| Domain | Knowledge-base statement | Quantitative/variant detail | Evidence type and key source |
|---|---|---|---|
| Disease identifiers | Thrombocytopenia-absent radius (TAR) syndrome is a Mendelian disorder characterized by thrombocytopenia with bilateral absence of the radii and preserved thumbs. | MONDO_0010121; OMIM 274000. Review text states TAR is characterized by platelet counts generally **<50 × 10^9/L** and bilateral absent radii with both thumbs present. | Disease ontology association and review evidence (pqac-00000000, pqac-00000015) |
| Defining phenotype | The hallmark clinical combination is **bilateral radial aplasia/agenesis with normal thumbs** plus neonatal/infant thrombocytopenia due to reduced megakaryocytes. | Distinguishes TAR from Fanconi anemia, Holt-Oram syndrome, and Roberts syndrome, where thumbs are absent or hypoplastic. Megakaryocytes are absent or reduced in marrow. | Clinical review/book chapter and review article (pqac-00000003, pqac-00000015) |
| Epidemiology | TAR is rare. | Approximate incidence reported as **1 in 240,000 live births** and **0.42 per 100,000** individuals/live births. Slight female predominance reported. | Clinical review/case-based summary (pqac-00000015, pqac-00000004, pqac-00000007) |
| Core genetics | TAR syndrome is caused by **compound inheritance** involving one **null RBM8A allele** and one **hypomorphic noncoding RBM8A allele**. | Null allele usually a **~200 kb 1q21.1 microdeletion**; hypomorphic alleles include **rs139428292** (5'UTR) and **rs201779890** (intron 1). | Molecular genetics review and disease-target evidence linking RBM8A to TAR (pqac-00000001, pqac-00000004, pqac-00000015, pqac-00000000) |
| Deletion statistics | The 1q21.1 deletion is necessary but not sufficient on its own. | In a 30-patient series, the heterozygous **1q21.1 ~200 kb deletion was found in 100%** of TAR cases; **75% inherited**, **25% de novo**; present in **32% of unaffected relatives**. | Summarized primary human genetics evidence from review (pqac-00000001) |
| Molecular mechanism | RBM8A encodes **Y14**, a component of the **exon-junction complex (EJC)**; reduced Y14 is the leading mechanistic explanation for hematopoietic defects in TAR. | Y14 is a **174-aa** protein in the 4-subunit EJC; functions include mRNA export, localization, translation efficiency, and nonsense-mediated decay. Y14 levels are reduced in patient platelets. Complete biallelic loss is thought incompatible with life. | Molecular review/mechanistic synthesis (pqac-00000001, pqac-00000015) |
| Megakaryopoiesis/TPO signaling | Thrombocytopenia reflects impaired megakaryopoiesis with abnormal thrombopoietin (TPO) signaling in at least a subset of patients. | In reported series, marrow megakaryocytes were **absent in 28/52** and **decreased in 22/58** cases; studies showed abnormal TPO-induced tyrosine phosphorylation and one report of failed **JAK2** phosphorylation. | Clinical review summarizing primary functional studies (pqac-00000003, pqac-00000005) |
| Phenotype frequencies (34-patient series) | TAR is multisystemic beyond the upper limbs. | In a **34-patient series**: **100%** bilateral radial aplasia, **100%** thrombocytopenia, **47%** cow’s milk intolerance, **47%** lower-limb involvement, **23%** renal anomalies, **15%** cardiac anomalies, **3%** cleft palate. Another review text reports cardiac and renal anomalies in **47%** and **23%**, respectively. | Clinical review/book chapter and review synthesis (pqac-00000005, pqac-00000015) |
| Temporal course and mortality | The highest bleeding risk is in early infancy; platelet counts often improve with age. | **50–60%** of hemorrhagic episodes occur in the **first week of life**; **82%** had platelet counts **<50,000/mm^3 at birth** in one series; mortality reported as **21/77** overall with **14 deaths within 4 months**, all with platelets **<30,000/mm^3**. Neonatal mortality of about **25%** from hemorrhage has also been reported. | Clinical reviews and prenatal case report (pqac-00000002, pqac-00000003, pqac-00000007, pqac-00000015) |
| Prenatal findings | TAR can be suspected prenatally from limb findings and confirmed with targeted fetal/genetic testing. | Prenatal ultrasound can identify bilateral radial defects with preserved thumbs; fetal platelet count of **40,000/mm^3** was documented by cordocentesis in one case. In a 2023 prenatal CNV cohort, **4** cases involved the TAR region among **26** 1q21.1 CNV pregnancies from **8,252** tested pregnancies. | Prenatal case report and 2023 prenatal cohort (pqac-00000006, pqac-00000002, pqac-00000012, pqac-00000004) |
| Diagnostic approach | Diagnosis is clinical-radiologic plus molecular confirmation. | Key tests: CBC/platelet count, bone marrow megakaryocyte assessment when needed, limb radiographs, and genetics with **chromosomal microarray** for 1q21.1 deletion plus **RBM8A** sequencing/genotyping for hypomorphic alleles. Differential diagnosis includes Fanconi anemia, Holt-Oram syndrome, Roberts syndrome, and other radial-ray disorders. | Review and prenatal molecular case report (pqac-00000015, pqac-00000004, pqac-00000006) |
| Standard treatment | Management is largely supportive, focused on bleeding prevention and orthopedic function. | **Platelet transfusion** is the main acute therapy, especially for severe thrombocytopenia, surgery, or infection-related drops; reconstructive hand surgery may benefit selected patients and is often delayed until bleeding risk falls. In utero platelet transfusion has been reported in severe prenatal thrombocytopenia. | Clinical management reports and 2023 review snippet (pqac-00000002, pqac-00000006, pqac-00000014) |
| Current research and trials | Current published progress is mainly in genetics, prenatal diagnosis, and patient-network/self-empowerment; there is little TAR-specific interventional evidence. | Retrieved ClinicalTrials.gov records were **observational**, not TAR-specific drug trials: **NCT00086476** (completed megakaryocyte study; actual enrollment **3**) and **NCT00027274** (recruiting IBMFS natural-history study; estimated enrollment **4000**). No retrieved TAR-specific gene, RNA, cell therapy, HSCT standard-of-care, or TPO-receptor-agonist trial evidence. | 2023 review and clinical-trial records (pqac-00000014, pqac-00000009, pqac-00000011, pqac-00000016) |


*Table: This table summarizes the most actionable disease-knowledge facts for thrombocytopenia-absent radius syndrome using only evidence already retrieved in the session. It highlights identifiers, defining features, epidemiology, RBM8A genetics, phenotype frequencies, natural history, diagnostic workup, standard care, and current evidence gaps.*