Published on: August 29, 2020
by Jessica Shugart for AlzForum:
As a group, women may be more likely than men to develop Alzheimer’s disease, but once they have it, they cope better and live longer. A study published August 26 in Science Translational Medicine credits a second X chromosome with some of that resilience—at least in mice. “This paper is an exciting and novel examination of the mechanisms that underlie sex differences in AD risk/resilience,” commented Rachel Buckley of Massachusetts General Hospital in Boston.
Scientists led by Dena Dubal at the University of California, San Francisco, studied a menagerie of mice with X, XX, XY, and XXY chromosomes, some of whom lacked gonads to avoid complications of sex hormones. The scientists found that two X chromosomes made for a longer life with less memory loss in mice with amyloidosis. The researchers pinned part of the protection on Kdm6a, an X chromosome gene that encodes a histone demethylase. Kdm6a evades X-inactivation, a process whereby most of the genes on one of a female’s two X chromosomes get silenced. Separately, the researchers found that people who carry a genetic variation that boosts expression of the gene slide more slowly on cognitive tests during aging and in preclinical AD.
The findings underscore the importance of genes on the X chromosome—typically overlooked in genetic studies—in shaping resilience to AD, Dubal told Alzforum. A mechanistic understanding of resilience may even point to therapeutic strategies that could benefit people regardless of how many Xs they have, she said.
The time between diagnosis and death varies markedly among people who get AD, ranging from five to 20 years (Jun 2020 news). Some people even remain cognitively normal for some years despite having large amounts of amyloid plaques and neurofibrillary tangles in their brains. What explains this resilience? Genetic studies are starting to uncover variants that boost resilience (Jul 2020 conference news).
Sex could play a role, too, as some studies suggest that men with AD succumb to the disease faster than women, or that men suffer more neurodegeneration and cognitive decline than women with a similar tangle burden (Lapane et al., 2001; Ossenkoppele et al., 2020; Digma et al., 2020). The literature is mixed, however, with some studies suggesting that women decline faster than men, especially in the later stages of AD (Aug 2018 conference news; Buckley et al., 2018).
Reasoning that sex differences could uncover novel mechanisms of resilience, co-first authors Emily Davis and Lauren Broestl started with a meta-analysis to confirm that there is a relationship between sex and AD duration. They integrated data from 16 longitudinal studies that measured the duration of disease after onset, and found that, overall, men died 62 percent sooner than women with the disease.
Similarly, in the hAPP mouse model of amyloidosis, males died earlier than females. This was true even when the researchers removed the ovaries or testes from the mice at 10 weeks of age, suggesting that this difference was not due to circulating sex hormones. In addition to dying young, male hAPP mice were more impaired than females on learning and memory tests despite having similar levels of plaques, soluble Aβ, and phospho-tau in the hippocampus. After 2 years of age, females had slightly more plaques than males.
How did female hAPP mice outperform and outlast their male counterparts? Davis and colleagues focused on the role of sex chromosomes. First, the researchers used the Four Core Genotypes (FCG) mouse model, in which the Sry gene is deleted from its typical location on the Y chromosome. Sry dictates development of the testes and male characteristics. The researchers moved the Sry gene to an autosomal location in some of the mice; this maneuver allowed them to control the development of male sex independently of the Y chromosome. They generated four sex genotypes: XX or XY, each with ovaries, i.e., -Sry, or testes, i.e. +Sry. The researchers generated hAPP mice with all four sex genotypes. They removed their gonads at around 3 months of age to remove the influence of sex hormones beyond development.
They found that hAPP mice with two X chromosomes lived longer than their XY counterparts, regardless of whether they expressed Sry. Similarly, XX-hAPP mice had better memory than XYs, regardless of whether they initially developed ovaries or testes.
Mouse Gender Jungle
Did XY, i.e., typical male hAPP mice flounder due to the presence of their Y, or absence of a second X? To answer this question, the researchers crossed hAPP mice to yet another gender model, XY* mice, which arise from a recombination snafu during meiosis. Offspring of XY* males and XX females produce four sex genotypes: XX and XO mice, both with ovaries, and XY and XXY mice, both with testes. XO mice have only a single sex chromosome. The researchers found that regardless of the presence of a Y chromosome, ovaries, or testes, hAPP mice with two X chromosomes, including the XXYs, lived longer and performed better on memory tests than mice with a single X, i.e., the X0s and XYs. The findings suggested that having two X chromosomes, as opposed to lacking a Y, conferred resilience to Aβ pathology.
