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13Molecular Genetics of Cell

Cycle and Cancer

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• There are two major parts in the cell cycle:Interphase: G1 = gap1 S = DNA synthesis G2 = gap2Mitosis: M

• There are two essential functions of the cell cycle: To ensure that each chromosomal DNA molecule is

replicated only once per cycle To ensure that the identical replicas of each

chromosome are distributed equally to the two daughter cells

The Cell Cycle

3Fig. 13.1

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The Cell Cycle• The cell cycle is under genetic control

• A fundamental feature of the cell cycle is that it is a true cycle: it is not reversible

• Many genes are transcribed during the cell cycle just before their products are needed

• Mutations affecting the cell cycle have helped to identified the key regulatory pathways

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The Cell Cycle• Progression from one phase to the next is propelled

by characteristic protein complexes, which are composed of Cyclins and cyclin-dependent protein kinases (CDK)

• Expression of mitotic cyclins E, A, and B are periodic, whereas cyclin D is expressed throughout the cell cycle in response to mitosis stimulating drugs (mitogens)

• The cyclin-CDK complexes phosphorylate targeted proteins, changing dramatically their activity

6Fig. 13.8

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The Retinoblastoma Protein• The retinoblastoma (RB) protein controls the initiation

of DNA synthesis.• RB maintains cells at a point in G1 called the G1

restriction point or start by binding to the transcription factor E2F, until the cell has attained proper size

• If the cycling cell is growing properly and becomes committed to DNA synthesis, several cyclin-CDK complexes inactivate RB by phosphorylation

• After cell enters S phase E2F becomes phosphorylated as well and loses its ability to bind DNA

8Fig. 13.10

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The Cell Cycle• The progression from G2 to M is controlled by a

cyclin B-CDK2 complex know as maturation-promoting complex

• Protein degradation (proteolysis) also helps regulate the cell cycle. The anaphase-promoting complex (APC/C), which is a ubiquitin–protein ligase responsible for adding the 76-amino-acid protein ubiquitin to its target proteins and marking them for destruction in the proteasome

• Proteolysis eliminates proteins used in the preceding phase as well as proteins that would inhibit progression into the next

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Checkpoints• Cells monitor their external environments and

internal state and functions

• Checkpoints in the cell cycle serve to maintain the correct order of steps as the cycle progresses; they do this by causing the cell cycle to pause while defects are corrected or repaired

• Checkpoints in the cell cycle allow damaged cells to repair themselves or to self-destruct

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Three Main Checkpoints

• A DNA damage checkpoint

• A centrosome duplication checkpoint

• A spindle checkpoint

12Fig. 13.12

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A DNA Damage Checkpoint• A DNA damage checkpoint arrests the cell cycle

when DNA is damaged or replication is not completed.

• In animal cells, a DNA damage checkpoint acts at three stages in the cell cycle: at the G1/S transition, in the S period and at the G2/M boundary

• The p53 transcription factor is a key player in the DNA damage checkpoint.

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A DNA Damage Checkpoint• In normal cells, level of activated p53 is very low

• Protein Mdm2 keeps p53 inactivated by preventing phosphorylation and acetylation of p53 and by exporting p53 from the nucleus

• Damaged DNA leads to activation of p53 and its release from Mdm2

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A DNA Damage Checkpoint• Activated p53 triggers transcription of a number of

genes - p21, 14-3-3s, Bax, Apaf1, Maspin,GADD45• DNA damage detected in G1 blocks cell G1/S

transition• DNA damage in S phase reduces processivity of DNA

polymerase and gives the cell time for repairProcessivity: number of consecutive nucleotides that replicate before polymerase detaches from template

• DNA damage detected in S or G1 arrests cells at G2/M transition

16Fig. 13.15

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A DNA Damage Checkpoint• DNA damage also triggers activation of a pathway

for apoptosis = programmed cell death

• When the apoptotic pathway is activated, a cascade of proteolysis is initiated that culminates in cell suicide

• The proteases involved are called caspases

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Centrosome Duplication Checkpoint• Monitors spindle formation

• Functions to maintain the normal complement of chromosomes

• Sometimes coordinates with the spindle checkpoint and the exit from mitosis

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The Spindle Checkpoint• Monitors assembly of the spindle and its

attachment to kinetochores

• The kinetochore is the spindle-fiber attachment site on the chromosome

• Incorrect or unbalanced attachment to the spindle activates spindle checkpoint proteins, triggers a block in the separation of the sister chromatids by preventing activation of the anaphase-promoting complex (APC/C)

