Among 172 participants, there were 79 APOE4 non-carriers (25 women and 54 men) and 93 APOE4 carriers (34 women and 59 men). The sample was 97% White, with a mean age of 75, and mean years of education of 15. In the overall sample, APOE4 carriers were younger, showed poorer global cognition (lower mean MMSE score), had higher p-Tau levels, were less likely to be cognitively normal and more likely to be AD dementia patients compared to non-carriers (ps<.05; Table 1). Mean testosterone level was lower in APOE4 carriers versus non-carriers, although not significantly (p=.09). When comparing men and women by APOE4 status, female APOE4 carriers were significantly younger than male APOE4 carriers (p=.002). As expected, mean total and free testosterone levels were lower in women than in men regardless of APOE4 status (ps<.001). In replication of previous findings, p-Tau levels were higher in women versus men but only among APOE4 carriers (p=.001).
Table 1. Sample characteristics by APOE4 carrier status and sex.
|
APOE4- (n=79)
|
APOE4+ (n=93)
|
p-value (effect size)a
|
APOE4-
|
APOE4+
|
|
|
|
|
Women
n=25
|
Men
n=54
|
p-value (effect size)a
|
Women
n=34
|
Men
n=59
|
p-value (effect size)a
|
Age, Mean (SD)
|
76.6 (7.2)
|
74.0 (6.7)
|
.02 (.37)
|
77.0 (6.3)
|
76.4 (7.7)
|
.74
|
71.3 (7.4)
|
75.6 (5.7)
|
.002 (.65)
|
Years of education, Mean (SD)
|
15.9 (3.0)
|
15.3 (3.2)
|
.20
|
15.7 (2.7)
|
16.0 (3.1)
|
.64
|
14.7 (2.9)
|
15.6 (3.4)
|
.17
|
White, n (%)
|
76
(96.2%)
|
91 (97.8%)
|
.54
|
25 (100%)
|
51 (94.4%)
|
.49
|
32 (94.1%)
|
59 (100%)
|
.06
|
Cognitive status
|
|
|
<.001 (.40)
|
|
|
.26
|
|
|
.65
|
Cognitively
normal, n (%)
|
22 (27.8%)
|
3
(3.2%)
|
|
10 (40.0%)
|
12 (22.2%)
|
|
1
(2.9%)
|
2
(3.4%)
|
|
MCI, n (%)
|
45 (57.0%)
|
52 (55.9%)
|
|
12 (48.0%)
|
33 (61.1%)
|
|
17 (50.0%)
|
35 (59.3%)
|
|
AD dementia, n (%)
|
12 (15.2%)
|
38 (40.9%)
|
|
3
(12.0%)
|
9
(16.7%)
|
|
16 (47.1%)
|
22 (37.3%)
|
|
Global cognition (MMSE), Mean (SD)
|
27.1 (2.2)
|
25.8 (2.5)
|
<.001 (.55)
|
27.6 (2.2)
|
26.9 (2.2)
|
.15
|
25.6 (2.5)
|
25.9 (2.5)
|
.56
|
BMI, Mean (SD)
|
26.5 (4.0)
|
25.8 (3.7)
|
.28
|
25.7 (4.9)
|
26.8 (3.5)
|
.24
|
25.5 (4.0)
|
26.0 (3.6)
|
.55
|
Self-reported history of cardiovascular events, n (%)
|
61 (77.2%)
|
62 (66.7%)
|
.13
|
16 (64.0%)
|
45 (83.3%)
|
.06
|
23 (67.6%)
|
39 (66.1%)
|
.88
|
Pulse pressureb, Mean (SD)
|
61.0 (18.1)
|
59.1 (14.7)
|
.41
|
61.9 (23.9)
|
60.8 (15.0)
|
.80
|
58.9 (15.8)
|
59.2 (14.1)
|
.93
|
Plasma total testosterone levelc (ng/mL), Mean (SD)
|
0.2 (0.4)
|
0.1 (0.5)
|
.09
|
-0.3 (0.4)
|
0.5 (0.1)
|
<.001 (2.74)
|
-0.4 (0.4)
|
0.4 (0.2)
|
<.001 (2.53)
|
Plasma free testosterone levelc (ng/mL), Mean (SD)
|
13.4 (24.8)
|
6.5 (27.7)
|
.09
|
-16.9 (19.5)
|
27.5 (9.8)
|
<.001 (2.9)
|
-22.0 (22.7)
|
23.0 (13.0)
|
<.001 (2.4)
|
CSF p-Tau181 level (pg/mL), Mean (SD)
|
26.8
(13.1)
|
35.9 (17.2)
|
<.001 (.59)
|
23.5 (8.6)
|
28.3 (14.5)
|
.13
|
43.7 (22.6)
|
31.5 (11.1)
|
.001 (0.68)
|
CSF Aβ1-42 level (pg/mL), Mean (SD)
|
1240.4 (702.9)
|
639.1 (292.2)
|
<.001 (1.12)
|
1286.7 (765.1)
|
1218.9 (678.7)
|
.69
|
658.7 (627.8)
|
627.8 (327.0)
|
.63
|
Note. aEffect sizes are provided for significant differences; Cohen’s d is provided for mean differences (0.2 = small, 0.5 = medium, 0.8 = large) and a phi coefficient is provided for differences in proportions (0.1 = small, 0.3 = medium, 0.5 = large). bPulse pressure = systolic – diastolic blood pressure. cTestosterone levels were normalized based on a Box-Cox transformation. MCI = mild cognitive impairment; AD = Alzheimer’s disease; APOE4 = apolipoprotein E ɛ4 allele; MMSE = Mini Mental Status Examination; BMI = body mass index. CSF = cerebrospinal fluid.
