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Gain-of-function mutations in the gene encoding the receptor tyrosine kinase RET have been identified as the aetiological factor for multiple endocrine neoplasia type 2A (MEN2A). MEN2A is a dominantly-inherited cancer predisposition syndrome characterized by medullary thyroid carcinoma, a tumour of the calcitonin-producing thyroid C-cells. There are three isoforms of RET: RET9, RET43 and RET51, and although in vitro evidence suggests they vary in cellular transformation activities, little is known about their function in tumorigenesis in vivo. To address this, we used RET51 cDNA to construct mice in which the most frequent MEN2A mutation, Cys-634-Arg, was expressed under the control of the human calcitonin promoter (CT-2A mice). These mice developed C-cell tumours resembling human MTC and follicular tumours resembling human papillary thyroid carcinoma (PTC) depending on the founder line examined. One founder line developed compound MTC/PTC at low frequency (8%) and pancreatic cystadenocarcinoma. CT-2A mice also displayed a developmental defect in thyroid follicular structure, in which much of the thyroid was occupied by large irregular cystic follicles thought to be derived from the ultimobranchial body, a developmental precursor of the thyroid gland. The CT-2A mice will provide a suitable model to further study the effects of the MEN 2A RET mutation in vivo.

Mice deficient for the mismatch repair (MMR) gene Msh2 show accelerated tumourigenesis and a reduced apoptotic response to DNA damage of methylation type. Here we examine the effect of mutation for Msh2 on in vivo mutation frequencies in the intestine as determined by loss of function at the Dolichos biflorus (Dlb-1) locus. Spontaneous mutation frequencies were scored in cohorts of ageing mice either wild type or mutant for Msh2. In mice less than 1 year old, mutation frequencies were only elevated in Msh2 null mice. However, beyond this age heterozygous Msh2 mice showed significantly higher mutation frequencies than controls. These findings implicate a gene dose dependent requirement for Msh2 in mutation suppression and prompted an analysis of young Msh2 mutants following exposure to DNA damage. Following exposure to N-methyl-N'-nitro-N-nitrosoguanidine (MNNG), Msh2 deficient mice show a reduced apoptotic response and an increase in mutation frequency. Heterozygotes did not differ from controls. Following exposure to cisplatin, no significant elevation was seen in mutation frequencies, even within homozygotes. This is particularly surprising given the association between cisplatin resistance and MMR deficiency. These findings therefore demonstrate a complex reliance upon functional Msh2 in mutation surveillance. We have identified three separate scenarios. First, where retention of both Msh2 alleles over an extended period of time appears critical to the suppression of spontaneous mutation; second, 3 weeks following exposure to MNNG, where only complete loss of Msh2 results in elevated mutation; and finally following cisplatin exposure, where induced levels of mutation are independent of Msh2 status.

Deficiency in genes involved in DNA mismatch repair increases susceptibility to cancer, particularly of the colorectal epithelium. Using Msh2 null mice, we demonstrate that this genetic defect renders normal intestinal epithelial cells susceptible to mutation in vivo at the Dlb-1 locus. Compared with wild-type mice, Msh2-deficient animals had higher basal levels of mutation and were more sensitive to the mutagenic effects of temozolomide. Experiments using Msh2-deficient cells in vitro suggest that an element of this effect is attributable to increased clonogenicity. Indeed, we show that Msh2 plays a role in the in vivo initiation of apoptosis after treatment with temozolomide, N-methyl-N'-nitro-N-nitrosoguanidine, and cisplatin. This was not influenced by the in vivo depletion of O6-alkylguanine-DNA-alkyltransferase after administration of O6-benzylguanine. By analyzing mice mutant for both Msh2 and p53, we found that the Msh2-dependent apoptotic response was primarily mediated through a p53-dependent pathway. Msh2 also was required to signal delayed p53-independent death. Taken together, these studies characterize an in vivo Msh2-dependent apoptotic response to methylating agents and raise the possibility that Msh2 deficiency may predispose to malignancy not only through failed repair of mismatch DNA lesions but also through the failure to engage apoptosis.

