Genetics and Gene Therapy

Sheila A.M. McLean

• Publisher: Routledge
• Year: 2017
• Print ISBN: 9780754620556
• eBook ISBN: 9781351934275
• Topic: Medicine
• Subtopic: Genetics in Medicine

Genetics and Gene Therapy shows the wide range of the debate and the very real significance that genetics and its associated developments have for human beings, individually and collectively. Few areas of science and medicine have resulted in the volume of academic and popular literature as has genetics. The so-called revolution in understanding of the causes of disease states, and even behavioural traits, has focussed public attention on the influence of genes in making us what we are. Rapidly, however, the potential benefits of such understanding were overtaken, in the public mind at least, by the question of the possible (negative) implications of genetic knowledge and associated technologies. The chapters in this volume show just how wide-ranging concern has become, ranging from regulation to cloning, with the fear of discrimination in between. Part One begins with a range of general discussions of about the genetic enterprise itself, followed by consideration of some specific questions. Part Two then addresses cutting edge debates in genetics.

Part I
Genetics — General

[1]

Human Genetics: The New Panacea?

Julian Kinderlerer and Diane Longley*

Considerable advances have been made in human genetics in recent years, often outstripping the knowledge and understanding of the medical professions as well as the general public and taking regulators by surprise. In this paper, we seek to give a realistic indication of the many developments in human genetics and of what might or might not be scientifically possible in time, without the clutter and sensationalism of media hype. This, we feel, is an essential exercise to enable certain key elements and concerns to be taken on board, putting developments in context prior to any fresh consideration of the need for and potential effectiveness of regulation of human genetics.

The elucidation of the structure of DNA during the 1950s¹ provided a model for understanding the process for the transfer of genetic information between generations of the same organism. In bacteria and fungi identification of a variety of enzymes capable of modifying this group of large molecules has made possible the science termed ‘modem biotechnology’, and opened up our understanding of the mechanisms which lead from information molecule to function. Until the identification of these new enzymes scientists had used a variety of mutagenic devices (including, for example, ultra-violet light) to modify the genetic information in bacteria and fungi and observe the change in characteristics (or phenotype). From the late 1970s it became possible both to insert and remove genes in bacteria and to observe the consequences. An understanding of the control mechanisms followed rapidly. It became possible to sequence and extract genes from higher organisms and insert them into bacteria. Numerous quantities of the bacteria could then be grown, which meant that the amount of both the gene and the protein derived from that gene was relatively large, allowing analysis.

In 1990 a fateful decision was made. The entire human genome was to be sequenced. A ‘genome’ is the complete set of genes and chromosomes of an organism.² The intention was to construct a ‘high-resolution genetic, physical and transcript map’ of the human, with ultimately, a complete sequence. The Human Genome Project is the largest research project ever undertaken with the intention of analysing the structure of human DNA and determining the location of the estimated 100,000 human genes. According to Hieter and Boguski:

The information generated by the human genome project is expected to be the source book for biomedical science in the 21st century and will be of immense benefit to the field of medicine. It will help us to understand and eventually treat many of the more than 4000 genetic diseases that afflict mankind, as well as the many multi-factorial diseases in which genetic predisposition plays an important role.³

About 40,000–50,000 genes have been identified, although for a majority the function is still unknown.⁴ Whilst more than 95 per cent of the human genome remains to be sequenced, the acceleration in the process as new techniques are introduced means that the 15 year timescale originally envisaged for the completed project is likely to be met.⁵

It should be pointed out that a complete sequence does not provide information that allows an understanding of the mass of data. It can (to some extent) be compared to the possession of a very large encyclopaedia written in an unknown language. The complete sequence will not be ‘sufficient to understand its functional organisation, neither for individual units nor at a more integrated level’.⁶ The emphasis will quickly shift from the huge databases that store the recorded information to a functional analysis. It is assumed that there are about 100,000 genes with specific functions in the genome. The function of most of these is unknown. ‘In the past we have had functions in search of sequence. In the future, pathology and physiology will become “functionators” for the sequences’.⁷

DNA profiling and similar techniques show very clearly that (virtually) no two individuals share the same genome. There will be differences in many of the genes on their chromosomes. Whose genome is, therefore, being sequenced? Sequences are not being determined for an individual, but rather for the genetic information of a large range of persons. This has resulted in an appreciation of the ‘polymorphism’ in our genetic make-up. Many of the amino-acids found in the linear sequence of a protein cannot be changed, for the change is likely to have a deleterious impact on the function. As proteins are directly coded in the DNA, there must be a similar constraint on the DNA. Many of the proteins found in humans are also found in other organisms. Even though the function is the same or similar, their sequence differs significantly. Hence exact replication is unnecessary. The DNA sequence that makes up many genes will differ from organism to organism, and even from person to person.⁸

Duboule⁹ raises the question that lies at the heart of this article. How will it be possible to assimilate the mass of newly available information and translate it into clinical practice ‘in a way that fulfils scientific criteria and respects ethical as well as social concerns’?

