Journal of Znternal Medicine 1991; 230: 97-100 ADONIS 095468209100126M

Editorial Internal medicine and molecular biology-molecular medicine

Internal medicine is now reaping the benefit of our rapidly accumulating knowledge of the molecular siechanisms of disease. Major breakthroughs in biotechnology have taken place during the last few decades, with a consequent explosion of new in- formation. Cornerstones in this development are techniques such as DNA hybridization, the poly- merase chain reaction (PCR), and the production of monoclonal antibodies that have enabled the identification, isolation and characterization of bio- medically important genes and gene products. This new field will provide us with information about the functions of normal proteins, a prerequisite for the understanding of abnormalities and disease. It is an area of major concern for the clinician, because it may lead us through the ‘back door ’ toward a better understanding of the mechanisms and pathogenesis of disease. The new technologies have furnished us with the instruments that we need to study cell regulation in health and disease. The cellular re- sponse following the interaction between a cell and other cells or cell products can now be described in molecular detail, from the interaction between re- ceptor molecules and their ligands via the induction and transduction of signals by a chain of molecules, to changes in the behaviour of the cell, and the production of effector molecules. In addition, the basis of an increasing number of diseases can be described in terms of defects in genes and their products. This enhanced understanding of human disease and the new biotechnology provide both the clinician and the patient with hope for better diagnosis and treatment of disease. The description of disease in molecular terms has been labelled ‘molecular medicine’. The potential of this field is certainly enormous.

The multidisciplinary nature of molecular medi- cine constitutes an excellent basis for research within the broad field of internal medicine. By providing the same basic knowledge and a limited battery of technologies it has already had an enormous impact on clinical research within such widely divergent fields as cardiology, rheumatology, oncology and

infectious medicine. Molecular medicine also forms a bridge between basic sciences such as molecular biology, immunology and cell biology on the one hand, and clinical research on the other, thus reducing the distance between basic scientific de- velopment and clinical application and progress. Molecular medicine is becoming a dominant field of clinical research during the 1990s. The responsibility has been laid on clinical scientists to assimilate the rapid progress of basic sciences in order to ensure the development of molecular medicine at a reasonable pace. To the clinicians, the concepts and techniques of molecular biology may appear complicated and far removed from clinical reality. However, basic mol- ecular biology contains a logic and simplicity within its conceptual frames. A limited number of techniques are available, which have now been standardized and are suitable for application in diagnostics and clinical research.

Molecular biology originated when James Watson and Frances Crick identified the structure of the DNA molecule in 1953. The initial stages of the evolution of the science were mainly devoted to developing our understanding of the principles of DNA, RNA and protein synthesis in bacteria. The last 1 5 years have seen the development of new methodologies with the potential to identify and modify DNA sequences in the human genome. A defined gene DNA fragment or a complete gene may be introduced to bacterial or phage DNA (hybrid-DNA), where it can be amplified to allow sequencing. Such gene fragments can be synthezised and labelled with radioactive phosphate and used as probes to identify the genes within biological material. Messenger RNA (mRNA) may also be used as the starting material to produce a DNA copy (cDNA probe). By the introduction of altered (mutated) genes or gene fragments into cells or mouse eggs, the effect of mutagenesis on a protein function can be studied in vitro and in vivo (transgenic mice). A major step forward was made when the polymerase chain reaction (PCK) was described in 198 7. This ingenious technique, capable of detecting a single molecule of nucleic acid, has resulted in a

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revolution in biomedical research. Not only does the sensitivity of the technique allow detection of muta- tions and other genetic changes at the single cell level, but also the rapidity of the analysis has rendered this technique indispensable in studies of the mol- ecular genetics of human disease. The method has been applied, together with conventional hybrid DNA technologies, for the prenatal diagnosis of phenylketonuria, beta-thalassaemia and haemo- philia, and is now finding applications in the diag- nosis of cancer, infectious diseases, autoimmunity, atherosclerosis and inflammation in man. It has also become an important tool in forensic medicine, for the identification of traces of DNA in biological material.

