Breast Cancer PY367

What are the risk factors for breast cancer?

• 60 years old

• Oestrogen exposure: HRT, oral contraception, early/late menopause

• 23% Linked to lifestyle factors like:

  • Overweight

  • Alcohol

  • Certain occupational exposures

• Family history

  • Inheritance of mutated BRCA gene (5% of cases)

• Breastfeeding and physical activity protect against breast cancer

What is the presentation of BC?

• Lump in breast

• Change in shape or size

• Dimpling of skin/thickening of tissue

• Inversion of nipple

• Rash on nipple

• Discharge from nipple

• Swelling or lump in armpit

What 2 symptoms are associated with BC?

• Abnormalities of overlying skin

• Unilateral bloody nipple discharge

How is it screened?

• Breast self-exam

  • Has high false positive and negative rates

  • Not decreased mortality

• Mammography

  • UK screening programme screens all women aged 50-70 every 3 years

  • Detection of early BC reducing mortality by 20-30%

Treatment Options

• Endocrine therapy

• Radiotherapy

• Chemotherapy

• MAbs and other targeted treatments

Patient pathway from appointment onwards

1. GP app. for suspicious lump

2. Physical exam, Radio, core biopsy, FBC, LFT, bone profile

   • Suspicion of metastasis leads to bone scan and chest abdo pelvis (CAP) CT scan

3. Immunohistochemistry - ER/PR/HER2

   • FISH (fluorescence in-situ hybridisation) part of HER2 testing using DNA primers linked to fluorescent marker

   • Detecting antigens in cells of tissue exploiting principle of Ab binding to them

     • Intensity of staining scores expression levels

4. BC MDM

5. Surgery

6. MDM post-surgery pathology - gene testing of tumour

7. Chemotherapy

What is TNM staging?

• Tumour size

• Lymph nodes

• Metastasis

• Used for all solid tumour growth and most common for BC

Explain T

• T1 = under 2cm

• T2 = 2-5

• T3 = over 5

• T4 = extension to chest wall or skin

Explain N

• N1 = Mobile ipsilateral lymph nodes

• N2 = fixed to one another or other structures

• N3 = Intraclavicular or ipsilateral internal mammary and axillary nodes

Explain M

• M0 = No distant metastases

• M1 = Contralateral lymph nodes or any distant metastases

• MX = Distant that cannot be assessed

Explain the numerical staging system

What is the grading pathology?

• Grade 1

  • Well differentiated/low grade

  • Cancer cells similar to normal cells and grow slowly

• Grade 2

  • Moderately differentiated

  • Look abnormal and slightly faster growing

• Grade 3

  • Poorly differentiated

  • Look very different and grow fast

Pathological Classification

• Ductal 70-80%

  • Worst and most invasive

  • Milk ducts

• Lobular 5-10%

  • Bilateral (lobules = glands that make the milk in outer breast)

  • Lesions in same area

• Tubular 10-20%

• Medullary 5-10%

  • Good prognosis because it grows slower than ductal and doesn’t usualy spread outside of breasts (i.e. lymph nodes) therefore easier to treat

  • Invasive - starts in milk ducts and then spreads into surrounding tissue

• Mucinous/colloid 1-2%

• Other 1-2%

• Inflamm 2%

  • 50% survival at 5 yrs due to blockage of lymphatic drainage = reason why we see peau d’orange

Immunohistochemistry

• ER and PR - Hormone dependent tumour

  • Favourable diagnosis

  • More likely to respond to treatments e.g. Tamoxifen

• HER2 positive - Transmembrane tyrosine kinase which regulates growth, survival and migration

  • More aggressive and less favourable

  • Responds to Trastuzamab

What is hormone receptor negative?

• No target to inhibit tumour growth (10-15%)

• Triple negative BC and more common in younger women, afro, BRCA mutation

• HER2+ tumours dependent on female hormones for growth and survival

What is luminal A?

