Stem Cells/ Cancer

Question 1

What are the 3 features of stem cells

Answer 1

• capable of dividing and long term self-renewal through mitotic cell division
• unspecialised, differentiate into specialised cell types under appropriate conditions

Question 2

What does long term self-renewal refer to + 2 types of divisions

Answer 2

• stem cells make identical copies of themselves via mitotic cell divisions for the lifetime of the organism (self-renewal)
• proliferation : repeated replication
• if resulting cells continue to be unspecialised like parent stem cells, cells are capable of LONG TERM self-renewal

2 types of division : asymmetric & symmetric
- symmetric : both daughter cells retain self-renewal property to ensure that pool of stem cells is constantly replenished in adult organ
- asymmetric : one remains a stem cell capable of self-renewal, other undergoes differentiation to become a specialised (progenitor) cell

Question 3

What is differentiation + importance

Answer 3

• unspecialised stem cells receive signals that lead to the expression of specific genes to form tissue specific structures on the specialised cell
• these new cells & tissues are used to repair or replace damaged or diseased cells in the body
• tissue specific structures : specific proteins found in certain types of cells that give them their specific functions
• signals : chemicals secreted by other cells, physical contact with neighbouring cells, certain molecules in the env

Question 4

What does potency refer to + types

Answer 4

Potency specifies the differentiation potential of the stem cell (potential to differentiate into different cell types)

Totipotent : differentiate into any cell type to form the whole organism
Pluripotent : differentiate into almost any cell type to form any organ/type of cell (except placenta or other extra-embryonic membranes)
Multipotent : differentiate into a limited range of cells and tissues appropriate to their location
Unipotent : differentiate to only one type of cell

Question 5

Type of stem cells

Answer 5

1. Zygotic stem cells
2. Embryonic stem cells
3. Adult stem cells
   
   • Hematopoietic stem cell
   • Bone marrow
   • Umbilical cord blood

Question 6

Zygotic stem cells (potency, source)

Answer 6

• totipotent : ability to differentiate into any cell type to form a whole organism
• derived from the morula during the zygotic stage of development (also pluripotent and multipotent)

Question 7

Embryonic stem cells (potency, source, forms what)

Answer 7

• pluripotent : differentiate into almost any cell type to form any organ/type of cell except extra-embryonic membranes
• derived from inner cell mass, which is part of the early (5-6 day) embryo called the blastocyst (consists of trophoblast & inner cell mass)
• form the entire foetus, but placenta / other extra-embryonic membranes cannot be formed = cannot form whole organism
• if cultured in lab = immortal, reproduce indefinitely, divide for long periods in an undifferentiated state

Question 8

Adult stem cell (potency, source, function, named example)

Answer 8

• multipotent : can renew itself + produce all the specialised cell types of the tissue from which it originated
• undifferentiated cell that occurs in a differentiated tissue
• function: replenish dying cells and regenerate damaged tissues

Examples
Hematopoietic stem cell (HSC)
Found in : bone marrow
: Umbilical cord blood

Question 9

Hematopoietic (blood) stem cell

Answer 9

• adult multipotent stem cell
• treat a range of blood disorders & immune system conditions like leukaemia and sickle cell anaemia
• source : bone marrow and umbilical cord blood
• differentiate into 2 types of multipotent stem cells : myeloid and lymphoid

Myeloid stem cells differentiate into
- WBCs, monocytes, macrophages, neutrophils, basophils, eosinophils, erythrocytes (RBCs) and platelets

Lymphoid stem cells differentiate into
- T cells (lymphocytes), B cells (lymphocytes) and natural killer cells

Question 10

Bone marrow as source of HSC

Answer 10

Bone marrow
- HSC in bone marrow divide mitotically into 2 kinds of cells
1. One remains as HSC
2. The other differentiates into a myeloid SC and lymphoid SC -> further differentiates into various kinds of blood cells (RBC, WBC, megakaryocytes)
- Differentiation path taken by cell is regulated by cytokines (protein) and/or hormones

Question 11

Umbilical cord blood as source of HSC

Answer 11

Umbilical cord blood
- blood from placenta and umbilical cord, rich in HSC
- utilised as a source of stem cells for transplantation

Advantage
- less prone to rejection, no immune response against cells (no need for immunosuppressants/test for blood and tissue match)
- cells in umbilical cord blood have not yet developed features that can be recognised and attacked by recipient’s immune system
- lacks well developed immune cells, less chance that transplanted cells will attack the recipient’s body
- no foreign antigens on cell membrane
- can be converted into specialised cells to treat specific diseases

Disadvantage
- limited storage capacity, high costs involved
- longevity of cells may be unclear
- freezer failure -> cells get damaged/mutate

Question 12

Uses of stem cells (5)

Answer 12

1. Replaced damaged tissue, replenish dying cells (specialised cells/tissues are transplanted into patient)
2. Testing of new drugs (differentiate into cell type that drug tests on)
3. Testing gene therapy methods (for genetic illnesses)
4. Study human development
5. Toxicity testing (degree to which a substance can damage a living or non-living organism)

Question 13

Why do stem cells need to be treated with chemicals to stimulate proliferation?

