PRESENTER: KHOZYA ZYAMBO

MODERATOR: DR SAMBO

Molecular and Cellular Biology of Cancer

Back Ground Oncology (Greek oncos = tumor) is the study

of tumors or neoplasms. Cancer is the common term for all malignant

tumors. Although the ancient origins of this term are somewhat uncertain, it probably derives from the Latin for crab, cancer—presumably because a cancer "adheres to any part that it seizes upon in an obstinate manner like the crab."

Back GroundCancer is a complex of diseases arising from

alterations that can occur in a wide variety of genes that result in Alterations in normal cellular processes such as:

1. Signal transduction, 2. Cell cycle control, 3. DNA repair,4. Cellular growth and differentiation, 5. Translational regulation, 6. Senescence and apoptosis (programmed cell

death).

Background Some stem cells may have a role in certain malignancies

such as CML, AML, ALL, gliomas, and breast cancer. These tumor-initiating cells have self renewal and

proliferative properties similar to nonmalignant stem cellsembryonic stem cells, derived from blastocystsadult stem cells, found in adult tissuescord blood stem cells, found in the umbilical cord.

The source of stem cells can also be subcategorized by‘potency’. This specifies the differentiation potential (the potential to differentiate into different cell types) of the stem cell.

Totipotent stem cells are produced from the fusion of an egg and sperm cell as well as the first few divisions of the fertilized egg are and can differentiate into embryonic and extraembryonic cell types.

Pluripotent stem cells are the descendants of totipotent cells and can differentiate into cells derived from any of the three germ layers.

Multipotent stem cells can produce only cells of a closely related family of cells (e.g. haematopoietic stem cells differentiate into RBC’s, WBC’s, platelets, etc.)

Unipotent cells can produce only one cell type, but have the property of self-renewal which distinguishes them from non-stem cells.

Human neoplasms that are most susceptible to chemotherapy are those with a high percentage of cells undergoing division.

Similarly, normal tissues that proliferate rapidly (e.g., bone marrow, hair follicles, and intestinal epithelium) are most subject to damage by cytotoxic drugs, which often limits their usefulness.

Conversely, slowly growing tumours with a small growth fraction (e.g., carcinomas of the colon or non–small cell lung cancer) often are less responsive to cycle-specific drugs.

Understanding of cell-cycle kinetics and the controls of normal and malignant cell growth is crucial to the design of current therapeutic regimens and the search for new drugs.

Cell Cycle

Phases of cell cycle

G0 quiescent non-cell replication phase; cells in this phase are stimulated by receptor mediated actions of growth factors (epithelial growth factor, platelet derived GF, insulin like GF)

G1 preparation phase; for DNA synthesis i.e. transcription of proteins necessary for DNA replication.

S synthetic phase; DNA synthesis (genome replication) the 46 chromosomes are duplicated into chromatids.

G2 gap phase; preparation phase for separating diploid chromosomes into daughter cells.

M mitosis phase; mitosis and formation of two daughter cells.

Cell cycling is modified by the cyclin family of proteins involved in DNA replication by phosphorylation (via kinases & phosphatase domains)

Flow chart depicting a simplified scheme of the molecular basis of cancer

GENES INVOLVED IN ONCOGENESIS

2 major classes of genes: Oncogenes and tumor suppressor genes.

Proto-oncogenes are cellular genes that are important for normal cellular function and code for various proteins, including transcriptional factors, growth factors, and growth factor receptors.

These proteins are vital components in the network of signal transduction that regulate cell growth, division, and differentiation.

Proto-oncogenes can be altered to form oncogenes that, when translated, can result in the malignant transformation of a cell.

Oncogenesis: Proto-oncogenes

The three main mechanisms by which proto-oncogenes can be activated include: 1. Amplification; myc a transcription

regulator protein (proto-oncogene) activated to Nmyc (oncogene) in neuroblastoma.

2. Point mutation; NRAS proto-oncogene in acute nonmyelogenous leukemias.

3. Translocation; t (9;22) the philadelphia chromosome in CML, t(2;13) or t(1;13) in alveolar rhabdomyosarcoma & t(4;18) the CMYC gene in Burkitt lymphoma.

