Two Neurological Diseases Are "Genetic Doppelgängers"

An international team led by Emory scientists has gained insight into the pathological mechanisms behind two devastating neurodegenerative diseases. The scientists compared the most common inherited form of amyotrophic lateral sclerosis and frontotemporal dementia (ALS/FTD) with a rarer disease called spinocerebellar ataxia type 36 (SCA 36).

Both of the diseases are caused by abnormally expanded and strikingly similar DNA repeats. However, ALS progresses quickly, typically killing patients within a year or two, while the disease progression of SCA36 proceeds more slowly over the course of decades. In ALS/FTD it appears that protein products can poison cells in the nervous system. Whether similar protein products exist in SCA36 is not known.

What Zachary McEachin, PhD, and Gary Bassell, PhD, from Emory’s Department of Cell Biology, along with a team of collaborators at Emory, Mayo Clinic Florida, and internationally from Spain and Japan, discovered have provided a new paradigm for thinking about how aberrant protein species are formed. Regardless of the disparate clinical outcomes between these diseases, this research could broaden the avenue of research toward genetically targeted treatments for such related neurodegenerative diseases.

Their study provides a guide to types of protein that build up in brain cells in both disorders, and which should be reduced if the new mode of treatment is working in clinical trials.

“We are thinking of these diseases as genetic doppelgängers,” says McEachin, a postdoctoral fellow in Bassell’s lab. “By that, I mean they are genetically similar, but the neurodegeneration progresses differently for each disease. We can use this research to understand each of the respective disorders much better – and hopefully help patients improve their quality of life down the road with better treatments.”

An estimated 16,000 people in the United States have ALS, a progressive neurodegenerative disease that affects nerve cells in the brain and spinal cord. The most common inherited form of ALS/FTD occurs because there is an abnormally expanded repeat of six DNA “letters” stuck into a gene called c9orf72.

Some of the degeneration of motor neurons comes from the expanded repeat being made into proteins that repeat two amino acids over and over. It’s like a printing press being forced to print entire pages of “GPGPGPGP”, “GAGAGAGA”…” or “PRPRPRPR” In c9 ALS, those proteins are thought to build up inside brain cells and poison them. A striking finding is that in c9 ALS, there are other chimeric variants e.g. “GAGAGAGPGPGP” suggesting these toxic proteins are more complex than previously thought.

“A major goal in the field has been to identify and characterize the various types of pathological aggregates that accumulate in patient brains as ‘bad actors’ that cause neurodegeneration,” Bassell says. “Here we identify fundamental differences in this process for these two neurological diseases with disparate clinical outcomes. I think this is a paradigm shift for how heterogenous this process is in ALS and points to possible therapeutic strategies that might mitigate multiple bad actors simultaneously.”

The two diseases (c9 form of ALS and SCA36) both have abnormally expanded 6-letter genetic repeats, but the letters that are being repeated are different. McEachin was able to show that the nonsensical protein products from the expanded repeats aggregate in c9 ALS patients’ cells but do not in SCA36.

McEachin says it was surprising because the proteins were predicted to be similar between these diseases.

“Studying the differences in the aberrant proteins gives insight into both diseases,” he says.

SCA36, one of several types of SCA, is a condition characterized by progressive problems with movement that typically begins in mid-adulthood. They also develop hearing loss and muscle twitches over time, including losing the ability to move their tongue. Their legs, forearms and hands atrophy as the condition worsens.

McEachin traveled to Spain and Japan where there are known pedigrees of SCA36 patients to learn more about the disorder. To obtain SCA36 patient samples, the authors reached out to Maria-Jesus Sobrido, M.D., and Manuel Arias, M.D., neurologists at the Hospital Clínico Universitario in Santiago de Compostela, Spain, and neurologist Koji Abe, M.D. of Okayama University in Okayama, Japan.

Emory University’s ALS Clinic, directed by Jonathan Glass, M.D., a close collaborator on this study, is the largest in the southeast and has curated a substantial biorepository of C9orf72 ALS and FTD biospecimens including those used in this study.

“The study would not have been possible without the interest of these clinicians and willingness of their patients to participate,” McEachin says. “We are truly grateful for their collaboration as this study wouldn’t have been possible otherwise.”

Reference

McEachin et al. (2020). Chimeric Peptide Species Contribute to Divergent Dipeptide Repeat Pathology in c9ALS/FTD and SCA36. Neuron. DOI: https://doi.org/10.1016/j.neuron.2020.04.011

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