Cancer genes can be divided into two main classes: oncogenes and tumor suppressor genes. Oncogenes promote cell growth and proliferation when activated by mutations, while tumor suppressor genes normally inhibit cell growth and their inactivation allows for unchecked cell division. Dysfunction of multiple cancer genes is typically required for malignant transformation, as an imbalance between oncogene and tumor suppressor gene activity leads to cancer development. Common oncogenes include ras, myc, and HER2, while tumor suppressor genes include RB, p53, and APC. Mutations in DNA repair genes can also contribute to cancer by allowing genetic errors to persist.

1. Cancer Genes
Mohammed Fathy Bayomy, MSc, MD
Lecturer
Clinical Oncology & Nuclear Medicine
Faculty of Medicine
Zagazig University

2.  Definition : normal DNA sequences which when altered will initiate or
complement process of cellular malignant transformation. Their normal
counterparts regulate cellular growth & differentiation in response to
environmental signals
 Classes
1) Oncogenes: arise from activation of normal proto-oncogenes & have
growth stimulatory effect
2) Tumour suppressor genes: normally have growth suppressive
function on cells. Inactivation of tumor suppressor genes will
contribute to neoplastic development
3) Apoptosis genes
4) DNA repair genes

3.  Function
* Oncogenes & tumor suppressor genes have opposing actions:
oncogenes favour cell proliferation & dedifferentiation and inhibit
apoptosis (e.g. bcl-2). Conversely, tumor suppressor genes inhibit
cellular proliferation, stimulate apoptosis, enhance cellular
differentiation & stimulate DNA repair
* Normally, there is balance between proto-oncogene & tumor
suppressor gene functions, hence stable cell population
* Imbalance between oncogene & tumor suppressor gene actions, with
predominance of former, results in malignant transformation

4.  Complementation of cancer genes
* Dysfunction of at least two cancer genes with different functions
are required (e.g. myc & ras)
* In case of carcinoma of colon, there is mutation of at least 5 cancer
genes (APC, ras, DCC, MCC, p53) in addition to other events as
mutation of mismatch DNA repair genes & hypomethylation of
DNA, both contributing to genetic instability

5. Oncogenes

6.  Definition: genes that promote autonomous cell growth in cancer
cells
 Derived by: mutations in proto-oncogenes
 Characterized by: ability to promote cell growth in absence of
normal growth-promoting signals
 Inheritance: dominant genes (mutation of single allele is sufficient to
cause cancer)
 Mechanism of activation of proto-oncogene to oncogene:
* Chromosomal translocation
* Gene amplification
* Gene mutation

8.  Their products: oncoproteins, resemble normal products of proto-
oncogenes except that oncoproteins are devoid of important
regulatory elements, & their production in transformed cells does
not depend on growth factors or other external signals
 They are classified according to their oncogenic protein products
1) Growth factors
2) Growth factor receptors
3) Signal transduction proteins
4) Transcription factors
5) Cell cycle regulators
6) Antiapoptosis

10. Growth factors
 Mutations of genes that encode growth factors can make their products
oncogenic & cancer cell is stimulated to proliferate by autocrine
mechanism
 Oncogene c-sis encodes beta chain of PDGF (reported in astrocytomas
& osteosarcomas)
 Oncogenes hst-1 & int-2 encode for FGF & are activated in several
gastrointestinal & breast tumors
 Mutations of ras causes overexpression of TGF-α & EGF
 It is noteworthy that stimulation of cells by growth factors is not
sufficient alone to cause cancer. They act as promoting factors through an
epigenetic mechanism

11. Growth factor Receptors
 Mutant receptors: associated with persistent activation of tyrosine
kinase activity of their cytoplasmic domain, even without binding of
receptor to growth factor  continuous mitogenic signal to nucleus
 c-erb-B2 (HER-2/neu): reported in adenocarcinoma of breast; ovary &
lung arises from mutation that alters single amino acid (valine to
glutamine) in transmembrane region of receptor, making receptor
constitutively active as kinase
 c-erb-B1: expressed in squamous carcinoma of lung, EGF receptor loses
its binding domain leading to hyperactive truncated protein

12. Signal Transduction Proteins
These membrane-bound signal transducers receive signals from receptor-
related tyrosine kinase & transmit signal to nucleus by secondary messenger.
They belong to two major categories:
 GTP-binding proteins (ras oncogene)
* Mutation of ras is most common oncogene encountered & is observed
in 30% of all human tumors of various types (90% of pancreatic
cancer, 50% of colon & 30% of lung carcinoma)
* Activated by point mutation
* Normally there is orderly cycling of ras between guanosine diphosphate
(ras-GDP, inactive form) & ras-GTP (active form)
* GAP (GTPase activating protein) causes hydrolysis of ras-GTP to its
inactive form

