APOE-dependent PET patterns of brain activation in Alzheimer disease

Abstract

Using H₂¹⁵O PET, the authors imaged 13 patients with Alzheimer disease (AD) while performing a serial nonverbal recognition memory task. Patterns of brain activation differed as a function of APOE genotype: ε4 carriers exhibited lower activation in the left lingual gyrus and higher activation in left cuneus, precuneus, parahippocampal, and right precentral gyrus. The APOE genotype seems to play a role in cerebral physiologic activity even after onset of clinical manifestations of AD.

Although the presence of the ε4 allele has been associated with earlier age of disease onset, its relation to different Alzheimer disease (AD) clinical and phenotypic subtypes is less clear. It is conceivable that the clinical heterogeneity observed in AD could derive from underlying genetic variability.

Whereas previous studies have investigated the association between APOE and patterns of brain activation in middle-aged and elderly individuals,¹ we explored the effect of the APOE genotype on brain activation in patients with AD.

Methods

Subjects

This was part of a larger imaging study, the details of which are described elsewhere.² Thirteen subjects with early AD (nine men and four women) who met National Institute of Neurological and Communication Disorders and Stroke/Alzheimer's Disease and Related Disorders Association for probable AD (reached at a consensus diagnostic conference of neurologists, psychiatrists, and neuropsychologists) were studied. The patients underwent MRI, appropriate laboratory tests, and extensive neuropsychological evaluation, part of which is shown in table E-1 (see table E-1 on the Neurology Web site at www.neurology.org). With the exception of two subjects (one ε4 carrier and one noncarrier) who were taking acetylcholinesterase inhibitors, patients were not receiving any other CNS-acting medications. APOE or PET activation results did not play any role in the diagnostic process.

Cognitive task

Because APOE-related differential activation during memory tasks has been previously reported¹ and because nonverbal memory deficits are among the earliest clinical features of AD, we chose a nonverbal episodic memory task for the neuroimaging component of this study. It comprised two conditions (figure, A).

Figure.

(A) Nonverbal memory task. Shape list size varied for the titrated demand condition so that each patient's performance was ∼75% accurate. For presentation purposes, a list size of three shapes during the titrated demand condition is illustrated here. During study phases, subjects were exposed to each stimulus for 5 seconds, while a 6-second response time limit was imposed during the recognition phase. (B) Graphs plot relative cerebral blood flow (rCBF) activation for the two groups in regions where global maxima of significance were noted. Note the clear rCBF separation between the ε4 carriers and noncarriers in these two regions. *The number of points in the graphs is less than the number of subjects because of close proximity of some activation values, which makes them appear as single points.

1. A low demand condition (LD), in which a single shape (different each time) was presented during the encoding phase. During the recognition phase, either the same shape as the encoding phase or a nonfamiliar foil was presented. Subjects were instructed to make a “new” or “old” response for each probe item by pressing one of two microswitches.

2. A titrated demand (TD) condition involved a serial presentation of several shapes during the encoding phase. The number of shapes was determined in a training session on the day preceding the PET scan, during which shape list size was adjusted in a staircase manner such that recognition accuracy was ∼75% for each subject. The recognition phase of the TD condition involved presentation of shapes studied during the preceding study phase intermixed with nonfamiliar foils.

PET scan acquisition and data processing

Multiple trials of each condition were acquired during scanning. For each scan, a bolus of 30 mCi H₂¹⁵O was injected IV, and nonquantitative counts (relative cerebral blood flow [rCBF]) were obtained. Two 30-second scan frames (which were subsequently averaged) were acquired in two-dimensional mode. The SPM99 program was used to implement realignment, spatial transformation, smoothing (isotropic, Gaussian kernel [full width at half-maximum = 12 mm]), and proportional scaling by global mean.

General linear model design

Multiple regression analyses were performed at each voxel with rCBF as the dependent variable. Independent variables were 1) the cognitive task condition (TD vs LD); 2) the APOE genotype (dichotomous form, presence vs absence of an ε4 allele); and 3) the cognitive task condition × APOE interaction. Our primary interest was the statistical significance of the condition × APOE interaction effect, which speaks to the hypothesis that functional activation (TD rCBF after subtracting out LD rCBF related to basic sensory and motor processing) differs between ε4 allele carriers and noncarriers. The false-positive rate was controlled at α = 0.05 per map (Bonferroni corrected for the number of resolution elements).

