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FGFR3 (Fibroblast Growth Factor Receptor 3) is a tyrosine kinase receptor that regulates endochondral ossification, cell proliferation, and differentiation, particularly in bone growth and brain development.

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Gain-of-function mutations in FGFR3 lead to a spectrum of skeletal dysplasias, ranging from mild short stature to lethal skeletal abnormalities. Rarely, loss-of-function mutations are associated with skin and cranial anomalies.

Key Points

Feature	Description
Gene locus	Chromosome 4p16.3
Protein function	Regulates bone growth by inhibiting chondrocyte proliferation
Mutation type	Typically gain-of-function mutations → excessive inhibition of growth plate cartilage

Spectrum of FGFR3-Related Disorders

Condition	Severity	Mutation	Key Features
Thanatophoric Dysplasia (TD)	Lethal	Various (R248C, K650E/M)	Extreme micromelia, narrow thorax, Type I: curved femurs, Type II: cloverleaf skull
Achondroplasia	Severe non-lethal	G380R (most common)	Rhizomelic dwarfism, frontal bossing, lumbar lordosis, trident hands
Hypochondroplasia	Mild	N540K (common)	Milder limb shortening, subtle features, normal cognition, often diagnosed later
SADDAN syndrome	Severe	K650M	Severe Achondroplasia with Developmental Delay and Acanthosis Nigricans
Crouzon syndrome with acanthosis nigricans	Craniosynostosis + skin	A391E	Premature fusion of skull sutures, proptosis, acanthosis nigricans
Muenke syndrome	Craniosynostosis	P250R	Coronal craniosynostosis, hearing loss, variable expressivity
Seborrheic keratoses / epidermal nevi	Somatic mosaic	Various	Localized FGFR3 mutations cause benign skin lesions

Radiology

Radiological hallmarks by condition:

Condition	Radiographic Features
Thanatophoric dysplasia	“Telephone receiver” femurs, platyspondyly, cloverleaf skull (Type II)
Achondroplasia	Rhizomelia, narrow interpedicular distance, trident pelvis, foramen magnum stenosis
Hypochondroplasia	Milder findings: short bones, interpedicular narrowing, squared iliac wings
SADDAN	Overlapping with achondroplasia + cerebral anomalies
Muenke/Crouzon syndromes	Cranial CT shows premature coronal suture fusion, facial bone abnormalities

Neurological & Systemic Associations:

Condition	Neurological / Systemic Impact
Thanatophoric dysplasia	Temporal lobe dysplasia, lethal at birth due to lung hypoplasia
Achondroplasia	Foramen magnum stenosis → hydrocephalus, brainstem compression
SADDAN syndrome	Intellectual disability, seizures, brain abnormalities
Muenke/Crouzon	Hearing loss, increased intracranial pressure

Diagnosis

Genetic Testing & Counseling:

Step	Relevance
Molecular testing (FGFR3 sequencing)	Confirms diagnosis; can be done prenatally or postnatally
Inheritance pattern	Most are autosomal dominant, often de novo mutations
Paternal age effect	Increased mutation risk with advanced paternal age
Recurrence risk	Usually <1% if parents are unaffected (unless mosaicism present)