826
7. Nairns RG. Therapy of hyponatremia: does haste make waste? N Engl J Med 1986; 314: 1573-75. 8. Laureno R, Karp BI. Pontine and extrapontine myelinolysis following rapid correction of hyponatraemia. Lancet 1988; i: 1439-41. 9. Fraser CL, Arieff AI. Fatal central diabetes mellitus and insipidus resulting from untreated hyponatremia: a new syndrome. Ann Intern Med 1990; 112: 113-19. 10. Moran WH, Miltenberger FW, Schuayb WA, Zimmermann B. The relationship of antidiuretic hormone secretion to surgical stress. Surgery 1964; 56: 99-108. 11. Chung H-M, Kluge R, Schrier RW, Anderson RJ. Post-operative hyponatremia: a prospective study. Arch Intern Med 1986; 146: 333-36. 12. Parvez H, Parvez S, eds. Advances in experimental medicine: a centenary tribute to Claude Bernard. New York: Elsevier/North Holland Biomedical Press, 1980: 18. 13. Felig P. Disorders of carbohydrate metabolism: other types of experimental diabetes. In: Bondy PK, Rosenberg LE, eds. Metabolic control of disease. 8th ed. Philadelphia: WB Saunders, 1980: 310-11. 14. Niijima A. Nervous regulation of metabolism. Prog Neurobiol 1989; 33: 135-47. 15. Bereiter DA, Rohner-Jeanrenaud F, Berthoud H-R, Jeanrenaud B. CNS modulation of pancreatic endocrine function. Diabetologia 1981; 20: 417-25. 16. Sterns RH, Thomas DJ, Herndon RM. Brain dehydration and neurologic deterioration after rapid correction of hyponatremia. Kidney Int 1989; 35: 69-75.
MOLECULAR TARGETS FOR CANCER THERAPY
During the past decade there have been remarkable developments in the application of molecular biology to cancer. The recognition that oncogenes encode proteins that are intimately involved in growth control stimulated a search for rationally designed agents that block oncoprotein function.! More recently, the role of tumour suppressor genes or anti-oncogenes in the genesis of common tumours such as those of the lung, breast, and colon has become evident.2 Although we are only just beginning to understand the physiology of growth control at a cellular level, already there are hints that even this limited information may soon result in clinical gain. The most hopeful prospective targets are those in which there are qualitative differences between tumour cells and their normal counterparts. Thus the autocrine secretion of growth factors,3 altered forms of growth factor receptors,4 and mutant ras and p53 oncoproteinss,6 are all potentially amenable to inhibition by rationally designed drugs. Interactive computer graphics which enable the three-dimensional structures of such proteins to be readily visualised have considerably enhanced our understanding of the molecular interplay within cells.7 Ultimately cancer is a disorder of gene transcription-the wrong genes are switched on at the wrong time in the wrong place harnessing powerful physiological processes such as growth, cell division, and invasion to the benefit of the tumour but not the individual. Learning how to switch off specific genes holds considerable promise for future therapeutic development. In this issue (p 808) Dr McManaway and her colleagues illustrate this possibility. Most Burkitt’s lymphomas have chromosomal translocations involving the c-myc oncogene. This gene encodes a nuclear acting oncoprotein that has been implicated in cell cycle control and differentiation. In most patients with this lymphoma, c-myc, which is usually present on chromosome 8, finds its way into the heart of the immunoglobulin heavy chain locus on chromosome 14. This is not the whole story because the pathogenesis of Burkitt’s lymphoma involves a complex interplay between Epstein-Barr virus infection, malaria, and probably
malnutrition. In some patients the breakpoint on chromosome 8 occurs within the c-myc gene. The c-myc gene itself has three exons connected by two intervening sequences (introns). Normal transcription begins at a promoter in the first exon. The resultant RNA is processed to leave only the information contained in the second two exons for translation into protein by ribosomes. When the translocation breakpoint occurs within the first exon or intron the remaining part of the gene becomes detached from its normal promoter. Within the first intron there is a hidden promoter that can now take over-but the synthesis of messenger RNA will of course start further downstream. When this happens, the normal splicing machinery is unable to recognise the remaining part of the first intron, which is then left in the message. However, it is not converted into protein since the initiation site is in the second exon. The mRNA with its unique intron sequence is therefore a molecular flag with complete specificity for tumour.
