Immortalisasi sel dan tumorigenesis

10/31/2014
IMMORTALISASI SEL
DAN
TUMORIGENESIS
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Dalam kultur sel/secara in vivo
 Ada sel yang masuk ke
stadium senescence sederhana/
replicative senescence
 Sel senescence  metabolisme
aktif tapi tidak dapat masuk
kembali ke dalam siklus sel

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Kapasitas
pembelahan
sel pada
organisme
menurun
dengan
bertambahnya umur
seseorang
 Mekanisme
penghitungan jumlah sel dalam
tubuh : cell autonomous 


intrinsik sel dan tidak dipengaruhi oleh interaksi
antar sel dengan lingkungan dan dengan tubuh
secara keseluruhan
~ “generational clock” bergantung pada molekul
intrasel :
Disintesis pada awal tahapan perkembangan
(developmental stage) dan tidak disintesis sesudah
stadium perkembangan
 Terdapat dalam konsentrasi yang tinggi dalam sel embrio
 Mengalami pengenceran dengan faktor 2 kali pada
keturunannya  hal ini yang mungkin menyebabkan
senescence karena senyawa tersebut berada di bawah
nilai ambang

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PEMBATAS REPLIKASI SEL :
STRESS FISIOLOGIS PADA SEL
 Replikasi sel dipengaruhi :

Kadar oksigen
Kadar oksigen yang rendah  peningkatan replikasi sel
 Kadar oksigen yang tinggi  Kemungkinan
terakumulasinya kerusakan oksidatif  senescence 
bentuk guanin teroksidasi 4 x lipat ; ROS


Feeder layer (kultur sel)  akibat CDK inhibitor

Level CDK inhibitor : p16INK4A dan p21Cip1 meningkat
pada kultur sel di atas plastik  senescence
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PEMBATAS PROLIFERASI : TELOMER

DNA linear  tidak stabil
 DNA linear yang ditransfeksikan ke dalam sel  berfusi
dengan DNA genom dengan bantuan nuklease dan ligase

Telomer 
 pada ujung kromosom  memungkinkan DNA linear stabil
dari kerja enzim
 Mencegah fusi ujung-ujung DNA dari kromosom  mencegah
fusi kromosom

Barbara McClintock : kromosom yang kehilangan telomer  fusi
 pembentukan megakromosom yang memiliki 2 atau lebih
sentromer
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
Telomer :


Terdiri atas :
hexanukleotida
5’-TTAGGG-3’, yang
berulang sampai
1000 kali dan
tersusun secara
tandem
Memendek pada
keturunan sel
berikutnya dalam
siklus sel  jadi
tidak melindungi
kromosom lagi
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TELOMER
Terdiri dari untai kaya G & untai kaya C
 Untai kaya G jauh lebih panjang  T-loop 
bantu lindungi ujung DNA linear

Struktur T-loop
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
DNA Telomer berikatan dengan protein-protein :


Pengontrol panjang telomer
Pelindung telomer
Telomerase
3 komponen telomerase
• Telomerase reverse transcriptase (TERT)
• Telomerase-associated protein 1 (TEP1) –
regulatory function (? not known for sure)
• Telomerase RNA subunit
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Pemendekan Telomer  berkaitan dengan
masalah replikasi ujung DNA
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Beberapa sel immortal dapat memelihara
telomer tanpa bantuan telomerase  10-15%
 Gunakan mekanisme ALT (alternative
lengthening of telomer


Terjadi pergantian telomer  tergantung pada
mekanisme tipe interkromosom copy choice 
Gunakan polimerase biasa untuk perpanjang telomer
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FUNCTION OF TELOMERASE AND
CONSEQUENCE OF ITS ACTIVATION
Function
Consequence examples of
telomerase activation
Elongation of telomeres
Elongation of cellular lifespan or
immortalization
Maintenance of chromosomal
structure
Telomerase is transiently expressed
in each S phase in normal cells
Addition of malignant potential
Tumor formation with
nontumorigenic ALT cells
Promotion of stem cell proliferation
e.g. increased hair growth
DNA repair?
Required to form DNA damage foci
following irradiation
Self-renewal capacity
Required to reprogram fibroblasts
to iPS cells
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
Sel pada tubuh manusia
mengalami mitosis 1016,
mencit 1011  resiko
manusia terkena kanker
lebih tinggi
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Regulation of telomerase in the hierarchy of the normal and leukemic
hematopoietic stem cell
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Besides losing the ability to
correctly determine if their
environment is appropriate for
division & the characteristics
necessary for the immune
system to remove them as
damaged cells, cells on the path
to tumor & cancer formation also
gain several capacities: they
evade apoptosis, produce their
own growth factors, become
insensitive to growth
suppressors & cell contact
signals, gain telomerase activity
& overcome the Hayflick limit, &
express angiogenic factors &
molecules needed during
metastasis.
Douglas Hanahan & Robert A. Weinberg,
The hallmarks of cancer,
Cell 100:57–70, 2000.
lung fibroblast 55 times,
heart 26, kidney 40, and skin 43
SEL KANKER IMMORTAL
Peningkatan jumlah sel pada kanker
secara teoritis
Kultur sel – 50-60 doubling
60 cells doubling 1018 sel = 109 cm3 = 106 kg (theoretically)
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
Pada kanker, tidak semua sel survive, ada sel-sel
apoptosis
TELOMERASE
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Carcinogenesis and immortalization of human cells in vivo. Normal
somatic cells, even stem cells or lymphocytes that have a capacity of
telomerase activation upon proliferation, cannot be immortalized in vivo.
On the contrary, once telomerase activation occurs in cancer cells, it is
usually irreversible and such cancer cells are easily immortalized. Whereas
key genes responsible for cellular transformation are heterogenous among
the individuals, those for cellular immortalization are considered to be
relatively monotonous, mostly ‘‘telomerase’’ except for ALT (alternative
lengthening of telomeres) or other rare events
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TELOMERASE AND POSSIBLE CAUSES TO ITS ACTIVITY IN
CANCER

