For centuries, farmers and breeders used to imitate the process of natural selection through breeding plants and animals of different descents; seeking for producing better strains of higher profitability and resistance to natural disasters. However, classical breeding usually takes a considerable amount of time, mostly decades, to generate desirable strains.
Although advances in genetic engineering and molecular biosciences speeded up the process of classical breeding, the introduction of exogenous DNA material into plants and livestock raises a lot of concerns for regulating authorities. So far, genetically engineered animals have not been affirmed for human consumption due to global concerns about its impact on human health and ecological balance.
In comparison to conventional genetic engineering, genome-editing technologies don’t account for the stable introduction of exogenous DNA into the animals’ genome. For instance, TALEN technology and CRISPR/Cas9 mediated genome-editing technology depend on certain enzymes that cut the intrinsic DNA at specific desired sites without integrating any external piece of DNA into the original genetic makeup of the animal. Thus, recent genome-editing technologies avoid the previous concerns of conventional genetic engineering.
Using TALEN technology, a group of Chinese and South Korean scientists announced the creation of “double-muscled” pigs through editing a single gene known as myostatin (MSTN). In normal conditions, MSTN is known to control the size of muscle cells and inhibiting their abnormal growth. However, in some cattle, dogs and humans, MSTN gets mutated, giving rise to abnormal bulk of muscle fibers.
The scientists introduced a mutation into MSTN of pig fetal cells where TALEN complex cuts the original copy of MSTN on the genome and subsequently, the fetal cells try to repair the cuts by ligating the DNA strands again. However, the cellular repair mechanism won’t be able to efficiently restore the original MSTN copy leading to dysfunction of that particular gene.
“We could do this through breeding, but then it would take decades”, says Jin-soo Kim, a project leader and molecular biologist at Seoul National University. Kim collaborates with Xi-jun Yin, an animal-cloning researcher at Yanbian University in Yanji, China, who transplanted the double mutated fetal cells into pig egg cell.
Yin succeeded in creating 32 cloned double-muscles piglets. Nevertheless, he encountered some worrying problems. For instance, the piglets suffered from birthing difficulties due to their enormous size. Out of the 32 cloned piglets, only 13 survived till the age of 8 months and eventually two are still alive.
Although Kim and Yin didn’t publish their results yet, they are not considering using their cloned piglets as a source of meat. Instead, they are thinking to make use of the piglets’ sperm by selling it to farmers for breeding with normal pigs. Hence, according to simple Mendelian Genetics rules, most of the expected offspring will have only one mutated copy of MSTN that will result in healthier and bit less muscular pigs.
The introduction of TALEN or CRISPR/Cas9 genome-edited animals into the markets may ignite a lot of debate, especially when the process of editing doesn’t include of vertical DNA transfer from one organism to another. Thus, some countries, such as China, are investing a lot of money in genome-editing technologies in order to supply the exhaustive demand of meat and agricultural commodities. In this way, Kim thinks that “China will go first”.
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