BIOCHEMISTRY: A TEXTBOOK FOR HIGHER EDUCATION - E. S. Severin - 2004

CHAPTER 16. ONCOGENESIS

IV. Mechanisms of Neoplastic Transformation

It is currently established that more than 100 different genes and approximately 10 tumor suppressor genes are involved in the Regulation of Cell growth and differentiation. Malignant transformation is not the result of a single event. Before a malignant cell emerges, it undergoes 5 to 7 stages that induce alterations in The Cell's genetic apparatus (the multistep carcinogenesis hypothesis).

A. Conversion of Proto-oncogenes into Oncogenes

Currently, five main mechanisms have been identified by which proto-oncogenes are converted into oncogenes, either through damage to Gene Structure or by altering the level of expression.

Incorporation of New Promoters into Genomic DNA

It has been noted previously that the genomes of DNA- and Introduction/7.html">RNA-containing Viruses integrate with the host cell DNA in the form of proviruses. It is known that proviral DNA features long terminal repeats (LTRs) at both ends, which act as METABOLISM/31.html">Transcription promoters (Fig. 16-10). For instance, after chicken B-lymphocytes are infected by certain avian leukosis viruses, proviruses sometimes integrate near the c-myc gene. As a result, the myc gene is activated, its transcription increases with The production of significant amounts of myc-mRNA, followed by enhanced Translation.

Class="center">Fig. 16-10. Appearance of new promoters in The Human Genome upon insertion of viral genetic material into the DNA molecule.

Appearance of New Enhancer Sequences

In A number of cases, a provirus integrates into the DNA molecule either upstream or downstream of the c-myc gene, or it may be oriented in the opposite direction; nevertheless, the c-myc gene becomes activated. This effect indicates that the provirus contains enhancer sequences that upregulate Gene Expression (Fig. 16-11).

Fig. 16-11. Insertion of a provirus containing an enhancer sequence into the human genome.

Gene Amplification

This phenomenon has been observed in a number of tumors. It has been shown that the amplification of c-ras oncogenes plays a significant role in tumor progression toward higher malignancy. Furthermore, Treatment with the antitumor drug methotrexate (a Dihydrofolate Reductase inhibitor) induces more than a 400-fold amplification of the dihydrofolate reductase gene during therapy, thereby reducing the sensitivity of tumor Cells to the drug. Cytological examination reveals that amplified genes can be detected as homogeneously staining regions on Chromosomes lacking centromeres.

Point Mutations

The c-ras oncogene, initially discovered in certain Retroviruses, encodes a protein with a Molecular Weight of 21 kD, designated as the P21 protein. Analysis of DNA sequences of the c-ras proto-oncogene from normal cells and the c-ras oncogene from a human Gallbladder tumor demonstrated that these genes differ by a single base pair, and their corresponding Proteins have different Amino Acids at position 12 of P21. Similar results were obtained when investigating The structure of the c-ras oncogene from other human tumors. The outcome in all cases was identical: a single missense mutation was found in the oncogene structure, although THE POSITION OF the mutation within the gene could vary. Nevertheless, this mutation altered the conformation of the encoded protein and reduced its GTPase activity.

The mutant protein caused prolonged stimulation of adenylate cyclase, an increase in intracellular cAMP concentration, and activation of cAMP-dependent protein Kinases.

Mutations have been discovered that cause constitutive activation of the cytosolic Tyr-PK oncogene src and the Ser/Thr-PK oncogenes mos and ret. As a result, these Enzymes phosphorylate an isoform of phospholipase C, switch on the Inositol phosphate signal Transduction pathway, and activate transcription factors that stimulate cell proliferation and division.

In breast and ovarian carcinomas, the erbB2 or neu oncogene is frequently detected, which is a homolog of the epidermal growth factor receptor. The receptor molecule contains three domains: an extracellular (or Ligand-binding) domain, a transmembrane domain, and an intracellular domain possessing Tyr-PK activity.

During transformation, this receptor gene is amplified and loses the fragment responsible for growth factor binding. Consequently, cells produce a protein with unregulated Tyr-PK activity (the "constitutive "on" switch effect"), which stimulates mitotic processes within the cell.

