Tag Archives: Genetic Engineering

Multiplex Automated Genome Engineering: Changing the world with MAGE

Humans have evolved a most unique mastery of toolmaking through advanced technology. As an extension of our biological bodies, technology has loosened the grip of natural selection. This is particularly true in the field of biomedicine and genetic engineering. We have the ability to directly alter the blueprint of life for any purpose we wish. Beginning in the 1970’s with the creation of recombinant DNA and transgenic organisms, genetic engineering has offered scientists the ability to study genes on a level that may not have seemed possible at the time. The field has provided a wealth of knowledge as well as practical implications, such as knockout mice and the ability to produce near-endless amount of human insulin for diabetics.

As of 2009, multiplex automated genome engineering (MAGE) has ushered in a new branch of genetic engineering – genomic engineering. We are no longer restricted to altering single genes, but rather are able to alter entire genomes by manipulating several genes in parallel. This new ability, brought about by MAGE technology, allows for nearly endless applications that stretch well beyond medicine or industry; agriculture, evolutionary biology, and conservation biology will benefit tremendously as MAGE technology progresses. Genetic engineering advancements such as MAGE are poised to revolutionize entire fields of science, including synthetic biology, molecular biology, and genetics by offering faster, cheaper, and more powerful methods of genome engineering.

Homologous Recombination

Genetic engineering underwent a revolutionary change in the 1980’s, largely due to the pioneering work of Martin Evans, Mario Capecchi, and Oliver Smithies. Evans and Kauffman were the first to describe a method for extracting, isolating, and culturing mouse embryonic stem cells. This laid the foundation for gene targeting, a method that was independently discovered by both Oliver Smithies and Mario Capecchi. Mario Capecchi and his colleagues were the first to suggest mammalian cells had the machinery capable for homologous recombination with exogenous DNA. Smithies took this a step further, demonstrating targeted gene insertion using the β-globin gene. Ultimately, the combined work of Evans, Smithies, and Capecchi on homologous recombination earned them the Nobel Prize in Physiology or Medicine in 2007. The science of homologous recombination has allowed for many scientific discoveries, primarily through the creation of knockout mice.

Homologous recombination works under many of the same principles are chromosomal recombination in meiosis, wherein homologous genetic sequences are randomly exchanged. The difference lies in the fact that homologous recombination works with exogenous DNA and on a gene level rather than chromosomal level.

1

2

The method works by using a double stranded genetic construct with flanking regions that are homologous to the flanking regions of the gene of interest. This allows for the sequence in the middle, containing a positive selection marker and new gene, to be incorporated. The positive control should be something that can be selected for, such as resistance to a toxin or a color change. Outside of one of the flanking regions of the construct should lie a negative selection marker; the thymidine kinase gene is commonly used. If homologous recombination is too lenient, and the thymidine kinase gene is incorporated into the endogenous DNA, it can be detected and disposed of. This is to prevent too much genetic information from being exchanged.

Using this method, knockout mice can be created. A knockout mouse is a mouse that is lacking a functional gene, allowing for elucidation of the gene’s function. Embryonic stem cells are extracted from a mouse blastocyst and introduced to the gene construct via electroporation. The successfully genetically modified stem cells are selected using the positive and negative markers. These are isolated and cultured before being inserted back into mouse blastocysts. The mouse blastocysts can then be inserted into female mice, producing chimeric offspring. These offspring may be mated to wild-type mice. If the germ cells of the chimeric mouse were generated from the modified stem cells, then the offspring will be heterozygous for the modified gene and wild-type gene. These heterozygous mice can then be interbred, with a portion of the offspring being homozygous for the modified gene. This is the beginning of a mouse line with the chosen gene “knocked-out.”

3

Multiplex Automated Genome Engineering Process

The major drawback of the previously described method of “gene targeting” is the inability to multiplex. The process is not very efficient, and targeting more than one gene becomes problematic, limiting homologous recombination to single genes. In 2009, George Church and colleagues solved this issue with the creation of multiplex automated genome engineering (MAGE). MAGE technology uses hybridizing oligonucleotides to alter multiple genes in parallel. The machine may be thought of as an “evolution machine,” wherein favorable sequences are chosen at a higher frequency than less favorable sequences. The hybridization free energy is a predictor of allelic replacement efficiency. As cycles complete, sequences become more similar to the oligonucleotide sequence, increasing the chance that those sequences will be further altered by hybridization. Eventually, the majority of endogenous sequences will be completely replaced with the sequence of the oligonucleotide. This process only takes about 6-8 cycles.

4

After the E. coli cells are grown to the mid log phase, expression of the beta protein is induced. Cells are chilled and the media is drained. A solution containing the oligonucleotides is added, followed by electroporation. This step is particularly lethal, killing many of the cells. However, the cells are chosen based on positive markers (optional, but increases efficiency) and allowed to reach the mid-log phase again before repeating the process. Church and his colleagues have optimized the E. coli strain EcNR2 to work with MAGE. EcNR2 contains a plasmid with the λ phage genes exo, beta, and gam as well as being mismatch gene deficient. When expressed, the phage genes will help keep the oligonucleotide annealed to the lagging strand of the DNA during replication, while the mismatch gene deficiency prevents the cellular repair mechanisms from changing the oligonucleotide sequence once it is annealed. Using an improved technique called co-selection MAGE (CoS-MAGE), Church and colleagues created EcHW47, the successor to EcNR2. In CoS-MAGE, cells that exhibit naturally superior oligo-uptake are selected for before attempting to target the genes of interest.

MAGE technology is currently in the process of being refined, but shows incredible promise in practical applications. Some of the immediate applications include the ability to more easily and directly study molecular evolution and the creation of more efficient bacterial production of industrial chemicals and biologically relevant hormones. Once the technique has been optimized in plants and mammals, immediate applications could be realized in GMO production and creation of multi-knockout mice that will give scientists the ability to study gene-gene interactions on a level previously unattainable. A more optimistic and perhaps grandiose vision could see MAGE working towards ending genetic disorders (CRISPR technology, an equally incredible genomic editing technique, may beat MAGE there) and serving as a cornerstone technique in de-extinction. The ability to alter a genome in any fashion brings with it immense power. The possibilities for MAGE are boundless, unimaginable, and are sure to change genomic science.

For more information on Homologous recombination, see:

http://www.bio.davidson.edu/genomics/method/homolrecomb.html

For more information on MAGE, see:

Wang, H. H., Isaacs, F. J., Carr, P. A., Sun, Z. Z., Xu, G., Forest, C. R., & Church, G. M. (2009). Programming cells by multiplex genome engineering and accelerated evolution. Nature, 460(7257), 894-898.

Wang, H. H., Kim, H., Cong, L., Jeong, J., Bang, D., & Church, G. M. (2012). Genome-scale promoter engineering by coselection MAGE. Nature methods, 9(6), 591-593.

For more information on CRISPR (which I highly recommend; it’s fascinating), see:

https://www.addgene.org/CRISPR/guide/

Advertisements