20Fig. 13.18

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Cancer• Cancer cells have a small number of mutations that

prevent normal checkpoint function

• Cancer is not one disease but rather many diseases with similar cellular attributes

• All cancer cells show uncontrolled growth as a result of mutations in a relatively small number of genes

• Cancer is a disease of somatic cells

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• 1% of cancer cases are familial: show evidence for segregation of a gene in pedigree

• 99% are sporadic: the result of genetic changes in somatic cells

• Within an organism, tumor cells are clonal, which

means that they are descendants from a single ancestral cell that became cancerous.

Cancer

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Cancer Cells vs. Normal Cells• In normal cells, cell-to-cell contact inhibits further

growth and division, a process called contact inhibition

• Cancer cells have lost contact inhibition: they

continue to grow and divide, and they even pile on top of one another

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Cancer Cells vs. Normal Cells • Even in the absence of damage, normal cells cease

to divide in culture after about 50 doublings = cell senescence

• Senescence of normal cells is associated with a loss of telomerase activity: the telomeres are no longer elongated, which contributes to the onset of senescence and cell death

• Cancer cells have high levels of telomerase, which help to protect them from senescence, making them immortal

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Key Mutational Targets• Many cancers are the result of alterations in cell

cycle control, particularly in control of the G1-to-S transition

• These alterations also affect apoptosis through their interactions with p53

• The major mutational targets for the multistep cancer progression are of two types: Proto-oncogenesTumor-suppressor genes

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Key Mutational Targets• The normal function of proto-oncogenes is to

promote cell division or to prevent apoptosis • The normal function of tumor-suppressor genes is

to prevent cell division or to promote apoptosis

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Oncogenes• Oncogenes are derived from normal cellular genes

called proto-oncogenes

• Oncogenes are gain-of-function mutations associated with cancer progression

• Oncogenes are gain-of-function mutations because

they improperly enhance the expression of genes that promote cell proliferation or inhibit apoptosis

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• Mdm2:Amplification of Mdm2 gene and overexpression of Mdm2 protein leads to inactivation of p53 geneAmplification of Mdm2 gene has been found in many tumors of adipose tissue, soft tissue sarcomas, osteosarcoma, and esophageal carcinoma

• Cyclin D and CDK4:Amplification and overexpression leads to unscheduled entry to S phaseAmplification and/or overexpression has been found in many esophageal carcinomas, bladder and breast cancers

Oncogenes

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Oncogenes• Growth-factor receptors:

Cellular growth factors stimulate growth by binding to a growth-factor receptor at the cell membraneThe binding activates a signal transduction pathway that acts through Ras, cyclin D, and its partner CDKs.

Amplification and overexpression of the gene that encodes the receptor for epidermal growth factor (EGFR) has been found in many malignant astrocytomas, glioblastomas, breast and ovarian cancers, head and neck cancers and melanomas.

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Oncogenes• Ras:

The Ras protein acts as a switch in stimulating cellular growth in the presence of growth factors Certain mutant Ras proteins lack GTPase activity and remain in the form of Ras–GTP. The signal for cellular growth is transmitted constitutively--unrestrained growth and division take place

Fig. 13.18

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Tumor-Suppressor Genes• Tumor-suppressor genes normally negatively

control cell proliferation or activate the apoptotic pathway

• Loss-of-function mutations in tumor-suppressor genes contribute to cancer progression.

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• p53: Loss of function of p53 eliminates the DNA checkpoint that monitors DNA damage in G1 and SThe damaged cells survive and proliferate and their genetic instability increases the probability of additional genetic changes and thus progression toward the cancerous state

p53 proves to be nonfunctional in more than half of all cancers Mutant p53 proteins are found frequently in

melanomas, lung cancers, colorectal tumors, bladder and prostate cancers, and astrocytomas.

Tumor-Suppressor Genes

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• p21: Loss of p21 function results in renewed rounds of DNA synthesis without mitosis, and the level of ploidy of the cell increases. Mutations in the p21 gene occur in

some prostate cancers.

