CHAPTER 23 Treatment of Genetic Disease
Many genetic disorders are characterized by progressive disability or chronic ill health for which there is, at present, no effective treatment. Consequently, one of the most exciting aspects of the developments in biotechnology is the prospect of new treatments mediated through gene transfer, RNA modification, or stem cell therapy. It is important, however, to keep a perspective on the limitations of these approaches for the immediate future and to consider, in the first instance, conventional approaches to the treatment of genetic disease.
Conventional Approaches to Treatment of Genetic Disease
Most genetic disorders cannot be cured or even ameliorated using conventional methods of treatment. Sometimes this is because the underlying gene and gene product have not been identified so that there is little, if any, understanding of the basic metabolic or molecular defect. If, however, this is understood then dietary restriction, as in phenylketonuria (p. 167), or hormone replacement, as in congenital adrenal hyperplasia (p. 174), can be used very successfully in the treatment of the disorder. In a few disorders, such as homocystinuria (p. 172) and some of the organic acidurias (p. 183), supplementation with a vitamin or co-enzyme can increase the activity of the defective enzyme with beneficial effect (Table 23.1).
Table 23.1 Examples of Various Methods for Treating Genetic Disease
| Treatment | Disorder |
|---|---|
| Enzyme Induction by Drugs | |
| Phenobarbitone | Congenital non-hemolytic jaundice |
| Replacement of Deficient Enzyme/Protein | |
| Blood transfusion | SCID resulting from adenosine deaminase deficiency |
| Bone marrow transplantation | Mucopolysaccharidoses |
| Enzyme/Protein Preparations | |
| Trypsin | Trypsinogen deficiency |
| α1-Antitrypsin | α1-Antitrypsin deficiency |
| Cryoprecipitate/factor VIII | Hemophilia A |
| β-Glucosidase | Gaucher disease |
| α-Galactosidase | Fabry disease |
| Replacement of Deficient Vitamin or Coenzyme | |
| B6 | Homocystinuria |
| B12 | Methylmalonic acidemia |
| Biotin | Propionic acidemia |
| D | Vitamin D–resistant rickets |
| Replacement of Deficient Product | |
| Cortisone | Congenital adrenal hyperplasia |
| Thyroxine | Congenital hypothyroidism |
| Substrate Restriction in Diet | |
| Amino acids | |
| Phenylalanine | Phenylketonuria |
| Leucine, isoleucine, valine | Maple syrup urine disease |
| Carbohydrate | |
| Galactose | Galactosemia |
| Lipid | |
| Cholesterol | Familial hypercholesterolemia |
| Protein | Urea cycle disorders |
| Drug Therapy | |
| Aminocaproic acid | Angioneurotic edema |
| Dantrolene | Malignant hyperthermia |
| Cholestyramine | Familial hypercholesterolemia |
| Pancreatic enzymes | Cystic fibrosis |
| Penicillamine | Wilson disease, cystinuria |
| Drug/Dietary Avoidance | |
| Sulfonamides | G6PD deficiency |
| Barbiturates | Porphyria |
| Replacement of Diseased Tissue | |
| Kidney transplantation | Adult-onset polycystic kidney disease, Fabry disease |
| Bone marrow transplantation | X-linked SCID, Wiskott-Aldrich syndrome |
| Removal of Diseased Tissue | |
| Colectomy | Familial adenomatous polyposis |
| Splenectomy | Hereditary spherocytosis |
SCID, Severe combined immunodeficiency.
Protein/Enzyme Replacement
If a genetic disorder is found to be the result of a deficiency of or an abnormality in a specific enzyme or protein, treatment could, in theory, involve replacement of the deficient or defective enzyme or protein. An obviously successful example of this is the use of factor VIII concentrate in the treatment of hemophilia A (p. 309).
For most of the inborn errors of metabolism in which an enzyme deficiency has been identified, recombinant DNA techniques may be used to biosynthesize the missing or defective gene product; however, injection of the enzyme or protein may not be successful if the metabolic processes involved are carried out within cells and the protein or enzyme is not normally transported into the cell. Modifications in β-glucocerebrosidase as used in the treatment of Gaucher disease enable it to enter the lysosomes, resulting in an effective form of treatment (p. 178). Another example is the modification of adenosine deaminase (ADA) by an inert polymer, polyethylene glycol (PEG), to generate a replacement enzyme that is less immunogenic and has an extended half-life.
Drug Treatment
In some genetic disorders, drug therapy is possible; for example, statins can help to lower cholesterol levels in familial hypercholesterolemia (p. 175). Statins function indirectly through the low-density lipoprotein (LDL) receptor by inhibiting endogenous cholesterol biosynthesis at the rate-limiting step that is mediated by hydroxymethyl glutaryl co-enzyme A (HMG-CoA) reductase. This leads to upregulation of the LDL receptor and increased LDL clearance from plasma.
