Evidence for a telomere-independent "clock" limiting RAS oncogene-driven proliferation of human thyroid epithelial cells

An initiating role for RAS oncogene mutation in several epithelial cancers is supported by its high incidence in early-stage tumors and its ability to induce proliferation in the corresponding normal cells in vitro. Using retroviral transduction of thyroid epithelial cells as a model we ask here: (i) how mutant RAS can induce long-term proliferation in an epithelial cell in contrast to the premature senescence observed in fibroblasts; and (ii) what is the "clock" which eventually triggers spontaneous growth arrest even in epithelial clones generated by mutant RAS. The early response to RAS activation in thyroid epithelial cells showed two features not seen in fibroblasts: (i) a marked decrease in expression of the cyclin-dependent kinase inhibitor (CDKI) p27(kip1) and (ii) the absence of any induction of p21(waf1). When proliferation eventually ceased (after up to 20 population doublings) this occurred despite undiminished expression of mutant RAS and was tightly correlated with a return to the initial high level of p27(kip1) expression, together with the de novo appearance of p16(ink4a). Importantly, neither the CDKI changes nor the proliferative life span of RAS-induced epithelial clones was altered by induction of telomerase activity through forced expression of the catalytic subunit, hTERT, at levels sufficient to immortalize human fibroblasts. These data provide a basis for cell-type differences in sensitivity to RAS-induced proliferation which may explain the corresponding tumor-type specificity of RAS mutation. They also show for the first time in a primary human cell model that a telomere-independent mechanism can limit not only physiological but also oncogene-driven proliferation, pointing therefore to a tumour suppressor mechanism additional, or alternative, to the telomere clock.

p53-Dependent growth arrest and altered p53-immunoreactivity following metabolic labelling with 32P ortho-phosphate in human fibroblasts

The tumour suppressor gene p53 plays a major role in the cellular response to DNA damage, mediating growth arrest and/or apoptosis. Phosphorylation of the protein occurs at numerous sites in vivo and is likely to be a major mechanism for modulation of its activity as a transcriptional transactivator. Not surprisingly, therefore, p53 has been intensively studied by 32P metabolic labelling. Here we show however, using normal human fibroblasts, that typical labelling conditions induce (i) a p53-dependent inhibition of DNA synthesis and (ii) an increase in the cellular content of p53 protein detectable by the phosphorylation-sensitive antibody DO-1 but not by antibody DO-12. These data demonstrate for the first time that 32P labelling is sufficient to induce a biologically-significant, p53-mediated cellular response and strongly suggest that it perturbs the phosphorylation state of p53 which it is being used to measure. This highlights the need to re-evaluate earlier data by non-radioactive approaches using phospho-specific antibodies.

Control of replicative life span in human cells: barriers to clonal expansion intermediate between M1 senescence and M2 crisis

The accumulation of genetic abnormalities in a developing tumor is driven, at least in part, by the need to overcome inherent restraints on the replicative life span of human cells, two of which-senescence (M1) and crisis (M2)-have been well characterized. Here we describe additional barriers to clonal expansion (Mint) intermediate between M1 and M2, revealed by abrogation of tumor-suppressor gene (TSG) pathways by individual human papillomavirus type 16 (HPV16) proteins. In human fibroblasts, abrogation of p53 function by HPVE6 allowed escape from M1, followed up to 20 population doublings (PD) later by a second viable proliferation arrest state, MintE6, closely resembling M1. This occurred despite abrogation of p21(WAF1) induction but was associated with and potentially mediated by a further approximately 3-fold increase in p16(INK4a) expression compared to its level at M1. Expression of HPVE7, which targets pRb (and p21(WAF1)), also permitted clonal expansion, but this was limited predominantly by increasing cell death, resulting in a MintE7 phenotype similar to M2 but occurring after fewer PD. This was associated with, and at least partly due to, an increase in nuclear p53 content and activity, not seen in younger cells expressing E7. In a different cell type, thyroid epithelium, E7 also allowed clonal expansion terminating in a similar state to MintE7 in fibroblasts. In contrast, however, there was no evidence for a p53-regulated pathway; E6 was without effect, and the increases in p21(WAF1) expression at M1 and MintE7 were p53 independent. These data provide a model for clonal evolution by successive TSG inactivation and suggest that cell type diversity in life span regulation may determine the pattern of gene mutation in the corresponding tumors.

