First report: A novel deep intronic mutation and treatment using hetrombopag olamine of a patient with MYH9 syndrome and immune thrombocytopenia

Abstract Introduction: MYH9-related diseases (MYH9-RD), caused by pathogenic variants in the MYH9 gene through an autosomal dominant inheritance pattern, are characterized by congenital thrombocytopenia, giant platelets, and Döhle-like inclusions in neutrophils, with variable features such as sensorineural hearing loss, cataracts, and nephropathy. Giant platelet thrombocytopenia, which is one of the most remarkable manifestations of MYH9 syndrome, remains poorly understood in terms of its mechanisms, and there are no unified treatment recommendations. The incidence of thrombocytopenia caused by MYH9 mutations is also not well established. Therefore, we conducted this study to shed light on potential treatment strategies. Here, we report the first documented case of MYH9-RD caused by a novel deep intronic mutation. The associated refractory thrombocytopenia was successfully managed with the thrombopoietin receptor agonist hetrombopag olamine, expanding both the genetic and therapeutic landscape of this rare disorder. Methods: A 22-year-old male with lifelong refractory thrombocytopenia was enrolled. Detailed clinical history and physical examination were reviewed, followed by routine laboratory investigations, including blood counts, peripheral smear, bone marrow cytology, and autoimmune workup, to exclude other causes and support a working diagnosis of immune thrombocytopenia. Whole-exome sequencing (WES) was conducted on the proband and his parents. A minigene splicing assay using pcMINI/pcMINI-N vectors was performed in HEK-293T and HeLa cells to evaluate splicing impact. Patient and maternal peripheral blood RNA were analyzed via nested RT-PCR, TA cloning, and Sanger sequencing. Structural modeling was used to visualize protein changes. Results: He had a baseline platelet count of 11×10⁹/L and recurrent mucocutaneous bleeding. He had been misdiagnosed with ITP for over a decade, showing poor response to steroids, eltrombopag, and rhTPO. Examination of peripheral blood smear revealed giant platelets and Döhle-like inclusions. The patient's laboratory tests revealed immune thrombocytopenia. WES identified a heterozygous deep intronic mutation c.2977-75C>T in MYH9, absent in population databases (gnomAD v4.1.1, ClinVar). The variant was maternally inherited (mother asymptomatic), and classified as a variant of uncertain significance (PM2_Supporting + PP3 per ACMG/AMP guidelines).Functional studies revealed that the mutation disrupted normal splicing, producing two aberrant transcripts: (1) a 138-bp intronic insertion leading to a truncated protein (p.Lys994_Glu1960delinsAsnProAsnGluLeuAspGlnAlaSerLysAlaLeuSerIleGlnProGlnIleSerGly; 1013 aa), and (2) a 62-bp retention leading to a frameshift and premature termination (p.Lys994AsnfsTer29; 1021 aa). Both abnormal isoforms lacked the C-terminal tail of NMMHC-IIA, critical for dimerization and cytoskeletal function. Hetrombopag olamine (5 mg daily) was initiated in August 2024. Platelet count rose from 22×10⁹/L at baseline to 34×10⁹/L at week 2, peaked at 92×10⁹/L at 1 month, and stabilized between 38–42×10⁹/L post-discontinuation. No adverse effects were observed during or after treatment. Conclusion: This is the first report of MYH9-RD caused by a deep intronic c.2977-75C>T mutation, which induces exonization and protein truncation. The positive clinical response to hetrombopag olamine demonstrates its therapeutic potential in MYH9-RD. Our findings support the inclusion of deep intronic variants in diagnostic pipelines and suggest TPO-RAs as a promising therapeutic approach in such cases.