Oncogenes: Identification and Role in Human Cancer, Study Guides, Projects, Research of Oncology

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ONCOGENESONCOGENES

Molecular Oncology - 2013

Michael Lea

ONCOGENES - Lecture Outline

I. Introductiont oduct o2. Identification of oncogenic genes in retroviruses3. Homologous sequences in transformed and untransformed cells4 Methods of transforming cells with oncogenes4. Methods of transforming cells with oncogenes5. Mechanisms for the activation of protooncogenes6. Naming of oncogenes7. Normal role of protooncogenes8. Cellular locations and functions of oncogenes9. Oncogenes coding for growth factors, truncated growth factorg

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receptors, non-receptor tyrosine kinases, guanine nucleotide bindingproteins and DNA binding proteins10. Genes associated with apoptosis

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11. Genes of DNA viruses12. Telomerase13 Micro RNAs13. Micro RNAs14. Summary of human cancer genes

IDENTIFICATION OF ONCOGENIC GENES IN

RETROVIRUSES

Oncogenes were first identified in some RNA viruses TheOncogenes were first identified in some RNA viruses. The

oncogenic RNA viruses contain reverse transcriptase and aredescribed as retroviruses. Retroviruses that possess oncogenes are arare and highly oncogenic group. While most viral oncogenes arerare and highly oncogenic group. While most viral oncogenes arebelieved to have originated from rare transductions of cellularsequences, there are exceptions. Thus in human T-cell leukemiaviruses (HTLV) there is a 1.5 kilobase region described as the X(

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region which lies between the env gene and the 3’LTR and is believedto mediate the transformation properties of the HTLV family. Thisregion does not appear to have a cellular homolog.

HOMOLOGOUS SEQUENCES ARE IDENTIFIED IN

DNA OF TRANSFORMED CELLSHybridization experiments revealed homology

between viral oncogenes and DNA sequences in normalbetween viral oncogenes and DNA sequences in normalcellular DNA. This was first established for the src gene.The viral and cellular genes were distinguished as v-srcd^

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and c-src, respectively. It appears that some viruses haveacquired cellular genes and that changes in the nucleotidesequences have caused the genes to have transformingq^

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properties. The acquired genes or oncogenes are notrequired for the virus to be infective or for viral replication.Transformation-defective variants which lack theTransformation defective variants which lack theoncogene can replicate.

MECHANISMS FOR THE ACTIVATION OF PROTOONCOGENES1. Mutations in the gene sequence may cause either achange in activity or an alteration in the function of the gene proteinproduct e.g. ras genes.2. Chromosomal translocation may put the gene in a differentregulatory environment e.g. myc. The first chromosomal translocationto be consistently associated with malignancy in humans was termedthe Philadelphia chromosome. This occurs in many cases of chronicmyelocytic leukemia and results from a translocation fromchromosome 9 to chromosome 22. Other frequently observed

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chromosomal translocations occur near the sites for c-myc and c-mos.These are 8:14 in 90% of Burkitt’s lymphoma and 8:21 in acutemyeloid leukemia, respectively.

Translocation of chromosomes 9 and 22 yielding thePhiladelphia chromosome seen in most cases of Chronic myelogenous leukemia

MECHANISMS FOR THE ACTIVATION OF PROTOONCOGENES3 Gene amplification is accompanied by two cytogenetic3. Gene amplification is accompanied by two cytogeneticchanges: double minute chromosomes and homogeneously stainingregions (HSRs). Changes of this type are seen infrequently in tumorsin vivo but are more frequent in cultured cells. N-myc and c-myc arein vivo but are more frequent in cultured cells. N myc and c myc arethe oncogenes which have been found most frequently to beamplified. Other genes which have been found to be amplified insome tumor cells include c-Ki-ras, c-myb, c-abl and c-erb B.Oncogene amplification may be associated with tumorprogression as seen with N-myc in neuroblastomas and with c-myc insmall cell lung carcinomas. In the case of c-myc the gene productmay antagonize cell differentiation. Amplification of genes isaccompanied by roughly proportional increase in the number oftranscripts. In the human promyelocytic leukemia cell line, HL60, thef^

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amplification of c-myc may decrease the tendency of promyelocyticleukemia cells to differentiate in culture. In HL60 cells, differentiatingagents such as 1,25-dihydroxyvitamin D3 can cause down-regulationof the amplified c myc genes and this is accompanied byof the amplified c-myc genes and this is accompanied bydifferentiation.

