Researchers at the National Institutes of Health have discovered that a previously unexplained fatal form of Osteogenesis Imperfecta--a disorder that weakens bones and which may cause frequent fractures--results from a genetic defect in a protein involved in the production of collagen.
The affected gene contains the information for cartilage associated protein, or CRTAP. The function of CRTAP is not well understood, but it is known to be part of a complex of proteins involved in the chemical transformation of collagen from simple protein "chains" into its final form.
The well-known forms of Osteogenesis Imperfecta (OI) result from a defect in the genes for type I collagen, which serves as a kind of molecular scaffolding that holds together bone, tendons, skin and other tissues. The collagen defects result from dominant mutations, requiring only one copy of a mutant gene to cause bone disease. The NIH researchers discovered that mutations in the CRTAP gene accounted for a recessive form of the disorder--requiring two copies of the affected gene to show a particular trait.
The NIH team was led by Joan Marini, M.D., Ph.D., Chief of NIH's Bone and Extracellular Matrix Branch and was assisted by colleagues at other institutions.
"This discovery provides a basis for counseling families that have lost a child to this previously unexplained form of Osteogenesis Imperfecta," said Duane Alexander, Director of the NIH institute that conducted the study, the National Institute of Child Health and Human Development. "It also offers insight into a crucial step needed in the formation of bone and other tissues."
There are several known forms of OI, which vary in severity. In the most severe forms, infants may die at or shortly after birth. In other forms, individuals may lead a relatively normal life, but have bones that fracture easily. In still other cases, affected individuals may have only a slightly increased risk for bone fracture. Additional information about OI is available from the National Institute of Arthritis and Musculoskeletal and Skin Diseases.
Dr. Marini explained that from 10 to 15 percent of OI patients do not have a mutation in one of the collagen genes known to cause the disorder. She noted that researchers couldn't explain these cases, and began searching for possible causes.
"The hypothesis was that there must be one or more recessive genes causing something that looked like OI," she said.
The first clue came when colleagues Roy Morello Ph.D., and Brendan Lee, M.D., Ph.D. also authors of the current paper, developed a laboratory mouse that lacked functional copies of CRTAP. The researchers developed the mouse to understand the function of the CRTAP protein, which Dr. Morello had isolated earlier. The mice had deformed brittle bones. Next, Drs. Morello and Lee studied a family with an unexplained form of OI, learning that a partially functioning copy of the CRTAP gene accounted for a moderate form of the disease, designated type VII OI.
After learning of Dr. Morello and Dr. Lee's findings, Dr. Marini and her coworkers examined tissue samples from three patients in Dr. Marini's NIH laboratory. The patients had all died during their first year of life and the cause for their disorder was unknown. Dr. Marini had stored the samples, one for nearly 15 years, in hopes that a cause would eventually be discovered. The three patients had symptoms unlike other OI patients. For example, the children all had small heads. Most OI patients have proportionally larger heads. Moreover, the sclerae, or "whites" of the infants' eyes were white in coloration; in most OI patients, the sclerae are blue.
Dr. Marini and her colleagues tested samples from the three infants and learned that their CRTAP genes were nonfunctional and the infants completely lacked the CRTAP protein. In the two sets of parents who were available for genetic testing, each parent carried one mutant CRTAP gene and one normal CRTAP gene, the classical pattern of recessive inheritance. Dr. Marini added that, on the basis of her samples, the mutation did not appear to be confined to a particular ethnic group.
OI is an uncommon disorder which occurs in 1 out of 15,000 to 20,000 births, Dr. Marini said. About one fourth of the known collagen mutations are lethal. She estimated that the recessive form of OI that she and her coworkers described in the current paper might occur in 2-3 percent of lethal OI cases.
Although there is no treatment for the disorder, the finding does allow OI experts to test families who have lost a child to OI for the presence of the recessive CRTAP gene. Couples who have lost a child to this form of OI could be apprised of their risk for conceiving another child with the disorder. Similarly, siblings of children affected by the recessive CRTAP gene can also be counseled about their likelihood of carrying the gene. The researchers have already tested the siblings of the infant who died 15 years ago. Dr. Marini said that she and her colleagues are investigating whether additional forms of OI might be caused by other recessive genes.
Dr. Marini said that the discovery of CRTAP's involvement with OI is exciting to researchers in the bone and genetics fields because it opens a new and unanticipated field of bone biology related to osteoporosis. She explained that the NIH team is now working to understand the role of this collagen-modification complex in normal bone development. Since CRTAP protein occurs elsewhere in the body, it may play an important role in the development of other tissues, Dr. Marini added.