Debate Follow-Up: Can Toxic Gain-of-Function (GOF) in Amyloid Pathologies Be Defended?

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We recently had a very interesting debate around the mechanisms of toxicity in neurodegenerative diseases (NDDs) and whether it is mainly a gain-of-function (GOF) or loss-of-function (LOF) toxicity due to amyloid aggregation. Before the debate, the inspiring patient advocate Marina Noordegraaf (@Sparks4PD) asked me if the GOF and LOF proponents could switch sides during the debate and try to defend the other point of view. I apologized to her saying that it would be very difficult for me to defend GOF since the most common GOF defenses are counterfactual. Here I will list some of the common GOF defenses that we usually get whenever we discuss LOF in papers, reviews, seminars, or on Twitter.  

1.  Knock out/down animals of the amyloidogenic proteins show no phenotype, thus not supporting LOF

This is just not true. While there is never going to be a perfect animal model that accurately reflects all aspects of the human pathology (and this is true for both GOF and LOF), there are literally 10s of papers showing the devastating effects of knocking out/down Aβ, Tau, α-synuclein, PrPC, SOD-1, TDP-43, and many others involved in multiple NDDs. We cite many of them in our recent review (1) and they are cited in several other reviews as well (2–4).

2. Genetic evidence indicates higher levels of the amyloidogenic proteins in familial disease forms, supporting GOF

Again, this is just not true. Clinically, patients with familial forms of Alzheimer’s disease (AD), both due to mutations (5) or gene duplication (Down syndrome (6)), have lower levels of soluble Aβ42 in the CSF. The same is true for familial forms of Parkinson’s disease (PD) and Creutzfeldt-Jakob disease (CJD), where soluble levels α-synuclein and PrPC are lower in patients (7, 8). This is also the case in the sporadic forms of AD and CJD (9, 10). For PD, figure 1 is an excerpt from a recent review showing how the reduction in soluble levels of α-synuclein in PD patients is very consistent even across different meta-analyses (11).

Figure 1. Excerpt from Parnetti,L. et. al., 2019 (11). 

3.   Prions/amyloids can self-replicate structural/conformational information

These are incorrect terms to describe a phenomenon of phase transition. And this is not only because in science we need to use the most accurate terms possible, but also because these terms underlie the conceptual framework for understanding etiology and pathogenesis and for designing experiments and therapeutics. The terms “self-replication” and “phase transition” are not just semantically different, they are conceptually different as well. The “protein-only self-replication/propagation” hypothesis for example cannot accommodate the fact that non-proteinaceous surfaces can directly induce phase transition independent of any protein seeds/prions via heterogeneous nucleation, which can be mediated by membranes, nanoparticles and viruses (1). In the eyes of the “protein-only self-replication/propagation”, surfaces are mere “cofactors” to a more fundamental protein-based structural replication process (12), which is incorrect, since HEN takes place in complete absence of proteinaceous seeds. Moreover, the exclusionary focus on a “self-replicating/propagating protein-only” species led the field to concentrate mainly on one pathway of amyloid aggregation; seeded nucleation, ignoring the HEN pathway, which is biophysically more common and in many cases biologically more relevant. Failing to describe the phenomenon in the correct terms and account for all the possible mechanisms led to the unnecessary exclusion of many potential causes and therapeutic targets for amyloid aggregation, which includes membrane pathologies and infections, ultimately narrowing the chances of full understanding and clinical success. Incorrect concepts are not benign, and historical reasons alone cannot justify their continued use. 

Conclusions

Science is not always about finding a middle-ground between different theories, especially if one theory is in contradiction with the available evidence. Advances are usually made by replacing old theories with better new ones, and this starts by highlighting the flaws in the old theories. In the case of GOF, false claims that are used to defend GOF at the expense of LOF, such as the points stated above, should be called out clearly and frequently. If there is something that both GOF and LOF agree on, it is that amyloids are not toxic in themselves. If we add to this to the animal and clinical evidence (points 1 and 2) that is unjustly dismissed by GOF, LOF emerges as not only plausible, but probably the only way forward. LOF is dealing with the same genes and proteins as GOF; however, it takes all the available evidence into account without selection. Additionally, LOF doesn’t need to invent vague entities and mechanisms such as oligomers or prion-replication to overcome its own contradictions like GOF. Within LOF, the lack of amyloid toxicity or the spread of amyloid aggregation can be explained by straightforward and accurate biological and physicochemical mechanisms. Such mechanisms or not only more accurate (HEN for example), they allow a more complete understanding of the etiology and pathophysiology by incorporating all the possible causes and pathways of amyloid aggregation. Most importantly, LOF opens a clear path for testing and monitoring new therapeutics targeting these pathways in addition to replacement therapies that restore the reduced levels of the proteins to the normal levels.

