Genetic Engineering

I

Introduction

Genetic Engineering, method of changing the inherited characteristics of an organism in a predetermined way by altering its genetic material. This is often done to enable micro-organisms, such as bacteria or viruses, to synthesize increased yields of compounds, to form entirely new compounds, or to adapt to different environments. Other uses of this technology, which is also called recombinant DNA technology, include gene therapy, which is the supply of a functional gene to a person with a genetic disorder or with other diseases such as acquired immune deficiency syndrome (AIDS) or cancer, and the cloning of whole organisms.

Genetic engineering involves the manipulation of deoxyribonucleic acid, or DNA. Important tools in this process are restriction endonucleases (so-called restriction enzymes) that are produced by various species of bacteria. Restriction enzymes can recognize a particular sequence of the chain of chemical units, called nucleotide bases, which make up the DNA molecule and cut the DNA at that location. Fragments of DNA generated in this way can be joined using other enzymes called ligases. Restriction enzymes and ligases therefore allow the specific cutting and reassembling of portions of DNA. Also important in the manipulation of DNA are so-called vectors, which are pieces of DNA that can self-replicate (produce copies of themselves) independently of the DNA in the host cell in which they are grown. Examples of vectors include plasmids, viruses, and artificial chromosomes. Vectors permit the generation of multiple copies of a particular piece of DNA, making this a useful method for generating sufficient quantities of material with which to work. The process of engineering a DNA fragment into a vector is called “molecular cloning”, because multiple copies of an identical molecule of DNA are produced. Another way of producing many identical copies of a particular (often short, for example, 100-3,000 base pairs) DNA fragment is the polymerase chain reaction. This method is rapid and avoids the need for cloning DNA into a vector.

II

Gene Therapy

Gene therapy involves supplying a functional gene to cells lacking that function, with the aim of correcting a genetic disorder or acquired disease. Gene therapy can be broadly divided into two categories. The first is alteration of germ cells, that is, sperm or eggs, which results in a permanent genetic change for the whole organism and subsequent generations. This “germ line gene therapy” is considered by many to be unethical in human beings. The second type of gene therapy, “somatic cell gene therapy”, is analogous to an organ transplant. In this case, one or more specific tissues are targeted by direct treatment or by removal of the tissue, addition of the therapeutic gene or genes in the laboratory, and return of the treated cells to the patient. Clinical trials of somatic cell gene therapy began in the late 1990s, mostly for the treatment of cancers and blood, liver, and lung disorders.

The history of human gene therapy is, however, not a particularly happy one. The effect of introducing a gene into cells rarely promotes more than small transient relief from the symptoms of the disease being treated. Worse still, there have been highly publicized cases where gene therapy trial patients have suffered as a consequence of the treatment itself. For example, in 1999 an 18-year-old gene therapy trial volunteer from Philadelphia died following a gene therapy trial. In addition, one of the few success stories of human gene therapy—the treatment of severe combined immune deficiency, X-SCID—has turned out to have unforeseen consequences. Bone marrow cells were taken from patients suffering from this disease and treated with a virus to introduce a functional copy of the defective gene. When the modified bone marrow cells were returned to patients, their immune systems were functional once more. However, some patients treated this way subsequently developed leukaemia, which most likely arises as a result of random insertion of a section of DNA into the human genome with the consequent disruption of nearby gene function.

III

Cloning Cells and Animals

In genetic engineering, the term “cloning” is now more commonly applied to the production of identical animals rather than molecular cloning of DNA fragments. Whole cell or animal cloning occurs through the transfer of the nucleus of an adult cell into an enucleated egg. This can result in the reprogramming of the adult cell DNA to produce a cloned animal. In 1997, a sheep named Dolly was born at the Roslin Institute in Edinburgh. She was created from the nucleus of a cultured mammary gland cell from a Finn Dorset sheep that was fused to an egg cell from a Scottish Blackface ewe that had had its own nucleus removed. The fused cell was implanted into a different Scottish Blackface ewe, and following a normal pregnancy, Dolly, a Finn Dorset sheep, was born. Nuclear transfer has subsequently been applied to produce a range of cloned animals including cows, goats, pigs, mice, and cats. The process is, however, not without its problems:

• Nuclear transfer is extremely inefficient. In the case of Dolly, only one of the 277 cell fusions produced was capable of developing into a lamb.
• Many of the offspring produced by nuclear transfer suffer gross abnormalities.
• Since Dolly was created from a cell that was potentially six-years old, what genetic age was she when she was born? Tests suggested that she had characteristics of an animal that was older than her chronological age. Indeed, Dolly developed arthritis at an early age and had to be put down following a lung infection in 2003 at the age of 6 years when the average life expectancy of a sheep of her breed is 12 years.

IV

Benefits

The process of genetic engineering has great potential. For example, the gene for insulin, normally found only in higher animals, can be introduced into a bacterial cell by way of a plasmid vector. The bacteria can then be grown in large quantities, giving an abundant source of so-called “recombinant” insulin at a relatively low cost. Production of recombinant insulin is also not dependent on the sometimes variable supply of pancreas tissue from animals. Another important use of genetic engineering is in the manufacture of recombinant factor VIII, the blood-clotting agent missing in patients with haemophilia. Virtually all haemophiliacs who received factor VIII before the mid-1980s have contracted AIDS or hepatitis from viral contaminants in the blood used to make the product. Since that time, donor blood has been screened for the presence of Human Immunodeficiency Virus (HIV) and hepatitis C virus, and the manufacturing process includes steps to inactivate these viruses if they should be present. The possibility of viral contamination is eliminated completely with the use of recombinant factor VIII. Other uses of genetic engineering include increasing the disease resistance of crops, producing pharmaceutical compounds in the milk of animals, generating vaccines, and altering livestock traits. Animal cloning can be used as a useful tool for the conservation of endangered species.