• p16/p19ARF: The p16 and p19ARF proteins are products of the same gene transcribed from different promoters The p16 product can inhibit the cyclin D–Cdk4 complex and help control entry into S phase The gene is deleted in many gliomas, mesotheliomas, melanomas, nasopharyngeal carcinomas, biliary-tract and esophageal carcinomas.

Tumor-Suppressor Genes

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• RB: The retinoblastoma protein controls the transition from

G1 to S phase by controlling the activity of the transcription factor E2F Loss of RB function frees E2F hence excessive rounds of DNA synthesis are continuously being initiated.

Loss of RB function is found in melanomas, small-cell lung carcinoma, osteosarcoma and liposarcomas

Tumor-Suppressor Genes

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• Bax: The Bax tumor-suppressor protein promotes

apoptosis Loss of Bax function is found particularly in gastric

adenocarcinomas and in colorectal carcinomas associated with microsatellite instability because of defective mismatch repair. Cells that are defective in mismatch repair are prone to undergo replication slippage leading to deletions or additions of nucleotides in runs of short tandem repeats

Tumor-Suppressor Genes

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• Bub l: Bub1 is a protein that is primarily involved in the

spindle checkpoint A subset of colon cancers show chromosomal

instability. Some of these unstable lines are defective in Bub1.

Tumor-Suppressor Genes

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Familial Cancers• Mutations that predispose to cancer can be

inherited through the germ line

• The presence of this mutation predisposes the individual to cancer, because it reduces the number of additional somatic mutations necessary for a precancerous cell to progress to malignancy

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Li–Fraumeni Syndrome• The Li–Fraumeni syndrome shows clear autosomal dominant

inheritance. However, the affected individuals have a range of different tumors and often have more than one, including osteosarcoma, leukemia, breast cancer, lung cancer, soft-tissue sarcoma, and brain tumors

• A large fraction of Li–Fraumeni families show segregation for

a mutation in the p53 gene.

• A situation analogous to the human Li–Fraumeni syndrome has been created in mice by experimental knockout (loss of function) of the p53gene via the germ-line transformation

39Fig. 13.24

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Retinoblastoma• RB protein in animal cells holds cells at

restriction point by binding to and holding E2F

• Alfred Knudson in 1971 suggested that loss of the wildtype allele of a tumor-suppressor gene might be the triggering event at the cellular level for tumors in heterozygous genotypes

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Retinoblastoma• Knudson suggested that genesis of a tumor in familial

cases of RB required a “single hit” in a somatic cell, whereas genesis of a tumor in sporadic cases required “two hits”

• RB is inherited in pedigrees as a simple Mendelian dominant. But Knudson’s hypothesis implied that even in familial cases, there must be another mutational event that triggers tumor development.

• Analysis of genetic markers around the gene in tumor cells revealed that the triggering event is the loss of the wildtype RB1 allele

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• At the organismic level, mutant gene is dominant. At the cellular level, it is recessive.

• Several mechanisms can uncover the mutant allele: chromosome loss, mitotic recombination, deletion, and inactivating nucleotide substitutions

• Uncovering of the recessive allele by various mechanisms is called loss of heterozygosity

• Retinoblastoma (RB) is an inherited cancer syndrome associated with loss of heterozygosity in the tumor cells.

Retinoblastoma

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Defects in DNA Repair• Genetic instability clearly contributes to the origin

of tumor cells• Some inherited cancer syndromes result from

defects in processes of DNA repair• Inherited skin cancer syndromes are called

xeroderma pigmentosum • Xeroderma pigmentosum cells are defective in

nucleotide excision repair• Individuals with this syndrome are very sensitive to

ultraviolet light

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Acute Leukemias• Acute leukemias are malignant diseases of the bone

marrow, spleen, and lymph nodes associated with uncontrolled proliferation of leukocytes and their precursors in the bone marrow

• Acute leukemias do not arise as a consequence of alterations in cell cycle regulation or checkpoints, nor are they familial

• Up to 60% of acute leukemias result from a chromosomal translocation that fuses a transcription factor with a leukocyte regulatory sequence

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Acute Leukemias• The translocations are of the two types:

promoter fusion—the coding region for a gene that encodes transcription factor is translocated near an enhancer for an immunoglobulin heavy-chain gene or a T-cell receptor gene

gene fusion—found more frequently in acute leukemia than a promoter fusion.

the translocation breakpoints occur in introns of genes for transcription factors in two different chromosomes. The result is a fusion gene called a chimeric gene composed of parts of the original gene