Unveiling MED12L: A Potential Novel Orchestrator of Platelet Dysfunction in Myeloproliferative Myeloid Neoplasms

Adequately treating, staging, and predicting disease progression remain challenging endeavors in chronic hematopoietic disorders such as Myeloproliferative Neoplasms (MPN), thus increasing public health costs and the frequency of patients experiencing thrombotic events or leukemic evolution. Over the years, deeper insights into somatic mutations have advanced our understanding of myeloid neoplasms but has not yet allowed to fully characterize them, leaving an unmet medical need.

A frequent feature of MPN is the somatic mutation V617F in the JAK2 gene, arising in patient's hematopoietic stem and progenitor cells (HSPC). In JAK2^V617F mice in which we deleted the stem cell transcription factor Pbx1, we showed that typical MPN features including thrombocythemia and erythrocytosis do not develop or disappear with time. One of the downregulated genes in the double mutant mouse model is Mediator complex subunit 12-like (MED12L), paralogue of MED12, an essential component of the transcriptional machinery of crucial stem cell genes. Therefore, we tested the hypothesis that MED12L plays a role in MPN.

To assess the physiological role of MED12L we performed in silico analysis of its expression along mouse and human hematopoietic hierarchy and took advantage of Med12l KO mice; to analyze a potential contribution of MED12L in myeloid neoplasms, we are studying MED12L expression in HSPC from MPN patients and the outcome of its inhibition in human MPN cell lines. Last, we crossed Med12l KO with JAK2^V617F mice (named JM mice) to analyze the consequences of Med12l absence in vivo in an MPN model.

MED12L is highly expressed in normal HSPC and megakaryocyte (Mk) precursors and it is upregulated in Mk-Erythroid progenitors of two additional MPN murine models according to published RNAseq data, in agreement with the hypothesis that its dysregulation, secondary to an established driver mutation, may contribute to the MPN phenotype. Blood analysis of Med12l KO mice through hemocytometer, flow cytometry and electron microscopy revealed higher Mean Platelets (PLT) Volume, altered PLT Distribution Width, increased PLT area and complexity and higher surface expression of the CD41 and CD61 integrins with respect to WT. Moreover, PLT from Med12l KO mice display a higher number of mitochondria, accounting for the increased internal complexity, and reduced activity per single mitochondrion. Larger size and increased complexity suggested defects in PLT maturation, which was confirmed by an increase in the frequency of reticulated PLT as seen upon measuring nucleic acids content. FACS analysis of all main bone marrow cells along the hematopoietic hierarchy revealed modestly increased frequency of hematopoietic stem cells in Med12l KO micecompared to WT, but no differences in more committed populations nor in Mk, suggesting that the higher frequency of reticulated PLT arise mainly from a defect in the last stage of their development. Analysis of MPN patient samples revealed higher MED12L expression with respect to healthy donors (HD), and increased expression in CD34⁺ compared to CD34^-cells, confirming also in patients a preferential expression in HSPC over mature cells as predicted by in silicodata on HD. Moreover, publicly available scRNAseq data indicate elevated and almost exclusive expression of MED12Lin PLT within the blood of MPN patients, resulting in a robust correlation between MED12L expression and both PLT features (CD41/CD61 expression) and activation path. Of note, also in our large cohort of myelodysplastic syndrome patients, we found a similar correlation between MED12L expression in CD34⁺ cells and PLT features and function.

These observations uncover a novel player in PLT biology and suggest a link between MED12L and PLT function in MPN, highlighting its potential as a therapeutic target and biomarker in the myeloid neoplasms landscape. Further investigation including the analysis of JM mice is ongoing to validate these findings and to explore the clinical value of MED12L in MPN management.