Heart2heart with pigs and baboons: What’s next?

While the whole world has its attention on the CRISPR babies, let’s shine the spotlight on the humble pig to baboon heart transplant!

In November 2018, the whole world was shaken by the news from China: “World’s first gene-edited babies created in China, claims scientist”. What was once thought to be science fiction became a reality. Now anybody could make designer babies just as they fancy, using this amazing technology called CRISPR-cas9 genome editing tool? No! Whether the claims by the Chinese scientist were true or not, this news definitely brought to light the need for better regulations in the scientific community. The topic was highly debated on and discussed until the end of the year and perhaps even now (I stopped following the topic because it was turning from science news into celebrity drama). But amongst all this excitement, another leap in medical science might have gone unnoticed — the cross-species organ transplantation, also called as xenotransplantation. This medical advancement could become a reality within a few years, according to scientists. If you are repulsed by the thought of having an animal organ, let alone a pig’s organ inside your body, then let me tell you the science behind it. And you will be surprised that CRISPR-cas9 can also be used for medical miracles other than creating designer human babies.

Before we talk about the science, let’s see some statistics.

• More than 114,000 people are on the waiting list per year for organs in the US as of April 2018
• In Croatia and Spain, the rate of organ donation is about 36 per million people.
• In China, the rate of organ donation is only 0.6 per million and in India 0.5 per million.

While in smaller countries the numbers for organ donation don’t look so bad, countries with larger populations like China and India have a relatively low rate of organ donations. Which means the amount of people on their waiting lists are probably quite high. These are but a few numbers. Yet it gives us an idea of the high demand for organs worldwide. To meet such high demands, some even resort to illegal trading of organs.

Other issues in the field of organ transplantation are the rejection of organs by the recipient’s immune system, sub-optimal quality of available organs and the inadequate methods of storage. [Note: Organs are not viable long term. They have a short shelf life and can be stored on ice only upto a few hours. Freezing to increase shelf life is also not an option because freezing causes ice crystals to form in cells and ultimately damage the cells.]

But what if we could solve all these problems by other means?

This is where science comes to the rescue. Nature News published an article on an exciting achievement by scientists in the medical field — successful transplantation of a genetically modified pig’s heart into a baboon and the baboon’s survival for more than 6 months. Non-human primates such as the baboon are our closest evolutionary cousins. And yet they are poor donors for a number of reasons. Firstly, there is the risk of transferring infectious diseases to humans when a primate organ is transplanted. Even if this difficulty could be tackled using modern technology, there is a second limitation. The primates cannot be easily grown in masses. The reason being that primates have similar reproductive cycles as humans. Meaning they have long gestation periods of about 8 months and give birth to only one offspring at a time. Ergo, the rising demand for organs will not be met by depending on our evolutionary cousins. So the next best substitute as donors are the pigs? Apparently yes.

Pigs have many similarities with humans such as body size or anatomical features like organs. Besides their similarities with humans, they can be produced in masses, produce large litters and have a relatively short development phase. They have been used as a model for genetic studies and biomedical research including organ transplantation for many years. However, using pigs as organ donors for humans have their own caveats. Pigs naturally carry viruses called porcine endogenous retroviruses (or often known by their infamous name PERVs). The viral genetic material in the pig’s cells could potentially integrate into human genome and become activated, thereby causing infection. Thus, before the pig’s organs could be used in humans, the pig’s genome needs to be modified so as to make it similar to ours. Moreover, pig’s genomes are easily edited using genetic tools such as the CRISPR-Cas9, which can disable the viral genetic material. So the scientists decided to put this often misconstrued animal to a better use. Scientists from Germany, Sweden, and Switzerland developed a modified protocol of xenotransplantation and have successfully broken through these barriers, with one baboon surviving for more than six months.

So what is the CRISPR-cas9 tool and how did the scientists use it in the xenotransplantation process? CRISPR-cas9 is shorthand for “clustered regularly interspaced short palindromic repeats and CRISPR-associated protein 9”. It was originally adapted from naturally occurring genome editing system in bacteria. When viruses invade bacteria, the bacteria captures short pieces of viral DNA and creates DNA segments known as CRISPR arrays, so as to “remember” the viruses. When the same virus or a similar one attacks again, the bacteria creates RNA segments from the templates encoded in the arrays and those RNA segments then target the virus’s DNA. The bacteria then use cutting enzymes like Cas9 or others to break down the viral DNA and disable the virus. [Note: DNA and RNA are molecules carrying genetic instructions for various functions involved in our growth and development.]

In the lab, this editing system is slightly modified. Here, viral DNA does not exist as the starting material. Hence using the cell’s own DNA sequence, scientists create short RNA “guide” sequences that bind to the target region of the DNA. The RNA also binds to the Cas9 enzyme and sort of “guides” the enzyme to cut at the targeted region. The scientists then either allow the cell to repair the DNA on its own or replace the cut targeted region with a customized DNA sequence.

The scientists used this very same technology with the pig’s heart. They employed the CRISPR-cas9 technology to deactivate PERVs in the pig’s genome, thereby generating a “human-like” heart. In addition to the genetic modification, a set of other amended procedures were also applied. When the pig’s heart is transplanted into a baboon, there is some amount of cell growth which produces tumors. A modified cocktail of immunosuppressants and other medications to inhibit cell proliferation were given to the baboons. The storage method of the organ was improved by constantly supplying oxygenated blood-based solution. With this improved transplantation protocol, scientists were able to successfully transplant the pig’s heart into baboons and even keep one of them alive for six months. Of course, futuristic approaches such as these come with their own limitations. The major ones are ethical concerns regarding the use of pig’s heart over the modern mechanical support devices that improve blood-circulation. Thus, the difficulty in applying this approach in clinical trials now seems to be rather practical than technical.

But despite some inhibitions concerning the use of CRISPR-cas9 after the CRISPR baby fiasco, the potential of this technology is irrefutable. And if we are successful in adapting xenotransplantation in humans, it would be a truly remarkable feat.