The Roles of Fibroblast Growth Factor Receptor 3 (FGFR3) in the Spectrum of Skeletal Dysplasia

The fibroblast growth factor receptor 3 (FGFR3) gene mutations were identified to be involved in the pathogenesis of most chondropathies. The FGFR3 gene encodes the FGFR3 receptor and is involved in the regulation of bone growth by limiting the ossification of long bones. Mutations of the FGFR3 gene result in abnormal cell proliferation and improper cartilage development. We aim to provide an overview of the roles of FGFR3 in skeletal dysplasia by highlighting the pathogenesis, clinical variants of skeletal dysplasia, diagnosis, and their management.

Achondroplasia is the most common form of chondropathies, occurring in approximately 1 in 20,000-30,000 live births, and it is the most common form of genetic dwarfism. In over 80% of cases of achondroplasia, the mutation is sporadic, and only 20% are inherited autosomal dominant. Achondroplasia results from a point mutation in the gene encoding the transmembrane portion of FGFR3. Two viable base substitutions are identified in achondroplasia, including a point mutation of guanine substituted for adenine (c.1138G>A); this is identified in approximately 98% of the affected individuals, and transversion of guanine to cytosine (c.1138G>C).

Hypochondroplasia is a milder form of chondropathies with an incidence between 1 in 33,000 and 1 in 47,000 live births. Missense mutations of FGFR3 (p.Asn540Lys) are isolated in tyrosine kinase domain I occurring in approximately 60% of cases, and missense mutation of FGFR3 (p.Lys650Asn) identified in the tyrosine kinase domain II of FGFR3. Thanatophoric dysplasia is the most lethal form of chondropathies, with neonatal fetal death secondary to pulmonary hypoplasia. In thanatophoric dysplasia, there is Lys650Met substitution in FGFR3 (Type I) with impairment of endochondral bone growth and a pathogenic variant of p.Lys650Glu substitution in FGFR3 (Type II). In addition, specific amino acid substitution in the FGFR3 gene (G380R) was identified to be associated with severe achondroplasia with developmental delay and acanthosis nigricans (SADDAN).

The diagnosis of achondroplasias is routinely made from clinical presentations and radiological findings. FGFR3 molecular genetic testing is performed in children with atypical presentations. In addition, a pre-implantation genetic diagnosis should be available for parents pursuing in-vitro fertilization and embryo implantation procedures. Management of chondropathies includes symptomatic treatment with drugs, surgical intervention, and lifelong follow-up care. Different pharmacological options have been used, including those that directly block FGFR3 activation or regulate signalling pathways controlling chondrocyte proliferation and differentiation.