Chromosomal Translocations

Chromosomal translocations are frequently observed in tumor cells when a fragment of one chromosome breaks off and is incorporated into another chromosome. If the second chromosome reciprocally exchanges a corresponding fragment with the first, such a translocation is termed a reciprocal translocation. Altering the position of a gene in a number of cases increases its expression and promotes malignant transformation. For instance, The Genome of patients with chronic myelogenous leukemia features a reciprocal translocation in which the abl proto-oncogene is relocated from chromosome 9 to chromosome 22 [t(9;22)], resulting in The formation of a shortened Philadelphia chromosome (Fig. 16-12, A), which is readily visible under a Microscope. As a result, a hybrid c-abl-bcr gene appears in the Philadelphia chromosome, encoding a protein with high Tyr-PK activity. This enzyme triggers a cascade of events that stimulate the malignant transformation of hematopoietic cells.

In Burkitt's lymphoma, the c-myc proto-oncogene is translocated from its position on chromosome 8 to chromosome 14 [t(8;14)]. This type of translocation is found in 90% of patients, while 10% of cases exhibit either t(8;2) or t(8;22). It has been established that in all cases, the c-myc gene is relocated to the region of a strong promoter of genes encoding immunoglobulin heavy (H) or light (L) chains. Consequently, cells undergo overexpression of the normal C-MYC protein, which is a transcription factor involved in the Cytology/cytology/16.html">Early stages of proliferation (Fig. 16-12, B).

Fig. 16-12. Chromosomal translocations and proto-oncogene activation. A — chronic myelogenous leukemia; B — Burkitt's lymphoma.

B. Mutations in tumor suppressor genes

Retinoblastoma (a rare pediatric eye Cancer affecting about 1 in 20,000 children) is characterized by a deletion in the rb1 locus of chromosome 13. Epidemiological studies and statistical analysis have established that retinoblastoma develops only when both alleles of the retinoblastoma gene acquire mutations (Fig. 16-13). DNA analysis of normal and tumor tissue samples obtained from patients during surgery has demonstrated that in normal tissue, the rb1 gene is heterozygous—containing one wild-type and one altered allele—whereas in tumor tissue, both alleles are altered, resulting in a loss of heterozygosity. This mutation inactivates the protein, causing it to lose its inhibitory effect on cell proliferation. Inactivation is also observed when

rb1 binds to viral inhibitory proteins.

Fig. 16-13. Development of hereditary (A) and sporadic (B) forms of retinoblastoma.

In hereditary retinoblastoma (~40% of cases), multiple tumors are common. This occurs because secondary mutations in the single remaining undamaged allele can arise independently in multiple retinoblasts. In the non-hereditary, sporadic form of the disease (~60% of cases), multiple tumors are rare, as the simultaneous inactivation of both gene alleles in a single cell is an extremely improbable event.

Mutations in the p53 gene are found in more than half of all cancer patients. Statistics show that functionally inactive p53 is present in 70% of Colorectal Cancer cases, 50% of lung cancers, and 40% of breast cancers.

Approximately 80% of p53 mutations are missense mutations affecting the most conserved Regions of the gene within CG dinucleotide sequences, encompassing both hereditary and non-hereditary origins.

Another distinctive feature of the p53 gene is that its mutations act in a dominant-negative manner, where structural disruption in only one of the diploid cell's gene copies is sufficient to cause functional impairment, unlike most other tumor suppressor genes, which follow a recessive mechanism.

In normal Tissues, the concentration of the p53 protein is extremely low. Mutations extend the half-life of this protein from a few minutes to several hours, leading to a marked accumulation of the mutant p53 form in the cell nuclei. This phenomenon has found Practical Application in immunohistochemical assays used to detect mutant protein in tumors.

Inactivation of the p53 protein can occur not only through structural damage to the gene itself, but also via the formation of inactive Protein Complexes with viral proteins (such as the SV40 large T-antigen) or through targeted protein degradation stimulated by oncoproteins produced during human papillomavirus infection.