Investigators found elevated levels of tau protein in the brains of women who initiated HT more than 5 years after menopause onset, while those who started the therapy earlier had normal levels.
Tau levels were also higher in women who started menopause before age 45, either naturally or following surgery, but only in those who already had high levels of beta-amyloid.
The findings were published online in JAMA Neurology.
Hotly debated
Previous research has suggested the timing of menopause and HT initiation may be associated with AD. However, the current research is the first to suggest tau deposition may explain that link.
“There have been a lot of conflicting findings around whether HT induces risk for Alzheimer’s disease dementia or not, and – at least in our hands – our observational evidence suggests that any risk is fairly limited to those rarer cases when women might delay their initiation of HT considerably,” senior investigator Rachel Buckley, PhD, assistant investigator in neurology at Massachusetts General Hospital and assistant professor of neurology at Harvard Medical School, Boston, told this news organization.
The link between HT, dementia, and cognitive decline has been hotly debated since the initial release of findings from the Women’s Health Initiative Memory Study, reported 20 years ago.
Since then, dozens of studies have yielded conflicting evidence about HT and AD risk, with some showing a protective effect and others showing the treatment may increase AD risk.
For this study, researchers analyzed data from 292 cognitively unimpaired participants (66.1% female) in the Wisconsin Registry for Alzheimer Prevention. About half of the women had received HT.
Women had higher levels of tau measured on PET imaging than age-matched males, even after adjustment for APOE status and other potential confounders.
Higher tau levels were found in those with an earlier age at menopause (P < .001) and HT use (P = .008) compared with male sex; later menopause onset; or HT nonuse – but only in patients who also had a higher beta-amyloid burden.
Late initiation of HT (> 5 years following age at menopause) was associated with higher tau compared with early initiation (P = .001), regardless of amyloid levels.
Surprising finding
Although researchers expected to find that surgical history (specifically oophorectomy) might have a greater impact on risk, that wasn’t the case.
“Given that bilateral oophorectomy involves the removal of both ovaries, and the immediate ceasing of estrogen production, I had expected this to be the primary driver of higher tau levels,” Dr. Buckley said. “But early age at menopause – regardless of whether the genesis was natural or surgical – seemed to have similar impacts.”
These findings are the latest from Dr. Buckley’s group that indicate that women tend to have higher levels of tau than men, regardless of preexisting amyloid burden in the brain.
“We see this in healthy older women, women with dementia, and even in postmortem cases,” Dr. Buckley said. “It really remains to be seen whether women tend to accumulate tau faster in the brain than men, or whether this is simply a one-shot phenomenon that we see in observational studies at the baseline.”
“One could really flip this finding on its head and suggest that women are truly resilient to the disease,” she continued. “That is, they can hold much more tau in their brain and remain well enough to be studied, unlike men.”
Among the study’s limitations is that the data were collected at a single time point and did not include information on subsequent Alzheimer’s diagnosis or cognitive decline.
“It is important to remember that the participants in this study were not as representative of the general population in the United States, so we cannot extrapolate our findings to women from a range of socioeconomic, racial and ethnic backgrounds or education levels,” she said.
The study’s observational design left researchers unable to demonstrate causation. What’s more, the findings don’t support the assertion that hormone therapy may protect against AD, Dr. Buckley added.
“I would more confidently say that evidence from our work, and that of many others, seems to suggest that HT initiated around the time of menopause may be benign – not providing benefit or risk, at least in the context of Alzheimer’s disease risk,” she said.
Another important takeaway from the study, Dr. Buckley said, is that not all women are at high risk for AD.
“Often the headlines might make you think that most women are destined to progress to dementia, but this simply is not the case,” Dr. Buckley said. “We are now starting to really drill down on what might elevate risk for AD in women and use this information to better inform clinical trials and doctors on how best to think about treating these higher-risk groups.”