Three potent proteins of the immune system, evolved to purge us of intestinal parasites, now often launch misguided attacks in our airways, triggering the congestion of asthma that leaves millions gasping for air.
By studying the genetic machinery that controls production of these immune soldiers called cytokines, a team of scientists has demonstrated a potential strategy to silence their misfiring and quell the asthma response.
In back-to-back papers in April and May, researchers at UC San Francisco and the Lawrence Berkeley National Laboratory (LBL) report that a stretch of DNA controlling all three cytokine genes is so similar in humans and mice that the mouse DNA can activate the three human genes inserted in a mouse.
The researchers showed too that the activity of all three genes can be at least partially blocked, suggesting that a single drug could be used to attack asthma at its genetic source. Such a drug could be reliably tested in mice, their study shows.
"The primary aim of our research has been to demonstrate that if non-coding regions of DNA (stretches containing no genes) have been conserved in species separated by many millions of years, they probably perform vital functions," said Richard Locksley, MD, investigator in the Howard Hughes Medical Institute and professor of medicine and microbiology/ immunology at UCSF.
"But in choosing a DNA region that modulates the genes for the cytokines IL-4, 5 and 13, we are dealing with genes that are dramatically expressed in asthma and other allergic diseases. Our experiments show that all three genes are regulated by the same non-coding DNA region, and interruption of this control affects all three genes at once. By blocking the activity of this region, we should be able to block the expression of all three genes."
Many drugs are now being designed to interfere with activity of a single gene or the protein coded by that gene, but developing a drug to treat a disease caused by at least three genes could be far more difficult, Locksley said. Targeting a region of the genome that controls expression of the three genes at once may offer a solution.
The demonstration that the human genes for IL-4, 5 and 13 can be faithfully expressed in mice under the regulation of the mouse DNA is being published in the May issue of the Journal of Immunology. Lead author is Dee A. Lacy, MD, PhD, research associate in the Howard Hughes Medical Institute at UCSF; senior author is Richard Locksley. Scientists at LBL collaborated on the research and the paper.
The publication comes a few weeks after the team, led by the LBL scientists, identified the high degree of similarity between the mouse and human stretches of DNA that regulate expression of the genes. These findings were published in the April 7 issue of the journal Science.
Over the past decade a powerful principle has emerged that genes shared by evolutionarily distant species are likely to play major roles for the organisms and may provide a window to understand basic genetic mechanisms. The research by the UCSF/LBL team extends that principle to the non-gene portions of the genome.
"If evolution conserved a sequence over the 70 to90 million years since mice and humans diverged, it likely has a function," says geneticist Kelly Frazer, PhD, senior scientist at LBL's Life Sciences Division, senior author on the Science paper and co-author on the immunology paper. "Whether its function is to determine the structure of a protein coded for by a gene or to regulate gene expression, we should be able to identify these sequences though mouse-to-human sequence comparisons."
In the research reported in the Journal of Immunology, the scientists inserted human chromosome segment 5q31which contains the genes for the three human interleukins (Ils) into the chromosome of a mouse that includes that species' versions of these genes.
"Remarkably," they report, "these human Ils were expressed faithfully in CD4+ T cells in vitro and in vivo. These data support the existence of conserved regulatory elements near the cytokine cluster itself that enables the activation and/or stable expression of the type 2 cytokine genes in a cell- and lineage-specific manner."
The resulting transgenic mouse strains showed normal lymph system development, the researchers reported. However, the mice did develop fewer mouse IL-4-producing cells, suggesting competition exists between the mouse and human cytokines genes for expression in the transgenic mice.
The fact that including an "extra" human copy of the genes cuts down on expression of the mouse copies suggests to Locksley that some "rate-limiting factor is scanning the DNA stretches to find these genes."
"The hope is that studies of this mechanism may guide new therapeutic strategies to shut down expression of these genes associated with allergic diseases and asthma," Locksley says.
Infections by hookworms and other parasites induce a potent response from cytokines Il-4 and IL-13, the researchers write. Hookworms are a major world parasite, infecting nearly a billion people. The half-inch worms commonly bore into the body through bare feet and make their way to the gut where they reproduce. A female can produce more than 20,000 eggs per day. The interleukins regulate the intestinal immune response to the worms, and combine to orchestrate smooth muscle contraction, production of mucus and attack by immune cells that together lead to expulsion of the worms, Locksley explains.
Intestinal worms infect essentially all infants and children in developing countries, Locksley says, and some scientists believe this type of immune response can become misdirected, no longer playing the role it probably evolved to play but focusing instead on viruses and allergens inhaled in the respiratory tract.
"The marked increase in asthma in developed countries may represent the price we pay for shoes and concrete, which have eliminated the usual pathogen target and the timing during which this type of immune response would normally become directed to the gut," Locksley says.
Co-authors with Lacy, Locksley and Frazer on the Journal of Immunology paper, and collaborators on the research, are Zhi-En Wang, MD, research specialist in the Howard Hughes Medical Institute (HHMI) at UCSF; Clifford J. McArthur, research specialist at UCSF; Derek J. Symula, PhD, research scientist at LBL; and Edward M. Rubin, MD, PhD, senior scientist at LBL.
The research was funded by the National Institutes of Health and the Howard Hughes Medical Institute.