In What Ways Can Proto-oncogenes Be Converted to Oncogenes?

The activation of oncogenes involves genetic changes to cellular protooncogenes. The consequence of these genetic alterations is to confer a growth advantage to the cell. Iii genetic mechanisms actuate oncogenes in homo neoplasms: (1) mutation, (two) gene amplification, and (3) chromosome rearrangements. These mechanisms result in either an alteration of protooncogene structure or an increment in protooncogene expression (Effigy half dozen-five). Considering neoplasia is a multistep process, more than than one of these mechanisms often contribute to the genesis of human tumors by altering a number of cancer-associated genes. Full expression of the neoplastic phenotype, including the capacity for metastasis, normally involves a combination of protooncogene activation and tumor suppressor gene loss or inactivation.

Figure 6-5

Schematic representation of the main mechanisms of oncogene activation (from protooncogenes to oncogenes). The normal gene (protooncogene) is depicted with its transcibed portion (rectangle). In the case of factor distension, the latter can be duplicated (more than...)

Mutation

Mutations actuate protooncogenes through structural alterations in their encoded proteins. These alterations, which normally involve disquisitional poly peptide regulatory regions, frequently atomic number 82 to the uncontrolled, continuous activity of the mutated protein. Various types of mutations, such as base substitutions, deletions, and insertions, are capable of activating protooncogenes.⁷⁶ Retroviral oncogenes, for example, often have deletions that contribute to their activation. Examples include deletions in the aminoterminal ligand-bounden domains of the erb B, kit, ros, met, and trk oncogenes.⁶ In human tumors, however, most characterized oncogene mutations are base substitutions (signal mutations) that change a single amino acid within the poly peptide.

Bespeak mutations are oft detected in the ras family of protooncogenes (Chiliad-ras, H-ras, and N-ras).⁷⁷ It has been estimated that as many equally xv% to twenty% of unselected homo tumors may contain a ras mutation. Mutations in K-ras predominate in carcinomas. Studies accept plant K-ras mutations in nigh 30% of lung adenocarcinomas, l% of colon carcinomas, and ninety% of carcinomas of the pancreas.⁷⁸ N-ras mutations are preferentially establish in hematologic malignancies, with up to a 25% incidence in acute myeloid leukemias and myelodysplastic syndromes.^{79, 80} The majority of thyroid carcinomas have been institute to accept ras mutations distributed among K-ras, H-ras, and N-ras, without preference for a single ras family member but showing an association with the follicular type of differentiated thyroid carcinomas.^{81, 82} The majority of ras mutations involve codon 12 of the gene, with a smaller number involving other regions such as codons 13 or 61.⁸³ Ras mutations in human tumors have been linked to carcinogen exposure. The consequence of ras mutations is the constitutive activation of the bespeak-transducing role of the ras poly peptide.

Another significant example of activating point mutations is represented past those affecting the ret protooncogene in multiple endocrine neoplasia type 2A syndrome (MEN2A).

Germline point mutations affecting i of the cysteines located in the juxtamembrane domain of the ret receptor have been plant to confer an oncogenic potential to the latter every bit a outcome of the ligand-independent activation of the tyrosine kinase activity of the receptor. Experimental evidences accept pointed out that these mutations involving cysteine residues promote ret homodimerization via the formation of intermolecular disulfide bonding, most likely as a result of an unpaired number of cysteine residues.^{84, 85}

Gene Amplification

Cistron amplification refers to the expansion in copy number of a gene within the genome of a cell. Gene amplification was first discovered as a mechanism by which some tumor cell lines tin acquire resistance to growth-inhibiting drugs.⁸⁶ The procedure of gene amplification occurs through redundant replication of genomic DNA, often giving ascension to karyotypic abnormalities called double-minute chromosomes (DMs) and homogeneous staining regions (HSRs).⁸⁷ DMs are characteristic minichromosome structures without centromeres. HSRs are segments of chromosomes that lack the normal alternate pattern of light- and dark-staining bands. Both DMs and HSRs represent large regions of amplified genomic Deoxyribonucleic acid containing up to several hundred copies of a gene. Amplification leads to the increased expression of genes, which in plough tin can confer a selective reward for cell growth.

