- Email: [email protected]
Screening for cancer genes
EDITORIAL Screening for Cancer Genes
W h e n the genetic screening of newborns for phenylketonuria (PKU) was getting u n d e r w a y some years ago, we asked Dick Hoefnagel what he thought about it. Hoefnagel, a veteran neurologist and clinical geneticist, replied in essence: " W h a t we really need to do n o w is to screen for diseases like neurofibromatosis and myotonic d y s t r o p h y and provide genetic counseling for these diseases." Hoefnagel's point was that these are autosomal dominant disorders. They are relatively c o m m o n ones as genetic diseases go. They are diseases that can have grave medical consequences, i n c l u d i n g mental retardation, and since they describe a vertical pattern of inheritance, can put a considerable cohort at risk. Finally, Hoefnagel argued that this type of clinical genetic screening could be done easily and inexpensively. Needless to say, not m a n y geneticists bought this strategy. Few went on to find the cases of autosomal d o m i n a n t diseases, locate their kin, and examine them in an aggressive way. We geneticists found it s i m p l e r to go the neonatal screening route in search of the one in 10,000 baby with PKU. A m o n g the advantages of n e w b o r n metabolic screening was that it could be m a n d a t e d by law and thus avoid the delicate matter of informed consent. Another obvious advantage of neonatal screening was the detection of diseases that could, we believed at the time, be completely treatable. A big advantage, too, was that the time-consuming and onerous task of screening could be relegated to h u m d r u m state laboratories, leaving the biochemical geneticists in medical school meccas to skim off the cream of the unusual cases for research. Cancer genetics is today at a comparable crossroads, a fork in the road between screening for cancer genes through clincial means or through laboratory assays. Belatedly we can begin to scour about in clinical search of i n d i v i d u a l s with elevated cancer risks or we can launch into development of mass cancer screening laboratories. Let's illustrate these alternative avenues. Consider two autosomal d o m i n a n t disorders k n o w n to cause cancer, n a m e l y the dysplastic nevus s y n d r o m e associated with cutaneous malignant m e l a n o m a and the hereditary adenomatous polyposis of the colon closely connected to carcinoma of the large bowel. We have talked with dermatologists and gastroenterologists, respectively, who care for patients with these conditions. We have asked these physicians whether they perform routine family studies. The answer is usually a startled No. The strategy of a cancer genetics screening laboratory can also be illustrated with two examples, n a m e l y the p53 gene and the ataxia telangiectasia (AT) gene. Heritable mutations at the p53 locus on the short arm of chromosome 188 Cancer Genet Cytogenet 60:188-189 (1992) 0165-4608/92/$05.00
17 appear to correspond with the Li-Fraumeni s y n d r o m e (and with a number of hepatic tumors associated with aflatoxin exposure as well). The Li-Fraumeni syndrome, an autosomal d o m i n a n t disorder, is clearly associated with an extraordinary assortment of malignancies. Even though the s y n d r o m e was originally ascertained by soft tissue carcinomas in children, it was certain from the start that other neoplasms, i n c l u d i n g breast cancer in adults, were part and parcel of the LiFraumeni s y n d r o m e [1, 2]. Mutant p53 alleles [3] are n o w suspected to be the genetic culprit. W h y not perform laboratory screening for p53 mutants? It could be done first w i t h i n Li-Fraumeni families and then be e x t e n d e d to the general population. Ataxia telangiectasia is an autosomal recessive disorder with the gene locus on the long arm of c h r o m o s o m e 11. Homozygotes for AT are particularly p r e d i s p o s e d to l y m p h o i d malignancies. However, AT homozygotes are rare; their frequency in the p o p u l a t i o n is p e r h a p s 1 in 20,000 persons. The finding by Swift et al. [4-7] that AT heterozygous w o m e n are at a