Researchers have deciphered the novel molecular structure of a protein that plays a critical role in determining male or female physical characteristics. Although the research was done in the fruit fly, Drosophila melanogaster, the researchers say the findings have implications in humans because similar genes have recently been found in the human genome.
The researcher team -- composed of members from Case Western Reserve University's School of Medicine, University of Chicago, and Gryphon Sciences -- report their results in a paper in the July 17 issue of the journal Genes and Development.
The structure binds DNA to turn on or off male genes or female genes. The finding is representative of how genome sequences are enabling a new generation of studies based on how the encoded proteins control the information in the genome.
Deletions in the human genes, found on Chromosome 9, are associated with sex reversal in children with normal Y chromosomes. Such children have a high risk of developing cancer in one or both gonads. The gene, called "doublesex" in the fly for its complementary roles in males and females, is also present in a wide range of multicellular animals, from insects and worms to fish and mice. In the male fruit fly, mutations in this new signal lead to a female pattern of some parts of the nervous system and bisexual orientation. The human protein was found to bind to a genetic control element in a fruit fly gene, indicating broad similarities of this new male signal.
"This structure extends known sex-specific signals, such as from estrogen or testosterone. The presence of similar genes in diverse kinds of animals suggests its primordial origins as a male signal," said Michael Weiss, the paper's senior author, and professor and chair of biochemistry at CWRU's School of Medicine.
"The newly obtained genomes of humans and other animals have revealed families of related genes, some of which may control aspects of the body plan shared by males and females."
Sex determination, whether an embryo becomes male or female, is regulated by different biological pathways in different kinds of animals. In humans, for example, males ordinarily have one X and one Y chromosome, whereas females ordinarily have two X chromosomes. In flies, by contrast, the Y chromosome does not determine maleness. Instead, sex is determined by the ratio of the number of the X chromosome to the other chromosomes in the cell. This led to the idea that each type of body plan (such as those of mammals or insects) had evolved its own type of male signals.
However, it turns out that the story of boys and girls may have common themes after all. Genes on other chromosomes, in addition to the sex chromosomes, play essential roles in determining the physical characteristics and behavior of animals. And these newly discovered genes, buried "downstream" of the sex chromosomes, may in part exhibit a common origin, structure, and function.
For about 30 years, for example, scientists have known about the doublesex gene in the fruit fly which controls some aspect of sexual development. "In the male, it turns on a class of male genes and turns off a class of female genes. In females, the reverse occurs," said Weiss.
Furthermore, he said, mutations can lead to behavioral changes. "Doublesex is representative of a class of genes in the fly that not only specify aspects of body plan, but also influence behavior," he said. "Whether this effect occurs in other animals is unclear. There is no known role in human sexual orientation."
Mutations in the doublesex gene, such as caused by radiation or chemicals, can lead to bisexual orientation in male flies. "The doublesex gene is postulated to control a hypothetical courtship command center in the fruit fly's brain, but whether or how this occurs at the molecular level is unknown," Weiss said.
"The same kind of gene has been found in every animal that has been examined so far, and found in every multi-cell animal," said Weiss. For example, another group of researchers found a similar gene in a worm. Expression of this gene, called MAB-3, is specific to males, and mutations in it cause partial feminization of the male just as in the fly.
Remarkably, the researchers found when they inserted the fly's male doublesex gene into mutated, hermaphroditic worms, they became male. This experiment indicated that an entire network of male genes was controlled in a similar fashion in flies and worms, animals as different from each other as they are from humans.
Humans carry a version of this gene on the short arm of Chromosome 9. Children with normal X and Y chromosomes, but with deletions or rearrangements on Chromosome 9, appear female and have a normal uterus, but are sterile due to lack of a functional ovary or testis.
Weiss and his colleagues used nuclear magnetic resonance technology to reveal the three-dimensional structure of the protein. They found a new kind of DNA binding structure (containing intertwined zinc binding sites) that regulates genes, that is turns them off or on.
Whereas the estrogen and testosterone receptors also contain zinc, the doublesex protein was found to bind to a different face of DNA, called the minor groove. This novel mode of binding permits coordinate control of genetic elements in the fly's DNA to combine sex-specific signals with other signals indicating where in the body the target gene is to be turned on or off, and at what stage of development.
"Being able to integrate distinct genetic signals is the key to how a genome can instruct an embryo to make the many different kinds of cells and organs, and all this in a coherent body plan," said Weiss.
He said that because this structure is so persistent across species, that it may be a more fundamental signal for maleness than testosterone.
"In fact, the human protein was found also to bind zinc like the fly's protein and even to recognize the fly's DNA control sites. The structure provides a foundation for analysis of DNA-binding mutations that can affect sex determination across species. Future studies of mutant flies may enable scientists to trace how mutations in a gene can alter the regulatory properties of a protein, in turn, alter the patterning of a nervous system, and, ultimately, how a nervous system makes possible a complex behavior. These are the types of molecular studies in simpler animals that will allow us to use the human genome map."
Other authors on the paper were Lingyang Zhu of the University of Chicago; Jill Wilken, Stephen M. Stratton, and Stephen B. Kent of Gryphon Sciences; and Nelson B. Phillips and Umadevi Narendra of CWRU's School of Medicine.
Journal
Genes & Development