Syndrome of the Month: Radioulnar Synostosis With Amegakaryocytic Thrombocytopenia Type 2.

Radioulnar synostosis with amegakaryocytic thrombocytopenia type 2 (RUSAT-2) is a rare inherited bone marrow failure syndrome characterized by congenital or progressive thrombocytopenia, frequent radioulnar synostosis, and variable multisystem involvement. It is caused by heterozygous germline pathogenic variants in the MDS1 and EVI1 Complex Locus (MECOM) gene, which encodes transcription factors essential for hematopoietic stem cell regulation and embryonic development. Disruption of highly conserved zinc finger domains within MECOM impairs long-term hematopoietic stem cell maintenance, leading to amegakaryocytic thrombocytopenia and, in many cases, progression to pancytopenia. Although MECOM is also implicated in leukemogenesis through somatic dysregulation, germline variants associated with RUSAT-2 result in a distinct developmental and hematologic phenotype with highly variable penetrance and age of onset. As of 2025, there were approximately 66 reported cases of RUSAT-2 reported in the literature, with clinical severity ranging from isolated thrombocytopenia to early-onset bone marrow failure requiring hematopoietic stem cell transplantation. This review summarizes the current understanding of the genetic basis, clinical manifestations, differential diagnosis, clinical course, management considerations, and outstanding mechanistic questions surrounding RUSAT-2.