• One of most common

• Start in the inner cells lining mammary ducts

• Tend to be ER+ and/or PR+ and HER2 -ve and tumour grade 1/2

• Less than 15% have p53 mutations = best prognosis and high survival rates

• ER+, treatement usually involves hormonal therapy

Explain Luminal B

• ER+ and/or PR+

• Tend to have mitotically active cells, positive for Ki67

• Often HER2+

• Diagnosed at younger age than luminal A

• Poorer prognosis with poorer TNM

Triple negative/basal like

• ER-, PR- HER2-

• Subsects like basal-like

  • Tumour have cells with similar features to basal cells

• Most BCA1 BC are both triple negative and basal like

• Younger women, Afro-american women and BRCA mutation

• Often aggressive and have poorer prognosis

• Treated with combo of surgery, radiation and chemo

HER2 Type

• Not same as HER2+ and not used to guide treatment

• These tend to be ER and PR negative with lymph node involvement and poor tumour grade

• Fairly poor prognosis and prone to early and frequent reoccurrence and metastases

• People tend to be diagnosed at a younger age than those with luminal

• HER2 tumours can be treated with Trastuzumab

What is claudin low

• Often - - - but distinct in there is low expression of cell-cell junction proteins including E-cadherin and there is frequent infiltration of lymphocytes

• Also enriched in mesenchymal and stem cell features

Explain Normal like

Usually small and tend to have good prognosis

How to lower risk of reoccurrence after surgery?

Adjuvant Chemo

Name a molecular assay and how is it used?

• Oncotype DX

• Real time PCR on 21 genes causing tumour proliferation

• Validated reoccurrence score indicating 10 year risk

• Validated prediction as to wether patient will have additional benefit from chemo compared to Tamoxifen alone

• Usually luminal A pts.

What is neo adjuvant chemo?

• Starts before surgery to shrink tumour and improve outcomes

• Used in:

  • Locally advanced tumours

  • Inflam tumours

  • Preserve tissue and facilitate less invasive surgery

What is used in adjuvant chemo HER2+ve?

• Cyclophosphamide

• Anthracycline

• Taxane

What are Cyclophosphamides?

• Alkylating agent - crosslinks DNA strands and inhibit DNA synthesis, transcription and replication

  • Alkyl groups to guanine base

  • Interferes with DNA replication by forming intrastrand and interstrand DNA crosslinks

• Non-cell cycle specific so works regardless of what part the cycle is in

• Pro-drug - activated in liver

What are anthracyclines?

• Create free radicals = Ox damage

• Intercalates between base pairs in DNA

• Inhibit action of topoisomerase 2 by stabilising DNA-topoisomerase 2 complex and preventing re-ligation of double helix

• Non-cell cycle specific

• Can cause red urine

What are Taxanes?

• Enhance polymerisation of tubulin

• Stabilise microtubule polymer preventing disassembly of mitotic spindle

• Blocks progression of mitosis resulting in apoptosis

• M phase of cell cycle

  • Stops spindle from separating into two cells because cell is frozen in M phase = cell death

How is combo chemo used?

• Drugs act at different cell cycle stages to maximised cytotoxic effect

• FEC-T regime

  • 5-Fluorouracil

  • Epirubicin

  • Cyclophosphamide

  • Docetaxel

    • 5-F could be removed without affecting efficacy due to new evidence but not adopted in practise yet

MOA of Trastuzumab

• Tags cell to IS

• Blocks receptor dimerisation with other receptors

• Stops signalling downstream into cell preventing proliferation

Issue with Trastuzumab?

• Too expensive (25k) per pt.

• Available as Biosimilar now though

  • Saving millions and opening up access

What is used with Trastuzumab?

• Pertuzumab and trastuzumab combination provides a more comprehensive block

• When used together they can block both sides of the HER2 receptor effectively to prevent dimerization as opposed to Herceptin alone

What can we learn about studying cell cycle control?