Human thyroid cancer cells as a source of iso-genic, iso-phenotypic cell lines with or without functional p53

Differentiated thyroid carcinomas (in contrast to the rarer anaplastic form) are unusual among human cancers in displaying a remarkably low frequency of p53 mutation and appear to retain wild-type (wt) p53 function as assessed by the response of derived cell lines to DNA damage. Using one such cell line, K1, we have tested the effect of experimental abrogation of p53 function by generating matched sub-clones stably expressing either a neo control gene, a dominant-negative mutant p53 (143ala) or human papilloma virus protein HPV16 E6. Loss of p53 function in the latter two groups was confirmed by abolition of p53-dependent 'stress' responses including induction of the cyclin/CDK inhibitor p21WAF1 and G1/S arrest following DNA-damage. In contrast, no change was detected in the phenotype of 'unstressed' clones, with respect to any of the following parameters: proliferation rate in monolayer, serum-dependence for proliferation or survival, tumorigenicity, cellular morphology, or tissue-specific differentiation markers. The K1 line therefore represents a 'neutral' background with respect to p53 function, permitting the derivation of functionally p53 + or - clones which are not only iso-genic but also iso-phenotypic. Such a panel should be an ideal tool with which to test the p53-dependence of cellular stress responses, particularly the sensitivity to potential therapeutic agents, free from the confounding additional phenotypic differences which usually accompany loss of p53 function. The results also further support the hypothesis that p53 mutation alone is not sufficient to drive progression of thyroid cancer to the aggressive anaplastic form.

Segregational fidelity of chromosomes in human thyroid tumour cells

Using fluorescence in situ hybridisation (FISH) we have analysed the segregational fidelity of all the human chromosomes during mitotic cell division. The losses and gains of chromosomes were analysed in human polyploid cell lines derived from a well-differentiated papillary thyroid cancer. These thyroid cells can be cultured for more than 300 population doublings. For the purpose of our study the polyploid nature of the cells may act as a protective buffer against the cell-lethal effects of the loss of individual chromosomes. To evaluate the role of the p53 gene product in maintaining the fidelity of chromosome segregation we compared the frequencies of chromosome loss and gain in cultures with wild-type p53 activity (K1E7neo3) and cultures transfected with plasmids expressing a mutant p53 product (K1E7scx6). Cultures were analysed for the presence of both structurally normal and rearranged chromosomes at both early and late passages. Cell cultures with defective p53 activity showed progressive chromosome loss from a median chromosome number of 87-97 to 75-86. Cell growth in cultures with wild-type p53 activity showed the loss of chromosomes 6, 7, and 8 and the gain of 17 and 20. Cultures expressing mutant p53 activity showed the loss of chromosomes 2, 5, 14 and 17 and the gain of 4 and 22. The combination of defective p53 and growth resulted in further destabilisation with the additional losses of chromosomes 3, 11, 15, 16 and 21. Chromosomes 1, 9, 10, 12, 13, 18, 19, X and Y segregated stably under all the culture conditions as did the structurally rearranged marker chromosomes. The study has demonstrated variation in the fidelity of mitotic chromosome segregation and the influence of p53 gene activity upon the segregation of individual human chromosomes.

