Opitz G/BBB syndrome is a genetic condition that causes several abnormalities along the midline of the body. "G/BBB" represents the first letters of the last names of the families first diagnosed with this disorder and "Opitz" is the last name of the doctor who first described the signs and symptoms. There are two forms of Opitz G/BBB syndrome, X-linked Opitz G/BBB syndrome and autosomal dominant Opitz G/BBB syndrome. The two forms are distinguished by their genetic causes and patterns of inheritance. The signs and symptoms of the two forms are generally the same.

X-linked Opitz G/BBB syndrome is caused by mutations in the MID1 gene. The MID1 gene provides instructions for making a protein called midline-1. This protein attaches (binds) to microtubules, which are rigid, hollow fibers that make up the cell's structural framework (the cytoskeleton). Microtubules help cells maintain their shape, assist in the process of cell division, and are essential for the movement of cells (cell migration). Midline-1 assists in recycling certain proteins that need to be reused instead of broken down. MID1 gene mutations lead to a decrease in midline-1 function, which prevents protein recycling. The resulting accumulation of proteins impairs microtubule function, leading to problems with cell division and migration. It is unclear how these changes disrupt normal development and cause the signs and symptoms of Opitz G/BBB syndrome.

X-linked Opitz G/BBB syndrome is thought to affect 1 in 10,000 to 50,000 males, although it is likely that this condition is underdiagnosed.

When caused by mutations in the MID1 gene, Opitz G/BBB syndrome has an X-linked pattern of inheritance. It is considered X-linked because the MID1 gene is located on the X chromosome, one of the two sex chromosomes in each cell. In males, who have only one X chromosome, a mutation in the only copy of the gene in each cell is sufficient to cause the condition. In females, who have two copies of the X chromosome, one altered copy of the gene in each cell can lead to less severe features of the condition or may cause no symptoms at all. Because it is unlikely that females will have two altered copies of the MID1 gene, females with X-linked Opitz G/BBB syndrome typically have hypertelorism as the only sign of the disorder. A characteristic of X-linked inheritance is that fathers cannot pass X-linked traits to their sons.

Published Date: January 01, 2015
Published By: National Institutes of Health