In Brain With Christchurch Mutation, More ApoE3 Means Fewer Tangles

Series - Alzheimer's Association International Conference (AAIC) - 2022:

Like so many others in her extended family with the misfortune of inheriting the pathogenic E280A variant in presenilin-1, Aliria Rosa Piedrahita de Villegas seemed destined to descend into Alzheimer's dementia before reaching her 50th birthday. Instead, she lived with her memory intact into her 70s, and passed away a month before her 78th birthday with only mild symptoms of dementia. Piedrahita de Villegas’ remarkable resilience likely stemmed from her inheritance of not one, but two copies of ApoE3 Christchurch—a rare, and apparently protective, mutation in this AD risk gene. But how did this apolipoprotein variant defend Piedrahita de Villegas against AD for nearly three decades? Some clues come from the postmortem examination of her brain, published in Acta Neuropathologica on July 15. Francisco Lopera of the Universidad de Antioquia in Medellín led the work, together with Kenneth Kosik of the University of California, Santa Barbara, and Yakeel Quiroz of Massachusetts General Hospital in Boston.

  • Postmortem of PSEN1-E280A carrier protected by ApoE3 Christchurch variant published.
  • Tau tangles crowded her occipital cortex, but spared frontal cortex.
  • ApoE expression higher in spared regions of the brain.
  • Microglia with distinct inflammatory profile seen in regions with tangles.

Speaking at the Alzheimer’s Association International Conference, held July 31-August 4 in San Diego, California, Lopera drew parallels between Piedrahita de Villegas from Angostura, Colombia, and Auguste Deter, a woman from Frankfurt, Germany, who died in 1906 at the age of 55 with what would later become known as Alzheimer’s disease, her brain riddled with plaques and neurofibrillary tangles. “These two women are most important in the study of Alzheimer’s disease,” Lopera said. “Auguste showed us how the brain is affected in AD, and Aliria showed us how the brain can be protected.”

The study identified an enormous burden of Aβ plaques, but an odd distribution of tau tangles. Piedrahita de Villegas’ frontal cortex was relatively free of tangles, while her occipital cortex was replete with them. Single-cell transcriptomics revealed that, intriguingly, regions of her brain that had managed to fend off tangles expressed more ApoE. Hinting at the power of the Christchurch variant, the researchers noticed that neurons considered exquisitely vulnerable to tau-induced neurodegeneration were doing fine in the frontal cortex and hippocampus. Tangle-free brain regions contained homeostatic astrocytes, while the occipital cortex was crawling with microglia of a distinctively inflammatory profile. Together, the findings suggest that the protective ApoE variant somehow severs an inflammatory cord linking Aβ plaques to tau tangles, buying at least this one homozygous carrier decades of life without dementia.

“This global resilience in the face of one of the most aggressive forms of AD underscores the apparently profound effect the APOE gene has on the time course of the AD pathophysiological process,” wrote Jacob Vogel of the University of Pennsylvania in Philadelphia, Rik Ossenkoppele of Amsterdam University Medical Center in the Netherlands, and Oskar Hansson of Lund University, Sweden, in a joint comment to Alzforum.

To neuropathologist Lea Grinberg of the University of California, San Francisco, the study speaks to the enduring importance of autopsy examination. “Methodological improvements in the last years have greatly enhanced the potential and impact of research using the human postmortem brain,” she wrote. “Hopefully studies like this will help to highlight the value of investing in autopsy programs in ADRD research.”

Affected, Protected. Auguste Deter (left) was the first woman to be diagnosed with what would later become known as Alzheimer’s disease. Aliria Rosa Piedrahita de Villegas was protected from the pathogenic AD mutation she carried by her unlikely inheritance of two copies of ApoE3-Christchurch mutation. [Courtesy of Francisco Lopera.]

