A Case of Thrombocytopenia with <italic>MYH9</italic> Gene Mutation Found in Siblings

Myosin heavy chain 9 (MYH9) related diseases are autosomal dominant diseases characterized by macrothrombocytopenia and inclusion bodies in leukocyte, which can be accompanied by extra-hematologic symptoms such as sensorineural hearing loss, renal dysfunction, and cataract. They are often diagnosed incidentally in adulthood or misdiagnosed as idiopathic thrombocytopenic purpura (ITP), leading to unnecessary treatment with intravenous high-dose gamma-globulin, steroids, or splenectomy. Here, we report the case of a brother and sister confirmed to have a MYH9 gene mutation during follow-up. An 8-year-old boy was confirmed to have thrombocytopenia at birth and treated with intravenous gamma-globulin under suspicion of ITP or sepsis. He was discharged after showing an increase in platelet count. Subsequently, during outpatient workup, he exhibited thrombocytopenia, large platelets, and neutrophilic inclusion bodies. His 10-year-old sister also presented with the same findings. In 2021, DNA analysis revealed that they share a mutation (c.4270G> A, p. Asp1424Asn), a pathogenic variant associated with MYH9-related disorder.