Achondroplasia - Causes

Causes

In normal figures, FGFR3 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 FGFR3 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 has to be present for the condition to occur. Therefore, a person with achondroplasia has a 50% chance of passing on the gene to his or her offspring, meaning that there will be a 50% chance that each child will have achondroplasia. Since it is fatal to have two copies (homozygous), if two people with achondroplasia have a child, there is a 25% chance of the child dying shortly after birth, a 50% chance the child will have achondroplasia, and a 25% chance the child will have an average phenotype. People with achondroplasia can be born to parents that do not have the condition. This is the result of a new mutation.

New gene mutations leading to achondroplasia are associated with increasing paternal age (over 35 years old). Studies have demonstrated that new gene mutations for achondroplasia are exclusively inherited from the father and occur during spermatogenesis; it is theorized that oogenesis has some regulatory mechanism that hinders the mutation from originally occurring in females (although females are still readily able to inherit and pass on the mutant allele). More than 99% of achondroplasia is caused by two different mutations in the FGFR3. In about 98% of cases, a G to A point mutation at nucleotide 1138 of the FGFR3 gene causes a glycine to arginine substitution (Bellus et al. 1995, Shiang et al. 1994, Rousseau et al. 1996). About 1% of cases are caused by a G to C point mutation at nucleotide 1138. The mutant gene was discovered by John Wasmuth and his colleagues in 1994.

There are two other syndromes with a genetic basis similar to achondroplasia: hypochondroplasia and thanatophoric dysplasia. Or it could just be a birth defect.