Oncogenesis: Tumor suppressor gene

Alteration in the regulation of tumor suppressor genes.Tumor suppressor genes are important

regulators of cellular growth and programmed cell death (apoptosis).

They have been called recessive oncogenes because the inactivation of both alleles of a tumor suppressor gene is required for expression of a malignant phenotype.

E.g. 1: RetinoblastomaKnudson's “two-hit” model of cancer development

was based on observation of the behavior of the tumor suppressor gene RB. In sporadic cases of retinoblastoma, both alleles of the RB gene must be inactivated.

However, in familial cases, children inherit an inactivated allele from one parent, and consequently require the inactivation of the only remaining normal allele.

This helps explain why familial cases of retinoblastoma present earlier in childhood than sporadic cases, since only one “hit” is required.

E.g. 2. P53Another major tumor suppressor protein is

p53, which is known as the “guardian of the genome” because it detects the presence of chromosomal damage and prevents the cell from dividing until repairs have been made.

In the presence of damage beyond repair, p53 initiates apoptosis and the cell dies.

More than 50% of all tumors have abnormal p53 proteins.

Mutations in the P53 gene are important in many cancers, including : breast, colorectal, lung, esophageal, stomach, ovarian, and prostatic carcinomas as well as gliomas, sarcoma,

and some leukemias

Several syndromes are associated with an increased

risk of developing malignancies, which can be characterized by different

mechanisms.

SYNDROMES PREDISPOSING TO CANCER

First mechanismOne mechanism involves the inactivation of

tumor suppressor genes, such as RB in familial retinoblastoma.

Interestingly, a patient with retinoblastoma in which one of the alleles is inactivated throughout all of his or her cells is at a very high risk for developing osteosarcoma.

A familial syndrome, Li-Fraumeni syndrome in which one mutant P53 allele (chromosome 17q) is inherited (autosomal dominant), also has been described in patients who can develop sarcomas, leukemias, and cancers of the breast, bone, lung, and brain.

Neurofibromatosis is characterized by the proliferation cells of neural crest origin, leading to neurofibromas.

These patients are at a higher risk of developing malignant schwannomas, pheochromocytomas acoustic neuromas, sarcomas, optic gliomas.

Inherited in an autosomal dominant fashion, although 50% of the cases present without a family history and develop because of the high rate of spontaneous mutations of the NF1 gene.

A second mechanism: Defects in DNA repair.

Syndromes associated with an excessive number of broken chromosomes due to repair defects include:1. Bloom syndrome (short stature, photosensitive

telangiectatic erythema-congenital telangiectatic erythema, primarily in butterfly distribution, of the face and occasionally of the hands and forearms)-increased risk of leukemia & lymphomas.

2. Ataxia-telangiectasia (childhood ataxia with progressive neuromotor degeneration)-increased risk of lymphoma & leukemia.

3. Fanconi anemia (short stature, skeletal and renal anomalies, pancytopenia). Increased risk of leukemia.

Due to the decreased ability to repair chromosomal defects, cells accumulate abnormal DNA that results in significantly increased rates of cancer, especially leukemia. 4. Xeroderma pigmentosum likewise increases the risk of skin cancer, owing to defects in repair to DNA damaged by ultraviolet light. These disorders display an autosomal recessive pattern (except dysplastic nevus synd; autosomal dominant, increased risk of melanoma)

The third category of inherited cancer predisposition

Defects in immune surveillance. E.g. Wiskott-Aldrich syndrome, SCID, common

variable immunodeficiency, and the X-linked lymphoproliferative syndrome.

The most common types of malignancy in these patients are lymphoma and leukemia.

NB: Cure rates for immunodeficient children with cancer are much poorer than for non-immunodeficient children with similar malignancies, suggesting a role for the immune system in cancer treatment as well as in cancer prevention.

