Biomarkers

Hippocampal volume is an important in vivo imaging marker for Alzheimer's disease (AD) risk and progression. Reported sex differences show accelerated hippocampal atrophy in females compared to males as AD-related pathology increases. However, genome-wide association studies (GWAS) of hippocampal volume have been mainly conducted in mid-life participants with no AD pathology and have not been systemically examined for sex-specific genetic effects. We investigated the sex-specific genetic architecture of hippocampal volume in eight aging and AD cohorts.

This study included 5,523 non-Hispanic White participants (N_Males=2,549; N_Females=2,974; mean baseline age=72 yrs; mean number of visits=2.3; 32.8% cognitively impaired). Hippocampal volume and estimated total intracranial volume (eTIV) were segmented from T₁-weighted MRIs using Hippodeep. Total hippocampal volume (Left+Right) and eTIV were harmonized using neuroCombat in R. Longitudinal slopes for hippocampal volume were calculated with linear mixed-effects models. GWAS were performed, at baseline and longitudinally, by cohort in all participants, sex-stratified, and sex-interaction models. Models covaried for baseline age, sex (in all participants), eTIV, and the first five genetic principal components. Longitudinal models also covaried for diagnosis conversion over time. Results were meta-analyzed across cohorts.

We identified three genome-wide significant genetic loci associated with hippocampal volume. Specifically, a chromosome 6 locus (index SNP rs62434269; MAF=0.33) near ARID1B was associated at baseline in all participants (β=0.10, p = 3.54x10^-8) (Figure 1). Further, we found a locus on chromosome 8 (rs34173062; MAF=0.09), which is a previously reported AD risk variant and eQTL for SHARPIN (β=-0.22, p = 1.68E-09), with significant effects in males (β_Males=-0.32, p_Males=2.14x10^-9) but not females (β_Females=-0.11, p_Females=0.03; p_{sex-interaction}=0.004) (Figure 2A-B). A chromosome 14 locus (rs75592630; MAF=0.05) near AKAP6, a gene previously associated with cognition, was associated with hippocampal volume change over time in females (β_Females=-0.28, p_Females=1.19x10^-9) but not males (β_Males=-0.01, p_Males=0.79; p_{sex-interaction}=0.003) (Figure 2C-D).

We extend findings of a previously reported AD risk variant near SHARPIN to hippocampal volume and provide evidence of novel sex-specific genetic effects. While replication is warranted, our results suggest the importance of genetic predictors and sex differences on imaging biomarkers.