Sex differences in p-Tau by APOE4 status
In line with hypotheses and our unadjusted analyses (Table 1), a significant sex by APOE4 interaction on p-Tau levels (B=-5.76, β=-0.24, standard error [SD]=2.96, p=.05) when adjusting for covariates (i.e., age, education and cardiovascular risk factors) indicated higher p-Tau levels in women versus men among APOE4 carriers only (B=-11.16, β=-0.31, SD=3.85, p=.005). Analyses stratified by APOE4 status actually revealed an opposing sex difference among non-carriers, whereby p-Tau levels were higher in men versus women, although not significantly (B=6.09, β=0.22, SD=3.22, p=.06; Figure 1).
Relationship between Testosterone and p-Tau by APOE4 status
In the overall sample, there was a significant relationship between lower total testosterone levels and higher CSF p-Tau levels (B=-13.26, β=-.39, p=.002; Figure 2), but, more importantly, there was a significant total testosterone X APOE4 status interaction on p-Tau levels (B=-17.78, β=-0.40, SD=4.9, p<.001). Analyses stratified by APOE4 status revealed that lower total testosterone levels were associated with higher p-Tau among APOE4 carriers (B=-17.36, β=-0.50, SE=5.41, p=.002) but not non-carriers (B=-4.45, β=-0.15, SE=6.4, p=.49). Results in the overall and the APOE4-stratified analyses were unchanged when substituting free for total testosterone and when including Aβ levels as a covariate in the model.
In sex-stratified analyses, the range of total testosterone levels were lower in women (range=-1.2-0.2, median=-0.28) versus men (range:-0.6-0.7, median=0.5) although overlapping. Within the distribution of lower testosterone levels in women (B=-13.83, β=-0.27, SE=5.88, p=.02) and the distribution of higher levels in men (B=-15.85, β=-0.24, SE=6.32, p=.01), there was a negative association between testosterone and p-Tau levels suggestive of a continuous, linear relationship (Figure 3). These associations occurred regardless of APOE4 status as indicated by non-significant testosterone by APOE4 interactions in women (B=-12.55, β=-.23, SE=13.13, p=.34) or men (B=10.19, β=0.20, SE=13.66, p=.46). However, the testosterone by APOE4 interaction on p-Tau in the overall sample appeared to be mostly driven by women in that the testosterone and p-Tau relationship was marginally significant among female APOE4 carriers (B=-18.06, β=-0.34, SE=8.89, p=.05) but not among female non-carriers (B=-0.27, β=-0.01, SE=5.1, p=.96; Figure 3). In contrast, the testosterone and p-Tau relationship was a trend in both male APOE4 carriers (B=-13.38, β=-0.26, SE=6.75, p=.053) and non-carriers (B=-24.24, β=-0.26, SE=13.78, p=.08), Despite the specificity of a testosterone and p-Tau link to female APOE4 carriers, we were likely underpowered to detect a APOE4 by testosterone interaction given the smaller sample size in female-specific analyses (n=53). Results in both men and women were unchanged when substituting free for total testosterone and when adjusting for Aβ levels.
Explanatory role of testosterone in sex difference in p-Tau
In testing the mediating role of testosterone in the sex difference in p-Tau levels, we found that the significantly higher p-Tau levels in female APOE4 carriers versus male APOE4 carriers was eliminated after adjusting for testosterone levels (B=3.21, β=0.09, SE=5.78, p=.58; Figure 1). Conversely, the trend for higher p-Tau levels in men versus women among APOE4 non-carriers changed minimally after adjusting for testosterone (B=9.47, β=0.34, SE=5.84, p=.10). Again, results were unchanged when substituting free for total testosterone and when adjusting for Aβ levels.