At the age of 73 and again at 75, Piedrahita de Villegas traveled to Boston to undergo brain scans. Much to the shock of researchers at the time, her brain contained a higher amyloid burden than that of any other carrier of the PSEN1-E280A variant, also known as the Paisa mutation, within the extended Colombian kindred (Nov 2019 news). However, tau-PET scans revealed a minuscule tangle burden, confined mostly to the medial temporal lobe and occipital cortex. Similar to a different protective variant—ApoE2—ApoE3Ch poorly bound to low-density lipoprotein receptors, and hardly latched on to heparin sulfate proteoglycans (HSPGs). These extracellular matrix proteins are thought to facilitate tau propagation, offering a potential mechanism underlying the scant tau tangle burden in her brain.

Piedrahita de Villegas developed short-term memory loss at the age of 72, and was diagnosed with mild dementia at age 75. She died from malignant melanoma at 77. Within 200 minutes of her passing away at her home, her brain was removed and carefully prepared for neuropathological examination. This examination was done by scientists across the world. Co-first author Diego Sepulveda-Falla of University Medical Center Hamburg-Eppendorf, Hamburg, Germany, was one of them. A neuropathologist who has examined the brains of many Paisa mutation carriers, Sepulveda-Falla recalled what he felt when Piedrahita de Villegas’ extremely unlikely—and serendipitous—combination of mutations was first discovered several years ago. “I was surprised that she existed at all,” Sepulveda-Falla said. “Aliria and her family were very aware of how unique she was.” 

What did the neuropathologists find in her brain? First, in keeping with her mild dementia, her brain was smaller than a typical 77-year-old’s, leading the Colombian scientists who removed the brain to classify it as having severe global atrophy. Atherosclerosis marred all major blood vessels. Co-first author Justin Sanchez of Massachusetts General Hospital in Boston and colleagues used immunohistochemistry to measure Aβ and tau pathology in 17 brain regions. In agreement with PET scans taken during life, Sanchez detected abundant Aβ plaques throughout the neocortex.

The distribution of tau tangles told a different story. Due to extensive p-tau accumulation in the isocortex, the researchers classified the pathology as Braak stage VI; alas, the overall distribution of tangles did not fit into the Braak staging model. The occipital cortex, which is typically affected in later Braak stages, had the greatest density of tangles, followed by the hippocampus and amygdala, which tend to deposit tangles much earlier on. Most strikingly, the frontal cortex, which typically becomes invaded by tangles in Braak stages V-VI, contained negligible tau pathology. Sepulveda-Falla noted that although tangles were found in the entorhinal cortex and hippocampus, their burden in these early Braak regions was mild compared to that observed in other Paisa mutation carriers.

This occipital-leaning distribution of tau pathology evokes a “posterior” subtype of AD, which is associated with a slower course of cognitive decline. It was recently reported by Vogel, Ossenkoppele, and Hansson (Apr 2021 news). “Future work will be needed to ascertain whether this similarity is a coincidence or if there is a consistent link between occipital phenotypes and slower clinical progression,” they wrote.

Aβ and Tau—A Severed Tie? Amyloid plaques (left panels) crowded Aliria Rosa Piedrahita de Villegas’ frontal cortex, and dotted her hippocampus and occipital cortex. P-tau aggregates were nearly absent from her frontal cortex, but abundant in her hippocampus and even more so in the occipital cortex. See map of p-tau regional intensity on right. [Courtesy of Sepulveda-Falla et al., Acta Neuropathologica, 2022.]

The scientists conducted a detailed accounting of the different morphological forms of each neuropathology throughout the brain. One result of this analysis? The occipital cortex was the lone neocortical region that contained Aβ plaques within blood vessels, aka cerebral amyloid angiopathy. CAA also cropped up in the amygdala and, where found, its extent correlated with the burden of tau pathology. This narrow range of CAA stands in contrast to the much broader distribution of CAA spotted in most other Paisa mutation carriers. The link between CAA and tau meshes with another recent postmortem study that tied CAA to tau and cognitive decline (Rabin et al., 2022).

Besides looking at Aβ plaques and tau tangles, the researchers also used immunohistochemistry to measure protein levels of ApoE and of glial markers. ApoE expression took on a plaque-like pattern, and was most intense in the hippocampus and occipital cortex. The microglial marker Iba1, along with the activation marker CD68, were both cranked up in the occipital relative to frontal cortex. Conversely, TMEM119, a marker of homeostatic microglia, gave the highest signal in the frontal cortex. The ratio of CD68 to TMEM119 was higher in the occipital relative to the frontal cortex, suggesting that microglia residing in occipital regions, where the tangles also were, tended to be more riled up than those in frontal regions.

