It’s estimated that, at birth, four to six out of every 10 eggs in females have the wrong number of chromosomes. Almost all of these aneuploidies, if later fertilized, will result in miscarriage or birth defects. Why and how does the body make so many mistakes?
The question dogs and energizes Paula E. Cohen, Biomedical Sciences, who suspects meiosis—the process of cell division that produces sperm and eggs. Meiosis may be where and when these errors most often occur. “It’s bloody scary,” says Cohen. “It’s amazing that this error rate persists and doesn’t seem to exist in any other organism as far as we’ve seen.”
The rate of miscarriages, infertility, and birth defects such as Down syndrome also increases as women age, and in the United States, many women, faced with difficult choices between career and family, wait to have children. By uncovering the regulatory mechanisms of meiosis, Cohen hopes to better understand infertility and to educate women about what precisely is going on in their bodies.
More broadly, the investigation into the health of eggs and sperm could have huge implications for understanding disease. “We don’t really know how the health of a germ cell impacts what’s going to happen post-fertilization,” Cohen says. “Some people wouldn’t agree—but I think the germ cell is the most important cell in the body.”
Meiosis, Crossovers, and DNA Repair
During the first stage of meiosis, the homologous chromosomes received from each parent—one maternal, one paternal—must find each other and bind before being divided into different cells. Getting the right number of chromosomes depends on successful division. Cohen is most interested in the mechanisms that make this process work: how the homologous chromosomes find each other and stay bound before division.
“They don’t come with little labels,” Cohen says. “They have to find each other some way, and it really is remarkable that they pair and undergo these physical connections, all for the purpose of then separating equally. It’s an amazing process and one we don’t understand well.”
Some of the binding is achieved through crossing over, whereby genetic sequences are exchanged between the two chromosomes. This results in the variability between siblings but also helps keep the chromosomes together.
While the mechanism of control for these crossovers is still mysterious, Cohen knows that the number of crossovers per cell is significant. “That number is beautifully fixed in mouse or any other organism,” says Cohen. “The reason we know something goes wrong in humans is that the number varies from one cell to the next. If something is that variable, it tells me that the control is lost in some way.”
To uncover what’s happening, Cohen wants to understand the mechanism of those crossovers, which involves hundreds of double-strand breaks on the DNA that are then repaired, some as crossovers and others not. This also makes the germ cell a perfect laboratory to study DNA repair. “Errors in DNA repair cause cancer and numerous aging-related diseases,” Cohen says. “So we study a variety of genes that regulate DNA repair pathways, and we find many that regulate these crossovers are also involved in cancer.”
Silencing Sex Chromosomes
One of the reasons homologous chromosomes can find each other is that they are roughly the same size and shape—except in the case of the X and Y, the sex chromosomes, where the Y is short and stubby, Cohen says. This disparity poses a challenge for the cell. “You have large regions on the X and Y that remain unpaired, and the cell has to see that as normal,” Cohen says. “Whereas if a non-sex chromosome were unpaired, the cell would sound an alarm and then die.”
The cell has a solution—it sequesters the X and Y chromosome in a region called the sex body and effectively silences it, stopping any transcription. While the mechanism is unknown, Cohen has some leads. With the help of a $4.7 million grant from the National Institutes of Health, Cohen and her collaborators at the Cornell Center for Reproductive Genomics and Weill Cornell Medicine are looking into the role that non-coding RNAs, small pieces of RNA that don’t make proteins, play in this silencing process, as well as more generally in the germ cell.
“I think our research is not so much about fertility, it’s about educated choice, because women have a huge role to play.”
Cohen found evidence of non-coding RNAs’ role in silencing when she discovered them inside the nucleus of germ cells. “This is a whole new concept because we had thought small RNAs mostly only functioned in the cytoplasm,” she says. “We’ve got a long way to go to prove it, but we think that small RNAs may silence the sex chromosomes.”
As with DNA repair, studies of non-coding RNAs in the germ cell could have repercussions beyond infertility. “We’ve always thought that diseases are caused by misregulation of the gene, but what if it’s actually messenger RNAs being degraded by these microRNAs?” says Cohen. “The impact for disease is huge.”
Stem Cell Health and Defects
Cohen admits that she’s “meiosis-centric,” suspecting that most of the chromosomal errors occur during meiosis. But she’s also taking a step back to ask whether the health of the stem cell, the cells that divide into gametes, might also be responsible for some defects.
“We’re really interested in studying these critical junctions at which the organism can check and make sure everything’s okay,” Cohen says. “For example, we want to look at what genes are involved in entry to meiosis and what genes regulate and monitor the health of the cell before entering meiosis. Or is it a done deal? Are they going to enter meiosis regardless of defects? We’re starting with a baseline of zero. We don’t know any genes, nothing.”
CRISPR-Cas9 gene editing is allowing Cohen and her group to do things they never could before—culturing stem cells and silencing various genes to screen for key players. “The stem cell CRISPR strategy is going to be groundbreaking,” Cohen says. “It’s going to really change the way we can look at things.”
The Mysteries Began with Family
Cohen was drawn to studying pregnancy and germ cells partly because miscarriages seemed to run in her family—her mother suffered seven before having her and her sister. And then in graduate school, she saw a startling graph. “The one that shows how the instances of aneuploidies takes a huge upturn when you hit 35,” she says. “At first I thought it was propaganda for my mother to force me to have babies early, but I also thought: what is that? It doesn’t make sense, and I still don’t think we’ve addressed it properly.”
This led Cohen to want to understand chromosomal abnormalities, both those that arise in a woman’s germ cell population before birth (during meiosis), as well as those that develop as they age. “To me the research has a huge social repercussion,” Cohen says. “Women have a really essential place in the workforce, and we need to understand these defects so that women have the best opportunities and can make their own decisions about when and if and how they have babies. And I still don’t think we’re there. We are still very much locked in our own bodies, and it’s not fair.”
In underdeveloped countries, girls are often having children too young, Cohen says. “For many, the concept of waiting to have children is unknown,” she explains. “Educating women in both of these circumstances is a basic human right and will help them make their own decisions.
“I always urge people in the lab to think about what they’re doing and why they’re doing it,” Cohen continues. “I think our research is not so much about fertility, it’s about educated choice, because women have a huge role to play.”