Achondrogenesis Explained
The Houston-Harris, Langer-Saldino, and Parenti-Fraccaro Types
Apr 9, 2009
Achondrogenesis is defined by a short stature, small extremities, and other skeletal irregularities. As a consequence of serious health issues, babies with achondrogenesis usually die ahead of birth, are stillborn, or die shortly after birth from respiratory problems. A few infants, nonetheless, have survived for a brief period of time with intensive medical care.
Investigators have reported at least three types of achondrogenesis, labeled as type 1A, type 1B, and type II. The forms are differentiated by their symptoms, inheritance pattern, and familial cause. However, types 1A and 1B are often very difficult to tell apart without genetic screening.
Achondrogenesis type 1A, which has also been referred to as the Houston-Harris type, is the least well researched of the three types. Affected patients have very short extremities, a narrow chest, short rib cages that break easily, and delicate skull bones. They also have deficiency conventional bone formation (ossification) in the spine and pelvic arch.
Achondrogenesis type 1B, also called the Parenti-Fraccaro type, is defined by extremely short arms, a narrow chest area, and a large, rounded abdomen. The toes and fingers are stubby and the feet may be turned inward. Affected babies often have a soft out-pouching near the belly button (an umbilical hernia) or adjacent to the groin (an inguinal herniation).
Patients with achondrogenesis type II, which is often called the Langer-Saldino type, have small legs and arms, a narrow chest with short ribs, and incomplete respiratory organs. This illness is also tied with a lack of ossification in the spinal column and pelvis.
Distinguishing facial characteristics include a prominent forehead, a short chin, and, in many instances, an opening in the roof of the oral cavity (a cleft palate). The abdominal cavity is enlarged, and affected patients frequently have an illness termed hydrops fetalis, in which excess liquid builds up in the body prior to birth.
How Common is Achondrogenesis?
Achondrogenesis types 1A and 1B are extremely rare genetic diseases, and their relative incidence is still unknown. Combined, achondrogenesis type II and hypochondrogenesis (a similar skeletal disease) happen in one in forty-thousand to sixty-thousand newborns.
Genes Associated with Achondrogenesis
Changes in the SLC26A2 and COL2A1 genes bring about achondrogenesis types 1B and II, respectively. The genetic agent of achondrogenesis type 1A is is still unknown.
Achondrogenesis type 1B is the most extreme skeletal illness caused by changes in the SLC26A2 gene. This gene gives instructions for creating a protein that is crucial for the normal growth of cartilage and for its transition to bone. Cartilage is a strong, flexible tissue that comprises much of the skeletal frame at the time of early development.
Most cartilage is later turned to bone, but not the the cartilage that goes on to cover and shield the edges of bones. The cartilage present in the nose and outer ear is also excluded here. Changes in the SLC26A2 gene affect the integrity of developing cartilage, keeping bones from developing properly and resulting in the skeletal issues characteristic of achondrogenesis type 1B.
Achondrogenesis type II is among a few skeletal problems that come from mutations in the COL2A1 gene. This gene gives instructions for producing a protein that constructs type II collagen. This form of collagen is located mostly in cartilage as well as in the gel-like material that occupies the eyeball (the vitreous).
This gene is much needed for the normal growth of bones and other tissues that constitute the body's supportive framing (connective tissues). Changes in the COL2A1 gene also result in the formation of type II collagen molecules that keep bones and related connective tissues from developing the right way.
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References:
Robert J. Gorlin, et al. "Syndromes of the Head and Neck." Oxford University Press. (4): September 2001
Jones, Kenneth and David W. Smith. "Smith's Recognizable Patterns of Human Malformation." W B Saunders Co. Sept. 2005
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