 501) compared with nondemented patients with PD (n = 1145; OR, 1.74; 95% CI, 1.36–2.23). However, interpretation of the results was made difficult because there was evidence of significant heterogeneity of ORs and publication bias, and the criteria used to define PDD varied substantially across individual studies. Williams-Gray and colleagues³⁵ also longitudinally assessed an incident cohort of 107 patients with PD for 5 years and found no effect of APOE ε4 on the risk for dementia or rate of cognitive decline. In contrast, a recent longitudinal study of 212 patients with PD reported that ε4 carriers displayed a more rapid decline in total score on the Mattis Dementia Rating Scale than noncarriers.³⁶ In a cross-sectional study of 937 patients with PD, we observed that the ε4 allele is associated with lower psychometric test scores across multiple cognitive domains after adjusting for disease duration.³⁷ Together with findings from the present study, we believe that the preponderance of the evidence indicates that APOE is a risk factor for cognitive dysfunction in PD. However, in our data set, the magnitude of the APOE ε4 effect was smaller for PDD than for LBD-AD and pDLB, as evidenced by the 2-fold (or more) larger ORs observed in the LBD-AD and pDLB groups (Table 3) and the results of the trend test. One explanation for these findings is that in the presence of synucleinopathy, APOE ε4 might increase the likelihood that dementia precedes parkinsonism, hence a clinical diagnosis of DLB rather than PDD is rendered.

This study had some limitations. Because most of the subjects died prior to the publication of the consensus clinical criteria for DLB, we were unable to fully apply these criteria retrospectively, in particular the fluctuating cognition and rapid eye movement sleep behavior disorder features.¹ Although we limited the sample to white subjects, we did not have data available for ancestry informative genetic markers, thus we cannot entirely exclude the possibility of unrecognized population structure in our data set.

We have shown that APOE ε4 increases the risk for dementia in subjects with LBDNCs but with no or low levels of ADNCs, which provides further rationale for exploring how apoE isoforms might modulate neurode-generation through mechanisms unrelated to Aβ metabolism. Longitudinal studies comparing cerebrospinal fluid profiles in subjects clinically diagnosed as having DLB or PDD and stratified by APOE genotype and amyloid burden by in vivo imaging might complement work in model systems to address this question in the future.