“Dripping Candle Wax” Bone Disease Genetic Cause Uncovered
Melorheostosis is a sporadic disease of uncertain etiology characterized by asymmetric bone overgrowth and functional impairment. It is characterized by excess bone formation in the classic “dripping candle wax” pattern on the surface of bone, revealed radiographically.
Now, NIH researchers have worked with 15 patients from around the world to shed light on the genetic basis of ‘dripping candle wax’ bone disease. The results offer potential treatment targets for this rare disease, provide important clues about bone development, and may lead to insights about fracture healing and osteoporosis.
“Scientists previously assumed that the genetic mutations responsible for melorheostosis occurred in all cells of a person with the disorder,” said co-senior author Timothy Bhattacharyya, M.D., head of the Clinical and Investigative Orthopaedics Surgery Unit at the National Institute of Arthritis and Musculoskeletal and Skin Diseases (NIAMS) at NIH. “Our team hypothesized that mutations might only occur in the affected bone tissue.”
“Most adults have the problem of weakening bones as they grow older. These patients have the opposite problem as some of their bones are rock hard and still growing,” said Bhattacharyya. “The prospect that we could somehow harness this pathway in the future is so exciting.
”The genetic condition is characterized by the thickening of the dense outer layer of the bone. On X-rays, the affected bones resemble a candle with wax flowing down its sides. According to the National Institutes of Health, only around 400 people worldwide have been diagnosed with the disease.
Fifteen unrelated adults with the condition underwent biopsies of both affected and unaffected bones at the NIH Clinical Center. Researchers compared samples of healthy and affected bone from each participant to look for differences in the exome, the portion of the genome that codes for proteins.
Comparing genetic information from both samples in each patient allowed the team to pinpoint even low levels of the mutations. The analysis revealed that 8 of the 15 participants had mutations in the MAP2K1 gene in the affected bone only. MAP2K1 produces the protein MEK1. The gene MAP2K1 has previously been linked to some types of cancerous growths as well as to conditions that lead to abnormal blood vessel formation in the head, face, or neck.
In melorheostosis, all the identified MAP2K1 mutations affect a region of the MEK1 protein that normally suppresses its activity, causing MEK1 to become overactive. The bone growth is considered benign and does not spread to other parts of the body.
“This is an exciting study of a very rare bone disorder that not only identified the responsible mutation in half of the patients, but uncovered fundamental information about the role of a cancer-related gene in the metabolic pathways of normal bone,” said study co-senior author Joan Marini, Ph.D., M.D., of NICHD. “When we started, we had no preconceived causative pathways, but the participation of the patients has really changed the scientific landscape on this topic. Further studies on how this pathway works in both normal and mutant bone cells may have broad implications that could benefit a wider population.”
