Hypometabolism in Alzheimer-Affected Brain Regions in Cognitively Healthy Latino Individuals Carrying the Apolipoprotein E ε4 Allele

Objective  To investigate with fluorodeoxyglucose positron emission tomography whether regional reductions in the cerebral metabolic rate for glucose (CMRgl) previously found in cognitively healthy late-middle-aged apolipoprotein E (APOE) ε4 carriers extend to members of the Latino Mexican American community.

Design  Prospective cohort study.

Setting  Banner Alzheimer's Institute, Phoenix, Arizona.

Patients or Other Participants  Eleven APOE ε4 carriers and 16 noncarriers from Arizona's Latino community (mean [SD] age, 54.6 [6.4] years) matched for sex, mean age, and educational level and who were predominantly of self-designated Mexican origin.

Main Outcome Measure  A brain mapping algorithm was used to compare cross-sectional regional CMRgl in Latino APOE ε4 carriers vs noncarriers.

Results  Participant groups had similar distributions for age, sex, education, family history of dementia, clinical ratings, and neuropsychological test scores. Latino APOE ε4 carriers had lower CMRgl than the noncarriers in the posterior cingulate, precuneus, and parietal regions previously found to be preferentially affected in patients with Alzheimer disease (AD) and cognitively healthy non-Latino APOE ε4 carriers. Additionally, the Latino APOE ε4 carriers had lower CMRgl in the middle and anterior cingulate cortex, hippocampus, and thalamus.

Conclusions  This study provides support for the relationship between APOE ε4 and risk of AD in Latino individuals. It illustrates the role of positron emission tomography as a presymptomatic endophenotype for the assessment of AD risk factors and supports the inclusion of Latino APOE ε4 carriers in proof-of-concept studies using fluorodeoxyglucose PET to evaluate promising presymptomatic treatments in cognitively healthy carriers of this common AD susceptibility gene.

Alzheimer disease (AD) is the most common form of dementia in elderly individuals.¹ Over the next few decades, it is projected that there will be dramatic increases in the number of elderly individuals, their racial and ethnic diversity,² and the number of patients with AD.³ Latino individuals appear to have a higher incidence of AD,⁴ which may be partly attributable to suggested AD risk factors including diabetes mellitus, obesity, cardiovascular disease, and hypertension, coupled with an earlier mean age at onset⁵ and a longer postdiagnosis survival time.⁶

Next to age, the apolipoprotein E (APOE) ε4 allele is the best established risk factor for late-onset AD.⁷ While this association has been confirmed in numerous case-control studies in Europe and the Americas, the association in different Latino groups continues to be clarified.^8-12 For instance, the association between AD and APOE ε4 is inconsistent for individuals from the Caribbean, perhaps because of differing levels of African admixture among these populations.^10,12-16

Moreover, some studies suggest that the strength of the association in Latino individuals is weaker compared with that in non-Latino white individuals.^8,17 The differences could be due to a multitude of factors, including whether there are differences in the frequency of APOE alleles by ethnic group,¹⁰ whether only APOE ε4 homozygosity increases the risk for AD in certain ethnicities or races,¹³ or whether APOE ε4 is only associated with late-onset familial AD rather than sporadic AD in certain populations.¹¹ Additionally, the accuracy of the self-designated origin, racial, or ethnic classifications and the homogeneity of the Latino groupings may be factors, as some Latino individuals are of African origin and the association between APOE ε4 and AD is inconsistent in Nigerian and African American individuals.^10,18,19 Lastly, other genetic variants, including variants in SORL1, may be associated with AD in certain Latino populations as well as non-Latino white individuals.^20,21

Fluorodeoxyglucose (FDG) positron emission tomography (PET) studies find that patients with AD have preferential and progressive reductions in the cerebral metabolic rate for glucose (CMRgl) in the posterior cingulate, precuneus, parietal, temporal, and prefrontal brain regions.^22-29 We and others have found that cognitively healthy APOE ε4 carriers exhibit CMRgl reductions in these AD-affected regions,^30-34 leading us to propose that FDG PET could be used as a promising biomarker (but not yet validated surrogate end point) for the evaluation of promising presymptomatic AD treatments in cognitively healthy APOE ε4 carriers.^33,35 Based on the finding that the CMRgl reductions in AD-affected regions are associated with the number of ε4 alleles in a person's APOE genotype (ie, 3 levels of genetic risk for AD),³⁶ we proposed using FDG PET to provide a quantitative presymptomatic endophenotype, a measurable feature that is more closely related to disease susceptibility than the clinical syndrome itself, to help assess the individual and aggregate effects of putative modifiers of AD risk.³³

This study compared FDG PET measurements of regional CMRgl in cognitively healthy late-middle-aged APOE ε4 carriers and noncarriers from the rapidly growing US Latino community.² In particular, it sought to determine whether cognitively healthy Latino APOE ε4 carriers have lower CMRgl than noncarriers in AD-affected regions, provide presymptomatic endophenotypic evidence for the role of the APOE ε4 allele as a late-onset AD susceptibility gene in this population, and support the inclusion of Latino ε4 carriers in our proposed presymptomatic AD treatment trials. Moreover, to address the potential heterogeneity within the Latino cohort, the cohort was tested for stratification between APOE ε4 carriers and noncarriers.

