Genomic Engineering and Xenotransplantation

by Andrew Zale

Given everything that "Back to the Future Part II" predicted incorrectly – hoverboards, flying cars and stationary bikes in coffee shops – the movie did correctly forecast the expansion of biotechnology and progress in the field of biomedical engineering.

Genomic engineering, described as the process of altering DNA so that the cell exhibits more favorable characteristics, is a substantial part of the field of biomedical engineering. More specifically, recent developments regarding the potential of the CRISPR/Cas9 system have been encouraging on many levels.

CRISPR, or clustered regularly interspaced short palindromic repeats, works with Cas9 as a main source of the prokaryotic immune system through acquired immunity. The complex was discovered within the genome of E. Coli in the 1980s and has been determined to control three important mechanisms within bacteria. The most prominent mechanism controls bacterial immunity against bacteriophages, or viruses that attack bacteria. Normally when viruses infect bacteria, their DNA is inserted into the bacteria’s DNA. This integration allows the virus’s genetic information to be transmitted so that the bacterial cell produces more viruses.

CRISPR, on the other hand, recognizes foreign DNA in the bacteria and splices it. When the viral DNA is placed within the bacterial DNA, the splices are separated by palindromic sequences, known as spacers (ie. AGGA). Cas9 recognizes this sequence as a warning that this DNA is foreign and dangerous for the bacteria. So the next time a virus with the same genome invades the bacterial cell, that DNA is destroyed. This discovery was quite significant because while human immunity is protein-based, which means that it is not heritable, bacterial immunity is found within its DNA, so it is transferred to its daughter cells down the next generation.

Dr. Alex Reis, researcher at the New England Bio Labs, explains that “The rapid progress in developing Cas9 into a set of tools for cellular and molecular biology research has been remarkable, likely due to the simplicity, high efficiency and versatility of the [CRISPR] system.”

The most expansive research has been on editing fetal mutations that may cause genetic disorders. However, investigation down this path has not come without government critique, limiting any federal research grants from funding the modification of human embryos. Looking beyond our own nation, the United Kingdom bans clinical genome editing but allows genome editing within a laboratory setting. Germany limits fetal embryo research and has pressed charges against researchers who have broken the law. Argentina, Japan, China, Ireland and India have moved towards bans, but the resulting legislation has been unenforceable.

Governmental interference is a major problem according to Reis, who says “Of the designer nuclease systems currently available for precision genome engineering, the CRISPR/Cas system is by far the most user friendly.”

With a controversial environment around fetal genome testing worldwide, researchers have focused on organ donation as a usage for CRISPR. Pigs have widely been considered as feasible candidates for donation given their similar organ size to humans and ability to breed. Just this past October, Dr. George Church, researcher at Harvard Medical School, published a paper hypothesizing that pig organs may be able to be altered in order to be compatible with humans.

The current issue is that pig DNA contains retroviral DNA, which can make humans sick. Retroviral DNA is an embedded strand of DNA from a virus that originated as a strand of mRNA, which was transcribed back to DNA. However, Church hypothesizes that CRISPR may target the retroviral DNA segments in the pig DNA and cut them, allowing the offspring of those pigs to have organs compatible with humans.

This hypothesis came to fruition just recently when Church confirmed the potential of CRISPR. He coded the pig genome and discovered only 62 segments of viral DNA that were all nearly identical, as they had been derived from a common ancestor. Church was able to use only one molecule of CRISPR to successfully remove all 62 viral DNA segments. Currently, Church is working with CRISPR to edit 25 genes that may decrease the risk of a patient rejecting the organ.

“This work brings us closer to a realization of a limitless supply of safe, dependable pig organs for transplants,” said Dr. David Dunn, transplantation expert at SUNY-Oswego.

Additionally, The Economist writes: “This is still a striking result. Not only does it demonstrate that it is possible to cleanse animal cells of unwanted viral passengers, thus helping remove one of the big barriers to cross-species organ transplants; it also shows the power of a genetic-engineering technique that has existed for only three years.”

Human progress may only be impeded by barriers that humans place upon themselves. With fetal embryo testing stringently regulated, one can only wonder what developments are yet to be made. Nevertheless, with twenty people dying daily in the United States from waiting for a transplant, the outlook on CRISPR still appears bright.