Oncogene

Illustration of how a normal cell becomes a cancer cell, when an oncogen is activated

An oncogene is an abnormal or activated gene that results from the mutation of an allele of a normal gene called a proto-oncogene. Oncogenes are responsible for the transformation of a normal cell into a malignant one that will develop a certain type of cancer. In humans, more than 62 oncogenes have been identified and sequenced in the different chromosomes of the genome, forming a very heterogeneous set of genes.

In a healthy human individual, there are more than 30,000 trillion cells that live in an interdependent condominium, mutually regulating their proliferation, to ensure that the size of the different organs is coordinated and in accordance with the size of the individual. Therefore, cells only proliferate when they receive very specific signals from other neighboring cells. Cancer cells, however, violate this scheme: they ignore all the signals they receive from the outside, and follow their own proliferation schemes, invading not only adjacent spaces, but also sites far from the place of origin, through the process of metastasis. From this point of view, cancer cells can be considered as "associative" cells, which do not follow the guidelines of the whole organism and even threaten its survival.

All the cells in a tumor come from a single cell (they belong to the same clone), a common ancestor that at a given moment (perhaps decades before the detection of the tumor) started an inappropriate program of proliferation. This malignant transformation is produced by the accumulation of mutations in a very specific set of genes. There are two classes of genes, which together represent a very small proportion of the whole genome, that play a fundamental role in the initiation of tumor progression. Under normal conditions, these genes are involved in regulating the cell's life cycle: the set of events that define when a cell should grow and proliferate. Regulatory genes can perform two types of functions:

• activate processes directed towards growth and proliferation -- these genes are called protooncogenes; they contribute to tumor progression when they suffer mutations that the activate permanently or constitutively, that is, when a function gain; this type of mutation has an effect dominant dominant: it is enough that one of the two wings of the cell is mutated to appear the activity;
• inhibiting such processes -- are so-called tumor suppressant genes; in this case, they intervene in the tumor process if they suffer mutations that the Inactivate, that is, if a loss of function; this type of mutation has an effect recessive: to eliminate the activity, the two alleles have to be mutated (see also Knudson hypothesis).

For a cancer to progress and develop, at least half a dozen mutations affecting various regulatory genes must be produced. However, other types of genes may also participate in malignancy, facilitating the invasive capacity of the tumor (for example example, mutations in cytoskeletal proteins that favor cell motility).

Oncogene concept

Oncogenes come from regulatory genes, the proto-oncogenes. Many proto-oncogenes participate in signaling cascades that receive, integrate, and transmit proliferation signals from abroad, executing specific programs through the expression of specific genes that activate the cellular machinery for growth and entry into the cell cycle. This signaling is transmitted from one cell to another in a tissue, and is normally initiated by the secretion of growth factors from different cell types (for example, fibroblasts during healing). These growth factors pass through the intercellular spaces, and are recognized by specific membrane receptors for that molecule. Membrane receptors are proteins that have one end towards the outside of the cell and the other towards the inside. When a growth factor associates with its receptor, it transmits a signal to the cytoplasm, producing a change in the conformation of one or several proteins, which is transmitted in a cascade, until activating the expression of the appropriate genes in the nucleus. to respond to the emitted signal. When mutations occur that deregulate some of these processes, so that they remain activated when they should remain detained, cell growth becomes anarchic.

Proto-oncogenes are therefore normal genes responsible for encoding nuclear, cytoplasmic and membrane proteins, which are involved in cellular homeostasis, that is, in maintaining the balance of cellular functions, for which reason their expression level it is strictly regulated. Many proto-oncogenes are highly expressed during certain stages of the cell cycle and/or closely related to certain phases of embryonic development.

In all cells of the body there are many proto-oncogenes and when a group of them is altered, it can precipitate the malignant transformation of the cell or the development of cancer. Proto-oncogenes exist in many species of multicellular organisms, being well conserved between different species, while different proto-oncogenes may not be similar within a particular species.

In some cases, viral oncogenes come from cellular genes that were once hijacked by the virus, and mutated, resulting in an oncogene. Therefore, non-mutated oncogenes found in normal cells are called proto-oncogenes, and mutated ones, oncogenes. Oncogenes are designated by three letters, for example src for Rous sarcoma virus. The viral or malignant form of the oncogene is preceded by a v (v-src) and the benign, normal, or cellular form is preceded by a c (c-src). A large number of oncogenes identified in retroviruses fall within this group, for example the oncogenes abl, erb-B, fes, fms, fos/jun, kit, raf, myc, H-ras, K-ras, rel and sis, in addition to src.