To identify Latino APOE ε4 carriers, a newspaper article about AD research and an accompanying advertisement, as well as other outreach activities, permitted us to recruit 81 (68 women and 13 men) cognitively healthy volunteers from Arizona's English-speaking Latino community, 47 to 68 years of age, irrespective of their reported family history of AD. Nearly all respondents were the result of the newspaper article and accompanying advertisement. Respondents understood that they would not receive any information about their APOE genotype, provided their informed consent, and were studied under guidelines approved by the human subjects committees at Banner Good Samaritan Medical Center and the Mayo Clinic. Venous blood samples were drawn and APOE genotypes characterized with analysis involving restriction fragment length polymorphisms.³⁷

The distribution of APOE genotypes in the Latino respondents is noted in Table 1. While 22 APOE ε4 carriers were identified, 8 declined to participate in the imaging studies and 3 did not meet our selection criteria for imaging study enrollment because of comorbid medical conditions (diabetes, stroke, and cancer). Of the 59 noncarriers, 14 declined to participate in the imaging studies and 11 did not meet our selection criteria because of diabetes or stroke. The 11 APOE ε4 carriers who agreed to participate in the imaging studies (1 with the ε4/ε4 allele, 1 with the ε2/ε4 allele, and 9 with the ε3/ε4 allele) were matched to 16 APOE ε4 noncarriers (1 with the ε2/ε3 allele and 15 with the ε3/ε3 allele) for sex, mean age, and educational level. Investigators who were unaware of the participants' APOE genotypes obtained data from medical and family histories as previously described,³¹ including a neurological examination, a structured psychiatric interview, the Mini- Mental State Examination,³⁸ the Hamilton Depression Rating Scale,³⁹ a battery of neuropsychological tests, and brain imaging studies.

The 27 participants in the imaging portion of the study were predominantly self-designated Mexican American (1 participant self-designated being of South American origin [Peru] and 2 self-designated being of Puerto Rican origin) and 10 reported a first-degree family history of dementia. At the time of their initial visit, all participants denied having impairment in memory or other cognitive skills, did not satisfy criteria for a current psychiatric disorder other than depression or anxiety, had no known cardiovascular or cerebrovascular disease, had scores of at least 28 on the Mini-Mental State Examination, had normal neurological examination results, and identified English as their primary language. To allow for inclusion of this underrepresented cohort into a study of this nature, we broadened our usual criteria to allow for self-reported depression or anxiety and pharmaceutical treatments of these disorders and did not require a first-degree family history of AD. Five participants reported using medication to treat depression or anxiety; however, all Hamilton Depression Rating Scale scores were within normal limits. In addition, 6 participants reported a history of hypertension or hypercholesterolemia.

Volumetric T1-weighted magnetic resonance imaging (MRI), performed to rule out structural lesions, and FDG PET were performed as previously described.^31,36 For 6 participants (2 APOE ε4 carriers and 4 noncarriers), PET was performed with an older 951/31 scanner (Siemens, Knoxville, Tennessee). This scanner records in 2-dimensional mode with intravenous injection of about 3.7 × 10⁸ Bq of FDG. The remaining 21 participants (9 APOE ε4 carriers and 12 noncarriers) were studied with an HR+ scanner (Siemens). The HR+ scanner simultaneously records data in a 3-dimensional mode with the intravenous injection of 1.85 × 10⁸ to 2.96 × 10⁸ Bq of FDG and permits the reconstruction of images consisting of 63 horizontal slices with a center-to-center slice separation of 2.46 mm, an axial field of view of 15.5 cm, an in-plane resolution of 4.2 to 5.1 mm full width at half maximum (FWHM), and an axial resolution of 4.6 to 6.0 mm FWHM. Regardless of scanner type, a 60-minute dynamic sequence of emission scans was acquired from each participant, who had fasted for at least 4 hours and was instructed to lay quietly with eyes closed in a darkened room. The emission image was reconstructed with measured attenuation correction and a 0.40-cycle per pixel Hanning filter, resulting in a final in-plane resolution of 10.5 mm FWHM. Regional analyses were performed using the PET images (counts relative to the whole-brain uptake) acquired during the last 30 minutes.