The frequent observation of DMs and HSRs in human tumors suggested that the amplification of specific protooncogenes may be a common occurrence in neoplasia.⁸⁸ Studies then demonstrated that three protooncogene families-myc, erb B, and ras-are amplified in a significant number of human being tumors (Table 6-two). About xx% to thirty% of breast and ovarian cancers show c-myc amplification, and an approximately equal frequency of c-myc amplification is constitute in some types of squamous cell carcinomas.⁸⁹ N-myc was discovered as a new member of the myc protooncogene family unit through its amplification in neuroblastomas.⁹⁰ Amplification of N-myc correlates strongly with advanced tumor stage in neuroblastoma (Tabular array six-three), suggesting a role for this cistron in tumor progression.^{91, 92} Fifty-myc was discovered through its amplification in pocket-sized-prison cell carcinoma of the lung, a neuroendocrine-derived tumor.⁹³ Amplification of erb B, the epidermal growth cistron receptor, is plant in up to 50% of glioblastomas and in 10% to 20% of squamous carcinomas of the head and neck.⁷⁷ Approximately fifteen% to 30% of breast and ovarian cancers have distension of the erbB-ii (HER-two/neu) factor. In breast cancer, erbB-2 amplification correlates with advanced stage and poor prognosis.⁹⁴ Members of the ras cistron family, including K-ras and North-ras, are sporadically amplified in various carcinomas.

Table 6-ii

Oncogene Amplification in Human being Cancers.

Chromosomal Rearrangements

Recurring chromosomal rearrangements are ofttimes detected in hematologic malignancies equally well as in some solid tumors.^{37, 95, 96} These rearrangements consist mainly of chromosomal translocations and, less frequently, chromosomal inversions. Chromosomal rearrangements can pb to hematologic malignancy via two dissimilar mechanisms: (ane) the transcriptional activation of protooncogenes or (2) the creation of fusion genes. Transcriptional activation, sometimes referred to equally factor activation, results from chromosomal rearrangements that move a proto-oncogene close to an immunoglobulin or T-cell receptor cistron (meet Effigy six-v). Transcription of the protooncogene then falls under control of regulatory elements from the immunoglobulin or T-cell receptor locus. This circumstance causes deregulation of protooncogene expression, which can and then lead to neoplastic transformation of the cell.

Fusion genes can exist created past chromosomal rearrangements when the chromosomal breakpoints fall within the loci of ii dissimilar genes. The resultant juxtaposition of segments from 2 different genes gives rise to a blended structure consisting of the head of one gene and the tail of some other. Fusion genes encode chimeric proteins with transforming action. In full general, both genes involved in the fusion contribute to the transforming potential of the chimeric oncoprotein. Mistakes in the physiologic rearrangement of immunoglobulin or T-cell receptor genes are thought to give rise to many of the recurring chromosomal rearrangements institute in hematologic malignancy.⁹⁷ Examples of molecularly characterized chromosomal rearrangements in hematologic and solid malignancies are given in Table 6-four. In some cases, the aforementioned protooncogene is involved in several different translocations (ie, c-myc, ews, and ret).

Table 6-iv

Molecularly Characterized Chromosome Rearrangements in Tumors.

Cistron Activation

The t(8;14)(q24;q32) translocation, found in about 85% of cases of Burkitt lymphoma, is a well-characterized example of the transcriptional activation of a proto-oncogene. This chromosomal rearrangement places the c-myc gene, located at chromosome band 8q24, under control of regulatory elements from the immunoglobulin heavy concatenation locus located at 14q32.⁹⁸ The resulting transcriptional activation of c-myc, which encodes a nuclear protein involved in the regulation of prison cell proliferation, plays a critical role in the development of Burkitt lymphoma.⁹⁹ The c-myc gene is also activated in some cases of Burkitt lymphoma by translocations involving immunoglobulin light-chain genes.^{100, 101} These are t(2;8)(p12;q24), involving the κ locus located at 2p12, and t(viii;22)(q24;q11), involving the κ locus at 22q11 (Figure 6-6). Although the position of the chromosomal breakpoints relative to the c-myc gene may vary considerably in private cases of Burkitt lymphoma, the consequence of the translocations is the aforementioned: deregulation of c-myc expression, leading to uncontrolled cellular proliferation.

