While traditional drugs are chemically synthesized, living organisms produce therapeutic proteins. The current generation of microbial and mammalian cell bioreactor-based systems will not be able to handle future demand. Expensive small-capacity units are too big a financial burden to produce and operate in the quantities needed. Manufacturing systems that use plants as bioreactors can meet the growing demand of the healthcare industry for therapeutic proteins at far lower cost.
Plant chloroplast transformation technology (CTT) can deliver quality proteins at low cost. Several of the advantages of this production platform are listed in Table 1. CTT in tobacco is now being validated as we explain below.
The chloroplast is a metabolic organelle designed to synthesize abundant proteins for energy generation and plant growth and development. The chloroplast's genome is a self-replicating circular double-stranded DNA molecule ranging in size from 110 to 220 kilobase pairs depending on the species of plant. In the majority of plants, the chloroplast genome has two inverted regions.2 With up to 100 genomes per chloroplast and up to 100 chloroplasts per cell, the integration of a transgene through homologous recombination into the inverted repeat region can generate up to 20,000 copies of the transgene per cell.
The chloroplast generates proteins in a manner similar to a prokaryotic cell, creating polycistronic messages and accepting large DNA inserts with multiple genes and repetitive elements without inducing silencing pathways. The chloroplast has been demonstrated to properly fold and assemble disulfide-bonded proteins, which can be sequestered away from cytosolic proteases during growth and development.9 However similar to bacterial expression systems, chloroplasts do not have the post-translational processing machinery for glycosylation of proteins.
The chloroplast transformation-expression vector is introduced into the chloroplast by particle bombardment of plant tissue, or polyethylene glycol treatment of protoplast.3,8 Transformation is initiated by transgene integration into a few copies of chloroplast genome within a single cell, followed by antibiotic selection and transgene replication during the next 15 to 20 cell divisions, which results in a homogeneous plastid genome population. Integration of the transgene expression cassette into one of the inverted repeat regions will initiate a copy number correction resulting in the duplication and insertion of another transgene cassette into the second inverted repeat region.