MECHANISMS FOR THE ACTIVATION OF PROTOONCOGENESOncogene amplification can be accompanied by gene rearrangement butmost amplified oncogenes are apparently normal on the basis of restrictionendonuclease mapping. Gene amplification arises from a segment of DNA replicatingmore than once during a single cell cycle There is evidence that there are preferredmore than once during a single cell cycle. There is evidence that there are preferredchromosomal positions for amplification of cellular genes and chromosomalrearrangements may facilitate gene amplification by placing a gene at a more favorablesite.4 I

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4. Insertional mutagenesis may operate if regulatory elements of a virus are inserted in proximity to a cellular protooncogene. This type of mechanism may occurwith avian leukosis virus which does not have an identified oncogene. There is evidencethat proviral integration in lymphomas can occur in the region of c-myc. Aviant^ i^

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retroviruses possess a well defined sequence which is responsible for the control of viralgene transcription. This long terminal repeat sequence (LTR) is located both upstreamand downstream of the viral structural genes in the integrated provirus. The LTRcontains potential transcription regulatory signals including a TATA box, al^ d^

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polyadenylation-termination signal and the initiation signal (cap site). There is evidencethat the downstream LTR is unable to act as an efficient promoter of transcription whena transcriptionally active upstream LTR is present. This transcriptional interference mayexplain the observation that only deleted proviruses have been observed adjacent to c-i^ l^

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myc in lymphomas of chickens induced by the leukosis virus.

NORMAL ROLE OF ONCOGENESThe cellular protooncogenes have shown great

evolutionary stability and presumably have an importantevolutionary stability and presumably have an importantrole in normal development and function. ras genes havebeen identified in yeast. There is activation of c-myc and

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ras genes in liver regeneration after partial hepatectomy.Protooncogene expression is not necessarily associatedwith growth. Differentiation of HL60 cells is accompaniedg^

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by a decrease in c-myc expression but there is anincrease in the expression of c-fos.

CELLULAR LOCATIONS AND FUNCTIONS OF ONCOGENE

PRODUCTS

The protein products of oncogenes differ in their cellular

locations. They may be associated with the cell membrane, thecytoplasm or the nucleus. Oncogene products have been describedcytoplasm or the nucleus. Oncogene products have been describedwith one of the following functions

Growth factorGrowth factor receptorGrowth factor receptorSignal transduction factorGTP-binding plasma membrane proteinTyrosine-specific protein kinaseSerine/threonine-specific protein kinaseTranscription factor

GROWTH FACTORS WITH ONCOGENIC POTENTIALCooper (Oncogenes, 2nd edition, 1995) has summarized growth factors withoncogenic potential as follows:Proto-oncogene productPDGF family:

A chain, B chain (sis) FGF family:

acidic FGF, basic FGF, int-2, hst, FGF- EGF family;

EGF, TGF-alpha Wnt family

wnt 1 (int 1) wnt 3 Wnt family

wnt-1 (int-1), wnt- Hematopoietic growth factors interleukin-2interleukin-3M-CSFGM-CSF Biologically active PDGF is a dimer (A:A, A:B or B:B). The c-sis gene encodes al^ tid

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polypeptide precursor of the B chain of platelet derived growth factor (PDGF). The v-sisgene product may have an intracellular action and not require binding to the surfacereceptor.

ONCOGENES CODING FOR TRUNCATED GROWTH FACTOR

RECEPTORS

The most extensively studied oncogene corresponding to a

truncated growth factor receptor is erb B. The proto-oncogene producti^

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consists of an extracellular receptor domain, a transmembrane domain andan intracellular tyrosine kinase domain. In the activated oncogene theextracellular domain has been lost and there is a constitutively active tyrosinekinase domain.kinase domain.

Other growth factor receptors that are proto-oncogenes are the

hepatocyte growth factor receptor (met), stem cell receptor (kit), NGF receptor(trk), CSF-1 receptor (fms). There are other genes for receptor like proteinsfor which the ligand is unknown and which can be activated to formoncogenes. These include ret, ros, sea and neu.

Some receptors lack tyrosine kinase activity and may act through

nonreceptor protein kinases particularly of the Jak family Others may actnonreceptor protein kinases, particularly of the Jak family. Others may actthrough GTP-binding proteins. An example is the product of the masoncogene which corresponds to the angiotensin receptor.