By David N. Leff

Editor¿s note: Science Scan is a roundup of recently published biotechnology-relevant research.

The idea of using HIV ¿ the human immunodeficiency virus ¿ as a DNA delivery vehicle for gene therapy may seem bizarre at first glance. But at second glance, dragooning this AIDS pathogen into a potential therapeutic gig has logic.

HIV belongs to the family of lentiviruses, which means, as the name implies, that they act relatively slowly over time, compared for example with the influenza virus, which can inflict illness on its victims in hours. What excites gene therapists at the prospect of making HIV a vector for delivering therapeutic genes to defective cells is the fact that lentiviruses, unlike most current viral vector candidates, can infect non-dividing cells ¿ notably, those of nerves and liver, as well as blood-forming stem cells. That gives them a possible leg up in treating, say, cancers of the brain, and exploiting the ability of liver to churn out a host of useful proteins.

What sparked this concept was the fact that HIV, which mainly attacks rapidly dividing T cells, can also infect macrophages, which are non-dividing.

But one leading gene therapist, Mark Kay of Stanford University, dampens this concept. He posits, in Nature Genetics for January 2000, the seemingly counter-intuitive proposition that lentiviruses can¿t really promote expression of foreign genes in liver cells unless they first coax those hepatocytes into dividing. His article bears the title: ¿Efficient lentiviral transduction of liver requires cell cycling in vivo.¿

Normally non-dividing liver cells spontaneously start proliferating when a portion of the organ is removed; then it grows back ¿ regenerates. Kay and his co-authors injected high-dose, replication-defective HIV viral vectors carrying the gene for beta-galactosidase into the livers of SCID mice, some of which had two-thirds of the organ removed. (Liver is one of the few tissues in the human body that spontaneously regenerates.)

His paper reports, ¿Mice partially hepatectomized before lentiviral infusion had a significant increase in transduction, compared with non-hepatectomized counterparts.¿ In contrast, he and his co-authors found, organs that don¿t regenerate ¿ brain, heart, lung, kidney and duodenum ¿ did not express the HIV-driven gene-therapy construct.

In a ¿News & Views¿ commentary on this paper, cell biologist Michael Emerman, at the Fred Hutchinson Cancer Research Center in Seattle, concluded, ¿The success of lentiviral vectors in gene therapy strategies will require target cells, such as neurons or macrophages, that can be normally transduced in a non-proliferative state, or a means of activating target cells, such as hepatocytes, without triggering proliferation.¿

Some Genes Elude Nature¿s Strategy For Equalizing Male, Female Gene Expression

Lyonization is not a misprint for lionization. Named for its discoverer, the pioneer British cytogeneticist, Mary Lyon, lyonization is the process that silences one of a female mammal¿s (including lioness¿s) two X chromosomes. It¿s evolution¿s way of equalizing gene expression between the sexes. In humans particularly, for some unknown reason, an increasing number of genes on the inactivated X chromosome ¿escape¿ this lyonization, and are expressed twice ¿ on both the active and inactive X chromosome.

That evasion risks messing up the orderly number of chromosomes in the escapee¿s genomic karyotype dealt by inheritance. To assess the frequency of this aberration, cytogeneticists at Case Western Reserve University in Cleveland surveyed a large sample of X-linked gene sequences. Of the 224 transcripts they tested, 34 appeared to have evaded silencing. The authors attribute this avoidance of lyonization to the different cards evolution dealt to the long and short arms of the ancestral human X chromosome, and suggest that ¿genetic imbalance of X¿s short arm may be more severe clinically than long-arm imbalance.¿

The researchers conclude that such escapes are not rare, but account for one-fifth of the genes on the human X. Their work appears in the Proceedings of the National Academy of Sciences (PNAS), dated Dec. 7, 1999, titled: ¿A first-generation X-inactivation profile of the human X chromosome.¿

A commentary on these findings observes, ¿Recent advances in the Human Genome Project now allow the inactivation status of many X-linked genes to be systematically studied ¿ as this analysis does.¿ It points out, ¿The existence of these patterns also has implications for the regulation and role of X-chromosomal genes in human disease.¿ Perhaps the best-known X-linked disease is hemophilia, which by definition occurs in human males only.

Malaria Parasite Ups Output Of Male Progeny To Trump Host¿s Red Blood Cell Output

Adjusting the sex ratio to meet their varying reproductive needs is a feat performed by malaria parasites. Many animals switch between male and female offspring according to environmental conditions. Not so Plasmodium, in which its victim¿s immune system decides how many male progeny it will produce.

When a malaria mosquito bites its human, animal or avian victim, the insect¿s saliva injects a multitude of pathogenic parasites into the wound. These proliferate in the hapless vertebrate host, through several life-cycle stages, and eventually wind up in the victim¿s bloodstream, awaiting return via the bite of the next mosquito that cruises along. At this sexual stage, the parasite consists of both male and female contingents, called gametocytes. When that hungry mosquito gorges itself on its target¿s blood, the ingested gametocytes head for the insect¿s midgut. There, male gametocytes produce eight progeny; females, only one, which a male gamete fertilizes. A single clone can make males or females.

To scope what guides this gender-preferring option, molecular biologists at the Pasteur Institute in Paris studied Plasmodium infection of jungle fowl and mice. Their report, ¿Sex determination in malaria parasites,¿ appears in Science dated Jan. 7, 2000. The co-authors found that the parasite¿s sex ratio becomes more male in parallel with the host victim¿s production of a hormone ¿ erythropoietin ¿ that stimulates red blood cells, as an immune response to the infection. This makes the blood a hostile environment for male gametes, limiting their ability to fertilize females. So in response, Plasmodium hikes the number of males it spawns. n