sixfold-normal risk of breast cancer was surprising. Ataxia telangiectasia h o m o z y g o u s females rarely if ever seem to develop breast cancer. Of course this may reflect the fact that m a n y AT females die in c h i l d h o o d and adolescence of other conditions before the diagnosis and the risk of breast cancer in patients w i t h AT can be identified. The AT heterozygous state is very common. Assuming that AT homozygotes have a frequency of one in 20,000, the Hardy-Weinberg formula indicates that AT heterozygotes are 283-fold more prevalent and involve one in every 71 persons in the population. Thus, the AT gene m a y actually predispose to a significant p r o p o r t i o n of breast cancer cases in the population. Routine detection of the AT gene in the laboratory has posed a pesky problem. As Lavin et al. of Brisbane point out [8], n u m e r o u s assays have been d e v e l o p e d to test for AT. They report a new assay based on the a n o m a l o u s l y long delay of AT heterozygous cells in G2 after irradiation. Using the assay, the Brisbane workers detected all six obligate AT heterozygotes and d i s t i n g u i s h e d them from all but one of 22 controls. (Is that one control an AT heterozygote?) The Brisbane assay appears to satisfy the requirements for screening, with no false negatives and few false positives, although this must be confirmed w i t h a larger sample. The Brisbane assay, according to the investigators, is "relatively simple and reliable and can be carried out in any laboratory with access to Epstein-Barr virus for transformation of l y m p h o c y t e s and a fluorescence activated cell analyzer." In addition, the laboratory must have a source to © 1992 Elsevier Science Publishing Co., Inc. 655 Avenue of the Americas, New York. NY 10010
F. Hecht a n d B. K. Hecht
189
irradiate the lymphoblastoid cells. However, it seems unlikely that a mass screening test can practicably be performed using Epstein-barr v i r u s - t r a n s f o r m e d cells that must then be irradiated and r u n through a fluorescence activated cell sorter. At this time, the excellent report by Lavin et al. [8] is primarily of research interest. A simple mass screening assay for the AT gene appears to await identification and characterization of the gene itself. Are we prepared to perform mass screening in the laboratory for any genes that predispose to cancer? No. We are not yet ready. The only effective way to initiate cancer genetic screening at present is to identify index cases and then perform genetic family studies. Except that this is time-consuming work and r e i m b u r s e m e n t for it may be hard to come by, it makes sense to l a u n c h a major effort to screen clinically for cancer genes. The laboratory assays can be added as we go along. FREDERICK HECHT BARBARA K. HECHT
Hecht Associates 4134 McGirts Boulevard Jacksonville, FL 32210
REFERENCES 1. Li FP, Fraumeni JF (1969): Soft tissue sarcomas, breast cancer and other neoplasms. A familial syndrome? Ann Intern Med 71:747-752. 2. Li FP, Fraumeni JF (1969): Rhabdomyosarcoma in children: An epidemiologic study and identification of a familial cancer syndrome. J Natl Cancer Inst 43:1364-1373. 3. Malkin D, Li FP, Strong LC, Fraumeni JF, Nelson CE, Kim DH, Kassel J, Gryka MA, Bischoff FZ, Tainsky MA, Friend SH (1990): Germ line p53 mutations in a familial syndrome of breast cancer, sarcomas, and other neoplasms. Science 250:1233-1238. 4. Swift M, Reitnauer PJ, Morrell D, Chase CL (1983): Breast and other cancers in families with ataxia-telangiectasia. Lancet i:1049-1050. 5. Swift M, Reitnauer PJ, Morrell D, Chase CL (1987): Breast and other cancers in families with ataxia-telangiectasia. N Engl J Med 316:1289-1294. 6. Swift M, Chase CL, Morrell D (1990): Cancer predisposition of ataxia-telangiectasia heterozygotes. Cancer Genet Cytogenet 46:21-27. 7. Morrell D, Chase CL, Swift M (1990): Cancers in 44 families with ataxia-telangiectasia. Cancer Genet Cytogenet 50:119-123. 8. Lavin MF, Poidevin PL, Bates P (1992): Enhanced levels of radiation-induced G2 phase delay in ataxia-telangiectasia heterozygotes. Cancer Genet Cytogenet 60:183-187.