The discovery and knowledge of oncogenes confirms that cancer is a genetic disease with the following caveats:

• The development of cancer is not due to the expression of a single oncogen. The accumulation of several oncogenes in a single cell (clonal theory) or a certain number of equal oncogenes in several cells is necessary to show the cancer.
• Oncogenes are not the only cause of cancer. The immune system is also one of the regulatory factors in eliminating cancer cells (which manifest oncogenes) or not recognizing malignant cells and allowing their survival and proliferation. Cancer is a set of multifactorial diseases, so oncogenes are not the only cause.

Classification of oncogenes

Oncogenes can encode proteins that act at different levels of the signaling cascade that activates cell proliferation:

Extracellular: excess production of growth factors

In this case, the oncogenes force the cell to produce an excess of growth factors; These factors influence not only neighboring cells, but can also activate the proliferation of the cells that produced them:

• sarcomas and gliomas (tumores of connective tissues and non neuronal brain cells, respectively) release a large amount of PDGF;
• other tumor types express too much TGF-alpha;
• oncogenes sis, int-2 and hst stimulate cell proliferation;

Membrane: modified receptors

Oncogenic versions of cell receptors for growth factors are produced, which transmit a proliferation signal into the cell in the absence of growth factors on the outside:

• breast tumor cells often express Erb-B2 receptors that work this way;
• other examples are oncogenes src or fms.

Cytoplasm: constitutive signaling cascades

Oncogenic versions of cytoplasmic proteins of the signaling cascade are generated that are always active:

• the best case studied is that of the Ras protein family; the Ras unen GTP family products, are associated with GTPasas and act as signal transducers for growth factor receptors on the cellular surface; the mutated Ras oncogen acts constitutively, always uniting GTP; hyperactive Ras forms are present in a quarter of all human tumors, including carcinomas (
• cytoplasmic proteins with kinase activity:
  • e.g. c-Raf protein can go to the core to exercise the function received in the activated membrane, acting as a second messenger; the oncogenic form of Raf has lost the regulatory sequences of the amino end and is constitutively active;
  • another type, c-Crk, is a cytoplasmic protein that stabilizes kinase tyrosine;

Core: transcription factors or constitutive associated sequences

Oncogenic versions of transcription factors or associated sequences are produced that function at all times:

• oncogenic alteration of transcription factors makes them oncogenic proteins with loss of their negative elements or loss of their active domain (negative dominant mutation):
  • is the case of the family of myc transcription factors); normally, cells only produce Myc when they are stimulated by growth factors, and once produced stimulate the transcription of genes that activate cell proliferation; however, in many types of cancer (especially in those associated with hematopoietic tissues), Myc levels remain high even in the absence of growth factors;
  • other oncogenes that encode for constitutive transcription factors are myb, fos, jun, erb-A and rel.
  • modification of regulatory sequences that are close to encoding genes, composed of short segments of DNA that serve as target for transcription factors that activate the encoding genes; many of these regulatory sequences are located outside the protein encoding sequences, in the non-coding DNA area or garbage DNA, which can represent 97% of the human genome.

Although nuclear genes are capable of perpetuating cell proliferation, they are not capable of forming malignant tumors. In order to acquire the tumorigenic capacity, the activation of a second oncogene, generally cytoplasmic, is necessary, so that for a malignant tumor to appear, the activation of several oncogenes is necessary.

Depending on the function of the encoded protein

• Oncogenes that encode G proteins: the most common is Ras. This gene encodes a monomeric G protein that, in protooncogen, when deactivated, hydrolyzes the GTP to GDP, deactivating cell proliferation. The oncogen, instead, keeps it in its active form. Thus, it cannot hydrolyze GTP and cell proliferation continues.
• Oncogenes that codify growth factors or their receptors: oncogen sis (symium sarcoma virus) codes the growth factor PDGF, whose excessive production stimulates cell proliferation. Erb-B (eritroblastosis aviar) governs the formation of a receptor for the growth factor EGF, which when altered acts as if it were permanently attached to EGF, stimulating cell proliferation.
• Oncogenes that encode kinase proteins of serina-treonin and thyrosine: Raf is a kinase of serina-treonin that acts at the beginning of the cyclic AMP cascade, which is the primary path of cell proliferation control. The oncogen keeps Raf in the active way, preventing proliferation from deactivating. The Src gene is a tyrosine kinase that produces intracellular signals, many of them related to cell proliferation.
• Oncogenes that codify nuclear transcription factors: the Myc gene causes G0 to G1 passage in a cell proliferation that should not occur.
• Oncogenes that encode products that affect apoptosis: the Bcl-2 gene, being overexpressed, suppresses apoptosis.

