About achondroplasia
What is achondroplasia?
Achondroplasia is the most common form of skeletal dysplasia. The features of achondroplasia include disproportionate short stature with shortened limbs.
What causes achondroplasia?
Achondroplasia is caused by a genetic change in the Fibroblast Growth Factor Receptor-3 or FGFR3 gene. Almost all people with achondroplasia have the same change in the same place in the gene, called G380R.
DNA is the master programmer of our bodies. It is the material inside our body’s cells. This master code acts like an instruction manual for making all of the body’s proteins. DNA is organized into thousands of genes. Genes are translated into proteins, and have very specific functions. The FGFR3 protein is active in an area of the bones called the growth plate. The growth plate plays a major role in bone and cartilage development in the body.
In achondroplasia, the DNA change within the FGFR3 gene impacts the function of the growth plate. Growth plates are present in the long bones, spine, and in the skull base. In individuals with achondroplasia, the change in FGFR3 causes excess signals, which tell the body to slow bone growth. The slower bone growth results in disproportionate short stature with shortened limbs in individuals with achondroplasia.
How is achondroplasia inherited?
Achondroplasia is inherited in an autosomal dominant manner. This means that an individual with achondroplasia has a single version of the FGFR3 gene with the typical change that causes achondroplasia.
Most children with achondroplasia, about 80%, are born to parents of average stature. These average statured parents do not carry the FGFR3 change. The change typically arises in the sperm (or less likely the egg) prior to conception. It is important that families understand that there is nothing that parents did to cause the FGFR3 change, nor anything that they could have done which would have prevented it.
When a person with achondroplasia has a child, there will be a 50% chance that the child will inherit a copy of the changed FGFR3 gene, and therefore also have achondroplasia.
What does the name achondroplasia mean?
The term “achondroplasia” in Latin roughly translates to a lack of cartilage formation. It was first proposed by Dr. Julius Parrot in 1878 to describe children with dwarfism. We now understand that name is technically incorrect in that people with achondroplasia do in fact, form cartilage.
How common is achondroplasia?
It is estimated that 1 in 15,000 to 40,000 children born have achondroplasia, with approximately 250,000 affected individuals worldwide.
What are the physical characteristics of achondroplasia?
Perhaps the most notable physical characteristic of achondroplasia, especially in older children and adults, is short stature. However, at birth, infants may have typical lengths. In the US population, the average height for adult men with achondroplasia is 129.9 cm or 4 foot 3 inches, and the average height for adult women with achondroplasia is 122.4 cm or 4 feet.
The short stature that occurs in achondroplasia is not proportionate. Disproportionate short stature means that the arms and legs are shortened compared to the length of the torso. Furthermore, within the arms and legs, the upper or proximal segments are shortened compared to the middle segments. This type of shortening is termed rhizomelia.
Additional features of achondroplasia include a head size that is usually larger than average and flattened features in the middle face. Most infants and children have increased joint mobility throughout, except at the elbows where there is decreased elbow extension.
How is achondroplasia diagnosed?
Classically, achondroplasia has been diagnosed by clinical and radiographic (X-ray) images, with most children being diagnosed within their first month of life. More and more commonly achondroplasia is diagnosed prenatally. The diagnosis may be considered based upon the findings of a fetal ultrasound, but rarely before the third trimester. DNA testing can be used in prenatal and postnatal settings to make or confirm the diagnosis as well. Fetuses can also now be diagnosed using a newer DNA testing technology called cell-free DNA testing.
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