W h e n the genetic screening of newborns for phenylketonuria (PKU) was getting u n d e r w a y some years ago, we asked Dick Hoefnagel what he thought about it. Hoefnagel, a veteran neurologist and clinical geneticist, replied in essence: " W h a t we really need to do n o w is to screen for diseases like neurofibromatosis and myotonic d y s t r o p h y and provide genetic counseling for these diseases." Hoefnagel's point was that these are autosomal dominant disorders. They are relatively c o m m o n ones as genetic diseases go. They are diseases that can have grave medical consequences, i n c l u d i n g mental retardation, and since they describe a vertical pattern of inheritance, can put a considerable cohort at risk. Finally, Hoefnagel argued that this type of clinical genetic screening could be done easily and inexpensively. Needless to say, not m a n y geneticists bought this strategy. Few went on to find the cases of autosomal d o m i n a n t diseases, locate their kin, and examine them in an aggressive way. We geneticists found it s i m p l e r to go the neonatal screening route in search of the one in 10,000 baby with PKU. A m o n g the advantages of n e w b o r n metabolic screening was that it could be m a n d a t e d by law and thus avoid the delicate matter of informed consent. Another obvious advantage of neonatal screening was the detection of diseases that could, we believed at the time, be completely treatable. A big advantage, too, was that the time-consuming and onerous task of screening could be relegated to h u m d r u m state laboratories, leaving the biochemical geneticists in medical school meccas to skim off the cream of the unusual cases for research. Cancer genetics is today at a comparable crossroads, a fork in the road between screening for cancer genes through clincial means or through laboratory assays. Belatedly we can begin to scour about in clinical search of i n d i v i d u a l s with elevated cancer risks or we can launch into development of mass cancer screening laboratories. Let's illustrate these alternative avenues. Consider two autosomal d o m i n a n t disorders k n o w n to cause cancer, n a m e l y the dysplastic nevus s y n d r o m e associated with cutaneous malignant m e l a n o m a and the hereditary adenomatous polyposis of the colon closely connected to carcinoma of the large bowel. We have talked with dermatologists and gastroenterologists, respectively, who care for patients with these conditions. We have asked these physicians whether they perform routine family studies. The answer is usually a startled No. The strategy of a cancer genetics screening laboratory can also be illustrated with two examples, n a m e l y the p53 gene and the ataxia telangiectasia (AT) gene. Heritable mutations at the p53 locus on the short arm of chromosome 188 Cancer Genet Cytogenet 60:188-189 (1992) 0165-4608/92/$05.00
17 appear to correspond with the Li-Fraumeni s y n d r o m e (and with a number of hepatic tumors associated with aflatoxin exposure as well). The Li-Fraumeni syndrome, an autosomal d o m i n a n t disorder, is clearly associated with an extraordinary assortment of malignancies. Even though the s y n d r o m e was originally ascertained by soft tissue carcinomas in children, it was certain from the start that other neoplasms, i n c l u d i n g breast cancer in adults, were part and parcel of the LiFraumeni s y n d r o m e [1, 2]. Mutant p53 alleles [3] are n o w suspected to be the genetic culprit. W h y not perform laboratory screening for p53 mutants? It could be done first w i t h i n Li-Fraumeni families and then be e x t e n d e d to the general population. Ataxia telangiectasia is an autosomal recessive disorder with the gene locus on the long arm of c h r o m o s o m e 11. Homozygotes for AT are particularly p r e d i s p o s e d to l y m p h o i d malignancies. However, AT homozygotes are rare; their frequency in the p o p u l a t i o n is p e r h a p s 1 in 20,000 persons. The finding by Swift et al. [4-7] that AT heterozygous w o m e n are at a sixfold-normal risk of breast cancer was