2007 Nobel Prize in Physiology or Medicine: Capecchi, Evans, and Smithies Usher in the Era of Gene-Edited Mice

What You Will Learn in This Article

The 2007 Nobel Prize in Physiology or Medicine was awarded to three individuals who developed a method for precisely targeting and editing specific genes in living mammalian organisms – the knockout mouse technology. They are Mario Capecchi of the University of Utah, Martin Evans of Cardiff University, and Oliver Smithies of the University of North Carolina at Chapel Hill. Capecchi and Smithies independently demonstrated that they could use homologous recombination to edit specific genes in mammalian cells at precise locations, while Evans was the first to establish mouse embryonic stem (ES) cells, which could be manipulated in this way. The combination of these three discoveries led to the birth of the first knockout mouse in 1989, and in the following three decades, tens of thousands of gene-specific knockout mice have been created, becoming a standard tool for studying human diseases.

Something Different from Common Knowledge: We Can Now Edit the Source Code of Living Mammals

Until the mid-20th century, researchers had limited options when trying to understand the function of a specific gene. They could wait for naturally occurring mutations, induce random mutations using chemicals and then select for the desired phenotype, or express or suppress genes in cell cultures. Precisely editing a specific gene in a living mammal was long considered a dream.

If we frame this in terms of computer science, the magnitude of this dream becomes clearer. The genomes of humans and mice are source codes containing approximately 3 billion base pairs. Within this, finding a specific function—a particular gene—and precisely deleting or replacing it, and ensuring that the edited code runs reliably in the entire living organism. From the perspective of a text editor, it seems like a simple find-and-replace operation, but precisely editing a specific location among 3 billion characters in a living cell presents a completely different challenge.

Capecchi, Evans, and Smithies solved this challenge by dividing it into two steps. First, they utilized homologous recombination, a natural cellular process used to repair DNA, to precisely replace the gene at the desired location with the desired sequence. This is the editing operation itself. Second, they used embryonic stem cells, which are ancestral cells capable of propagating the modifications to all cells in the body. The editing, which starts with a single cell, is then propagated to all tissues of a living mouse and passed on to subsequent generations. The combination of these two steps made it possible to edit the source code of living mammals for the first time.

The Landscape of the Time: The Year the iPhone Appeared and Subprime Collapsed

The world in 2007 was caught between two major events. On January 9, Steve Jobs unveiled the first iPhone at the Macworld Conference, and it was officially released on June 29. This was the introduction of the concepts of touchscreens and smartphones to the general public, and it marked a turning point that defined the grammar of the future mobile computing era. In November, Amazon released the Kindle, launching the e-book market. It was the dawn of a new era of consumer interface innovation.

At the same time, the subprime mortgage crisis began to surface in the summer. Several large hedge funds showed signs of collapse, and Bear Stearns began to falter. The 2008 financial crisis was foreshadowed in that year, and the global economy was beginning to feel the tremors of the coming storm. The commodity supercycle reached its peak, and oil prices approached $100 per barrel.

South Korea also experienced a tumultuous year. From October 2nd to 4th, President Roh Moo-hyun visited Pyongyang for the second inter-Korean summit with Kim Jong-il, the Supreme Leader of North Korea. The October 4th Joint Declaration was announced, outlining a broad framework for inter-Korean economic cooperation, although most of it was put on hold after the change in government. On December 19th, Lee Myung-bak was elected president. It marked the beginning of a pragmatic government, and the Grand Canal project, deregulation, and market-friendly policies emerged as new priorities.

On December 7th, the oil tanker Hebei Spirit collided with a crane barge off the coast of Taean, South Korea, causing a massive oil spill. It was recorded as the worst marine pollution incident in the West Sea, and for several months, hundreds of thousands of volunteers were involved in removing the black oil from the rocks and sand. The incident reaffirmed the Korean characteristic of citizen mobilization in the face of disaster.

In the scientific community, knockout mouse technology had already become a standard tool by the early 2000s, but the Nobel Prize was awarded to the three individuals nearly 20 years after the first knockout mouse was created in 1989. This was due to the sheer number of disease models and discoveries that had been made using this technology, and it took some time for the Nobel Committee to decide which individuals to award the prize to and when.