An automated algorithm (SPM5; Wellcome Department of Cognitive Neurology, London, England) was used to linearly and nonlinearly deform each person's PET image into the coordinates of a standard brain atlas. Images were further smoothed using a 3-dimensional gaussian filter to a spatial resolution of 12 mm FWHM. The images were normalized for the variation in whole-brain measurements by using proportionate scaling. Two-sample t tests were used to examine the differences in CMRgl between the Latino APOE ε4 carriers and noncarriers (P < .005, uncorrected for multiple comparisons) on a voxel by voxel basis. The statistical map was superimposed onto a map of CMRgl reductions in previously studied patients with AD²² and a spatially standardized, volume-rendered MRI. Significance levels were then adjusted for the number of resolution elements in the AD-affected posterior cingulate, precuneus, parietotemporal, and frontal brain regions using the small-volume correction procedure in SPM (P < .05, corrected fo multiple comparisons). Findings in other brain regions were not corrected for multiple comparisons and are considered exploratory. The statistical analysis was first performed using all data, covarying for the coding of the 2 scanners. A post hoc analysis was conducted using only the data (n = 21) from the HR+ scanner to confirm findings independent of any potential confounds associated with the use of the 2 different scanners. Post hoc voxel-based analysis was conducted using data from a predominately non-Latino white cohort⁴⁰ (11 APOE ε4 heterozygotes and 22 noncarriers) to confirm findings independent of an interaction between APOE ε4 carrier/noncarrier status and Latino/non-Latino status. Additional post hoc analyses were conducted. First, voxelwise post hoc analyses were conducted to determine the effect of positive first-degree family history of dementia on CMRgl reductions, given that this may impart additional risk above APOE.^41-43 Second, an analysis was conducted to examine whether the observed CMRgl reductions were associated with performance on the Rey Auditory Verbal Learning Test–Long-Term Memory,⁴⁴ a measure we have shown to be the most sensitive to age-related memory decline in APOE ε4 carriers.⁴⁵ For the later, data from the voxel with the most significant reduction of CMRgl (1 AD-predicted region and 1 additional region) were extracted from each subject and used to calculate Pearson correlation coefficients.

Twenty-five of the 27 Latino samples were successfully genotyped with the Affymetrix 6.0 array (Affymetrix Inc, Santa Clara, California) using standard methods and Birdsuite⁴⁶ was used to call single-nucleotide polymorphism (SNP) genotypes from CEL files. The Latino cohort was tested for stratification between APOE ε4 carriers and noncarriers in PLINK version 1.06⁴⁷ using a clustering approach with pairwise identity-by-state distance measures. For this analysis, we removed SNPs with Hardy-Weinberg equilibrium of P ≤ .001, missing genotypes more than 10%, and minor allele frequencies less than 1% for a total of 764 108 SNPs used for stratification analysis. Ten thousand label-swapping permutations were performed between APOE ε4 carriers and noncarriers and similarity P values were calculated.

The distribution of APOE genotypes in the 81 subjects is shown in Table 1. The APOE genotype frequencies are similar to those found in other samples of Mexican American individuals,^48,49 and the percentage of APOE ε4 homozygotes is similar to that found in the general population.⁵⁰ The characteristics of the APOE ε4 carriers and noncarriers enrolled in the imaging study are shown in Table 2. The APOE ε4 carriers and noncarrier groups did not differ significantly in their sex, reported family history of dementia, mean age, educational level, Mini-Mental State Examination score, other clinical ratings and neuropsychological test scores, or in the scanners used to acquire their FDG PET data.

As predicted, Latino APOE ε4 carriers had significantly lower CMRgl relative to the noncarriers bilaterally in brain regions previously found to be preferentially affected by AD, including the posterior cingulate (P < .05, corrected for multiple comparisons using small-volume correction), precuneus, and parietal cortex (P < .005, uncorrected for multiple comparisons) (Table 3) (Figure). In addition, compared with noncarriers, the APOE ε4 carriers had CMRgl reductions in the middle and anterior cingulate, hippocampus, and thalamus (P < .005, uncorrected for multiple comparisons) (Table 3) (Figure). The latter findings should be considered exploratory since the findings were not predicted and were not subject to small-volume correction. For each of the locations specified in Table 3, the mean CMRgl was 6.9% to 16.0% lower in the APOE ε4 carriers than in the noncarriers.

