p.Ser348Cys mutation in FGFR3 gene leads to "Mild ACH /Severe HCH" phenotype

Achondroplasia (ACH) and hypochondroplasia (HCH) are genetic bone disorders known to be caused by gain-of-function mutations in the fibroblast growth factor receptor 3 (FGFR3) gene. Both conditions share radiographic and phenotypical features. HCH is a milder form of ACH. 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. Other rare mutations have been reported, however no additional hot-spot has been identified. We report an 8-month-old infant, with the heterozygous mutation, c.1043C > G, leading to an amino acid change from serine at 348 to cysteine (p.Ser348Cys). Clinical diagnosis of the patient is intertwined with "mild ACH" or "severe HCH". He did not demonstrate acanthosis nigricans (AN). This mutation has been reported in two different patients and it is located in the Ig-III domain of the FGFR3 region near other mutations associated with ACH. Among the two the 8-year old one also demonstrated AN without evindece of hyperinsulinem. This report emphasizes the benefit of whole gene sequencing for FGFR3 in individuals with suspected "mild ACH/severe HCH". This child will be monitored for future occurrence of AN.