SADDAN

Overview
SADDAN (severe achondroplasia with developmental delay and acanthosis nigricans) is a rare inherited disorder of bone growth characterized by skeletal, brain, and skin abnormalities.

All people with this condition are extremely short with particularly short arms and legs. Other signs and symptoms include unusual bowing of the leg bones; a small chest with short ribs and curved collar bones; short, broad fingers; and folds of extra skin on the arms and legs. Structural abnormalities of the brain lead to seizures, profound developmental delay, and mental retardation. Acanthosis nigricans, a progressive skin disorder characterized by thick, dark, velvety skin, develops in infancy or early childhood. People with SADDAN often live into childhood and adulthood.

Many of the features of SADDAN are similar to those seen in other skeletal disorders, specifically achondroplasia and thanatophoric dysplasia.

Epidemiology
This disorder is very rare; it has been described in only a small number of individuals worldwide.

Genetics
Mutations in the FGFR3 gene cause SADDAN. The protein made by the FGFR3 gene is a receptor that plays a role in the development and maintenance of bone and brain tissue. A mutation in the gene may overactivate the receptor, which leads to disturbances in bone growth. Researchers have not determined how the mutation disrupts brain development or causes acanthosis nigricans.

SADDAN is considered an autosomal dominant disorder because one mutated copy of the FGFR3 gene in each cell is sufficient to cause the condition. The few described cases of SADDAN have been caused by new mutations in the FGFR3 gene. No individuals with this disorder are known to have had children; therefore, the disorder has not been passed to the next generation.