Cancer IV-V: stem cells Flashcards
What is the approximate number of cells associated with each endpoint during a person’s lifetime? (relative values): growth cells, stem cell regeneration, cancer cells
Bottom line: WAY MORE stem cell divisions occur during a person’s lifetime to keep us healthy than division occurring for either growth or in cancer
what are the 2 major types of stem cells?
- embryonic stem cells (ESCs)
- adult stem cells (ASCs)
embryonic stem cells (ESCs)
Fertilized egg → blastocyst (5 days old) → inner cell mass → stem cells
Derived from the inner cell mass of the blastocyst, a hollow ball of cells formed about 5 days after fertilization
Adult stem cells (ASCs)
aka somatic stem cells
exist throughout the body after embryonic development and are found inside of different types of tissues: ie. brain, bone marrow, blood (vessels), skeletal muscles, skin, and liver
what are the 3 historical key stem cell properties?
clonal succession
self renewal
asymmetric division
clonal succession
Clonal succession: once a stem cell differentiates, it cannot go backwards (aka it cannot give rise to a more “potent” stem cell)
self renewal
stem cell divisions results in a more differentiated cell and an identical cell
asymmetric division definition and explanation
one daughter cell differentiates while the other stays the same due to the presence of a stem cell niche (aka the 2 daughter cells are DIFFERENT)
The daughter cells ARE identical IMMEDIATELY after cell division, but the subsequent fate of one daughter cell is influenced by displacement into a different MICROENVIRONMENT
stem cell niche
the specialized microenvironment where stem cells reside and it provides structural and biochemical support, regulates stem cell behavior, and maintains their ability to self renew and differentiate
the location where stem cells “survive” for decades into and through adulthood
what is a well studied example of asymmetric division and the role microenvironments play on cell fate?
spermatogenesis
spermatogenesis definition and description (include sertoli cells)
definition: the process by which male gametes, or sperm, are produced in the testes
Sperm “stem cells” exist in a “stem cell niche”
Sertoli cells: “nurse cells” that nourish developing sperm cells through the stages of spermatogenesis as they “pass by”
Key point: conversion of sperm stem cells to mature sperm requires interaction with sertoli cells
all 4 cell fate processes occur during spermatogenesis, what are the 4 cell fate processes?
- Replication
- Differentiation
- Migration
- Apoptosis (~200 million sperm per day undergo apoptosis → that is ~2 out of 3!)
approx how much sperm per day undergoes apoptosis?
~200 million –> approx 2 out of 3 sperm undergo apoptosis
what are the 4 “__potent” stem cell types?
totipotent
pluripotent
multipotent
unipotent
totipotent stem cells (TSCs)
toti = all
Can differentiate into ALL cell types found in an adult and those that support embryonic development (ie. embryonic stem cells)
aka 1. Can differentiate into all cell types found in an adult (>400 cell types)
2. Can give rise to a new organism
what type of potency is a fertilized egg?
totipotent
what potency are cultured embryonic stem (ES) cells?
pluripotent
In humans, totipotency is lost during embryonic stem (ES) cell isolation
Pluripotent stem cells (PSCs)
pluri = many
PSCs can differentiate into MANY but NOT all cell types
ie. Muscle, liver, cardiac, nerve, bone, cartilage, gene therapy, fat, angiogenesis/anti-apoptosis
aka can differentiate into all cell types found in an adult (ie. adipose and mesenchymal stem cells)
give 2 examples of pluripotent stem cells?
