The deliberate modification of the genome is known as designed genomics. It involves targeted, specific modification of the genetic information of living organisms. The technologies that were earlier developed for the insertion of a gene into a living cell were found to have certain limitations. Apart from reproducibility, random nature of the insertion of the new sequence into the genome and blind positioning of the new gene that may disturb the functioning of other genes were a few limitations. They could also trigger the process of ‘cancerisation’. The major advantage of designed genomics is precision and reproducibility. Due to the recent developments in the area of genome engineering, it is now possible to enhance the de novo assemblies of DNA at a reduced cost. “These developments promise genetic engineering with unprecedented levels of design originality and offer new avenues to expand both our understanding of the biological world, and the diversity of applications for societal benefit”, write P A Carr and G M Church of MIT’s Media Lab.
Designed genomics, a very active field of research at present, involves introducing a gene into a chromosome to obtain a new function. The insertion of gene is also to compensate for a defective gene, particularly by making it possible to manufacture a functional protein if the protein produced by the patient is defective. Designed genomics also involves inactivation or ‘knock-out’. The purpose of inactivation or ‘knock-out’ is to know the function of a gene by observing the anomalies that occur as a result of its inactivation. Correction aims to remove and replace a defective gene sequence with a functional sequence. Another aspect of designed genomics is ‘correction’. It is intended to correct short sequences (sometimes just a few nucleotides), such as in the case of sickle cell anaemia.
Designed genomics, a technology based on gene targeting, has a wide range of possible applications, like the correction of a gene carrying a harmful mutation, the production of therapeutic proteins and the development of new generations of genetically modified plants. Gene targeting technology can bring precise changes into the protein coding part of a gene if one knows the mutation that causes the disease.
The use of gene targeting to evaluate the function of genes in the living mouse is now a routine procedure. A mouse could also be used to produce a human version of protein. We now know that more than 90 per cent of the genes have a function shared between a mouse and a man. More than 10,000 genes have been targeted in mice. More than 500 different models of human diseases, including models for hypertension, atherosclerosis, cancer, diabetes and cystic fibrosis have been produced by gene targeting. Gene targeting has also been used to understand the role of individual genes in embryonic development, adult physiology and aging.
Mario Capecchi of the University of Utah, Martin Evans of the Cardiff University and Oliver Smithies of the University of North Carolina at Chapel Hill have been jointly awarded the Nobel Prize in Physiology or Medicine for their discoveries of “principles for introducing specific gene modifications in mice by the use of embryonic stem cells.” Capecchi demonstrated that defective genes could be repaired by homologous recombination with the incoming DNA. Homologous recombination is a type of genetic recombination in which nucleotide sequences are exchanged between two similar or identical molecules of DNA. Capecchi discovered genes crucial for mammalian organ development and the body plan in general. Capecchi’s research has shed light on the causes of several birth defects. Evans discovered that chromosomally normal cell cultures could be established directly from early mouse embryos, referred to as embryonic stem (ES) cells, and developed mouse models for human diseases. Smithies discovered that all genes may be accessible to modification by homologous recombination. Smithies used gene targeting to develop mouse models for inherited diseases such as cystic fibrosis and the blood disease thalassemia. He also developed numerous mouse models for common human diseases such as hypertension and atherosclerosis.
Creating designed genomic modifications has three major advantages for introducing mutations into mice, writes Capechchi. The investigator can choose which genetic locus to mutate. The technique takes full advantage of all the resources provided by the known sequences of the mouse and human genomes. The investigator has complete control of how to modulate the chosen genetic locus. Gene targeting in mice has pervaded all fields of biomedicine, and as Capechchi concludes “The repertoire of biological phenomena that can be studied through the use of gene targeting is only limited by the imagination of the investigator.” zz
(The writer is a biotechnologist and ED, Birla Institute of Scientific