Evidence is linking genetic mutations to Achilles tendon and anterior cruciate ligament injuries as researchers try to connect complex motor control processes to small segments of DNA. But genetic testing is still a long way from becoming a clinical tool.
By Larry Hand
News arrives almost daily about how discoveries in genetics and genomics point to potential treatments for diseases or suggest explanations for why some people are more susceptible than others to certain types of cancer. Yet, until recently, researchers have not focused on the genetics of lower extremity musculoskeletal injuries. Now studies are starting to reveal how some genetic mutations may help predict musculoskeletal injury risk.
Researchers have found that some gene mutations are associated with Achilles tendinopathy (AT), which can develop from overuse, and with anterior cruciate ligament (ACL) injury. However, these researchers’ goal is not for genetic testing to be used in a vacuum to identify individuals at risk of injury, but to roll that genetic information into a complex disease model that accounts for the multiple factors that contribute to risk.
Using the totality of the information from the model, practitioners could then develop a management program in which the nonmodifiable (genetic) factors might be ameliorated by modifiable factors through personalized training or reduced exposure to risk.
“This is a very young area of research,” said Malcolm Collins, PhD, chief specialist scientist at the South African Medical Research Council in Cape Town and associate professor in the University of Cape Town’s Research Unit for Exercise Science and Sports Medicine in Newlands, South Africa. “It is still in its infancy. We published the first paper1 that showed a specific genetic element was associated with Achilles tendon injuries in 2005.”
That study compared 114 physically active people with Achilles symptoms and 127 asymptomatic physically active people. The researchers found the presence of symptoms was associated with the frequency of two gene base chemicals (guanine [G] and thymine [T]) within the tenascin-C (TNC) gene, which encodes for tendon structure. The genotypes, the complex collections of DNA inherited from both parents, of symptomatic people differed significantly from those of asymptomatic people in the number of G and T repetitions in a DNA sequence.
“We have used the term Achilles tendinopathy to refer to a chronic painful overuse injury of the Achilles tendon,” Collins explained. “We have also investigated the association of the same genes with a small cohort of Achilles tendon ruptures. Although we need to be very cautious in overinterpreting the results because of the sample size, there appear to be similarities and also differences between the genetic factors associated with a rupture, which is an acute injury, and AT, which is a chronic injury. Again, with caution, some of our findings2 suggest that there might be more genetic similarities between Achilles rupture and ACL rupture, another acute injury.”
Collins and his University of Cape Town colleagues have published the bulk of original research in this area since 2005. Most recently they described how genomics might be applied to the prevention, treatment, and management of Achilles tendon injuries and ACL ruptures, reporting on this work in sports research in one of a series of articles on genetics (the study of individual genes) and genomics (the study of all of a person’s genes, or their genome).2 This work was published by the journal Recent Patents on DNA & Gene Sequences to coincide with the recent London Olympics.2 Previous studies from the Cape Town group have found that Achilles tendinopathy and ACL injuries are both associated with the same variant of the COL5A1 gene.3,4
“We are just starting to get at what the genetic underpinnings may be for increased risk of an injury like an anterior cruciate ligament injury,” said Timothy Hewett, PhD, professor and director of research at the Ohio State University Sports, Health, and Performance Institute in Columbus, and director of the Sports Medicine Biodynamics Center at Cincinnati Children’s Hospital.
Hewett and colleagues published a study in 2010 that pointed out some of the physical attributes of the lower extremity that may be determined by genetics.5 The study focused on two fraternal twin sisters, both of whom played high school soccer and basketball and who were part of large cohort in a study of high school athletes. One sister ruptured the ACL in her nondominant (left) knee and the other sister ruptured the ACL in her dominant (right) knee. The twins also reported that an older sister had previously experienced an ACL injury.
“Twin studies are of great advantage when you’re looking at genetics,” Hewett said. “The common factors are most likely to be genetic. What the study tells us is these physical characteristics may be genetically determined or related to genetics in some way.”
When the researchers compared the twins’ landing mechanics with those of 72 other school-based athletes, they found that the twins had higher peak knee abduction angles and knee abduction range of motion but lower peak knee flexion angles. Joint laxity was similar in both twins but greater than in controls. Femoral intercondylar notch width, measured intraoperatively during ACL reconstruction, was relatively narrow (12 mm and 13 mm, respectively) compared with control values in the literature.
“Together they had increased joint laxity and decreased neuromuscular control and the notch at the distal end of the femur was more A-shaped and smaller than average and similar between the two girls,” Hewett said.
