MDM4 haploinsufficiency leads to p53-mediated bone marrow failure.

Bone marrow failure (BMF) syndromes are heterogeneous diseases characterized by impaired hematopoiesis and a risk of evolution to myelodysplastic syndrome (MDS) and leukemia. We report 6 unrelated individuals with variable BMF phenotypes and hypocellular MDS, presenting at a median age of 10 years (range, 4 weeks to 53 years). Genomic analysis revealed germ line heterozygous variants in mouse double minute 4 (MDM4), including 4 null (frameshift, nonsense, and splice site resulting in premature truncation confirmed by RNA sequencing) and 2 missense variants, of which 1 had previously been associated with a familial BMF syndrome. Mechanistically, MDM4 mutations are loss-of-function mutations leading to enhanced p53 activation. We used CRISPR/Cas9 to delete MDM4 in healthy donor hematopoietic stem and progenitor cells (HSPCs). The resulting MDM4-haploinsufficient HSPCs exhibited increased p53 activity, impaired colony-forming capacity, and reduced engraftment potential in immunodeficient mice. Complementation studies revealed both p53-binding and RING-finger domains as necessary for MDM4-mediated hematopoietic regulation. To study variant effects in a confounder-free genetic background, we introduced patient-specific MDM4 variants into induced pluripotent stem cells (iPSCs). MDM4-mutant iPSCs yielded significantly reduced erythroid and myeloid cells and exhibited increased p53 activity, as evidenced by elevated p21 expression, confirming the role of MDM4 in regulating hematopoiesis through p53. Transcriptome analysis of iPSC-derived hematopoietic cells revealed upregulation of the p53 pathway. Importantly, 1 patient with MDS acquired loss-of-function TP53 mutations, suggesting maladaptive somatic rescue. Our findings establish MDM4 deficiency as a TP53-activating syndrome, with features of BMF and variable hematopoietic manifestations. This study also highlights the critical role of the MDM4-p53 axis in maintaining hematopoietic homeostasis.