- Email: [email protected]
A cool tumour-suppressor gene
Literature
Chip-ping away at cancer classification Understanding the molecular mechanisms underlying cancer will allow the development of new therapeutic strategies that are tailored to a specific type of tumour, thereby minimizing the toxic effects of current anti-cancer drugs. A shift from histopathological analysis to rapid molecular diagnosis will enable the precise definition of cancer classes and the accurate prediction of tumour type and choice of optimal treatment. Golub and colleagues1 have applied microarray technology (commonly referred to as ‘DNA chips’) to test whether gene expression profiles can be used to distinguish between sets of patients suffering from two different types of cancer, acute myeloid leukaemia (AML) or acute lymphoblastic leukaemia (ALL). They chose these leukaemias because they are currently diagnosed by immunochemistry and cytogenetic analysis and require distinct clinical treatment protocols. The researchers analysed the expression levels of over 6000 known genes, to identify approximately 1000 that were specifically associated with either AML or ALL. Sophisticated statistical analysis allowed them to define a ‘class predictor’ set of genes capable of automatically diagnosing whether tumour samples represented ALL or AML
MOLECULAR MEDICINE TODAY, FEBRUARY 2000 (VOL. 6)
ALL
AML
–3 –2.5 –2 –1.5 –1 –0.5 0 0.5 1 1.5 Low Normalized expression
2
2.5
3 High
Figure 1. Genes distinguishing ALL from AML. The 50 genes most highly correlated with the ALL–AML class distinction are shown. Rows correspond to genes; columns correspond to expression levels in samples. Expression levels greater than the mean are in red, those below are in blue. Reproduced with permission from Ref. 1. Copyright 1999 American Association for the Advancement of Science. Permission for electronic format not granted.
(Fig. 1). They also showed that this approach can be used for unbiased characterization of new classes of cancer: ‘class discovery’. In this way, the methodology was able to further divide ALL tumour samples into those of either T- or B-cell origin. This study elegantly demonstrates how an unbiased, genome-wide analysis of gene-expression profiles can be used to discover new classes of cancer, independently of tumour histopathology, and offers two types of promise for future cancer research. The microarray analysis identifies genes not previously associated with the molecular mechanisms of cancer, offering novel drug development targets. Furthermore, it allows us to explore common players in oncogenesis which provide a molecular link between tumour classes with distinct histoimmunopathology. In the future, this knowledge of a multigene molecular signature could be effectively linked with careful record-keeping of patient responses to drug treatment, helping doctors to refine targeted therapeutic drug protocols. 1 Golub, T.R. et al. (1999) Molecular classification of cancer: class discovery and class prediction by gene expression monitoring. Science 286, 531–537
Jonathan B. Weitzman PhD [email protected]
A cool tumour-suppressor gene Retinoblastoma (RB) is a rare intraocular tumour that occurs predominantly in children below the age of five. There are heritable and sporadic forms, both resulting from inactivation of the tumour-suppressor gene RB1. The sporadic form occurs when both copies of the RB1 gene are inactivated within the same retinoblast cell. As this is a rare event, the disease is typically unilateral and unifocal. In heritable retinoblastoma, an inherited or newly acquired germline mutation inactivates one allele in every cell. Mutation or loss of the second allele is then very likely in at least one retinoblast, and multiple tumours are common. Thus, familial RB usually shows dominant inheritance with high penetrance and bilateral disease. A small subset of families with heritable RB exhibit reduced penetrance. Obligate carriers
50
either remain disease-free or they only develop a unifocal tumour more typical of sporadic RB. Most mutations identified in these families are predicted to alter, but not abolish, protein function. Otterson et al.1 studied the effects of three RB1 mutations associated with low-penetrance RB. All the mutations are situated within the pocket-binding domain of the protein, which is crucial for tumour-suppressor activity, and have previously been shown to have defective pocket-binding. However, in assays carried out in yeast grown at 308C, they found that the pocket-binding activity was significantly greater than that observed at 378C. This was true for each of the three mutations, and binding was further enhanced at 248C. Temperature-sensitivity is not a new phenomenon, but this study documents significant fluctuations in pocket-
binding activity simply by altering in vitro culture temperature. Minor variations in ambient conditions, temperature or otherwise, might similarly influence activity of the mutant protein in vivo. These subtle mutations will aid further understanding of RB protein function but, most excitingly, one might speculate that if the pocketbinding activity could be stably enhanced, then perhaps retinoblastoma could be prevented in patients carrying these particular mutations. 1 Otterson, G.A. et al. (1999) Temperature-sensitive RB mutations linked to incomplete penetrance of familial retinoblastoma in 12 families. Am. J. Hum. Genet. 65, 1040–1046 Micheala A. Aldred PhD, DipRCPath [email protected]
1357-4310/00/$ - see front matter © 2000 Elsevier ScienceLtd. All rights reserved.