adipose stem cells (ASCs)
mesenchymal stem cells (MSCs)
why are adipose stem cells so popular? (3 reasons)
aka fat stem cells
ASCs are pluripotent which makes them useful for many applications
ASCs are adult stem cells, avoiding ethical issues
ASCs are “easy” to get (ie. can get them from liposuction)
mesenchymal stem cells (MSCs) function (8)
MSCs function as “nature’s drug store”…
Increase reactive astrocytosis: astrocyte proliferation and activation
Angiogenesis: increase cerebral blood vessels
Myelination: increase axonal remyelination
Synaptogenesis: increase synaptic connections
Trophic factors: secretion of neurotrophic and angiogenic factors
Apoptosis: reduce apoptotic cell death
Neurogenesis: increase neuronal growth and differentiation
Inflammation: reduce T-lymphocyte activation; Reduce macrophage infiltration and microglia activation
mesenchymal stem cells (MSCs) types (3)
osteoblasts
adipocytes
chondrocytes
medicinal mesenchymal stem cells (MSCs)
a type of stem cell that possess therapeutic potential due to their ability to secrete bioactive molecules that can modulate the immune system, promote tissue regeneration, and establish a regenerative microenvironment
what type of bioactive molecules do medicinal MSCs secrete? (give 4 examples)
Bioactive molecules secreted by medicinal MSCs are IMMUNOMODULATORY and affect a variety of immune cell lineages
Immunomodulation: Medicinal MSCs secrete molecules that influence immune cell function. These cells can interact with various types of immune cells, such as T cells, B cells, macrophages, and dendritic cells, to suppress inflammation and regulate immune responses.
examples
T cells
B cells
dendritic cells
T-regs
etc…
medicinal MSCs can secrete molecules that establish what?
Other secreted molecules establish a regenerative microenvironment by establishing a powerful trophic field
Medicinal MSCs play a key role in creating a “trophic field”—a supportive environment that facilitates tissue repair and regeneration
regenerative microenvironment
- anti-apoptotic
- anti-scarring
- angiogenic
- mitotic
where do medicinal MSCs come from?
pericytes
pericyte –> MSC –> activated MSC –> medicinal MSC
pericytes
pluripotent stem cells associated with blood vessels and are found throughout the body
multipotent stem cells
can differentiate into all types of adult cells belonging to a particular group (ie. hematopoietic stem cells)
Multipotency: a type of “restricted potential” stem cell that can only differentiate into a single family of cells
one example of multipotent stem cell
hematopoietic stem cells (HSCs)
→ HSCs CANNOT differentiate into brain or cardiac (heart) cells but can differentiate into many types of blood cells
Unipotent stem cells (aka progenitor cells) and one example
can only differentiate into ONE type of adult cell (ie. spermatogonium)
give an example for each: totipotent, pluripotent, multipotent, unipotent
totipotent: fertilized egg, embryonic stem cells
pluripotent: mesenchymal stem cells, adipose stem cells
multipotent: hematopoietic stem cells
unipotent: spermatogonium
stem cell and cancer risk equation/model
The equation calculates the probability of cancer (p) with 5 parameters: number of cell divisions (d)
number of stem cells (N*m)
number of critical rate limiting pathway driver mutations (k)
mutation rate (u)
In this model, progression to cancer STARTS AT CONCEPTION (aka at birth) and mutations accumulate with cell division
Transformation (aka fully fledged or clinically relevant cancer) occurs when a critical number of rate limiting pathway mutations first accumulates within a single STEM cell
what is the Implications of cancer starting at conception?
Means that we all have cancer cells growing in us
→ relates back to the MIT lecture quote: “everybody in this room who does not die of something else first WILL die of CANCER” (aka we all have cancer but 2/3 or 3/4 of us will die of something else first)
What do all these dividing/divided stem cells do?
regeneration
aka divided stem cells constantly help to repair and rebuild our bodies to keep us healthy
satellite progenitor cells
a type of adult stem cell found in skeletal muscle that play a crucial role in muscle regeneration and repair
They are a form of muscle stem cell that is activated in response to muscle injury or stress, helping to regenerate and repair damaged muscle tissue
muscle tissue is completely replaced every how many years?
Muscles continuously regenerate → muscle tissue is completely replaced every 15 years or so (as long as the satellite progenitor cells are not depleted)
How do we know how long it takes for muscle tissue to be completely replaced?
The cold war enabled measurement of a cell’s lifetime
explain the cold war implications
Above ground weapons testing led to a spike in atmospheric Carbon (isotope 14) which labeled the DNA of anyone alive at the time
People undergoing rapid growth during these years had up to 2% of their biomolecules (ie. DNA) labeled with 14C
what is the turnover rate for…
Cerebellum neurons
Occipital-cortex neurons
Jejunum non-epithelial
Skeletal muscle
Cerebellum neurons: < or = lifetime
Occipital-cortex neurons: younger than cerebellum neurons
Jejunum non-epithelial: 10.7 years
Skeletal muscle: 15.1 years
stem cells help maintain virtually all tissues, including ___
Cerebellum neurons
Occipital-cortex neurons
Jejunum non-epithelial
Skeletal muscle
liver!
