By David N. Leff
While the U.S. government deploys intelligence assets (i.e., spies) to ferret out rogue threats of anthrax and other biological horror weapons, a small multinational bio-espionage group is beginning to penetrate one global cabal of mayhem-sowing bio-killers.
Their targets are not human, but in their effects, inhuman. They comprise an advance echelon of insects and a follow-on array of one-cell parasites. The former are needle-nosed, blood-sucking sandflies; the latter, diverse species of Leishmania. (See BioWorld Today, Oct. 6, 1995, p. 1.)
These protozoa cause a spectrum of tropical diseases across Asia, Africa and Latin America. They come in two types: One, cutaneous leishmaniasis, inflicts grotesque skin ulcerations all over the body, resembling leprosy at its worst. Over time ¿ a long time ¿ many of these lesions tend to heal spontaneously. Leishmania¿s other, visceral form, attacks the body¿s inner organs, with 95 percent fatality unless treated.
That treatment ¿ consisting of drugs containing antimony or arsenic as their active ingredient ¿ is costly beyond the means of tropical populations, and its side effects can be life-threatening.
Like the malarial parasite, Leishmania species enter their human or animal victims via insect-bite injection. Then, like the long-lived tuberculosis bacterium, they hole up in the macrophage cells of the immune system, where they defy immune-response eviction.
In a microbiological effort to blow the parasite¿s cover, the World Health Organization recently provided seed money to organize the Leishmania Genome Network. Its mission: to sequence the entire DNA underpinning of Leishmania major, the best-known species. Now, a charter member of that group, the Seattle Biomedical Research Institute, has announced sequencing the first, and smallest, of L. major¿s 36 chromosomes.
Its report appears in the current Proceedings of the National Academy of Sciences (PNAS), published March 16, 1999. The paper¿s title is, ¿Leishmania major Friedlin chromosome 1 has an unusual distribution of protein-coding genes.¿ The article¿s senior author is molecular parasitologist Kenneth Stuart, director of the Seattle Institute, and chairman of pathobiology at the University of Washington.
¿This was the first eukaryotic diploid chromosome,¿ Stuart told BioWorld Today, ¿to be completely sequenced in a pathogen, human or animal. The most startling finding,¿ he added, ¿is that of the 79 putative protein-coding genes on this 269-kilobase chromosome, the first 29 are all on one DNA strand, and the other 50 are all on the other strand. It¿s the first case of such absolute strand-location gene polarity.¿
Another surprise was the relative density with which those 79 genes are packed on their DNA strands. ¿For example,¿ Stuart recalled, ¿we have a colleague who has sequenced about 1.5 million bases in one region of a human chromosome, and found three genes. We sequenced 300,000 bases and found 79 genes. So there¿s one-fifth the amount of DNA for 25 times as many genes ¿ about 75-fold greater gene density.¿
Next Chromosomes In Lineup
Since his paper was communicated to PNAS at the end of last year, Stuart and his co-authors have nearly completed work on L. major¿s chromosome 3, which turns out to have a similar gene density.
¿We¿ve already started chromosomes 27 and 35,¿ he said, ¿and are scheduled to begin sequencing 7 and 8 in the near future.¿ The consortium aims to finish sequencing the parasite¿s 9,800 genes, on a total genome of 34 megabases, Stuart observed, ¿within another 3.5 years ¿ but that¿s totally dependent on funding.¿
He made the point that of those 79 genes discovered on chromosome 1, ¿45 percent have no discernible function and may encode unknown proteins.¿ One gene with known function encodes arsenate reductase.
¿This is of particular interest,¿ Stuart observed, ¿because arsenical drugs are used in treating leishmaniasis. Such heavy-metal compounds,¿ he pointed out, ¿are effective against this organism. It¿s likely that there¿s going to be genes in there that can deal with heavy metals, [genes that] either resist them or are susceptible to the drug.¿
These genomic findings, Stuart said, ¿should interest the biotech industry. A lot of the genes that we¿re identifying, for example, have signatures that are traditional drug targets. Others are probably going to be related to human or animal genes, and therefore will provide good model systems for studying functions, and developing pharmaceutical compounds, even before these genes are identified in humans.
Potential Payoffs For Drugs, Vaccines
¿And also,¿ he continued, ¿the fact that these leishmania are pathogens of the immune system, that they infect the macrophages, provides an opportunity to find gene products that affect macrophage function, and so may allow manipulation of the immune system for both vaccines and immunotherapy.¿
The Seattle Biomedical Research Institute, which Stuart directs, is a non-profit institution, he observed. ¿We see ourselves as a resource for the biotech community, with which we¿re anxious to collaborate.¿ In 1994, the institute spun off Seattle-based Corixa Corp. He noted, ¿The intellectual property ¿ at least part of Corixa¿s beginning ¿ was developed here.¿
As for the institute¿s enthusiastic participation in the Leishmania Genome Network, Stuart said, ¿Even though we¿re a small laboratory, with our tuned technology, we can perform the sequencing at a cost that is competitive with the large labs.
¿As for various tropical diseases,¿ he concluded, ¿you can see this is a bit of a dilemma, because there¿s not much of a market.¿