• Development

• Stem cells

• Differentiation - occurs whether cells get the right cell

• Cloning and processes of differentiation for cloning tissues or whole organisms

• Division capacity - after division for a certain amount of times, cells go into senescence

• Senescence

• Oxidative damage effects

• Opportunities for therapeutic intervention

  • Stopping cell division that leads to carcinogenesis

3 ways of studying cell cycle

1. In whole organisms e.g. yeast

2. In cell-free extracts e.g. frog eggs

3. In cell culture e.g. mammal cells

Phases of the cell cycle

• G1 - 1st gap phase

  • Cells increase in size

  • Production of RNA and protein

  • Damage check

• S - synthesis phase

  • Takes 6-8 hours

  • All DNA replicated

  • Replication bubbles and forks in different directions

• G2 - 2nd gap phase

  • 2-6 hours

  • Cell growth - protein production and sorting of organelles

  • Check for replication errors

• M - Mitosis phase

  • Shortest phase

  • Compartmentalisation

  • Chromosomes are partitioned into two daughter cells

Why do we use yeast models to look in between phases?

• Rapid production

• Genome size

• Amenable to genetic manipulation so we can easily change to track part of cells

  • Gene deletions, replacement or alteration

  • Can use fluorescence to track better

• Can proliferate in haploid state

• Can make temp sensitive mutants which stop functioning at certain temperatures

  • Helps understand cyclin dependent kinases and cyclins

  • Pauses the replication cycle at a certain stage so we can see what involved within that stage (e.g. pause in G1 and then see what is involved in that stage)

3 ways of identifying cell cycle stages

1. Radiolabelled nucleotides and X-ray photography

   1. Cells take these up at replication so they’re more easily visible to see stages

2. Artificial analogues and Ab staining

   1. Modded by adding fluorescent ab which allows tracking of cell division

3. Flow cytometry

   1. Stain all DNA with fluorescence indicating the phase

   2. Cells in G1 phase contain half the DNA of cells after G2 and M

   3. Cells in S phase contain intermediate quantity

What 2 major points are cell cycle events controlled at?

1. Entry point to cell cycle

2. Critical checkpoints

What choices does a cell have when its at division?

• Divide

• Not divide

• Apoptosis

• Quiescence - Cell steps out and goes to G0 phase and has ability to return to cell cycle when environment is right

• Senescence

Entry into cell cycle due to growth factor availability

• Restriction point at late G1 due to growth factor availability

• If no growth factors, cell enter quiescent stage G0

• If cell passes restriction point = division irreversible

• Cells normally have limited proliferation capacity = enter G0 permanently called senescence

• If cycles are disturbed or damaged they will not enter replicative senescence = IMMORTALITY

What happens at each checkpoint?

1. G1 detects presence of damaged DNA and leads to cell cycle arrest

   1. Checks if DNA intact and nothing wrong

   2. Cascades activated if damage to CKIs, tumour suppressor proteins

2. G2 checkpoint arrests cells in response to damaged or unreplicated DNA

   1. Depending on extent of damage, cell goes into senescence or apoptise or repairs

   2. Ensures all DNA replicated, if parts missing = improper proliferation

3. M phase checkpoint arrests mitosis if the daughter chromosomes are not properly aligned on mitotic spindle

   1. Misaligned = pause cycle

   2. If they cannot realign, cell does not continue - important in cancer targeting

What is cell cycle progression determined by?

• Cyclin dependent kinases

• These are protein kinases whose activity will rise and fall during cell cycle

• Phosphorylation of intracellular proteins/enzymes initiate or regulate major events of cell cycle

  • This going wrong causes issues

• CDKs partially activated by cyclins and regulated by multiple processes

What happens to cyclin levels over time?

• Increase during interphase and decrease during mitosis

• Accumulate during interphase and rapidly degrade towards end of mitosis

What is the process of removal of cyclins?

• After phosphorylation, cyclins are degraded by ubiquitin/protease system

• Ubiquitin tags cyclins after phosphorylation event

Why is the availability of them key to cell cycle control?

