Summary: Researchers discovered an unexpected link between the progression of ALS and PEG10, a protein traditionally known for its role in placental development. Overabundance of this protein in nerve tissue has been observed to alter cell behavior contributing to ALS. The team is now studying the molecular pathways involved with a view to inhibiting this rogue protein, potentially paving the way for new therapeutics.
The ancient, virus-like protein PEG10, typically associated with placental development, has been linked to the progression of ALS when present in high levels in nerve tissue.
The research marks the first time PEG10 has been connected to ALS, with its accumulation seen as a hallmark of the disease.
Research findings suggest that inhibiting the rogue PEG10 protein could potentially lead to a new class of therapeutics for ALS.
Source: University of Colorado
More than 5,000 people are diagnosed annually with ALS (amyotrophic lateral sclerosis), a fatal, neurodegenerative disease that attacks nerve cells in the brain and spinal cord, gradually robbing people of the ability to speak, move, eat and breathe.
To date, only a handful of drugs exist to moderately slow its progression. There is no cure.
But CU Boulder researchers have identified a surprising new player in the disease—an ancient, virus-like protein that is best known, paradoxically, for its essential role in enabling placental development.
The findings were recently published in the journal eLife.
“Our work suggests that when this strange protein known as PEG10 is present at high levels in nerve tissue, it changes cell behavior in ways that contribute to ALS,” said senior author Alexandra Whiteley, assistant professor in the Department of Biochemistry.
Whiteley’s lab is now working to understand the molecular pathways involved and to find a way of inhibiting the rogue protein.
“It is early days still, but the hope is this could potentially lead to an entirely new class of potential therapeutics to get at the root cause of this disease.”
Ancient viruses with modern-day impact
Mounting research suggests about half the human genome is made up of bits of DNA left behind by viruses (known as retroviruses) and similar virus-like parasites, known as transposons, which infected our primate ancestors 30-50 million years ago. Some, like HIV, are well known for their ability to infect new cells and cause disease.
Others, like wolves who have lost their fangs, have become domesticated over time, losing their ability to replicate while continuing to pass from generation to generation, shaping human evolution and health.
PEG10, or Paternally Expressed Gene 10, is one such “domesticated retrotransposon.” Studies show it likely played a key role in enabling mammals to develop placentas—a critical step in human evolution.
But like a viral Jekyll and Hyde, when it’s overly abundant in the wrong places, it may also fuel disease, including certain cancers and another rare neurological disorder called Angelman’s syndrome, studies suggest.
Whiteley’s research is the first to link the virus-like protein to ALS, showing that PEG10 is present in high levels in the spinal cord tissue of ALS patients where it likely interferes with the machinery enabling brain and nerve cells to communicate.
“It appears that PEG10 accumulation is a hallmark of ALS,” said Whiteley, who has already secured a patent for PEG10 as a biomarker, or way of diagnosing, the disease.
Too much protein in the wrong places
Whiteley did not set out to study ALS, or ancient viruses.
Instead, she studies how cells get rid of extra protein, as too much of the typically good thing has been implicated in other neurodegenerative diseases, including Alzheimer’s and Parkinson’s.
Her lab is one of a half-dozen in the world to study a class of genes called ubiquilins, which serve to keep problem proteins from accumulating in cells.
In 2011, a study linked a mutation in the ubiquilin-2 gene (UBQLN2) to some cases of familial ALS, which makes up about 10% of ALS cases. The other 90% are sporadic, meaning they are not believed to be inherited.
But it has remained unclear how the faulty gene might fuel the deadly disease.
Using laboratory techniques and animal models, Whiteley and colleagues at Harvard Medical School first set out to determine which proteins pile up when the UBQLN2 misfires and fails to put the brakes on. Among thousands of possible proteins, PEG10 topped the list.
Then Whiteley and her colleagues collected the spinal tissue of deceased ALS patients (provided by the medical research foundation Target ALS) and used protein analysis, or proteomics, to see which if any seemed overexpressed.
Again, among more than 7,000 possible proteins, PEG10 was in the top five.
In a separate experiment, the team found that with the ubiquilin brakes essentially broken, the PEG10 protein piles up and disrupts the development of axons—the cords which carry electrical signals from the brain to the body.
PEG10 was overexpressed in the tissue of individuals with both sporadic and familial ALS, the study found, meaning the virus-like protein may be playing a key role in both.
“The fact that PEG10 is likely contributing to this disease means we may have a new target for treating ALS,” she said. “For a terrible disease in which there are no effective therapeutics that lengthen lifespan more than a couple of months, that could be huge.”
The research could also lead to a better understanding of other diseases that result from protein accumulation, as well as keener insight into how ancient viruses influence health.
In this case, Whiteley said, the so-called “domesticated” virus could a be rearing its fangs again.
“Domesticated is a relative term, as these virus-like activities may be a driver of neurodegenerative disease,” she said. “And in this case, what is good for the placenta may be bad for neural tissue.”
UBQLN2 restrains the domesticated retrotransposon PEG10 to maintain neuronal health in ALS
Amyotrophic Lateral Sclerosis (ALS) is a fatal neurodegenerative disease characterized by progressive motor neuron dysfunction and loss. A portion of ALS cases are caused by mutation of the proteasome shuttle factor Ubiquilin 2 (UBQLN2), but the molecular pathway leading from UBQLN2 dysfunction to disease remains unclear.
Here, we demonstrate that UBQLN2 regulates the domesticated gag-pol retrotransposon ‘paternally expressed gene 10 (PEG10)’ in human cells and tissues.
In cells, the PEG10 gag-pol protein cleaves itself in a mechanism reminiscent of retrotransposon self-processing to generate a liberated ‘nucleocapsid’ fragment, which uniquely localizes to the nucleus and changes the expression of genes involved in axon remodeling. In spinal cord tissue from ALS patients, PEG10 gag-pol is elevated compared to healthy controls.
These findings implicate the retrotransposon-like activity of PEG10 as a contributing mechanism in ALS through the regulation of gene expression, and restraint of PEG10 as a primary function of UBQLN2.