Dissociation of telomere dynamics from telomerase activity in human thyroid cancer cells

Prevention of telomere erosion through acquisition of telomerase activity is thought to be an essential mechanism in most human cancer cells for avoidance of cellular senescence and crisis. It has been generally assumed that once telomerase has been activated, no further telomere shortening should ensue. We show here, however, that a much more complex pattern of telomere dynamics can exist in telomerase-positive immortal cancer cells. Using a panel of subclones derived from a human thyroid cancer cell line, K1E7, we found that some clones show persistent decline in mean telomere restriction fragment (TRF) length by up to 2 kb over 450 population doublings (pd), despite sustained high telomerase activity (as assessed by the in vitro "TRAP" assay). TRF length subsequently stabilized at around 5 kb, but with no corresponding increase in telomerase activity. One clone showed an even more unexpected biphasic time course, with the mean TRF length initially increasing by 1.5 kb over 90 pd, before "plateauing" and then returning over a similar period to its original value, again without any correlation to TRAP activity. Such dissociations between telomere dynamics and telomerase activity support the existence of additional controls on telomere length in the intact cell. Our observations are consistent with current negative-feedback models of telomere length regulation by telomere binding proteins and these cell lines should prove useful experimental tools for their further evaluation.

The tumour suppressor gene p53 as a regulator of proliferative life-span and tumour progression

This review develops the concept of p53 as a transcription factor mediating growth arrest or cell death in response to long-term (senescence-related) as well as acute (DNA damage) signals. Evidence is presented to support the importance of both functions in tumour development. The role of p53 in senescence is discussed in the context of the telomere theory and in relation to its more established function as a 'guardian of the genome'. Finally, data indicating important tissue-specific differences in the control of proliferative life-span by p53 are reviewed, together with potential clinical implications.

S phase cell-cycle arrest following DNA damage is independent of the p53/p21(WAF1) signalling pathway

It is now likely that the cyclin-kinase inhibitor, p21(WAF1/SD11), is a key effector of p53-mediated cell-cycle arrest at the G(1)/S checkpoint following DNA damage. More recently, however, in vitro data has suggested that this pathway may also mediate the acute inhibition of DNA synthesis seen in cells already in S phase. Here we address this question in an intact cell system using normal human diploid fibroblasts in which p53 function is manipulated by expression of a dominant-negative mutant (ala(143)) introduced by a retroviral vector. Induction of DNA strand breaks in normal control fibroblasts by exposure to bleomycin led as expected to G(1)/S cell cycle arrest, induction of p2l(WAF1) and a rapid reduction in the rate of DNA synthesis in cells already in S phase. Stable expression of mutant p53 abrogated the G(1)/S (but not the G(2)/M) cell cycle checkpoint and abolished the induction of p21(WAF1), but had no significant effect on the inhibition of DNA replication in S phase nuclei. We conclude that, despite the in vitro evidence for inhibitory activity on PCNA/polymerase delta, p21(WAF1) induction does not appear to be essential for the acute inhibition of DNA synthesis in the intact cell following strand-break damage in S phase.

High frequency deletion of the tumour suppressor gene P16INK4a (MTS1) in human thyroid cancer cell lines

p16INK4a (MTS1) is an important negative regulator of mammalian cell proliferation, acting via inhibition of CDK4/cyclin D-dependent phosphorylation of pRb to prevent progression through the G1 phase of the cell cycle. Loss of p16 activity by either gene deletion, mutation or transcriptional inactivation has now been found in a wide range of human cancers of both epithelial and mesenchymal origin, at a frequency rivalling that of p53 mutation. As a first step towards investigating its possible role as a tumour suppressor gene in thyroid tumorigenesis, we have carried out a Southern blot analysis of the p16 gene locus in a series of cell lines derived from differentiated human thyroid cancers. Homozygous deletion of the entire p16 coding sequence was observed in two of three follicular and two of four papillary cancer cell lines, but not in normal tissue or normal cells immortalised by SV40 T antigen. Given the co-existence of p16 abnormalities in primary tumours and cell lines observed in other tumour types, this high frequency of deletion suggests that p16 is a key tumour suppressor gene in the genesis of differentiated thyroid cancer.