Comparison of results derived from transgenic animal gene-mutation assays with those from mutation analyses in endogenous genes is an important step in the validation of the former. We have used lambda lacZ transgenic mice to study alkylation-induced mutagenesis in vivo in (a) lacZ and hprt in spleen cells, and (b) lacZ and Dlb-I in small intestine from lambda lacZ+/0/Dlb-Ia/b mice. Induction of mutations by ethyl- and methylnitrosourea (ENU, MNU) and ethyl methanesulphonate (EMS) was investigated at 7 weeks after a single i.p. dose of each of these chemicals. In the small intestine, treatment with various dosages of ENU (10-150 mg/kg) resulted in a linear dose-response in both lacZ and Dlb-I. MNU (30 mg/kg) was also mutagenic in lacZ and Dlb-I, while EMS (250 mg/kg) did not significantly induce mutations in either gene. In spleen, ENU gave a linear dose-related response in both lacZ and hprt, MNU induced mutation sin both lacZ and hprt, and EMS was only positive for lacZ. No differences in response were observed between single and split-dose treatment with ENU (1 x 50 or 5 x 10 mg/kg with a 1- or 7-day interval), both in spleen and small intestine, except for lacZ in small intestine, where the single high dose gave a significantly higher induction than the split dose with the 7-day interval. The overall results suggest that mutagenic effects of fractionated doses are generally additive. In most cases, the induction factor (ratio treated over controls) for mutations in lacZ was lower than that for hprt and Dlb-I, presumably due to a higher background in lacZ and/or a lower mutability of lacZ. The general concordance between the data for lacZ and the endogenous genes indicates that lambda lacZ transgenic mice are a suitable model to study induction of gene mutations in vivo.

The ability to process damaged DNA may vary between cells depending on their differentiated status. However, there is little in vivo data available and it is not intuitively obvious how the activity of specific repair pathways may vary between different subpopulations (e.g. stem cells and proliferative, committed and differentiated cells) of a particular tissue. To obtain such information for the intestinal epithelium, we have developed an assay that detects differences in the way different regions of the crypt (stem, proliferative and maturation zones) respond to DNA damage. The assay is a variant of the 'comet' assay, which detects DNA strand breaks by measuring the proportion of DNA migrating from individual cells, or in this case intact isolated crypts, in an electrophoretic field. The method is quantitative, with the amount of migrating DNA being proportional to the number of strand breaks. Isolated crypts are repair competent and spatial differences are apparent with some agents. The assay has the potential to characterize the repair properties of cells at different stages of differentiation within the crypt, determine the characteristics that might predispose them to damage and may help in understanding the route of stem cell mutation.

Normal function of the p53 gene is integral to the cellular response to genotoxic stress. One prediction arising from this is that p53 deficiency results in an increased mutation frequency. However, limited evidence has been produced in support of this idea. In order to further investigate the in vivo role of p53 in surveillance against mutation, and particularly to address the significance of p53-dependent apoptosis, we scored mutation frequency at the Dlb-1 locus within cells of the intestinal epithelium of animals which were wild type, heterozygous or null for p53 and heterozygous (a/b) at the Dlb-1 locus. Using this assay we have shown that loss of a p53-dependent apoptotic pathway is associated with the detectable acquisition of mutations, but only at high levels of DNA damage. These results question the significance of the immediate 'wave' of p53-dependent apoptosis seen in this tissue, particularly as there was a delayed p53-independent apoptotic pathway. We conclude that loss of p53 function only becomes relevant to the in vivo acquisition of mutations and thus tumorigenesis in certain circumstances.

Mutations in long lived stem cells are critical events in carcinogenesis. The Dlb-1 assay detects intestinal stem cell mutation at the Dlb-1 locus in Dlb-1a/b heterozygous mice by visualizing mutated clones of epithelial cells in situ which do not bind the lectin Dolichos biflorus agglutinin. We have used this assay to show that the food-derived heterocyclic amine 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) is a potent intestinal mutagen when administered either i.p. or p.o. This contrasts with the inactivity of the structurally related mutagen 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline in the assay which we have described previously. Immunocytochemical localization of the P-450 enzyme CYP1A2, which is responsible for the primary activation of these mutagens, shows that in untreated mice it is present in liver hepatocytes and in occasional villus epithelial cells but is absent from the target intestinal stem cell population. In addition, liver microsomes, unlike intestinal microsomes, were able to convert PhIP to the proximate mutagen N-hydroxy-PhIP. CYP1A2 immunoreactivity in beta-napthoflavone-induced animals was elevated in liver hepatocytes and increased to a lesser extent in duodenal villus epithelial cells. Treatment with beta-napthoflavone produced an unexpected 46% decrease in the number of Dlb-1 mutations in response to PhIP. Following treatment with PhIP, there was no difference in the number of Dlb-1 locus mutations between the proximal and distal ends of the small intestine in uninduced animals, indicating that the bile duct is unlikely to be responsible for transport of mutation inducing metabolites of PhIP to the small intestine. Our results demonstrate that metabolic activation of an indirect acting genotoxic agent can occur at a site other than the target tissue, and absence of the enzymes required for activation of a mutagen does not necessarily protect that tissue from its genotoxic effects.