ApoE3-Ch: Protector of Vulnerable Neurons, Blocker of Tangles?
To explore which molecular mechanisms might have been at play in different cell types, scientists in Kosik's lab conducted single-nuclei RNA sequencing from cells in the hippocampus, and frontal and occipital cortices. This analysis brought out different transcriptional clusters of excitatory and inhibitory neurons. Strikingly, a transcriptional cluster emerged of excitatory neurons marked by high expression of RORB, but only in the hippocampus and frontal cortex. RORB+ excitatory neurons earlier this year were reported to be exquisitely vulnerable to tau-induced neurodegeneration (Jan 2021 news).

Grinberg, a senior author on that study, commented that both studies highlight the importance of this neuronal class for AD pathogenesis. That these supposedly vulnerable neurons had been alive and well in the brain of a person with mild AD could reflect protection by the ApoE3-Ch variant, Sepulveda-Falla suggested. These RORB+ neurons abundantly expressed genes involved in neurodevelopment, while containing a dearth of transcripts encoding genes involved in synaptic function.

What about glia? Working with a single brain, the researchers had limited numbers of cells for the analysis, relying on 364 microglia, 598 astrocytes, and 1,250 oligodendrocytes to tell the tale of differential gene expression across three brain regions. They identified a single transcriptional cluster for each cell type, and then compared the expression levels of individual genes in each cell type by region. This identified subsets of genes that were differentially expressed in the frontal and occipital cortices. ApoE was prominent among them. In both astrocytes and microglia, ApoE expression was highest in the frontal cortex and lowest in the occipital cortex, with intermediate expression in the hippocampus. In other words, the more ApoE glia expressed in a given region, the less tau tangled there.

Curiously, another study, by Victor Montal of Hospital de la Santa Cru in Spain and Jorge Sepulcre-Bernad at Massachusetts General Hospital in Boston, recently uncovered the opposite association, reporting that regions of the brain with higher ApoE expression contained more tangles (Jul 2022 news). However, Montal and Sepulcre-Bernad believe that findings from the two studies complement each other. Both tied ApoE expression to tau accumulation. While ApoE3-Ch warded off tau tangles, common ApoE variants might beckon them to form. The MGH study did not distinguish between ApoE variants.

Sepulveda-Falla and colleagues also checked whether other genes might follow ApoE’s regional expression pattern in each glial cell type. For astrocytes, a cadre of homeostatic genes tracked closely with ApoE expression, such that astrocytes residing in the frontal cortex had a more homeostatic profile than astrocytes in the occipital cortex.

In microglia, genes involved in immune regulation tracked with ApoE expression. Focusing on microglia, the researchers found distinct profiles of immune genes expressed in the frontal and occipital cortices. These profiles did not match perfectly with those reported in mice. That said, microglia in Piedrahita de Villegas’ occipital cortex assumed a profile resembling a chronic inflammatory state reported in a mouse model of amyloidosis, while those in her frontal cortex expressed a suite of genes involved in acute immune responses (Sep 2017 news).

Sepulveda-Falla said it’s unclear what these different microglial profiles mean, and whether they were beneficial or harmful given the distinct neuropathological issues confronting microglia in each region. In the frontal cortex, microglia were dealing with extensive Aβ plaques, while in the occipital cortex, the cells faced both plaques and tangles.

Piedrahita de Villegas did ultimately develop characteristic symptoms of AD. That she did despite her atypical tangle distribution raises questions about the relationship between tau pathology and the clinical manifestations of AD. “Whatever was driving cognitive impairment in this case seems not to be defined by the distribution of tau pathology,” Sepulveda-Falla said. “Pathology is not the whole story.” He also wondered what role Piedrahita de Villegas’ malignant melanoma may have played in bringing on her Alzheimer's symptom. Though there were no tumors in her brain, systemic health crises are known to speed up the timeline of AD and other dementias.