Using the techniques of molecular genetics, the broad field of molecular medicine may be applied to the study of genetic diseases as well as to the study of diseases caused by non-genetic dysregulation of cell functions.

Molecular genetics informs us about various muta- tions and other defects of the genomic DNA that may be inherited, leading to lack of a protein or to the expression of abnormal proteins. If the abnormality is critical, disease may ensue, or even death. About 4200 genetic diseases are known, but the gene defects of only a few monogenic diseases have yet been identified. An example is cystic fibrosis, where the gene for synthesis of a membrane transport protein on chromosome 7 is defective. The most advanced area appears to be that of the monogenic diseases of low density lipoprotein (LDL) receptor expression, where defects have been localized at the levels of receptor structure, internalization and pro- cessing. This results in severe consequences in terms of hypercholesterolaemia. Other monogenic diseases are haemophilia, alpha-l-antitrypsin deficiency and phenylketonuria. The extensive studies of mutations of the LDL receptor and its clinical effects represent a most illuminating case. However, most human diseases, such as atherosclerosis, cancer, neurological diseases and immunological-inflammatory disorders are mainly multifactorial in their aetiology, and involve environmental factors acting in concert with a multigenetic background.

The accumulation of certain malignant diseases within families is well known. Molecular genetic studies have shown that retinoblastoma is related to the inactivation of a recessive gene on chromosome number 13. This gene is required for normal cell differentiation, and tumours develop when it ceases

to function. Retinobiastoma has served as a model for hereditary cancer. Our knowledge of the genetic factors underlying human cancers is now developing rapidly. Genetic factors have been identified in many adult tumours, such as mammary carcinoma, colon cancer, hypernephroma and multiple endocrine neo- plasia.

Diabetes of the non-insulin-dependent type (type II) may also belong to this group, the genetic defects possibly including products involved in the insulin production machinery or in the insulin effector steps with receptors and glucose transporters. Monogenic defects have been identified at several levels, but only in very rare cases and with questionable relevance to the typical diabetes patient. It is of interest that no defects at the gene level have yet been identified in Pima Indians, where the majority of the population becomes diabetic with increasing age. Other American Indians, as well as circumscribed popu- lations in the Pacific Islands, also exhibit an extremely high prevalence of diabetes. Such a parallel between ethnicity and the prevalence of diabetes is indicative of defects at the level of the gene.

A genetic background is likely for a number of other prevalent diseases in internal medicine. Recent studies have provided evidence for the involvement of genetic factors in salt-sensitive hypertension, obesity and abnormalities of body fat distribution.

Most genetic diseases lack effective therapies. With the development of the science of molecular biology, treatment by transfer of genetic material is moving from speculation to reality. In fact, the first clinical trial of gene therapy has been approved by the National Institutes of Health (NIH), and is in progress in patients with severe combined immunodeficiency due to adenosine deaminase deficiency. To date, only monogenetic diseases have been potential candidates for gene therapy. The healthy gene is introduced in vitro into the patient’s cells, which are then returned to the body. Other diseases that are candidates for similar treatment include thalassaemias, hae- mophilias, familial hypercholesterolaemia, cystic fibrosis, and Duchenne’s muscular dystrophy.

The ethical aspects of gene therapy are of prime importance, and manipulation of the genes of the gonads has not been contemplated. It is interesting to note that genetic engineering by the transfer of autologous cells to which genetic material has been introduced does not differ in principle from the introduction of new DNA into the body by bone marrow transplantation, and by transplantation of

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solid organs. In-vitro fertilization may also be regarded as an example of gene transfer.

It will be much easier to understand the patho- genetic mechanisms of polygenic disease when the reference frame for normality in genomic terms has become available. With the development of new techniques, the accumulation of nucleotide sequence data on the human genome in databanks is in- creasing. During the last 15 years the DNA data base has grown by a factor of ten every 5 years. The aim of the Human Genome Organization (HUGO) project was to sequence the entire human genome. This will further accelerate the accumulation of sequence data. It is expected that our knowledge of the human genome will be complete at around the turn of the century. This project will dramatically increase our understanding of a number of genetic diseases. both monogenetic and polygenetic, at the molecular level.

