KIF21A

The KIF21A gene provides instructions for making a protein called kinesin family member 21A. Proteins in the kinesin family are essential for the transport of materials within cells. Kinesin proteins function like freight trains that transport cargo, and their structure is suited for this cargo-carrying function. One end of the protein, called the motor domain, provides power to move the protein and its cargo along a track-like system made from structures called microtubules. The other end of the protein attaches (binds) to specific cargo, such as groups of proteins, for transport. The two ends of each kinesin are connected by a flexible region known as the stalk.

Kinesin family member 21A is found in developing nerve cells (neurons). Researchers believe that this protein carries cargo that is needed for the normal development and function of nerves in the head and face. In particular, this kinesin plays a critical role in the development of cranial nerve III, which emerges from the brain and controls several of the muscles that surround the eyes (extraocular muscles). These muscles direct eye movement and determine the position of the eyes.

congenital fibrosis of the extraocular muscles - caused by mutations in the KIF21A gene

At least 11 mutations in the KIF21A gene have been identified in people with congenital fibrosis of the extraocular muscles. These mutations cause the most common form of the disorder, CFEOM1, and are a rare cause of another form of the condition called CFEOM3.

Each of the known KIF21A mutations changes a single protein building block (amino acid) in kinesin family member 21A. Most of these changes occur in the stalk region of the protein. These mutations alter the protein's structure, which likely interferes with its ability to transport cargo within neurons. As a result, several cranial nerves and the extraocular muscles they control do not develop normally. Abnormal development and function of these muscles leads to the characteristic features of congenital fibrosis of the extraocular muscles, including restricted eye movement and related problems with vision.