what is an example of natural regeneration?
revival
The varying regenerative properties of amphibian, reptile, and mammal tails result from differences in neural stem cells found in the animals’ spinal cords
Tissue engineering definition and 3 requirements
Tissue engineering: the creation of new tissues or organs in the lab to replace diseased, worn out, or injured body parts
Requirements for tissue engineering:
Cells of the correct type
Biomaterials / scaffold
Control of cell fate
relation between tissue engineering and stem cells
Tissue engineering exploits stem cells for regeneration
Where and how does a tissue engineer obtain stem cells?
stem cells can be derived from differentiated cells
Embryonic stem cells? → NO because there are ethical issues involved in using ES cells
Adult stem cells → NO because they are difficult to obtain (with the exception of fat, but even so, the donor-host immune matching is problematic)
what are cancer stem cells NOT?
normal/healthy stem cells (despite contributing to cancer risk) are NOT cancer stem cells (CSCs)
AND not all cancer cells are CSCs
4 key features of cancer stem cells (CSCs)
DEFY the principle of clonal succession (a more highly differentiated cell becomes LESS differentiated)
- can generate stem cells from differentiated cells and use these differentiated cells to make mature tissue
Possess all of the hallmarks of cancer/rate-limiting hits (aka has ALL of the properties needed to rapidly regenerate the tumor)
Are relatively SLOW growing (due to the metabolic burden of expressing additional genes)
Are drug resistant
what are 2 hypothetical examples of tumor cells that lack CSC properties?
not all cells in a tumor need to express VEGF (angiogenesis supplies all cells with blood)
not all cells in a tumor need to express PD-L1 (the cells that do express PD-L1 can inactivate the “killer” T cells and thus protect non expressing cells)
key point: individual cells in a tumor may NOT have all the “hallmarks of cancer” but CSC presumably DO have all the hallmarks
how often do cancer stem cells divide and why are they relatively slow?
An observation from in vitro cell culture experiments where CSCs divide for example, once a week instead of once a day
CSCs needs to express additional genes (ie. VEGF, PD-L1): the expression of these genes presumably comes at the expense of faster replication
^^ Note: the key word is relatively: if the CSCs can divide once a week in the body like they do in cell culture, it can take only a few months instead of 60-70 years
clinical significance of cancer stem cells being slow-growing and drug resistant? (2)
- Conventional therapies target RAPIDLY growing cells –> killing the majority of cells in a tumor which often leads to clinical remission
- CSCs disproportionately avoid eradication and rapidly regenerate the tumor (often within months) instead of 50-70 years
Future therapies for cancer regarding cancer stem cells?
future cancer therapies are proposed to target the CSCs
- Drugs with less toxicity can be developed to selectively target the CSCs
- Without CSCs and their ability to regenerate the tumor, it will safely and harmlessly “melt away” over time
induced pluripotent stem cell (iPSC)
Induced pluripotent stem cells (aka iPSCs): terminally differentiated cells that have been converted into pluripotent stem cells via transfection of Oct-¾ SOX2, c-Myc, and Klf4 genes
researchers can convert terminally differentiated stem cells into pluripotent stem cells
HOWEVER, researchers have not been able to generate totipotent stem cells yet
the transfection of which 4 genes can convert terminally differentiated cells into pluripotent cells?
Oct-¾, SOX2, c-Myc, Klf4
what are the 2 distinct ideas/strategies of iPSC technology?
iPSC technology application 1: disease models
- The “disease in a dish” model can be used to study genetic diseases when patient samples are not available (ie. for muscle or neurological disorders)
- Best of all, this application is safe to use RIGHT NOW
iPSC technology application 2: replacement cells/tissues
- iPSCs hold tremendous potential of cell and tissue-based regenerative therapies
- HOWEVER, safety is a significant concern for iPSC therapies
what is the number one safety concern for iPSC reimplantation therapy and where does it come from?