Biographical Narratives: From a Child on the Streets of Italy to a Laboratory in Utah

Mario Capecchi (1937–) has one of the most dramatic stories in the history of 20th-century science. He was born in Verona, Italy, and his mother, Lucy Ramberg Capecchi, was an American poet. His mother was deported to the Dachau concentration camp during World War II for writing anti-Nazi poems, and young Capecchi survived for four years on the streets of Italy from the age of five. He did not attend school and often went without bathing. After the war, his mother found him, and the two immigrated to Philadelphia, USA.

The child of the streets showed remarkable talent when he entered school and went on to attend Antioch College and Harvard University (in James Watson's laboratory) before becoming a professor at the University of Utah. His ambitious question was: "Is it possible to precisely edit only specific genes in mammalian cells?" In the late 1970s, the standard observation was that when foreign DNA was introduced into cells, it was inserted at random locations in the genome. Capecchi challenged this. "Cells use homologous sequences as a reference when repairing their own DNA. If we attach a sequence homologous to a specific gene in the cell to foreign DNA, the cell will use this as a reference and insert it at the precise location."

The NIH was skeptical of this idea and rejected Capecchi's research grant application. Capecchi secured other funding and continued his experiments, and in 1988, his team reported the successful targeting of genes by homologous recombination in mammalian cells. In a later account, the NIH review committee admitted that they were wrong and sent him a letter acknowledging his success. This anecdote is remembered as a moment that demonstrates the humility of the scientific funding system.

Oliver Smithies (1925–2017) was born in Yorkshire, England. He received his bachelor's and doctoral degrees from Oxford and then worked at the University of Wisconsin before settling at the University of North Carolina at Chapel Hill. Smithies was initially known for his research on the structure and genetics of insulin, and he also developed the standard method for starch gel electrophoresis. In his 60s, he turned to a new problem: gene targeting by homologous recombination. Almost simultaneously with Capecchi, but completely independently, Smithies also demonstrated the principle of this technology in mammalian cells. The two papers appeared in scientific journals around the same time, and the Nobel Committee recognized both of these independent discoveries.

Martin Evans (1941–) was born in Stroud, England. He received his Ph.D. from Cambridge and settled at Cardiff University. His question was on a different track: "How can the edited cells be propagated to the entire living organism?" The answer was embryonic stem cells (ES cells). These cells, derived from the inner cell mass of the early embryo, can differentiate into all cells of the body, can be cultured indefinitely in vitro, and can be introduced back into the embryo after being edited, allowing them to be incorporated into all tissues of the adult mouse. In 1981, Evans and his student Matthew Kaufmann first established a method for culturing mouse ES cells, and this cell line became the key tool for knockout technology.

The combination of the achievements of these three individuals led to the birth of the first knockout mouse in Capecchi's laboratory in 1989. Since then, one human disease-related gene after another has been manipulated in mice, creating tens of thousands of mouse models for Parkinson's disease, Alzheimer's disease, cardiovascular disease, cancer, autoimmune diseases, and developmental disorders.

Key Achievements: The Gene Editing Deployment Pipeline as Seen Through the Lens of Computer Science

The process of creating a knockout mouse can be viewed as a pipeline:

• Source Preparation (Targeting vector): A DNA fragment is created by sequentially linking the 5' homologous sequence of the target gene, the editing insert (e.g., a resistance gene or a gene-disrupting sequence), and the 3' homologous sequence. This serves as the editing instruction.
• Isolated Editing (ES cell targeting): This vector is introduced into mouse ES cells. In most cells, the vector is inserted at a random location in the genome, but in some cells, homologous recombination occurs, and the cell uses the homologous sequence in the vector to perform the editing at the precise location. Cells that have successfully edited are selected using a resistance gene.
• Verification: The editing location of the remaining cells is verified by PCR and Southern blotting. It is confirmed that the desired location has been precisely edited and that it has not been inserted elsewhere. This is the assertion step for successful compilation.
• Deployment (Chimera formation): The edited ES cells are injected into the early embryo (blastocyst) of another mouse and implanted into the uterus of a surrogate mother. The resulting mouse is a chimera, with some tissues derived from the edited ES cells and other tissues derived from the original embryo.
• Inheritance (Germline transmission): If the editing has been reflected in the germ cells of the chimera, some of its offspring will inherit the edited gene in a heterozygous state. By mating these offspring, homozygous knockout mice are produced. This organism is the final production deployment that we desired.

