Thrombocytopenia Absent Radius Syndrome

Disease Overview

Thrombocytopenia-absent radius (TAR) syndrome is a rare disorder that is present at birth (congenital). It is characterized by low levels of platelets in the blood (thrombocytopenia) and absence (aplasia) of the long, thin bones of the forearms (radii) but with presence of thumbs. Other abnormalities are often present including additional skeletal defects such as absence or underdevelopment of the other bone of the forearm (ulna), structural malformations of the heart (congenital heart defects) and kidney (renal) defects. Affected individuals may be short for their age (short stature) and have cow’s milk intolerance. TAR syndrome is inherited as an autosomal recessive genetic disorder and caused by deletion and/or variants in the RBM8A gene.

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Synonyms

• radial aplasia-amegakaryocytic thrombocytopenia
• radial aplasia-thrombocytopenia syndrome
• TAR syndrome
• thrombocytopenia-absent radii syndrome

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Subdivisions

• tetraphocomelia-thrombocytopenia syndrome

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Signs & Symptoms

TAR syndrome can potentially affect multiple systems of the body, but it is especially associated with blood (hematological) and bone (skeletal) abnormalities. The two main findings are thrombocytopenia and radial aplasia. A variety of additional symptoms also occur. The specific symptoms vary from patient to patient. Affected individuals will not have all of the symptoms listed below. Some symptoms improve over time and may cause little or no problems in adulthood. Most affected individuals have normal intelligence, are able live independently and many have married and have had their own children.

Approximately 90% of affected individuals develop symptoms related to low levels of the platelets in the blood during the first year of life. Platelets are specialized blood cells that clump together to form clots to stop bleeding. In TAR syndrome certain specialized cells in the bone marrow known as megakaryocytes are defective or improperly developed (hypoplastic). Megakaryocytes normally develop into platelets. The normal maturation of megakaryocytes into platelets does not occur in individuals with TAR syndrome, causing the low levels of platelets, which may be referred to as (hypomegakaryocytic thrombocytopenia). The exact reason why megakaryocytes fail to develop into platelets is unknown.

In individuals with TAR syndrome, the level of platelets in the blood goes up and down. Episodes of thrombocytopenia are most frequent during the first two years of life. Episodes may be preceded or triggered by certain infections, such as viral illnesses (particularly digestive illnesses), surgery, stress or other factors, such as intolerance to cow’s milk (see below).

Low platelet levels can result in severe bleeding episodes (hemorrhaging). Specific symptoms of thrombocytopenia include frequent nosebleeds or gastrointestinal bleeding, which can result in the vomiting of blood (hematemesis) or bloody stools. In addition, affected individuals may develop bleeding (hemorrhages) within skin (dermal) layers or layers below the mucous membranes (submucosal), resulting in easy bruising (ecchymoses) and/or the appearance of pinpoint-sized, purplish or reddish spots on the skin (petechiae). In severely affected patients, bleeding episodes, particularly in the brain (intracranial hemorrhaging), may lead to potentially life-threatening complications during infancy. In addition, intellectual disability has been reported in some individuals who had a history of intracranial hemorrhaging. Otherwise, intelligence in individuals with TAR syndrome is usually unaffected.

As mentioned above, thrombocytopenia typically is most severe during the first year of life. By adulthood, platelet levels may improve to almost normal ranges. Therefore, adults may have few associated symptoms; however, affected females may have unusually heavy or prolonged menstrual periods (menorrhagia).

In addition to platelets, the two other main blood cell lines (red and white cells) may also be affected. Red blood cells deliver oxygen to the body and white blood cells help in fighting off infections. Low levels of circulating red cells (anemia) may occur. Anemia is associated with fatigue, pale skin and weakness. Some affected children may have an excessive amount of white blood cells called a “leukemoid reaction”. This occurs in infants with extremely low platelet levels. There may also be enlargement of the liver and spleen (hepatosplenomegaly). In some patients, increased levels of a specific type of white blood cell called an eosinophil (eosinophilia) may also occur. The cause of eosinophilia is not known. It is often associated with allergy or asthma and may occur in children with TAR syndrome who have cow’s milk intolerance.