CANCER
iPSCs and cancer: risk from the reprogramming factors
iPSC reprogramming factors are ___
iPSC reprogramming factors ARE cancer genes
c-Myc
recall Hallmark 8: metabolic reprogramming → however as shown below, MYC actually does much more than metabolic reprogramming in cancer
Oct3/4
Oct3/4: important during embryonic development and for the maintenance of CSCs
SOX2
also promotes cancer develop and contributes to several hallmarks of cancer
KLf4
Klf4: a “two faced” molecule that can be a tumor suppressor (left) but can also participate in cancer stem cell maintenance (right)
Severe Combined Immunodeficiency (SCID) aka “boy in the bubble” syndrome
a genetic disorder where the immune system is severely weakened or absent → leaves individuals highly vulnerable to infections and is caused by mutations in genes that are essential for the development and function of T cells and B cells
describe the initial success and then complications of the gene therapy used for SCID
An early gene therapy test in the late 1990s cured most children (8/10 in one study)
In the next few years, a high number (4/9 patients in one study) of children developed a specific type of leukemia
During gene therapy, the IL2RG gene which was needed to cure SCID integrated into part of the genome that regulated the LOM2 gene and activated this cancer gene
what were the 1st generation safety issues of iPSC therapy?
Genes needed for iPSC induction were integrated into the host cell’s genome
Advantage: long term expression for the genes required for iPSC creation
Disadvantage: integration potentially can activate cancer genes (ie. LMO2)
describe the iPSCs 2nd generation technology
Used non-integrative DNA which was safer but inefficient
Integration of the gene into the host cell’s genome does not occur → INCREASED SAFETY
however the tradeoff is that gene expression is transient (for a small amount of time) which DECREASED EFFICIENCY
4 examples of cancer genes that are turned off in healthy adults
VEGF turned off after angiogenesis is complete
PD-L1 turned off (except for macrophages and dendritic cells)
Telomerase turned off (expect in rapidly dividing progenitor cells)
iPSC reprogramming factors turned off in fully differentiated cells
what turns genes off and then on again in cancer?
EPIGENETICS turn genes off and then turned them back on in cancer
are gain-of-function “hits” genetic?
NO!
→ bottom line: gain of function hits almost never are genetic!
instead they are epigenetic
ie. gain-of-function cancer gene: VEGF, telomerase, PD-L1
why does any early stage tumor grow very slowly (ie. cells only divide once every 12-24 months)
it could be waiting until a gain of function hit kicks in: ie. gain of VEGF for angiogenesis, telomerase to confer immortality, or PDL-1 to avoid immunesurveillance
Epigenetics
the changes caused by the modification of GENE EXPRESSION rather than by direct alteration of the genetic code (DNA) `
describe epigenetic regulation in development and in cancer
During development and oncogenesis, many genes are turned on → after development, many genes are turned off
ie. Genes like VEGF and telomerase need to be turned on during early development, but then these genes are turned off in an adult and then turned back on in cancer
Summary: during development, cancer genes are imprinted (turned off) through a long and complicated process but they CAN be turned back on through epigenetic regulation in cancer and iPSC production
These enzymes and other involved proteins include the iPSC reprogramming factors!
Gain of function hits hypothesis
Gain of function hits → cancer is an EPIGENETIC disease
^^ This idea was developed by Johns Hopkins BME Andy Feinberg
Imprinted genes are linked to __
Imprinted genes (genes that are turned off) are linked to DNA methylation
- Imprinted genes are regulated by DNA methylation, which silences one allele (either the maternal or paternal copy)
- The addition of methyl groups to cytosines in promoter regions of genes results in gene inactivation
DNMTs methylate DNA → certain cytosines are methylated → specifically, DNA methylation in promoter regions of genes results in inactivation
imprinted genes definition
genes where only one allele (either the maternal or paternal copy) of the gene is active, while the other is silenced
This selective silencing is an example of epigenetic regulation, where the gene expression is controlled by mechanisms like DNA methylation and histone modifications
what is a hit?
hit: a mutation that inactivates a proto-oncogene to create a cancer gene: ie. Rb, p53
This is a genetic effect (aka a direct change in the DNA sequence)
→ HOWEVER in other cases, a genetic explanation doesn’t make sense
Consider: what if any gain of function cancer gene (ie. VEGF, telomerase, PD-L1) suffers a genetic hit (aka experiences a mutation)
^^ bottom line: gain of function hits almost never are genetic! –> instead they are epigenetic
