Biomarkers

Background: The APOE ε4 variant is the largest known genetic risk factor for late-onset sporadic Alzheimer's disease (AD). Recent blood biomarker models include APOE ε4 status with plasma p-tau217 for higher accuracy for AD pathology. Thus, protein assays that can accurately determine ε4 carriership simultaneously with plasma p-tau217 would be advantageous for clinical use. This study aims to evaluate the concordance between the NULISA ApoE4 protein assay and conventional genetic testing for APOE measurement.

Method: We included two independent cohorts; cohort 1 (Translational Biomarker for Aging and Dementia Cohort [TRIAD]) consisted of 341 participants (mean [SD] age, 64.8[16.1] years; 213 females[62.4%]) and cohort 2 consisted of 38 participants (72.1[16.1] years; 21 females[55.2%]). APOE genotyping was determined by the TaqMan® SNP Genotyping Assay. In cohort 1, ApoE4 and p-tau217 levels (NPQ) were quantified by the NULISAseq CNS disease panel from Alamar Biosciences. In cohort 2, ApoE4% (ApoE4/ApoE^total) were quantified by a prototype singleplex assay from Alamar Biosciences RESULT: In cohort 1, we included 341 participants with APOE genotyping (ε4 non-carriers=221; ε4 carriers=120). ApoE4 NPQ values correctly identified 88.3% of carriers and 94.1% of non-carriers (Figure 1A). Homozygous ε2 and ε4 carriers were entirely identified. Higher ApoE4 NPQ levels were observed in ε4/ε4 carriers (median [SD], 16.4[0.52]) compared to ε3/ε4 carriers (12.3[5.11]; P<0.0001) but an overlap remained. In these participants, the simultaneously measurement of NULISA plasma p-tau217 was significantly increased in Aβ+ participants compared to Aβ- participants (AUC=0.941; P<0.0001). In cohort 2, we included 38 participants with APOE genotyping (ε3/ε3, n=26; ε3/ε4, n=9; ε4/ε4, n=6) and calculated the ApoE4% from a prototype singleplex NULISA method. Here, ε3/ε3 (0.03 [1.43]), ε3/ε4 (9.37 [1.61] and ε4/ε4 (41.9 [18.0]) individuals were classified with 100% accuracy (Figure 1B).

Conclusion: Our study assessed ApoE protein assays to determine APOE genotype. The ApoE4 assay within the NULISAseq CNS panel demonstrated high accuracy in distinguishing APOE ε4 carriers from non-carriers, with some discordance. The reason for the discordance subject to further studies. However, ApoE4% quantification using a prototype singleplex assay distinguished ε4 homozygous, ε4 heterozygous, and non-carriers with 100% accuracy. This pilot study demonstrates the ability to concurrently determine APOE status and p-tau217 levels for more accurate diagnostic models of AD pathology.