Mutation From Proto-Oncogenes to Oncogenes

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Although retroviruses can induce tumour development in animals, only a few instances are known of human proto-oncogenes being mutated into oncogenes by retroviral insertion. Nevertheless, various forms of genetic mutation and alteration can convert a human proto-oncogene into an oncogene. Three main mechanisms have been identified: chromosomal translocation, gene amplification, and point mutation.

Chromosomal translocation has been linked to several types of human leukemias and lymphomas. Through chromosomal translocation one segment of a chromosome breaks off and is joined to another chromosome. As a result of such an event, two separate genes can be fused. In some cases the newly created gene leads to tumour development. Such is the case with the so-called Philadelphia chromosome, the first translocation to be linked to a human cancer—chronic myelogenous leukemia. The Philadelphia chromosome is found in more than 90 percent of patients with chronic myelogenous leukemia. This well-known example of translocation involves the fusion of a proto-oncogene called c-ABL, which is located on chromosome 9, to a site on chromosome 22 known as a breakpoint cluster region (BCR). BCR and the c-ABL gene produce a hybrid oncogene, BCR-ABL, which produces a mutant protein that aberrantly regulates cellular proliferation. The exact mechanism by which the newly created BCR-ABL protein gives rise to leukemia is not yet understood.

Sometimes translocations do not generate a new gene but instead place an intact gene under the control of a regulatory element that normally acts on another gene. This situation occurs in about 75 percent of cases of Burkitt lymphoma. In the cells of patients with this cancer, a proto-oncogene called c-MYC is moved from its site on chromosome 8 to a site on chromosome 14. In its new location the c-MYC gene is positioned next to the switch signal, or promoter region, for the immunoglobulin G gene. As a result, the MYC protein encoded by the c-MYC gene is produced continuously.

Gene amplification is another type of chromosomal abnormality exhibited by some human tumours. It involves an increase in the number of copies of a proto-oncogene, an aberration that also can result in excessive production of the protein encoded by the proto-oncogene. Amplification of the N-MYC proto-oncogene is seen in about 40 percent of cases of neuroblastoma, a tumour of the sympathetic nervous system that commonly occurs in children. The higher the copy number of the N-MYC gene, the more advanced the disease. Amplification of the proto-oncogene c-ERBB2 (HER2) is seen in some breast cancers.

Another mechanism by which a proto-oncogene can be transformed into an oncogene is point mutation. To understand what a point mutation is, it must first be explained that DNA molecules — and hence the genes found along their length — are composed of building blocks called nucleotide bases. A proto-oncogene may be converted into an oncogene through a single alteration of a nucleotide. This alteration may be the deletion of a base, the insertion of an extra base, or the substitution of one base for another. Point mutations also can be caused by radiation or chemicals that disrupt the DNA. However, regardless of the type or cause of such a mutation, it usually changes the amino acid sequence of the encoded protein and thus alters protein function.

A point mutation can increase protein function — as occurs with the ras family of proto-oncogenes — or it can interrupt protein synthesis so that little or no protein is made. Point mutations are common mechanisms of inactivation of tumour suppressor genes.

Tumour suppressor genes, like proto-oncogenes, are involved in the normal regulation of cell growth; but unlike proto-oncogenes, which promote cell division and differentiation, tumour suppressors restrain them. If proto-oncogenes are the accelerators of cell growth, tumour suppressor genes are the brakes.

Just as the term oncogene is somewhat misleading because it suggests that the main function of the gene is to cause cancer, the name tumour suppressor gene wrongly suggests that the primary function of these genes is to stem tumour growth. This terminology has to do with the history of their discovery; loss of function of these genes was seen in practically all tumours, and restoration of their function inhibited tumour growth.

Unlike proto-oncogenes, which require that only one copy of the gene be mutated to disrupt gene function, both copies (or alleles) of a particular tumour suppressor gene must be altered to inactivate gene function. In many tumours one copy of a tumour suppressor gene is mutated, producing a gene product that cannot work properly, and the second copy is lost by allelic deletion.

Two of the most studied tumour suppressor genes are RB and p53. The RB gene is associated with retinoblastoma, a cancer of the eye that affects 1 in every 20,000 infants. The gene also is associated with bone tumours (osteosarcomas) of children and cancers of the breast, prostate, lung, uterine cervix, and bladder in adults. The p53 gene, which is named for the molecular weight of its protein product (53 kilodaltons), is the most commonly mutated gene in tumours. Practically every person who inherits a mutated copy of a tumour suppressor gene will develop some form of cancer.