ARTICLE IN BRIEF
Researchers reported that human endogenous retrovirus-K (HERV-K) is activated in the cortical neurons of patients with sporadic amyotrophic lateral sclerosis (ALS). Activated HERV-K proved neurotoxic both in vitro and in vivo in a transgenic mouse model, triggering pathological changes consistent with ALS.
In a development that leading experts are calling “important” and “exciting,” investigators have found that human endogenous retrovirus-K (HERV-K) plays a significant role in the neuropathology of sporadic amyotrophic lateral sclerosis (ALS), prompting the launch of a pilot clinical trial of combination antiretroviral therapy.
Whether the retrovirus, incorporated into precursors of the human genome millions of years ago and normally dormant, plays a direct causal role in ALS remains to be seen. But in a series of studies described in the September 30 issue of Science Translational Medicine, the researchers said they found elevated levels of proteins encoded by the HERV-K genome (env) in postmortem cortical motor neurons from patients with sporadic ALS, and that activated HERV-K proved neurotoxic both in vitro and in vivo in a transgenic mouse model, triggering pathological changes consistent with ALS.
ALS experts not involved in the study said the findings will need to be replicated and investigated further, ultimately in a clinical trial of antiretroviral drugs. Such a trial is already underway, the senior author of the paper told Neurology Today.
“We have a clinical trial in place for enrolling 10 patients to give them a combination of four antiretroviral drugs to see what happens to HERV-K,” said Avindra Nath, MD, clinical director of the National Institute of Neurological Disorders and Stroke (NINDS) and chief of the NINDS section of infections of the nervous system.
Preclinical results show that combination antiretroviral therapy is partially effective in suppressing the HERV-K virus, Dr. Nath said.
“We have unpublished data showing that they do have an effect, but not as strong as they have on HIV,” he told Neurology Today. “When you treat HIV, if you don't get the virus down close to zero, it has no effect on the disease clinically. We want to see in the pilot clinical trial if combination antiretroviral therapy makes any difference to the HERV-K virus in vivo.”
Dr. Nath and others emphasized that although many patients are already requesting antiretroviral therapy, it should not be prescribed to ALS patients outside of clinical trials.
The new paper described the results of a series of experiments. First, Dr. Nath's group found that a protein encoded by the HERV-K gene, called env, was expressed in the postmortem cortical and spinal neurons of 10 patients diagnosed with sporadic ALS, but not in neurons from healthy controls or from people with Alzheimer's disease.
Next, in in vitro studies, they used the latest gene editing technique, CRISPR, to transfect either the whole HERV-K genome or the env gene alone into cultured human neurons. Both resulted in the triggering of neurite retraction and neuronal death, hallmarks of ALS pathology. The same effect was observed after activating the endogenous HERV-K genome by forcing expression of the protein VP16, which interacts with the HERV-K long terminal repeat.
They also documented neurotoxicity in a transgenic mouse expressing the env gene of HERV-K. In behavioral tests, the mice showed progressive motor dysfunction. They traveled shorter distances in an open field and rested for longer periods, and fell faster in a rotarod performance test, displaying evidence of spasticity and weakness of the limbs. The transgenic mice showed a progressive decline in motor function from three to six months of age; 50 percent of them died by 10 months.
Magnetic resonance imaging and histochemical analyses revealed that the mice developed “selective loss of volume of the motor cortex, decreased synaptic activity in pyramidal neurons, dendritic spine abnormalities, nucleolar dysfunction, and DNA damage,” the study authors noted. “Injury to anterior horn cells in the spinal cord was manifested by muscle atrophy and pathological changes consistent with nerve fiber denervation and reinnervation.”
An accompanying commentary by Robert H. Brown, MD, a professor and chair of neurology at the University of Massachusetts Medical Center and Medical School, where he directs the Day Neuromuscular Research Laboratory, described the results as being “of considerable importance” and “exciting,” a view echoed by other neurologists who specialize in ALS.
