12 Mar 2022

Tau aggregation lies at the heart of many neurodegenerative diseases, but why this protein goes rogue remains mysterious. At the Tau2022 conference, held virtually February 22-23, speakers homed in on the role of the tau haplotype in neurodegeneration. The larger region of the genome that includes the tau gene, MAPT, comes in two forms, H1 and H2, with the former associated with neurodegenerative disease risk, and the latter protective. Because the region contains around a dozen genes, the causal factors are unknown. Kathryn Bowles at the Icahn School of Medicine, Mount Sinai, New York, described a multicenter effort to dissect the source of this risk. Although the project is in its infancy, it has already turned up previously unknown differences between the haplotypes.

Meanwhile, Peter Heutink at the German Center for Neurodegenerative Diseases in Tübingen, suggested that one culprit in H1 might be weaker binding of a master regulator of oxidative stress, which leaves cells more vulnerable to ferroptosis, a type of cell death caused by iron toxicity.

H2 is an evolutionarily ancient 1.5 MB inversion of H1 that contains duplicated genes. Because of the inversion, H1 and H2 are unable to recombine with each other, and so these haplotypes are inherited separately as blocks. Researchers have long known that H1 associates with several primary and secondary tauopathies, including progressive supranuclear palsy, corticobasal degeneration, Parkinson’s disease, and APOE4-negative Alzheimer’s disease (Sep 2005 news; Mar 2019 news; Strickland et al., 2020). 

Mirror Images. The H2 haplotype (bottom) is a 1.5 MB inversion of H1 (top) and also contains some duplicated genes. This large region, which contains the MAPT gene, is inherited as a block. [Courtesy of Bowles et al., 2019.]

At Tau2022, Bowles described a large consortium, called the “Tau Centers Without Walls.” It is trying to track down the source of this risk by making iPSC lines from people with each haplotype and performing omics analyses. Bowles, working in the lab of Alison Goate at Mount Sinai, has made lines from people with European ancestry. The H2 inversion is most common in this population, occurring in 10 to 36 percent of people, depending on the exact group studied. It is less common in African populations and nearly nonexistent in South Asians. In fact, some have suggested this inversion is a legacy from Neanderthal ancestors (Hardy et al., 2005).

In the European lines, Bowles identified previously unreported differences in the noncoding, antisense versions of some genes. These may serve a regulatory function, she noted. Expression of the noncoding variants varied by brain cell type as well as haplotype. For example, MAPT antisense was more highly expressed in H1 than H2 neurons, but no differently expressed in other brain cell types. On the other hand, an antisense version of the chromatin-modifying gene KANSL1 was up in all three H2 cell types examined: neurons, astrocytes, and microglia.

Bowles also identified numerous splicing differences between genes in H1 and H2. The biggest was for KANSL1, where H2 cells tended to retain an intron. These findings have to be confirmed in human brain, as the immature neurons generated from iPSCs can splice genes differently than do mature neurons.

Overall, H2 neurons and astrocytes suppressed genes involved in protein translation compared to H1 cells, whereas H2 microglia boosted them. “H2 is associated with differences in splicing that affect protein localization and trafficking,” Bowles concluded. She expects disease risk will ultimately involve far more genes than just MAPT. “I believe there are effects in multiple genes in both neurons and glia that all interact to contribute to influencing disease risk,” Bowles wrote to Alzforum.

Heutink focused on one particular aspect of risk, i.e., the susceptibility of H1 cells to oxidative stress. He generated neural precursor cells from H1 and H2 iPSCs, then allowed them to mature into neurons in culture. When antioxidants were omitted from the culture medium, H1 neurons developed swollen axons and died, while H2 neurons stayed healthier. Heutink believes this is due to the transcription factor NRF2, a master regulator of the oxidative stress response. When a cell experiences oxidative stress, NRF2 enters the nucleus and binds an antioxidant response element (ARE) upstream of MAPT. Notably, this ARE is mutated in the H1 haplotype, resulting in weaker binding of NRF2 (Wang et al., 2016). 