• Many different types of cyclins with different roles

• Controls activity of CDKs and promotes cell progression

• When CDKs are complexed with appropriate M-phase cyclin, mitosis machinery is triggered

• When CDKs are complexed with appropriate S-phase cyclin, trigger DNA replication

Cyclin sub-groups and CDKs

• Cyclin A = CDK 1/2

• B = 1

• D = 4/6

• E = 2

• Precise timing of each step is essential and many cycle proteins are degraded after they have carried out their functions

How is activity of these complexes negatively modulated at either of the checkpoints?

In G0, Cyc D is present at low concentration and Rb protein is hypophosphorylated restraining cell at checkpoint 1 by inhibition of several proteins which are critical for cycle progression

What is the Rb protein and what does it do?

• Tumour suppressor protein

• Binds transcription factors and prevents them from promoting expression of genes required for DNA replication during S-phase

Each molecule present during different phases of cell stages

CDK1 only one required to drive through cell stages

Cyclin B1 and A2 essential for normal cell cycle

Phase	CDK	Cyclin	Inhibitor
G1	CDK 4, 6	Cyclin D	P16, P21, P27 & P15
G1/S	CDK 2	Cyclin E	P21, P27
S	CDK 1, 2	Cyclin A	P21, P27
M/G2	CDK 1	Cyclin B	(none)

What is the mechanism of CDK regulation?

1. Association of CDK with its cyclin

2. Phosphorylation events

   1. Phosphate attached to threonine amino acid

   2. Causes repellent force on amino acid causing conformational change in their structure allowing for inhibition (thr14+tyr15) or activation (thr160)

3. Association with CKI stops action of CDK/CYC complex

Positive CDK regulation by phosphorylation

• Without cyclin, CDK remains inactive therefore by moving T-loop, active site opens allowing cyclin to bind

• Once CAK phosphorylates Thr160, T-loop is fully removed which opens active site

Negative CDK regulation by phosphorylation

• If complex is made early in cell, needs to remain inactive till required

• WEE1 provides inhibitory phosphorylation

• CDC25 dephosphorylates afterwards to allow for cell progression

• Processes determine wether WEE1 or CDC25 is up or down regulated

How do CKIs regulate CDKs and two family types?

• They bind to complex and distort active side by inserting into ATP binding site

1. Ink4 family - P15, 16, 18, 19 Inhibit CDK4/6 = entry into cell

2. Cip/Kip family - P21, 27, 57 Inhibit CDK1/2 = whole cycle

• P21 and 16 dominant inhibitors of cell proliferation in senescent cells

Control of cell cycle entry

• Growth factors regulate cycle progression through G1 restriction point

• Cyclin D via MAPK signal path

  • IF NOT CONTROLLED = UNCONTROLLED CELL DIVISION = CANCER

• CDK4/6 with cyclin D drive the passage through restriction point

• Receptor protein key area dysregulated in cancer

What is the cell signalling pathway?

1. Signal binds to receptor on RTK

2. Tyrosine kinase dimerise and autophosphorylate forming hyperphosphorylated tyrosine kinase complex

3. Adapter protein activated signalling and activating Ras activating protein

4. This happens indirectly from conversion of GDP to GTP

5. Activated Ras phosphorylates other molecules and starts intracellular cascade of signals

What does Ras now do?

• Feeds into intracellular signal pathways e.g MAPK

  • Acting as phosphorylation cascade

  • Eventually alters activity of target proteins causing gene expression changes

  • Rafs → MEKs → ERKs

How does this initiate cell division?

• MAPK cascade leads to transcription factors expression

  • Myc promotes transcription of cyc D → activates CDK4/6

  • G1 complex made and activated due to initial growth factor

What does the CDK4/6 complex do now regarding the Rb protein?

