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RNA Technology Thwarts HIVResearchers at HMS and MIT have used RNA interference to inhibit HIV infection in host cells, raising hopes that the technology can be developed to complement available antiretroviral therapies.
RNA vs. HIV. In a recent study by HMS and MIT researchers, a CD4 siRNA blocked CD4 receptors of the host cell so the cell was less susceptible to HIV entry. And a p24 siRNA blocked mRNA of the virus so viral protein synthesis and replication were inhibited. (Illustration adapted from original by C.D. Novina and H. Cargill (MIT) RNA interference (RNAi) is a naturally occurring phenomenon by which cells guard themselves against viruses. The process involves post-transcriptional gene silencing in which specific RNA sequences get chopped into small pieces after binding to complementary short interfering RNAs (siRNA). These siRNAs can target either host mRNAs or viral genomic or messenger RNAs. As a consequence, gene expression and protein synthesis are blocked, inhibiting viral infection (see Focus, April 19, 2002). "It is interesting that this ancient, natural defense mechanism against viruses can be harnessed against HIV," said Premlata Shankar, an HMS assistant professor of pediatrics at the Center for Blood Research and a lead author on the study, appearing in print in the July Nature Medicine and currently online. Researchers transfected host cells with siRNAs designed to target RNA sequences coding for the CD4 host cell receptor for the virus and the viral core gag protein. The siRNAs inhibited HIV infection by two mechanisms. The ones targeted to the CD4 molecule blocked viral entry into cells. An eightfold decrease in CD4 molecule production resulted in a fourfold decrease in viral entry. Host cells transfected with siRNAs directed against viral gag sequences showed a fourfold decrease two days after infection in the number of virus-infected cells and a 25-fold reduction in production of viral protein compared to controls. The study also showed that cells that were already latently infected with virus could be inhibited from making new viral particles. Judy Lieberman, an HMS associate professor of pediatrics at the CBR and Children's Hospital and a co-author on the paper, said, "We can attack different parts of the life cycle of the HIV virus with this technology through preventing infection by inhibiting viral entry or by destroying the mRNA of the virus or the viral genome." There are obstacles to using the technology, however. "This is not like a drug that you can give people to drink," Lieberman said. "Figuring out how to deliver siRNAs efficiently into lymphocytes in vivo is a major obstacle for this therapeutic approach. This is an obstacle that all forms of gene therapy need to surmount." Also, HIV is constantly mutating and siRNA needs to be developed that will bind to those parts of the virus that are stable.
Compounds May Improve on Standard MS TherapyOne of the few effective treatments for multiple sclerosis is Copolymer 1, a synthetic polypeptide that reduces relapse frequency by 30 percent in relapsing-remitting MS patients. The drug, also called glatiramer acetate and Copaxone, counters the autoimmune process that leads to demyelination in MS. Scientists believe it accomplishes this by outcompeting the myelin basic protein for binding to the MS-associated histocompatibility antigen, HLA DR2, thereby suppressing the activation of myelin-targeting T cells. After binding it may also deactivate T cells or induce formation of regulatory T cells. In a study published in the June 15 Journal of Clinical Investigation, Harvard researchers made copolymers that bind to HLA DR2 better than Copolymer 1 does. Using a mouse model of MS, they found that these new copolymers also suppressed the disease more efficiently. An unusual compound, Copolymer 1 is a mixture of four amino acids--alanine, lysine, glutamic acid, and tyrosine--in a fixed ratio but no fixed sequence. The scientists--lead author Masha Fridkis-Hareli, HMS instructor in medicine at the Dana-Farber Cancer Institute; senior author Jack Strominger, the Higgins professor of biochemistry at Harvard University and a member of the Department of Cancer Immunology and AIDS at DFCI; and colleagues--began by looking at the structure of the peptide-binding groove of HLA DR2, the portion of the antigen that binds to myelin basic protein in MS and to Copolymer 1. They found that Copolymer 1's ability to bind HLA DR2 was far from optimal, because tyrosine is too large to fit in an important pocket on the antigen. Armed with this information, they made copolymers with different combinations of amino acids--substituting valine or phenylalanine for tyrosine, for example--that more closely mimicked the shape of the myelin basic protein. Some of the novel copolymers completely protected the mice from experimental autoimmune encephalomyelitis. Others delayed its onset longer than Copolymer 1, which is relatively ineffective in mice. An accompanying editorial notes that "given the precedent of Copolymer 1's safety and efficacy in people with MS, the use of other copolymers, perhaps optimized to target an individual's MHC haplotype, seems an attractive scenario for MS and perhaps other autoimmune diseases." Said Strominger, "Although mice are not men, I am excited about the potential for therapy of MS." --Tom Reynolds
Most Americans Would Get Smallpox Vaccination If It Were AvailableIf a smallpox vaccine were made available, three in five Americans say they would get vaccinated as a precaution against a smallpox bioterrorist attack, according to a recent survey conducted by HSPH and the Robert Wood Johnson Foundation.The results are particularly interesting since the federal government is debating offering voluntary smallpox vaccinations to the public. Vaccinations were discontinued in the late 1970s when the disease was globally eradicated, but after Sept. 11, the government is preparing for a possible smallpox bioterrorist attack. Although the number of Americans worried about a smallpox bioterrorist attack is down by 10 percent since November 2001, two out of five Americans are still concerned. About three fourths of Americans are mildly optimistic that they would survive if they contracted smallpox. A majority believe that their doctor would recognize the symptoms and their local hospital emergency room is equipped to treat smallpox victims. Almost two thirds believe that the local health department would prevent the spread of smallpox if there were an outbreak. The smallpox vaccine is a live-virus vaccine that is highly effective against the disease, which is extremely contagious and spreads by person-to-person contact and through aerosols containing the virus. The Centers for Disease Control and Prevention held a series of public forums on smallpox vaccine usage from June 6 to 11. The researchers took the survey by interviewing 2,000 people nationwide. Data are available online. Robert Blendon, project director and HSPH professor of health policy and management, said that this is the first of a series of surveys on smallpox vaccination that he will conduct. --Sena Desai |
