Biology Flashcards
What is a cancer?
Cancer is an uncontrolled growth of abnormal cells in the body. Cancerous cells are also called malignant cells. The balance shifts towards proliferation and survival - accumulation of abnormal cells - increase in division and decrease in cell death
What are the types of gene mutation?
Change in genetic information by either addition, removal, or swapping of nucleotides
Can be harmful, beneficial or neutral
Function of proto-oncogenes?
- Increase cell division
- Decrease cell death
Function of tumour suppressor genes?
- Reduce cell division
- Induce cell death
What happens when a mutation occurs in a proto-oncogene?
May cause it to become an oncogene
Oncogene: a gene that is mutated (changed) form of a gene involved in normal cell growth. Oncogenes may cause the growth of cancer cells
What happens when a tumour suppressor gene is mutated?
Mutation = lose protective abilities Mutations in this gene that allows control of growth may lead to uncontrolled cell growth = cancer A third class of important genes (often included in tumour suppressor genes) - encode proteins that maintain DNA stability by repairing DNA and protecting against accumulation of mutations
Name two tumour supressors
p53 (TP53, tumour protein 53)
RB1 (retinoblastoma 1)
Tell me about p53
- blocks cell cycle in response to cellular damage
- induces apoptosis if DNA damage is irreparable
- transcription factor
- most commonly mutated gene in cancers
- activators of p53: low oxygen conc, DNA damage, chemotherapeutic agents, other stress
Tell me about RB1
- blocks cell cycle by binding (and inhibiting) E2F transcription factors - cell cycle can’t proceed past G1/S
- inhibits transcription of genes needed for cell cycle progression
- is inhibited by phosphorylation (e.g. by cyclin D-CDK4)
- leads to changes in gene expression
- can be inactivated by phosphorylation
How is p53 structured?
Homotramer
- four identical subunits interact at their C-terminal complexing domains
- central part of each subunit can bind DNA at specific regulatory sequences
- N terminal transactivation domain switches the gene on by recruiting RNA polymerase
What are the two main proto-oncogenes?
MYC
RAS
Lets talk about about MYC!
- transcription factor (partners with MAX)
- promotes cell growth
- leads to changes in gene expression
- produced in response to oncogenic signals
- MYC protein will then regulate numerous pathways, positively and negatively
Tell me about RAS
- G protein - binds to GDP (inactive) or GTP (active)
- activated by growth factors
- activates downstream signaling pathways
- leads to changes in gene expression
- single subunit small GTPase
- not to be confused with GPCRs
- GTP bound = on; GDP bound = off
- mutated in ~ 25% of all cancers
Two ways cells can die?
necrosis or apoptosis
Explain necrosis
Spillage of cell contents due to inflammation
Caused by damage, infection, ischaemia, cancer
Can release pro-inflammatory signals - immune cells recruited whih can promote angiogenesis
Growth factors released which promote proliferation
Necrosis can promote cancer - cell death not always good
Explain apoptosis
Programmed cell death
Cellular components are degraded and removed
Two pathways - intrinsic and extrinsic apoptosis pathway
How are cancer cells resistant to apoptosis?
- fine balance between pro- and anti- apoptotic signals
- often found on mitochondrial membrane
- upreguation of survival signals (anti-apoptotic) e.g. Bcl2
- downregulation of pro-apoptotic signals e.g. Bax
What happens to a cell’s telomeres every time it divides?
Get shorter
- unless telomerase is active to restore them
- ~90% of malignant cells express telomerase
- telomerase only normally expressed in germ cells and stem cells
Purpose of telomeres?
RNA primers tell DNA polymerase where to start and are degraded in the process
What is the alternative to telomere lengthening?
- recombination or fusion between the ends of different chromosomes
- cells that survive telomere shortening have chromosome rearrangements (e.g. fused chromosomes; deletions; amplifications) -> oncogenic changes
What does EGFR stand for?
Epidermal Growth Factor Receptor
Purpose of EGFR?
Senses growth signals
Lead to increase in proteins needed for cell division
What is the phosphorylation cascade induced by EGFR binding?
Must be a conformational change cause by dimerisation
Activated by phosphorylation
EGFR phosphorylation (due to EGF binding) -> RAS ->. RAF -> MEK -> switch on ~100 genes for cell division -> ERK
Alternative route following EGFR activation?
PI3K -> AKT -> protein synthesis, apoptosis, TP53 (provides instruction to code for p53) - allow cell cycle to continue
What happens when EGFR mutates?
EGFR permanently activated - behaves as if bound to EGF
- causes the cell to divide
How do drugs target AGFR?
- stop EGF binding to EGFR: cetuximab, an antibody that blocks the receptor
- stop EGFR activation: erlotinib & gefitinib, looks like ATP (which is required for activation), don’t behave like ATP (block activation)
What are steroid hormone receptors?
Steroid hormones are derived from colesterol (a membrane component)
They can diffuse into cells - no need for cell surface receptors
Steroid hormone receptors are part of the nuclear receptor superfamily
They are transription factors which become activated by steroid hormones e.g. estrogen receptor, androgen receptor, progesterone receptor, retinoic acid receptor
Tell me about ER (estrogen receptor)
- an example of a type 1 steroid hormone receptor (activated in the cytoplasm)
- ligand (estradiol) diffuses into cell
- binds to the receptor, displacing associated chaperone proteins e.g. HSP90
- ER dimerises and migrates to the nucleus
- ER dimer associates with co-activators or co-repressors to modify transcriptions of target genes
Explain the mode of action of tamoxifen
- a prodrug
- metabolised to active metabolites by CYP2D6
- binds to ER with much higher affinity that estrogen
- estrogen antagonist, but with some agonism
- mostly anti-estrogenic effect (but estrogenic in uterus & bone)
- selective ER modifier (SERM)
Explain the mode of action of Fulvestrant
- “pure” anti-estrogen (no agonism)
- prevents dimerisation, activation
- increases degradation
- Selective ER down-regulator (SERD)
What is epigenetics?
- changes that affect gene expression without changing DNA base sequence e.g. histone modifications (acetylation, methylation) or DNA modifications (cytosine methylation -> promotor repression)
- can be inherited by daughter cells
- alter the accessibility of DNA for transcription
Acetylation opens chromatin domains (more susceptible to transcription) whereas methylation closes chromatin domains
Mode of action for HDAC inhibitors
HDACs (histone deacetylases) remove acetyl groups from histones
It is an attractive drug target
HDAC inhibitors have proven to be effective, but unsure of MOA
Induce apoptosis, is ant-angiogenic and causes cell cycle arrest