OTHER FACTORS ASSOCIATED WITH

ONCOGENESIS

Ionizing radiation

VIRUSESEpstein-Barr virus (EBV) and Burkitt lymphoma and

nasopharyngeal carcinoma was identified >30 yr ago, although EBV infection alone is not sufficient for malignant transformation.

EBV also is associated with mixed cellularity and lymphocyte-depleted Hodgkin disease, as well as some T-cell lymphomas, which is particularly intriguing because EBV normally does not infect T lymphocytes.

The most conclusive evidence for a role of EBV in lymphogenesis is the direct causal role of EBV for B-cell lymphoproliferative disease in immunocompromised persons, especially those with AIDS.

EBV also is associated with leiomyosarcoma in immunocompromised persons.

In children with chronic hepatitis B infection (HBsAg-positive), the risk of developing hepatocellular carcinoma is increased >200-fold.

In adults, the latency period between viral infection and the development of hepatocellular carcinoma approaches 20 yr. However, in children with perinatal transmission, the latency period can be as short as 6–7 yr.

The additional factors required for the malignant transformation of the virally infected hepatocytes have not been determined.

Hepatitis C virus infection also is a risk factor for hepatocellular ca and is associated with splenic lymphoma.

Human papillomavirus (HPV).

Papillomaviruses type 16 and 18 are highly associated with cervical cancer. Types 31, 33, 35, 45, and 56 are less likely to cause cervical cancer.

The low-risk papillomaviruses, including types 6 and 11, which commonly are found in genital warts, rarely are associated with malignancies.

The mechanism by which HPV 19 induces malignant transformation is thought to involve P53 and RB tumor suppressor gene, which regulate cell cycle by acting as gatekeepers of the G1/S and G2/M checkpoints. By interfering with these proteins, HPV alters the regulation of cell growth.

Human herpesvirus 8 (HHV8) is associated with Kaposi's sarcoma, primary effusion B-cell lymphoma, and the plasma cell variant of Castleman disease, all of which occur primarily among persons with AIDS.

Human T-cell leukemia virus 1 (HTLV-1) is associated with adult T-cell leukemia and lymphoma

GENOMIC IMPRINTING

Genomic imprinting; is the selective inactivation of one of two alleles of a particular gene.

Which gene is inactivated is determined by whether the gene is inherited from the mother or father.

Normally the IGF2 (insulin-like growth factor receptor 2) gene is inactivated. The inactivation is thought to be due to methylation of specific CpG sequences upstream of the IGF2 promoter, which interferes with the transcription of the IGF2 gene.

In some Wilms tumors, there is loss of methylation in the upstream area of the maternal gene, which, in turn, allows transcript expression of the maternal IGF2 gene.

At the same time the H19 gene (whose function is not yet clear), a previously actively transcribed maternal gene, is silenced by methylation.

Beckwith-Weidemann Syndrome, an overgrowth syndrome characterized by macrosomia, macroglossia, hemihypertrophy, omphalocele, and renal anomalies also is associated with an increased risk of Wilms tumor, hepatoblastoma, rhabdomyosarcoma, neuroblastoma, and adrenal cortical carcinoma.

The increased risk of developing cancer is associated with changes in the methylation pattern of genes on the 11p15 chromosome.

In vitro fertilization has been associated with imprinting defects and the development of some cases of Beckwith-Weidemann syndrome-associated Wilm tumor and retinoblastoma.

TELOMERASETelomeres are a series of tens to thousands of TTAGGG

repeats at the ends of chromosomes that are important for stabilizing the chromosomal ends and limiting breakage, translocation, and loss of DNA material.

With DNA replication there is a progressive shortening of telomere length, which is a hallmark of cellular aging and may be a senescence signal.

In some instances telomerase, an enzyme that adds telomeres to the ends of chromosomes, becomes active.

The addition of telomeres can be found in immortalized cell lines and most tumor types, and as a consequence, these cells may have a survival advantage that allows them to undergo additional cell divisions.

Therapy aimed at inhibition of telomerase activity may result in cell death.

Cellular responses to telomere shortening.

References: Nelson’s Text book.Paediatric in ReviewKumar & Clerk.

The end

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