“They bring new technology to exploring the role of a retrovirus in ALS,” said Benjamin Rix Brooks, MD, director of the Carolinas Neuromuscular/ALS-MDA Center in Charlotte, NC. “I and others have spent a lot of time looking at exogenous retroviruses, but the signals we saw in ALS patients were not so different from controls. What this paper does, step by step, is demonstrate that a unique endogenous retrovirus is expressed in the target organ and causes slowly progressive motor neuron disease in transgenic mice. These are important pieces of evidence, but other pieces will have to come before we know how significant these findings are.”
Dr. Brooks added that in motor neuron degeneration caused by exogenous retrovirus superinfection, increased novel single-stranded DNA forms are seen at their highest levels in the areas affected by paralysis.
“If such markers are present in endogenous retrovirus,” Dr. Brooks said, “more insight into ALS pathogenic mechanisms may be forthcoming.”
Hiroshi Mitsumoto, MD, FAAN, the Wesley J. Howe professor of neurology and director of the MDA/ALS Center at Columbia University Medical Center, praised the study's methodology and called the results convincing. But, he emphasized, the results must now be replicated by an independent laboratory.
“If we find the same results from an independent laboratory, this becomes very, very important,” Dr. Mitsumoto said.
He pointed out that ALS is not a single disease. “Almost everyone agrees it is made up of different diseases, and that the symptoms are different from case to case,” Dr. Mitsumoto said. “Probably the causes are different from case to case, so this endogenous retrovirus might be the cause of only some cases. It might be an important cause, but we can't be sure of that yet.”
While agreeing that the results cannot yet be considered definitive, the director of the Duke ALS Clinic called the study “by far the best of the papers demonstrating a possible connection with different endogenous retroviruses.”
Richard Bedlack, MD, PhD, FAAN, who is also the outgoing chair of the North American ALS Research Group, added: “Instead of just showing that this virus was present in neurons of patients with ALS, they took it one step further. In in vitro with cultures and in vivo with an animal model, they demonstrated that it can kill neurons and cause disease. Those steps were missing before. It is potentially a very big deal.”
Jeremy A. Garson, MD, PhD, a clinical microbiologist and virologist at University College London, called the new study “substantial and impressive,” but said in an email that many questions about the findings remain unanswered.
“Is elevated HERV-K expression found in familial ALS cases as well as sporadic?” he asked. “Is there a particular phenotype of ALS associated with elevated HERV-K expression? Is there evidence of HERV-K expression in the blood and/or CSF of ALS patients, or is it restricted to the brain and spinal cord? Can its expression levels in brain, blood, or CSF be used as a diagnostic marker?”
Dr. Garson also pointed out that the researchers failed to measure expression of another human endogenous retrovirus, HERV-W, which has been previously implicated in human neurological and psychiatric disorders, including MS and schizophrenia.
In response to that observation, Dr. Nath told Neurology Today: “Besides HERV-K, we also looked for HERVs P, R, and E. They were not activated. Dr. Garson's suggestion to look for HERV-W is reasonable and we will be glad to look at it as well.”
EXPERTS: ON THE ROLE OF AN ENDOGENOUS RETROVIRUS ON SPORADIC ALS
AN EVOLVING CONCEPT: ALS AND ENDOGENOUS RETROVIRUSES
Retroviruses have been suspected to play a role in ALS pathophysiology since the 1970s, and drew particular attention in the 1980s when a number of AIDS patients developed neurological symptoms that mimicked ALS. A 2001 paper in Neurology reported that the same combination antiretroviral therapy that suppressed HIV was also completely successful in reversing the ALS-like syndrome.
But a 2005 paper in Neurology, the only published study to test an antiretroviral drug (indinavir) in ALS patients who were not also infected with HIV, found it ineffective and harmful. And no published trial has yet tested the combination therapy that has become the standard of care in HIV: highly active antiretroviral therapy, or HAART.
In 2011, Avindra Nath, MD, who was then a professor of neurology at Johns Hopkins University School of Medicine, and colleagues reported in the Annals of Neurology that they had found mRNA transcripts from retroviruses in postmortem brain tissue from patients with ALS, as well as those who died from chronic system diseases such as cancer, accidental causes, and Parkinson's disease. The patients with ALS had HERV-K transcripts in areas surrounding the motor cortex of the brain, while the other patients' transcripts were spread more diffusely throughout the brain, they reported.