surprising. Ataxia telangiectasia h o m o z y g o u s females rarely if ever seem to develop breast cancer. Of course this may reflect the fact that m a n y AT females die in c h i l d h o o d and adolescence of other conditions before the diagnosis and the risk of breast cancer in patients w i t h AT can be identified. The AT heterozygous state is very common. Assuming that AT homozygotes have a frequency of one in 20,000, the Hardy-Weinberg formula indicates that AT heterozygotes are 283-fold more prevalent and involve one in every 71 persons in the population. Thus, the AT gene m a y actually predispose to a significant p r o p o r t i o n of breast cancer cases in the population. Routine detection of the AT gene in the laboratory has posed a pesky problem. As Lavin et al. of Brisbane point out [8], n u m e r o u s assays have been d e v e l o p e d to test for AT. They report a new assay based on the a n o m a l o u s l y long delay of AT heterozygous cells in G2 after irradiation. Using the assay, the Brisbane workers detected all six obligate AT heterozygotes and d i s t i n g u i s h e d them from all but one of 22 controls. (Is that one control an AT heterozygote?) The Brisbane assay appears to satisfy the requirements for screening, with no false negatives and few false positives, although this must be confirmed w i t h a larger sample. The Brisbane assay, according to the investigators, is "relatively simple and reliable and can be carried out in any laboratory with access to Epstein-Barr virus for transformation of l y m p h o c y t e s and a fluorescence activated cell analyzer." In addition, the laboratory must have a source to © 1992 Elsevier Science Publishing Co., Inc. 655 Avenue of the Americas, New York. NY 10010
F. Hecht a n d B. K. Hecht
189
irradiate the lymphoblastoid cells. However, it seems unlikely that a mass screening test can practicably be performed using Epstein-barr v i r u s - t r a n s f o r m e d cells that must then be irradiated and r u n through a fluorescence activated cell sorter. At this time, the excellent report by Lavin et al. [8] is primarily of research interest. A simple mass screening assay for the AT gene appears to await identification and characterization of the gene itself. Are we prepared to perform mass screening in the laboratory for any genes that predispose to cancer? No. We are not yet ready. The only effective way to initiate cancer genetic screening at present is to identify index cases and then perform genetic family studies. Except that this is time-consuming work and r e i m b u r s e m e n t for it may be hard to come by, it makes sense to l a u n c h a major effort to screen clinically for cancer genes. The laboratory assays can be added as we go along. FREDERICK HECHT BARBARA K. HECHT
Hecht Associates 4134 McGirts Boulevard Jacksonville, FL 32210
REFERENCES 1. Li FP, Fraumeni JF (1969): Soft tissue sarcomas, breast cancer and other neoplasms. A familial syndrome? Ann Intern Med 71:747-752. 2. Li FP, Fraumeni JF (1969): Rhabdomyosarcoma in children: An epidemiologic study and identification of a familial cancer syndrome. J Natl Cancer Inst 43:1364-1373. 3. Malkin D, Li FP, Strong LC, Fraumeni JF, Nelson CE, Kim DH, Kassel J, Gryka MA, Bischoff FZ, Tainsky MA, Friend SH (1990): Germ line p53 mutations in a familial syndrome of breast cancer, sarcomas, and other neoplasms. Science 250:1233-1238. 4. Swift M, Reitnauer PJ, Morrell D, Chase CL (1983): Breast and other cancers in families with ataxia-telangiectasia. Lancet i:1049-1050. 5. Swift M, Reitnauer PJ, Morrell D, Chase CL (1987): Breast and other cancers in families with ataxia-telangiectasia. N Engl J Med 316:1289-1294. 6. Swift M, Chase CL, Morrell D (1990): Cancer predisposition of ataxia-telangiectasia heterozygotes. Cancer Genet Cytogenet 46:21-27. 7. Morrell D, Chase CL, Swift M (1990): Cancers in 44 families with ataxia-telangiectasia. Cancer Genet Cytogenet 50:119-123. 8. Lavin MF, Poidevin PL, Bates P (1992): Enhanced levels of radiation-induced G2 phase delay in ataxia-telangiectasia heterozygotes. Cancer Genet Cytogenet 60:183-187.