MCL-1 is a pro-survival BCL-2 protein that is frequently overexpressed in human cancer and many cancers depend on its expression for their survival and sustained growth. Therefore MCL-1 represents a promising target in cancer therapy 1. Amongst the pro-survival BCL-2 family members MCL-1 is unique in being essential for early embryonic development and the survival of many cell types, including haematopoietic stem/progenitor cells, neurons and intestinal epithelial cells 2-10. Even though most tissues express high levels of several pro-survival BCL-2 proteins, the loss of other pro-survival proteins is tolerated in most tissues. To date it remains unclear which unique characteristics of MCL-1, such as protein half-life (MCL-1=30 min, BCL-XL/BCL-2=24 h), differences in binding to pro-apoptotic BCL-2 family members, or alleged apoptosis-unrelated functions (e.g. in mitochondrial energy production) underly its essential role.
To identify the unique functions of MCL-1 we generated “gene-swap” mice by replacing its coding region with that for BCL-XL, BCL-2 or A1. As control, the MCL-1 coding region was replaced with an Mcl-1 cDNA. We found that in contrast to the E3.5 embryonic lethality of Mcl-1-/- mice, homozygous Bcl-X>Mcl-1 and Bcl-2>Mcl-1 gene-swap mice developed until E12 and beyond. This reveals that the alleged apoptosis-unrelated functions of MCL-1 are dispensable for early embryogenesis. Flow cytometric analysis demonstrated that adult heterozygous Bcl-X>Mcl-1 and Bcl-2>Mcl-1 gene-swap mice have increased numbers of certain haematopoietic cell subsets. This reveals that replacement of short-lived MCL-1 with long-lived BCL-XL or BCL-2 causes increased survival and accumulation of cells. Thus, the different pro-survival BCL-2 family members with their distinct biochemical properties (protein lifespan, binding to pro-apoptotic BCL-2 family members) must have evolved to provide adequate control of cell survival in a cell type specific, developmental checkpoint specific and stress condition specific manner.