Answer 13

Removal of stem cells from patient’s body : absence of natural growth factors / hormones to stimulate cell division

Addition of chemicals stimulate cell division by binding to cell receptors & stimulating cell division pathways

Question 14

Ethical implications of stem cells

Answer 14

1. Destruction of embryo (some consider it a life)
2. Donors of oocytes or embryos may not have informed consent regarding the use for research
3. Potential of medical complications or health risks to oocyte donors (not informed)
4. Time consuming, expensive -> widen social divisions

-> induced pluripotent stem cells

Question 15

Induced pluripotent stem cells

Answer 15

IPSCs are pluripotent stem cells generated directly from adult cells

Certain adult stem cells may be able to generate cell types of a completely different tissue under right conditions -> plasticity or trans-differentiation

Makes use of 4 protein factors (specific transcription factors) which are introduced into differentiated cells by retroviruses

Overcome ethical complications : no destruction of embryo, skin biopsy is less invasive = fewer risks, iPSCs made in patient-matched manner = no risk of immune rejection

Question 16

What are cancer cells

Answer 16

• cells that have escaped from cell cycle control, uncontrolled cell growth and division (unrestrained cell proliferation)
• immortal if continued supply of nutrients is given
• caused by mutation in genes regulating the cell division cycle (proto-oncogenes and tumour suppressor genes)

Question 17

Characteristics of cancer cells vs normal cells

Answer 17

Growth factor required?
- normal : external growth factors required to divide. When synthesis of gf is stopped by normal cell regulation, cell division stops
- cancer : no need for gf, do not behave as part of the tissue = independent cells

Contact inhibition
- normal : yes = respond to contact with other cells by ceasing cell division (eg when gap is filled w enough cells)
- cancer : no, continue to grow after touching other cells = large mass of cells, disordered multi-layered cell patterns

Limit on number of cell divisions
- normal : age and die via apoptosis, replaced by new cells
- cancer : telomerase activated, can undergo unlimited no of cell divisions without apoptosis being triggered
* telomeres (non-coding repetitive sequences) located at end of chromosome protects coding sequences due to end replication problem -> once critical length of telomere is reach, cell undergoes apoptosis. Telomerase = extends telomeres = telomeres never reach critical length

Divide when dna is damaged?
- normal : division ceased, undergo apoptosis
- cancer : division continues, damaged DNA (mutations) accumulates

Question 18

Proto-oncogenes and oncogenes

Answer 18

Proto-oncogenes : code for proteins that send signal to nucleus to stimulate cell division (eg growth factors, receptor proteins for growth factors, g-protein, intracellular protein kinases, transcription factors)
- when turned on at wrong place/time = oncogenes : code for proteins that lead to overstimulation of cell growth & division

Gain of function mutation
- most oncogenes arise from dominant mutations (single copy of oncogene is sufficient for trait expression)
- cells with mutant form of protein have gained a new function
- presence of oncogene in germ line cell (egg/sperm) = inherited predisposition for tumours in offspring BUT single oncogene is not enough to cause cancer
- Point mutations
- chromosomal rearrangement
- gene amplification
- insertional mutagenesis

Question 19

Gain of function mutation (proto-oncogene) : point mutations

Answer 19

In coding region
- small change in base sequence (substitution/deletion)
- change in 3D conformation of protein, altered protein becomes hyperactive (still made in normal amounts)

Ras gene : codes for G-protein found on cell membranes. (Ras+GTP=active ; Ras+GDP=inactive)
- normally, binding of appropriate growth factor to a receptor = activation of ras protein, GTP molecule displaces GDP in ras protein
- active ras protein passes on signal to series of cytoplasmic kinases which activate transcription factors that turn on genes for proteins that stimulate cell cycle
- to turn pathway off, ras proteins hydrolyses its bound GTP to GDP and becomes inactive
- point mutation to ras GENE = change in 3d conformation of the ras PROTEIN = loss of ability to hydrolyse GTP to GDP
- altered ras protein is constitutively active (hyperactive), continuously delivers signal for cell growth and division (uncontrolled)

