ONCOGENE AND TUMOUR SUPPRESSOR GENE

ONCOGENE AND PROTOONCOGENE
P53 GENE AND ITS APPLICATION IN CANCER ETIOLOGY
TUMOUR SUPPRESSOR GENE AND BCA AND BAC GENE AND ITS APPLICATION ON THE APOPTOSIS AND DEATH RECEPTORS

1. ONCOGENE
&
TUMOUR
SUPPRESSOR GENE
RVS Chaitanya Koppala
Assistant Professor
Vignan Institute of Pharmaceutical Technology
Visakhapatnam

2. ONCOGENE AND TUMOR SUPPRESSOR GENES
 Cancer is a genetic disease and is mostly caused by somatic mutations.
 The characteristic properties of cancer cells - consequences of genetic changes in the
tumor cells - Genomic instability
 Self-sufficient proliferation of growth
 Refractory to inhibitory signals
 Survival without survival signals
 Unlimited replicative potential
 Recruitment of blood supply
 Invasion and metastasis
 Cancer cells contain multiple alterations in the number and structure of genes and
chromosomes, mainly acquired by mutations in somatic cells.
 Individual genes display point mutations such as base changes, insertions and deletions,
or can be affected by chromosomal translocations or inversions.
These changes lead to the
 expression of altered gene products,
 decreased or increased gene expression
 novel gene products like fusion proteins.
Two classes of genes affected by genetic and epigenetic alterations in cancer cells are
oncogenes and tumor suppressor genes.
 Oncogenes contribute to tumor development by increased or misdirected activity.
 Tumor suppressors - insufficient or lost function supports tumor development.
 Typically, in human cancers activation of oncogenes and inactivation of tumor
suppressor genes - both are observed.

3.  Oncogenes are cellular or viral (i.e., inserted into the cell by a virus) genes; their
expression can cause the development of cancer.
 Proto-oncogenes are normal cellular genes - conversion to oncogenes occur via several
mechanisms. Gain-of-function mutations of protooncogenes stimulate cells to multiply.
 Tumor suppressors (anti-oncogenes) are cellular genes; their inactivation increases the
probability of tumor formation. Loss-offunction mutations – relieve cells of control in
replication.
 About one hundred potential oncogenes (cellular and viral) and thirty tumor suppressors
have been recognized.
Tumor suppressors are guardians against DNA damage.
 Slows or inhibits cell proliferation and prevents damaged or abnormal cells from
becoming malignant.
 Monitor DNA damage and help repair the damage before cells divide.
 Induce growth arrest at two DNA damage checkpoints: the first gap phase (G1) and the
second gap phase (G2) of the cell cycle. This enables cells to check the integrity of the
chromosomes before proceeding into DNA replication (S phase) or division (M phase).

4. FUNCTIONS OF TUMOR-SUPPRESSOR PROTEINS
 Repression of genes that are essential for the continuing of the cell cycle,
effectively inhibiting cell division.
 Coupling the cell cycle to DNA damage. Cell should not divide with damaged DNA
- the cell cycle continue once it is repaired.
 If the damage cannot be repaired, the cell should initiate apoptosis (programmed cell
death) to remove the threat it poses for the greater good of the organism.
 Metastasis suppressors prevent tumor cells from dispersing, block loss of contact
inhibition, and inhibit metastasis.
 DNA repair proteins are usually classified as tumor suppressors. HNPCC, MEN1 and
BRCA.
 The functional inactivation of tumor suppressors by Genetic (gene mutation or
deletion, mutations in the promoter of genes) or epigenetic alterations (promoter
methylation, mutations in the promoter of genes) – creates imbalance between
proliferation, cell death, and differentiation programs that facilitates tumorigenesis.
 Individuals born with mutations in tumor-suppressor genes are predisposed to cancer.
 Tumor suppressors represent about 0.001% of the total number of genes (around
30,000) that make up the entire mammalian genome. About 30 tumor suppressors are
known.
THE “TWO HITS” HYPOTHESIS: LOSS OF HETEROZYGOSITY
 The Knudson hypothesis - cancer is the result of accumulated mutations to a cell's
DNA.
 Two mutations are required for the development of retinoblastoma
 Familial retinoblastoma
 Mutated allele from one parent (the first hit) in retinoblastoma (RB) gene -
Heterozygosity - not sufficient to cause tumor - child is initially asymptomatic.
 A second mutation or deletion on the other RB allele (second hit) – loss of function of
both alleles –loss of heterozygosity (LOH) –development of tumor.
 Nonfamilial or “sporadic” retinoblastoma
 Somatic mutations on both alleles (two hits) in retinal cells - disease is not transmitted
to the next generation.