To inhibit the function of tumour c-myc RNA, a 21-oligonucleotide sequence, chosen to bind to the transcribed part of the first intron, was synthesised. Such antisense oligonucleotides were shown to inhibit not only c-myc protein synthesis but also proliferation in cell lines derived from lymphomas bearing the relevant breakpoint. Normal lymphocytes or lymphoma lines without the appropriate translocation were not affected. There are now several examples in which the antisense technology has been applied to switch off genes involved in growth control including c-myc8 and fibroblast growth factor.9 Although growth inhibition was shown, it was not specific for tumour cells. Here an understanding of the detailed molecular genetics of Burkitt’s lymphoma has led to a novel strategy for selective inhibition. There are two immediate questions. How many similar targets are there in the spectrum of human cancer and can the technology be applied in vivo? As the molecular understanding of cancer becomes more detailed more targets are likely to emerge. Mutation, gene rearrangement, amplification, and enhanced transcription have all been observed in human tumours and correlated with tumour behaviour, and it seems likely that further genetic changes will be found in cancer cells. It remains to be seen whether the problems of getting antisense oligonucleotides into tumour cells in patients can be overcome by use of modified probes with novel delivery systems.
1. Gullick WJ, Sikora K. Oncogenes as clinical tools. Cancer Topics 1988; 6: 136-42. 2. Stanbridge EJ. Identifying tumor suppressor genes in human colorectal cancer. Science 1990; 247: 12-13. 3. Carney DN. Bombesin and its receptors. In: Waxman JH, Sikora K, eds. The molecular biology of cancer. Oxford: Blackwell, 1989: 54-64. 4. Stemberg MJE, Gullick WJ. New receptor dimerisation. Nature 1989; 339: 587. 5. Rodenhillis S, Wetering ML, Mooi WJ, Evers SG, Bos JL. Mutational activation on the K-ras oncogene. N Engl J Med 1987; 317: 929-35. 6. Iggo R, Gatter K, Bartek J, Lane D, Harris AL. Increased expression of mutant forms of p53 oncogene in primary lung cancer. Lancet 1990;
335: 675-79.
7. Stemberg MJE. Protein modelling using data bases European conference on biotechnology Verona: European Institute of Technology, 1988: 97-101. 8. Heikkila R, Schwab G, Wickstrom E, et al. A c-myc antisense oligodeoxynucleotide inhibits entry into S phase but not progress from G0 to G1. Nature 1987; 328: 445-49. 9. Becker P, Meier CB, Herlyn M. Proliferation of human malignant melanoma inhibited by antisense oligodeoxynucleotide targeted against basic fibroblast growth factor. EMBO J 1989; 8: 3685-91.
7. Nairns RG. Therapy of hyponatremia: does haste make waste? N Engl J Med 1986; 314: 1573-75. 8. Laureno R, Karp BI. Pontine and extrapontine myelinolysis following rapid correction of hyponatraemia. Lancet 1988; i: 1439-41. 9. Fraser CL, Arieff AI. Fatal central diabetes mellitus and insipidus resulting from untreated hyponatremia: a new syndrome. Ann Intern Med 1990; 112: 113-19. 10. Moran WH, Miltenberger FW, Schuayb WA, Zimmermann B. The relationship of antidiuretic hormone secretion to surgical stress. Surgery 1964; 56: 99-108. 11. Chung H-M, Kluge R, Schrier RW, Anderson RJ. Post-operative hyponatremia: a prospective study. Arch Intern Med 1986; 146: 333-36. 12. Parvez H, Parvez S, eds. Advances in experimental medicine: a centenary tribute to Claude Bernard. New York: Elsevier/North Holland Biomedical Press, 1980: 18. 13. Felig P. Disorders of carbohydrate metabolism: other types of experimental diabetes. In: Bondy PK, Rosenberg LE, eds. Metabolic control of disease. 8th ed. Philadelphia: WB Saunders, 1980: 310-11. 14. Niijima A. Nervous regulation of metabolism. Prog Neurobiol 1989; 33: 135-47. 15. Bereiter DA, Rohner-Jeanrenaud F, Berthoud H-R, Jeanrenaud B. CNS modulation of pancreatic endocrine function. Diabetologia 1981; 20: 417-25. 16. Sterns RH, Thomas DJ, Herndon RM. Brain dehydration and neurologic deterioration after rapid correction of hyponatremia. Kidney Int 1989; 35: 69-75.
