From Mice to Men: ‘Knockout’ Rodents Five KO to Disease
One mouse is called Methuselah. Another is called Frantic.
A whiskered cousin of theirs goes by the less poetic name of p53.
These and thousands of other genetically altered – or “knockout” – mice play a critical role in health research today, earning the Nobel Medicine Prize this week for the trio of humans who invented them.
Researchers depend crucially on these unsung rodents to decipher disease, devise new treatments and explore the mysteries of the genetic code.
“Most of our profound understanding of how genes cause disease in humans has come by identifying particular genes in the mouse, knocking them out, and then looking at what disease develops in the mouse and how it develops,” said Steve Brown, head of the Mammalian Genetics Unit of Britain’s Medical Research Council (MRC).
“Without this toolkit, we would be considerably hampered,” said Brown, a specialist on the genetics of deafness, who described the technique as a “grand, groundbreaking achievement.”
The Nobel was awarded to Mario Capecchi and Oliver Smithies of the United States and Martin Evans of Britain.
In the 1980s, they devised a technique to remove a functioning gene from the mouse’s genetic code and replace it with a flawed counterpart to replicate human disease.
Knockout mice have been used as a model for various cancers, heart disease, diabetes, Parkinson’s, Alzheimer’s and scores of other conditions.
“Methuselah mice,” which enjoy extraordinary longevity for rodents, are helping to find the keys to ageing, while so-called “Frantic mice” are shedding light on the genetic roots of anxiety. Others carry the name of the disease-causing gene, such as the p53 cancer gene.
The targeting technique is so smart that engineers can choose to manipulate a gene that is a notorious culprit in one disease, or a selection of genes that play a supporting role in a harmful molecular cascade.
“You can knock out one gene and observe what happens, or introduce two or three copies of that gene, to get a more subtle picture of what occurs in the proteins controlled by the gene,” said Nadine Bouby-Bouzidi of France’s National Institute for Medical Research (Inserm), who worked on diabetes with Nobel laureate Oliver Smithies.
So far more than 10,000 mouse genes – roughly half of the genes in the mammalian genome – have been knocked out. “Ongoing international efforts will make ‘knockout mice’ for all genes available within the near future,” the Nobel jury said.
“We are now on the cusp of being able to knockout every gene in the mouse genome,” said Brown, who is part of an international consortium of European, US and Canadian scientists working towards this end.
Knockout mice, while extremely useful, have their limitations as a mirror of what happens among humans, which are more complex creatures.
“But in terms of their fundamental developmental processes – their biochemistry, their physiology – the two organisms are extraordinarily similar,” said Brown.
Disabling, or knocking out, a gene is a two-step process.
The first is to snip out a functioning gene from the animal’s genome, using chemical “scissors” such as an enzyme.
The next is to replace that gene with the modified one – the gene whose flaws will cause the disease to be studied.
The task is to coax this introduced piece of DNA into the right slot in the chromosome.
Little more than two decades ago, the prevailing wisdom that this challenge was impossible in mammalian cells and that the DNA would park itself in the chromosome almost randomly.
The breakthrough came when Evans isolated mice embryonic stem cells – the all-purpose master cells whose manipulation could create in theory any mutation of choice.
The contribution of Capecchi and Smithies was to target genes through “homologous recombination” – “homologous” means the introduced DNA sequence lines up with its mirror target sequence in the mouse chromosome, while “recombination” means the incoming and target sequences break and then rejoin.Filed under: Health