Sepulveda-Falla and other neuropathologists are still carrying out more extensive comparisons of tau pathology in this unique brain to brains of AD cases who had other combinations of ApoE variants, including heterozygous carriers of ApoE3Ch. For more on the life and death of this remarkable woman, see Jennie Erin Smith's NYT story. —Jessica Shugart

Comments

  1. This is a fascinating report of an individual harboring a rare combination of genetic mutations/variants (i.e., PSEN1 E280A + APOE3 Christchurch homozygosity). These genetic features together putatively resulted in a phenotype of extreme resilience to AD, as symptom onset in this patient was delayed by three decades compared to other PSEN1 E280A mutation carriers. The fact that such a case was not only identified, but also deeply phenotyped with multimodal longitudinal imaging and cognitive testing during life, as well as neuropathological and multi-omic analysis at death, is a testament to the impressive ongoing international collaborations between these research centers.

    While this study is chock-full of fascinating insights and analyses, there is an emphasis placed on the patient's striking occipital-predominant pattern of tau pathology. Both in vivo and neuropathological analyses confirm prominent occipital tau pathology with a relative sparing of frontal and medial parietal regions, which are brain areas that are typically compromised in PSEN1-E208A mutation carriers. The finding of primarily occipital tau expression in a resilient individual is reminiscent of our recent tau subtyping work, which described a “posterior” subtype with prominent occipital tau-PET binding and an attenuated rate of cognitive decline relative to the other tau subtypes (Vogel et al., 2021). 

    Future work will be needed to ascertain whether this similarity is a coincidence or if there is a consistent link between occipital phenotypes and slower clinical progression. The present finding potentially linking occipital CAA to occipital tau is in accordance with a recent postmortem analysis (Rabin et al., 2022) and represents a compelling lead worth pursuing further.

    However, the focus on the occipital predominance of tau pathology somewhat overshadows what, in our opinion, might be the main message of this work: This individual displayed reduced tau pathology relative to other PSEN-E208A carriers in almost every region. Close visual inspection of figures 1C and 2D seem to indicate that this is true both in terms of ante-mortem longitudinal tau PET accumulation and the total amount of tau pathology at death. This is in spite of having excessive and widespread brain amyloid deposition consistent with the genotype.

    This global resilience in the face of one of the most aggressive forms of AD underscores the apparently profound effect the APOE gene has on the time course of the AD pathophysiological process.

    This paper includes many other interesting analyses. We look forward to future work comparing results from the omics analyses to those of other individuals carrying different APOE haplotypes and causal AD mutations. We hope these analyses will ultimately help us achieve a better understanding of AD pathogenesis and may eventually lead to the identification of targets for boosting resilience against AD pathology.

    References:

    Vogel JW, Young AL, Oxtoby NP, Smith R, Ossenkoppele R, Strandberg OT, La Joie R, Aksman LM, Grothe MJ, Iturria-Medina Y, Alzheimer’s Disease Neuroimaging Initiative, Pontecorvo MJ, Devous MD, Rabinovici GD, Alexander DC, Lyoo CH, Evans AC, Hansson O. Four distinct trajectories of tau deposition identified in Alzheimer's disease. Nat Med. 2021 May;27(5):871-881. Epub 2021 Apr 29 PubMed.

    Rabin JS, Nichols E, La Joie R, Casaletto KB, Palta P, Dams-O'Connor K, Kumar RG, George KM, Satizabal CL, Schneider JA, Pa J, Brickman AM. Cerebral amyloid angiopathy interacts with neuritic amyloid plaques to promote tau and cognitive decline. Brain. 2022 Aug 27;145(8):2823-2833. PubMed.

    View all comments by Jacob Vogel View all comments by Rik Ossenkoppele View all comments by Oskar Hansson

  2. This is a very important case report because it brings the neuropathological findings of a woman who had a PSEN1 mutation with extremely high presented and expected age of clinical onset in the fifth decade, who remained practically asymptomatic until her eighth decade of life. Previous work showed that a rare ApoE3 Christchurch variant could be protecting her against dementia. Now this careful neuropathological examination adds to the history by showing that, indeed, the levels of APOE in her brain are different from those in other members of this family who have come to autopsy, and by confirming an unexpected distribution of tau pathology.

