Which Of The Following Genetic Changes Cannot Convert A Proto-oncogene Into An Oncogene?

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. Three genetic mechanisms activate oncogenes in human neoplasms: (1) mutation, (two) cistron distension, and (three) chromosome rearrangements. These mechanisms result in either an amending of protooncogene structure or an increase in protooncogene expression (Figure vi-5). Considering neoplasia is a multistep process, more 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 chapters for metastasis, unremarkably involves a combination of protooncogene activation and tumor suppressor cistron loss or inactivation.

Effigy six-5

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

Mutation

Mutations activate protooncogenes through structural alterations in their encoded proteins. These alterations, which unremarkably involve critical protein regulatory regions, often lead to the uncontrolled, continuous activeness 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-binding domains of the erb B, kit, ros, met, and trk oncogenes.^{half dozen} In human tumors, nevertheless, nigh characterized oncogene mutations are base substitutions (betoken mutations) that change a single amino acid within the protein.

Point mutations are frequently detected in the ras family of protooncogenes (K-ras, H-ras, and N-ras).⁷⁷ It has been estimated that every bit many as xv% to 20% of unselected human tumors may incorporate a ras mutation. Mutations in K-ras predominate in carcinomas. Studies accept found Thou-ras mutations in nearly thirty% of lung adenocarcinomas, 50% of colon carcinomas, and 90% of carcinomas of the pancreas.⁷⁸ N-ras mutations are preferentially institute in hematologic malignancies, with upwardly to a 25% incidence in acute myeloid leukemias and myelodysplastic syndromes.^79, ^eighty The majority of thyroid carcinomas have been found to have ras mutations distributed among K-ras, H-ras, and North-ras, without preference for a single ras family member but showing an clan 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 equally codons 13 or 61.⁸³ Ras mutations in human tumors have been linked to carcinogen exposure. The effect of ras mutations is the constitutive activation of the signal-transducing part of the ras protein.

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

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

Factor Distension

Gene amplification refers to the expansion in copy number of a gene within the genome of a jail cell. Gene amplification was first discovered as a mechanism by which some tumor jail cell lines can acquire resistance to growth-inhibiting drugs.⁸⁶ The process of gene amplification occurs through redundant replication of genomic DNA, oftentimes giving ascension to karyotypic abnormalities called double-infinitesimal 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 nighttime-staining bands. Both DMs and HSRs represent large regions of amplified genomic Dna containing up to several hundred copies of a gene. Amplification leads to the increased expression of genes, which in plow tin confer a selective advantage 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 tumors (Table vi-2). Virtually 20% to xxx% of chest 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 through its distension in neuroblastomas.⁹⁰ Distension of N-myc correlates strongly with advanced tumor stage in neuroblastoma (Table 6-3), suggesting a role for this gene in tumor progression.^91, ⁹² L-myc was discovered through its amplification in small-cell carcinoma of the lung, a neuroendocrine-derived tumor.⁹³ Amplification of erb B, the epidermal growth factor receptor, is establish in upwards to 50% of glioblastomas and in x% to twenty% of squamous carcinomas of the head and neck.⁷⁷ Approximately 15% to xxx% of breast and ovarian cancers have distension of the erbB-2 (HER-2/neu) gene. In chest cancer, erbB-two amplification correlates with advanced stage and poor prognosis.⁹⁴ Members of the ras factor family, including K-ras and N-ras, are sporadically amplified in diverse carcinomas.

Table half dozen-2

Oncogene Amplification in Homo Cancers.

Chromosomal Rearrangements

Recurring chromosomal rearrangements are oft detected in hematologic malignancies equally well equally in some solid tumors.^37, ^95, ⁹⁶ These rearrangements consist mainly of chromosomal translocations and, less ofttimes, chromosomal inversions. Chromosomal rearrangements can lead to hematologic malignancy via two different mechanisms: (i) the transcriptional activation of protooncogenes or (2) the creation of fusion genes. Transcriptional activation, sometimes referred to as gene activation, results from chromosomal rearrangements that motion a proto-oncogene close to an immunoglobulin or T-cell receptor gene (meet Figure 6-5). Transcription of the protooncogene then falls nether command of regulatory elements from the immunoglobulin or T-cell receptor locus. This circumstance causes deregulation of protooncogene expression, which tin can then pb to neoplastic transformation of the cell.

Fusion genes can exist created past chromosomal rearrangements when the chromosomal breakpoints autumn inside the loci of two different genes. The resultant juxtaposition of segments from two different genes gives ascension to a composite construction consisting of the head of one gene and the tail of another. Fusion genes encode chimeric proteins with transforming activeness. In 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 establish in hematologic malignancy.⁹⁷ Examples of molecularly characterized chromosomal rearrangements in hematologic and solid malignancies are given in Table half-dozen-four. In some cases, the aforementioned protooncogene is involved in several different translocations (ie, c-myc, ews, and ret).

