Production of biologically active GM-CSF in sugarcane: a secure biofactory

Over 300 transgenic sugarcane plants representing approx. 200 independent lines producing the human cytokine granulocyte macrophage colony stimulating factor (GM-CSF) were analyzed for recombinant protein accumulation and activity levels. Expression constructs differed in use of the maize polyubiquitin 1, Mubi-1, or the sugarcane polyubiquitin 9, SCubi9, promoters; presence or absence of a C-terminal HDEL tag for ER retention; and presence or absence of a 6X Histidine tag for metal ion affinity purification. Accumulation of GM-CSF protein ranged from undetectable to 0.02% of total soluble protein. No significant difference was observed between the two promoters; however, the ER retention tag was required for higher accumulation levels. Human bone marrow cells (TF-1), which require GM-CSF for cell division, proliferated when growth media was supplemented with transgenic sugarcane extracts. Comparison to purified commercially produced GM-CSF indicated the sugarcane-produced protein had essentially identical activity levels. In a 14-month field trial, accumulation levels remained stable. This is the first report of field production of GM-CSF. During the field trial, no flowering of the trial plants occurred; no pollen or seed was produced. Drying, burning, and burial of the test plants effectively blocked possible routes for the transgenic sugarcane to enter the environment or food supply. Sugarcane may provide a highly secure system for biofactory production of pharmaceutical proteins.