ACHONDROPLASIA

Achondroplasia — the most common skeletal dysplasia and now the first with a targeted growth-modifying therapy (vosoritide), where molecular confirmation of the FGFR3 variant is required for treatment eligibility.

Whole genome sequencing confirms the specific FGFR3 variant — distinguishing achondroplasia from hypochondroplasia, thanatophoric dysplasia, and other FGFR3-related skeletal conditions that present with overlapping features but require different management.

FGFR3

About this condition

Achondroplasia

Achondroplasia is the most common form of disproportionate short stature, affecting approximately 1 in 15,000-40,000 births worldwide. It is caused by a specific gain-of-function missense variant in FGFR3 (fibroblast growth factor receptor 3) on chromosome 4p16.3 — p.Gly380Arg (c.1138G>A, accounting for ~98% of cases, or c.1138G>C, accounting for ~1-2%). FGFR3 is a negative regulator of endochondral ossification: the gain-of-function variant constitutively activates the receptor, suppressing chondrocyte proliferation and differentiation at the growth plate, producing rhizomelic (proximal) limb shortening.

Achondroplasia is autosomal dominant with complete penetrance. Approximately 80% of cases arise as de novo mutations — both parents are of average stature. Advanced paternal age is a risk factor for de novo FGFR3 variants. Clinical features include rhizomelic short limbs, macrocephaly with frontal bossing, midface hypoplasia, trident hand configuration, and lumbar lordosis. Medical complications include foramen magnum stenosis (requiring neurosurgical assessment in infancy), obstructive sleep apnea, recurrent otitis media, and spinal stenosis in adulthood. Adult height averages approximately 131cm in males and 124cm in females.

Vosoritide (Voxzogo), a C-type natriuretic peptide analog that counteracts FGFR3-mediated growth plate suppression, was approved by the FDA in 2021 and the EMA in 2021 for treatment of achondroplasia in children aged 5 and older with open growth plates. Clinical trials demonstrated a mean increase in annualized growth velocity of approximately 1.57 cm/year compared to placebo. Vosoritide is the first targeted, disease-modifying therapy for any skeletal dysplasia — and molecular confirmation of the FGFR3 achondroplasia variant is required for prescribing eligibility.

Homozygous achondroplasia (both FGFR3 alleles carrying p.Gly380Arg) is a lethal condition with severe skeletal abnormalities and respiratory failure. Couples where both partners have achondroplasia have a 25% risk of a homozygous-affected pregnancy — genetic counseling is essential.

Gene locus: FGFR3 (4p16.3)

Clinical diagnosis of achondroplasia is usually straightforward, but molecular confirmation is required for vosoritide eligibility and distinguishes achondroplasia from other FGFR3 skeletal dysplasias with different prognoses.

Vosoritide prescribing requires confirmed FGFR3 achondroplasia variant — clinical diagnosis alone is not sufficient

Vosoritide (Voxzogo) is indicated specifically for achondroplasia confirmed by FGFR3 molecular testing. Other FGFR3-related skeletal dysplasias — hypochondroplasia (FGFR3 p.Asn540Lys), thanatophoric dysplasia types I and II (various FGFR3 variants), and SADDAN (Severe Achondroplasia with Developmental Delay and Acanthosis Nigricans) — are caused by different FGFR3 variants with different functional consequences. Vosoritide clinical trial data specifically applies to the achondroplasia p.Gly380Arg variant. Molecular confirmation ensures that the therapy is being applied to the correct genotype and that clinical trial efficacy data is applicable.

Couples where both partners have achondroplasia face a 25% risk of lethal homozygous disease — genetic counseling requires confirmed genotype

When both parents have achondroplasia, each pregnancy has a 25% chance of homozygous achondroplasia (lethal), 50% chance of heterozygous achondroplasia (like the parents), and 25% chance of average stature. Prenatal molecular diagnosis through CVS or amniocentesis can distinguish these outcomes, and PGT-M with IVF can select against the homozygous genotype. This reproductive counseling requires confirmed FGFR3 variant characterization in both parents. Additionally, some patients clinically diagnosed with achondroplasia may actually have hypochondroplasia or another skeletal dysplasia — molecular confirmation resolves diagnostic ambiguity.

Achondroplasia — the most common skeletal dysplasia and now the first with a targeted growth-modifying therapy (vosoritide), where molecular confirmation of the FGFR3 variant is required for treatment eligibility.

Achondroplasia

Clinical diagnosis of achondroplasia is usually straightforward, but molecular confirmation is required for vosoritide eligibility and distinguishes achondroplasia from other FGFR3 skeletal dysplasias with different prognoses.

Vosoritide prescribing requires confirmed FGFR3 achondroplasia variant — clinical diagnosis alone is not sufficient

Couples where both partners have achondroplasia face a 25% risk of lethal homozygous disease — genetic counseling requires confirmed genotype

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