Genetic Testing

Hypochondroplasia (HCH) is an autosomal dominant skeletal dysplasia caused by pathogenic variants in the FGFR3 gene (fibroblast growth factor receptor 3) located on chromosome 4p16.3. 

Molecular confirmation is achieved through identification of a heterozygous pathogenic variant in FGFR3, supporting diagnostic clarification and differentiation from other skeletal dysplasias.

Genetic testing should be considered in individuals presenting with:

• Disproportionate short stature
• Reduced growth velocity in early childhood
• Radiological features of skeletal dysplasia
• Clinical overlap with achondroplasia or related FGFR3 disorders
• Family history suggestive of autosomal dominant inheritance

Testing is also appropriate in prenatal settings where skeletal dysplasia is suspected, in individuals with mild or atypical phenotypes, and where clinical and radiographic findings are inconclusive.

Diagnosis is based on combined clinical, radiological, and molecular assessment.

Hypochondroplasia results from gain‑of‑function variants in FGFR3, leading to impaired endochondral ossification and altered skeletal growth. 

The most common variant is c.1620C>A (p.Asn540Lys; N540K), accounting for approximately 60% of molecularly confirmed cases.

The disorder demonstrates genetic heterogeneity, and not all individuals with clinical HCH have a detectable variant using standard testing approaches.

Targeted testing for recurrent FGFR3 variants, particularly N540K, may be used in cases with strong clinical suspicion. This approach provides rapid results but has limited sensitivity for rare or novel variants.

Where targeted testing is negative or inconclusive, full sequencing of FGFR3 is recommended. This is typically performed using next‑generation sequencing and may include deletion/duplication analysis. It improves detection of rare or novel variants and is important given the recognised genetic heterogeneity in HCH.

Multigene skeletal dysplasia panels may be used where diagnosis is uncertain. These panels include FGFR3 along with other genes associated with skeletal dysplasias and can assist in differentiating overlapping phenotypes.

Prenatal testing may be offered when a familial pathogenic variant is known or when ultrasound findings suggest skeletal dysplasia. Testing is performed using chorionic villus sampling or amniocentesis, allowing direct molecular analysis of fetal DNA.

Once a pathogenic variant is identified, cascade testing can be offered to relatives to determine inheritance pattern. Given autosomal dominant inheritance, there is a 50% recurrence risk for offspring of affected individuals.

mRNA testing is not part of routine diagnostic evaluation in hypochondroplasia. Most pathogenic variants in FGFR3 associated with HCH are missense variants that alter protein function rather than affecting RNA expression or splicing. As a result, DNA-based sequencing remains the primary diagnostic modality.

RNA-based analysis may be considered in specific contexts, including assessment of variants of uncertain significance where splicing effects are suspected, or in research settings evaluating transcript behaviour and downstream signalling effects.

Limitations of RNA-based approaches include the requirement for high-quality RNA samples, tissue-specific expression of FGFR3, and lack of standardisation in clinical diagnostics. At present, RNA analysis remains a specialised or research-level tool rather than a routine clinical test.

Pathogenic or likely pathogenic variants confirm the molecular diagnosis and support clinical management and genetic counselling.

Variants of uncertain significance require correlation with phenotype and imaging and may require further familial or functional studies.

A negative result does not exclude hypochondroplasia and may reflect undetected variants, technical limitations, or an alternative diagnosis.

Approximately 70% of individuals with clinically diagnosed hypochondroplasia have identifiable FGFR3 variants. 

Rare or novel variants may not be detected or fully characterised, and interpretation challenges persist, particularly for variants of uncertain significance. Clinical assessment remains central to diagnosis.

Genetic counselling is recommended both before and after testing. It supports understanding of inheritance, recurrence risk, and the implications of test results, including uncertain findings.