Mutant p53 rescues human diploid cells from senescence without inhibiting the induction of SDI1/WAF1

Although the cyclin-dependent kinase inhibitor p21SDI1 (WAF1/CIP1) has been proposed as the mediator of p53-induced cell cycle arrest following DNA damage, several stimuli now appear to induce SDI1 independent of p53 function. We have examined the behavior of p53 and SDI1 in an isogeneic model by manipulating p53 status in normal diploid human fibroblasts using an amphotropic retroviral vector. Following DNA strand break damage induced by bleomycin, both SDI1 induction and G1-S cell cycle arrest are p53 dependent, consistent with SDI1 being the key mediator. In contrast, in cellular senescence (and following UV irradiation), induction of SDI1 occurs independent of p53 function yet growth arrest is still p53 dependent. We conclude (a) that redundant pathways exist for induction of SDI1, but that (b) SDI1, while perhaps necessary, is not sufficient for inhibition of cell cycle progression, requiring the cooperation of an additional factor (possibly another cyclin-dependent kinase inhibitor) whose expression, at least in the case of senescence, is strictly p53 dependent.

Interaction between p53 and TGF beta 1 in control of epithelial cell proliferation

Although loss of sensitivity to transforming growth factor beta (TGF beta) may be a key step in the escape of epithelial tumours from normal growth control, the intracellular signals determining responsiveness remain controversial, particularly the role of p53. We have investigated this question using thyroid epithelial lines as a model. We analysed (i) human thyroid cancer cell lines having either wild-type (wt) or mutant p53; (ii) rat thyroid lines derived by spontaneous immortalisation following introduction of mutant H-ras, which exhibit high levels of wt p53 but loss of p53-mediated cell-cycle control. Loss of response to TGF beta 1 was found in all human lines bearing mutant p53, and in the majority of the functionally equivalent rat lines, consistent with a role of wt p53 in mediating response. However, introduction of a dominant negative p53 mutant into TGF beta 1 responsive human lines containing wt p53 did not reduce responsiveness, demonstrating that p53 function is not necessary for TGF beta 1 response. On the other hand, expression of a temperature-sensitive (ts) p53 gene in a partially-responsive rat line demonstrated a highly significant modulation of TGF beta response, which fell from 65% inhibition of 3H-thymidine labelling index at 32.5 degrees C (wt p53 conformation) to only 14% at 37.5 degrees C (mutant conformation). The results suggest that p53 and TGF beta generate separate but interacting inhibitory signals, i.e. that p53 modulates but does not mediate TGF beta response. This conclusion explains previous conflicting data and is consistent with current models of cell cycle control by multiple inhibitors of cyclin-dependent kinases.

Evasion of p53-mediated growth control occurs by three alternative mechanisms in transformed thyroid epithelial cells

Using the thyroid as a model of multistep epithelial tumorigenesis, we have used representative cell lines to correlate the degree of malignant transformation with the functional status of p53 and the integrity of cell-cycle check-points. Three distinct phenotypes were observed: Type I lines, derived from poorly-differentiated human thyroid cancers, expressed high levels of mutant p53 protein; Type II, also poorly-differentiated but derived from rat, showed over-expression of wild-type (wt) p53 with marked cell-cell heterogeneity: Type III, from well-differentiated human cancers, contained uniformly low levels of wt p53. All cell lines containing wt p53 retained a near-normal induction of p53 by DNA damage. However, the ability to undergo growth arrest differed strikingly. Whereas Type I and II lines had lost both G2/M and G1/S check points, Type III cells retained both. In Type III cells, as in diploid human fibroblasts, mutant p53 expression specifically abrogated G1/S check-point function with no other change in phenotype. These data demonstrate 3 mechanisms for evasion of p53 growth control: (i) direct mutation (ii) indirect inactivation, or (iii) 'avoidance' of activation, most probably due to failure to reach a critical threshold of DNA damage.

