Allan Balmain, PhD

Genetic dissection of the mechanisms of invasion and metastasis.
Tumours develop in both mice and humans as a consequence of sequential genetic alterations at critical genomic loci. These changes involve mutations in oncogenes such as ras, and in tumour suppressor loci, including the p53and p16 genes. Our strategy has been to investigate the genetic alterations which take place during mouse skin tumour development, and subsequently to use both transgenic and knock-out mice to test the functions of candidate genes in vivo. Genes implicated in multistage carcinogenesis include H-ras, p53 and Transforming Growth Factor Beta (TGFß). Mice expressing TGFßdevelop fewer papillomas than non-transgenic littermates, but have elevated rates of tumour progression. This indicates that TGFß can act either positively or negatively at different stages of carcinogenesis. We have proposed that TGFß can act as a negative regulator of cell cycle progression during the early phase of tumorigenesis, but positively induces invasion and angiogenesis during progression to malignancy. This possibility is supported by studies involving transfection of dominant negative TGFß type II receptor constructs into invasive carcinoma cells. This results in suppression of the invasive phenotype and the restoration of expression of some adhesion proteins such as E-cadherin. We have shown that TGFß signalling can also be upregulated by genetic alterations in highly malignant spindle carcinoma cells. Many of the features characteristic of the invasive, metastatic phenotype of these cells can be reversed by the introduction of dominant negative interfering mutants that disrupt Smad signalling. Thresholds of Smad activity are important at distinct stages of tumor progression, cooperating with activated ras to induce invasion or dissemination to distant sites (refs 1-4).

Identification of Genetic Modifiers of Cancer Susceptibility
Another class of genes that are important in tumor development are those that control genetic predisposition. Studies of mouse models of human cancer have demonstrated the existence of multiple tumor modifiers that influence important parameters of cancer susceptibility such as tumour multiplicity, size, or the probability of malignant progression. It is highly likely that such tumor modifiers also segregate in humans, and play a major role in determining cancer predisposition or the outcome of therapeutic intervention. In previous work on skin carcinogenesis, we have mapped at least ten independent tumor resistance loci in the spretus genome. Some of these affect the early development of tumors, while others influence primarily tumor progression. A specific subset of three of these modifier loci dramatically prolongs the survival of tumor-bearing mice, suggesting that they modify either tumor growth rate or the ability to invade and metastasize. Candidate genes have been identified for several of these loci and further studies on their identification are in progress (refs 5-8). Additional experiments are in progress to map modifiers of susceptibility to mouse tumors of the lung, prostate and lymphoid system, as well as to investigate candidate human tumor modifiers using DNA samples from human cancer patients and control populations.