In regulatory region (eg promoter)
- over-expression of gene = overproduction of the normal functional protein

Question 20

Gain of function mutations (proto-oncogene) : chromosomal rearrangement

Answer 20

Translocation : breakage and rejoining of DNA
- change the protein-coding region = hyperactive fusion protein OR alter control regions for a gene so normal protein is over-produced

Question 21

Gain of functions mutation (proto-oncogene) : gene amplification

Answer 21

Error in DNA replication = extra gene copies
- due to selective replication of a region of a chromosome (many copies made) -> genes within amplified portion of chromosome can be transcribed to produce normal protein. Translation of these genes leads to overproduction of the normal proteins
- process occurs in CANCER cells (not normal cells) -> if oncogene is included in amplified region, over-expression of that gene = deregulated cell growth

Question 22

Gain in function mutation (proto-oncogene) : insertional mutagenesis

Answer 22

Insertion of retrovirus into DNA causes over-expression of a proto-oncogene -> becomes oncogene
- retrovirus integrates into host DNA at region near proto-oncogene -> proto-oncogene comes under control of active retroviral promoter sequences
- over-expression occurs as retroviral sequences do not respond to environmental signals that normally regulate proto-oncogene expression -> tumorous state

Question 23

Tumour suppressor genes

Answer 23

Code for proteins that send appropriate signals to halt cell cycle, carry out DNA repair, and induce cell death (apoptosis)

Loss of function mutation
- mutated genes are no longer able to inhibit cell growth
- mutations are usually recessive, not expressed unless both copies of normal allele are mutated
- loss of heterozygosity (when remaining normal allele undergoes mutation)

Example : p53 tumour suppressor gene codes for p53 protein (transcription factor that binds directly to DNA in nucleus)
- can activate DNA repair proteins when DNA is damaged (maintains genetic stability)
- holds cell cycle at G1 checkpoint (cell cycle arrest) on recognition of DNA damage
- initiate programmed cell death (apoptosis)
- mutations = non-functional proteins synthesised = DNA damage is allowed to accumulate within a cell = increase risk of cancer formation

Question 24

Development of cancer

Answer 24

Multi step process, accumulation of 4-6 independent mutations in key cell-cycle regulatory genes
- need to inactivate several regulatory genes -> cancers develop over decades
- tumours have degree of malignancy

For cell to turn cancerous
- Gain of function mutation in at least 1 proto-oncogene
- Loss of function mutation in several tumour suppressor genes
- accumulation of mutations in these genes occur over time = overstimulation of cell growth and division + inability to halt cell cycle, carry out DNA repair, initiate apoptosis

Question 25

Possible steps in cancer development (7)

Answer 25

1. Single cell in tissue suffers a mutation in a gene involved in cell cycle - gain/loss of function mutation in respective gene
2. Mutated cell has a slight growth advantage over other cells in tissue
3. Any DNA damage from mutated cells is passed on to daughters
4. Over time, some descendent cells may suffer another mutation in another cell cycle regulatory gene
5. Further deregulates cell cycle of cell and descendants
6. Rate of mitosis in that line of clones increases = chances of further DNA damage increases
7. Accumulation of mutations = uncontrolled proliferation of cells to form tumors

Question 26

How do malignant tumours develop

Answer 26

Benign (localised) tumours lack ability to invade other tissues

Through further mutations, cells in tumour may gain ability to invade normal tissues & migrate to other parts of the body (becomes malignant)
1. METASTASIS : tumour cells can penetrate blood or lymphatic vessels -> circulate through circulatory system -> proliferate at another site
2. ANGIOGENESIS : other mutations can induce formation of new blood vessels that play a role in supplying nutrients and oxygen to the growing tumour at the new site

Question 27

Causative factors of cancer

Answer 27

Internal
- Loss of immunity
- Genetic predisposition (born w 1 defective copy of TSG = germline mutation, inherited from both parents)
- Hormones

External
- Chemical carcinogens (damage/alter DNA eg tobacco smoke)
- UV and ionising radiation (IR can penetrate nucleus and form damaging ions inside the cell that can mutate DNA)
- Viruses (human papillomaviruses HPV, human immunodeficiency virus HIV)
- Agents that stimulate rate of mitosis (chronic tissue injury = increased mitosis to repair damage)
- Agents that cause chronic inflammation (generates DNA damaging oxidising agents in cell)