Heutink screened a library of 1,430 chemical compounds for any that could improve the oxidative stress response. He found 87 compounds that made H1 neurons more viable; about half generated consistent dose-response curves, suggesting a robust effect. Reviewing the literature on these compounds, Heutink found a number of them were involved in ferroptosis, a type of cell death that results from iron accumulation and oxidative stress (for review, see Li et al., 2020). These included curcumin, ethinyl estradiol, deferasirox, idebenone, lapatinib, zileuton, and Vitamin E.

Did the H1 neurons die by ferroptosis? Indeed, staining techniques confirmed that dying H1 neurons displayed features of ferroptosis, such as axon swelling, mitochondrial damage, and the presence of lipid peroxidase. In addition, death swept through the cultures in a wave, which is characteristic of ferroptosis. In contrast, apoptosis hits cells in a random pattern. Also, compounds that suppressed apoptosis did not prevent neuron death.

Heutink is now examining downstream effects of these compounds on specific genes in the H1 haplotype, including MAPT and KANSL1, to determine the mechanisms involved in the increased vulnerability to ferroptosis. In answer to an audience question, he said he plans to check for iron accumulation in the dying cells as well.

Bowles noted that several different MAPT mutation lines, all of which have the H1 haplotype, also show increased susceptibility to oxidative stress compared with wild-type H1 tau. “This could demonstrate a similar risk-associated mechanism in both sporadic and familial FTD and PSP,” she told Alzforum.—Madolyn Bowman Rogers

Comments

1. • Debomoy Lahiri
     Indiana University School of Medicine
   • Posted: 13 Apr 2022

   The role of the tau haplotype in neurodegeneration is critical, and this recent work on different biological effects of haplotypes is intriguing. The H2 haplotype of the MAPT gene region of chromosome 17 has long been known to confer protection from neurodegenerative disorders such as AD; however, its overall differences in neurological disorders are complex. H2 also associates with a reduced risk of PD, association with 17q21.31 microdeletion syndrome, and earlier age of onset and faster progression for FTD.

   Mechanisms of such differences have recently been proposed by the Tau Centers Without Walls project. Specifically, H2 associates with reduced ferroptosis and potentially noteworthy differences in transcription of natural antisense noncoding RNAs (ncRNAs) and splicing variation from genes in H1.

   The H2 haplotype can be most simply viewed as an inversion of a region of Chr 17. The H1 sequence is ~1.5Mb in length, while H2 is ~1.7Mb. H2 is most common in European populations, rare in Africans, and generally unknown in Asians. While this has led to speculation that H2 would be a “Neanderthal” relic lineage, there is no specific evidence for this. In addition to the chromosomal inversion, the H2 MAPT promoter sequence has several potentially important differences from the H1 MAPT promoter. In particular, eight potential transcription factor sites in the H1 MAPT promoter are absent in H2, while eight different transcription factor sites are present in H2 MAPT that are absent in H1 (Maloney and Lahiri, 2012).

   Of particular interest, the Tau Center identified that H1-containing iPSCs were more prone to ferroptosis than were H2 iPSC. This potentially vital discovery links an outcome with broad effect (ferroptosis) to a specific and well-defined genetic difference, particularly given the link between ferroptosis and AD (Rogers and Cahill, 2020).

   Talking about ncRNAs, a specific link between microRNA regulation (e.g., miR-346) of APP dependent on ferrohomeostatic status (Long et al., 2019) suggests potential interaction between H1/H2 and broader neurometabolism in the etiology and progression of neurodegenerative disorders. Likewise, it would be interesting to test how tau's regulation by a specific microRNA (e.g., miR-298, Chopra et al., 2020) depends on ferrohomeostatic status.

   Studies that examine potential interactions of H1/H2 haplotype and iron-sensitive aspects of APP regulation could help zero in on “cross-condition” treatments, particularly in early stages, of “pre-breakdown,” where misregulation of fundamental processes like ferrohomeostasis could lead to or prevent clinical presentation of a full degenerative condition.

