A study in the May 29 issue of Neurology is the third to find that a viral protein known as reverse transcriptase is more frequently found in the blood of people with amyotrophic lateral sclerosis (ALS) than in those who don't have the disease.
Reverse transcriptase is an enzyme that retroviruses (a viral family that includes HIV) use to replicate themselves.
When Daniel MacGowan at Beth Israel Medical Center in New York and colleagues compared serum (the fluid portion of blood) from 23 ALS patients and 21 without ALS, they found reverse transcriptase in 13 (56 percent) in the ALS group and four (19 percent) in the non-ALS group. All the ALS patients tested negative for the HIV (AIDS) virus.
An earlier report, published in 2000, found that 33 out of 56 (59 percent) of ALS-affected study participants had evidence of reverse transcriptase in their serum, compared to three out of 58 (5 percent) of those without the disease. They didn't find any known viruses, including HIV, in the serum samples.
A 2005 investigation reported reverse transcriptase activity in the serum of 47 percent (14 out of 30) of ALS patients tested compared to 18 percent (five out of 28) unrelated people without the disease.
However, this research group also found the enzyme in six of 14 (43 percent) of blood relatives of ALS patients who did not themselves have ALS, weakening somewhat the hypothesis that its presence could be related to the disease.
“The reason for the increased frequency [of reverse transcriptase] in ALS and its importance remains unknown,” the authors of the 2007 study write. They note that no known retrovirus has so far been found to help explain the phenomenon, and they suggest further testing to see whether reverse transcriptase enzyme activity increases as ALS progresses.
"It's important to continue studies to understand this robust finding and its relevance to the cause and possible treatments for ALS," said Merit Cudkowicz at Massachusetts General Hospital in Boston, who was part of the 2005 study.
The ALS Therapy Development Institute (ALS TDI) in Cambridge, Mass., in partnership with MDA, plans to use new technology to scan for viral and bacterial genes in blood and other tissues from ALS patients.