Gene therapy with biodegradable polymers
Novel biodegradable, synthetic and non-viral polymers, manufactured by researchers at MIT, USA, could pave the way to achieving safe and effective gene therapy. Gene therapy involves inserting new genes into patients’ cells to fight diseases such as severe combined immunodeficiency.
Using poly(beta-amino esters), composed of chains of alternating amine and diacrylate monomers, scientists at MIT are producing reactions with various diamines to create a positively charged material. This binds to and condenses negatively charged deoxyribonucleic acid (DNA) into nanoparticles that are ready to be injected into the body.
Performance is enhanced 100-fold by adding amine-containing, positively charged molecules to the ends of polymer chains to bind the DNA more tightly. ‘Tighter DNA binding leads to the formation of smaller nanoparticles and [in turn] increased cellular uptake and gene delivery efficacy,’ says Jordan Green, a graduate student in biological engineering and first co-author on the research paper.
Gene therapy has seen intense research for over 20 years but has yet to realise its full potential, partly due to safety concerns over using viruses as gene carriers.
‘Clinical trials with viral gene therapy have [sometimes] caused cancer (such as T-cell acute lymphoblastic leukemia) and death in humans,’ says Green. ‘Cancer can result from the virus inserting its own gene inside an endogenous healthy gene, causing loss of function.’
Furthermore, viruses have a protein coat that is recognised by the body as a foreign virus and triggers an immune response. This increases the immune system’s resistance to therapy. Moreover, a severe immune response could be dangerous.
Green explains that while viruses have an upper DNA payload limit of eight-kilobases, the material developed at MIT could have a much larger capacity or allow for different combinations of DNA. This is because the polymers do not have a pre-set limit to the size of DNA that they can bind to.
Furthermore, the polymeric nanoparticles do not contain proteins or peptides and are therefore not seen as foreign by the immune system. The same polymers could thus be used in one patient on multiple occasions.
‘The problem has been that viruses are normally much more efficient at delivering genes than artificial constructs,’ comments Dr David Matthews, Lecturer in Virology at the University of Bristol, UK. ‘Many artificial systems look very promising in the laboratory but do not perform well in animal studies where [they] come in contact with body fluids. Real viruses have evolved alongside us and are therefore much better at surviving.’
He says, ‘The data provided by MIT seems to indicate a significant breakthrough in non-viral gene therapy, as they appear to have overcome some of the biggest problems and matched the efficiency of viral-based systems.’
The polymeric nanoparticles are internalised in the endosomes (membrane-bound compartment) of the cells. Green explains, ‘Protons are typically pumped into endosomes. Our polymers, via their tertiary amines, are able to buffer this acidification, maintaining a near-neutral pH.’ This aids rupture of the endosomes and release of the polymer particles into the cytoplasm, where polymer/DNA unbinding thermodynamism and polymer degradation in water enables release of the genes, which then enter the nucleus of the cell.
‘Many other biomaterials are unable to facilitate endosomal escape and their DNA is degraded in the endosomes, resulting in poor gene delivery,’ adds Green. ‘The polymers also work in the presence of serum proteins which are known to inactivate delivery particles.’
Targetted delivery to cells in vivo could be ensured, for example, by injecting the material into the organ or area to be treated, or by using DNA sequences that are only expressed in certain tissues, explains Daniel Anderson of the Center for Cancer Research at MIT.
The team has conducted preliminary tests in mice, and so far the materials have been found to be safe for use, benign to healthy muscle with minimal inflammation.