In others, avoidance of certain drugs or foods can prevent the manifestation of the disorder, for example sulfonamides in glucose-6-phosphate dehydrogenase (G6PD) deficiency (p. 187). Drug therapy might also be directed at a subset of patients according to their molecular defect. An example is a trial in which gentamicin was administered via nasal drops to patients with cystic fibrosis. Aminoglycoside antibiotics such as gentamicin or PTC124 cause read-through of premature stop codons in vitro and only patients with nonsense mutations (p. 25) showed evidence of expression of full-length cystic fibrosis transmembrane conductance regulator (CFTR) protein in the nasal epithelium. However, although gentamicin and PTC124 were effective in the mdx mouse model of Duchenne muscular dystrophy (DMD), a clinical trial with 174 patients failed to show convincing functional improvement after daily treatment for 48 weeks.
Therapeutic Applications of Recombinant DNA Technology
Biosynthesis of Gene Products
Insulin used in the treatment of diabetes mellitus was previously obtained from pig pancreases. This had to be purified for use very carefully, and even then it occasionally produced sensitivity reactions in patients. However, with recombinant DNA technology, microorganisms can be used to synthesize insulin from the human insulin gene. This is inserted, along with appropriate sequences to ensure efficient transcription and translation, into a recombinant DNA vector such as a plasmid and cloned in a microorganism such as Escherichia coli. In this way, large quantities of insulin can be made. An artificial gene that is not identical to the natural gene needs to be constructed for this purpose. However, synthetically produced genes cannot contain the non-coding intervening sequences, or introns (p. 17), found in the majority of structural genes in eukaryotic organisms, as microorganisms such as E. coli do not possess a means for splicing of the messenger RNA (mRNA) after transcription.
Recombinant DNA technology is being employed in the production of a number of other biosynthetic products (Table 23.2). The biosynthesis of medically important peptides in this way is usually more expensive than obtaining the product from conventional sources because of the research and development involved. For example, the cost of treating one patient with Gaucher disease can exceed £100,000 per year. However, biosynthetically derived products have the dual advantages of providing a pure product that is unlikely to induce a sensitivity reaction and one that is free of the risk of chemical or biological contamination. In the past, the use of growth hormone from human cadaver pituitaries has been associated with the transmission of Creutzfeldt-Jakob disease, and human immunodeficiency virus (HIV) has been a contaminant in cryoprecipitate containing factor VIII used in the treatment of hemophilia A (p. 309).
Table 23.2 Proteins Produced Biosynthetically Using Recombinant DNA Technology
| Protein | Disease |
|---|---|
| Insulin | Diabetes mellitus |
| Growth hormone | Short stature resulting from growth hormone deficiency |
| Factor VIII | Hemophilia A |
| Factor IX | Hemophilia B |
| Erythropoietin | Anemia |
| α-Galactosidase A | Fabry disease (X-linked lysosomal storage disorder) |
| β-Interferon | Multiple sclerosis |
Gene Therapy
Gene therapy has been defined by the UK Gene Therapy Advisory Committee (GTAC) as ‘the deliberate introduction of genetic material into human somatic cells for therapeutic, prophylactic, or diagnostic purposes’. It includes techniques for delivering synthetic or recombinant nucleic acids into humans; genetically modified biological vectors (such as viruses or plasmids), genetically modified stem cells, oncolytic viruses, nucleic acids associated with delivery vehicles, naked nucleic acids, antisense techniques (e.g., gene silencing, gene correction or gene modification), genetic vaccines, DNA or RNA technologies such as RNA interference, and xenotransplantation of animal cells (but not solid organs).
Regulatory Requirements
There has been much publicity about the potential uses and abuses of gene therapy. Regulatory bodies have been established in several countries to oversee the technical, therapeutic, and safety aspects of gene therapy programs (p. 368). There is universal agreement that germline gene therapy, in which genetic changes could be distributed to both somatic and germ cells, and thereby be transmitted to future generations, is morally and ethically unacceptable. Therefore all programs are focusing only on somatic cell gene therapy, in which the alteration in genetic information is targeted to specific cells, tissues or organs in which the disorder is manifest.
More than 1500 clinical trials of gene therapy have been approved for children and adults for a variety of genetic and non-genetic disorders. For the most part, these appear to be proceeding without event, although the unexpected death of a patient in one trial in 1999 and the development of leukemia in 3 of 20 children who received gene therapy for X-linked severe combined immunodeficiency (XL-SCID) (p. 201) has highlighted the risks of gene therapy.