Genetic and biological advances

For obvious reasons, development of modem biotechnology proceeded apace in bacteria, viruses and fungi much earlier than in higher organisms. Within bacteria, fungi and plants it is now possible to move almost any ‘gene’¹⁰, from any one organism to any other. The use of ‘gene’ here includes the coding sequence that provides the information necessary to produce the gene product (protein)¹¹ and any sequences that identify when, where (in which tissue, within the cell or in interstitial fluids) and how much of the product will be produced. Scientists are now able to choose a known gene product from virtually any source, identify its DNA sequence, manufacture it in the laboratory modifying the sequence so that it will be better expressed in the organism into which it is to be placed — attach to it the various signal sequences needed to identify:

• when in the life cycle of the cell or organism that product will be expressed;

• where in the cell or organism expression will occur; and

• how much will be produced; and

insert this new construct into an organism of choice. Clearly non-human biotechnology genetic modification knows few bounds. As a consequence, efforts have been made to establish international regulatory measures, it being considered inappropriate to leave regulation to the market and industrial initiatives in most countries. There are negotiations currently in progress to produce a protocol to the Convention on Biological Diversity¹² to ensure the safe transport of genetically modified organisms between countries, and the United Nations Environmental Programme (UNEP) has produced a set of guidelines which define minimum standards for the safe manufacture and use of modified organisms.¹³

The transformation of bacteria and viruses is now routine. Viruses carry a small number of genes, and for many viruses the function of most of these is known. Insertion of a gene into a specific position is easily accomplished, and in general, the effect is predictable. Live viruses are often used as vaccines — the pathogenic impact for human and animal viruses having been attenuated by a variety of techniques. However, in many cases, the mechanism of attenuation is still poorly understood, and the impact on the virulence of the organism due to an inserted gene has to be investigated before the modified virus may be used on human or animal tissue.

There remain many limitations to the use of genetic modification in higher organisms. The transformation of some plants remains difficult, and insertion of genes into large animals is problematic as the normally long generation time means that a very high yield of transformed animals is needed if the technique is to be used effectively. In general we cannot choose where the construct is inserted into the genome of the host plant or animal, even if the complete sequence of that genome is known. In many instances the insert may go into the middle of a vital gene, rendering the cell incapable of growing. Techniques presently available for insertion of genes into plant cells make it likely that a number of copies of the gene will be inserted randomly into the genome. For plants, traditional breeding practices (over a period of time) allow choice of a plant carrying both a small number of insertions and with as little loss of other characteristics as possible. As these separate ‘constructs’ are inserted into different places within the plant genome, selection over time means that they segregate, and it is possible to choose plants that demonstrate a particular, desirable characteristic.

The efficiency of transformation of plant cells is extremely low. Only a small proportion of the targeted cells is usually modified successfully depending on the technique.¹⁴ There are, however, a very large number of cells in the tissue targeted for transformation and in most plants it is possible to regenerate a complete plant from a single cell. By using markers such as antibiotic resistance, or herbicide resistance, it is possible to select those cells which have survived and are not susceptible to (say) the antibiotic — implying the resistance marker is present and is being expressed. Even where the conversion is only one in a million, selection systems are able to find the successfully transformed single cell and grow it into the entire organism, or grow numerous copies.

The insert used is usually a small piece of DNA. Even for a large protein, the number of bases¹⁵ in the ‘gene’ is likely to be less than 5000. In addition most of the DNA in a chromosome has no known function and is thought of as ‘junk’. Insertion of a gene into these regions is unlikely to have a significant impact on the characteristics of the organism.

Table 1 shows the size (in base pairs) of the genome or chromosome for a number of differen...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Table of Contents
• Acknowledgements
• Series Preface
• Introduction
• Part I Genetics - General
• Part II Gene Therapy/Testing/Cloning
• Name Index