A retroviral marker has been quantified in serum from patients with amyotrophic lateral sclerosis (ALS) but scientists say that it is still not clear what the virus is or whether it is damaging motor neurons.
A team of British scientists, led by Jeremy A. Garson, MD, PhD, of the University College London Center for Virology, developed a powerful assay to measure reverse transcriptase activity — a marker for the enzyme expressed by retroviruses — in serum and CSF. Fifty percent of the ALS patients had more than 6 x 10-8 units/mL of this enzyme activity in their serum compared to 7 percent of healthy controls, according to the report in the Jan. 22 Neurology. Project ALS, a nonprofit philanthropy that supports ALS research, funded the study.
If scientists can identify a viral cause, “it absolutely opens us a whole new door to treatment,” said Kenneth Tyler, MD, the Ruler-Lewin Professor of Neurology at the University of Colorado, in a telephone interview with Neurology Today.
Dr. Tyler, who was not involved in the study, said: “Dr. Garson's group has developed an improved assay for reverse transcriptase that could help scientists identify viral activity. These studies continue to provide intriguing evidence that an unidentified retrovirus may have some role in the disease.”
Earlier Work with the Enzyme
In 2000, Dr. Garson and his colleagues screened for the enzyme in a small sample of British ALS patients and controls — using an older assay that measured the presence of reverse transcriptase activity — and found that a much higher proportion of patients expressed serum reverse transcriptase than did healthy controls.
In 2005, Dr. Garson and Ammar Al-Chalabi, MD, of Kings College in London, had replicated similar findings in American ALS patients, including samples from the Harvard ALS team, led by Robert H. Brown Jr., MD, PhD.
But with the new assay, investigators could use the molecular biological technique — real-time polymerase chain reaction — to measure the amount of DNA that the reverse transcriptase present in the patient's serum can synthesize from RNA in a period of 90 minutes.
“In the new study, we could quantify the amount of reverse transcriptase activity present in the blood of ALS patients and compare it to the amount present in the blood of patients infected with HIV,” Dr. Garson explained. In addition, the new study looked at CSF for the first time and also at the novel human gamma retrovirus — xenotropic MuLV-related virus (XMRV) — that had not yet been discovered in 2005.
The idea that a virus can damage motor neurons is not new. Decades ago, it was clear that poliovirus had a special affinity for anterior horn cells but there was no convincing evidence that survivors of poliomyelitis were at increased risk for ALS.
Joseph Berger, MD, and his colleagues at the University of Kentucky at Louisville reported in a 2006 paper in the Journal of the Neurological Sciences that symptoms similar to ALS in AIDS patients disappeared with anti-retroviral therapy. Also, scientists have used retroviruses in animals to reproduce motor neuron damage and ALS symptoms. These same retroviruses trigger an increase in lymphomas, according to a 1997 report in Neurology.
Is XMRV the Elusive Virus?
These findings — plus a 2006 report online in the Pubic Library of Science Pathology of a newly-identified XMRV found as yet only in some men with prostate cancer — prompted Dr. Garson and colleagues to determine whether they could replicate their findings in another cohort of ALS patients. They wanted to determine whether XMRV might be the elusive virus.
Dr. Garson tested 22 blood samples and 25 CSF samples from ALS patients — mostly supplied by Dr. Brown's group at the Massachusetts General Hospital. Investigators also obtained serum from two patients with HIV from hospitals in London.
The samples were sent to the British investigators who were blinded to the sample sources. The ALS patients had no family history of motor neuron disease.
Fifty percent of the serum samples from ALS patients contained reverse transcriptase — greater than 6 x 10-8 units/mL — compared to 7 percent of the controls. The levels in the ALS patients matched those measured in samples from the HIV-infected patients. Only one of 25 CSF samples tested positive for reverse transcriptase, and it was from an ALS patient. The scientists concluded that serum is the better fluid to look at for reverse transcriptase activity in the future.
But why didn't the investigators find reverse transcriptase activity in all patient samples? “The level of serum RT activity in some ALS patients may fall below the lower detection limit of the current assay,” the investigators said. And, “there may be a proportion of patients with sporadic ALS in whom the disease is not associated with serum RT activity because different pathogenic mechanisms may be involved.”
The retrovirus, however, remains elusive. The serum and CSF tested negative for XMRV.
Can Viruses Cause ALS?
So that brings scientists back to the vexing question: Can viruses cause ALS? “As new viruses emerge, this question gets re-explored,” Dr. Tyler said. “For a disease with limited therapeutic options, this is an important line of research.”
Over the years, scientists have looked for evidence of known viruses, such as polio, as a trigger for human sporadic ALS. The identification of retroviruses in human disease has sent a new generation of virologists back into the laboratory. Retroviruses do not follow ordinary rules of genetics. They possess an RNA genome and replicate through a DNA intermediary to survive in a host. And they do this through reverse transcriptase.
While the most famous retrovirus is HIV, there are many others that may be involved in human disease. Some have been linked to cancers and autoimmune diseases but many just invade the genome and appear to do nothing.
“It's an unexplored field,” Dr. Tyler explained. What's more, there is evidence of endogenous human retroviruses that may have piggybacked on to the human genome.
Implications for Future Research
”The problem is where we go from here?” said Dr. Brown, a professor of neurology at Harvard Medical School. He is a co-author on the Jan. 22 Neurology paper, but gives sole credit to the British investigators for the finding.
“It makes the story solid enough and worth figuring out what the retrovirus is,” Dr. Brown said. “But the process of going from the source of enzyme activity to the actual virus producing that activity is extremely labor-intensive. Scientists can work for many years and find nothing. Of course, the dream would be that something could be identified like the HIV virus that causes AIDS.”
If there were a retrovirus in the blood, would anti-viral therapy work? These treatments have not yet been rigorously tested, but Dr. Brown said doctors at Beth Israel Medical Center in Manhattan tried retroviral therapy in a small group of their ALS patients and the treatment didn't show much effect. The damage may have already been done, Dr. Tyler added.
The next step in unraveling a viral theory for ALS, experts agreed, is to attempt to isolate and identify the mysterious retrovirus.
ARTICLE IN BRIEF
✓ Fifty percent of ALS patients had more than 6 × 10−8 units/mL of a marker for a retrovirus — reverse transcriptase enzyme activity — in their serum compared to 7 percent of healthy controls, according to a new study.