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Any cell has three choices of behavior:

1) it can progress through the cell cycle to complete

another round of division,

2) it can choose out of the cell cycle to become a non-

dividing cell,

3) or it can die (apoptosis).

Some cells also have the option of following a

program of differentiation, like the blood cell types.

Cells select one of these options in response to

internal and external signals.

Cell behaviors

Dysregulation of the cell cycle is the cardinal feature

of cancer cells.

These cells progress persistently through round after

round of mitosis, even when it would be more

appropriate to pause, exit from the cycle, or commit

suicide by apoptosis.

The genes that generate and interpret the signals

controlling cell cycle progression feature strongly in

lists of oncogenes and tumor suppressor genes.

Progression through the cell cycle is controlled by

cyclins and cyclin dependent kinases (Cdks) and

regulated at a series of checkpoints.

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Upstream of the Cdks is a crowd of activators and

inhibitors that act on the cyclin-Cdk complexes.

The whole network of kinases, cyclins, inhibitors, and

activators integrates many different signals, ensuring a

flexible and responsive control of the cell cycle.

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If the cells receive an anti-proliferative stimulus

they may exit from the cell cycle altogether to

enter a modified G1 phase called G0 phase .

Cells in G0 phase are in a prolonged non-dividing

state, but they are not inactive.

They can become terminally differentiated; that is,

irreversibly committed to serve a specialized

function.

Most cells in the body are in this state, but they

often actively synthesize and secrete proteins and

may be highly motile.

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G1 is the phase in which cells spend most of their

time and do most of their work, even if they are

intended eventually to proceed to S phase and so

through the cycle.

Controls on progression through G1 are especially

important targets of oncogenic mutations.

Progression from one phase of the cycle to the next

is controlled at a series of checkpoints.

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These ensure that a cell can only progress round

the cycle when it, and in particular its DNA, is in a

suitable condition.

There are three main checkpoints.

1) The G1/S checkpoint.

2) The G2/M checkpoint.

3) The spindle (or mitotic) checkpoint.

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This checkpoint is especially important, because a

cell that passes the G1/S boundary is committed to

mitosis.

The checkpoint is controlled by Cdk2/ cyclin E.

Entry into S phase is blocked when there is

unrepaired DNA damage. Irreparable damage leads

to apoptosis.

Within S phase there are additional checkpoints

(cyclin A) at which DNA damage prevents new

origins of replication from becoming active.

The G1/S checkpoint

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Cells are blocked from entering mitosis unless

DNA replication and the repair of any damage are

complete.

Entry into mitosis depends on the activation of

Cdk1/cyclin B by the phosphatase Cdc25C.

Incomplete DNA replication or unrepaired damage

generates a signal that activates inhibitors of

Cdc25C, thus preventing Cdk1 from becoming

active.

The G2/M checkpoint

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Separation of chromatids at anaphase of mitosis is

triggered by the anaphase-promoting complex

(APC) or cyclosome.

This multiprotein ubiquitin ligase degrades cyclins

A and B, and (indirectly) the cohesin glue that

holds sister chromatids together.

Kinetochores that are not attached to spindle

microtubules secrete a signal that inhibits the APC.

The spindle (or mitotic) checkpoint

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If this signaling is defective, chromatids can start to

separate before all of them have been correctly

attached to spindle fibers, and there is then no way

of ensuring that exactly one chromatid of each

chromosome goes into each daughter cell.

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Note that APC can refer either to the anaphase-

promoting complex or to the APC tumor

suppressor gene, that is mutated in familial and

sporadic colon cancer.

This is confusing, because one function of the APC

gene product is to stabilize the attachment of

microtubules to chromosomes, and APC mutations

cause chromosome instability.

But the APC protein is not part of the anaphase-

promoting complex.

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In cancer cells, all the checkpoint mechanisms are

typically defective.

Cells replicate their DNA despite damage, enter

mitosis with unrepaired damage, and become

inefficient at segregating their chromosomes

correctly.

All of this destabilizes the genome and lays the

ground for further evolution toward full-blown

malignancy.

