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A DAB hand at DNA delivery

30 Nov, 2011

ResearchBlogging.orgAs a kid I always wanted to be Spider-Man (secretly I still do). In fact Peter Parker’s love of science as a student was one of the things that influenced me to choose scientific subjects at school. That’s where our stories diverge. Parker, of course, was bitten by a radioactive spider, which introducing its DNA into his cells and give him super powers. I, on the other hand, became a bacteriologist.

In the real world, the introduction in DNA into human cells is a rapidly advancing field that may hold the key to curing a multitude of diseases, including cancer. But what is it? How does it work? I had the opportunity to talk to Dr Christine Dufès, Lecturer in Drug Delivery at the Strathclyde Institute of Pharmacy and Biomedical Sciences, who is making huge strides at delivering DNA into tumour cells safely and efficiently.

Using DNA to treat cancer is known as ‘gene therapy’. In clinical terms, it involves the introduction of a gene or genes into a cancer cell. These encode proteins that help destroy a tumour, either by being outright poisonous to the tumour cell; making it more susceptible to chemotherapy drugs; or making it more identifiable to the body’s immune system.

How do we introduce these genes into the cancer cells? Thankfully not with the aid of a radioactive arachnid. Often viruses are used, for example the adenovirus, as they are great at both attaching to and entering human cells. Despite their undoubted promise, problems with viral vectors remain – production can be difficult; the human immune response can prevent repeated treatments; and non-tumour cells can be targeted as a side effect.

So what else can be used? Well, in response to the potential hurdles that need to be overcome with using viruses, researchers have been developing ‘non-viral gene delivery systems’. These are molecules that bind DNA and are able to transport it into cells. Although these molecules have a reputation as being less efficient at gene delivery than viruses, they do have the marked advantage of not inducing an immune response. They are non-toxic and have the potential for large-scale production.

Dufès is working on a group of non-viral delivery systems called dendrimers. In particular, she researches ‘generation 3 diaminobutyric polypropylenimine dendrimer’ or DAB for short (thankfully). These spherical polymers consist of a core molecule from which a large number of highly branched arms originate.

The DAB molecule has shown promise at delivering small circles of DNA, known as plasmids, into tumour cells. DAB has an abundance of amino-groups at the end of the dendrimer branches. These allow the molecule to carry large amounts of DNA, or can be chemically modified to change the characteristics of DAB.

In vitro results showed that DNA attached to amino acid modified DAB was more likely to be taken up by cancer cells than unmodified DAB, so in a recent paper, Dufès’ group describe how they attached different amino acids to the ends of the DAB branches, to try and increase both the uptake of the DAB-DNA complex into cancer cells and the expression of the genes introduced.

They tested three amino acids, arginine leucine and lysine, and it was the latter that proved most effective at transporting DNA to tumour cells. This effect was shown in two types of cells: epidermoid carcinoma cells and glioblastoma cells.

In addition to being more efficient at delivering DNA, the modified DABs were better at killing cancer cells than the controls. The researchers introduced a plasmid containing the gene TNFα (tumour necrosis factor) into the two cancer cell types via the modified DABs. Again, the lysine modified molecule worked best, killing the cancer cells. This was 47 times more effective than the unmodified molecule at killing glioblastoma cells.

Finally the group tested the modified DABs in vivo to assess the levels of gene expression in a mouse model. Gene expression was highest in tumours, but was also seen in other organs, notably the liver and heart.

Dufès’ other work may hold the key to tumour specificity. Modifying DAB by adding a transferrin molecule makes it more efficient at targeting cancer cells, which have more transferrin receptors than ‘regular’ cells.

The results are encouraging, but as Dufès told me, there’s still a lot of work to be done before this becomes a treatment. “We need to improve tumour specificity. [But] gene therapy, for me, holds the promise of curing cancer.”

“Ultimately I want to develop a stable, powerful medicine that works well and goes straight to the tumour. My long term objective will be to perform clinical trials, but this is still an early stage – I’m moving step-by-step.”

Intriguingly, why this works is unknown. We’ve no idea what modifying DAB with amino acids does to increase both the uptake of the molecule into cancer cells and the levels of gene expression. This activity has been demonstrated by other scientists using other non-viral gene delivery systems, but the mechanisms remain a mystery.

What is clear is that these molecules have the potential to deliver DNA directly to tumours with minimal side effects. And not a web-slinger in sight.

Benjamin Thompson

Aldawsari H, Edrada-Ebel R, Blatchford DR, Tate RJ, Tetley L, & Dufès C (2011). Enhanced gene expression in tumors after intravenous administration of arginine-, lysine- and leucine-bearing polypropylenimine polyplex. Biomaterials, 32 (25), 5889-99 PMID: 21596431

One Comment leave one →
  1. Samme permalink
    4 Nov, 2014 1:51 pm

    Another interesting advance in delivery is this by Andaloussi et al
    http://www.ncbi.nlm.nih.gov/pubmed/23154783

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