Table vi-iv

Molecularly Characterized Chromosome Rearrangements in Tumors.

Gene Activation

The t(8;14)(q24;q32) translocation, establish in near 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 cistron, located at chromosome band 8q24, under command of regulatory elements from the immunoglobulin heavy chain locus located at 14q32.⁹⁸ The resulting transcriptional activation of c-myc, which encodes a nuclear poly peptide involved in the regulation of cell proliferation, plays a disquisitional role in the development of Burkitt lymphoma.⁹⁹ The c-myc cistron is also activated in some cases of Burkitt lymphoma by translocations involving immunoglobulin light-chain genes.^100, ¹⁰¹ These are t(2;viii)(p12;q24), involving the κ locus located at 2p12, and t(8;22)(q24;q11), involving the κ locus at 22q11 (Figure 6-6). Although the position of the chromosomal breakpoints relative to the c-myc cistron may vary considerably in individual cases of Burkitt lymphoma, the consequence of the translocations is the same: deregulation of c-myc expression, leading to uncontrolled cellular proliferation.

Effigy 6-6

C-myc translocations found in Burkitt lymphoma. A, t(8;14)(q24;q32) translocation involving the locus of immunoglobulin heavy-chain gene located at 14q32. B, t(8;xiv)(q24;q32) translocation where only 2 exons (Ex) of c-myc are translocated under regulatory (more...)

In some cases of T cell astute lymphoblastic leukemia (T-ALL), the c-myc gene is activated by the t(8;14)(q24;q11) translocation. In these cases, transcription of c-myc is placed under the control of regulatory elements within the T-cell receptor α locus located at 14q11.¹⁰² In improver to c-myc, several protooncogenes that encode nuclear proteins are activated by 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 past these genes are thought to function as transcription factors through Deoxyribonucleic acid-binding and protein-protein interactions. Overexpression or inappropriate expression of these proteins in T cells is idea to inhibit T-jail cell differentiation and lead to uncontrolled cellular proliferation.

A number of other protooncogenes are also activated past chromosomal translocations in leukemia and lymphoma. In most follicular lymphomas and some large prison cell lymphomas, the bcl-2 gene (located at 18q21) is activated every bit a consequence of t(xiv;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 Drapery cell lymphomas are characterized by the t(11;14)(q13;q32) translocation, which activates the cyclin d1 (bcl-1) factor located at 11q13.^106, ¹⁰⁷ Cyclin D1 is a G1 cyclin involved in the normal regulation of the cell cycle. In some cases of T prison cell chronic lymphocytic leukemia and prolymphocytic leukemia, the tcl-1 gene at 14q32.1 is activated by inversion or translocation involving chromosome 14.¹⁰⁸ The tcl-1 gene production is a small cytoplasmic protein whose role is not yet known.

Factor Fusion

The first 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 factor, normally located at 9q34, with the bcr gene at 22q11 (Figure 6-seven).¹¹⁰ The bcr/abl fusion, created on the der(22) chromosome, encodes a chimeric poly peptide of 210 kDa, with increased tyrosine kinase activity and abnormal cellular localization.¹¹¹ The precise mechanism by which the bcr/abl fusion protein contributes to the expansion of the neoplastic myeloid clone is not even so known. The t(9;22) translocation is also found in up to 20% of cases of acute lymphoblastic leukemia (ALL). In these cases, the breakpoint in the bcr gene differs somewhat from that found in CML, resulting in a 185 kDa bcr/abl fusion protein.¹¹² It is unclear at this time why the slightly smaller bcr/abl fusion protein leads to such a large deviation in neoplastic phenotype.

Figure 6-7

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

In addition to c-abl, ii other genes encoding tyrosine kinases are involved in distinct gene fusion events in hematologic malignancy. The t(2;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 unit. The NPM protein is a nucleolar phosphoprotein involved in ribosome associates. The NPM/ALK fusion creates a chimeric oncoprotein in which the ALK tyrosine kinase activeness may be constitutively activated. The t(5;12)(q33;p13) translocation, characterized in a case of chronic myelomonocytic leukemia, fuses the tel gene (12p13) with the tyrosine kinase domain of the PDGF receptor b gene (PDGFR-b at 5q33).¹¹⁴ The tel gene is thought to encode a nuclear DNA-bounden protein similar to those of the ets family of protooncogenes.