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Three key tumor suppressor genes control

events in G1 phase

pRb: a key regulator of progression through G1

phase

p53: the guardian of the genome

CDKN2A: one gene that encodes two key

regulatory proteins

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Events in G1 phase are particularly critical for

carcinogenesis, because this is when cells make the

decision whether or not to divide.

Three proteins, the products of the RB1, TP53, and

CDKN2A genes, have central roles in controlling

progression through G1 phase.

Somatic mutations in these three genes are among

the most common genetic changes in tumor cells.

In addition, inherited mutations in each are the

cause of well-known familial cancer syndromes.

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pRb: a key regulator of progression through G1 phase

The RB1 gene was identified through its role in

retinoblastoma, but it is widely expressed and helps

control cycling of all cells.

The gene product, pRb, is a 110 kD nuclear protein.

Some cells contain two related proteins, p107 and

p130, giving some redundancy in the Rb pathway.

Lack of this redundancy in certain cells probably

explains why a loss of RB1 function results in very

specific types of tumor.

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pRb binds and inactivates the cellular transcription

factor E2F, function of which is required for cell

cycle progression.

At 2-4 hours before a cell enters S phase,

complexes of D cyclins and Cdk4 or Cdk6

phosphorylate pRb. This inactivates it, allowing

E2F to become free.

Once free, E2F stimulates the transcription of a

variety of genes whose products are necessary for

progression into S phase, including particularly

cyclin E.

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Controls on cell cycle progression and genomic integrity

mediated by the RB1, TP53, and CDKN2A gene products

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In cells with loss-of-function mutations in RB1,

E2F is inappropriately activated.

Several viral oncoproteins (adenovirus E1A, SV40-

T antigen, and human papilloma virus E7 protein)

achieve the same result by binding and

sequestering or degrading pRb, thus favoring cell

cycle progression.

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p53: the guardian of the genome

The p53 transcription factor, encoded by the TP53 gene, has been called the guardian of the genome

because of its central role in preventing

inappropriate cell cycling.

Tumor cells with absent or nonfunctional p53 may

continue to replicate damaged DNA and do not

undergo apoptosis. Normally, p53 levels in a cell

are low.

The Mdm2 protein ubiquitylates p53, which targets

it for degradation.

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MDM2 is itself a transcriptional target of p53, so

there is a negative feedback loop that keeps p53

concentrations low.

MDM2 is an oncogene that is amplified in many

sarcomas.

Signals from a whole range of cellular stress

sensors, including sensors of DNA damage, lead to

phosphorylation of p53.

Phosphorylated p53 is no longer a substrate for

Mdm2, and hence the level of p53 in the cell rises.

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This increases p53-dependent transcription of

genes such as that encoding p21WAFlICIP1, an

inhibitor of Cdk2 and hence of cell cycling, and of

genes such as PUMA, EAX, and NOXA that

control apoptosis.

p53 has two relatives, p63 and p73, with functions

partly overlapping those of p53.

However, TP53 is the major target of mutations in

cancer. Loss or mutation of TP53 is probably the

commonest single genetic change in cancer.

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Controls on cell cycle progression and genomic integrity

mediated by the RB1, TP53, and CDKN2A gene products

TP53 maps to 17p13, and this is one of the

commonest regions of loss of heterozygosity in a

wide range of tumors.

Tumors that have not lost TP53 very often have

mutated versions of it.

To complete the picture of TP53 as a tumor

suppressor gene, constitutional mutations in TP53

are found in families with the dominantly inherited

Li-Fraumeni syndrome (OMIM 151623).

Affected family members suffer multiple primary

tumors, including sarcomas, osteosarcomas, tumors

of the breast, brain, and leukemia.

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CDKN2A: one gene that encodes two key

regulatory proteins

The remarkable CDKN2A gene at 9p21 uses

alternative promoters and first exons to encode two

structurally unrelated proteins.

Exons 1α, 2, and 3 encode the p16INK4A protein.

This is an inhibitor of Cdk4/6 and hence serves to

keep pRb in its active, dephosphorylated state.

