73 Sex Differences in Verbal Memory and Alzheimer’s Disease Biomarkers in Clinically Normal Older Adults: Role of SNAP-25 Genetics

Abstract

Objective:

Females outperform males on verbal memory tests across the lifespan. Females also exhibit greater Alzheimer’s disease (AD) pathology at preclinical stages and faster atrophy and memory decline during disease progression. Synaptic factors influence the accumulation of AD proteins and may underpin cognitive resilience against AD, though their role in sex-related cognitive and brain aging is unknown. We tested interactive effects of sex and genetic variation in SNAP-25, which encodes a presynaptic protein that is dysregulated in AD, on cognition and AD-related biomarkers in cognitively unimpaired older adults.

Participants and Methods:

Participants included a discovery cohort of 311 cognitively unimpaired older adults (age mean [range]=70 [44-100]; 56% female; education mean=17.3 years; 24% APOE-e4+), and an independent, demographically-comparable replication cohort of 82 cognitively unimpaired older adults. All participants completed neurological examination, informant interview (CDR=0), neuropsychological testing, and blood draw. Participants were genotyped for the SNAP-25 rs105132 (T→C) single-nucleotide polymorphism via Sequenom (discovery cohort) or Omni 2.5M (replication cohort). In vitro models show the C-allele is associated with increased SNAP-25 expression compared to T/T genotype. A subset of the discovery cohort completed structural MRI (n=237) and florbetapir Aβ-PET (n=97). Regression analyses across cohorts examined the interaction of sex and SNAP-25 genotype (T/T homozygotes [53% prevalence] vs. C-carriers [47% prevalence]) on cognitive z-scores (verbal memory, visual memory, executive function, language), adjusting for age, education, APOE-e4, and APOE-e4 x sex. Discovery cohort models also examined sex-dependent effects of SNAP-25 on temporal lobe volumes and Aβ-PET positivity.

Results:

SNAP-25 T/T vs. C-carriers did not differ on demographics or APOE-e4 status across cohorts or within sexes. Sex interacted with SNAP-25 to predict verbal memory (p=.024) and language (p=.008) in the discovery cohort, with similar verbal memory differences observed in the replication cohort. In sex-stratified analyses, C-carriers exhibited better verbal memory than T/T carriers among females (d range: 0.41 to 0.64, p range: .008 to .046), but not males (d range: 0.03 to 0.12, p range: .499 to .924). In SNAP-25-stratified analyses, female verbal memory advantages were larger among C-carriers (d range: 0.74 to 0.89, p range: <.001 to .034) than T/T (d range: 0.13 to 0.36, p range: .022 to .682). Sex also interacted with SNAP-25 to predict Aβ-PET positivity (p=.046) such that female C-carriers exhibited the lowest prevalence of Aβ-PET positivity (13%) compared to other groups (23% to 35%). C-carriers exhibited larger temporal lobe volumes across sex, yet this effect only reached statistical significance among females (females: d=0.41, p=.018; males: d=0.26, p=.179). In post-hoc analyses, larger temporal lobe volumes were selectively associated with better verbal memory in female C-carriers (β=0.36, p=.026; other groups: |βs|<0.10, ps>.538).

Conclusions:

Among clinically normal older adults, we demonstrate female-specific advantages of carrying the SNAP-25 rs105132 C-allele across cognitive, neural, and molecular markers of AD. The rs105132 C-allele putatively reflects higher endogenous levels of SNAP-25. Our findings suggest a female-specific pathway of cognitive and neural resistance, whereby higher genetically-driven expression of SNAP-25 may reduce likelihood of amyloid plaque formation and support verbal memory, possibly through fortification of temporal lobe structure.