Gene therapy of MPL deficiency: challenging balance between leukemia and pancytopenia

Area of Science:

• Hematology
• Gene Therapy
• Oncology

Background:

• Thrombopoietin receptor (MPL) signaling is vital for hematopoietic stem cells (HSCs) and megakaryocyte differentiation.
• Loss-of-function MPL mutations cause congenital amegakaryocytic thrombocytopenia (CAMT), a severe blood disorder.
• Assessing retroviral Mpl expression's safety is crucial for developing gene therapy for CAMT.

Purpose of the Study:

• To evaluate the toxicity of retroviral Mpl expression in a murine bone marrow transplantation (BMT) model.
• To develop safer retroviral vectors for potential gene therapy of CAMT.

Main Methods:

• Murine bone marrow transplantation (BMT) model used to assess retroviral Mpl expression.
• Initial vectors led to high Mpl expression, causing adverse hematological effects.
• Improved self-inactivating gamma-retroviral vectors with reduced Mpl expression (PGK or Mpl promoter) were constructed.

Main Results:

• High Mpl expression caused transient multilineage hematopoiesis elevation, chronic myeloproliferative disorder (CMPD)-like features, and progressive pancytopenia.
• Erythroleukemia developed in 10% of mice due to insertional activation of Sfpi1 and Fli1.
• Mice receiving BM cells with Mpl expressed from the Mpl promoter showed no adverse reactions.

Conclusions:

• High retroviral Mpl expression can lead to myelodysplastic syndrome (MDS)-like disorders and leukemia in a BMT model.
• Reduced and physiological Mpl expression using self-inactivating vectors and the Mpl promoter is a safer approach.
• This strategy holds promise for developing effective gene therapy for CAMT.