Escape from senescence in human diploid fibroblasts induced directly by mutant p53

Cellular senescence is thought to be a key restraint on the progression of human tumours, escape from which involves loss of function of tumour suppressor genes. The number and nature of the genes involved however is uncertain, in particular the role of p53 mutation, which is commonly correlated with tumour progression. To address this question, we used the novel approach of directly assessing the effect of mutant p53 on 'pre-aged' human diploid fibroblasts (HDF), thereby avoiding the uncertainty of additional cooperating events, inherent in transgenic models. HDF were passaged till near-senescent and then infected with an amphotropic retroviral vector encoding an ala143 human mutant p53. The results show conclusively that p53 mutation alone is sufficient to extend the proliferative lifespan of normal fibroblasts by approximately 17 population doublings, but has no phenotypic effect on 'young' fibroblasts. We conclude that a key tumour-limiting function of wild-type p53 is to mediate growth arrest after a given number of cell divisions, in agreement with data implicating a p53-regulated gene, WAF-1/sdi-1, in cellular senescence. This may be reconciled with its 'guardian of the genome' role, if telomere erosion, a key change in senescence, is perceived by the cell as a form of DNA 'damage'.

In vitro reconstruction of tumour initiation in a human epithelium

Knowledge of tumour initiation in human epithelia is limited by sample availability and difficulty in experimental manipulation of human cells. The thyroid is a useful model since, in addition to multiple tumour stages, it presents two distinct 'pathways' of tumorigenesis: 'follicular' tumours, in which ras oncogene mutations occur at high frequency and 'papillary' tumours, associated with ret (or trk) activation. We have used these observations to reconstruct early thyroid tumorigenesis, using amphotropic retroviral vectors. When introduced into normal thyroid epithelial cells, mutant ras induces self-limiting growth of well-demarcated, differentiated colonies--a phenotype consistent with follicular adenoma. Activated ret on the other hand induces smaller, poorly-demarcated colonies with a morphology consistent with early papillary tumours. Mutant p53--which occurs only in the latest stages of thyroid cancer--was without effect. Our results provide the first direct experimental evidence in a human epithelium for alternative initiating oncogenes and their determination of the subsequent 'direction' of tumour development.

A variant epithelial sub-population in normal thyroid with high proliferative capacity in vitro

We describe the existence in normal human primary thyroid cultures of a hitherto unrecognised sub-population of epithelial cells. This variant phenotype is characterised by squamoid morphology, absence of thyroglobulin, and an altered profile of intermediate filament expression. We suggest that these cells are derived from scattered foci of squamous metaplasia present in the normal gland. Although they are initially present at a frequency of less than 10(-4), their very high proliferative capacity enables them to outgrow the 'classical' follicular cells and confers a much increased capacity for gene transduction. Recognition of these cells is therefore crucial in the interpretation of long-term thyroid culture experiments and those involving in vitro gene transfer.

Direct growth stimulation of normal human epithelial cells by mutant p53

We developed a high-titer amphotropic retroviral vector that expresses mutant (Ala143) human p53 to test directly the response of genetically normal human epithelial cells to p53 mutation. Contrary to our prediction, we found that in pancreatic epithelium (whose tumors display a high frequency of p53 mutation) but not in thyroid (whose tumors show an exceptionally low mutation frequency), expression of mutant p53 induced a dramatic, though self-limiting, proliferative response. This result questions the assumption that p53 mutation is relevant only to the later stages of tumorigenesis.

A phenotypically and karyotypically stable human thyroid epithelial line conditionally immortalized by SV40 large T antigen

Primary cultures of normal human neonatal thyroid follicular cells were transfected with a plasmid expressing a temperature-sensitive (tsA58) mutant of SV40 large T antigen. An epithelial cell line, designated B-thy-ts.1, was obtained which showed tight temperature-dependent growth. In sharp contrast to previous such lines, which were derived from adult thyroid, B-thy-ts.1 has retained a well-differentiated phenotype as reflected in its morphology and cytokeratin expression pattern. In addition to phenotypic stability the line also displays an unusually stable karyotype, lacking the usual clastogenic effects of SV40, which we speculate to result from a greater DNA repair capacity of its cell of origin. B-thy-ts.1 should be a particularly useful tool with which to study the effects of activated oncogenes on epithelial growth and differentiation.