Figure half-dozen-vi

C-myc translocations establish in Burkitt lymphoma. A, t(viii;14)(q24;q32) translocation involving the locus of immunoglobulin heavy-concatenation gene located at 14q32. B, t(8;xiv)(q24;q32) translocation where only ii exons (Ex) of c-myc are translocated under regulatory (more than...)

In some cases of T cell acute lymphoblastic leukemia (T-ALL), the c-myc cistron is activated by the t(8;14)(q24;q11) translocation. In these cases, transcription of c-myc is placed nether the control of regulatory elements within the T-cell receptor α locus located at 14q11.¹⁰² In addition to c-myc, several protooncogenes that encode nuclear proteins are activated past various chromosomal translocations in T-ALL involving the T-cell receptor α or β locus. These include HOX11, TAL1, TAL2, and RBTN1/Tgt1.^103–105 The proteins encoded by these genes are thought to function as transcription factors through DNA-binding and poly peptide-poly peptide interactions. Overexpression or inappropriate expression of these proteins in T cells is thought to inhibit T-cell differentiation and lead to uncontrolled cellular proliferation.

A number of other protooncogenes are also activated past chromosomal translocations in leukemia and lymphoma. In near follicular lymphomas and some big cell lymphomas, the bcl-2 gene (located at 18q21) is activated as a consequence of t(fourteen;18)(q32;q21) translocations.^{72, 73} Overexpression of the bcl-2 protein inhibits apoptosis, leading to an imbalance between lymphocyte proliferation and programmed cell death.⁷⁴ Mantle prison cell lymphomas are characterized by the t(11;fourteen)(q13;q32) translocation, which activates the cyclin d1 (bcl-1) gene located at 11q13.^{106, 107} Cyclin D1 is a G1 cyclin involved in the normal regulation of the jail cell cycle. In some cases of T cell chronic lymphocytic leukemia and prolymphocytic leukemia, the tcl-1 gene at 14q32.1 is activated by inversion or translocation involving chromosome fourteen.¹⁰⁸ The tcl-1 gene product is a pocket-size cytoplasmic protein whose part is not nevertheless known.

Gene Fusion

The starting time example of gene fusion was discovered through the cloning of the breakpoint of the Philadelphia chromosome in chronic myelogenous leukemia (CML).¹⁰⁹ The t(9;22)(q34;q11) translocation in CML fuses the c-abl gene, commonly located at 9q34, with the bcr gene at 22q11 (Figure 6-7).¹¹⁰ The bcr/abl fusion, created on the der(22) chromosome, encodes a chimeric protein of 210 kDa, with increased tyrosine kinase activity and abnormal cellular localization.¹¹¹ The precise mechanism by which the bcr/abl fusion poly peptide contributes to the expansion of the neoplastic myeloid clone is not yet known. The t(nine;22) translocation is also constitute in up to 20% of cases of acute lymphoblastic leukemia (ALL). In these cases, the breakpoint in the bcr gene differs somewhat from that plant in CML, resulting in a 185 kDa bcr/abl fusion protein.¹¹² Information technology is unclear at this time why the slightly smaller bcr/abl fusion protein leads to such a large difference in neoplastic phenotype.

Figure six-vii

Gene fusion. The t(9;22)(q34;q11) translocation in chronic myelogenous leukemia (CML) determines the fusion of the c-abl gene with the bcr gene. Such a gene fusion encodes an oncogenic chimeric protein of 210 kDa. Chr = chromosome.