Bill Ribbans, MChOrth, PhD, an orthopedic surgeon at Northampton General Hospital in Northampton, UK, and a professor of sports medicine at the University of Northampton, describes the current status of genetics of musculoskeletal injuries this way: “I liken it to a large jigsaw puzzle and we still have identified only a few pieces and are trying to find out where they fit and their relationship to each other.”
Ribbans was a coauthor with Collins and others on two studies linking the matrix metalloproteinase (MMP) genes to AT6 and ACL injury.7 The presence of MMP systems is necessary for normal health and integrity of tendons. In the ACL study in 2009, they found that a variant within one of the MMP genes, MMP3, interacts with another gene, COL5A1, which encodes for collagen to alter the risk of tendinopathy. Collagen proteins strengthen the structure of tissues and bones but, in patients with tendinopathy, collagen doesn’t repair properly after injury.
“The extracellular matrix genes, especially collagen genes, are the clearest links, as distinguished by the South African research group,” said Jill Cook, PhD, a physical therapist and professor of medicine, nursing, and health sciences at Monash University in Victoria, Australia. She too has been a coauthor with Collins and others on studies that validated some of the South African studies in Australian populations.4,8
In a 2009 study4 they found the variant of the COL5A1 gene that was associated with Achilles tendon injury risk in a South African population was also associated with AT in an Australian population. And in a study this year they found that variants of the CASP8 gene, which encodes for a component of the apoptosis signaling cascade, a biological pathway involved in development of Achilles tendon injuries, are associated with risk of Achilles tendon injuries in South African and Australian populations.
In a 2011 study, Collins, Cook, and colleagues published a study showing that interactions among a COL5A1 gene variant and variants within interleukin (IL) genes IL-1ß, IL-6, and IL-1 in combination increase the risk of developing AT. Interleukins are highly involved in the body’s effective response to inflammation. The researchers characterized their finding as preliminary evidence that genetic factors may influence the inflammatory pathway in pathogenesis of Achilles tendon injuries.9
At the moment, Cook said, there are few musculoskeletal injuries or conditions for which there is any evidence for a genetic base.
“Most injuries are multifactorial and there are extrinsic/environmental and intrinsic factors that can be altered that would have a direct impact on injuries, some that may even be used to prevent injury,” she said.
And, while there is not much a practitioner can do in the clinic to address the issue of genetic effects on musculoskeletal issues, there are a few things that can be done. Collins provided a review of these issues in a 2010 article in the British Journal of Sports Medicine.10
“This is controversial, but we believe that genetic testing can be used by clinicians as one of the tools at their disposal to determine risk of injury,” Collins said. “It is important to note that the genetic tests will never be diagnostic: they can only be used together with other clinical, training, and other information to determine risk of injury. We are in the process of developing a test that clinicians can use.”
Genetic tests, however, should not be used in an information vacuum, multiple sources told LER. Even though some companies are already offering genetic testing directly to the public, interpreting their results can be tricky.
Collins and colleagues offered a bit of guidance in their most recent article.2 They suggested that practitioners could become familiar with the types of tests that are available and advise their patients and clients about the potential and limitations of the tests. They emphasized, however, that an individual’s unique genetic information may have the greatest benefit when it is interpreted within the context of that individual’s previous medical history, family history, and environmental risk factors.
“The key will be to develop a scoring system involving numerous genetic combinations for a particular condition,” Ribbans said. “The difficulty is to be able to confidently apply the appropriate influence/weighting of each gene pattern to allow an accurate protection/risk score to be made. Another area that needs to be explored further is the influence of the environment on genetic expression, epigentics. For instance, what is the influence of diet and training load on the way genes behave?”
Ethics and costs
Then there are the ethical issues, such as possibly excluding someone from playing a sport because of a genetic predisposition or of exposing private information.
“Would you stop a child from playing sports if they had a genetic predisposition to tear an ACL when, if you could improve their hip strength and educate about landing and change of direction, you might partially or fully ameliorate that risk?” Cook asked.
Although the price of sequencing a personal genome has collapsed in recent years, costs are still a major concern.
“Genotyping is relatively expensive,” Hewett said. “The best-case scenario price is still in the range of $1500 [per genotyping sequence]. That’s coming down not long ago from $5000. But that’s just the genotyping and doesn’t include the analysis. But if someone is so inclined and has funding, you could find out if you have these mutations in the COL5A1 gene family that might put you at risk. From a clinical perspective, there is the potential to look at that as a risk factor.”
