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Gene Involved in Brittle Bone Disease

Researchers discovered a third gene that accounts for previously unexplained forms of osteogenesis imperfecta, a genetic condition that weakens bones, results in frequent fractures and is sometimes fatal.

Osteogenesis imperfecta affects between 20,000 and 50,000 Americans. About 85% of all cases are caused by mutations in the genes needed to make the protein collagen. Collagen functions as a molecular scaffolding that holds together bone, skin and other tissues.

Dr. Joan Marini ar Â鶹´«Ã½'s Eunice Kennedy Shriver National Institute of Child Health and Human Development (NICHD) and her colleagues had earlier found that osteogenesis imperfecta could also be caused by defects in the protein complex that modifies collagen before it is secreted from cells to become a part of bone. Mutations in the genes for 2 proteins in this complex—cartilage associated protein (CRTAP) and prolyl 3-hydroxylase 1 (P3H1)—could result in severe and sometimes fatal forms of osteogenesis imperfecta.

The new study involved a collaboration between researchers at NICHD, led by Marini, and at the Hospital for Special Surgery in New York City, led by Dr. Cathleen Raggio. Scientists at the University of Washington in Seattle and NIH's National Institute of Human Genome Research (NHGRI) also took part. Additional support came from other Â鶹´«Ã½ institutes.

The researchers diagnosed a 12-year-old boy and his 4-year-old sister as having a novel form of osteogenesis imperfecta, as described in the online edition of the New England Journal of Medicine on January 20, 2010. The children's parents were immigrants from Senegal living in New York. Although the children's bones were brittle and highly susceptible to fracturing, they did not have shortening of the upper portion of limbs seen in children with mutations in CRTAP and P3H1.

The researchers analyzed the children's DNA and found that they had a mutation in a gene coding for Cyclophilin B that causes a complete lack of the protein. Cyclophilin B is part of the same complex with CRTAP and P3H1. Like the mutations in those genes, the mutation in Cyclophilin B is recessive, requiring 2 defective copies to cause the disorder.

A previous study found that Cyclophilin B was essential for folding collagen into its final form. Marini and her colleagues, however, discovered that collagen from the 2 children was folded into its usual configuration. Cyclophilin B, then, must not be the only protein to have this role in collagen folding.

Marini notes that additional research is needed to determine why, despite the seemingly normal collagen folding, the children developed osteogenesis imperfecta.

"The discovery provides insight into a previously undescribed form of osteogenesis imperfecta," says NICHD Acting Director Dr. Alan E. Guttmacher. "The advance also provides new information on how collagen folds during normal bone formation, which may also lead to greater understanding of other bone disorders."