In add-on to c-abl, two other genes encoding tyrosine kinases are involved in distinct cistron fusion events in hematologic malignancy. The t(ii;5)(p23;q35) translocation in anaplastic large cell lymphomas fuses the NPM gene (5q35) with the ALK gene (2p23).¹¹³ ALK encodes a membranespanning tyrosine kinase similar to members of the insulin growth factor receptor family. The NPM poly peptide is a nucleolar phosphoprotein involved in ribosome associates. The NPM/ALK fusion creates a chimeric oncoprotein in which the ALK tyrosine kinase action may exist constitutively activated. The t(5;12)(q33;p13) translocation, characterized in a example of chronic myelomonocytic leukemia, fuses the tel gene (12p13) with the tyrosine kinase domain of the PDGF receptor b cistron (PDGFR-b at 5q33).¹¹⁴ The tel factor is thought to encode a nuclear DNA-binding protein like to those of the ets family unit of protooncogenes.

Gene fusions sometimes pb to the formation of chimeric transcription factors.^{68, 95} The t(1;19)(q23;p13) translocation, found in childhood pre-B-cell ALL, fuses the E2A transcription factor gene (19p13) with the PBX1 homeodomain gene (1q23).¹¹⁵ The E2A/PBX1 fusion protein consists of the amino-terminal transactivation domain of the E2A protein and the DNA-binding homeodomain of the PBX1 protein. The t(15;17)(q22;q21) translocation in acute promyelocytic leukemia (PML) fuses the PML gene (15q22) with the RARA gene at 17q21.¹¹⁶ The PML poly peptide contains a zinc-binding domain called a Ring finger that may be involved in protein-protein interactions. RARA encodes the retinoic acid alpha-receptor protein, a member of the nuclear steroid/thyroid hormone receptor superfamily. Although retinoic acid binding is retained in the fusion protein, the PML/RARA fusion protein may confer altered Deoxyribonucleic acid-binding specificity to the RARA ligand circuitous.¹¹⁷ Leukemia patients with the PML/RARA gene fusion respond well to retinoid handling. In these cases, treatment with all-trans retinoic acid induces differentiation of PML cells.

The ALL1 gene, located at chromosome band 11q23, is involved in approximately five% to 10% of acute leukemia cases overall in children and adults.^{118, 119} These include cases of ALL, acute myeloid leukemia, and leukemias of mixed cell lineage. Among leukemia genes, ALL1 (too called MLL and HRX) is unique because it participates in fusions with a large number of different partner genes on the various chromosomes. Over xx different reciprocal translocations involving the ALL1 gene at 11q23 have been reported, the virtually common of which are those involving chromosomes 4, vi, 9, and xix.¹²⁰ In approximately five% of cases of astute leukemia in adults, the ALL1 cistron is fused with a portion of itself.¹²¹ This special blazon of gene fusion is chosen self-fusion.¹²² Cocky-fusion of the ALL1 gene, which is thought to occur through a somatic recombination mechanism, is establish in loftier incidence in acute leukemias with trisomy xi equally a sole cytogenetic aberration. The ALL1 gene encodes a large poly peptide with DNA-binding motifs, a transactivation domain, and a region with homology to the Drosophila trithorax protein (a regulator of homeotic gene expression).^{123, 124} The diverse partners in ALL1 fusions encode a diverse group of proteins, some of which announced to exist nuclear proteins with Dna-binding motifs.^{125, 126} The ALL1 fusion protein consists of the aminoterminus of ALL1 and the carboxyl terminus of i of a variety of fusion partners. It appears that the critical feature in all ALL1 fusions, including cocky-fusion, is the uncoupling of the ALL1 amino-terminal domains from the residual of the ALL1 protein.