This in turn prevents E2F from stimulating the

progression of the cell through G1 toward the G1/S

boundary.

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The two products of the CDKN2A gene

This gene (also known as MTS and INK4A) encodes two

completely unrelated proteins. p16INK4A is translated from exons lα,

2, and 3, and p14ARF from exons 1β, 2, and 3 -but with a different

reading frame of exons 2 and 3. The two gene products are active in

the pRb and p53 arms of cell cycle control, respectively.

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Thus, p16 is a tumor suppressor protein, whose loss

allows inappropriate cell cycling.

A second promoter starts transcription further

upstream, at exon 1β.

Exon 1β is spliced on to exons 2 and 3, but the

reading frame is shifted so that an entirely

unrelated protein p14ARF (ARF for alternative

reading frame) is encoded.

p14ARF mediates G1 arrest by destabilizing Mdm2,

the oncoprotein responsible for keeping p53 levels

low.

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Loss of p14ARF function leads to excessive levels of

Mdm2, excessive destruction of p53, and hence

loss of cell cycle control.

Rare inherited CDKN2A mutations, usually

affecting just p16INK4A, are seen in some families

with multiple melanoma, but somatic mutations are

very much more frequent.

Tumor cells probably need to inactivate both the

pRb and p53 arms of the system for the cell to bypass the usual checks on cycling and to avoid

triggering apoptosis.

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Homozygous deletion of the CDKN2A gene is an

efficient way of achieving this and is a very

common event in tumorigenesis.

Some tumors have mutations that affect p16INK4A

but not p14ARF (e.g. specific inactivation of the 1α

promoter by methylation).

Those tumors tend also to have TP53 mutations,

showing the importance of inactivating both arms

of the control system.

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Instability of the Genome

Genomic instability is an almost universal feature

of cancer cells. Instability may be of two types:

Chromosomal instability (CIN) is the commonest form.

Tumor cells typically have abnormal karyotypes, with

extra and missing chromosomes, rearrangements, and so

on. Tumor cell lines are often chromosomally unstable,

acquiring new changes during culture.

Microsatellite instability (MIN) is a DNA-level

instability seen in certain tumors, especially some colon

carcinomas.

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Instability is probably necessary to enable a cell to

a mass enough mutations to complete the micro

evolution from a normal somatic cell to an invasive

cancer cell.

However, tumors normally show either CIN or

MIN but not both, and this suggests that instability

is not a chance feature but is the result of selection.

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Three main mechanisms for the chromosome

instability and abnormal karyotypes

The many chromosomal abnormalities seen in

tumor cells are mostly random, although they

provide material for further selection of faster-

growing variants.

They probably arise in three ways:

In many tumor cells the spindle checkpoint is

defective. Chromatids can start to separate before all of them

are correctly attached to spindle fibers.

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There is then no way of ensuring that exactly one

chromatid of each chromosome goes into each

daughter cell.

Cancer cells often contain extra centrosomes,

which may produce abnormal spindles.

Failure of the spindle checkpoint is probably the

main source of the many numerical abnormalities

seen in cancer cells.

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Tumor cells are able to progress through the cell

cycle despite having unrepaired DNA damage.

Normally unrepaired damage generates a signal

that stalls the cell cycle until the damage is

repaired, and irreparable damage triggers apoptosis.

In cancer cells, either the damage signaling system

or the apoptotic response is often defective.

Structural chromosome abnormalities can be a by-

product of attempts at DNA replication or mitosis

with damaged DNA.

Tumor cells may replicate to the point that

telomeres become too short to protect chromosome

ends, leading to all sorts of structural abnormalities.

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Telomeres are essential for chromosomal stability

The ends of all human chromosomes carry a repeat

sequence (GGGTTA) n.

This forms a special DNA structure that binds

specific proteins and protects chromosome ends

from being treated by the cell as DNA double-

strand breaks.

Telomere length declines by 50-100 bp with each

cell generation because of the inability of DNA

polymerase to use the extreme 3' end of a DNA

strand as a template for replication.

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Telomerase extends the

TG-rich strand of

telomeres by DNA

synthesis using an

internal RNA template.