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

The ALL1 gene, located at chromosome band 11q23, is involved in approximately v% to ten% of acute leukemia cases overall in children and adults.^118, ¹¹⁹ These include cases of ALL, astute myeloid leukemia, and leukemias of mixed prison cell lineage. Among leukemia genes, ALL1 (as well called MLL and HRX) is unique because information technology participates in fusions with a large number of different partner genes on the various chromosomes. Over 20 different reciprocal translocations involving the ALL1 gene at 11q23 accept been reported, the well-nigh common of which are those involving chromosomes 4, 6, 9, and 19.¹²⁰ In approximately v% of cases of acute leukemia in adults, the ALL1 cistron is fused with a portion of itself.¹²¹ This special type of cistron fusion is chosen self-fusion.¹²² Self-fusion of the ALL1 gene, which is thought to occur through a somatic recombination mechanism, is found in loftier incidence in acute leukemias with trisomy 11 every bit a sole cytogenetic abnormality. The ALL1 gene encodes a large protein with DNA-binding motifs, a transactivation domain, and a region with homology to the Drosophila trithorax poly peptide (a regulator of homeotic gene expression).^123, ¹²⁴ The various partners in ALL1 fusions encode a diverse group of proteins, some of which announced to exist nuclear proteins with DNA-bounden motifs.^125, ¹²⁶ The ALL1 fusion poly peptide consists of the aminoterminus of ALL1 and the carboxyl terminus of one of a multifariousness of fusion partners. It appears that the critical feature in all ALL1 fusions, including self-fusion, is the uncoupling of the ALL1 amino-terminal domains from the remainder of the ALL1 protein.

Solid tumors, specially sarcomas, sometimes have consequent chromosomal translocations that correlate with specific histologic types of tumors.¹²⁷ In general, translocations in solid tumors result in gene fusions that encode chimeric oncoproteins. Studies thus far bespeak that in sarcomas, the majority of genes fused by translocations encode transcription factors.¹²⁸ In myxoid liposarcomas, the t(12;16)(q13;p11) fuses the FUS (TLS) gene 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 poly peptide, which is a dominant inhibitor of transcription, contributes a protein-bounden domain and a presumptive DNA-binding domain to the fusion. Despite knowledge of these structural features, the mechanism of action of the FUS/CHOP oncoprotein is not nonetheless known. In Ewing sarcoma, the t(eleven;22)(q24;q12) fuses the EWS gene at 22q12 with the FLI1 gene 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 DNA-binding domain of FLI1. In alveolar rhabdomyosarcoma, the t(2;thirteen)(q35;q14) fuses the PAX3 gene at 2q35 with the FKHR gene at 13q14.¹³¹ The PAX3 poly peptide, a transcription cistron that activates genes involved in development, is a paired-box homeodomain protein with two distinct Deoxyribonucleic acid-binding domains. The FKHR protein encodes a conserved Deoxyribonucleic acid-bounden motif (the forkhead domain) similar to that first identified in the Drosophila forkhead homeotic gene. The PAX3/FKHR fusion protein is a chimeric transcription gene containing the PAX3 Deoxyribonucleic acid-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, equally we have discussed before, the commencement chromosomal abnormality in solid tumors to be characterized at the molecular level as a fusion protein was an inversion of chromosome 10 institute in papillary thyroid carcinomas.¹³² In this tumor, ii 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 2 abnormalities represent the cytogenetic mechanisms which activate the protooncogene ret on chromosome 10, forming the oncogenes RET/ptc1 and RET/ptc2, respectively. Alterations of chromosome ane in the same tumor type have and so been associated to the activation of NTRK1 (chromosome i), an NGF receptor which, like RET, forms chimeric fusion oncogenic proteins in papillary thyroid carcinomas.¹³³ A comparative analysis of the oncogenes originated from the activation of these two tyrosine kinase receptors has allowed the identification and label of common cytogenetic and molecular mechanisms of their activation. In all cases, chromosomal rearrangements fuse the tK portion of the two receptors to the 5′ end of dissimilar genes that, due to their general effect, accept been designated every bit activating genes. In the bulk of cases, the latter belong to the same chromosome where the related receptor is located, 10 for RET and 1 for NTRK1.

Furthermore, although functionally different, the various activating genes share the following three properties: (1) they are ubiquitously expressed; (two) they display domains demonstrated or predicted to be able to grade dimers or multimers; (3) they translocate the tK-receptor-associated enzymatic activeness from the membrane to the cytoplasm.

These characteristics can explicate the mechanism(s) of oncogenic activation of ret and NTRK1 protooncogenes. In fact, following the fusion of their tK domain to activating gene, several things happen: (1) ret and NTRK1, whose tissue-specific expression is restricted to subsets of neural cells, become expressed in the epithelial thyroid cells; (two) their dimerization triggers a constitutive, ligand-independent transautophosphorylation of the cytoplasmic domains and as a consequence, the latter tin recruit SH2 and SH3 containing cytoplasmic effector proteins, such as Shc and Grb2 or phospholipase C (PLCγ), thus inducing a constitutive mitogenic pathway; (iii) the relocalization in the cytoplasm of ret and NTRK1 enzymatic action could permit 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 abnormal expression of their associated enzymatic (tK) action.¹³⁴