Chip-ping away at cancer classification Understanding the molecular mechanisms underlying cancer will allow the development of new therapeutic strategies that are tailored to a specific type of tumour, thereby minimizing the toxic effects of current anti-cancer drugs. A shift from histopathological analysis to rapid molecular diagnosis will enable the precise definition of cancer classes and the accurate prediction of tumour type and choice of optimal treatment. Golub and colleagues1 have applied microarray technology (commonly referred to as ‘DNA chips’) to test whether gene expression profiles can be used to distinguish between sets of patients suffering from two different types of cancer, acute myeloid leukaemia (AML) or acute lymphoblastic leukaemia (ALL). They chose these leukaemias because they are currently diagnosed by immunochemistry and cytogenetic analysis and require distinct clinical treatment protocols. The researchers analysed the expression levels of over 6000 known genes, to identify approximately 1000 that were specifically associated with either AML or ALL. Sophisticated statistical analysis allowed them to define a ‘class predictor’ set of genes capable of automatically diagnosing whether tumour samples represented ALL or AML
MOLECULAR MEDICINE TODAY, FEBRUARY 2000 (VOL. 6)
ALL
AML
–3 –2.5 –2 –1.5 –1 –0.5 0 0.5 1 1.5 Low Normalized expression
2
2.5
3 High
Figure 1. Genes distinguishing ALL from AML. The 50 genes most highly correlated with the ALL–AML class distinction are shown. Rows correspond to genes; columns correspond to expression levels in samples. Expression levels greater than the mean are in red, those below are in blue. Reproduced with permission from Ref. 1. Copyright 1999 American Association for the Advancement of Science. Permission for electronic format not granted.
(Fig. 1). They also showed that this approach can be used for unbiased characterization of new classes of cancer: ‘class discovery’. In this way, the methodology was able to further divide ALL tumour samples into those of either T- or B-cell origin. This study elegantly demonstrates how an unbiased, genome-wide analysis of gene-expression profiles can be used to discover new classes of cancer, independently of tumour histopathology, and offers two types of promise for future cancer research. The microarray analysis identifies genes not previously associated with the molecular mechanisms of cancer, offering novel drug development targets. Furthermore, it allows us to explore common players in oncogenesis which provide a molecular link between tumour classes with distinct histoimmunopathology. In the future, this knowledge of a multigene molecular signature could be effectively linked with careful record-keeping of patient responses to drug treatment, helping doctors to refine targeted therapeutic drug protocols. 1 Golub, T.R. et al. (1999) Molecular classification of cancer: class discovery and class prediction by gene expression monitoring. Science 286, 531–537
Jonathan B. Weitzman PhD [email protected]
A cool tumour-suppressor gene Retinoblastoma (RB) is a rare intraocular tumour that occurs predominantly in children below the age of five. There are heritable and sporadic forms, both resulting from inactivation of the tumour-suppressor gene RB1. The sporadic form occurs when both copies of the RB1 gene are inactivated within the same retinoblast cell. As this is a rare event, the disease is typically unilateral and unifocal. In heritable retinoblastoma, an inherited or newly acquired germline mutation inactivates one allele in every cell. Mutation or loss of the second allele is then very likely in at least one retinoblast, and multiple tumours are common. Thus, familial RB usually shows dominant inheritance with high penetrance and bilateral disease. A small subset of families with heritable RB exhibit reduced penetrance. Obligate carriers
50
either remain disease-free or they only develop a unifocal tumour more typical of sporadic RB. Most mutations identified in these families are predicted to alter, but not abolish, protein function. Otterson et al.1 studied the effects of three RB1 mutations associated with low-penetrance RB. All the mutations are situated within the pocket-binding domain of the protein, which is crucial for tumour-suppressor activity, and have previously been shown to have defective pocket-binding. However, in assays carried out in yeast grown at 308C, they found that the pocket-binding activity was significantly greater than that observed at 378C. This was true for each of the three mutations, and binding was further enhanced at 248C. Temperature-sensitivity is not a new phenomenon, but this study documents significant fluctuations in pocket-
binding activity simply by altering in vitro culture temperature. Minor variations in ambient conditions, temperature or otherwise, might similarly influence activity of the mutant protein in vivo. These subtle mutations will aid further understanding of RB protein function but, most excitingly, one might speculate that if the pocketbinding activity could be stably enhanced, then perhaps retinoblastoma could be prevented in patients carrying these particular mutations. 1 Otterson, G.A. et al. (1999) Temperature-sensitive RB mutations linked to incomplete penetrance of familial retinoblastoma in 12 families. Am. J. Hum. Genet. 65, 1040–1046 Micheala A. Aldred PhD, DipRCPath [email protected]
1357-4310/00/$ - see front matter © 2000 Elsevier ScienceLtd. All rights reserved.