13. * With mutation of ras, GAP can not hydrolyse ras-GTP which remains in
active state & stimulate another downstream mitogenic signal pathways
namely Raf-1 & MAPK (mitogen activated protein kinase)
 Non-receptor associated Kinase (abl oncogene)
* In chronic myeloid leukemia (CML), proto-oncogene (abl) is
translocated from its normal location on chromosome 9 to chromosome
22 where it fuses with bcr (break point cluster region) gene
* Resulting hybrid gene produce chimeric protein with potent mitogenic
tyrosine kinase activity
* Chromosome resulting from this reciprocal translocation is called
Philadelphia chromosome

14. Transcription factors
 C-myc
* In Burkitt's lymphoma, c-myc is translocated from its normal
place on chromosome 8 to chromosome 14 where it is placed
close to immunoglobulin heavy chain gene. In this new location
c-myc in subjected to stimulation by enhancer element of
immunoglobulin gene
* Following translocation, c-myc protein is rapidly translocated to nucleus
forms heterodimer with another protein (max)
* c-myc/max complex activates transcription of several cell-cycle-related
genes resulting in cellular proliferation
 N-myc: amplified in neuroblastoma
 L-myc: amplified in small cell lung cancer

15. Cell Cycle regulators
 Orderly progression of various stages of cell cycle is under control of two
classes of proteins, namely: cyclins (regulatory units) & cyclin-
dependent-kinases (catalytic units). CDKs are expressed continuously,
but different cyclins are expressed periodically according to phase of cell
cycle
 In mantle cell lymphoma, translocation t (11; 14)  activation of PRAD-
1 gene  overexpression of cyclin-Dl  activates CDK-4 
phosphorylation of Rb protein  release of E2F which drives cell to enter
S-phase (activates transcription of several genes whose products are
essential for progression through S-phase)

16. Antiapoptosis
 Genes which inhibit apoptosis lead to cell immortalization & cell
accumulation, hence favoring neoplastic development
 In follicular lymphoma translocation t ( 14; 18) in which causes
activation of antiapoptotic gene bcl-2

17. Tumor suppressor genes

18.  Definition: genes that act as negative regulators, or brakes, of cell
cycle & cellular proliferation
 Inheritance: autosomal recessive traits (mutations in both alleles are
required to get disease), change of heterozygous to homozygos state =
cancer develops after loss of heterozygosity (LOH)
 Regard to cancer risk: considered dominant (single mutant allele is
associated with significant cancer risk)
 Classified according to mechanism of action
1) Growth inhibition factor (TGF-β)
2) Inhibition of signal transduction (NF-1 for ras, APC for β catenin)
3) Augmentation of cell adhesion (E-cadherin)
4) Negative regulators of cell cycle (RB, p53, WT-1, p16)
5) Increase DNA repair (HNPCC, BRCA, MMR genes)

19.  Mechanism of loss of function of tumor suppressor gene
* Loss of heterozygosity (LOH) in tumour suppressor genes may occur
by one of 4 mechanisms which results in loss of remaining normal
allele:
(1) deletion due to non-disjunction
(2) non-disjunction & duplication
(3) unequal crossing over
(4) gene mutation
* Inactivation of their protein products there is loss of tumour
suppressor gene function in heterozygous state. This is explained by
"dominant negative effect" phenomenon. Protein product of mutant
suppressor gene inhibits protein product of wild suppressor gene

21. Retinoblastoma gene (RB)
 Located in: long arm of chromosome 13
 Loss of function of Rb gene: involved not only in retinoblastoma, but
also in several other cancers as osteosarcoma, breast, lung and bladder
 Most retinoblastomas (90%) are sporadic but about 10% are familial
 Knudson double hit hypothesis: in familial cases, one genetic change
(first hit) is inherited from affected parent (germline mutation) &
therefore present in all somatic cells. Second mutation (second hit) occurs
in one of retinal cells leading to loss of heterozygosity (LOH) & tumor
formation. Conversely, in sporadic type of retinoblastoma, both mutations
are acquired after birth in somatic cells

23.  Product of Rb gene (pRb)
* Nuclear phosphoprotein that regulates cell
cycle
* Normally, it acts as brake of cell cycle by
binding with a transcription factor E2F. Thus,
in its active hypophosphorylated state pRb
prevents cell replication by binding with
transcription factor E2F. This binding occurs in
special site of Rb molecule called Rb pocket.
Any factor which inhibits Rb phophorylation
(e.g. p53 action) will keep E2F in bound state
with arrest of cell cycle in G1

