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Diseases of the Skeletal System
The human skeletal system is a complex and important body structure made up of bones, cartilages, and ligaments. It provides the framework to the human body, protects internal organs from mechanical damage, and helps muscles to get the body moving. Moreover, production of red blood cells and storage of important minerals occur in the skeletal system. The skeletal system may not perform these essential functions if it is afflicted with diseases.
Skeletal diseases are caused by various factors including age, lifestyle, nutrition, genetics, and others. There are common and rare skeletal diseases. For now, let us look at a rare skeletal disease called achondrogenesis type 2.
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Overview of Achondrogenesis Type 2: A Skeletal Disease
Achondrogenesis type 2 is a severe skeletal disease that affects the normal development of cartilage and bone. This skeletal disease causes infant death before birth,or children die soon after birth due to respiratory failure. However, some infants have lived for a while under an intensive care regime.
Characteristics of infants with achondrogenesis type 2 include underdeveloped lungs, a narrow chest with short ribs, and short arms and legs. Important bones of their bodies such as the skull, pelvis, and the vertebrae do not ossify which makes them soft and easily broken. They also have prominent foreheads, flat and oval-shaped face, small chins, widely spaced eyes, and cleft palates. They can also have an enlarged abdomen because of the condition called hydrops fetalis wherein excess fluid builds up in the infant's body before birth.
Achondrogenesis type 2 is caused by the absence of type 2 collagen produced by chondrocytes. Type 2 collagen is an important protein for cartilage and bone development. The absence of this protein is the result of COL2A1 gene mutation. Mutated COL2A1 gene produces a protein (procollagen) with an altered amino acid sequence: the amino acid glycine is replaced by another amino acid.
Procollagen is the precursor protein for the alpha 1 (II) chain of type 2 collagen. This is a long polypeptide and part of the triple helix structure of type 2 collagen. If the polypeptide is altered, the assembly of type 2 collagen into its triple helix structure will not happen and no functional type 2 collagen will be produced.
Chondrocytes replace type 2 collagen with either type 1 or type 3 collagen but these types of collagen are unable to promote the normal development of cartilage, bones and other connective tissues of the skeletal system.
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Inheritance of Skeletal Disease: Achondrogenesis Type 2
Usually a person with achondrogenesis type 2 has no family history of the skeletal disease. This is because the disease is caused by a new mutation of the COL2A1 gene in the infant. However there are instances where family members of the patient are carriers of the mutated gene but they are asymptomatic.
Achondrogenesis type 2 is an autosomal dominant genetic disease which means a single copy of altered COL2A1 gene in chromosome 12 is enough for a human embryo to develop the skeletal disease. The defected gene is not transmitted to the next generation because individuals who have it do not live long enough to produce children.
Prenatal diagnosis of achondrogenesis type 2 involves mutational analysis of DNA from the chorionic villi. The skeletal disease can also be detected in the fetus by an experienced ultrasonographer at 12-14 weeks gestation.