Achondroplasia is a type of genetic disorder that is a common cause of dwarfism. People with this condition have short stature, usually reaching a full adult height of around 4'0" (1.2 metres).
The disorder is a result of an autosomal dominant mutation in the fibroblast growth factor receptor gene 3 (FGFR3), which causes an abnormality of cartilage formation. FGFR3 normally has a negative regulatory effect on bone growth. In achondroplasia, the mutated form of the receptor is constitutively active and this leads to severely shortened bones.
People with achondroplasia have one normal copy of the fibroblast growth factor receptor 3 gene and one mutant copy. Two copies of the mutant gene are invariably fatal before or shortly after birth. Only one copy of the gene needs to be present for the disorder to occur. Thus, a person with achondroplasia has a 50% chance of passing on the gene to their offspring, meaning that 1 in 2 of their children will have achondroplasia. Since two copies are fatal, if two people with achondroplasia have a child, there's a 25% chance of it dying shortly after birth, a 50% chance the child will have achondroplasia, and a 25% chance the child will have a normal phenotype. However, in 3 out of 4 cases, people with achondroplasia are born to parents who don't have the condition. This is the result of a new mutation.
New gene mutations are associated with increasing paternal age (over 35 years). Studies have demonstrated that new gene mutations are exclusively inherited from the father and occur during spermatogenesis (as opposed to resulting from a gonadal mosaicism).
There are two other syndromes with a genetic basis similar to achondroplasia: hypochondroplasia and thanatophoric dysplasia. Both of these disorders are also caused by a genetic mutation in the FGFR3 gene.
Growth hormone (GH) therapy has been proposed as a possible treatment for the short stature of achondroplasia. However, the people who participated in the studies on the subject have not yet reached adult size, so this type of therapy has unknown results.
Early experience with surgical limb lengthening procedures resulted in a high incidence of complications, but recent experiences have improved results considerably.
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