Telomere length is restored by a special RNA-

containing enzyme system, telomerase, that is

present in the human germ line but absent from

most somatic tissues.

In prolonged culture, normal cells reach a point of

senescence, at which they stop dividing.

Cells with certain genetic defects (e.g. fibroblasts

with deficiency of both p53 and pRb, or harboring

viral oncoproteins) continue beyond senescence

and hit crisis, which probably represents the point

at which telomeres are so short that they can no

longer protect chromosome ends.

The cell then treats chromosome ends as double-

strand DNA breaks, which it attempts to repair by

homologous recombination or nonhomologous end

joining.

Random joining of chromosomes produces

translocations, including chromosomes with two

centromeres (dicentric chromosomes).

These may be pulled in two directions at anaphase

of mitosis, forming bridges.

The bridges eventually break, creating new broken

ends and triggering further rounds of fusion and

breakage. The result is chromosomal disorder.

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At crisis most cells die, but the 1 in 107 or

thereabouts that survive have gross chromosomal

abnormalities. Additionally, they have become

immortal.

In one way or another, tumor cells always acquire

the ability to replicate indefinitely while

maintaining their telomeres.

Of full-blown metastatic cancers, 85-90% have

fixed to re-express telomerase; the remainder use

an alternative (ALT) mechanism, based on

recombination.

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DNA damage sends a signal to p53, which

initiates protective responses

The DNA of every cell is constantly suffering

damage from exogenous agents such as ionizing

radiation and ultraviolet radiation, and from

endogenous processes such as depurination,

deamination of cytosines, and attack by reactive

oxygen species.

The normal response of a cell to such damage is to

stall the cell cycle until the damage is repaired, and

to trigger apoptosis if the damage is irreparable.

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Proteins encoded by several well known tumor

suppressor genes are involved in the system that

senses DNA damage and organizes the cellular

response.

Inherited loss-of- function mutations in these genes

are associated with an elevated risk of cancer.

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ATM: the initial detector of damage

The ATM protein may be the primary detector of

damage.

This 3056-residue protein kinase is somehow

activated by DNA double-strand breaks-maybe by

changes in the chromatin conformation.

Once activated, the ATM kinase phosphorylates

numerous substrates, including the p53, NBS1

(Nibrin), CHEK2, and BRCA1 proteins.

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Part of the signaling mechanism by which

DNA damage cause cell cycle arrest

The ATM kinase is activated by

DNA double-strand breaks

(DSB) and phosphorylates

numerous substrates, some of

which are involved in DNA

repair (e.g. Nibrin and BRCA1).

ATM also activates the CHEK2

protein, which in turn activates

p53, the principal target of the

DNA damage detection system.

Phosphorylated p53 acts to block

further progress through the cell

cycle and also promotes

apoptosis.

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Loss of function of the ATM gene causes ataxia

telangiectasia (AT; OMIM 208900).

Affected homozygotes have a predisposition to

various cancers, plus immunodeficiency,

chromosomal instability, and cerebellar ataxia.

Heterozygous carriers of certain specific ATM

mutations are at increased risk of breast cancer.

Presenting tricky problems of medical management

because they are also sensitive to radiation, so X-

ray mammography may increase the risk of breast

cancer.

Nibrin and the MRN complex Nibrin is phosphorylated by ATM and forms a

complex (MRN) with the MRE11 and RAD50

proteins.

The complex localizes to sites of damage and may

help to recruit repair enzymes.

Lack of Nibrin causes Nijmegen breakage syndrome

(NBS; OMIM 251260).

NBS is clinically rather similar to AT but includes

microcephaly and growth retardation in place of

ataxia. MRE11 mutations also result in an AT-like

disorder (ATLD; OMIM 604391).

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Nijmegen breakage syndrome (NBS) is a rare

autosomal recessive condition with chromosomal

instability.

Clinical and biological overlap between Fanconi

anemia and ataxia telangiectasia has been reported.

Chromosomal analysis of bone marrow cells

revealed tetraploidy, which indicates progression

towards leukemia.

