By David N. Leff
Editor's note: Science Scan is a roundup of recently published biotechnology-relevant research.
In his 14-year career (1925-1939) as first baseman for the New York Yankees, Lou Gehrig broke most of baseball's records - from playing in 2,130 consecutive games to runs batted in to homers. Although he was elected to baseball's Hall of Fame in 1939, Gehrig's name became immortal because of the disease that killed him in 1941, at age 38.
Lou Gehrig's disease - amyotrophic lateral sclerosis (ALS) - is an incurable, degenerative neurological disorder of motor cells in the spinal cord and brain. It leads inevitably to death in three to 10 years after diagnosis. Neurologists have found that in its familial form, 15 percent to 25 percent of ALS patients have mutations in a gene that encodes copper ions joined to superoxide dismutase (SOD). This finding strongly implicates reactive oxygen radicals in the little-understood pathogenesis of ALS. (See BioWorld Today, Sept. 12, 1995, p. 1.)
Copper is essential to life, but too much of it in the body can bring death. In excess, the metal instigates the production of toxic hydroxyl (-OH) radicals, which wreak oxidative havoc in proteins, lipids and DNA. One of nature's antidotes to this deadly process is de-fanging the mutated gene via its life-saving superoxide dismutase component.
Moreover, it has evolved an elaborate system for delivering copper to enzymes in cells that need it, shielded from the oxidative onslaught. Metallochaperone proteins pick up their copper cargo, then bind to their individual target proteins, forming a heterodimeric molecule. To explore this copper-trafficking process, structural biologist Amy Rosenzweig, at Northwestern University in Evanston, Ill., solved the structure of the yeast copper chaperone for SOD by X-ray crystallography. She and her co-authors report this work in the Aug. 1999 issue of Nature Structural Biology, under the title "Crystal structure of the copper chaperone for superoxide dismutase."
Biological studies have suggested that motor neuron degeneration, as in ALS, may be due to a gain of function in the deformed SOD protein expressed by the mutant gene. If this process turns out to involve a copper-mediated reaction, then the structural data Rosenzweig presents "could lead," her paper concludes, "to the design and development of drugs that inhibit copper delivery."
A Vaccine Against Human-Targeting Herpesvirus Falters After Initial Optimistic Reaction In Mice
Besides mononucleosis - the adolescent "kissing disease" - Epstein-Barr virus (EBV) causes Burkitt's lymphoma in African children, and other lymphomas, largely in HIV-infected individuals with deficient immune defenses. EBV is one of the two gamma-herpesviruses (gHV) known to infect humans. The other, human herpesvirus 8, causes Kaposi's sarcoma in AIDS patients. However, until the advent of the HIV epidemic, both of these large DNA viruses were well controlled throughout life.
The pathogens lie latent in antibody-producing B lymphocytes and other cell types, from which they emerge to destructively infect epithelial cells, and replicate, during their lytic phase. Studies aimed at developing anti-gHV vaccines have found that every component of the specific host immune response plays a part in controlling these viruses. Both CD8+ and CD4+ T cells producing interferon-gamma limited the acute phase of lung infection after respiratory exposure to the virus. Antibodies apparently modulated the consequences of viral reactivation from latency.
Immunologists at St. Jude Children's Research Hospital in Memphis, Tenn., set out to determine whether CD8+ T cells "remembered" the p56 epitope, an antigen prominent during the virus's replicative phase. They vaccinated mice with a recombinant vaccinia virus that expressed this p56 peptide. One month later, the animals' lungs swarmed with expanded populations of p56-specific CD8+ T cells, whereupon they were promptly challenged intranasally with a virulent strain of gamma-herpesvirus.
Despite a gratifying initial reaction - a massive reduction of respiratory-tract infection, and lower levels of latency - these effects waned. So did repeated immunization experiments with higher vaccine levels. The St. Jude researchers' paper in the current Proceedings of the National Academy of Sciences (PNAS), dated Aug. 3, 1999, tellingly relates the outcome of their vaccine attempt. Its title: "A gamma-herpesvirus sneaks through a CD8+ T-cell response primed to a lytic-phase epitope." The co-authors concluded, "[A] high level of CD8+ T cell memory to lytic-phase epitopes alone does not protect against the longer-term consequences of this gHV infection."
Consortium Tackles Yeast's Recently Sequenced Genome To Identify Gene Functions, Antifungals
Although only a lowly fungus, yeast (Saccharomyces cerevisiae) has a lot in common with humans (Homo sapiens), the most highly evolved specimens of life on earth. For one thing, both organisms are eukaryotes, comprised of cells containing nuclei. For another, yeast cells divide much as human cells do. It's now over three years since an international consortium sequenced the complete 12,057,500-base-pair genome of S. cerevisiae, in early 1996. Those gene-counters determined that up to 30 percent of the yeast's 6,000-odd genes are closely related to those of humans. But in about 3,000 of the yeast genes, the functions of the proteins they encoded were unknown. (See BioWorld Today, April 25, 1996, p. 1.)
Now, a 52-member consortium at 14 institutions in six countries of Europe and North America has begun to make inroads into this field of ignorance, using wholesale rather than retail techniques. Their progress report, in Science dated Aug. 6, 1999, bears the title: "Functional characterization of the S. cerevisiae genome by gene deletion and parallel analysis." Its lead authors, Elizabeth Winzeler and Ronald Davis, are genomicists in the Stanford University School of Medicine.
During an earlier interview with BioWorld Today, Winzeler explained their large-scale approach: "If you're using the traditional method of cloning genes, which is complementation - putting little pieces of DNA into the yeast and trying to figure out which piece causes the change back to wild type - this can only be done with one gene at a time. So if you have multiple genes contributing to a trait, you'll never be able to figure it out with that method. Examining the whole genome simultaneously," she said, "provides a really strong way to look at multigenic traits."
To investigate the function of more than one-third of the new genes, the co-authors engineered 6,925 different yeast strains, each missing a different gene, and marked each with a molecular "barcode." They determined that 17 percent of them were essential for the fungus to survive. By identifying the genes necessary for yeast's viability - particularly those without human counterparts - the paper pointed out, it might be possible to design new antifungal drugs that target those genes.