Mutation of the p53 gene in a differentiated human thyroid carcinoma cell line, but not in primary thyroid tumours

The p53 gene has been implicated as a tumour suppressor, with mutations occurring in many carcinomas, such as colon, breast and lung. We have sequenced exons 5, 7 and 8 containing conserved gene regions in the only available differentiated thyroid follicular carcinoma cell line and found a mutation at position 273, Arg----His, with no normal allele present. The same mutation was also present in DNA from the tumour of origin. However immunohistochemical analysis of 129 human thyroid tumours using a panel of p53 antibodies was unequivocally negative. Southern blotting in 20 cases failed to demonstrate any deletion or rearrangement, and direct genomic sequencing of 20 carcinomas showed normal DNA sequence for exons 5, 7 and 8. Thus p53 abnormalities may not be important in human thyroid carcinogenesis, in contrast to colon, breast and lung. However, the FTC 133 cell line was only established after 132 unsuccessful attempts with other differentiated thyroid follicular tumours. Since this line and the corresponding tumour of origin have a p53 mutation, we propose that p53 mutation may confer on thyroid follicular tumour cells the ability to grow in culture. This has potential applications for the future development of thyroid carcinoma cell lines.

In vitro responsiveness to serum growth factors is inversely related to in vivo malignancy in human thyroid epithelial cells

We have examined the proliferative response (DNA synthesis) of primary thyroid epithelial cultures to serum and a defined serum-substitute. These cultures were derived from normal human thyroid and from thyroid adenomas and carcinomas. All normal cultures showed a dose-dependent response, with a maximum 3H-thymidine labelling index of around 50%. Three out of the four adenomas demonstrated a much reduced or delayed response under the same conditions. In two carcinomas, labelling was never more than 5% and in one case was undetectable. This inverse relationship between the degree of in vivo malignancy and proliferative response in vitro has important implications for the interpretation of tissue culture models of epithelial neoplasia and also offers the potential for isolating novel growth factors specific for thyroid cancer cells.

Correlated abnormalities of transforming growth factor-beta 1 response and p53 expression in thyroid epithelial cell transformation

Using the thyroid follicular cell as a model for multi-stage carcinogenesis, we have investigated the role of two potential negative growth regulators ('anti-oncogenes') in epithelial tumour progression--transforming growth factor-beta 1 (TGF beta 1) and p53. Normal follicular cells, as expected, showed marked growth inhibition in response to TGF beta 1. Adenoma cells were equally inhibited. In contrast, spontaneously and SV40-immortalised follicular cell lines showing features of malignant transformation (notably loss of growth factor dependence) had lost all responsiveness to TGF beta 1, accompanied by a partial loss of its receptors. p53 protein was below detectable limits in normal and in adenoma cells but in contrast very high levels were observed in all three transformed lines. In the SV40-immortalised cells, this was expected in view of the known stabilising effect of the viral large T protein. In the spontaneous line we found strong evidence for point mutation of p53, which is known to have the same effect. Both mechanisms result in loss of p53 tumour suppressor function despite increased protein content. We conclude that loss of inhibition by TGF beta and inactivation of p53 are important steps in in vitro immortalisation and/or in vivo tumour progression in human thyroid follicular cells, and speculate that p53 may mediate or be required for the inhibitory signal normally induced by TGF beta 1.

Partial transformation of human thyroid epithelial cells by mutant Ha-ras oncogene

We have previously shown that activation of ras oncogenes by mutation is a frequent early event in human thyroid neoplasia. Using amphotropic retroviral vectors to achieve gene transfer, we demonstrate here that human primary thyroid epithelial cells can be partially transformed by an activated cellular or viral Ha-ras oncogene, in the absence of a cooperating oncogene. The transformation event induced by ras involves temporary rescue from senescence for up to 20 rounds of cell division together with morphological alteration, growth factor independence and anchorage independence. It has therefore been possible to reconstruct in vitro a key early event in the genesis of human epithelial neoplasia.

