The human genome is littered with scraps of millennia-old retroviruses, which hopped in, never hopped back out, and now pass from parents to children. Some of those retroviruses may wake up and contribute to amyotrophic lateral sclerosis, according to researchers at Johns Hopkins University in Baltimore, Maryland. Their paper was accepted November 8 by the Annals of Neurology and is available online. If confirmed, the results suggest that retroviral genes and proteins could be a useful biomarker for ALS.
First author Renée Douville, who has since moved to the Lady Davis Institute in Montréal, Canada, led the research in the Johns Hopkins laboratory of senior author Avindra Nath. She hunted down the retroviral gene pol, which encodes reverse transcriptase, in a collection of frozen brain samples: 28 cases of ALS (25 sporadic, three familial); 12 cases of chronic systemic disease such as coronary artery disease or cancer; 10 control cases of accidental death, mostly traffic accidents; and 12 cases of Parkinson’s disease. She found the retrovirus HERV-K was active, to varying levels, in nearly all people who had ALS, plus the chronic disease cases. Viral proteins could have been toxic to neurons or upset cellular metabolism, Nath suggested. However, he noted that it is too early to do much more than speculate about the results. “So far, all we have shown is association,” Nath said. “The caveat of all this is it could be a non-specific finding.”
The study is not the first to link ALS and retroviruses. After all, TAR-DNA binding protein 43, currently starring in many an ALS research program, was first identified because it binds the HIV gene TAR (Ou et al., 1995). Scientists have found reverse transcriptase in serum from people with ALS before (MacGowan et al., 2007; McCormick et al., 2008), and hunted for evidence of infectious environmental viruses such as HIV, which can cause ALS-like symptoms (see ARF related news story on MacGowan et al., 2001 and Moulignier et al., 2001; also Verma and Berger, 2006). “They never found anything,” Nath said. Instead, he and Douville sought evidence of endogenous retroviruses, which jumped into human DNA and stayed put millions of years ago (reviewed in Bannert and Kurth, 2006). “Nine percent of the human genome is retroviral sequences,” Nath said.
Nath studies neurological complications of retroviruses such as HIV; he collaborated with Jeffrey Rothstein, an ALS researcher in the laboratory next door, who provided the brain tissue. Douville screened mRNAs, via RT-PCR, with a handful of primers for endogenous retroviral pol. She found that HERV-K pol was transcribed in the ALS cases—sporadic and familial—as well as the systemic disease samples. PD and accidental death samples did not evince RT pol transcription. HERV-K has also been implicated in multiple sclerosis (Dolei et al., 2009; Perron and Lang, 2010). Although HERV-K was the strongest hit, the researchers note that their findings do not eliminate the possibility that other retroviruses are active, too. HERV-K pol mRNA showed up strongly in the prefrontal and sensory cortex, with less in the motor cortex. Nath suggested this is because so many motor cortex cells have already died. The researchers have not yet examined spinal cord tissue.
Is HERV-K expression a cause of neurodegeneration, or merely the swan song of neurons dying for a different reason? The researchers do not know, Nath said, although he pointed to the lack of HERV-K pol transcription in Parkinson’s cases as evidence that it does not show up in every neurodegenerative disease.
Douville further examined pol expression by immunostaining. Ten of 13 ALS cases were positive for RT, versus three of 10 systemic disease cases. Reverse transcriptase tended to show up in clusters of neurons, in the kind of staining pattern one might expect if the virus were made in one cell and released to attack neighboring neurons, Nath noted. He added that the idea was purely hypothetical.
Dozens of HERV-K sequences are sprinkled throughout the human genome, so the researchers attempted to pinpoint the ALS-related loci by sequencing DNA. They used primers that started in the retroviral genes and followed the sequence through into the native human DNA. They found hits on the seventh chromosome, among others. ALS cases, in particular, expressed pol genes at 7q34 and 7q36.1. Tantalizingly, these loci sit within an unidentified motor neuron disease (MND) locus in the 7q34-7q36 region (Gopinath et al., 2007); Douville and Nath suggest the unknown ALS gene could be a retroviral polymorphism.
Garth Nicholson of the University of Sydney, Australia, authored the 2007 paper identifying the 7q34-36 MND locus, and he was skeptical of the retroviral connection. “Our locus causes a non-lethal but disabling motor neuron disease,” he wrote in an e-mail to ARF. “It would need to be a functional gene (rather than a virus relic) and have a disease-specific mutation to be relevant to our disorder.” Further, Nicholson cautioned, “Every now and again retroviruses have been said to cause neurological disorders, but they have not stood the test of time.” Most recently, researchers hypothesize a link between Herpes viruses and Alzheimer’s disease (Porcellini et al., 2010; for review, see also Itzhaki and Wozniak, 2010).
Jeremy Garson of University College London, U.K., has studied RT expression in ALS serum (Andrews et al., 2000; Steele et al., 2005; McCormick et al., 2008). He said this is the first study to identify a specific retrovirus, but agreed the results are very preliminary. “If the findings are independently confirmed and extended by others, it is possible that HERV-K expression might in future become a useful biomarker or diagnostic/prognostic indicator (assuming, of course, that brain biopsy is not required to measure it),” he mused in an e-mail to ARF. “Aberrant HERV-K expression might prove important in understanding the pathogenesis of ALS,” he wrote.
Nath noted that drug companies already possess libraries of retroviral drugs originally developed for HIV. If, indeed, HERV-K or another retrovirus proves important in ALS, it might be worth testing on endogenous retroviruses, he suggested.—Amber Dance
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