Most people with the disorder have average-size parents, which means that achondroplasia is caused by a new mutation in the fibroblast growth factor receptor 3 (FGFR3) gene. Scientists do not know why this mutation occurs.
(Click FGFR3 Gene for more information.)
Achondroplasia can also be inherited in an autosomal dominant pattern, which means that one copy of the altered gene in each cell is sufficient to cause the disorder. In these cases, one of the parents with achondroplasia passes the FGFR3 gene to the child.
If one parent has achondroplasia, children have a 50 percent chance of inheriting the FGFR3 gene. If both parents have it, children have a one in four chance of inheriting the gene from both parents. Newborns who inherit both genes are considered to have a severe form of achondroplasia, where survival is usually less than 12 months after birth.
(Click Cause of Achondroplasia for more information.)
Although achondroplasia can be inherited, 80 percent of cases are due to new, sporadic mutations involving the FGFR3 gene. The protein made by the gene is a receptor that regulates bone growth by limiting the formation of bone from cartilage (a process called ossification), particularly in the long bones. Researchers believe that mutations in the FGFR3 gene cause the receptor to be overly active, which interferes with ossification and leads to the disturbances in bone growth seen with this disorder.
This theory is supported by the knockout mouse model in which the FGFR3 receptor is absent. Therefore, the negative regulation of bone formation is lost. The result is a mouse with excessively long bones and elongated vertebrae, resulting in a long tail. Achondroplastic mouse models are useful tools in developing potential treatments.
(Click Achondroplasia Genetics for more information.)