Activation of oncogenes

The activation of a proto-oncogene and its transformation into an oncogene is produced by mutations caused by physical causes such as ionizing radiation, chemical causes such as carcinogens, biological causes such as oncogenic viruses or hereditary causes, due to mutations transmitted along generations or by failure in any of the DNA repair mechanisms.

The mechanisms by which a proto-oncogene can be transformed into an oncogene are quantitative and qualitative.

Quantitative Mechanisms

1. Insertion of a viral promoter: Some retrovirus contains a developer sequence called LTR (Long Terminal Repeat, in English), that when it is incorporated into the DNA of the infected cell adjacent to the regulatory sequences of a protooncogen, there is an increase in the expression of that gene that remains under the control of the viral promoter LTR, causing alterations in cell growth and differentiation.
2. chromosomal translocation or reordering: It is the change of location of a chromosomal portion, with the genes that it carries to another different location within the same chromosome or another, which can affect the expression or biochemical function of a protooncogen. Translocations often occur in hematological tumors such as lymphomas and leukemias. For example, protooncogen c-myc is located on chromosome 8 and can be moved to chromosome 14. This new position produces a protein overexpression that encodes, giving rise to Burkitt lymphoma. Also chronic myeloid leukemia occurs by the reciprocal translocation between chromosome 9 and 22, producing a hybrid oncogene between the c-abl gene of chromosome 9 and the bcr region of chromosome 22, giving rise to chromosome Philadelphia.
3. Amplification: It is the increase in the number of copies of the same protoonchogen of the genome, even several thousand times. The chromosomes of the amplified oncogene tumors have structural disorders that are easily visualized in the cariotype as regions with abnormal bands, homogenously dyed regions (a)Homogeneously Staining Regions, HSR in English or “double tiny”double minutes, DM in English) that are small extrachromosomal fragments of variable size that are automatically replicated. Several tumors have detected oncogen amplification and the degree of amplification is very related to the stage and prognosis of the tumor. Overexpression by amplification of oncogen n-myc produces neuroblastoma, although it is also found in other tumors. The increase in the number of copies of an oncogen in addition to producing an increase in the protein that encodes and acts as a growth factor also produces a greater increase in receptors to the growth factor. Protooncogenes amplified in human tumors mainly belong to one of these three families: erb B, ras or myc. It is still unknown if the protooncogenic amplification is caused or causes malignancy in a tumor, for example to acquire the ability to metastatize or is an effect of the malignant transformation of a tumor as it occurs in large, undifferentiated tumors and have metastases, qualities that increase the probability of malignant transformation of the amplification.
4. Hypomethylation: It is estimated that between 2 and 7% of cytosine residues in DNA are methylated. When methyl groups (CH)₃) are located in sequences of DNA promoters of genes, the initiation of transcription is mechanically interfered, being the degree of transcription inversely proportional to methylation. The decrease of methyl groups in the cytosine bases of a protooncogen-promoting sequence activates their transcription and the possible malignant transformation to an oncogene.

Qualitative mechanisms

1. On-site: Replacement of a nitrogenous base in a gene's DNA can produce a change in the amino acid identified by the codon that presents the mutation, which causes a structural change in the protein synthesized by that gene, altering its function, so replacing a single nitrogen base in the DNA chain can transform a protooncogen into an oncogen. For example, oncogen ras modifies a reading condom that converts glycine into valine. Homologist oncogenes such as H-RAS, K-RAS and N-RAS also have punctual mutations in other locations. The points where such mutations are produced are critical for the control of normal cell growth, since in the case of oncogen ras, mutations prevent the conversion of the active to inactive form, with the consequent alteration in the control of cell proliferation.
2. Deletion of genetic material: The loss of genetic material from a chromosome can activate an oncogen through three mechanisms:

1. The loss can be from an inhibitory sequence of a protooncogen, which causes overexpression of the oncogen product.
2. The loss can cause the oncogen to be closer to a promoter sequence, also producing an overexpression.
3. The loss can be from a tumor suppressant gene, and is usually the most important mechanism by which a chromosomal loss can activate an oncogen.