Conditional immortalization of human thyroid epithelial cells: a tool for analysis of oncogene action

To overcome the difficulty of assessing oncogene action in human epithelial cell types, such as thyroid, which have limited proliferative potential in culture, we have explored the use of temperature-sensitive (ts) mutants of simian virus 40 (SV40) early region to create conditionally immortalized epithelial cell lines. Normal primary cultures of human thyroid follicular cells were transfected with a plasmid containing the SV40 early region from mutant tsA58. Expanding epithelial colonies were observed after 2 to 3 months, all of which grew to greater than 200 population doublings without crisis. All showed tight temperature dependence for growth. After switch-up to the restrictive temperature (40.5 degrees C), no further increase in cell number was seen after 1 to 2 days. However, DNA synthesis declined much more slowly; the dissociation from cell division led to marked polyploidy. Viability was maintained for up to 2 weeks. Introduction of an inducible mutant ras gene into ts thyroid cells led, as expected, to morphological transformation at the permissive temperature when ras was induced. Interestingly, this was associated with a marked reduction in net growth rate. At the restrictive temperature, induction of mutant ras caused rapid cell death. These results demonstrate the utility of a ts SV40 mutant to permit the study of oncogene action in an otherwise nonproliferative target cell and reveal important differences in the interaction between ras and SV40 T in these epithelial cells compared with previously studied cell types.

Immunocytochemically detectable TGF-beta associated with malignancy in thyroid epithelial neoplasia

The possible role of changes in TGF-beta expression in the multistage development of thyroid cancer was assessed. The presence of TGF-beta 1 in thyroid epithelial cells was analyzed in sections of normal and tumor tissue using an immunoperoxidase technique employing an antibody directed against the amino-terminal 30 amino acids of mature TGF-beta 1. Specific immunostaining was clearly detected in epithelial cells in 58% of malignant thyroid tumours (including follicular, papillary, and anaplastic variants). However, no positive cells were seen in any of 7 benign tumors nor in any normal thyroid epithelium. Within the cancer group as a whole, there was no significant correlation with pathological grade or clinical stage of tumor but in one subgroup--follicular carcinomas--a significant association was noted between TGF-beta immunostaining and the presence of a specific mutation of the H-ras oncogene (codon 61, gln----arg). We conclude that a major alteration in expression of TGF-beta occurs specifically in the malignant stage of tumor development in thyroid follicular epithelium and speculate on its possible role in this process.

Absence of abnormalities of the c-erbB-1 and c-erbB-2 proto-oncogenes in human thyroid neoplasia

The c-erbB-1 and c-erbB-2 proto-oncogenes are frequently activated by gene amplification and overexpression in a variety of human cancers. In an analysis of a large series of benign and malignant thyroid tumours, no abnormalities of structure or expression of either of c-erbB-1 or c-erbB-2 were found. Activation of these oncogenes is not a necessary event in neoplasia of this epithelial system.

Papillary and follicular thyroid carcinomas show a different pattern of ras oncogene mutation

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Structure and expression of nuclear oncogenes in multi-stage thyroid tumorigenesis

We have investigated the possibility that structural alterations of the 'nuclear' oncogene family (c-myc, N-myc, L-myc, fos, myb and p53) leading to aberrant expression might, as in several other tumour types, play a role in the multi-stage development of tumorigenesis in the human thyroid follicular cell. Direct analysis of expression by slot and Northern blot RNA hybridisation showed that normal thyroid expresses surprisingly high levels of fos, and to a lesser extent c-myc, c-myc expression was markedly increased in all tumours, both benign and malignant, but no increase was seen in any other nuclear oncogene. fos expression was reduced specifically in one type of malignant tumour-follicular carcinoma-in inverse correlation with differentiation. Southern blot analysis showed no evidence of rearrangement or amplification of c-myc, or of any other 'nuclear' oncogene in any thyroid tumour. We conclude that there is no evidence that a primary abnormality of these genes plays a role in thyroid follicular cell tumorigenesis and suggest that the observed changes in expression can be adequately explained as secondary consequences of the tumour phenotype.

