Thrombocytopenia-absent radius syndrome

Thrombocytopenia-absent radius (TAR) syndrome is characterized by the absence of a bone called the radius in each forearm. Affected individuals also have a reduction in the number of blood cells involved in clotting (platelets). This platelet deficiency (thrombocytopenia) usually appears during infancy and becomes less severe or returns to normal over time.

Thrombocytopenia prevents normal blood clotting, resulting in easy bruising and frequent nosebleeds. Potentially life-threatening episodes of severe bleeding (hemorrhages) may occur in the brain and other organs, especially during the first year of life. Affected children who survive this period and do not have damaging hemorrhages in the brain usually have a normal life expectancy and normal intellectual development.

People with TAR syndrome have thumbs, which are missing in other types of malformations involving an absent radius. However, affected individuals may have short stature and additional skeletal abnormalities, including underdevelopment of other bones in the arms and legs. They may also have malformations of the heart and kidneys. This disorder is associated with unusual facial features including a small lower jaw (micrognathia), a prominent forehead, and low-set ears. About half of affected individuals experience difficulty digesting cow's milk.

TAR syndrome is a rare disorder, affecting fewer than 1 in 100,000 newborns.

Everyone diagnosed with TAR syndrome has had a deletion of about 200,000 DNA building blocks (200 kilobases, or 200 kb) from the long (q) arm of chromosome 1 at position 1q21.1. This section of chromosome 1 contains 11 genes. The loss of multiple genes in this region is believed to be responsible for the signs and symptoms of TAR syndrome.

Researchers believe that the 1q21.1 200 kb deletion is needed to cause TAR syndrome but that some other, unknown genetic change must also be present.

Read more about chromosome 1.

The inheritance pattern of TAR syndrome is unclear. The deletion of genetic material on chromosome 1 associated with this condition can be passed through generations in families. Affected individuals usually have the deletion in only one copy of chromosome 1. Some deletions occur during early development in people with no history of the disorder in their family.

Because an additional genetic change is also believed to be involved in this disorder, the inheritance pattern is complex. Not all people who inherit the chromosome 1 deletion associated with TAR syndrome will develop the disorder. Even within a single family, some people with the deletion may have TAR syndrome while others are unaffected.