The nature of this pipeline is compile-time source editing → sandbox testing → controlled deployment → germline propagation. Homologous recombination, a natural cellular process used to repair DNA, is used to ensure that the editing instructions are applied precisely to a specific location, and ES cells, which are ancestral cells, are used to ensure that the edited source can be propagated to all cells in the living organism.

Cre-loxP conditional knockout is an extension of this architecture. Instead of deleting the entire gene, short sequences called loxP are inserted at both ends of the gene, and when the Cre recombinase is expressed at the desired time, the gene between the two loxP sites is deleted. This allows for the deletion of genes only in specific tissues or at specific times, acting as a feature flag. It is critical for dealing with genes that are essential for embryonic development but can be eliminated in adults. With the establishment of this technology, the scope of application for knockout mice has expanded dramatically.

However, the limitations of this analogy should also be acknowledged. Knockout mice are not perfect models of human diseases. Mice and humans are species that diverged 65 million years ago, and the phenotype that results from deleting a specific gene can be quite different in the two species. A knockout that causes severe disease in mice may only cause mild symptoms in humans, and vice versa. Therefore, the results obtained from knockout mice are reference materials for understanding the human condition, but they are not answers that can be directly applied to clinical practice.

Why It Matters: Broadening the Horizon of Understanding Human Diseases

First, it became the standard model for human disease. Over the past 30 years, tens of thousands of knockout mice have been created, and the International Knockout Mouse Consortium (IKMC) is conducting a project to secure knockout mice for each gene in the mouse genome. The goal of this project is to observe the function of each gene in a living organism, which is an ambitious attempt to experimentally complete the gene catalog itself.

Second, it has become an essential tool in the drug development pipeline. The first experimental gateway to confirm whether a newly discovered potential drug target actually causes disease is the knockout mouse of that target. If removing the target does not induce the disease or, conversely, unexpected side effects appear, the drug development strategy for that target needs to be reconsidered. Most of the new drugs developed in the 21st century, including palbociclib (2001, CDK story), Helicobacter pylori treatment, and patisiran (2006, RNAi story), underwent knockout mouse validation in the early stages of development.

Third, it completed the map of developmental biology. The most reliable way to answer the question of which genes are expressed when and where, and which are essential for the development of specific tissues, is conditional knockout. The developmental map of the heart, brain, kidneys, and immune system has been meticulously drawn over the past 30 years, providing a theoretical basis for regenerative medicine and cell therapy. The 2012 Nobel Prize (Yamanaka and Gurdon, iPS cells) is understood within this map, and today's organoid research is also conducted on it.

Fourth, it became the conceptual ancestor of gene editing technology. The CRISPR-Cas9 revolution of the 2010s is based on the principle of homologous recombination with a much more efficient cutting tool. When Cas9 cuts DNA at a specific location, the cell activates homologous recombination to repair the damage. The homologous recombination protocol created by Capecchi and Smith is reused here. Even in the CRISPR era, the conceptual roots of knockout mice remain in the 2007 discovery of these three individuals.

Fifth, it is a human story of science. The fact that Capecchi, who spent his childhood on the streets of Italy, created knockout mice in a laboratory in Utah and received the Nobel Prize is one of the most moving narratives in the history of science in the late 20th century. His reflection on how the four years from the age of five, when he was separated from his mother, to the age of nine, when they reunited, left a mark on his life and how he became a pioneer of genetic engineering, which requires extreme precision, is a serious reflection on how science takes root in human life.

The moment it became possible to edit the source code of living mammals using knockout mice, human disease research shifted from the era of observation to the era of manipulation. The sixth Nobel Prize in Physiology or Medicine of the century was given to the three pioneers of this transformation, and the stories that followed—HPV, telomeres, IVF, iPS—all unfold on this foundation.

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