Proteins encoded by oncogenes

Proto-oncogenes encode the necessary proteins involved in the control of most of the mechanisms by which cell proliferation is regulated, such as:

• Growth factors.
• Receptors of growth factors.
• Hormone receptors.
• Intracellular transmission factors or second messengers such as:

• Proteins with thynoid activity.
• Triphosphate guanosine binding proteins.
• Nuclear transcription factors.

Types of oncogenes

History of Oncogenes

The beginning of the discovery of oncogenes was through the studies of the pathologist Francis Peyton Rous who worked at the Rockefeller Institute in New York in 1910. Rous transmitted chicken sarcoma, by injecting dozens of hens with an extract of tumor cell culture, which did not contain living cells. With this procedure he was able to reproduce the tumor and suspected that the causative agent should be smaller than cells and bacteria, so it could correspond to a virus, although he did not call it that, but rather a carcinogen. He later discovered that it was a virus and for his discoveries he was awarded the Nobel Prize in Medicine in 1966.

The Rous sarcoma virus (src) is the prototype of retroviruses, demonstrating that genetic information is not only transferred unidirectionally from DNA to RNA and from this to proteins, but rather that retroviruses through the reverse transcriptase enzyme They are capable of synthesizing DNA from RNA. In addition to discovering that src was carcinogenic, it was discovered that it was made up of four genes, three of which are essential for the multiplication of the virus and the fourth gene, v-src, which does not perform any function in the virus, but which in cells undergo malignant transformation when infected by the virus. In 1975, the work of Drs Ferrer-Roca O. and Egozcue Cuixart J. on the importance of ubiquitous viruses developed the theory of tumor karyotype correlation within the somatic-viral theory of cancer, which stipulated that the insertion of the oncogenic viruses caused the chromosomal abnormalities of the tumors and then underwent a process of clonal selection. In 1989 John Michael Bishop, who also received the Nobel Prize in Medicine in 1989, discovered through genetic engineering techniques that homologous sequences of v-src are also found in the DNA of normal cells, both from birds and many vertebrates and e.g. even in humans, thus demonstrating that the src oncogene comes from a normal animal gene, a proto-oncogene, so that the cancerous transformation occurs in normal genes, the proto-oncogenes that perform very important functions in cell proliferation and differentiation. The study of the nucleotide sequence of the v-src, as it possesses introns and exons, which are exclusive sequences of animal and non-viral DNA, it was deduced that this gene does not belong to the virus, but must have been dragged by the virus after joining and shed the DNA of some infected animal cell during evolution. The c-src proto-oncogene of normal cells encodes a protein called by Bishop pp60c-src that is tightly bound to the inner surface of the cell membrane, capable of tyrosine phosphorylation. Cancer cells containing the active v-src oncogene are known to have an abnormal pp60c-src protein, which is missing a tyrosine residue at a particular location in its conformation and is phosphorylated in the normal version of the protein. to block the phosphorylation of other proteins in signal transmission. That's why the mutated pp60c-src is continually at work, adding phosphate groups to these signaling proteins in cancer cells.

The final demonstration that cancer is a genetic disease dates back to the early eighties, thanks to Robert A.Weinberg, Geofrey M. Cooper, Michael Wigler and Mariano Barbacid belonging to different research teams. These four scientists isolated DNA samples from different human tumors and added it to non-tumor mouse fibroblast cell cultures. Normal fibroblast cells stop multiplying when they come into contact with each other, a phenomenon called contact inhibition, while cell groups of fibroblast cultures containing tumor DNA multiplied uncontrollably, losing inhibition by contact. contact and also formed tumor nodules when inoculated into immunosuppressed mice, a phenomenon that did not occur when injecting normal fibroblasts. The fibroblastic cells transformed into tumors were isolated again, fragmenting their DNA by means of restriction enzymes, and the obtained DNA fragments were injected again into normal fibroblasts, some of them becoming cancerous and others not. The process of isolating tumor cells, fragmenting their DNA, and inoculating it into normal cells was repeated several times, until more and more fragments of human DNA that cause cancer, that is, oncogenes, were isolated.