24. * This braking mechanism is lost with release of E2F from its bound
state under 3 conditions
1) Phosphorylation of Rb as a result of ras gene action or loss of p53
gene function
2) Human papilloma virus onocprotein (E7) which binds with Rb
pocket
3) Mutation of Rb gene which typically involves the Rb pocket. Under
these conditions, E2F is released and induce cyclin E formation and
cell enter S phase (cell proliferation)

25. p53 gene
 Located in: short arm of chromosome 17
 Inactivation of p53 gene: the most common gene alteration in human
cancer (>50% of human malignant tumors)
 Product of p53 gene (p53)
* Following DNA damage, wild p53 normally causes arrest of cell cycle
to allow for DNA repair. If this initial mechanism fails, p53 induces
apoptosis to eliminate damaged cells & keep genomic integrity
(policeman of DNA or guardian of genome)
* Nuclear phosphoprotein, under normal conditions (wild p53) it is
concerned with negative control of cell cycle, DNA repair & apoptosis
* Wild, p53 is activated following gamma or UV irradiation,
chemotherapy or hypoxia

26. * Inhibitory action of wild p53 on cell cycle is mediated through Rb
protein
* p53  activates gene WAF-1  produce inhibitory protein p21 
inhibits cyclin/cyclin-dependent kinase (cycl-Dl/ CDK-4) 
hypophosphorylation of pRb  keeps transcription factor E2F in
bound state of pRb with arrest of cell cycle in Gl

27. * Results in abnormal protein devoid of its normal function
* Characterized by remarkably long half life
* There is loss of cell cycle control with unchecked cellular proliferation
* Policeman action of p53 is lost  cellular mutations are propagated
rather than eliminated. Hence, genetic instability observed in malignant
tumors
 Loss of p53 function (or inactivation) occurs under 3 conditions
 Mutant p53 gene or inactivated p53
1) Missense mutation, which may be germline (Li- Fraumeni
syndrome) or somatic (in several tumors)
2) Inhibitory effect of p53 gene product e.g. MDM2, kind of feedback
auto-regulation
3) Inhibitory effect of human papilloma virus oncoprotein products (E6)

28. Apoptosis genes

29. Apoptosis
 Definition: Apoptosis is defined as a distinctive active, genetically
programmed cell death which eliminates unwanted cells. Literally,
apoptosis in Greek language means falling of leaves of trees in winter
 Character
* May be physiological or pathological
* Resulting from mild injuries
* Affects single isolated cells
* Nonviable cell constituents shrink &
are enclosed by cell membranes, 
absence of inflammatory reaction

31.  Pathogenesis: enzymes are activated by calcium ions, cytochrome-C,
Apaf-1 & ceramide
* Enzymatic cleavage of cytoskeleton protein by cysteine proteases
(caspases)
* Protein cross-linking by transglutaminases
* Cleavage of DNA by several agents, including enzymes
endonuclease
 Pathways: two enzymes activation pathways
1) Extrinsic (Death receptor) pathway
FAS ligand (soluble or membrane-bound) will bind to FAS cell
membrane receptor or cytokine tumor necrosis factor (TNF) will bind to
its receptor (TNFR) on cell membrane  activation of membrane-bound
protein FADD  caspase activation

32. 2) Intrinsic (Mitochondrial integrity) pathway
Increase of mitochondrial permeability  release of calcium &
cytochrome-C from mitochondria to cytosol  assembly of apoptotic
protease activating factor (Apaf-1)  activates caspases

33. Genetic control of Apoptosis
 Bcl-2 family members are decision-makers that integrate pro-apoptotic
and anti-apoptotic signals to determine whether cell should commit
suicide
1) Anti-apoptotic Bcl-2 type proteins
* Including Bcl-2, Bcl-xL, Bcl-wL
* Two ways of antagonizing death signals
- They insert into outer mitochondrial membrane to antagonize
channel-forming pro-apoptotic factors  cytochrome c release
- Bind cytoplasmic Apaf so that it cannot form apoptosome complex

34. 2) Pro-apoptotic Bcl-2 type proteins
* Pro-apoptotic ion channel forming members (Bax, Bak, Bok,
Bcl-xs) when they dimerize with pro-apoptotic BH3-only members
in outer mitochondrial membrane, they form an ion channel that
promotes cytochrome c release rather than inhibiting it
* Pro-apoptotic BH3-only proteins (Bad, Bid, Bom/Bim) activates
pro-apoptotic family members (Bax) & inactivates anti-apoptotic
members (Bcl-2)

