TAR syndrome is characterized by a transient Thrombocytopenia (low platelet count) and a bilateral Absent Radius from which is derived the acronym of this condition (TAR). Majority of affected individuals have a minimal 200kb deletion of region 1q21.1 (which comprise the RBM8A gene) on one copy and another genetic change in the RBM8A gene on the second copy. In this way, both copies of the RBM8A gene do not function what causes the observed manifestations in this syndrome.
Although the radius is absent in affected individuals, the thumbs are still present. Upper and lower (hip, patella, long bones), ribs, and vertebra can be present.
Between others, renal abnormalities and malformations of the female genitalia can also be present.
- Transmission: TAR syndrome is an autosomal recessive disorder. This means that both copies of the RBM8A gene do not function.
- In 50-70% of cases, the 200 kb deletion is inherited from an asymptomatic parent. In this situation, the risk for brothers and sisters to be also affected of TAR syndrome is 25% in each pregnancy if both parents carry a genetic change.
- In 25-50% of cases, the 200kb deletion happened de novo in the affected individual. The brothers and sisters will then have a 50% risk to be carrier but will not present symptoms of the TAR syndrome.
- Prenatal diagnosis: Prenatal diagnosis is possible if the mutations are known in the familily either by chorionic villus sampling from 11-14 weeks or amniocentesis from 15 weeks. It is also possible to evaluate the fetus by ultrasound by the observations of the limbs to see if abnormalities associated with the syndrome is present (absent radius).
Notes that little is know about the disease and this is why it's very important for studies to understand how this region can cause difficulties for some individuals carrying a certain rearrangement, and save others with the opposite one.
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