A comparison between GOF and LOF is shown in table 1.

I think it’s time to stop blaming brain complexity, prolonged disease duration, clinical trial design for the lack of therapeutic success in NDDS and address the elephant in the room; GOF inconsistencies and contradictions. While the complexities are real, in science we cannot change the world to fit our theories, but we can change our theories to fit the world, and this is what needs to be done now. 

Table 1. GOF vs. LOF comparison

References

1. Malmberg,M., Malm,T., Gustafsson,O., Wright,A., Andaloussi,S. El and Ezzat,K. (2020) Disentangling the Amyloid Pathways : A Mechanistic Approach to Etiology. Front. Neurosci., 14, 1–11.

2. Kent,S.A., Spires-Jones,T.L. and Durrant,C.S. (2020) The physiological roles of tau and Aβ: implications for Alzheimer’s disease pathology and therapeutics Springer Berlin Heidelberg.

3. Benskey,M.J., Perez,R.G. and Manfredsson,F.P. (2016) The contribution of alpha synuclein to neuronal survival and function - Implications for Parkinson’s disease. J. Neurochem., 137, 331–359.

4. Saccon,R.A., Bunton-Stasyshyn,R.K.A., Fisher,E.M.C. and Fratta,P. (2013) Is SOD1 loss of function involved in amyotrophic lateral sclerosis? Brain, 136, 2342–2358.

5. Bateman,R.J., Xiong,C., Benzinger,T.L.S., Fagan,A.M., Goate,A., Fox,N.C., Marcus,D.S., Cairns,N.J., Xie,X., Blazey,T.M., et al. (2012) Clinical and biomarker changes in dominantly inherited Alzheimer’s disease. N. Engl. J. Med., 367, 795–804.

6. Portelius,E., Hölttä,M., Soininen,H., Bjerke,M., Zetterberg,H., Westerlund,A., Herukka,S.K., Blennow,K. and Mattsson,N. (2014) Altered cerebrospinal fluid levels of amyloid β and amyloid precursor-like protein 1 peptides in Down’s syndrome. NeuroMolecular Med., 16, 510–516.

7. Kasuga,K., Tokutake,T., Ishikawa,A., Uchiyama,T., Tokuda,T., Onodera,O., Nishizawa,M. and Ikeuchi,T. (2010) Differential levels of α-synuclein, β-amyloid42 and tau in CSF between patients with dementia with Lewy bodies and Alzheimer’s disease. J. Neurol. Neurosurg. Psychiatry, 81, 608–610.

8. Dorey,A., Tholance,Y., Vighetto,A., Perret-Liaudet,A., Lachman,I., Krolak-Salmon,P., Wagner,U., Struyfs,H., De Deyn,P.P., El-Moualij,B., et al. (2015) Association of cerebrospinal fluid prion protein levels and the distinction between Alzheimer disease and Creutzfeldt-Jakob disease. JAMA Neurol., 72, 267–275.

9. Meyne,F., Gloeckner,S.F., Ciesielczyk,B., Heinemann,U., Krasnianski,A., Meissner,B. and Zerr,I. (2009) Total prion protein levels in the cerebrospinal fluid are reduced in patients with various neurological disorders. J. Alzheimer’s Dis., 17, 863–873.

10. Stomrud,E., Minthon,L., Zetterberg,H., Blennow,K. and Hansson,O. (2015) Longitudinal cerebrospinal fluid biomarker measurements in preclinical sporadic Alzheimer ’ s disease : A prospective 9-year study. 1, 403–411.

11. Parnetti,L., Paciotti,S., Farotti,L., Bellomo,G., Sepe,F.N. and Eusebi,P. (2019) Parkinson’s and Lewy body dementia CSF biomarkers. Clin. Chim. Acta, 495, 318–325.

12. Supattapone,S. (2020) Cofactor molecules : Essential partners for infectious prions 1st ed. Elsevier Inc. PMID: 32958241

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