BioWorld International Correspondent
LONDON - The SARS epidemic apparently may be over, but the search for drugs to combat the coronavirus that causes this new infectious disease has just begun.
A team in Germany has just published the first examination of how the proteins involved in the replication of the virus' genetic material are expressed. They conclude that some of the key enzymes represent attractive targets for antiviral therapies.
Severe acute respiratory syndrome was first recognized earlier this year. The disease probably emerged in the Guangdong Province in China and within a few months had spread around the world. By July 3, the World Health Organization had recorded more than 8,400 cases and 812 deaths due to SARS.
The virus that causes SARS is a member of the coronaviruses. Those large RNA viruses were already known to cause respiratory and enteric diseases in many different hosts, including cattle, pigs, cats, mice and fowl, as well as an estimated 20 percent to 30 percent of cases of the common cold in humans.
John Ziebuhr, virologist and biochemist at the Institute of Virology and Immunology at the University of Wurzburg in Wurzburg, Germany, together with colleagues there and collaborators elsewhere in Germany and in the Netherlands, comprise just one of the teams around the world that has been trying to elucidate the workings of the new coronavirus, which has been called SARS-CoV. Their latest data will appear in the September edition of the Journal of General Virology, in a paper titled "Mechanisms and enzymes involved in SARS coronavirus genome expression."
Ziebuhr told BioWorld International, "This is the first time that anyone has looked at the RNAs that are produced in SARS-CoV infected cells, and now that we have done so, we can make solid predictions about the proteins that will be expressed."
The group worked with an isolate known as Frankfurt 1, which was obtained from a 32-year-old male physician who was admitted with typical symptoms of SARS to Frankfurt University Hospital in March. The RNA of this isolate has been completely sequenced by Ziebuhr's team. Other teams have sequenced other isolates obtained from other patients.
The latest findings of the German team have now confirmed what had been predicted from the available sequences, although, Ziebuhr said, his team also found other SARS-CoV RNAs in infected cells of which no one had predicted the existence.
One of the first actions of coronaviruses upon infecting a cell is to make a huge complex of proteins called the replicase complex. The components of this complex are cleaved by proteases. "We have identified the two proteases that are active in SARS-CoV," Ziebuhr said, "and, more importantly, we have shown that the substrate specificities of these proteases are exactly the same as those already established for other coronaviruses."
That is good news, because it means that the substrate-binding sites must therefore be the same in SARS-CoV and the other coronaviruses. Since the crystal structures of the equivalent sites in two other viruses are already available, "we can start rational drug design for compounds that will fit into the binding sites of these two other coronavirus proteases, and be confident that these inhibitors will also work for SARS-CoV proteases," Ziebuhr said.
Coronaviruses also have enzymes called helicases, which unwind double-stranded RNA. These enzymes are common to many RNA viruses, and, in most cases, they are very unspecific, Ziebuhr said. "For this reason, we do not think that it is possible to develop antiviral drugs mimicking natural substrates of these enzymes," he said.
The team has, however, discovered that the helicase of SARS-CoV is capable of unwinding DNA as well as RNA. The researchers doubt that this function of the enzyme has any biological significance, but it could be useful to researchers who want to find helicase inhibitors using DNA-based assays, rather than RNA-based assays, because DNA is much more stable in the test tube than RNA.
"This characteristic is useful," Ziebuhr said, "because we now have a simple assay that can be used for high-throughput screening of large compound libraries by pharmaceutical companies, in order to identify candidate compounds that inhibit the helicase."
The group is now keen to work with pharmaceutical companies that would like to pursue these approaches in order to identify drugs that would be able to treat any new outbreak of SARS.
"In the future," Ziebuhr said, "we want to determine the atomic structure of the SARS-CoV protease as well as the structures of other essential enzymes that the virus uses to replicate its genome. This structural information will then be used to design inhibitors that specifically block the functions of the viral proteins without interfering with the activities of related proteins present in the cell. In this way, we hope to find drugs that block the replication of SARS-CoV but have no toxic side effects on the cell."