Terence D. Capellini has been interested in how joints work for almost three decades. Part of it is due to personal experience, having sustained several joint injuries as a college ice hockey player and recently developing knee osteoarthritis. But the principal investigator of Harvard's Developmental and Evolutionary Genetics Lab has also seen the pain and limited mobility of loved ones who've received similar diagnoses and injuries.
"We have all these joints in the body and they don't look the same from one another," said Capellini, the Richard B. Wolf Associate Professor in the Department of Human Evolutionary Biology. "Two very interesting inter-related questions are: Why do certain joints naturally acquire some disease while others do not, and why are some joints more prone to injury than others?"
These type of questions, along with a passion to understand the skeleton, motivated Capellini to lead a study on joint disorders that published Tuesday in Nature Communications. The findings from the paper could one day lead to therapeutics for two difficult-to-treat joint disorders that primarily affect the world's oldest and youngest populations.
The report details regulatory variants found near a gene, which plays a crucial role joint formation called GDF5. The study pinpoints two separate mutations near the gene, one that can cause knee osteoarthritis in older adults and another that can cause hip dysplasia in babies. Knee osteoarthritis, a degenerative disease in the knee joint, affects more than 30 percent of people over the age of 65, while hip dysplasia, a structural hip disorder, is present in one out of 1,000 newborns.
The findings can lay the groundwork for possible screenings of these diseases, which could allow for medical and non-medical interventions since patients will know they are at risk. The results could also lead to the creation of potential treatments because scientists will be able to better understand, target, and possibly affect the expression of the gene mutations to prevent the diseases.
"We can begin to understand whether we can start targeting these genetic regions for therapy," Capellini said.
The team of researchers say the mutations that lead to the diseases originate within two distinct regulatory switches near the GDF5 gene. One of the switches affects a population of cells that make the knee joint form its shape, while the other switch affects the cells that make the bones growing around the hip fit correctly.
"You can think of these switches as similar to the light switches in your house, with the light bulb being the gene," Capellini said. "You may have the same 100-watt light bulb in each room, but the kitchen switch only turns on the light bulb in the kitchen, while the living room switch only turns it on in the living room."
The discovery gives the researchers clues about the biological importance of these switches as target areas for therapies because they are prone to disruption.
The report is a collaboration between scientists from Harvard's Faculty of Arts and Sciences, the Broad Institute, the Harvard School of Dental Medicine, Harvard Medical School, Boston Children's Hospital, and medical researchers in China. Pushpanathan Muthuirulan, a research associate in Capellini's lab, was first author on the study, and Ata Kiapour, an assistant professor of orthopedic surgery at HMS and Children's Hospital, did the bulk of the medical imaging work.
The research team started by examining data acquired from genome-wide association studies on knee osteoarthritis and hip dysplasia. The goal of these types of studies are to detect and map genetic mutations across the genome that are associated with disease risk or variation in a trait. They chose the GDF5 gene because it's a region of the genome associated with about 20 different skeletal traits and has hundreds of potential disease-causing mutations.
Capellini and his colleagues identified the regulatory on and off switches across the genome responsible for building different joints in the human body, such as the hip, knee, shoulder, and elbow. With that information and the genome-wide association data, they used CRISPR gene editing techniques to modify human cartilage cells in a dish and to engineer mice to have separate mutations in each of the knee and hip switches they found.
These humanized mouse models allowed the researchers to study how each mutation impacts the formation and pathology of each joint. They were able to see if the DNA sequences in those switches possessed genetic mutations that have been associated with risk of knee osteoarthritis or risk of hip dysplasia. Through the process of elimination and arduous functional testing, they found the two separate mutations that can lead to both diseases.
The researchers also applied similar computational methods to other genes beyond GDF5. They found that nearly three-quarters of examined genes that are involved in multiple muscular skeletal diseases show the same patterns between potential disease-causing mutations and separate on and off switches for different joints.
The next steps for the researchers involve continuing to broaden their study to apply these experimental techniques to other regions of the genome to see if these patterns hold when it comes to knee osteoarthritis, hip dysplasia, and other skeletal diseases.
"On the broader scale, we want to explore more all these other regions of the genome because the risk for osteoarthritis and hip dysplasia is not just due to one mutation in one region of the genome," Capellini said. "It's a complex polygenic trait, so there's lots of regions of the genome that are conferring your risk for hip dysplasia or osteoarthritis. We want to be able to start modeling these mutations in more complex ways."
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Journal
Nature Communications