Generalizability of Tau and Amyloid Plasma Biomarkers in Alzheimer's Disease Cohorts of Diverse Genetic Ancestries

Introduction

Plasma phosphorylated threonine-181 of Tau and amyloid beta are biomarkers for differential diagnosis and preclinical detection of Alzheimer disease (AD). Given differences in AD risk across diverse populations, generalizability of existing biomarker data is not assured.

Methods

In 2,086 individuals of diverse genetic ancestries (African American, Caribbean Hispanic, and Peruvians) we measured plasma pTau-181 and Aβ42/Aβ40. Differences in biomarkers between cohorts and clinical diagnosis groups and the potential discriminative performance of the two biomarkers were assessed.

Results

pTau-181 and Aβ42/Aβ40 were consistent across cohorts. Higher levels of pTau181 were associated with AD while Aβ42/Aβ40 had minimal differences. Correspondingly, pTau-181 had greater predictive value than Aβ42/Aβ40, however, the area under the curve differed between cohorts.

Discussion

pTau-181 as a plasma biomarker for clinical AD is generalizable across genetic ancestries, but predictive value may differ. Combining genomic and biomarker data from diverse individuals will increase understanding of genetic risk and refine clinical diagnoses.

Dissecting the role of Amerindian genetic ancestry and the ApoE ε4 allele on Alzheimer disease in an admixed Peruvian population

Alzheimer disease (AD) is the leading cause of dementia in the elderly and occurs in all ethnic and racial groups. The apolipoprotein E (ApoE) ε4 is the most significant genetic risk factor for late-onset AD and shows the strongest effect among East Asian populations followed by non-Hispanic white populations and has a relatively lower effect in African descent populations. Admixture analysis in the African American and Puerto Rican populations showed that the variation in ε4 risk is correlated with the genetic ancestral background local to the ApoE gene. Native American populations are substantially underrepresented in AD genetic studies. The Peruvian population with up to ~80 of Amerindian (AI) ancestry provides a unique opportunity to assess the role of AI ancestry in AD. In this study, we assess the effect of the ApoE ε4 allele on AD in the Peruvian population. A total of 79 AD cases and 128 unrelated cognitive healthy controls from Peruvian population were included in the study. Genome-wide genotyping was performed using the Illumina Global screening array v2.0. Global ancestry and local ancestry analyses were assessed. The effect of the ApoE ε4 allele on AD was tested using a logistic regression model by adjusting for age, gender, and population substructure (first 3 principal components). Results showed that the genetic ancestry surrounding the ApoE gene is predominantly AI (60.6%) and the ε4 allele is significantly associated with increased risk of AD in the Peruvian population (odds ratio = 5.02, confidence interval: 2.3-12.5, p-value = 2e-4). Our results showed that the risk for AD from ApoE ε4 in Peruvians is higher than we have observed in non-Hispanic white populations. Given the high admixture of AI ancestry in the Peruvian population, it suggests that the AI genetic ancestry local to the ApoE gene is contributing to a strong risk for AD in ε4 carriers. Our data also support the findings of an interaction between the genetic risk allele ApoE ε4 and the ancestral backgrounds located around the genomic region of ApoE gene.

Linkage of Alzheimer disease families with Puerto Rican ancestry identifies a chromosome 9 locus

The genetic admixture of Caribbean Hispanics provides an opportunity to discover novel genetic factors in Alzheimer disease (AD). We sought to identify genetic variants for AD through a family-based design using the Puerto Rican (PR) Alzheimer Disease Initiative (PRADI). Whole-genome sequencing (WGS) and parametric linkage analysis were performed for 100 individuals from 23 multiplex PRADI families. Variants were prioritized by minor allele frequency (<0.01), functional potential [combined annotation dependent depletion score (CADD) >10], and co-segregation with AD. Variants were further ranked using an independent PR case-control WGS dataset (PR10/66). A genome-wide significant linkage peak was found in 9p21 with a heterogeneity logarithm of the odds score (HLOD) >5.1, which overlaps with an AD linkage region from two published independent studies. The region harbors C9orf72, but no expanded repeats were observed in the families. Seven variants prioritized by the PRADI families also displayed evidence for association in the PR10/66 (p < 0.05), including a missense variant in UNC13B. Our study demonstrated the importance of family-based design and WGS in genetic study of AD.

Use of local genetic ancestry to assess TOMM40-523' and risk for Alzheimer disease

OBJECTIVE

Here, we re-examine TOMM40-523' as a race/ethnicity-specific risk modifier for late-onset Alzheimer disease (LOAD) with adjustment for local genomic ancestry (LGA) in Apolipoprotein E (APOE) ε4 haplotypes.

METHODS

The TOMM40-523' size was determined by fragment analysis and whole genome sequencing in homozygous APOE ε3 and APOE ε4 haplotypes of African (AF) or European (EUR) ancestry. The risk for LOAD was assessed within groups by allele size.

RESULTS

The TOMM40-523' length did not modify risk for LOAD in APOE ε4 haplotypes with EUR or AF LGA. Increasing length of TOMM40-523' was associated with a significantly reduced risk for LOAD in EUR APOE ε3 haplotypes.

CONCLUSIONS

Adjustment for LGA confirms that TOMM40-523' cannot explain the strong differential risk for LOAD between APOE ε4 with EUR and AF LGA. Our study does confirm previous reports that increasing allele length of the TOMM40-523' repeat is associated with decreased risk for LOAD in carriers of homozygous APOE ε3 alleles and demonstrates that this effect is occurring in those individuals with the EUR LGA APOE ε3 allele haplotype.

The Puerto Rico Alzheimer Disease Initiative (PRADI): A Multisource Ascertainment Approach

Introduction

Puerto Ricans, the second largest Latino group in the continental US, are underrepresented in genomic studies of Alzheimer disease (AD). To increase representation of this group in genomic studies of AD, we developed a multisource ascertainment approach to enroll AD patients, and their family members living in Puerto Rico (PR) as part of the Alzheimer’s Disease Sequencing Project (ADSP), an international effort to advance broader personalized/precision medicine initiatives for AD across all populations.

Methods

The Puerto Rico Alzheimer Disease Initiative (PRADI) multisource ascertainment approach was developed to recruit and enroll Puerto Rican adults aged 50 years and older for a genetic research study of AD, including individuals with cognitive decline (AD, mild cognitive impairment), their similarly, aged family members, and cognitively healthy unrelated individuals age 50 and up. Emphasizing identification and relationship building with key stakeholders, we conducted ascertainment across the island. In addition to reporting on PRADI ascertainment, we detail admixture analysis for our cohort by region, group differences in age of onset, cognitive level by region, and ascertainment source.

Results

We report on 674 individuals who met standard eligibility criteria [282 AD-affected participants (42% of the sample), 115 individuals with mild cognitive impairment (MCI) (17% of the sample), and 277 cognitively healthy individuals (41% of the sample)]. There are 43 possible multiplex families (10 families with 4 or more AD-affected members and 3 families with 3 AD-affected members). Most individuals in our cohort were ascertained from the Metro, Bayamón, and Caguas health regions. Across health regions, we found differences in ancestral backgrounds, and select clinical traits.

Discussion

The multisource ascertainment approach used in the PRADI study highlights the importance of enlisting a broad range of community resources and providers. Preliminary results provide important information about our cohort that will be useful as we move forward with ascertainment. We expect that results from the PRADI study will lead to a better understanding of genetic risk for AD among this population.