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Promotion of tumorigenesis by
heterozygous disruption of the
beclin 1 autophagy gene
The journal of Clinical Investigation
112:1809-1820 (2003)
Lien Hsu
Outlines
► Introduction----
Autophagy
Beclin 1
Hypothesis
► Methods and Results
► Discussion
► Critics
Introduction----what is autophagy?
Autophagy (autonomous
phagocytosis)
Functions:
I. allows cells to survive during starvation
II.enables cells to undergo structural remodeling
during differentiation and development
III.prevents aging
► Defects of autophagy--?--Development of
cancer
Malignant cells----lower basal autophagic
activity ; no increased protein degradation rates
Beclin 1
I. promotes starvation-induced autophagy in
human breast carcinoma cells
II. 17q21, a tumor-susceptibility locus
III. Monoallelically deleted----in 40-75% of cases
of human sporadic breast, ovarian, and prostate
cancer
Hypothesis
Inference----tumor suppressor?
*biallelic mutations of beclin 1 have not been
demonstrated in human cancer~~ haplo-insufficient
tumor suppressor gene?
Methods and Results
►
►
Knock-out mice beclin 1 +/+/- x +/- => F1----embryonic lethality of homozygous-deficient
mice
Beclin 1 heterozygous disruption in mice results in increased
spontaneous tumorigenesis
Macroscopic
malignancy
All malignancies
+/ +/+
Prevalence of
macroscopic
malignancies
any malignancy
lung carcinoma
hepatocellular
carcinoma
Lymphomas (gray) and
lymphoproliferative
disease (black or white)
Lung carcinoma
well-differentiated papillary lung
carcinoma in beclin 1(+/-)
anti-Beclin 1(lung)
anti-TTF-1(lung carcinoma):
specific transcription factor in
bronchial and type II alveolar
epithelial cells
Hepatocellular carcinoma
Gross pathology of liver tumor
anti-Beclin 1(hepatocellular carcinoma)
inset shows lymphoma adjacent to
normal kidney
anti-Pax5 (dark purple
anti-CD3 (brown): DLCL
Lymphoproliferative
disease in the thymus
Lymphomas
anti-BCL-6: transcriptional repressor
controls germinal center formation: human B
cell lymphoma
Southern blot to detect wt and disrupted
beclin 1 allele in tumor and normal tissuse
*no deletion or rearrangement of remaining wt beclin 1
allele
Results suggest:
► functional inactivation of one beclin 1 is
sufficient to promote tumorigenesis
► beclin
gene
1 is a haplo-insufficient tumor-suppressor
Beclin 1 heterozygous disruption in mice “accelerates” the
development of HBV (hepatisis B virus)-induced premalignant
lesions
The model---I. Cross beclin +/- X beclin +/+ with HBV transgenesis (13m)
II.liver is a major site of nutrient starvation-induced autophagy
preneoplastic small-cell dysplasia in the liver Extent of small-cell
dysplasia in liver HBV transgenic mice (13m)
(beclin 1+/- express HBV)
+/+ HBV trangenic mice(white)
+/- HBV transgenic mice(black)
Results suggest:
► Beclin 1 heterozygous disruption in mice
accelerates the development of HBVinduced premalignant lesions
Beclin 1 heterozygous disruption results in increased cellular
proliferation in vivo
intraepithelial
Epithelial duct
beclin 1 heterozygous
deficiency results in
abnormal cellular
proliferation in the
TEBs and mammary
ducts.
Studies for proproliferation affects
in germinal center
formation: B
lymphocyte
neoplasia
adenomyoepithelioma acinar neoplasia
Terminal
end bud
TEB
Mammary
ducts
Number
Size
Result suggest:
► beclin 1 heterozygous disruption increases cellular
proliferation in vivo, beginning at an early age.
Inference:
the increased cellular proliferation in beclin 1+/– mice
may increase the number of genetic mutations that
occur over the lifetime of the animals, thereby
contributing to the increased spontaneous tumorigenesis
that occurs in older beclin 1+/– mice
Q: whether beclin 1 +/- affects its known function in autophagy?
Beclin 1 heterozygous disruption decrases autophagy in vivo
GFP-LC3 marker---Upon stimulation of autophagy, LC3 localizes to preautophagosomal membranes
* The muscle has been shown to be an important site of starvation-induced autophagy
2m old 24hr starvation
Q: whether beclin 1 heterozygous deletion affects autophagy in any of
the tissues associated with increased spontaneous tumorigenesis?
►
Lymphocyte---no; liver---variably expressed; lung----typeII aveolar
and bronchial epithelial cells
►
Well-differentiated papillary lung carcinoma----show in bronchial
cell origin
Results suggest---► beclin
1 heterozygous deletion reduces
autophagic activity in a tissue that undergoes
starvation-induced increases in autophagy (i.e.
muscle)
Discussion
► Autophagy
genes may represent a novel class of tumorsuppressor genes.
► The
precise mechanisms by which the autophagy fuction
of Beclin 1 contributes to tumor suppression is not
known.
► Autophagy
may also contribute to tumor suppression by
degrading specific cellular organelles and long-lived
proteins that are essential for regulating cell growth,
thereby functioning as a brake on cell growth in response
to mitogenic signals.
Critics
► No
normal histologic slides to compare.
► Why
didn’t the authors mention if expression
of Beclin 1 decreases in all neoplastic lesions
or not?
► Is there any other possible autophagy-related
gene involved in tumorigenesis?
► Is
tumorigenesis really through any funtion of
autophagy? or just because of beclin 1?