Basic Science and Pathogenesis

Alzheimer's disease (AD) is characterized by abnormal blood levels of specific proteins associated with amyloid plaques, tau pathology, and neurodegeneration, which are considered biomarkers of AD. To better understand the genetic architecture of AD in South Asians, we assessed the association between single nucleotide polymorphisms (SNPs) previously associated with AD and AD biomarkers with blood-level measures of seven AD biomarkers in 2,547 South Asians from the Harmonized Diagnostic Assessment of Dementia for the Longitudinal Aging Study in India (LASI-DAD).

We performed SNP-based association analysis between 152 AD- and 9 AD biomarker-associated SNPs with 7 AD biomarkers, including amyloid beta 40 (Ab40), amyloid beta 42 (Ab42), Ab42/Ab40 ratio, total tau (tTau), tau phosphorylated at threonine 181 (pTau181), glial fibrillary acidic protein (GFAP), and neurofilament light chain (NfL). The model adjusted for age, sex, the first 10 genetic principal components (PCs), and incorporated the genetic relationship matrix and plate as random effects. Significance was declared at Benjamini-Hochberg FDR q<0.1. Identified loci were followed up with haplotype analysis to understand the potential driving factors.

Among 152 AD-associated SNPs, rs199515_C (WNT3/MAPT) was associated with tTau in the expected direction (beta=0.21, p = 0.00052), whereas rs2718058_A (NME8) was associated with pTau181 in the unexpected direction (beta=0.13, p = 0.00057). Among the 9 biomarker-associated SNPs, rs242557_A (MAPT) was associated with tTau (beta=-0.139, p = 2.3e-08) and pTau181 (beta=0.099, p = 1.4e-04), and rs429358_C (APOE-e4) was associated with Ab42/Ab40 ratio (beta=-0.141, p = 0.0012). Follow-up analysis in MAPT, the gene that encodes the tau protein, identified a significant association between H1/H2 haplotype and tTau (global-p=0.00205). Specifically, H1j (p = 0.0015), H2 (p = 0.027), H1r (p = 0.031), and H1b (p = 0.041) were associated with lower levels of tTau, whereas H1c (p = 0.04) and H1o (p = 0.012) were associated with higher levels. Moreover, APOE-e4 stratified analysis detected significant H1j-tTau (score=-3.61, p = 0.00031) and H1c-pTau181 (score=2.55, p = 0.01) associations in e4 carriers only.

The analysis highlights a critical role of MAPT in tau pathology in South Asians. The observed association does not seem to be fully driven by known risk haplotypes. It also provides evidence to suggest modification by APOE-e4. Further investigation of MAPT haplotype structure in this population is warranted.