Results

Demographic/behavioral

Four (of nine) men and two (of four) women were ε4 carriers (p = 0.85). All patients had early AD: mean modified Mini-Mental State Examination score was 46 of 57 (corresponding to a Folstein Mini-Mental State Examination score of ∼24; see table E-1 on the Neurology Web site at www.neurology.org). Although age did not differ between the groups (p = 0.09), two ε4 carriers had early age of disease onset (symptoms starting at ages 45 and 53 years), resulting in ε4 carriers being on average 10 years younger. Because patients with early-onset AD are considered by many to represent a separate group not only in terms of age but also in terms of many other clinical or biologic measures, we explored the effect of removing the early-onset patients from the ε4 carrier group (which resulted in almost identical age distributions in both APOE groups): the imaging analysis results were unchanged. Education and neuropsychological performance did not differ statistically among the groups. To further ensure that the two groups were cognitively equivalent, we included Selective Reminding Test delayed recall performance as a covariate in supplementary imaging analyses: the results were similar. The following shape list sizes were used during the TD condition: 2 (two subjects), 3 (four subjects), 4 (one subject), 5 (two subjects), 6 (three subjects), and 9 (one subject).

PET data

Compared with the subjects without the ε4 allele, ε4 carriers exhibited significant deactivation in the left lingual gyrus (see table and figure, B; also see figure E-1 on the Neurology Web site at www.neurology.org). At the same time, ε4 carriers manifested significantly higher activation in left cuneus, precuneus, right precentral, and parahippocampal gyri.

Table. Areas where significant associations between brain activation and APOE (presence vs absence of ε4) were detected (p < 0.05 Bonferroni corrected) in the SPM analyses.

	Talairach coordinates			T values (df = 30)	Cluster size (# voxels)	R2 (variance explained)	Location (Brodmann area)
	x	y	z
Areas where ε4 carriers exhibit lower rCBF activation	−28	−64	3	7.7	19	0.84	Left lingual gyrus (19)
Areas where ε4 carriers exhibit higher rCBF activation	−18	−76	33	6.8	8	0.80	Left cuneus (7)
	50	−8	34	6.6	40	0.80	Right precentral gyrus (6)
	−38	−43	−1	5.6	3	0.74	Left parahippocampal gyrus (19)
	−18	−56	45	5.4	3	0.72	Left precuneus (7)

rCBF = relative cerebral blood flow.

Discussion

The relation of APOE to different AD clinical and phenotypic subtypes is debatable. APOE-related clinical heterogeneity after AD onset is supported by observations reporting that presence of the ε4 allele is associated with increased risk for psychosis,³ less risk for development of extrapyramidal signs,⁴ lower rates of cognitive decline, and lower mortality.⁴ AD patients with the ε4 allele have also been reported to carry increased β-amyloid and neurofibrillary tangles.⁵ There is controversy regarding APOE effects on brain tissue atrophy with some structural MRI studies reporting no APOE-related differences in hippocampal volumes,⁶ whereas others have reported greater atrophy in medial temporal structures in AD patients carrying the ε4 allele.⁷ Similarly, resting functional imaging studies after onset of AD have reported mixed results with some reporting decreased,⁸ increased,⁹ or unchanged¹⁰ cerebral blood flow or metabolism for the ε4 carriers.

We detected an association between APOE genotype and cerebral physiologic activity during cognitive activation. This was not a function of differential effort because memory task difficulty was experimentally equated in the two APOE groups. The detected activation differences indicate altered memory processing in AD patients with the ε4 allele despite the common underlying pathology. The results argue for an APOE-dependent neurophysiologic heterogeneity among subjects with AD even after the onset of clinical manifestations of the disease.

Areas with differential activation in the ε4 carriers may reflect malfunctioning (taking the form of either overactivation or deactivation) because of more severe AD pathologic involvement for ε4 carriers in these regions (i.e., parahippocampal gyrus). Alternatively, some of these regions may still be spared by AD pathology (i.e., precentral gyrus) and recruited for task performance by ε4 carriers because of more severe pathologic involvement in other regions. It is also possible that differential activation is not directly related to the degree or localization of pathology but reflects APOE-related cerebral physiologic heterogeneity.

This study has limitations. Because the PET scanning session encompassed the encoding and recognition phase of the task, exact APOE-related neuroanatomic localization for each cognitive process is not possible. Because no arterial sampling was performed, we had to rely on relative flow values rather than absolute quantification of the activations. This study included white, highly educated patients recruited from a university-based dementia referral center, which might limit generalizability to the population. Finally, the small number of participants may have limited power to detect significant age and cognitive differences between carriers and noncarriers; therefore, results should be replicated with a larger sample size.