Molecular medicine also includes cell function in health and disease. The distance between basic scientific progress and clinical application is par- ticularly short in this field. The new biology has led to extraordinarily rapid development of our know- ledge of the biological effects of drugs and hormones, cytokines and other signal substances. The new biotechnology has enabled molecular mapping of the signal transduction chain from the triggering of the receptor to the effector phase, under both physio- logical and pathological conditions. This progress has widened the possibilities for more selective pharmaco- and biotherapy.

Cell receptor mechanisms have been an area of major concern since the early 1970s, when the steroid and nicotinic receptor systems were first described. The steroid receptors belong to a super- family of intracellular receptor proteins. Although the natural ligands and their detailed functions are not known, steroid receptors are now suitable for clinical studies, and are used for routine screening in connection with endocrine treatment of malig- nancies. It may be possible to tailor new drugs with such signal systems that are sufficiently well known. Receptor blockade of the P-adrenergic system has already demonstrated the potential success of this therapeutic principle. Other examples include ' anti- hormones' in the treatment of human cancer. The use of the interleukin-2-receptor as a target for toxin-conjugated monoclonal antibodies, or inter- leukin-2 in immunosuppression, illustrates another new therapeutic principle. The recent discovery of endothelin and its corresponding receptors opens

exciting possibilities for clinical progress in the treatment of hypertension and cardiovascular dis- ease.

In addition, elucidation of the molecular events following receptor activation which lead to the biological effect will provide clinically important information. For example, quantitative or qualitative changes in GTP-binding G-proteins have been described in various diseases, such as pseudo- hyperparathyroidism, diabetes and dilated cardio- myopathy.

An area of molecular medicine in which the distance to clinical applicability already appears to be very short is the study of growth factors. A number of different growth factor families have now been defined, and the mechanism of action is partly known for several of them. Cell proliferation appears to play an important role in the pathogenesis of a number of clinical entities, such as inflammatory diseases, fibrotic diseases and atherosclerosis, where smooth muscle proliferation represents a major part of the plaque formation. The role of growth factors in proliferative diseases, and the possibilities for treat- ment of disease by antibodies or antagonists to growth factors are potential goals for clinical re- search.

How is molecular medicine best introduced into internal medicine ? This question was discussed during a recent s~mposium in StockhoI~ on ' Molecular research in internal medicine during the 1990s', with the participation of scientists rep- resenting the fields of molecular biology, immuno- logy, pharmacology, clinical genetics and internal medicine. It was agreed that laboratory facilities and scientific competence would be optimal if molecular biologists, immunologists and cell biologists worked alongside clinicians in integrated units. Such an arrangement would allow the close co-operation and cross-fertilization of ideas necessary for the develop- ment of clinical science. Such units or centres would benefit from the participation of clinical scientists trained in pure science, as well as clinically trained pure scientists. The field of molecular medicine will continue to provide internal medicine with large, new areas for scientific exploration. It is therefore important that internists involved in the direction and leadership of research are informed about these new developments, and it should be ensured that internists start their training in this area, and that the necessary contacts with pure scientists are created. With this knowledge and organization,

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research projects will be more direct and penetrating, thus optimizing the potential for obtaining important clinical information.

Instrumental to this biotechnical revolution is the development of monoclonal antibodies and other immunological techniques to enable the identi- fication and purification of antigens. Together with the development of biochemistry and cell biology, we now have an instrumentarium which allows the description of disease and disease processes at the molecular and cellular levels, and consequently

enables these processes to be treated with more precise therapeutic measures.

PER BJORNTORP, J A N NILSSON & G O R A N HOLM

Department of Medicine I. Sahlgrenska Hospital, 9413 4 5 Goteborg, and Department oi Medicine,

Karolinsku Hospital, S-10401, Stockholm,

Sweden