Background and Introduction
In 1692, French physician Guy Patin was said to have reported a woman who “turned to wood,” with gradual bone growth in place of muscle. Occurring in about one out of every two million people, Fibrodysplasia Ossificans Progressiva (FOP), commonly known as Stoneman Syndrome, is a rare genetic disorder marked by abnormal bone development and heterotopic ossification (HO), or ectopic bone development (where it otherwise should not be) in extraskeletal soft tissue (Hüning & Gillessen-Kaesbach, 2014). This results in a shortening of life and often quality of life due to progressive impairment or even loss of mobility as the ectopic bone continues to spread throughout the body. In 2006, a study by Shore et al. found the cause of FOP to be a random, autosomal dominant mutation in the ACVR1 gene. Under normal circumstances, it is responsible for producing bone morphogenic protein BMI type I receptors: proteins that manage the growth and development of bone and muscles, including facilitating normal, controlled ossification. Mutations in the ACVR1 gene, however, impair regulators of the receptor’s activity, resulting in excessive and uncontrolled activity. This results in an overgrowth of bone and cartilage formation, as well as unregulated ossification that together produce the symptoms associated with FOP. Today, there are approximately. 800 living cases of FOP, with intergenerational cases being extremely rare. Meanwhile, correlations such as prevalence among a given sex or ethnicity, or geographic predisposition have yet to be studied in detail (Shaikh et al., 2023).
Symptoms and Diagnosis
The most common features leading up to diagnosis of FOP include malformed, swollen toes and progressive heterotopic endochondral ossification. Symptoms of FOP generally manifest at a young age in the form of noticeable abnormal bone growth in the neck and shoulders, before progressing into the body and the limbs/extremities. Detection can emerge around a decade into a person’s life, and can be painless at first, but often presents long-term challenges for the patient, including eventual malnutrition due to difficulties with eating caused by limited mouth mobility. Breathing difficulties are common due to ectopic bone formation around the rib cage and restricted lung expansion. As FOP continues to progress in more extreme cases, patients would likely suffer from the inability to walk and perform basic tasks due to a loss of motor control. Patients live a median of 50-60 years, and can express stable forms of the condition, though they often experience sensitivity to external trauma, which can result in reactivation of a more latent form or cause significant flare-ups. In several cases, surgery for counteracting FOP can be extremely risky, as attempting to remove the ectopic bone can result in stimulated ossification starting at the sites of excision. In addition to clinical evaluations, an FOP diagnosis can be confirmed by genetic testing to detect the mutation of the ACVR1 gene and next-generation genetic sequencing. Other forms of diagnosis include imaging through X-rays and advanced 3D-enhanced CT scans, with the potential of focused biomarkers for early detection in the future.
Current Treatments and Future Research Challenges
Despite research on FOP developing for centuries, the physiological understanding of the condition remains shallow due to the genetic complexity and rarity in numbers, and as of today, there are no effective medical treatment options for treating heterotopic ossification (HO) significant to FOP. In 2024, however, the first approved targeted treatment method for FOP became available in the US, Canada, Australia, and the UAE: a drug called palovarotene, which was found to limit bone accrual by 60% as opposed to those in a separate clinical study without intervention. While a promising solution, palovarotene comes with several costs and concerns. Beyond limited accessibility, financial burdens, and existing data constraints, palovarotene is currently discouraged from usage by young children due to potential complications, which means the drug is unable to target FOP in early onset, with childhood being a critical point in the disease development. Here, there is a great need for research in finding viable solutions for the critical stages of young patients, as well as methods to counteract existing ectopic bone growth actively. Other challenges and considerations for the future of FOP research prompt greater awareness and recognition by pharmaceutical companies and medical institutions. As there is no centralized treatment for Fibrodysplasia Ossificans Progressiva, and there are few cases worldwide, it would be difficult to orchestrate future targeted clinical trials on a wide scale. This could introduce restrictions to the scope of drug tests and ongoing studies. Still, clinical development and trials of other drugs to counteract variations in the ACVR1 gene are also underway. Currently, there are medications to reduce inflammation and flare-ups, while physical and occupational therapy, along with mobility aids, can significantly improve the quality of life for affected patients. Every small step of focused awareness, research, and dedication can make a difference towards helping patients worldwide manage the life-altering effects of FOP.
As shared by Frederick Kaplan, MD, of UPenn Medicine, “I hope we can turn a catastrophic disease into an inconvenience.”