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What Causes Achondroplasia?

People concerned about the condition in their families may wonder, "What causes achondroplasia?" In most cases, this common type of short-limbed dwarfism is related to a genetic mutation in the fibroblast growth factor receptor 3 gene. However, one or both parents passing the altered gene to their children is also what causes achondroplasia.

What Causes Achondroplasia? -- An Overview

There are two possible causes of achondroplasia. One possible cause is a genetic mutation in the fibroblast growth factor receptor 3 (FGFR3) gene located on chromosome 4. The other possible cause is that the condition can be inherited from a parent with achondroplasia.

What Causes Achondroplasia? -- Genetic Mutation

Most people with achondroplasia have average-size parents, which means that the cause of achondroplasia is from a new mutation in the FGFR3 gene. Scientists do not know why this mutation occurs.

What Causes Achondroplasia? -- Inheritance

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 on 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 achondroplasia, 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.

What Causes Achondroplasia? -- The FGFR3 Gene

Although achondroplasia can be inherited, 80 percent of all cases are due to a sporadic mutation involving the gene encoding FGFR3, situated on chromosome 4.
The protein made by the FGFR3 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 receptor is absent, and so 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.)
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What Is Achondroplasia?

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