Researchers investigate genetic mutations in sperm cells to help prevent birth defects

Aug 10 2017

It's about preventing birth defects.

Associate pharmaceutical sciences professor Wenfeng An and his team at South Dakota State University are investigating mobile DNA segments, known as L1s, in sperm cells with the goal to treat families at risk for genetic mutations before they conceive.

"We know sporadic insertions can cause genetic diseases, with hemophilia being the best known examples," explained An. In the case of L1-induced hemophilia, the insertion interrupts production of the clotting factor. An, who has done research on L1s since 2003, is the first College of Pharmacy and Allied Health Professions endowed scholar, a cancer research position that was created in 2014 through a trust established by SDSU alumni Barry and Sharon Markl.

An and his team are investigating L1 insertions during sperm production with the long-term goal of preventing birth defects by treating at-risk individuals, prior to conceiving a child. The project is partially supported by a grant from the National Institute of Child Health and Human Development.

Controlling L1 insertions

An, whose lab is part of the Center for Systems Biology of Retrotransposition funded by the National Institute of General Medical Sciences, describes the replication of L1 segments as a "copy-and-paste mechanism." The original DNA segment remains but it is first transcribed to RNA, a simplified version of the original, and then converted back into DNA before being inserted somewhere else within the genome.

Though these insertions normally contribute to genetic diversity, they can sometimes lead to dysfunction and disease, An explained. Normally, the cells of an organism exert tight control over this replication process, but during certain developmental stages, these controls are relaxed.

L1 insertions are low in normal humans, approximately one new insertion per 100 to 200 live births, he noted. When L1 insertions land inside a gene, they may inadvertently shut down its normal function.

To investigate genetic mutations in sperm cells due to L1 insertions, the researchers combined their L1 transgene mouse model with one from the lab of professor P. Jeremy Wang at the University of Pennsylvania. Wang's mouse model has a mutated MOV10L1 segment and, hence, a non-functional piRNA pathway, which leads to increased L1 expression.

Under normal conditions, two L1 insertions are produced from the transgene in every 10,000 cells of the mouse testes, but the mutant mouse adult testes had a nearly 300-fold increase in L1 insertions, according to postdoctoral research associate Simon Newkirk. "That was pretty significant."

The good news is that males with high numbers of L1 insertions are sterile, according to An. However, the males who have an abnormal number of L1 insertions, yet are able to reproduce, are at risk for passing on the potentially harmful genetic mutations.

Using the mutant-mouse model, researchers can also identify drugs that prevent L1 insertions and determine the window of time, before conception, within which they must be administered.

Examining sperm cell development

To understand the mechanisms at work, the researchers examined sperm cell development in offspring from birth to sexual maturity. They observed no increase in L1 insertions at Day 7 after birth, followed by a 70-fold increase at Day 14 in mutant mouse testes.

"From birth to Day 7, the sperm producing factories are just waking up," Newkirk explained. At birth, the testes have about 90 percent body or somatic cells and 10 percent germ cells; in an adult male, it's the opposite.

At Day 14, the production line that had been increasing germ cell numbers through a duplication process called mitosis has already moved on to meiosis, which reduces the number of chromosomes by half, ultimately resulting in sperm that can fertilize an egg. This entry into meiosis coincided with the enormous increase in L1 insertions.

"Because the piRNA pathway, which controls DNA methylation, is knocked out, we don't expect to see high DNA methylation at both Day 7 and Day 14," Newkirk said. Thus, other control mechanisms must be involved. The researchers found changes in histone modifications are correlated with the dramatic increase in RNA expression and L1 insertions between Day 7 and 14.

"Multiple levels of regulation have to be overcome at the transcriptional level- predominantly DNA methylation and histone modifications," Newkirk explained. Other post-transcriptional regulators must also be overcome before L1s are activated. The researchers discussed these factors in a recent book "Human Retrotransposons in Health and Disease," which is edited by Gael Cristofari from University of Nice.

"DNA wraps around histones, like a spool of rope. It's a method of packaging DNA," Newkirk explained. "In meiotic cells, we see that large jump in insertions, because both DNA methylation and one of the histone marks is lost."

After the prepubescent stage, sperm cells are being produced continuously through meiosis in adult mice. The researchers showed the same is true in the adult stage- an increase in L1 insertions only happens in meiotic cells. Their research has important implications on preventing L1-induced mutations in sperm cells.

Antiretroviral drugs, such as those used to treat HIV, work well at inhibiting L1 activity in cell cultures, An noted. These drugs prevent the conversion of RNA into a DNA copy that can be inserted into the genome.

In a pilot experiment, treating at-risk male mice with anti-HIV drugs before the critical transition period between Day 7 and Day 14, reduced the number of L1 insertions dramatically. "We can, theoretically, decrease the number of insertions similarly in humans," Newkirk said.