Detailed molecular studies are essential for

accurate diagnosis and management of this disease.

Nijmegen breakage syndrome

CHEK2: a mediator kinase

CHEK2 is a mediator kinase. It is activated when it

is phosphorylated by ATM.

It then relays the ATM signal to other substrates

including p53.

A frameshift mutation in CHEK2 was mentioned as

an inherited factor in breast cancer susceptibility,

giving carriers a relative risk of 2.34.

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The role of BRCA 1/2

BRCA1 protein, the product of the first gene

implicated in familial breast cancer, localizes to

sites of DNA damage.

A multi-protein complex, the BASC (for BRCA1-

associated genome surveillance complex), has been

isolated that includes proteins involved in DNA

damage response and repair.

BRCA1 also has functions in recombination,

chromatin remodeling, and control of transcription.

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The BRCA2 gene encodes another very large

protein (3416 residues) with multiple functional

domains, and a major role in familial breast cancer.

It has no structural similarity to BRCA1 protein but

shares many functions.

One function is to load many molecules of the

RAD51 protein onto single-stranded DNA at sites

of damage, as part of the recombinational repair

mechanism.

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As well as being the cause of some hereditary

breast cancer, a particular class of homozygous

BRCA2 mutations cause the D1 form of Fanconi

anemia (OMIM: 227650 and 605724).

This is an autosomal recessive syndrome of

congenital abnormalities, progressive bone marrow

failure, cellular hypersensitivity to DNA damage,

and predisposition to cancer.

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Clinical and diagnostic features of the Fanconi anemia patient.

(A)Fanconi facies with tapering jaw.

(B) Polydactyly.

(C) Chromosomal breaks in peripheral blood.

Fanconi anemia

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p53 to the rescue p53 is a principal downstream target of the DNA

damage detection system.

DNA damage causes p53 to be phosphorylated. This

stabilizes p53, preventing Mdm2 from flagging it for

degradation.

The raised level of p53 leads to activation of the

Cdk2 inhibitor p21WAF1/CIP1, which prevents further

progress toward S phase of the cell cycle.

Raised levels of p53 can also lead to transcription of

pro-apoptotic genes.

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When the signaling system is defective, the G1/S

checkpoint fails and cells may attempt to replicate

damaged DNA.

Cells with defects in the proteins described here

seem to be particularly bad at repairing double-

strand breaks.

It may be that other types of damage do not rely so

heavily on detection by this system for their repair.

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Defects in the repair machinery underlie a

variety of cancer-prone genetic disorders

Tumor cells may fail to signal DNA damage, or

alternatively the signal may be intact but the cell

may have a defect that makes it unable to repair

certain types of damage and different repair

mechanisms.

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Defects in the repair system associated with

specific forms of cancer

Base excision repair (BER)/Colon cancer

Nucleotide excision repair (NER)/XP

Double-strand break repair/BRCA1-2

Replication error repair/FAP-APC

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Base excision repair (BER)

BER corrects much the commonest type of DNA

damage (of the order of 20,000 altered bases in

each nucleated cell in our body each day).

BER glycosylase enzymes remove abnormal bases

by breaking the sugar–base bond.

Humans have at least eight genes encoding

different DNA glycosylases, each responsible for

identifying and removing a specific kind of base

damage.

After removal of the damaged base, an

endonuclease and a phosphodiesterase cut the

sugar–phosphate backbone at the position of the

missing base and remove the sugar–phosphate

residue.

The gap is filled by resynthesis with a DNA

polymerase, and the remaining nick is sealed by

DNA ligase III.

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Defects in Base Excision Repair (BER)

The defects are not common in human cancers, but

a survey of patients with colon cancer showed that

rare homozygous defects in the MUTYH repair

enzyme were associated with a 93-fold increased

risk of colon cancer.

Overall, these accounted for about 0.5% of all

cases of colon cancer.

Nucleotide Excision Repair (NER)

Nucleotide excision repair (NER) removes

thymine dimers and large chemical adducts.

NER removes and resynthesizes a large patch

around the damage, rather than just a single base

as in BER.