But how does that second X chromosome do that? In female mammalian cells, X-inactivation silences most genes on one of the X chromosomes to ensure their expression is not doubled. However, a few genes manage to elude this epigenetic crackdown, riffing off transcripts from both Xs. One such gene—in both mice and people—is Kdm6a (Greenfield et al., 1998). Also called Utx, it encodes a histone demethylase, enzymes that open up chromatin to activate transcription. People with loss-of-function mutations in the gene develop Kabuki syndrome, a disorder that causes intellectual disability. Mice without the gene have deficits in synaptic plasticity and memory (Miyake et al., 2013; Tang et al., 2017).
The researchers confirmed that KDM6A was transcribed from both X chromosomes in female mouse neurons, and that the female hippocampus had 30-50 percent more of the mRNA and protein than did the male hippocampus. In subsequent series of experiments, the researchers found that boosting KDM6A expression in the brains of male hAPP mice relieved memory deficits. Overexpressing KDM6A in primary neurons from male mice shielded the cells from Ab oligomer toxicity, while dampening its expression in female neurons rendered the cells more vulnerable. In all, the data suggested that, in the mouse brain, KDM6A explained part of the female resilience to Ab pathology, and that raising its expression could protect neurons from it.
To check whether this could be relevant in the human brain, the scientists consulted three public datasets that catalog gene expression in postmortem brain samples. From a total of more than 500 subjects, they found higher expression of KDM6A in the temporal cortex, parahippocampal gyrus, and superior temporal gyrus in people with AD than in controls. KDM6A expression did not differ between cases and controls in brain regions typically spared in AD, such as the cerebellum. Dubal hypothesized that this increased expression in AD brain regions could reflect a compensatory response. In keeping with their double dose of the gene, KDM6A expression was also higher in women, with or without AD.
Besides the doubling of expression due to a second copy of the gene, there could be other types of genetic variation that boost KDM6A expression. To check for this, the researchers turned to the Genotype-Tissue Expression (GTEX) project, an online portal with gene expression and genome sequencing data on tissues from nearly 1,000 people. They searched for variants associated with elevated KDM6A expression in the brain, and, sure enough, identified a common variant, rs12845057, whose minor allele, an adenosine, is carried by roughly 14 percent of the global population and associates with more KDM6A expression in the brain.
To see if this variant associated with cognitive resilience, the researchers looked for it among 778 volunteers in the Alzheimer’s Disease Neuroimaging Initiative (ADNI). The KDM6A minor allele was equally distributed among cognitively normal people, those with mild cognitive impairment, and AD, suggesting it did not alter a person’s risk of getting AD. But when considering men and women combined, the researchers found that 78 people with one copy of the allele declined more slowly on the mini mental state examination (MMSE) than did 692 noncarriers. The eight women in this sample who had two copies of the variant even seemed to improve their scores over time (see image above). When the researchers limited their analysis to men, the allele had no effect. Dubal said that this is likely due to lack of statistical power, exacerbated by the fact that men can only carry a single copy of the gene.
Dubal’s lab is investigating how KDM6A protects cultured neurons from Aβ toxicity, and whether the protein shields against insults from tau, α-synuclein, ApoE4, and age-related cellular stress. She plans to investigate whether other X-inactivation escapees in the human brain are also involved in resilience. “If we understand what makes one sex more resilient than another, then maybe we can harvest that knowledge and apply it to both sexes,” she said.
Buckley noted that KDM6A’s escape from X-inactivation is likely to have different consequences in different cell types, given that its expression in T cells was linked to autoimmunity in women (Itoh et al., 2019). “This won’t be a one-size-fits-all risk gene,” she cautioned.
Why has KDM6A not turned up in the large genetic association studies of AD? Michelle Mielke of the Mayo Clinic in Rochester, Minnesota, noted that most GWAS have historically excluded the sex chromosomes. “This study highlights the need to examine genes on the X and Y chromosomes for risk and resiliency to AD and other dementias,” Mielke wrote. She added that while the study clearly establishes a role for sex chromosomes, as opposed to hormones, in resilience in the hAPP mouse model, it does not examine the potential influence of hormonal treatments on this resilience.
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