Susan Parkhurst, Ph.D., and Miriam Rosenberg, both of Fred Hutchinson's Basic Sciences Division, describe in the May 17 issue of Cell how a gene called Sir2, long studied in yeast and worms, has a counterpart in fruit flies that plays a dynamic role in the genetic regulation of early development.
"Because of the important role of the Sir2 gene to basic life functioning, this gene has been the focus of laboratories worldwide," said Parkhurst, a member of Fred Hutchinson's Basic Sciences Division.
"This finding represents a major step in understanding what the Sir2 gene does in a complex, multi-cellular system and further establishes the fruit fly as an important animal model for the study of cancer genetics in humans," she said.
Cancer is a disease of genetic mistakes. Errors in the DNA blueprint that cause single genes or entire chromosomal regions to switch off at the wrong time or turn on inappropriately can result in uncontrolled cell growth and malignancy. Understanding the mechanisms that turn genes on and off during normal development is crucial for decoding - and ultimately correcting - any flawed and potentially fatal operating instructions within cells that may lead to cancer and developmental defects.
Until now, the Sir2 gene, short for "silent information regulator No. 2," has been known to act as a silencer, quieting large regions of the genome for extended periods to shut down the production of proteins that are no longer needed in the course of early development.
For the first time, Parkhurst and colleagues report that in Drosophila, or fruit flies, Sir2 appears to play a dual role. It also acts as a repressor gene, turning off the short-term expression of specific gene products throughout the complex cellular choreography of normal growth.
"These results support the idea that in higher organisms, the mechanisms of silencing and dynamic gene regulation - once believed to be separate functions governed by separate genes - in fact represent two sides of the same coin," said Parkhurst, a developmental biologist. While Sir2 has been studied extensively in yeast, analogous versions of the gene exist in many higher life forms as well. Human cells, for example, have seven such silencing genes. Because some of the key genetic components of Sir2's gene-silencing pathway in lower organisms are identical to those in humans, scientists can exploit the power of yeast and fruit fly genetics to study complex human processes, from early development to cancer growth.
"It appears that Sir2 in higher organisms, including humans, can get the right gene to shut on or off at the right time. This is crucial for maintaining the integrity and normal functioning of cells," said Parkhurst, also an affiliate associate professor of zoology at the University of Washington College of Arts and Sciences.
Mutations, or defects, in Sir2 result in disastrous effects. In lower organisms, for example, such genetic errors have been associated with a shortened life span.
In the Parkhurst lab, Sir2 mutations have been linked to drastic developmental defects and foiled gender assignment, taking a significant toll on equality of the sexes, at least as far as the fruit fly population is concerned. "We noticed that flies with Sir2 mutations weren't too happy," said Rosenberg, a graduate student in the Parkhurst lab. "There were far too few males in the fly stocks."
"In the absence of Sir2, there's the wrong kind of gene expression for the sex chromosomes present. A female can express as if she's a male, and vice versa. That can lead to all kinds of problems. In flies, it leads to death," Parkhurst said.
In humans, inappropriate gene silencing is thought to account for various sex-linked genetic disorders, in which the affected genes are located on the X chromosome. Such developmental abnormalities, which nearly always affect males, include xeroderma pigmentosum (a rare, inherited skin disease that causes extreme sensitivity to sunlight and increased risk of skin cancer), color blindness and hemophilia.
Malfunctions in gene silencing also have been implicated in several human cancers, including acute myelogenous leukemia, colon cancer and several forms of breast cancer.
Sir2 is an attractive drug target because it has been found to modulate the function of p53, an important tumor-suppressor protein which, when defective, can increase cancer risk in humans. One drug under investigation at Fred Hutchinson, called Splitomicin, is a potent inhibitor of gene silencing. It works by interfering with Sir2's ability to cloak the chromosome in proteins that shield it from factors needed for activation. The compound is highly specific, halting the function of Sir2. Such precision is an attractive feature in drug design because it increases a drug's effectiveness and decreases its side effects. Fred Hutchinson researchers scientifically reported the drug's discovery last December and the center has filed patent protection for it.
In the Cell paper, Parkhurst and Rosenberg report that the drug, which already has been shown to reverse global gene silencing in yeast, also works in fruit flies.
"With fruit flies, we now have a multi-cellular model system that is more complex than yeast that will allow us to study all of the functional components of the pathway that are required for the drug to work," Parkhurst said. "Now that we know the drug works on fruit fly Sir2, this opens up all kinds of possibilities that we wouldn't have thought to try before." For example, this finding now hints at the drug's potential for treating diseases that result from the faulty regulation of a single protein, such as certain forms of leukemia.
Fred Hutchinson researchers also have found the compound to be effective in sensitizing human cells to DNA-damaging agents, a finding that could be exploited to increase the effectiveness of cancer chemotherapy, since many anti-cancer drugs inflict DNA damage. Another potential clinical application may be activating silent tumor-suppressor genes, such as p53, to fight cancer growth.
In addition to treating certain cancers, silencing inhibitors such as Splitomicin may be effective against sickle-cell anemia, which arises from defects in the gene for the adult form of hemoglobin. Sufferers do, however, possess a normal version of the fetal hemoglobin gene, which gets silenced early in life as part of normal development. Reversing the silencing of fetal hemoglobin could potentially compensate for the lack of functioning hemoglobin in those with the disease. Likewise, Parkhurst envisions using Sir2 inhibitors to compensate genetically for various certain sex-linked conditions.
The Splitomicin used in Parkhurst's laboratory was developed by Fred Hutchinson researchers Antonio Bedalov, M.D., Ph.D., a research associate the Clinical Research and Human Biology divisions; Dan Gottschling, Ph.D., a member of the Basic Sciences Division; and Julian Simon, Ph.D., a member of the Clinical Research Division. Grants from the National Institutes of Health supported this work.
The Fred Hutchinson Cancer Research Center, home of two Nobel Prize laureates, is an independent, nonprofit research institution dedicated to the development and advancement of biomedical technology to eliminate cancer and other potentially fatal diseases. Fred Hutchinson receives more funding from the National Institutes of Health than any other independent U.S. research center. Recognized internationally for its pioneering work in bone-marrow transplantation, the center's four scientific divisions collaborate to form a unique environment for conducting basic and applied science. Fred Hutchinson is the only National Cancer Institute-designated comprehensive cancer center in the Pacific Northwest and is one of 41 nationwide. For more information, visit the center's Web site at www.fhcrc.org.
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