Características de las medidas cautelares

Two distinct classes of genes known as proto-oncogenes and tumour suppressor genes regulate the proliferation o f normal cells. Proto-oncogenes are growth-promoting genes whereas tumour suppressor genes inhibit cell growth. Mutations that potentiate the activities o f proto-oncogenes create the oncogenes that promote the growth o f tumour cells. Conversely, genetic lesions that inactivate suppressor genes liberate the cell from the growth constraints imposed by these genes, resulting in uncontrolled cell growth. Oncogenes are dominantly acting at the cellular level in that only one mutated copy of the gene causes tumourigenesis, whereas tumour suppressor genes are recessive and require both copies of the gene to be inactivated.

1.7.1 Oncogenes

Evidence for the existence of oncogenes has been obtained from the study of tumour-inducing retroviruses, chromosomal rearrangements in tumours and DNA transfection-transformation assays. There are two types o f tumour-inducing retroviruses. One group, the acutely transforming retroviruses, harbour viral oncogenes which are derived from the transduction of a cellular proto-oncogene. For example, the oncogene w-src encoding a tyrosine kinase and isolated from the Rous sarcoma virus (Martin, 1970), is derived from a normal gene of the chicken, which is the host o f this retrovirus (Stehelin et al., 1976). The other group are the chronically transforming retroviruses, which lack oncogenes but act via proviral integration in the host genome to disrupt cellular proto­ oncogene sequences and their transcriptional control. Included in this group are the proto­ oncogenes int\ (Nusse and Varmus, 1982) and in ti (Dickson et al., 1984) which are the

common integration sites in mouse mammary tumour virus induced breast tumours, and pim \ (Cuypers et al., 1984) the integration site in murine leukaemia virus induced T-cell

lymphomas.

Cytogenetic analyses have identified specific chromosomal rearrangements in several different kinds o f human cancer, many of which result in the activation o f proto­ oncogenes. The chromosome translocation t(9;22) occurring in chronic granulocytic leukaemia was one of the first reciprocal translocations to be characterised (Rowley, 1973). Analysis o f a somatic cell hybrid containing the derivative chromosome (known as the Philadelphia chromosome) as the only human component, revealed that the c-abl proto-oncogene on chromosome 9 had been translocated to chromosome 22, generating

a chimeric fusion protein which has elevated tyrosine kinase activity (de Klein et al., 1982; Kurzrock et al., 1988). In Burkitt's lymphomas, the c-myc proto-oncogene on chromosome 8q24 is translocated to one o f the immunoglobulin gene loci at 14q32, 2pl3 or 22ql 1 (Dalla-Favera et al., 1982; Taub et al., 1982). In addition, translocations involving the chromosomal region l l q l 3 have been found to be associated with the development of parathyroid adenomas (Arnold et al., 1989) and also B-cell lymphomas (Rosenberg et al., 1991) (discussed further in section 1.9.1).

Cellular oncogenes were also identified from DNA transfection-transformation assays, in which DNA from chemically transformed rodent cells (Shih et al., 1979), from human tumour cell lines and from primary human tumours (Krontiris and Cooper, 1981; Shih et al., 1981) was used to transfect normal cells in culture. Cellular transformation o f the transfected cells was identified by the appearance o f foci o f transformed cells on a background of non-transformed cells. In these experiments, it was usually activated ras genes which were responsible for the transformed phenotype although, other genes

including ret (Takahashi et al., 1985) and hst (Sakamoto et al., 1986) have been identified by this approach.

1.7.2 Tumour suppressor genes

Historically, three lines of evidence namely, somatic cell hybrid studies, the occurrence of familial cancers and loss o f heterozygosity in tumours, have supported the existence o f tumour suppressor genes whose products negatively regulate cell proliferation (Marshall, 1991).

The earliest indication that malignant transformation might involve loss o f gene function was provided by somatic cell hybrid studies. In these experiments, Harris et al. (1969) observed that when non-malignant cells from a mouse fibroblast line were fused with highly malignant mouse carcinoma or sarcoma cells, the hybrids were non- tumourigenic when injected into nude mice. Therefore, the normal fibroblasts were able to suppress the tumour phenotype. Human-human hybrids generated by fusing normal fibroblasts with malignant HeLa cells also demonstrated a similar suppression of malignancy (Stanbridge et al., 1976). However, some o f the hybrid cells showed a tendancy to re-express tumourigenicity and this was associated with a loss of chromosomes (Harris et al., 1969). Subsequent studies have shown that these revertants lose specific chromosomes (Stanbridge et al., 1981; Benedict et al., 1984) which support the theory that loss or inactivation o f a tumour suppressor gene allow tumourigenesis to proceed.

Tumour suppressor genes have been identified through the study o f inherited cancer syndromes. In 1971, Knudson put forward his two-hit hypothesis for the development of inherited malignancies. An epidemiological study of the rare childhood malignancy

retinoblastoma, revealed that the development o f bilateral tumours were more frequent in patients with the dominantly inherited form of the disease. Knudson proposed that these patients had inherited a germline mutation, the "first hit", which predisposed them to develop retinoblastoma. The "second hit" necessary for tumour development was a somatic mutation occuring in the cells o f the developing retina. In patients with no family history of the disease, tumour formation occurred as a result o f both the first and second hits occuring in the same retinal cell. Thus, the risk o f tumour development in individuals without the inherited mutation is very small because both hits have to coincide in the same somatic cell. The apparent paradox that retinoblastoma is a dominantly inherited disorder which is recessive at the cellular level i.e. both copies o f the gene must be mutated for tumour development, is e^glained by the fact that individuals who have inherited a germline mutation will develop a second mutation in at least one o f the cells in the developing retina due to the high frequency of somatic mutational events in dividing cells.

The retinoblastoma gene was the first tumour suppressor gene to be cloned. Initial localisation o f the gene to the chromosomal region 13ql4 was established by the rare cytogenetic observations of germline deletions of 13ql4 in hereditary retinoblastoma patients (Fran eke, 1978). More frequently, loss of heterozygosity (LOH) for polymorphic loci on chromosome 13 was observed in tumour DNA from patients with familial retinoblastoma (Cavanee et al., 1983; Dryja et al., 1986) and furthermore, the alleles which were lost were those inherited from the unaffected parent demonstrating this was the second hit. LOH analysis involves the comparison o f tumour and normal (usually leucocyte) DNA using polymorphic markers. Markers which demonstrate loss of alleles with the tumour DNA help to define the chromosomal region which contains a putative

tumour suppressor gene. The detection o f LOH has proved very useful in the mapping of a number of tumour suppressor genes, notably those involved with colorectal cancer (Solomon et al., 1987; Stanbridge, 1990), Wilms' tumour (Fearon et al., 1984; Koufos et al., 1984) and M EN l (Larsson et al., 1988; Friedman et al., 1989; Thakker et al., 1989).