Based on his twins study4 and his group’s continued work on related issues, Hewett said some things can be done in the clinic now to make assessments and develop preventive measures. For instance, the increased knee abduction and decreased knee flexion during a vertical drop landing that appear to be risk factors for ACL injury in the twins have also been associated with ACL injury risk in nongenetic studies.11
“If she has a small femoral notch showing on an x-ray, she has probably increased risk,” Hewett said. “We’re combining these parameters together into predictive models that we’ve shown to be in excess of 80% sensitive and specific to young people at relative risk of ACL injury.”12
Practitioners could take measurements of the key parameters—most of which are relatively inexpensive to obtain, certainly less expensive than genetic testing—and use the predictive tool to help identify risk factors to address, he said. That wouldn’t be delving into genetics, but it would be evaluating the physical elements that genetics may determine.
Hewett said his institution is genotyping 44 family members of a pair of female twins studied after the ones in the previous study. The twins in the current study both had ACL ruptures, and one had two ACL ruptures. In addition, their father was one of triplets, all three of whom had ACL ruptures. Of the 44 family members, 14 had ACL ruptures.
As they begin to analyze the voluminous data set, the researchers are seeing six genetic variants that appear to be associated with ACL injury risk. They’re beginning to find gender-based differences in risk factors, and both passive and dynamic systems are involved.
“We’re trying to track some of these relatively complex motor patterns to small combinations of gene variants,” Hewett said. “Of course, it’s going to generate more questions than it does answers, but that’s OK. At the same time it’s going to give us some very salient data sets that we’re going to be able to mine further to get at the genetic underpinnings of motor control in humans and how they relate to musculoskeletal injuries.”
From genetics to gene therapy
As genetics research marches on, could gene therapy be far behind for musculoskeletal problems?
Christopher E. Evans, DSc, PhD, director of the Center for Advanced Orthopaedic Studies at Beth Israel Deaconess Medical Center and Maurice Edmond Mueller Professor of Orthopaedic Surgery at Harvard Medical School in Boston, says he believes it is highly possible.
“But with the exception of arthritis and arguably, bone healing, it isn’t going to happen any time soon. The reason is that they’re not simple genetic diseases where one gene has gone wrong,” Evans explained.
One gene therapy application in development for osteoarthritis, for cartilage repair and ligaments, is moving along.13
“For arthritis, we are quite far along. There are clinical trials in progress. Bone healing is also at an advanced preclinical stage,” he said.
In a 2010 opinion article, Evans and coauthors described the status of arthritis gene therapy at that time: “Numerous preclinical studies confirm its efficacy and safety in animal models and there is preliminary evidence of its efficacy and safety in humans. With sufficient resources and regulatory pragmatism, arthritis gene therapy stands a good chance of success.”14
But, in most other cases, Evans said, “we’re still trying to figure out which genes to use and which vectors to use to get them in the right place. There are still a lot of fundamental questions in the context of ligament and tendon cartilage.
For instance, to what degree would you want to block an MMP gene variant?
“Part of the repair process involves clearing away the damage, which requires MMPs and other components of an inflammatory response. So whatever we deliver [as a gene transfer] to repair an injury, timing will be a critical factor, depending on the setting,” Evans said.
Evans described the plight of orthopedic gene therapy in a recent article.15 He wrote that gene therapy’s reputation for being dangerous is unfounded, and based on reactions by regulators, media, and others to one-time adverse events that number only four out of 1700 clinical trials. In fact, he wrote, the science behind orthopedic gene therapy is generally sound, and the preclinical data are strong.
The future of gene therapy, Evans said, depends on clinical accessibility.
“We need a better roadmap to getting into the clinic,” he said. “We know what we’re doing in the lab and we can come up with promising approaches.”
The US Food and Drug Administration (FDA) approval process is long and confusing, however, and the scientists in labs who are unfamiliar with FDA issues need a translational environment in which to facilitate moving lab findings into the clinic.
Larry Hand is a writer based in Massachusetts.
1. Mokone GG, Gajjar M, September AV, et al. The guanine-thymine dinucleotide repeat polymorphism within the tenascin-C gene is associated with Achilles tendon injuries. Am J Sports Med 2005;33(7):1016-1021.
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11. Hewett TE, Myer GD, Ford KR et al. Biomechanical measures of neuromuscular control and valgus loading of the knee predict anterior cruciate ligament injury risk in female athletes. Am J Sports Med. 2005;33(4):492-501.
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