    In 2020, my and Martin Kampmann’s labs at UCSF showed that RORB+ excitatory neurons have a higher vulnerability to AD pathology than other excitatory neurons. This work corroborates our findings and adds a spotlight on the importance of this neuronal class for AD pathogenesis.

    Postmortem examination remains the gold standard for diagnosing neurodegenerative diseases. Methodological improvements in the last years have greatly enhanced the potential and impact of research using the human postmortem brain. Hopefully studies like this will help to highlight the value of investing in autopsy programs in ADRD research.

    View all comments by Lea T. Grinberg

  3. This is an exciting follow-up report from the homozygote APOE3ch case reported in 2019 (Nov 2019 news). Notably, the authors highlight the histological differences in the pattern of tau accumulation and the single-cell gene-expression profiles along the cortical mantle compared to other AD forms.

    The study advances histological analyses to clarify how the Christchurch mutation might confer a protective effect against developing AD pathology—as the previous report had mainly focused on neuroimaging. The authors confirm that the APOE3ch case accumulates tau at a lower degree in most of the cerebral cortex, except for the occipital lobe, where they noted significant accumulations. These results align with recent findings (Jul 2022 news) in which we showed a tight spatial intersection between tau cortical spreading and the spatial gradient of APOE expression. As the occipital cortex displays low topological APOE expression, it is not surprising that the potential beneficial effect of the APOE Christchurch mutation is sparse in that cerebral region.

    Additionally, the authors highlight the selective vulnerability of excitatory RORB+ neurons, which significantly differ from expression profiles in other AD presentations.

    Understanding the mechanism by which APOE-specific mutations might affect genetic profiles and AD pathology in individual cases is essential to grasp the big picture of AD risk. Whereas the current results highlight the genetic co-expression between APOE and other genes in a cell-specific manner, it would be necessary to further evaluate such associations in APOE3 non-Christchurch participants.

    In summary, this study highlights the pathological implications of specific forms of APOE and opens new avenues for developing APOE-centered treatments in AD.

    View all comments by Victor Montal View all comments by Jorge Sepulcre-Bernad

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References

Mutations Citations

  1. PSEN1 E280A (Paisa)

News Citations

  1. Can an ApoE Mutation Halt Alzheimer’s Disease?
  2. Forget Typical Alzheimer's: AI Finds Four Types.
  3. Selective Vulnerability News: RORB Neurons Are First Victims of Tangles
  4. Does Tangle Progression Follow ApoE Expression?
  5. ApoE and Trem2 Flip a Microglial Switch in Neurodegenerative Disease

Paper Citations

  1. Rabin JS, Nichols E, La Joie R, Casaletto KB, Palta P, Dams-O'Connor K, Kumar RG, George KM, Satizabal CL, Schneider JA, Pa J, Brickman AM. Cerebral amyloid angiopathy interacts with neuritic amyloid plaques to promote tau and cognitive decline. Brain. 2022 Aug 27;145(8):2823-2833. PubMed.

External Citations

  1. Jennie Erin Smith's NYT story

Further Reading

Papers

  1. Wardell MR, Brennan SO, Janus ED, Fraser R, Carrell RW. Apolipoprotein E2-Christchurch (136 Arg----Ser). New variant of human apolipoprotein E in a patient with type III hyperlipoproteinemia. J Clin Invest. 1987 Aug;80(2):483-90. PubMed.

Primary Papers

  1. Sepulveda-Falla D, Sanchez JS, Almeida MC, Boassa D, Acosta-Uribe J, Vila-Castelar C, Ramirez-Gomez L, Baena A, Aguillon D, Villalba-Moreno ND, Littau JL, Villegas A, Beach TG, White CL 3rd, Ellisman M, Krasemann S, Glatzel M, Johnson KA, Sperling RA, Reiman EM, Arboleda-Velasquez JF, Kosik KS, Lopera F, Quiroz YT. Distinct tau neuropathology and cellular profiles of an APOE3 Christchurch homozygote protected against autosomal dominant Alzheimer's dementia. Acta Neuropathol. 2022 Sep;144(3):589-601. Epub 2022 Jul 15 PubMed.

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