A variety of skeletal abnormalities occur in individuals with TAR syndrome. The characteristic finding is absence (aplasia) of one of the bones of the forearm (radius). The radii of both arms are affected (bilateral). The radius is a long thin bone that extends from the elbow to the thumb side of the wrist. The thumbs are present in individuals with TAR syndrome, a finding that distinguishes it from other disorders involving radii. The hands, fingers and thumbs are almost always unaffected, although the fingers may be abnormally short.

Additional skeletal abnormalities may also occur including underdevelopment or absence of the other bone of the forearm, the ulna. Sometimes the long bone of the upper arm (humerus), which extends from the shoulder to the elbow, may be underdeveloped. In some patients, the shoulder girdle may also be underdeveloped and affected individuals may have reduced upper body strength. In severe cases, the arms may be missing, and the hands may be joined to the trunk by small, irregularly shaped bone (phocomelia).

In some patients, the lower limbs may be involved. The severity may range from barely noticeable changes to significant malformations. Affected individuals may exhibit abnormalities of the knees including a loose kneecap that does not slide properly within its groove (patellar subluxation) and can potentially slide completely out of the socket (dislocate), absence of the kneecap (patella) or, rarely, the bones of the knees may be fused together. Dislocation of the hip, in which the head of the long bone of the upper leg (femur) does not fit properly into its socket in the hip, may also occur. Additional lower limb abnormalities often occur including improper inward rotation of the long bones of the legs (femoral and tibial torsion), bowing of the legs and abnormalities affecting the feet and toes. Lower limb abnormalities can potentially affect the ability to walk (mobility). Most individuals with severe involvement of the upper limbs are more likely to have abnormalities of the lower limbs.

In addition, cow’s milk intolerance or allergy has frequently been reported in association with TAR syndrome. In these children, introduction of cow’s milk to the diet may cause thrombocytopenic, eosinophilic and/or “leukemoid” episodes (see above). Cow’s milk intolerance can also cause a variety of gastrointestinal symptoms including nausea, vomiting, diarrhea and failure to gain weight and grow at the expected rate (failure to thrive).

Approximately one third of affected infants also have structural malformations of the heart (congenital heart defects). Such cardiac defects may include an abnormal opening in the fibrous partition (septum) that divides the upper chambers of the heart (atrial septal defect) or a malformation known as tetralogy of Fallot. The latter describes a combination of heart defects, including abnormal narrowing (stenosis) of the opening between the pulmonary artery (which carries blood to the lungs) and the lower right chamber (ventricle) of the heart, an abnormal opening in the partition between the lower chambers of the heart (ventricular septal defect); displacement of the major artery that transports oxygen-rich blood to most of the body (i.e., aorta) and enlargement of the right ventricle (hypertrophy).

Some individuals with TAR syndrome may have short stature. A variety of additional physical abnormalities have been reported to be associated with TAR syndrome including an abnormally small jaw (micrognathia), incomplete closure of the roof of the mouth (cleft palate), one or more pink or dark red irregularly shaped patches of skin (hemangiomas) on the face caused by dense collections of small blood vessels (capillaries) or minor abnormalities affecting the spine and ribs. Kidney (renal) defects may also be present, such as a malformation in which the two kidneys are abnormally joined at the base (horseshoe kidney) as well as underdevelopment (hypoplasia) and improper function of the kidneys. Some of these findings have only occurred in a few reported patients and researchers do not know whether these are coincidental occurrences or whether individuals with TAR syndrome have a greater risk of developing these manifestations.

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Causes

TAR syndrome is inherited as an autosomal recessive genetic disorder and caused by two different types of changes (variants) in the RBM8A gene.

Recessive genetic disorders occur when an individual inherits two copies of a disease-causing variant, one from each parent. If an individual receives one normal gene and one gene variant for the disease, the person will be a carrier for the disorder but usually will not show symptoms. However, reports have been published that describe an affected child born to an affected parent. The risk for two carrier parents to both pass the gene variant and have an affected child is theoretically 25% with each pregnancy, but because the RBM8A gene deletion associated with TAR syndrome is often not inherited but occurs as a new variant in about 25% of those affected, the risk for affected sibs is lower. The risk of having a child who is a carrier like the parents is 50% with each pregnancy. The chance for a child to receive normal genes from both parents is 25%. The risk is the same for males and females.

Some researchers suggest that TAR syndrome may result from variants of different disease genes (genetic heterogeneity). Further research is required to determine the underlying genetic cause or causes of the disorder.

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Affected populations

The prevalence of TAR syndrome is estimated at 1:200,000-1:100,000.

The disorder was originally described in siblings in the 1950s. Over 100 cases have since been recorded in the medical literature.

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Disorders with Similar Symptoms

Symptoms of the following disorders may be similar to those of thrombocytopenia-absent radius (TAR) syndrome. Comparisons may be useful for a differential diagnosis:

Fanconi anemia, also known as Fanconi pancytopenia syndrome, is a rare genetic disorder characterized by deficiency of all blood cell types (pancytopenia), including red blood cells, white blood cells and platelets. Such abnormalities may lead to abnormal bleeding and easy bruising, paleness of the skin (pallor), recurrent infection and other findings that typically become apparent from approximately three to 10 years of age. However, in some patients, pancytopenia and associated abnormalities (hematologic findings) may occur as early as infancy or as late as the third decade of life. Individuals with Fanconi anemia may also have abnormal, patchy, brownish discolorations of the skin, short stature and certain congenital defects. These may include underdevelopment or absence of the thumbs and/or the bones on the thumb side of the forearms (radii), congenital hip dislocation, kidney (renal) malformations, cardiac defects and/or other abnormalities. There are several different subtypes (complementation groups) of Fanconi anemia, each of which is thought to result from variants in different disease genes. The disorder is known to be associated with a high frequency of chromosomal breakage, a finding that may be helpful in distinguishing Fanconi anemia from other disorders with similar symptoms and findings. Fanconi anemia is inherited in an autosomal recessive pattern. (For further information on this disorder, choose “Fanconi” as your search term in the Rare Disease Database.)

Holt-Oram syndrome (HOS), also known as Cardiac-Limb syndrome, is a rare genetic disorder characterized by malformations of bones of the forearms and hands (upper limbs) and/or heart abnormalities. The thumbs may be absent (aplastic), underdeveloped (hypoplastic) or have an extra bone (triphalangy). Additional upper limb malformations may include defects of certain bones of the wrists (carpals), the middle portion of the hands (metacarpals), the thumb side of the forearms (radii) and/or the pinky side of the forearms (ulnae). Some affected individuals also have additional musculoskeletal abnormalities, such as malformations of the bones of the upper arms (humeri), shoulder blades (scapulae) and collarbones (clavicles), other skeletal defects and restricted range of movements at the shoulders and elbows. Characteristic heart abnormalities may include structural cardiac defects, such as an abnormal opening in the fibrous partition (septum) between the upper (atrial) or lower (ventricular) chambers of the heart or abnormal transmission of electrical impulses that coordinate the heart’s muscular contractions (electrocardiographic conduction defects). In some patients, other abnormalities are also present. Holt-Oram syndrome may be inherited in an autosomal dominant pattern or appear to occur spontaneously due to a new gene variant. (For further information, choose “Holt Oram” as your search term in the Rare Disease Database.)

Roberts syndrome is a rare genetic disorder characterized by growth delays before and after birth, malformations of the arms and legs (limbs), distinctive differences of the skull and facial (craniofacial) region and/or other symptoms and findings. In infants with Roberts syndrome, the arms and legs may be incompletely developed (limb reduction abnormalities). Such abnormalities may range from phocomelia of all four limbs (tetraphocomelia) to less severe degrees of limb reduction, such as underdevelopment and/or absence of bones of the upper arms (humeri), forearms (radii and/or ulnae), thighs (femurs), shins (tibiae) and/or outside of the lower legs (fibulae). In addition, certain fingers and toes may be unusually short or absent (oligodactyly). Characteristic craniofacial differences may include an unusually small, broad head (microbrachycephaly), incomplete development of the upper lip (bilateral cleft lip), incomplete development of both sides of the roof of the mouth (bilateral cleft palate), thin, small nostrils (hypoplastic nasal alae) and/or low-set, malformed (dysplastic) ears. Additional abnormalities may also be present, such as low levels of platelets in the blood (thrombocytopenia), congenital heart defects, kidney (renal) abnormalities, intellectual disability and/or other findings. In some patients, the disorder is associated with distinctive changes involving certain chromosomal regions (e.g., premature centromere separation). Roberts syndrome is inherited in an autosomal recessive pattern. (For further information, use “Roberts” as your search term in the Rare Disease Database.)

Baller-Gerold syndrome is a rare genetic disorder characterized by distinctive differences of the skull and facial (craniofacial) area and bones of the forearms and hands. In affected infants, there is premature fusion of the fibrous joints (cranial sutures) between certain bones in the skull (craniosynostosis). As a result, the head may appear unusually short and wide and/or pointed at the top (turribrachycephaly) or relatively triangular in shape (trigonocephaly). Additional craniofacial malformations may include a prominent forehead, downslanting eyelid folds (palpebral fissures) and/or small, malformed (dysplastic), low-set ears. Baller-Gerold syndrome is also characterized by underdevelopment (hypoplasia) or absence (aplasia) of the bone on the inner (or thumb) side of the forearms (radii). In addition, the bone on the pinky side of the forearms (ulnae) is unusually short and curved and the thumbs may be underdeveloped or absent. Some affected individuals also have congenital heart defects, such as an abnormal opening in the fibrous partition that separates the lower or upper chambers of the heart (ventricular or atrial septal defects) and/or patent ductus arteriosus (PDA). The latter is characterized by an abnormal opening between the artery that transports oxygen-rich blood to most of the body (aorta) and the pulmonary artery, which carries oxygen-deficient blood to the lungs. Some individuals with the disorder may also have kidney (renal) malformations, additional physical abnormalities, and/or intellectual disability. Baller-Gerold syndrome is inherited in an autosomal recessive pattern. (For further information, choose “Baller Gerold” as your search term in the Rare Disease Database.)

Additional congenital disorders may be characterized by malformations of bones of the upper and lower limbs, hematologic abnormalities, short stature, congenital heart defects and/or other symptoms and findings similar to those potentially associated with TAR syndrome. (For more information on these disorders, choose the exact disease name in question as your search term in the Rare Disease Database.)

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Diagnosis

In most cases, the diagnosis of TAR syndrome is made at birth based upon a thorough clinical examination, identification of characteristic physical findings, and a variety of specialized tests. Such testing may include blood studies to confirm the presence of thrombocytopenia, anemia, and/or other hematologic abnormalities as well as a radiograph (X-ray) of the forearm and renal ultrasonography of the kidneys.

The first step in molecular genetic testing is deletion/duplication analysis for the region of chromosome band 1q21 that contains the RBM8A gene. Diagnosis of TAR syndrome is confirmed if a deletion is present in an individual with bilateral absence of the radius and presence of thumbs. However, lack of identification of this deletion is not sufficient to rule out the diagnosis. Sequence analysis of the RBM8A gene should be done if no deletion is identified, or to identify the second RBM8A gene variant for confirmation of the diagnosis.

Clinical Testing and Work-Up

Cardiac evaluation may also be recommended to detect any heart abnormalities that may be associated with the disorder. Such evaluation may include a thorough clinical examination, during which heart and lung sounds are assessed through use of a stethoscope, and specialized tests that enable physicians to evaluate the structure and function of the heart (e.g., X-ray studies, electrocardiography [EKG], echocardiography, cardiac catheterization).

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Standard Therapies

Treatment

The treatment of TAR syndrome is directed toward the specific symptoms that are apparent in each individual. Such treatment may require the coordinated efforts of a team of medical professionals, such as pediatricians, surgeons, physicians who diagnose and treat disorders of the skeleton, joints, muscles, and related tissues (orthopedists), specialists in the study of the blood and blood-forming tissues (hematologists), physicians who specialize in heart disease (cardiologists) and/or other health care professionals.

Physicians may recommend preventive measures to help affected infants and children avoid infection, stress, or other factors that may precipitate thrombocytopenia. In addition, experts indicate that cow’s milk should be avoided, since its introduction may cause thrombocytopenic, eosinophilic, or “leukemoid” episodes. (For further information, please see the “Symptoms” section of this report above.)

Management of the disorder may include ongoing monitoring and supportive hematologic measures as required, such as platelet transfusions or transfusions with whole blood products. In some patients, certain medications or other measures may be recommended to help prevent or treat hematologic complications. As noted above, thrombocytopenia typically improves with age.

In individuals with TAR syndrome, various orthopedic techniques may also be recommended, such as splints, corrective braces and/or certain surgical measures. In some patients, adaptive and/or artificial devices (prosthetics) and mobility aids such as wheelchairs or motorized carts may also be beneficial.

For affected individuals with congenital heart defects, treatment with certain medications, surgical intervention and/or other measures may be necessary. The surgical procedures performed will depend upon the severity and location of the anatomical abnormalities, their associated symptoms and other factors.

Early intervention may be important to ensure that children with TAR syndrome reach their potential. Special services that may be beneficial include special education, physical therapy and/or other medical, social, or vocational services.

Genetic counseling is recommended for affected individuals and their families. Other treatment for this disorder is symptomatic and supportive.

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Clinical Trials and Studies

Adults and children with inherited bone marrow failure syndromes, including thrombocytopenia absent radius syndrome, are being recruited for a research study at the National Institutes of Health in Bethesda, MD, aimed at increasing understanding of the causes of cancer susceptibility in these disorders. For information, contact The IBMFS Study (cancer.gov)

Information on current clinical trials is posted on the Internet at www.clinicaltrials.gov. All studies receiving U.S. Government funding, and some supported by private industry, are posted on this government web site.

For information about clinical trials being conducted at the NIH Clinical Center in Bethesda, MD, contact the NIH Patient Recruitment Office:

Tollfree: (800) 411-1222
TTY: (866) 411-1010
Email: prpl@cc.nih.gov

Some current clinical trials also are posted on the following page on the NORD website: https://rarediseases.org/living-with-a-rare-disease/find-clinical-trials/

For information about clinical trials sponsored by private sources, contact: www.centerwatch.com

For information about clinical trials conducted in Europe, contact: https://www.clinicaltrialsregister.eu/

Contact for additional information about Thrombocytopenia-absent radius syndrome:

Helga V. Toriello, PhD
Director, Clinical Genetics
25 Michigan St., Suite 2000
Grand Rapids, MI 49503
phone: 616-391-2700
fax: 616-391-3114

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References

TEXTBOOKS
Jones KL. Smith’s Recognizable Patterns of Human Malformation. 5th ed. Philadelphia, PA: W.B. Saunders Company; 1997:298-299, 316-317, 320-323, 430-431.

Behrman RE, et al., eds. Nelson Textbook of Pediatrics. 15th ed. Philadelphia, PA: W.B. Saunders Company; 1996:1359-1361, 1432.

Buyse ML. Birth Defects Encyclopedia. Dover, MA: Blackwell Scientific Publications, Inc.; 1990:1665-1666.

JOURNAL ARTICLES

Boussion S, et al. TAR syndrome: Clinical and molecular characterization of a cohort of 26 patients and description of novel noncoding variants of RBM8A.  Hum Mutat. 2020; 41:1220-1225.

Christensen CP, et al. Lower extremity deformities associated with thrombocytopenia and absent radius syndrome. Clin Orthop. 2000;375:202-206.

Letestu R, et al. Existence of a differentiation blockage at the stage of a megakaryocyte precursor in the thrombocytopenia and absent radii (TAR) syndrome. Blood. 2000;95:1633-1641.

Tongsong T, et al. Prenatal diagnosis of thrombocytopenia-absent-radius (TAR) syndrome. Ultrasound Obstet Gynecol. 2000;15:256-258.

Bradshaw A, et al. Thrombocytopenia and absent radii (TAR) syndrome associated with horseshoe kidney. Pediatr Nephrol. 2000;14:29-31.

McLaurin TM, et al. Management of thrombocytopenia-absent radius (TAR) syndrome. J Pediatr Orthop. 1999;19:289-296.

Shelton SD, et al. Prenatal diagnosis of thrombocytopenia absent radius (TAR) syndrome and vaginal delivery. Prenat Diagn. 1999;19:54-57.

Martinez-Frias ML, et al. An epidemiological study of the thrombocytopenia with radial aplasia syndrome (TAR) in Spain. An Esp Pediatr. 1998;49:619-623.

Sekine I, et al. Thrombocytopenia with absent radii syndrome: studies on serum thrombopoietin levels and megakaryopoiesis in vitro. J Pediatr Hematol Oncol. 1998;20:74-78.

Urban M, et al. Bilaterally cleft lip, limb defects, and haematological manifestations: Roberts syndrome versus TAR syndrome. Am J Med Genet. 1998;79:155-160.

Ballmaier M, et al. Thrombopoietin in patients with congenital thrombocytopenia and absent radii: elevated serum levels, normal receptor expression, but defective reactivity to thrombopoietin. Blood. 1997;90:612-619.

Strauss G, et al. Significance of thrombopoietin and its receptor c-Mpl in regulation of thrombocytopoiesis in thrombocytopenia. Klin Padiatr. 1996;208:168-171.

Donnenfeld AE, et al. Prenatal diagnosis of thrombocytopenia absent radius syndrome by ultrasound and cordocentesis. Prenatal Diag. 1990;10:29-35.

Ward RE, et al. Parent to child transmission of the thrombocytopenia absent radius (TAR) syndrome. Am J Med Genet Suppl. 1986;2:207-214.

Anyane-Yeboa K, et al. Tetraphocomelia in the syndrome of thrombocytopenia with absent radii (TAR syndrome). Am J Med Genet. 1985;20:571-576.

Schoenecker PL, et al. Dysplasia of the knee associated with the syndrome of thrombocytopenia and absent radius. J Bone Joint Surg. 1984;66:421-427.

Ray R, et al. Lower limb anomalies in the thrombocytopenia absent-radius (TAR) syndrome. Am J Med Genet. 1980;7:523-528.

Whitfield MF, et al. Cow’s milk allergy in the syndrome of thrombocytopenia with absent radius. Arch Dis Child. 1976;51:337-343.

Pfeiffer RA, et al. The phocomelia-thrombocytopenia syndrome: a follow-up report. Humangenetik. 1975;26:157-158.

Shaw S, et al. Congenital hypoplastic thrombocytopenia with skeletal deformities in siblings. Blood. 1959;14:374-377.

INTERNET

Petit F, Boussion S. Thrombocytopenia Absent Radius Syndrome. 2009 Dec 8 [Updated 2023 Nov 2]. In: Adam MP, Feldman J, Mirzaa GM, et al., editors. GeneReviews® [Internet]. Seattle (WA): University of Washington, Seattle; 1993-2024. Available from: https://www.ncbi.nlm.nih.gov/books/NBK23758/ Accessed Jan 30, 2024.

Online Mendelian Inheritance in Man (OMIM). The Johns Hopkins University. Thrombocytopenia-Absent Radius Syndrome; TAR. Entry No: 274000. Last Edited 03/17/2023. Available at: http://omim.org/entry/274000 Accessed Jan 30, 2024.

Wu JK, Wong MP, Williams S. Thrombocytopenia-Absent Radius Syndrome. Medscape. Updated: Mar 07, 2019. http://emedicine.medscape.com/article/959262-overview Accessed Jan 30, 2024.

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