Although 98% of ACH cases are accounted for by a single G380R substitution in the TM, a common mutation (N540K) in the TK1 region is detected in only 60-65% of HCH cases.
More than 95% of 242 cases reported so far are accounted for by a single Gly380Arg mutation.McKusick et al. proposed that achondroplasia and hypochondroplasia are allelic based on the similarities in phenotype between the two disorders and the identification of a severely dwarfed individual whose father had achondroplasia and whose mother had hypochondroplasia.
Most individuals with ACH have the recurrent mutation (p.Gly380Arg) in the transmembrane (TM) domain of the receptor and individuals with HCH show the common mutation (p.Asn540Lys) in the tyrosine kinase 1 (TK1) region.
The father has achondroplasia due to the common G1138A (G380R) mutation in the fibroblast growth factor receptor 3 (FGFR3) gene, while the mother has hypochondroplasia due to the C1620G (N450K) mutation in the FGFR3 gene.
The mapping of the achondroplasia locus to the short arm of chromosome 4 and the subsequent identification of a recurrent missense mutation (Gly380Arg) in the gene encoding fibroblast growth factor receptor 3 (FGFR-3) has been followed by the detection of common FGFR-3 mutations in two clinically related disorders: thanatophoric dysplasia (TD; types I and II) and hypochondroplasia.
The mother has achondroplasia and carries the common G1138 (G380R) mutation in the FGFR3 gene; the father has hypochondroplasia due to the C1620A (N540K) mutation in the same gene.
This assay, which is performed on the LightCycler thermocycler, enables the rapid and reliable detection of the two most common FGFR3 mutations associated with ACH (1138G --> A and 1138G --> C; G380R) and HYCH (1620C --> A and 1620 C --> G; N540K) in a single test.