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Thrombocytopenia-absent radius syndrome
(often shortened to TAR syndrome)
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Reviewed October 2012
What is TAR 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 shortage (deficiency) of blood cells involved in clotting (platelets). This platelet deficiency (thrombocytopenia) usually appears during infancy and becomes less severe over time; in some cases the platelet levels become normal.
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. Hemorrhages can damage the brain and lead to intellectual disability. 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.
TAR syndrome is unusual among similar malformations in that affected individuals have thumbs, while people with other conditions involving an absent radius typically do not. TAR syndrome is also associated with short stature and additional skeletal abnormalities, including underdevelopment of other bones in the arms and legs. Affected individuals may also have malformations of the heart and kidneys. TAR syndrome is associated with unusual facial features including a small lower jaw (micrognathia), a prominent forehead, and low-set ears. About half of affected individuals have allergic reactions to cow's milk that may worsen the thrombocytopenia associated with this disorder.
How common is TAR syndrome?
TAR syndrome is a rare disorder, affecting fewer than 1 in 100,000 newborns.
What are the genetic changes related to TAR syndrome?
Mutations in the RBM8A gene cause TAR syndrome. The RBM8A gene provides instructions for making a protein called RNA-binding motif protein 8A. This protein is believed to be involved in several important cellular functions involving the production of other proteins.
Most people with TAR syndrome have a mutation in one copy of the RBM8A gene and a deletion of genetic material from chromosome 1 that includes the other copy of the RBM8A gene in each cell. A small number of affected individuals have mutations in both copies of the RBM8A gene in each cell and do not have a deletion on chromosome 1. RBM8A gene mutations that cause TAR syndrome reduce the amount of RNA-binding motif protein 8A in cells. The deletions involved in TAR syndrome eliminate at least 200,000 DNA building blocks (200 kilobases, or 200 kb) from the long (q) arm of chromosome 1 in a region called 1q21.1. The deletion eliminates one copy of the RBM8A gene in each cell and the RNA-binding motif protein 8A that would have been produced from it.
People with either an RBM8A gene mutation and a chromosome 1 deletion or with two gene mutations have a decreased amount of RNA-binding motif protein 8A. This reduction is thought to cause problems in the development of certain tissues, but it is unknown how it causes the specific signs and symptoms of TAR syndrome. No cases have been reported in which a deletion that includes the RBM8A gene occurs on both copies of chromosome 1; studies indicate that the complete loss of RNA-binding motif protein 8A is not compatible with life.
Researchers sometimes refer to the deletion in chromosome 1 associated with TAR syndrome as the 200-kb deletion to distinguish it from another chromosomal abnormality called a 1q21.1 microdeletion. People with a 1q21.1 microdeletion are missing a different, larger DNA segment in the chromosome 1q21.1 region near the area where the 200-kb deletion occurs. The chromosomal change related to 1q21.1 microdeletion is often called the recurrent distal 1.35-Mb deletion.
Read more about 1q21.1 microdeletion.
Can TAR syndrome be inherited?
TAR syndrome is inherited in an autosomal recessive pattern, which means both copies of the gene in each cell are altered. In this disorder, either both copies of the RBM8A gene in each cell have mutations or, more commonly, one copy of the gene has a mutation and the other is lost as part of a deleted segment on chromosome 1. The affected individual usually inherits an RBM8A gene mutation from one parent. In about 75 percent of cases, the affected person inherits a copy of chromosome 1 with the 200-kb deletion from the other parent. In the remaining cases, the deletion occurs during the formation of reproductive cells (eggs and sperm) or in early fetal development. Although parents of an individual with TAR syndrome can carry an RBM8A gene mutation or a 200-kb deletion, they typically do not show signs and symptoms of the condition.
Where can I find information about diagnosis or management of TAR syndrome?
These resources address the diagnosis or management of TAR syndrome and may include treatment providers.
General information about the diagnosis and management of genetic conditions is available in the Handbook. Read more about genetic testing, particularly the difference between clinical tests and research tests.
To locate a healthcare provider, see How can I find a genetics professional in my area? in the Handbook.
Where can I find additional information about TAR syndrome?
You may find the following resources about TAR syndrome helpful. These materials are written for the general public.
You may also be interested in these resources, which are designed for healthcare professionals and researchers.
What other names do people use for TAR syndrome?
What if I still have specific questions about TAR syndrome?
Where can I find general information about genetic conditions?
The Handbook provides basic information about genetics in clear language.
These links provide additional genetics resources that may be useful.
What glossary definitions help with understanding TAR syndrome?
autosomal ; autosomal recessive ; blood clotting ; cell ; chromosome ; clotting ; deficiency ; deletion ; disability ; distal ; DNA ; gene ; inherited ; kb ; lower jaw ; Mb ; micrognathia ; motif ; mutation ; platelets ; protein ; recessive ; reproductive cells ; RNA ; short stature ; sperm ; stature ; syndrome ; thrombocytopenia
You may find definitions for these and many other terms in the Genetics Home Reference Glossary.
See also Understanding Medical Terminology.
References (7 links)
The resources on this site should not be used as a substitute for professional medical care or advice. Users seeking information about a personal genetic disease, syndrome, or condition should consult with a qualified healthcare professional. See How can I find a genetics professional in my area? in the Handbook.