Using a human gene, UCSF scientists were able to correct a defect in infertile flies that prevented them from carrying out a key step in the creation of sperm. The finding advances the effort to identify the genes involved in human male infertility, the researchers say, and suggests a possible target for a male contraceptive.
In their study of the fly, published in the January 15 issue of Human Molecular Genetics, the researchers focused on a gene that in male flies regulates meiosis, a key step in the creation of sperm and egg in all animals, including humans. In the male fly, loss of the gene, known as boule, leads to meiotic arrest, and hence infertility. Predictably, when the scientists inserted a normal copy of the gene into the flies, meiosis resumed. More notably, however, when they inserted the human form of boule, known as BOULE, into the meiosis-defective flies, development of the fly sperm also resumed.
The UCSF researchers had previously discovered the existence of BOULE in humans and had suggested that it was the distant relative of the fly boule gene, based on a similarity in the genes’ protein sequences and gene expression patterns. This finding was significant in itself, as it illuminated an ancient genetic lineage tracing back through the mouse to the fly, which evolved 600 million years ago. But nothing was known about BOULE’s function.
The new finding is the first demonstration that a human reproductive gene can “rescue” reproductive defects in the fly, and strongly suggests that the human gene BOULE also regulates meiosis in human sperm development, the researchers say.
This is significant because some 30 percent of infertile men have meiotic arrest during sperm development.
“The identification of genes such as BOULE that are required for meiotic function are likely to be important clinically,” says Eugene Yujun Xu, PhD, UCSF assistant research geneticist in the laboratory of senior author Renee A. Reijo Pera, PhD, a leading researcher on human infertility and a UCSF assistant professor of obstetrics, gynecology and reproductive sciences, physiology, urology and a member of the programs in human genetics and cancer genetics.
“We’ve opened the box on the BOULE molecular pathway. Now we must look further to see what other conserved genes are regulating human meiosis,” says Xu.
The researchers are hopeful, they say, that other genes involved in male and female reproductive development also have been conserved, and that the fly will reveal additional causes of infertility, as well.
While the researchers’ recent small study of infertile men suggests that BOULE is not often mutated, it is likely, they say, that other genes in the molecular pathway in which BOULE functions may also play a role in regulating meiosis, and that some of these genes, when mutated, will be linked to meiotic arrest in sperm development.
Identification of such a mutated gene could lead to the development of drugs that would assist the immature sperm cells in finishing meiosis, thereby overcoming infertility.
On another front, the human BOULE gene could prove a target for a male contraceptive, says Xu. While the adult human male carries millions of mature sperm in the testes, the supply is continuously replenished through a process of new sperm maturation. It’s possible, says Xu, that drugs could be developed that would disable BOULE in the testes and thus block the progression of meiosis in developing sperm. After being administered for several months - the period of time it would take for the mature sperm in the testes to proceed through the natural cycle to death - the drug would theoretically inhibit further sperm maturation.
To confirm their finding regarding BOULE, the researchers are engineering a mouse model to examine whether deletion of the equivalent gene in mice - which is known to exist - causes meiotic arrest in the animals’ developing sperm. If it does, they will examine whether inserting the human BOULE gene reactivates meiosis.
The discovery of BOULE’s likely role in human sperm meiosis is particularly fascinating, the researchers say, because it suggests that a gene that evolved in man’s ancient brethren has been maintained not only in terms of its genetic sequence, but its function. Such conservation is highly unusual in reproductive genes, both within and among species, as the competitive pressure to clinch the reproductive niche prompts constant changes in genes responsible for sperm production.
Sperm evolve quickly, to become faster, stronger, wilier all around.
In this case, says Xu, the scientists discovered a gene whose role in sperm development has been so crucial that it has been maintained over more than a half billion years.
The finding builds on previous research by the UCSF researchers aimed at identifying genes required for men and women to make germ cells, the sperm and eggs that ultimately fuse to form an individual. The exploration has offered a glimpse into a prehistoric class of genes known as DAZ, which were discovered by Reijo Pera several years ago and have been found to have family members required for female and male fertility in diverse organisms. BOULE is a member of this family. While boule is required for male fertility in the fly, it is required for female fertility in worms. In tracing the DAZ gene’s genetic heritage, the researchers have discovered in BOULE a gene that may be essential for germ cell development in such organisms as flies, worms, frogs, fish, mice and humans.
Co-authors of the study were Douglas F. Lee, BS, research associate and Paul J. Turek, MD, associate professor of urology and obstetrics, gynecology and reproductive sciences; and Ansgar Klebes, PhD, postdoctoral fellow in the laboratory of Tom B. Kornberg, PhD, a professor of biochemistry and biophysics.
The study was funded by the National Institutes of Health.