Genetic Screens Yield Potential Therapies for Neurodegenerative Diseases

SAN DIEGO—Cross-species genetic screens are helping researchers find molecules that modulate the proteins that cause adult neurodegenerative disease, according to a lecture delivered at the 142nd Annual Meeting of the American Neurological Association. Such screening thus reveals potential therapeutic targets and augments scientific understanding of the biology of these proteins.

The research raises the possibility that clinicians will be able to delay or prevent neurodegenerative disease in the future through the early administration of molecules that target these proteins. “We need to identify those at risk and begin the therapy … before the symptoms develop, just like you would treat somebody with statins if they have high cholesterol before they have a heart attack,” said Huda Y. Zoghbi, MD, an investigator with the Howard Hughes Medical Institute; Professor of Pediatrics, Molecular and Human Genetics, Neuroscience, and Neurology at Baylor College of Medicine; and Director of the Jan and Dan Duncan Neurological Research Institute in Houston.

Research Into a Rare Disorder

Studying the rare disorder spinocerebellar ataxia type 1 (SCA1) has yielded information that could be applicable to more common neurodegenerative diseases, said Dr. Zoghbi. SCA1 is characterized by a loss of Purkinje cells and brainstem neurons that impairs balance and coordination and increases the risk of premature death. In 1993, Dr. Zoghbi; Harry Orr, PhD, Professor and Tulloch Chair in Genetics at the University of Minnesota in Minneapolis; and colleagues discovered that a CAG repeat expansion in ATXN1 causes SCA1 by producing an abnormally long version of the ataxin-1 protein. They also found that neurodegeneration results if levels of normal ataxin-1 are increased by 20% or 30%. The brain thus is highly sensitive to ataxin-1 levels, said Dr. Zoghbi.

Huda Y. Zoghbi, MD

Borrowing a technique from cancer research, the investigators performed genetic screening using fruit fly and human cells to find targets that would reduce ataxin-1 levels when inhibited. They found approximately 30 relevant genes in each organism, and about 12 were common to both organisms. All of the shared genes operate in the mitogen-activated protein (MAP) kinase pathway, and inhibiting each gene lowered ataxin-1 in cells and flies.

Inhibiting Enzymes

The researchers also observed that the enzymes MSK1 and MSK2 intervene in the pathway and promote ataxin-1 accumulation. Inhibiting MSK1 produced clinical improvement in SCA1 mouse models, and inhibiting MSK1 and MSK2 together produced still more improvement. A small molecule that inhibited MSK1 therefore could help patients with SCA1, said Dr. Zoghbi.

Inhibiting enzymes such as MSK1 and MSK2 for years at a time could raise safety concerns, however. The investigators thus decided to look for other modulators of ataxin-1, on the theory that targeting modulators that function in different pathways could reduce the amount of inhibition required and decrease the risk of adverse events.

Further screening revealed that PKA1 appeared to modulate ataxin-1 by a mechanism similar to that of MSK1. An animal study indicated that inhibiting MSK1 and inhibiting PKA1 produced equivalent reductions on ataxin-1, but that inhibiting both in tandem yielded a greater reduction. The investigators then found that the PAK1 enzyme promoted ataxin-1 accumulation through a pathway different from that of MSK1 and PKA1. Inhibiting PAK1 reduced ataxin-1 levels, and inhibiting PAK1 and MSK1 simultaneously had a still greater effect.

Targeting Tauopathies

Their research into kinases and enzymes prompted Dr. Zoghbi and colleagues to consider whole genome screening as a method of targeting proteins that cause neurodegenerative diseases other than SCA1. Overexpression of tau, for example, causes neurodegeneration, regardless of whether the overexpression results from a mutation in tau-encoding genes. “Tau is a true culprit in dementias, and we thought that if we could find something to lower it, we could help patients with these disorders,” said Dr. Zoghbi.

Another genetic screen suggested that the enzyme Nuak1 stabilizes tau by phosphorylating it at Ser356. The investigators observed that inhibiting Nuak1 reduced tau levels and suppressed neurodegeneration in human cells and in fruit flies. Tau accumulation decreases fruit flies’ climbing ability, and inhibiting Nuak1 improved this ability in flies with tau accumulation.

In a subsequent mouse study, Dr. Zoghbi and colleagues found that inhibiting Nuak1 significantly reduced levels of phosphorylated tau and provided smaller reductions in total tau and endogenous tau. They also observed that mice with tauopathy took longer than wild-type mice to complete a water maze task. Inhibiting Nuak1 in mice with tauopathy improved their performance on this task. Nuak1 inhibition also restored synaptic plasticity in these mice to a level similar to that of wild-type mice. Finally, Nuak1 inhibition reduced tau tangle pathology and increased survival. Dr. Zoghbi and colleagues are now searching for small-molecule Nuak1 inhibitors that could provide protection against tauopathy.

In Search of More Targets

The investigators next looked for genes that influence tau. Successive levels of genetic screening identified 59 genes that “robustly lower tau levels,” said Dr. Zoghbi. She and her colleagues prioritized 12 of the genes for investigation.

They used adenoassociated viral vectors to deliver therapies that can knock out mouse genes for as long as a year. With this technique, the investigators confirmed that all of the initial 12 genes decreased tau levels. In principle, this strategy could enable researchers to scan the whole genome for genes that modulate tau, said Dr. Zoghbi.

—Erik Greb

Suggested Reading

Lasagna-Reeves CA, de Haro M, Hao S, et al. Reduction of Nuak1 decreases tau and reverses phenotypes in a tauopathy mouse model. Neuron. 2016; 92(2):407-418.

Park J, Al-Ramahi I, Tan Q, et al. RAS-MAPK-MSK1 pathway modulates ataxin 1 protein levels and toxicity in SCA1. Nature. 2013;498(7454):325-331.

Rousseaux MW, de Haro M, Lasagna-Reeves CA, et al. TRIM28 regulates the nuclear accumulation and toxicity of both alpha-synuclein and tau. Elife. 2016 Oct 25;5. pii: e19809