MOLECULAR TARGETS FOR CANCER THERAPY
During the past decade there have been remarkable developments in the application of molecular biology to cancer. The recognition that oncogenes encode proteins that are intimately involved in growth control stimulated a search for rationally designed agents that block oncoprotein function.! More recently, the role of tumour suppressor genes or anti-oncogenes in the genesis of common tumours such as those of the lung, breast, and colon has become evident.2 Although we are only just beginning to understand the physiology of growth control at a cellular level, already there are hints that even this limited information may soon result in clinical gain. The most hopeful prospective targets are those in which there are qualitative differences between tumour cells and their normal counterparts. Thus the autocrine secretion of growth factors,3 altered forms of growth factor receptors,4 and mutant ras and p53 oncoproteinss,6 are all potentially amenable to inhibition by rationally designed drugs. Interactive computer graphics which enable the three-dimensional structures of such proteins to be readily visualised have considerably enhanced our understanding of the molecular interplay within cells.7 Ultimately cancer is a disorder of gene transcription-the wrong genes are switched on at the wrong time in the wrong place harnessing powerful physiological processes such as growth, cell division, and invasion to the benefit of the tumour but not the individual. Learning how to switch off specific genes holds considerable promise for future therapeutic development. In this issue (p 808) Dr McManaway and her colleagues illustrate this possibility. Most Burkitt’s lymphomas have chromosomal translocations involving the c-myc oncogene. This gene encodes a nuclear acting oncoprotein that has been implicated in cell cycle control and differentiation. In most patients with this lymphoma, c-myc, which is usually present on chromosome 8, finds its way into the heart of the immunoglobulin heavy chain locus on chromosome 14. This is not the whole story because the pathogenesis of Burkitt’s lymphoma involves a complex interplay between Epstein-Barr virus infection, malaria, and probably
malnutrition. In some patients the breakpoint on chromosome 8 occurs within the c-myc gene. The c-myc gene itself has three exons connected by two intervening sequences (introns). Normal transcription begins at a promoter in the first exon. The resultant RNA is processed to leave only the information contained in the second two exons for translation into protein by ribosomes. When the translocation breakpoint occurs within the first exon or intron the remaining part of the gene becomes detached from its normal promoter. Within the first intron there is a hidden promoter that can now take over-but the synthesis of messenger RNA will of course start further downstream. When this happens, the normal splicing machinery is unable to recognise the remaining part of the first intron, which is then left in the message. However, it is not converted into protein since the initiation site is in the second exon. The mRNA with its unique intron sequence is therefore a molecular flag with complete specificity for tumour.
To inhibit the function of tumour c-myc RNA, a 21-oligonucleotide sequence, chosen to bind to the transcribed part of the first intron, was synthesised. Such antisense oligonucleotides were shown to inhibit not only c-myc protein synthesis but also proliferation in cell lines derived from lymphomas bearing the relevant breakpoint. Normal lymphocytes or lymphoma lines without the appropriate translocation were not affected. There are now several examples in which the antisense technology has been applied to switch off genes involved in growth control including c-myc8 and fibroblast growth factor.9 Although growth inhibition was shown, it was not specific for tumour cells. Here an understanding of the detailed molecular genetics of Burkitt’s lymphoma has led to a novel strategy for selective inhibition. There are two immediate questions. How many similar targets are there in the spectrum of human cancer and can the technology be applied in vivo? As the molecular understanding of cancer becomes more detailed more targets are likely to emerge. Mutation, gene rearrangement, amplification, and enhanced transcription have all been observed in human tumours and correlated with tumour behaviour, and it seems likely that further genetic changes will be found in cancer cells. It remains to be seen whether the problems of getting antisense oligonucleotides into tumour cells in patients can be overcome by use of modified probes with novel delivery systems.
1. Gullick WJ, Sikora K. Oncogenes as clinical tools. Cancer Topics 1988; 6: 136-42. 2. Stanbridge EJ. Identifying tumor suppressor genes in human colorectal cancer. Science 1990; 247: 12-13. 3. Carney DN. Bombesin and its receptors. In: Waxman JH, Sikora K, eds. The molecular biology of cancer. Oxford: Blackwell, 1989: 54-64. 4. Stemberg MJE, Gullick WJ. New receptor dimerisation. Nature 1989; 339: 587. 5. Rodenhillis S, Wetering ML, Mooi WJ, Evers SG, Bos JL. Mutational activation on the K-ras oncogene. N Engl J Med 1987; 317: 929-35. 6. Iggo R, Gatter K, Bartek J, Lane D, Harris AL. Increased expression of mutant forms of p53 oncogene in primary lung cancer. Lancet 1990;
335: 675-79.
7. Stemberg MJE. Protein modelling using data bases European conference on biotechnology Verona: European Institute of Technology, 1988: 97-101. 8. Heikkila R, Schwab G, Wickstrom E, et al. A c-myc antisense oligodeoxynucleotide inhibits entry into S phase but not progress from G0 to G1. Nature 1987; 328: 445-49. 9. Becker P, Meier CB, Herlyn M. Proliferation of human malignant melanoma inhibited by antisense oligodeoxynucleotide targeted against basic fibroblast growth factor. EMBO J 1989; 8: 3685-91.