35. DNA Repair genes

36. Mutation
 Definition: any permanent & heritable change in DNA base sequence
 Classification
* Germline mutation: mutation occurs in gamete cells, mutation
will be passed on to all cells of body
* Somatic mutation: mutation affects somatic cell it will be more
restricted & passes to only descendents of that cell lineage in
particular organ
1) According to cell type affected
2) According to structural changes
* Point mutation: involves single base that substitute one amino
acid for another
* Deletions or insertions: remove or add one or more bases
* Inversion mutation: inverts segment of DNA sequence

37. * Missense mutation: substitutes one amino acid for another thus
resulting in abnormal protein
* Nonsense mutation: substitutes a stop codon for amino acid
codon leading to incomplete or truncated protein
* Frameshift mutation: adds or deletes base with shift of base
sequence, hence, changing reading frame, resulting in
mistranslation of all amino acid sequence beyond mutation
* Splicing mutation: will act on splice junction usually causing
major changes in m-RNA & encoded protein
3) According to consequence of mutation on encoded protein

38.  Pathogenesis
* Mispairing of bases ( A-C pair instead of normal A-T pair) during
DNA synthesis ( replication error ) or recombination
* Slippage of newly synthesized DNA strand during/replication
with loop formation is another replication error resulting in
copying of bases twice
* Loss of purine bases ( depurination) occurs as a result of effect
of body heat and cellular metabolism
* De-amination of cytosine leading to C-T substitution
1) Spontaneous mutations

39. * Ultraviolet radiation produce pyrimidine dimers in which
adjacent thymine residues in same DNA strand become covalently
attached
* Exposure to ionizing radiation may produce various lesions such
as : base damage (e.g. conversion of thymine to thymine glycol or
change of guanine to hydroxy guanine), single or double strand
breaks in DNA phosphate-sugar backbone, and cross-links between
the two strands of DNA or between DNA & proteins
* Chemotherapeutic-agents may cause base damage (alkylating
agents), strand breaks (bleomycin) or various adducts or cross-
links (cisplatin)
2) Induced mutations

40. DNA Repair
 Cells have evolved an elaborate array of enzymatic systems to maintain
integrity of their genetic material in the face of numerous agents that alter
DNA structure or base sequence
 Cellular repair of macromolecule is known to occur only for DNA, vital
molecule for cell survival
 Tumor suppressor gene p5 3 plays key role in this regard, hence it is
called guardian of genome. Thus, with DNA injury, p53 causes arrest of
cell cycle to give time for DNA repair enzymes. If DNA lesions are
severe & irrepairable, p5 3 eliminates mutant cells by stimulating
apoptosis

41. DNA Repair Systems
 Damaged DNA is reverted to its original state without replacement
of constituents
 Example
1) Damage Reversal
* Repair of base damage, by alkyl transferase enzyme, removing
methyl & larger alkylating groups from DNA bases
* Rejoining single strand break by ligase
2) Mismatch repair (MMR)
 Four types of mismatch repair genes: hMSH-2 (2p), hMCH-1
(3p), hPMS-1, hPMS-2
 During replication, mismatch protein gene products act as "spell
checkers" and if there is replication error (erroneous pairing of G-T
instead of A-T), this system corrects defect

42.  Mismatched lesion is recognized by hMSH-2 protein, or hMSH-2:
GTBP/P160 protein heterodimer
 Removal of mismatch, followed by resynthesis & ligation completes
repair process
3) Excision repair
A) Base Excision Repair (BER)
 Limited to small lesions (de-aminated cytosine, single strand breaks
 Repair patches consist usually of only one or two nucleotides (short
patch repair)
 Base damage is recognized & excised by glycosylase, removal of
sugar residue by endonuclease & exonuclease, followed by
resynthesis & ligation

43. b) Nucleotide Excision Repair (NER)
 Involves replacement of long patches of DNA of order of thirty
nucleotides (long patch repair)
 There are 5 steps in NER process, namely: recognition of DNA
lesion, incision of damaged strand on each side of lesion, removal of
damaged nucleotides, synthesis of new nucleotides, & its ligation
4) Recombination repair
 Double strand breaks are most difficult to repair due to absence of
template strand
 Unrepaired or misrepaired double strand breaks may lead to serious
consequences: end-to-end joining of nonhomologous chromosomes, or
formation of new telomeres generating chromosomal rearrangements
deletions, use of nonhomologous DNA strands as template results in
formation of abnormal base sequence (mutation)

44.  Double strand breaks, in view of difficulty of repair, is most
common cause of cell lethality following genotoxic agents
 Repair requires recruitment of homologous or heterologous DNA
strands with formation of Holliday junction caused by crossing over
of single strands from two adjacent DNA duplexes
 This junction requires resolution, by enzymatic cutting of crossing
point, before repair & duplex separation are complete