Modelling the consequences of interactions between tumour cells.

Classical models of tumorigenesis assume that the mutations which cause tumours to grow act in a cell-autonomous fashion. This is not necessarily true. Sometimes tumour cells may adopt genetic strategies that boost their own replication and which also influence other cells in the tumour, whether directly or as a side-effect. Tumour growth as a whole might be enhanced or retarded. We have used mathematical models to study two non-autonomous strategies that tumour cells may use. First, we have considered the production by tumour cells of an angiogenesis growth factor that benefits both the cell from which it originates and neighbouring cells. Second, we have analysed a situation in which tumour cells produce autocrine-only or paracrine-only growth factors to prevent programmed cell death. In the angiogenesis model, stable genetic polymorphisms are likely to occur between cells producing and not producing the growth factor. In the programmed cell death model, cells with autocrine growth factor production can spread throughout the tumour. Production of paracrine-only growth factor is never selected because it is 'altruistic' (that is of no benefit to the cell that makes the growth factor), despite being potentially beneficial to tumour growth as a whole. No polymorphisms can occur in the programmed cell death model. Production of angiogenesis and other growth factors in tumours may be under stable genetic, rather than epigenetic, control, with implications for therapies aimed at such targets. Many of the mutations observed in tumours may have non-autonomous effects

Allele loss occurs frequently at hMLH1, but rarely at hMSH2, in sporadic colorectal cancers with microsatellite instability.

Mutations at the hMSH2 and hMLH1 mismatch repair loci have been implicated in the pathogenesis of colorectal cancer. Tumours with two allelic mutations at a mismatch repair locus develop replication errors (RERs). In the hereditary non-polyposis colorectal cancer (HNPCC) syndrome, one mutation is inherited and the other acquired somatically: in RER+ sporadic colorectal cancers, both mutations are somatic. RER+ tumours tend to have a low frequency of allele loss, presumably because they acquire most mutations through RERs. However, before a second mismatch repair mutation has occurred somatically, there is no reason to suppose that allele loss occurs less frequently in tumours that are to become RER+. Indeed, this second mutation might itself occur by allele loss. We have searched for allele loss at the hMSH2 and hMLH1 loci in RER+ and RER- sporadic colorectal cancers. Loss occurred at the hMLH1 locus in 7/17 (41%) RER+ tumours, compared with 6/40 (15%) RER- cancers (chi2=3.82, P approximately 0.05). At hMSH2, 2/22 RER+ sporadic cancers (9%) had lost an allele, compared with 2/40 (5%) RER- cancers (chi2=0.03, P>0.5). Taken together with previous studies which focused on colorectal cancers from HNPCC families, the data suggest that allele loss at hMLH1, but not at hMSH2, contributes to defective mismatch repair in inherited and sporadic colorectal cancer

CDX2 mutations do not account for juvenile polyposis or Peutz–Jeghers syndrome and occur infrequently in sporadic colorectal cancers

Peutz–Jeghers syndrome (PJS) and juvenile polyposis (JPS) are both characterized by the presence of hamartomatous polyps and increased risk of malignancy in the gastrointestinal tract. Mutations of the LKB1 and SMAD4 genes have been shown recently to cause a number of PJS and JPS cases respectively, but there remains considerable uncharacterized genetic heterogeneity in these syndromes, particularly JPS. The mouse homologue of CDX2 has been shown to give rise to a phenotype which includes hamartomatous-like polyps in the colon and is therefore a good candidate for JPS and PJS cases which are not accounted for by the SMAD4 and LKB1 genes. By analogy with SMAD4CDX2 is also a candidate for somatic mutation in sporadic colorectal cancer. We have screened 37 JPS families/cases without known SMAD4 mutations, 10 Peutz-Jeghers cases without known LKB1 mutations and 49 sporadic colorectal cancers for mutations in CDX2. Although polymorphic variants and rare variants of unlikely significance were detected, no pathogenic CDX2 mutations were found in any case of JPS or PJS, or in any of the sporadic cancers. © 2001 Cancer Research Campaign www.bjcancer.co

The APC variants I1307K and E1317Q are associated with colorectal tumors, but not always with a family history

Classical familial adenomatous polyposis (FAP) is a high-penetrance autosomal dominant disease that predisposes to hundreds or thousands of colorectal adenomas and carcinoma and that results from truncating mutations in the APC gene. A variant of FAP is attenuated adenomatous polyposis coli, which results from germ-line mutations in the 5' and 3' regions of the APC gene. Attenuated adenomatous polyposis coli patients have "multiple" colorectal adenomas (typically fewer than 100) without the florid phenotype of classical FAP. Another group of patients with multiple adenomas has no mutations in the APC gene, and their phenotype probably results from variation at a locus, or loci, elsewhere in the genome. Recently, however, a missense variant of APC (I1307K) was described that confers an increased risk of colorectal tumors, including multiple adenomas, in Ashkenazim. We have studied a set of 164 patients with multiple colorectal adenomas and/or carcinoma and analyzed codons 1263-1377 (exon 15G) of the APC gene for germ-line variants. Three patients with the I1307K allele were detected, each of Ashkenazi descent. Four patients had a germ-line E1317Q missense variant of APC that was not present in controls; one of these individuals had an unusually large number of metaplastic polyps of the colorectum. There is increasing evidence that there exist germ-line variants of the APC gene that predispose to the development of multiple colorectal adenomas and carcinoma, but without the florid phenotype of classical FAP, and possibly with importance for colorectal cancer risk in the general population

Rare genetic variants and the risk of cancer.

