Learning Objectives

• Explain the molecular basis of Achondroplasia involving the FGFR3 mutation.
• Differentiate between the effects on endochondral and membranous ossification.
• Identify the genetic inheritance pattern and associated risk factors like paternal age.
• Recognize the clinical phenotype of short-limbed dwarfism.

1. Pathophysiology & Bone Growth

Achondroplasia is the most common cause of dwarfism. It is specifically a disorder of the epiphyseal plate (growth plate).

• Endochondral Ossification: There is a total failure of longitudinal bone growth. Because long bones (arms/legs) rely on a cartilage scaffold, they remain short.
• Membranous Ossification: This process is unaffected. Because the bones of the skull (calvarium) and face form directly from mesenchyme without cartilage, they grow normally.
• Clinical Result: Disproportionate dwarfism—short limbs with a large head (macrocephaly) relative to the body.

2. Molecular Genetics

The condition is caused by a specific gain-of-function mutation in the Fibroblast Growth Factor Receptor 3 (FGFR3) gene on chromosome 4.

• The “Paradox”: Under normal conditions, FGFR3 inhibits bone growth. In achondroplasia, the receptor is constitutively active (always “on”). This constant signaling inhibits the proliferation of chondrocytes (cartilage cells), preventing bone lengthening.
• Inheritance:
  • Autosomal Dominant: It has full penetrance.
  • Lethality: Homozygosity () is lethal in utero or shortly after birth due to respiratory failure (tiny chest cavity).
  • Sporadic Mutations: > 85% of cases are new mutations (de novo), often associated with advanced paternal age.

Activity: Achondroplasia Fact Check

Activity: