Thrombocytopenia-Absent Radius Syndrome 

Thrombocytopenia-absent radius (TAR) syndrome is a rare condition in which thrombocytopenia is associated with bilateral radial aplasia. TAR syndrome was first described in 1951. An autosomal recessive inheritance pattern was proposed because TAR affected more than one member of some families. In 1969, TAR was defined as a syndrome and further classified as the association of hypomegakaryocytic thrombocytopenia and absent radii. The expression varies and includes abnormalities in the GI, skeletal, hematologic, and cardiac systems. See the images below.

Some have proposed that the association of seemingly disparate skeletal and hematologic abnormalities is related to the simultaneous development of the heart, the radii, and the megakaryocytes at 6-8 weeks' gestation. The similarity of TAR syndrome to congenital rubella suggests intrauterine injury when the involved systems develop, but a common etiologic agent has not been identified. As an alternative, the contiguous gene model is based on the premise that phenotypic findings are related when genes responsible for each defect are geographically related in a chromosome. This mechanism, if true, is independent of the anatomic association and the degree of involvement in either system.

The exact pathophysiology of the thrombocytopenia is still unclear. The platelet abnormality reflects platelet hypoproduction, for which numerous explanatory theories have been proposed. One suggestion is that a failure in production of humoral or cellular stimulators of megakaryocytopoiesis (eg, thrombopoietin) is responsible for inhibiting platelet production. However, studies by Ballmaier and colleagues and Sekine and associates showed comparable or increased levels of thrombopoietin in patients with TAR compared with healthy control subjects.^{[1, 2]} These findings suggest that the thrombocytopenia is due to a lack of response to thrombopoietin, especially given the observation of normal thrombopoietin receptor expression on megakaryocytes. Letestu and colleagues suggested that the defect was a blockage in cell differentiation at an early stage.^[3]

Other theories for platelet hypoproduction include an abnormal response to stimulators of megakaryocytopoiesis involving an abnormal signal-transduction pathway, decreased numbers and sizes of megakaryocytic progenitor cells,^[2] abnormal progenitor cells with a maturational defect or receptor defect, and the presence of humoral or cellular inhibitors of megakaryocytopoiesis.

No causative mutation has been identified despite investigations of the c-mpl gene in patients with TAR.^[4] Another proposed candidate gene is a HOX gene. The HOX family of genes plays a major role in embryogenesis and cell differentiation, including differentiation of hematopoietic cell lines. However, Fleischman and colleagues did not detect mutations in the coding sequence of HOX genes known to affect radial development.^[5] An interstitial microdeletion of chromosome 1q was identified in 30 patients with TAR syndrome.^[6] All patients and 75% of unaffected parents in this cohort had the microdeletion, suggesting co-inheritance of an additional modifier gene for disease expression. The observation that platelet counts improve during infancy and that they may even normalize with age has led to the suggestion that abnormal genes may be developmentally regulated.

TAR syndrome is generally considered an autosomal recessive disease. Some have suggested that the inheritance pattern may be autosomal dominant with variable penetrance. Urban and associates postulated that, given the phenotypic overlap between Roberts syndrome and TAR syndrome, allelic heterogeneity might cause both. In this postulate, TAR syndrome is the compound heterozygous form, with a mild and a severe mutation, whereas Roberts syndrome is the homozygous form with the severe mutation. However, genetic heterogeneity and environmental factors cannot be completely ruled out.

Frequency

United States

TAR syndrome rarely occurs in the United States.

International

The frequency of TAR syndrome is 0.42 case per 100,000 live births in Spain.

Mortality/Morbidity

The major cause of mortality in TAR syndrome is hemorrhage. The incidence of hemorrhage is limited to the first 14 months of life. In a study by Hedberg and associates, 18 of 20 deaths in 76 patients were due to hemorrhagic events; most of patients who died had platelet counts < 10 X 10⁹/L.^[7]

Bleeding and hemorrhage can also result in clinically significant morbidity, especially intracranial hemorrhage. Hand and upper-extremity function is usually good if radial aplasia is the only skeletal abnormality. However, patients require plastic surgery, occupational therapy, and physiotherapy.

Race

No ethnic or racial predilection is reported.

Sex

The male-to-female ratio is 1:1. Greenhalgh and associates reported an excess of females (27:7),^[8] as did Hall and colleagues (26:14).^[9]

Age

TAR syndrome is congenital, and patients usually present with symptomatic thrombocytopenia in the first week of life.