The ability of four mutagenic/carcinogenic chemicals administered as single doses to induce a programmed form of cell death (apoptosis) in the BDF1 mouse large bowel was studied and compared with a previous study on the small intestine using the same mice. The number of apoptotic cells was counted following treatment with the direct-acting agents N-nitroso-N-methylurea (NMU) and N-nitroso-N-ethylurea (NEU) and two agents which require metabolic activation 1,2-dimethylhydrazine (DMH) and N-nitrosodimethylamine (NDMA). DMH (80 mg/kg) was the most effective at inducing acute cell death and this was closely followed by NMU (200 mg/kg). The least effective agent in the large bowel was NDMA. The peak yield of apoptosis occurred between 4 h (NEU) and 8 h (DMH) after treatment. An analysis of the changing shapes of the frequency plots of apoptosis at each cell position in the crypt at various times after exposure permits an estimate to be made of the position in the crypt of the primary target cells for the cytotoxic action at time t = 0. For the agents studied, this is in the range of the 5th to the 10th position from the base of the crypt. This distribution for the target cells for apoptotic cell death is not coincident with that for the presumptive stem cells, which is at cell position 1 or 2. Comparisons with results previously obtained in the small intestine (ileum) of the same mice show that the relative cytotoxic effectiveness of the four agents differs. Furthermore, the position of the target cells is at about the 4th position from the bottom of the crypt in the ileum, and here the distribution is coincident with that presumed for the stem cells. Our interpretation of the data is that damaged cells in the stem cell region of the small bowel are removed by the activation of a cell suicide programme, which effectively removes potentially harmful genetic alterations. In contrast, in the large bowel, cell death is not initiated particularly strongly in the stem cell region but tends to occur higher in the crypt. The absence of this selective deletion process may result in the perpetuation of deleterious mutations in the colonic stem cell population and this may explain in part, the higher incidence of cancers observed in the large bowel.

We have identified a minor population of crypts in small intestine which do not appear to export cells to villi. These crypts can be observed in whole-mounts of small intestine prepared from C57BL/6J<-->SWR mouse aggregation chimaeras stained with a peroxidase conjugate of the lectin Dolichos biflorus agglutinin (DBA-Px). In preparations where by chance the C57BL/6J epithelium (positive staining) forms only a minor component of the chimaera, occasional crypts occur which are isolated from larger patches of C57BL/6J epithelium and are surrounded by SWR (non-staining) epithelium. Fifty-one of 383 isolated C57BL/6J crypts (13%) did not appear to export cells to villi, although the crypt mouth is surrounded by a small patch of C57BL/6J epithelium on the intervillus gut floor.

Two direct-acting mutagens, N-nitroso-N-methylurea (NMU) and N-nitroso-N-ethylurea (NEU), and two agents requiring metabolic activation, 1-2-dimethylhydrazine (DMH) and N-nitrosodimethylamine (NDMA), were administered i.p. to mice. Sections of crypts of the small intestine were assayed for acute histological cell death at various times up to 12 h after treatment. Dead or dying cells exhibited the typical light microscopic morphological features of apoptosis. The incidence of apoptosis at each cell position along the side of longitudinal crypt sections was recorded and frequency plots of the incidence against cell position were determined. NEU (50 mg/kg) produced the highest incidence of cell death but this was closely followed by NDMA (50 mg/kg) and NMU (200 mg/kg). DMH (40 or 80 mg/kg) was the least cytotoxic but even here significantly elevated levels of cell death were observed. The highest incidence of cell death occurred 4-5 h after treatment with NEU, NMU and DMH and at 6 h after NDMA. The data obtained at 4 h after NEU suggest that approximately 22 cells out of a total crypt population of 250 cells are killed, but that for some cell positions near the crypt base (stem cell regions) up to 24% of the cells may be killed. Analysis of the changing shape of the frequency plots with time after treatment enabled the target cell position in the crypts for cytotoxicity to be estimated. This was at cell position 4 for NEU, NMU and DMH and at cell position 5 for NDMA. The stem cells in the crypts are believed to be located at the fourth cell position and hence at least NEU, NMU and DMH are targeting the stem cells with some specificity.

