Genome Screen in Worm Sheds Light on Molting

Parasitic nematodes cause over 130 million human infections and $80 billion of worldwide crop damage annually. Researchers from HMS have recently uncovered dozens of genes involved in the nematode molting process, representing a leap forward in the understanding of nematode biology as well as presenting possible targets for anti-infectious drugs and pesticides. The research was performed by lead author and research fellow Alison Frand and graduate student Sascha Russel, both members of the laboratory of Gary Ruvkun, HMS professor of genetics at Massachusetts General Hospital. It appears in the October issue of PLoS Biology.

Courtesy PLoS Biology

Genetic steps in molting. A C. elegans larva is unable to shed its old cuticle (molt) after RNA interference of the gene acn-1 (A). The GFP-tagged (green) mlt-11 gene is not expressed before (B) or after (D) the first molt, but is precisely expressed during the molt and in the correct epithelial cells (C).

The Ruvkun lab is interested in the mechanisms of hormonal control of development, like puberty in humans and molting in insects and nematodes. Because molting has remained a mysterious process in worms, the researchers chose to look for the genetic players through an RNA interference screen designed to silence each of the 19,427 predicted worm genes. “The RNAi screen gave us the power to systematically knock down each gene in the genome,” said Frand. “One potential benefit may be to use the genetic information to develop more specific nematocides, since the drug that is commonly used to treat nematode infections targets a protein that is conserved in humans.”

Frand and Russel obtained a library of bacterial clones, each expressing a single double-stranded RNA designed to silence a specific worm gene. Each clone was then fed to C. elegans larvae, which were examined for molting defects as they developed. A total of 159 genes made larvae unable to molt when knocked down; they code for transcription factors, secreted signaling molecules, proteases and antiproteases, components of the protein secretion machinery, structural molecules, and novel proteins. The investigators selected eight representative genes and examined their expression patterns in living worms. The promoters of the selected genes were fused with rapidly degrading GFP, and transgenic animals were made for each construct. The spatial and temporal patterns of fluorescence confirmed that the genes were expressed in epithelial cells involved in molting and in a pattern that corresponded with each new molting stage.

“We had envisioned the discovery of molting genes with no human counterparts that could be targets for drug development. The fact that Alison and Sascha found so many proteases and antiproteases in the screen is very promising, since these proteins are already drug targets in other diseases, like HIV,” said Ruvkun.

—Jillian Lokere

Alpert Symposium Recounts Taxol Story

Susan Horwitz, professor of cancer research and co-chair of the Department of Molecular Pharmacology at the Albert Einstein College of Medicine, was awarded the 17th annual Warren Alpert Foundation Prize on Sept. 29 at a symposium honoring her scientific contributions. The prize, established for discoveries that have significantly advanced medicine and patient care, recognizes her “pioneering work,” which illuminated the molecular action of paclitaxel, or Taxol, a plant-derived compound that inhibits tumor growth, and launched its development as a cancer therapeutic, said HMS dean Joseph Martin. The first blockbuster drug in oncology, paclitaxel is now a routine therapy in treating cancers, including tumors of the ovary, lung, and breast.

Horwitz presented an account of her efforts to elucidate paclitaxel’s role in stabilizing microtubules—the hollow, rodlike polymers of tubulin that buttress the cell, assist in its motility, and ensure the proper allocation of chromosomes during cell division. She recalled her early fascination with its chemical configuration, explaining, “One always hopes that with an unusual and unique structure you will have an unusual mechanism.” Current efforts in her own laboratory focus on deepening our understanding of its function in blocking tumor cell growth, which may lead to more effective therapies that treat a broader spectrum of cancers. Horwitz and her colleagues also are studying other microtubule-stabilizing drugs that, like paclitaxel, have been isolated from natural sources and may be exploited in a clinical setting.

Bruce Chabner, HMS professor of medicine at Massachusetts General Hospital, and Larry Norton, the Norna S. Sarofim chair in clinical oncology at the Memorial Sloan–Kettering Cancer Center and professor of medicine at the Weill Medical College of Cornell University, also spoke at the symposium, recounting the drug’s influence on the pharmaceutical industry and on patient care. Chabner explained the difficulties encountered in implementing paclitaxel as a cancer therapy, some of which have resulted in adverse reactions in patients. Despite its success in treating some cancers, he emphasized the need for developing improved paclitaxel derivatives that might mitigate some of the drug’s undesired effects. Norton elaborated on the clinical perspective, focusing on the research that helped promote paclitaxel as a breast cancer therapy, for which it has proven particularly effective. He also discussed a new drug delivery method involving a protein-coupled nanoparticle that shows improved efficacy over the original formulation. In contemplating paclitaxel’s influence, Norton said, “This is tantamount to a cure for breast cancer. There are people walking around today free of cancer because of this drug development and because of the proper application of this drug.”

—Nicole Davis

Bipolar Disorder Linked to Two Chromosomal Regions

By giving an “extreme makeover” to the accrued genetic information on bipolar disorder, scientists have gained a fresh look into the hereditary underpinnings of the psychiatric illness. In the October American Journal of Human Genetics, a multinational team, led by HSPH professor of biostatistics Nan Laird, describes a comprehensive reevaluation of existing genetic data that links two regions in the genome with the disease. “These two regions will help prioritize the search for genetic variants underlying bipolar disorder,” said lead author Matthew McQueen.

Bipolar disorder is a prevalent disease with clear genetic trends, but the pattern of inheritance is complicated and likely to involve multiple genes. Though intensively studied, these genetic dimensions have remained obscure since little consensus has emerged regarding the key chromosomal locations associated with the disease. In a unifying approach, Laird and her colleagues sought a wide-angle view of the extensive genetic data contributed by several previous studies. But rather than survey statistical figures, they relied on the original genotyping information. Such a strategy, they reasoned, would minimize sources of heterogeneity, which can dampen the signs of genetic linkage in similar meta-analyses.

To begin, Laird’s group structured a uniform classification for patients according to the disease’s two diagnostic categories, distinguished by the severity and frequency of mood swings. Then they gathered the DNA profiles of more than 5,000 individuals and condensed this information into a standardized genetic map. After integrating the data, they employed statistical methods to determine the genomic addresses where genetic variability correlates with the disease phenotype. Through this process, Laird and her group uncovered two regions with significant linkage to bipolar disorder on chromosomes 6 and 8. Moreover, they observed a correlation between the linkage indicator on chromosome 6 and one of the clinical subgroups of bipolar disorder, which may point to distinct genetic origins within the disease. The investigators also noted weak linkage signals on chromosomes 9 and 20.

This careful inspection represents the largest and most thorough genetic analysis of a psychiatric illness and may form a template for localizing the relevant sites in other complex genetic diseases. For Laird and her team, the challenge now is to hunt down the candidate genes that reside in these hotspots, which may help guide new strategies for treatment and, perhaps, prevention. “No one really understands what is biologically relevant for bipolar disorder, at least at the genetic level,” McQueen said.

—Nicole Davis

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