High frequency of ras oncogene activation in all stages of human thyroid tumorigenesis

Using polymerase chain reaction amplification and oligonucleotide probing, the activation of ras oncogenes in 24 benign and 20 malignant human thyroid neoplasms was examined. The frequency of ras oncogene activation was similar at all stages of tumorigenesis in this system, being found in 33% of adenomas overall (50% of microfollicular tumours), 53% of differentiated follicular carcinomas and 60% of undifferentiated carcinomas. This supports the contention that mutation of these oncogenes occurs at an early step in tumorigenesis. The predominant amino acid substitution in the differentiated tumours was glutamine to arginine at position 61 of Ha-ras or N-ras, but this mutation was not found in any of the undifferentiated tumours. It was noted that while transition mutations predominated in differentiated tumours (both benign and malignant), transversions were more common in the undifferentiated tumours.

Activated ras oncogenes in human thyroid cancers

Human thyroid epithelial (follicular) cells give rise to two malignant tumors--"follicular" carcinomas, which metastasize almost exclusively via the bloodstream, and "papillary" carcinomas, which metastasize predominantly via lymphatics (Williams, E. D. In: W. Duncan (ed.), Recent Results in Cancer Research: Thyroid Cancer, pp. 47-55. Berlin: Springer-Verlag, 1980). We have investigated whether this contrast in biological behavior might be associated with different patterns of oncogene activation. DNA transfection analysis of five follicular and ten papillary cancers indeed showed a statistically significant difference in the pattern of genes responsible, activated ras oncogenes being found in 80% of follicular tumors but only 20% of papillary tumors. In addition, in follicular cancers we have found activation of all three ras oncogenes (H-ras, K-ras, and N-ras), the first time that this has been demonstrated in a primary human tumor type (as opposed to cell lines). We suggest therefore that ras activation may be an important determinant of metastatic capability in these epithelial cancers.

N-myc expression in neoplasia of human thyroid C-cells

In view of the frequent reports of the increased expression of myc oncogenes in several neuroendocrine tumor types, we have investigated c- and N-myc expression in human medullary carcinoma, a malignant tumor derived from the neuroendocrine "C"-cell subpopulation of the thyroid gland. In situ nucleic acid hybridization was used to permit analysis not only of tumors but also of normal C-cells which form a tiny, scattered, minority of the thyroid epithelial cell population. N-myc expression was readily demonstrable in 6 of 21 tumor samples and c-myc in one case, whereas neither N- nor c-myc mRNA was ever detected in normal C-cells. We conclude that N-myc expression is a specific feature of C-cell tumors and is not merely a differentiation marker of their cell of origin. The data therefore strengthen the hypothesis that myc oncogene activation plays a role in neuroendocrine neoplasia.

RFLP for the human erb-A beta locus

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Ha-ras restriction fragment length polymorphisms in colorectal cancer

The possibility of an association between restriction fragment length polymorphisms (RFLPs) at the Ha-ras gene locus and susceptibility to develop colorectal cancer has been investigated. Leucocyte DNA from 46 carcinoma patients and 49 controls was analysed by Southern blotting to determine the size distribution of restriction fragments containing the variable tandem repeat 3' to the Ha-ras gene. Four predominant allelic fragments were found in both groups (in AvaII digests having sizes of 1.55, 2.0, 2.65 and 3.15 kilobases [kb]), together with a variety of 'rare' alleles (with individual frequencies less than 5%). The overall prevalence of rare alleles was not significantly different between cancer and control groups. The distribution of the common alleles, however, differed significantly. The combined frequency of the two larger alleles (a3 and a4) was approximately twice as high in the cancer group (34%) as in controls (18%) (P less than 0.025), which was reflected in a highly significant increase in the proportion of individuals carrying an a3 or a4 allele.