p53 inactivation

C-myc expression

Steroid hormones
www.biocarta.com/pathfiles/h_tertPathway.gif
Sel kanker mengekspresikan
telomerase  crisis dapat
diatasi
 Sel embrionik  diferensiasi
 telomerase menurun

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Telomere biology in CML model: Biological properties of CML cells in
three hematologically different stages (normal, chronic phase, and blastic
phase) are shown in the triangle. The peak telomere lengths in each stage
are shown in the lower part of the figure, showing the telomere attrition
and disease progression in LSC.
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TELOMERASE AND CANCER

Normal Cells
little to no Telomerase
activity
 Limited life span
 Exception: highly
proliferative tissues


Cancer Cells
High telomerase
Activity
 Immortalized

X Yuan, et al. 1999
HAYFLICK LIMIT AND CRISIS
William C. Hahn, 2003
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Clinical significance: Cancer
or absent
telomere due to
progressive
shortening with
DNA replication
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MEKANISME SIKLUS
PEMATAHAN-FUSI KROMOSOM
(BREAKAGE-FUSION-BRIDGE /
BFB CYCLE)
Kromosom disentrik

Pada manusia
terjadi pula BFB
cycles pada saat
fungsi p53 hilang
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CLINICAL SIGNIFICANCE: CANCER
SELF-RENEWAL OF EPITHELIAL CELL
POPULATION BY REPEATED CELL DIVISION
Telomeres shorten and uncap
Normal p53 cell cycle
checkpoint control
Normal
senescent cells
stop dividing
Loss of p53 and cell
cycle checkpoint control
Mutant cell survives and proliferates
Chromosome
translocation
Cell dies due to
catastrophic
genomic
instability and
DNA damage
Chromosome
fusion
Chromosome
bridge
Chromosome
breakage
Massive chromosomal damage
Telomerase reactivated
CHROMOSOME
BREAKGE-FUSIONBRIDGE CYCLE
Chromosomes are partially
stabilized and cell survives
with many mutations
CANCER

Pada sel kanker, telomerase ~ onkogen
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TA = telomerase activity
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
Tidak ada telomerase  bisa menurunkan dan
sekaligus meningkatkan kerentanan terhadap
kanker
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Hypothetical model of telomeres and telomerase in primary and metastatic
lesions of human cancer. Human cancers may be developed from
telomerase-negative normal cells (upper) and telomerase-positive normal
cells, typically from normal stem cells through cancer stem cells (lower). In
the former mechanism, the population of cancer cells that have activated
telomerase in mutational manner increases according to tumor development
through clonal selections, while in the latter, cancer cells have high
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telomerase activity from an early stage
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Transformation and immortalization of human cells in vitro. TERT alone
transfection sometimes immortalizes normal fibroblasts but not normal
epithelial cells. SV40 early region (SV40ER) immortalizes neither.
Although cotransfection of TERT and SV40ER can immortalize both, they
do not have tumorigenicity. Addition of oncogenic ras, mutated H-ras or
K-ras, makes
them genuine immortal cancer cells with tumorigenicity
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a, p53-/- mice. The number of tumours
identified (t) and the total number of mice
(n) in each cohort is indicated. Hatched line,
G1–G2 mTERC-/-; triangles, G5–G6 mTERC-/-;
circles, G7–G8 mTERC-/-. b, p53+/- mice.
Hatched line, mTERC+/+, mTERC+/- or G1–
G2 mTERC-/-; triangles, G5–G6 mTERC-/-;
circles, G7–G8 mTERC-/- .
a, Breast cancer, G5 mTERC-/- p53+/-mouse; b, squamous cell carcinoma, G6 mTERC-/- p53+/mouse. c, Gross view of caeca from mTERC+/ - p53+/- (left), G6 mTERC -/- p53+/- (middle), and G5
mTERC-/- p53+/- (right). d, Histology of normal caecum, mTERC +/- p53+/-, shows typical colonic villi and
ordered nuclei. . e, Adenomatous polyp in the caecum, G5 mTERC-/- p53 +/-. Inset, glands remain
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round with basal nuclei. f, Caecal adenocarcinoma, G5 mTERC-/- p53+/-. Inset, disordered glands and
pleiomorphic nuclei. . g, Invasive adenocarcinoma of colon, G5 mTERC-/- p53 +/-. Inset, tumour cells (t) with
poor glandular organization invading muscle fibres (m)
Nature 406, 641-645(10 August 2000)
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Karyotype chaos in cancer cells
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