The ability of three model carcinogens, 1,2-dimethylhydrazine, dimethylnitrosamine, and 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline, to induce mutation in a novel in vivo assay in mouse intestine has been examined. The assay is based on mutations at the Dlb-1 locus which determines the tissue specific pattern of expressio of the binding site for the lectin Dolichos biflorus agglutinin. In C57BL/6J x SWR F1 mice Dlb-1 mutants are recognized as clones of epithelial cells not staining with a peroxidase conjugate of D. biflorus agglutinin. Chronic administration of 1,2-dimethylhydrazine (20 mg/kg/week s.c. for 10 weeks) induced Dlb-1 mutants, whereas administration of a single dose did not. Similarly, chronic dimethylnitrosamine treatment p.o. (0.001% in drinking water for 8 weeks) induced Dlb-1 mutants, but acute administration did not. In contrast, neither chronic nor acute treatment of the mice with 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline induced Dlb-1 mutations. The activities of 1,2-dimethylhydrazine, dimethylnitrosamine, and 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline in the Dlb-1 assay more accurately reflect their carcinogenic potential than do many in vitro bioassays.

Electron microscopy was used to investigate the changes induced by bilateral adrenalectomy (ADX) in rat hepatocytes. Animals were killed at 1 week post-ADX, either with or without a prior 24 h fast. Further studies were carried out at 4 to 5 weeks after ADX to investigate the effect of administering deoxycorticosterone acetate (DOCA) for 4 weeks on the ADX-induced alterations. At 1 week, fasting had a profound effect on the response of hepatocytes to ADX in that either fasting or ADX alone caused proliferation of smooth endoplasmic reticulum (ER) and reduced glycogen but did not alter the rough ER, while ADX together with fasting resulted in the total depletion of glycogen, the virtual absence of smooth ER and the disaggregation of the rough ER into single cisternae. In contrast to the changes seen at 1 week after ADX in unfasted animals, at 4 weeks the hepatocytes contained increased glycogen in large, dense areas and had a scanty smooth ER, suggesting that some endogenous source of steroid hormones active in carbohydrate metabolism becomes available in male ADX rats by this time. This interpretation was consistent with the finding that DOCA-treatment of ADX rats prevented the glycogen accumulation seen at 4 weeks and also induced some smooth ER proliferation.

A quantitative electron microscope study was made of the effects of bilateral adrenalectomy (ADX) and deoxycorticosterone acetate (DOCA) on the state of aggregation of hepatocyte rough ER cisternae in both untreated and 3'MeDAB-fed Leeds strain rats. Disaggregation of hepatocellular rough ER appears to be a common response of the rat liver to hepatocarcinogenic insult, while ADX and DOCA treatment are known to inhibit the chemical induction of neoplasia in this species. In untreated animals both ADX and DOCA significantly increased the mean number of cisternae per array or stack, while an even more pronounced effect was obtained from a combination of the two. The carcinogen 3'MeDAB induced a highly significant loss of aggregation, which was prevented by the combination of ADX and DOCA. In a separate experiment, a single dose of cortisone acetate was also found to partially reverse 3'MeDAB-induced rough ER disaggregation. In the rat hepatocyte, rough ER stacking or aggregation appears to be at least partially under hormonal control.

We have previously shown that the epithelium of each adult intestinal crypt in chimaeric mice is derived from a single progenitor cell. Whether the crypts are monoclonal from the outset-that is, are formed by the proliferation of a single cell-or whether their formation is initiated by several cells was not known. Here we report that many crypts contain cells of both chimaeric genotypes in the neonatal period indicating a polyclonal origin at this stage of morphogenesis. The cellular organization of the early neonatal crypt is therefore different from that of the adult crypt, which includes a zone of 'anchored' stem cells above the crypt base. Within 2 weeks, however, the crypt progenitor cell and its descendants displace all other cells from the crypt and the crypt attains monoclonality. The distribution of enterocytes on chimaeric villi in the neonate shows a mottled pattern of mosaicism which is progressively replaced by coherent sheets of cells from the crypts, and within two weeks the orderly adult clonal pattern is established.

