Achondrogenesis type II: A severe inherited disorder of bone growth characterized by a short body and limbs and a lack of bone formation in the spine and pelvis.

Infants with this disorder have short arms and legs, a small chest with short ribs, and underdeveloped lungs. The skull bones may be soft, but they often appear normal on X-ray images. In contrast, bones in the spine (vertebrae) and pelvis do not harden, or ossify. Typical facial features include a prominent forehead, a small chin, and, in some cases, an opening in the roof of the mouth (cleft palate). The abdomen is enlarged, and excess fluid builds up in the body before birth (a condition called hydrops fetalis). Infants with this disease are usually premature and stillborn or die shortly after birth from respiratory failure. Some infants have lived for a time, however, with intensive medical support.

Mutations in a gene called COL2A1 cause achondrogenesis type II, which is the most severe condition in a spectrum of disorders caused by mutations in the COL2A1 gene. The protein made by this gene forms type II collagen, a molecule found mostly in cartilage and in the clear gel that fills the eyeball (the vitreous). Type II collagen is essential for the normal development of bones and other connective tissues (tissues that form the body's supportive framework). Mutations in the COL2A1 gene interfere with the assembly of type II collagen molecules, which prevents bones from developing properly. Achondrogenesis type II is thus a collagenopathy.

Achondrogenesis type II is caused by a new mutation in the COL2A1 gene. The disorder is an autosomal dominant disorder because only one copy of the altered gene is necessary to cause the condition. The disorder is not passed on to the next generation, however, because affected individuals do not live long enough to have children.

The symbol for achondrogenesis II is ACG2. The disease is also known as achondrogenesis, Langer-Saldino type; chondrogenesis imperfecta; and achondrogenesis-hypochondrogenesis, type II.