Solid tumors, especially sarcomas, sometimes have consequent chromosomal translocations that correlate with specific histologic types of tumors.¹²⁷ In full general, translocations in solid tumors result in gene fusions that encode chimeric oncoproteins. Studies thus far indicate that in sarcomas, the bulk of genes fused by translocations encode transcription factors.¹²⁸ In myxoid liposarcomas, the t(12;16)(q13;p11) fuses the FUS (TLS) factor at 16p11 with the CHOP factor at 12q13.¹²⁹ The FUS protein contains a transactivation domain that is contributed to the FUS/CHOP fusion protein. The CHOP protein, which is a ascendant inhibitor of transcription, contributes a protein-bounden domain and a presumptive Deoxyribonucleic acid-bounden domain to the fusion. Despite knowledge of these structural features, the mechanism of activity of the FUS/CHOP oncoprotein is non all the same known. In Ewing sarcoma, the t(11;22)(q24;q12) fuses the EWS gene at 22q12 with the FLI1 cistron at 11q24.¹³⁰ Like FUS, the EWS protein contains three glycine-rich segments and an RNA-binding domain. The FLI1 protein contains an ets-like DNA-binding domain. The EWS/FLI1 fusion protein combines a transactivation domain from EWS with the Deoxyribonucleic acid-binding domain of FLI1. In alveolar rhabdomyosarcoma, the t(two;13)(q35;q14) fuses the PAX3 factor at 2q35 with the FKHR gene at 13q14.¹³¹ The PAX3 protein, a transcription gene that activates genes involved in evolution, is a paired-box homeodomain protein with two distinct DNA-binding domains. The FKHR poly peptide encodes a conserved Deoxyribonucleic acid-bounden motif (the forkhead domain) similar to that first identified in the Drosophila forkhead homeotic factor. The PAX3/FKHR fusion protein is a chimeric transcription factor containing the PAX3 DNA-binding domains, a truncated forkhead domain, and the carboxy-terminal FKHR regions.

In DP, an infiltrating skin tumor, both a reciprocal translocation t(17;22)(q22;q13) and supernumerary ring chromosomes derived from the t(17;22) have been described.

Although early successful studies in this field have been performed with lymphomas and leukemia, every bit nosotros have discussed before, the first chromosomal abnormality in solid tumors to exist characterized at the molecular level as a fusion protein was an inversion of chromosome 10 institute in papillary thyroid carcinomas.¹³² In this tumor, two main recurrent structural changes have been described, including inv(10) (q112.2; q21.2), as the more frequent alteration, and a t(10;17)(q11.2;q23). These two abnormalities represent the cytogenetic mechanisms which activate the protooncogene ret on chromosome x, forming the oncogenes RET/ptc1 and RET/ptc2, respectively. Alterations of chromosome 1 in the same tumor type take then been associated to the activation of NTRK1 (chromosome 1), an NGF receptor which, like RET, forms chimeric fusion oncogenic proteins in papillary thyroid carcinomas.¹³³ A comparative assay of the oncogenes originated from the activation of these 2 tyrosine kinase receptors has allowed the identification and characterization of common cytogenetic and molecular mechanisms of their activation. In all cases, chromosomal rearrangements fuse the tK portion of the 2 receptors to the 5′ stop of different genes that, due to their full general effect, have been designated as activating genes. In the majority of cases, the latter belong to the aforementioned chromosome where the related receptor is located, x for RET and 1 for NTRK1.

Furthermore, although functionally different, the diverse activating genes share the post-obit three properties: (1) they are ubiquitously expressed; (2) they brandish domains demonstrated or predicted to be able to form dimers or multimers; (3) they translocate the tK-receptor-associated enzymatic activeness from the membrane to the cytoplasm.

These characteristics tin explain the machinery(s) of oncogenic activation of ret and NTRK1 protooncogenes. In fact, post-obit the fusion of their tK domain to activating factor, several things happen: (i) ret and NTRK1, whose tissue-specific expression is restricted to subsets of neural cells, become expressed in the epithelial thyroid cells; (2) their dimerization triggers a constitutive, ligand-independent transautophosphorylation of the cytoplasmic domains and as a consequence, the latter can recruit SH2 and SH3 containing cytoplasmic effector proteins, such equally Shc and Grb2 or phospholipase C (PLCγ), thus inducing a constitutive mitogenic pathway; (3) the relocalization in the cytoplasm of ret and NTRK1 enzymatic activity could let their interaction with unusual substrates, perhaps modifying their functional properties.

In conclusion, in PTCs, the oncogenic activation of ret and NTRK1 protooncogenes following chromosomal rearrangements occurring in breakpoint cluster regions of both protooncogenes could be defined as an ectopic, constitutive, and topologically aberrant expression of their associated enzymatic (tK) activity.¹³⁴

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Source: https://www.ncbi.nlm.nih.gov/books/NBK12538/