A new method has been developed for the quantitative analysis of an ultrastructural change in hepatocyte rough endoplasmic reticulum (ER) that is induced by liver carcinogens. Hitherto only subjective observations of this alteration had been made. Male inbred Leeds rats were fed a diet containing 0.06% of the carcinogenic azo dye 3'-methyl-4-dimethyl-aminoazobenzene (3'MeDAB). Groups of treated rats, together with untreated controls, were sacrificed after 10 days, 4 weeks, 10 weeks and 17 weeks. Samples of liver tissue from each animal and from 10 3'MeDAB-induced hepatocellular carcinomas (HCC), were examined by electron microscopy. A quantitative study was carried out to investigate the effect of chronic exposure to 3'MeDAB upon the state of aggregation of the hepatocyte rough ER into parallel arrays. As early as 10 days after the start of treatment, the rough ER showed a highly significant degree of disaggregation: the mean number of ER cisternae per array fell from the control value of 6.20 to 3.73. This change was sustained throughout the experiment. At 17 weeks, comparison of the mean array size in the HCC cells with that in the surrounding hepatocytes revealed a further significant decline, from 3.46 to 2.12. Further groups of rats were fed other azo dyes for 4 weeks and subjected to the same assay. The carcinogens 4'-methyl-4-dimethylaminoazobenzene (4'MeDAB) and N,N-dimethyl-4-amino-N-acetyl-N-monomethyl-4-aminoazobenzene (DAAMAB) resembled 3'MeDAB with respect to the degree of rough ER disaggregation they induced. In contrast, the non-carcinogen 3'-trifluoromethyl-4-dimethylaminoazobenzene (3'TFMeDAB) had no such effect, while the weak initiator 2-methyl-4-dimethylaminoazobenzene (2MeDAB) induced disaggregation to a lesser degree than the strong carcinogens. At least with the azo dyes used in this study, the extent of rough ER disaggregation appears to be related to hepatocarcinogenesis.

A method for the quantitative analysis of the degree to which hepatocyte rough endoplasmic reticulum (ER) is arranged into parallel arrays was used to study the effect of fasting on rough ER aggregation in rat liver cells following either bilateral adrenalectomy or administration of the carcinogenic azo dye 3'-methyl-4-dimethylaminoazobenzene (3'MeDAB). One group of male inbred Leeds strain rats was subjected to bilateral adrenalectomy (ADX): after 1 week half of the animals were fasted for 24 h whereupon the whole group was killed. A second group of rats, fed for 4 weeks on a diet containing 0.06% of the carcinogenic azo dye 3'MeDAB, was similarly divided into two groups that were killed either with or without a prior 24-h fast. Untreated control groups of rats, both fasted and unfasted, were also killed. A quantitative electron microscope study was carried out to investigate the effect of each treatment on the degree to which the hepatocyte rough ER was aggregated into parallel arrays. ADX alone was without effect but caused a dramatic fall in rough ER aggregation when combined with fasting. At least as great an effect was induced by 3'MeDAB, with or without fasting, while fasting alone had a significant but much more modest effect than either ADX or the carcinogen. Thus, two disparate treatments induced morphologically identical responses in hepatocyte rough ER. The implications of this are discussed in terms of known interrelations between glucocorticoids and chemical carcinogenesis in the rat liver.

We have previously proposed that an in vivo mutagenicity assay could be based on the detection of mutations affecting the Dlb-1 locus in the small intestine of C57BL/6J x SWR F1 mice. F1 mice are heterozygous Dlb-1b/Dlb-1a and have only a single allele (Dlb-1b) which specifies expression of the binding site for the lectin Dolichos biflorus agglutinin (DBA) in intestinal epithelia. In whole-mount preparations of small intestine stained with a DBA--peroxidase conjugate, mutated stem cells and their progeny can be recognized as ribbons of unstained cells against a stained background. These DBA-negative ribbons can be quantified. The present investigation characterizes the responses of F1 mice to high- and low-dose rate gamma radiation (1.8 and 0.01 Gy/min respectively) and shows that the induction of ribbons is dose dependent in both cases. Dose sparing is evident at the lower dose rate: 20 ribbons/10(4) villi can be induced by 4 Gy at high-dose rate and by 7.1 Gy at low-dose rate, a dose sparing of 1.76. Age-matched untreated mice had a background level of 3.7 ribbons/10(4) villi. As expected, no ribbons were observed in homozygous (Dlb-1b/Dlb-1b) C67BL/6J mice, in which each stem cell has two alleles specifying lectin binding. The results further validate this system as a sensitive in vivo mutagenicity assay and suggests that the target cells are capable of repairing radiation damage.