5. HAPLOINSUFFICIENCY IN CANCER
 Appearance of a phenotype in cells or an organism when only one of the two gene copies
(alleles) is inactivated.
 For some tumor suppressor genes, the loss of a single allele is sufficient to induce
susceptibility to tumor formation - these are haploinsufficient tumor-suppressor
genes.
 Inactivation of one allele can be achieved by genetic (i.e. mutation or deletion) or
epigenetic (transcriptional silencing by methylation, repressor complexes, or mutations
in the promoter
 region) events.
EPIGENETIC EVENTS
 Epigenetic regulation of genes - methylation, repressor complexes, or mutations of the
promoter regions – reduce messenger RNA transcription - reduction or loss of protein
expression.
 Methylation occurs on the cytosine nucleotide of CpG pairs usually located in promoter
regions, resulting in transcriptional silencing.
 Mutations in the promoter region can affect binding of transcription factors to specific
DNA binding domains.

6.  Repressor protein complexes bind to the promoter regions of affected genes -
Transcriptional repression .
 Epigenetic events are linked to cancer development and are equally important in the
process of carcinogenesis.
 Eg. tumor suppressor p16INK4A or p14ARF - silenced by methylation of their
promoters or by active transcriptional repression by repressor protein complexes.
RETINOBLASTOMA PROTEIN is important in the control of proliferation and
development of not only retinal cells, but also other tissues.
 Retinoblastoma - large deletions of coding exons or point mutations - loss of the
messenger RNA or the expression of a nonfunctional pRb protein.
 Children with germ-line RB mutations are at higher risk of developing osteosarcomas
later in adolescence.
 Certain sporadic human tumors with RB mutations - carcinomas of the bladder, breast,
and in particular small-cell lung carcinomas.
 The retinoblastoma protein plays a critical role in the control of neoplasia in a variety
of tissues.

p53/TP53
 “The guardian of the genome” - a transcription factor – its loss leads to genomic
instability and increased mutagenesis.
 Restrains uncontrolled cell growth – block cell cycle and induce apoptosis.
 Sense multiple cellular stresses (genotoxic stress e.g., UV, X-ray, carcinogens, and
cytotoxic drugs) or oncogenic stresses (hyperproliferative signals from oncogenes ).

7.  In response to stress, the p53 protein is stabilized and activated - induce a set of genes
involved in cell cycle arrest, DNA repair, or apoptosis.
 p53 is inactivated in most human cancers (50% of tumors) as a result of mutations in
the TP53 gene, or through binding to viral proteins.
 Tumor suppressor activity of p53 is due to its ability to induce Apoptosis
The phosphatase PTEN (phosphatase and Tensin homolog) negatively regulates cell
proliferation.
 Loss of PTEN function is common in several cancer types.
 Somatic inactivating mutations in PTEN are frequently found in glioblastoma,
endometrial carcinoma, and prostate adenocarcinoma, in sporadic cancers of the
breast, thyroid, lung, stomach, and blood.
BRCA1 and BRCA2 are tumor suppressor genes and are involved in DNA repair of double-
strand breaks.
 Mutations in BRCA1 (chromosome 17) and/or BRCA2 (chromosome 13) cause
decreased stability of the human genome and result in dangerous gene rearrangements
that can lead
 to hematologic cancers.
 A BRCA mutation is a mutation in either of the genes, BRCA1 and BRCA2.

9.  Heterozygous germline mutations in either the BRCA1 or BRCA2 genes - high risk
for breast and ovarian cancer phenotypes - exhibit loss of heterozygosity (LOH).

REGRESSING TUMORS BY ACTIVATING TUMOR SUPPRESSOR GENES – A
THERAPEUTIC TARGET
 Preclinical studies both in vitro and in vivo have shown that restoring p53 function can
induce apoptosis in cancer cells.
 P53 gene therapy for lung cancer, hepatocellular carcinoma using viral delivery
system.
 However, resistance develops in many patients, suggesting that combination of such
gene-targeted therapies may be required.