The sugar–phosphate backbone is cleaved at the

site of the damage, and exonucleases remove a

large stretch of the surrounding DNA.

As in BER, the gap is filled by resynthesis and

sealed by DNA ligase.

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Nucleotide excision repair is also used to correct

single-strand breaks in the sugar–phosphate

backbone.

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Defects in Nucleotide Excision Repair (NER)

It is defective in several cancer-prone syndromes,

particularly the various forms of xeroderma

pigmentosum (XP; OMIM 278700).

Patients with XP are homozygous for inherited

loss-of-function mutations in one or other of the

genes involved in nucleotide excision repair.

They are unable to repair DNA damage caused by

ultraviolet radiation.

They are exceedingly sensitive to sunlight and

develop many tumors on exposed skin.

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Xeroderma pigmentosum (XP)

Clinical characteristics

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XP is characterized by: Sun sensitivity (severe

sunburn with blistering, persistent erythema on

minimal sun exposure in ~60%

of affected individuals), with marked freckle-like

pigmentation of the face before age two years in

most affected individuals; Sunlight-induced ocular

involvement (photophobia, keratitis, atrophy of the

skin of the lids);Greatly increased risk of sunlight-

induced cutaneous neoplasms (basal cell

carcinoma, squamous cell carcinoma, melanoma).

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Approximately 25% of affected individuals have

neurologic manifestations (acquired microcephaly,

diminished or absent deep tendon stretch reflexes,

progressive sensorineural hearing loss, and

progressive cognitive impairment).

The most common causes of death are skin cancer,

neurologic degeneration, and internal cancer.

The median age at death in persons with XP with

neurodegeneration (29 years) was found to be

younger than that in persons with XP without

neurodegeneration (37 years).

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Diagnosis/testing

The diagnosis of XP is made on the basis of

clinical findings and family history and/or by the

identification of biallelic pathogenic variants

in DDB2, ERCC1, ERCC2, ERCC3, ERCC4, ER

CC5, POLH, XPA, or XPC.

Genetic counseling

XP is inherited in an autosomal recessive manner.

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Two different ways to repair DNA double-strand breaks

Direct reversal of the DNA damage is an

infrequently used mechanism of DNA repair in

humans.

Three human genes have been implicated in this

mechanism, of which the best-characterized

encodes the O-6-methylguanine-DNA

methyltransferase, which is able to remove methyl

groups from guanines that have been incorrectly

methylated.

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In many organisms, UV radiation-produced

thymine dimers can be directly resolved by the

enzyme photolyase using the energy of visible

light (photo reactivation).

Although mammals possess enzymes related to

photolyase, they use them for a quite different

purpose, to control their circadian clock.

In the repair processes described above, the

second, undamaged DNA strand acts as a template

for accurate reconstruction of the damaged strand.

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Damage that affects both strands of DNA requires

different mechanisms.

There are two main processes.

In homologous recombination, a single strand

from the homologous chromosome invades the

damaged DNA and acts as a template for accurate

repair.

This type of repair normally occurs after DNA

replication but before cell division, and involves

the sister chromatid.

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Homologous recombination

The correct sequence is

reconstructed using the

intact sister chromatid as

a template (molecular

details are not shown).

This produces an

accurate repair but is not

always possible.

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The eukaryotic machinery for recombination

repair is less well defined than the excision repair

systems.

Human genes involved in this pathway include

NBS (Nijmegen breakage syndrome; OMIM

602667), BLM (Bloom syndrome; OMIM

604610), and the breast cancer susceptibility genes

BRCA2 (OMIM 600185) and BRCA1 (OMIM

113705).

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In nonhomologous end joining, large multiprotein

complexes are assembled at broken ends of DNA

molecules, and DNA ligases rejoin the broken ends

regardless of their sequence.

It is always available, but

the repaired strand has extra

or missing nucleotides at the

junction.

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There is always some loss of DNA sequence from

the broken ends.

This is a desperate measure that is likely to cause

mutations or chromosomal rearrangements, but it

is better than leaving breaks unrepaired.

One reason why chromosomal telomeres require a

special structure is to protect normal chromosome

ends from this double-strand break response.