A post hoc voxel-based analysis failed to identify an interaction between APOE ε4 carrier/noncarrier status and Latino/non-Latino status, as the CMRgl reductions observed in the Latino APOE ε4 carriers compared with noncarriers were not significantly different from CMRgl reductions in non-Latino white APOE ε4 heterozygotes compared with noncarriers. Findings from the post hoc analysis of the data from the 21 participants scanned with the HR+ scanner were nearly identical to the results obtained from all 27 participants who were scanned on either the HR+ scanner or 951/31 scanner, indicating that the findings are not attributable to any confounds associated with the use of 2 scanners. Similarly, the post hoc voxel-based analyses that controlled for first-degree family history of dementia were nearly identical to those shown in Table 3 and the Figure, indicating that the findings are not solely attributable to this confound (data not shown). Lastly, reduction in CMRgl was not correlated with Rey Auditory Verbal Learning Test–Long-Term Memory score, nor was there a significant interaction with APOE ε4 carrier status.

Pairwise identity-by-state clustering across 764 108 SNPs showed no significant similarity or difference between APOE ε4 carriers and noncarriers (P = .17 less similar and P = .83 more similar). Additionally, using the identity-by-state approach, we confirmed the 25 genotyped Latino individuals clustered into a single cluster with no significant outliers based on nearest neighbor calculations.

In our previous FDG PET studies, we found that cognitively healthy late-middle-aged APOE ε4 carriers have significantly lower CMRgl than noncarriers in brain regions preferentially affected by AD.^31,33,36 The present study extends our findings to a cohort of Latino Mexican American individuals and supports the relationship between the APOE ε4 allele and the risk of AD in this rapidly growing North American community. Furthermore, the genetic findings confirmed the relative homogeneity of our Latino cohort, with no stratification between the APOE ε4 carriers and noncarriers.

Among certain Latino groups, the association between the APOE ε4 allele and AD continues to be clarified, particularly for those of Caribbean origin.^10,12-14 Some studies suggest that the strength of the association in Latino individuals is weaker compared with that in non-Latino white individuals^8,17 or that the degree of association among Latino individuals may be intermediate between that in African American and non-Latino white individuals,^12,52 consistent with the African admixture in Latino individuals of Caribbean origin.⁵³ If inconsistent findings persist, they may be attributable to AD-related environmental and genetic differences (eg, dietary differences or African vs European origin) within the heterogeneous characterization of “Latino,” perhaps warranting the use of genome-wide genetic analyses in future studies, with the caveat that there may be cultural sensitivities to the issue specific to different ethnic groups.

In addition to the predicted CMRgl reductions in AD-related brain regions, the Latino APOE ε4 carriers exhibited hypometabolism in the middle and anterior cingulate cortex, hippocampus, and thalamus. This pattern of hypometabolism is somewhat different than what we previously observed in a predominately non-Latino cohort of APOE ε4 carriers.^31,40 However, in a post hoc comparison of the interaction between Latino/non-Latino status and APOE ε4 carrier/noncarrier status, these CMRgl reductions in Latino individuals were not significantly greater than those in the predominately non-Latino cohort and so are of uncertain significance. Accordingly, there is a need for additional studies with a larger Latino sample size to confirm that the more extensive regional findings remain significant. If they do, there are other differences besides Latino status that may account for the findings. For instance, there is no requirement for family history of AD in our Latino cohort as there is in our predominately non-Latino cohort, thereby potentially causing us to slightly underestimate changes related to AD risk in the non-Latino cohort.

This study has some limitations. First, since our findings are restricted to participants who identified English as their primary language, were predominately self-designated Mexican American, were recruited using targeted outreach efforts, and had relatively high levels of education and included only 11 APOE ε4 carriers and more women than men, additional population-based studies are needed to determine the extent to which they are generalizable to other Latino subjects and communities. Still, our findings are likely to be relevant to those Mexican American individuals who express interest in proof-of-concept presymptomatic AD treatment studies, discussed later. Second, we have not yet sought to determine the extent to which our findings are solely attributable to the combined effects of atrophy and partial-volume averaging, MRI white matter intensities, or vascular risk factors. Still, we previously demonstrated that APOE ε4–related CMRgl reductions are not solely attributable to brain atrophy, subjects in our imaging studies do not have clinically significant MRI abnormalities, and the carriers and noncarriers did not differ significantly in their reported vascular risk factors.