There are good reasons to expect that common genetic variants do not explain all of the inherited risk of the common cancers, not least of these being the relatively low proportion of familial relative risk that common cancer SNPs currently explain. One promising source of the unexplained risk is rare, low-penetrance genetic variants, a class that ranges from low-frequency polymorphisms (allele frequency < 5%) through subpolymorphic variants (frequency 0.1-1.0%) to very low frequency or 'private' variants with frequencies of 0.1% or less. Examples of rare cancer variants include breast cancer susceptibility loci CHEK2, BRIP1 and PALB2. There are considerable challenges associated with the discovery and testing of rare predisposition alleles, many of which are illustrated by the issues associated with variants of unknown significance in the Mendelian cancer predisposition genes. However, whilst cost constraints remain, the technological barriers to rare variant discovery and large-scale genotyping no longer exist. If each individual carries many disease-causing rare variants, the so-called missing heritability of cancer might largely be explained. Whether or not rare variants do end up filling the heritability gap, it is imperative to look for them along side common variants

Failure of programmed cell death and differentiation as causes of tumors: some simple mathematical models.

Most models of tumorigenesis assume that the tumor grows by increased cell division. In these models, it is generally supposed that daughter cells behave as do their parents, and cell numbers have clear potential for exponential growth. We have constructed simple mathematical models of tumorigenesis through failure of programmed cell death (PCD) or differentiation. These models do not assume that descendant cells behave as their parents do. The models predict that exponential growth in cell numbers does sometimes occur, usually when stem cells fail to die or differentiate. At other times, exponential growth does not occur: instead, the number of cells in the population reaches a new, higher equilibrium. This behavior is predicted when fully differentiated cells fail to undergo PCD. When cells of intermediate differentiation fail to die or to differentiate further, the values of growth parameters determine whether growth is exponential or leads to a new equilibrium. The predictions of the model are sensitive to small differences in growth parameters. Failure of PCD and differentiation, leading to a new equilibrium number of cells, may explain many aspects of tumor behavior--for example, early premalignant lesions such as cervical intraepithelial neoplasia, the fact that some tumors very rarely become malignant, the observation of plateaux in the growth of some solid tumors, and, finally, long lag phases of growth until mutations arise that eventually result in exponential growth

Rare genetic variants and the risk of cancer.

There are good reasons to expect that common genetic variants do not explain all of the inherited risk of the common cancers, not least of these being the relatively low proportion of familial relative risk that common cancer SNPs currently explain. One promising source of the unexplained risk is rare, low-penetrance genetic variants, a class that ranges from low-frequency polymorphisms (allele frequency < 5%) through subpolymorphic variants (frequency 0.1-1.0%) to very low frequency or 'private' variants with frequencies of 0.1% or less. Examples of rare cancer variants include breast cancer susceptibility loci CHEK2, BRIP1 and PALB2. There are considerable challenges associated with the discovery and testing of rare predisposition alleles, many of which are illustrated by the issues associated with variants of unknown significance in the Mendelian cancer predisposition genes. However, whilst cost constraints remain, the technological barriers to rare variant discovery and large-scale genotyping no longer exist. If each individual carries many disease-causing rare variants, the so-called missing heritability of cancer might largely be explained. Whether or not rare variants do end up filling the heritability gap, it is imperative to look for them along side common variants

Chromosome 11q in sporadic colorectal carcinoma: patterns of allele loss and their significance for tumorigenesis.

AIMS: To analyse the frequency of loss of heterozygosity (allele loss, LOH) in a large sample of colorectal carcinomas using highly informative markers along chromosome 11q. METHODS: One hundred paired samples of colorectal cancer and normal tissue were genotyped at six microsatellite markers on chromosome 11q (cen-D11S1313-D11S901-DRD2/NCAM-D11S29- D11S968-tel). The high levels of heterozygosity at these markers allow allele loss to be determined in about 80% of cases at any one locus. The frequency of replication errors (RERs, microsatellite instability) has also been determined. RESULTS: LOH was found at frequencies of 25% and 29% at the distal D11S968 (11qter) and D11S29 (11q23.3) loci, slightly above the accepted baseline of 0-20%. Allele loss at NCAM, DRD2, D11S901, and D11S1313 was not raised above baseline levels. The probable genetic mechanism of allele loss--chromosomal non-disjunction, mitotic recombination, deletion, or gene conversion--seemed to vary between tumours and no consistent mechanism of mutation was found. Microsatellite instability was found in 23 (23%) tumours. No associations were found between LOH and clinical data (patient sex, age at presentation, tumour site, and Duke's stage). CONCLUSIONS: Although gene(s) on 11q may have a role in the development of a minority of colorectal carcinomas, this study provides evidence against the general importance of allele loss on chromosome 11q in the pathogenesis of colorectal cancer. The results also have implications for the importance of 11q in other cancers: it seems less likely that a single tumour supressor gene at this location promotes the growth of all types of tumour when lost. Rather, one or more genes with tissue specific effects may be involved