A final repair mechanism is concerned with

correcting mismatches caused by replication

errors.

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Cells deficient in mismatch repair have mutation

rates 100–1000-fold normal, with a particular

tendency to replication slippage in homopolymeric

runs.

In humans, the mechanism involves at least five

proteins, and defects cause Lynch syndrome

(OMIM 120435 and OMIM 609310).

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Defects in replication error repair system

The defects cause a large generalized increase in

mutation rates.

These defects came to light when studies of

familial colon cancer revealed the phenomenon of

microsatellite instability.

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Microsatellite instability in familial colon cancer

Most colon cancer is sporadic. The rare familial

cases fall into two main categories:

Familial adenomatous polyposis (FAP) or adenomatous

polyposis coli (APC)

Hereditary non-polyposis colon cancer (HNPCC)

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Familial adenomatous polyposis or adenomatous polyposis coli (FAP or APC; OMIM 175100)

It is an autosomal dominant condition in which the

colon is carpeted with hundreds or thousands of

polyps.

The polyps (adenomas) are not malignant, but if

left in place one or more of them is virtually

certain to evolve into invasive carcinoma.

The cause is an inherited mutation in the APC

tumor suppressor gene.

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Rare Familial Cancer caused by Tumor Suppressor

gene mutations

Hereditary non-polyposis colon cancer/HNPCC (Lynch syndrome I; OMIM 120435 and 120436)

It is also autosomal dominant and highly penetrant

but, unlike FAP, there is no preceding phase of

polyposis.

The causative genes are hMSH2, hMLH1, hPMS1,

and hPMS2 were mapped by linkage analysis in

affected families.

Studies on FAP tumors and the polyps that precede

them revealed losses of heterozygosity (LOHs) that

led to a pioneering and influential description of the

molecular events involved in the multi-stage

evolution of colorectal tumors.

Lynch Syndrome Genes

Human

Gene

Chromosomal

Location

Base Pairs

in cDNA

No. of

Exons

Protein Size (aa-

Amino Acids)

hMSH2 2p15-16 2727 16 934aa (105kDa)

hMSH6 2p15-16 4245 10 1360aa (160kDa)

hMLH1 3p21.3 2268 19 756aa (85kDa)

hPMS2 7p22 2586 15 862aa (96kDa)

دکتر یاسایی

However, similar LOH studies on the tumors in

patients with Lynch syndrome I produced

unexpected, counter intuitive results.

Rather than lacking alleles present in the

constitutional DNA, tumor specimens seemed to

contain extra, novel, alleles of the microsatellite

markers used.

LOH is a property of particular chromosomal

regions, and may reveal the locations of tumor

suppressor genes, but the microsatellite instability

(MIN) in Lynch syndrome I is general.

دکتر یاسایی

Electropherograms of two PCR-amplified microsatellite markers.

Upper traces: blood DNA. Lower traces: tumor DNA.

Note: the extra peaks in the tumor DNA

Microsatellite instability

دکتر یاسایی

Microsatellite instability is seen in 10-15% of

colorectal, endometrial, and ovarian carcinomas,

but only occasionally in other tumors.

Patients with Lynch syndrome I are constitutionally

heterozygous for a loss-of-function mutation,

almost always in MLHl or MSH2.

Their normal cells still have a functioning

mismatch repair system and do not show the MIN+

phenotype.

دکتر یاسایی

In a tumor, the second copy is lost by one of the

mechanisms described for retinoblastoma or, at

least in the case of MLH1, it may be intact but

silenced by promoter methylation.

Mechanism of mismatch repair

Replication errors can produce

mismatched base pairs or

small insertion/deletion loops.

These are recognized by

hMutSα, a dimer of MSH2

and MSH6 proteins, or

sometimes by the MSH2-

MSH3 dimer hMutSβ.

The proteins translocate along

the DNA, bind the MLH1-

PMS2 dimer hMutLα, then

assemble the full repairosome,

which strips back and

resynthesizes the newly

synthesized strand .

دکتر یاسایی

دکتر یاسایی