We have proposed how FDG PET and other brain imaging measurements could be used in cognitively healthy APOE ε4 carriers as quantitative presymptomatic endophenotypes, measurable features that are more closely related to disease susceptibility than the clinical syndrome itself, to help evaluate the individual and aggregate effects of putative genetic and nongenetic modifiers of AD risk.³⁶ We are currently investigating the possibility of using multivariate statistical methods, including partial least squares and our recently reported multimodal partial least squares method,⁵⁴ to characterize the patterns of FDG PET and other imaging changes associated with APOE ε4 gene dose, characterize the presymptomatic brain imaging phenotype in a single-subject score that reflects this pattern, and use them to evaluate suggested genetic and nongenetic modifiers of AD risk using our presymptomatic brain imaging phenotype with superior statistical power and freedom from the type I error associated with multiple comparisons.

We have also proposed how FDG PET and other brain imaging measurements could be used to evaluate the effectiveness of putative presymptomatic AD therapies to slow down the progressive regional CMRgl declines in proof-of-concept studies without having to study thousands of healthy late-middle-aged persons or wait many years to determine whether or when persons in the clinical trial develop symptoms.³⁵ As a complement to observational studies of older patients with AD and controls, our proposed endophenotype could provide prospective evaluation of putative risk modifiers, help address the potentially confounding effects of differential survival related to the risk modifiers, provide information about the individual or aggregate effects of risk factors, and permit the accurate measurement and real-time evaluation of a putative risk factor years before the onset of symptoms. As a complement to prospective cohort studies, this imaging endophenotype could potentially be used as a surrogate marker for presymptomatic AD treatment development, decreasing the number of healthy participants needed or length of treatment needed to observe drug effects.

The present study illustrates the use of this endophenotype by showing an AD-related pattern of hypometabolism in APOE ε4 carriers from Arizona's Latino community. Consistent with previous findings in non-Latino white individuals, the predominately Mexican American APOE ε4 carriers have reduced glucose metabolism in AD-related brain regions, which, coupled with the projected rapid increase in the racial and ethnic diversity of older adults,² supports the inclusion of Latino APOE ε4 carriers in proof-of-concept studies using FDG PET to evaluate promising presymptomatic treatments in cognitively healthy people at increased risk for AD.

Correspondence: Jessica B. S. Langbaum, PhD, Banner Alzheimer's Institute, 901 E Willetta St, Phoenix, AZ 85006 (jessica.langbaum@bannerhealth.com).

Accepted for Publication: September 9, 2009.

Author Contributions:Study concept and design: Langbaum, Caselli, Huentelman, Fleisher, and Reiman. Acquisition of data: Bandy, Burns, Kaszniak, Reeder, Corneveaux, Allen, Pruzin, and Reiman. Analysis and interpretation of data: Langbaum, Chen, Lee, Reschke, Alexander, Kaszniak, Corneveaux, Huentelman, Fleisher, and Reiman. Drafting of the manuscript: Langbaum, Bandy, Corneveaux, and Reiman. Critical revision of the manuscript for important intellectual content: Langbaum, Chen, Caselli, Lee, Reschke, Alexander, Burns, Kaszniak, Reeder, Allen, Pruzin, Huentelman, Fleisher, and Reiman. Statistical analysis: Langbaum, Chen, Lee, Reschke, Alexander, Kaszniak, and Corneveaux. Obtained funding: Caselli and Reiman. Administrative, technical, and material support: Caselli, Reschke, Bandy, Burns, Kaszniak, Reeder, Allen, Pruzin, Fleisher, and Reiman. Study supervision: Langbaum, Bandy, Burns, Huentelman, Fleisher, and Reiman.

Financial Disclosure: None reported.

Funding/Support: This work was supported by National Institute of Mental Health grant R01MH57899 (Dr Reiman), National Institute on Aging grants R01AG031581 and P30AG19610 (Dr Reiman), the Evelyn G. McKnight Brain Institute (Dr Alexander), the state of Arizona (Drs Reiman, Caselli, Alexander, and Chen), and contributions from the Banner Alzheimer's Foundation and Mayo Clinic Foundation.

Previous Presentation: Portions of this study were presented at the 11th International Conference on Alzheimer's Disease; July 27, 2008; Chicago, Illinois.

Additional Contributions: We thank Richard Gerkin, MD, MS, Napatkamon Ayutyanont, PhD, Patti Aguilar, David Branch, AAS, CNMT, ARRT, Sandra Goodwin, CNMT, NMTCB(PET), ARRT, CRT, Bruce Henslin, BA, Debbie Intorcia, Jennifer Keppler, CNMT, MBA, Xiaofen Liu, MS, Anita Prouty, BS, Oded Smilovici, MS, Desiree Van Egmond, AS, Justin Venditti, BA, and Sandra Yee-Benedetto, BA, for assistance.