A genetically engineered virus is taking on a missionimpossible: to resurrect the nearly extinct American chestnuttree. The creators of the virus plan to attack and weaken thevirulent chestnut-blight fungus that wiped out billions of thetrees in the first half of this century.

Virologist Donald L. Nuss and plant pathologist Gil H. Choi at theRoche Institute of Molecular Biology reported in Friday'sScience how they converted a double-stranded native RNAvirus into a synthetic full-length cDNA copy of the virus andinserted it into the fungal genome.

"We demonstrated for the first time," Nuss told BioWorld, thatthe virus is responsible for reducing virulence. "By making thesynthetic DNA form," he added, "we can manipulate mutations,and by putting the cDNA into fungal chromosomes, create ahypovirulent strain, more readily able to spread through theenvironment and convert virulent strains." He explained, "Thevirus sets up a persistent infection by replicating in the fungus,thus lowering the latter's pathogenic ability."

Because the virus is indwelling, the researchers could not takeit out for study and transfection. Instead, they had to switchfrom the native RNA to a synthetic DNA version, which can betransmitted from fungus to fungus by sexual mating as well asby fusing with the mass of fungal filaments. The RNA virus canspread only by the latter route.

It is conceivable, Nuss declared, "that the same strategy mighteventually do as much for Dutch elm disease, which a differentfungus, carrying a different virus, causes."

What about human fungal infections, from athlete's foot toCandida?

"Possible," Nuss predicts, "but it will take much more researchto determine feasibility, because viruses are less prevalent inmedically significant fungi."

He and the Roche Institute are in close consultation with theU.S. Department of Agriculture, seeking clearance to conductgreenhouse, then open-field, trials of the geneticallyengineered, attenuated C. parasitica fungus. "USDA is beingvery helpful," he said, "and we are ready to go into thegreenhouse as soon as February 1993."

Pending next year's field trials, Nuss and his team are "makinglarge batteries of mutants of infectious DNA clones to constructa variety of hypovirulent strains to spread the sexuallytransmitted disease to more fungal spores.

At an International Chestnut Conference last month inMorgantown, W.Va., 125 researchers from a dozen countriesheard Nuss report the hypovirulence induction work publishedlast week in Science. The event was sponsored by the AmericanChestnut Foundation, most of whose 1,700 members are tryingto restore the vanished tree by back-crossing shoots thatsprout from stumps with Chinese species resistant to the blight.

Ironically, as John H. Herrington, the Foundation's executivedirector, told BioWorld, the devastation began in 1904, when ashipment of ornamental chestnut trees, probably from China,reached the Bronx Botanical Gardens. "By the 1930s and '40s,"Herrington stated, "most of the mature chestnut trees in NorthAmerica were dead." Aside from the aesthetic loss, it ended aunique traditional source of rot-resistant lumber forconstructing homes and fences.

Chestnut researchers hope to re-create a 94 percent originalAmerican species, with blight resistance, in five generations,Herrington said. Each such breeding generation takes three tofive years.

Biologist David L. Mulcahy at the University of Massachusetts,observed, "I'm in reasonably good health, but at that rate, Iwon't live to see the end-product of back-crossing." His genetic-engineering solution for speeding up the process is to generate"lots of molecular markers" by DNA probes and PCR. Then, "bypooling DNA samples, we'll soon have populations withindividuals scored for susceptibility or resistance." Once he'scaptured sample resistance genes, Mulcahy declares, "I wouldcross them into seedlings sprouted from stumps throughout thespecies range, from Maine to Mississippi," to ensurebiodiversity.

The Foundation's Herrington asserts, "We're convinced we'regoing to lick this blight, and restore the chestnut, by combiningall three approaches -- back-crossing, hypovirulence andgenetic engineering."

-- David N. Leff Science Editor

(c) 1997 American Health Consultants. All rights reserved.