   See also Tau haplotypes hint at transcriptional changes, ferroptosis. 

   References:

   Maloney B, Lahiri DK. Structural and functional characterization of H2 haplotype MAPT promoter: Unique neurospecific domains and a hypoxia-inducible element would enhance rationally targeted tauopathy research for Alzheimer's disease. Gene. 2012 Jan 30; PubMed.

   Rogers JT, Cahill CM. Iron-responsive-like elements and neurodegenerative ferroptosis. Learn Mem. 2020 Sep;27(9):395-413. Print 2020 Sep PubMed.

   Long JM, Maloney B, Rogers JT, Lahiri DK. Novel upregulation of amyloid-β precursor protein (APP) by microRNA-346 via targeting of APP mRNA 5'-untranslated region: Implications in Alzheimer's disease. Mol Psychiatry. 2019 Mar;24(3):345-363. Epub 2018 Nov 23 PubMed.

   Chopra N, Wang R, Maloney B, Nho K, Beck JS, Pourshafie N, Niculescu A, Saykin AJ, Rinaldi C, Counts SE, Lahiri DK. MicroRNA-298 reduces levels of human amyloid-β precursor protein (APP), β-site APP-converting enzyme 1 (BACE1) and specific tau protein moieties. Mol Psychiatry. 2020 Jan 15; PubMed.

   View all comments by Debomoy Lahiri

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References

News Citations

1. Tau Shows Subtle Hints of Genetic Association 16 Sep 2005
2. Unequal: Some Tau Haplotypes Carry More Risk Than Others 22 Mar 2019

Paper Citations

1. Strickland SL, Reddy JS, Allen M, N'songo A, Burgess JD, Corda MM, Ballard T, Wang X, Carrasquillo MM, Biernacka JM, Jenkins GD, Mukherjee S, Boehme K, Crane P, Kauwe JS, Ertekin-Taner N, Alzheimer's Disease Genetics Consortium. MAPT haplotype-stratified GWAS reveals differential association for AD risk variants. Alzheimers Dement. 2020 Jul;16(7):983-1002. Epub 2020 May 13 PubMed. Correction.
2. Bowles KR, Pugh DA, Farrell K, Han N, TCW J, Liu Y, Liang SA, Qian L, Bendl J, Fullard JF, Renton AE, Casella A, Iida MA, Bandres-Ciga S, Gan-Or Z, Heutink P, Siitonen A, Bertelsen S, Karch CM, Frucht SJ, Kopell BH, Peter I, Park YJ, Crane PK, Kauwe JSK, Boehme KL, Höglinger GU, PART working group, International Parkinson’s Disease Genomics Consortium, Progressive Supranuclear Palsy Genetics Consortium, Charney A, Roussos P, Wang JC, Poon WW, Raj T, Crary JF, Goate AM. 17q21.31 sub-haplotypes underlying H1-associated risk for Parkinson’s disease are associated with LRRC37A/2 expression in astrocytes. bioRxiv. November 30, 2019 bioRxiv
4. Wang X, Campbell MR, Lacher SE, Cho HY, Wan M, Crowl CL, Chorley BN, Bond GL, Kleeberger SR, Slattery M, Bell DA. A Polymorphic Antioxidant Response Element Links NRF2/sMAF Binding to Enhanced MAPT Expression and Reduced Risk of Parkinsonian Disorders. Cell Rep. 2016 Apr 13; PubMed.
5. Li J, Cao F, Yin HL, Huang ZJ, Lin ZT, Mao N, Sun B, Wang G. Ferroptosis: past, present and future. Cell Death Dis. 2020 Feb 3;11(2):88. PubMed.

External Citations

1. Tau Centers Without Walls

Further Reading

News

1. Tau Gene Confirmed to Amp Up Alzheimer’s Risk and Neurodegeneration 11 Mar 2015
2. Genetics Tie ALS into the Frontotemporal Dementia Spectrum 19 Apr 2018
3. Tau2020: Meeting for Tauopathies Debuts Genetic Variants 3 Mar 2020