X-linked Chondrodysplasia Punctata 2
X-linked chondrodysplasia punctata 2 is an uncommon genetic disorder affecting bone, skin, and eye development that occurs almost exclusively in females due to mutations that disrupt cellular cholesterol production.
Overview
X-linked chondrodysplasia punctata 2 is a genetic disorder characterized by abnormalities involving the bones, skin, and eyes. The condition occurs almost exclusively in females, who typically experience normal intelligence and a standard lifespan. Although rare, a much more severe and often life-threatening form has been documented in a small number of males.
Signs and Symptoms
The signs and symptoms of X-linked chondrodysplasia punctata 2 vary considerably among affected individuals, but commonly include features affecting several body systems:
Skeletal Abnormalities
- Chondrodysplasia punctata: An abnormality observed on x-rays as spots, or stippling, near the ends of bones and in cartilage. Stippling characteristically involves the long bones of the arms and legs, ribs, vertebrae, and tracheal cartilage. It appears during infancy and disappears in early childhood.
- Rhizomelia: Shortening of the bones in the upper arms and thighs, which is frequently asymmetrical (affecting the left and right sides differently).
- Spinal curvature: Progressive abnormal curvature of the spine.
- Short stature: Reduced overall height resulting from these cumulative skeletal irregularities.
Skin and Hair Features
- Ichthyosis: Dry, scaly patches of skin presenting at birth in a linear or spiral (whorled) pattern. These patches fade over time.
- Follicular atrophoderma: Abnormally colored blotches of skin lacking hair, which remain after the scaly patches fade.
- Scalp involvement leading to patches without hair.
Eye and Vision Abnormalities
- Clouding of the lens of the eye present from birth or early childhood.
- Microphthalmia: Unusually small eyes.
- Microcornea: Abnormally small corneas (the clear front surface of the eye).
- Vision impairment resulting from these ocular malformations.
Features in Males
In the small number of males identified with the condition, symptoms are far more severe and frequently life-threatening. Alongside the abnormalities seen in females, affected males may experience:
- Hypotonia: Weak muscle tone.
- Structural changes in the brain.
- Moderately to profoundly delayed development.
- Seizures.
- Distinctive facial features and other congenital birth defects.
Frequency
The condition is estimated to affect fewer than 1 in 400,000 newborns. However, the true prevalence may be higher because the condition is likely underdiagnosed, especially in females who exhibit mild signs and symptoms. More than 95 percent of identified cases occur in females, with approximately a dozen affected males reported in scientific literature.
Causes & Genetics
X-linked chondrodysplasia punctata 2 is caused by mutations in the EBP gene. This gene provides instructions for producing the enzyme 3β-hydroxysteroid-Δ8,Δ7-isomerase, which is required for one of the final stages in the synthesis of cholesterol.
Cholesterol is essential for typical embryonic development and plays critical roles both before and after birth, serving as a structural component of cell membranes and assisting in hormone and digestive acid production. Mutations in EBP reduce enzyme activity, leading to insufficient cholesterol production and the accumulation of potentially toxic metabolic byproducts. This combination disrupts normal growth and development across multiple body systems, though the exact biological process producing the specific signs of the disorder remains unknown.
Inheritance
This condition follows an X-linked dominant pattern of inheritance. The EBP gene resides on the X chromosome, one of the two sex chromosomes.
- In females: A mutation in one of the two copies of the EBP gene in each cell is sufficient to produce the disorder. Because of X-inactivation, some cells generate a normal amount of the enzyme while others produce none, leading to an overall enzymatic deficiency.
- In males: Males possess only one X chromosome. A mutation in their single copy of the EBP gene typically leads to a complete absence of the 3β-hydroxysteroid-Δ8,Δ7-isomerase enzyme. This total loss is usually lethal during early development, explaining why very few males are born with the condition.
Other Names
- CDPX2
- Chondrodysplasia punctata 2, X-linked
- Conradi-Hünermann syndrome
- Conradi-Hünermann-Happle syndrome
- Happle syndrome
- X-linked dominant chondrodysplasia punctata