1. Rb usually binds and inactivates E2F proteins and is responsible for G1 restriction point

2. When cell is ready to divide at G1 restriction point, G1-CDK complex phosphorylates pRb-E2F complex to free E2F to act as transcription factor

   1. CDK2/cycE complex also hyperphosphorylates pRb in G1

3. E2F acts on S-phase gene transcription making cyc E and A leading to cell cycle entry and DNA synthesis as cell can now go from G1 to S-phase

• pRb protein now hyperphosphorylated and therefore inactivated

How does positive feedback loop ensure process continues when required?

• Hyperphosphorylation of pRb is required to free E2F because it is a key transcription factor

• Blocking CDK is not enough to block cell cycle as pRb can be phosphorylated by other pathways

• E2F activates genes like PCNA and cyclins

Why is there DNA replication once per cycle?

• Do not want extra parts which have more copies than others

  • Cannot get re-replication

  • Cannot start more cycles whilst one is running

  • Cycles can only go in linear forward direction

What do cancer chemo agents do?

• Stop division by blocking DNA replication/damage DNA/block mitosis

• Causes growth arrest by triggering cell cycle checkpoints with damage and errors

• They also act via growth factor signal inhibition and reduce cell entry e.g. Herceptin

7 chemo agents and their actions

1. Methotrexate - Prevent folate synthesis for purine and pyramidines

2. Etoposide - Topoiso 2 inhibitors prevents religation of DNA

3. Irinotecan - As above for Topoiso 1

4. Bleomycin - Strand scission

5. Doxorubicin - Intercalator which prevents Topoiso 2 action and alters membrane fluidity

6. Emtansine - Trastuzumab helps enter cell and broken down to release mertanisine

7. Paclitaxel - Stabilise microtubules so no disassembly for elsewhere use

What is the human epidermal growth factors receptor?

• Works same way as cell signal pathway above

• When growth factor stimulates HER2 ligands, TKase domains dimerise and hyperphosphorylate which trigger same Ras/Raf/Mek/Erk early response pathway to drive cell cycle

What is the alternate P13k pathway?

• Goes through AKT which is linked to apoptosis inhibition and mTOR which is linked to protein biosynthesis

• Also blocks CKIs

How does herceptin work?

1. Many HER2 receptors proliferate causing too much cell growth

2. Herceptin blocks these to stop signal

Cyclin timing

1. Decision to replicate requires E2F initiation by growth signals

2. These stimulate CDK activity by antagonising CKI and expressing cyc D

3. Rb inactivates E2F

4. Commitment to replication requires cyc A

5. Metaphase decision - replication stress checkpoint

What happens at G1 /DNA damage checkpoint?

• G1 checkpoint mediated by P53

• P53 levels increase in response to levels of DNA damage

• Cancers occur to P53 mutations as they bypass checkpoint

What does DNA damage lead to an increase of?

• Increase of P53 which increases P21 levels

• This blocks machinery driving restriction point

What is P21?

• Effector

• Acts on G1, G1/S and S-phase CDKs

• Can be activated independently (without P53)

• Can change regulation cyc D activity

What are the roles of P53?

• Causes transcription of P21 which is also a CDK inhibitor

• CDK inhibition via P21 prevents cell cycle progression

• P21 can also bind and inhibit PCNA

  • PCNA is a component of DNA replication machine that prevents it

• It can also stop cell division to allow for DNA repair

  • If irreparable, it initiates signalling pathway which causes cell apoptosis

Apoptosis process

• P53 transcriptionally activate pro-apoptotic genes

  • Bax

  • Fas

• Bcl-2 promotes survival by inhibiting apoptosis factors but is downregulated when a cell needs to be apoptosed

  • This can go wrong in cancer

    • Bcl-2 upregulated and Bax downregulated

What are the 2 routes of cell death?

1. Necrosis

   1. Cells spill contents into surrounding tissue and evoke an inflamm response

2. Apoptosis

   1. DNA is systemically fragmented

   2. Cell contents are packed into membrane vesicles and phagocytosed by adjacent cells

How is G2 checkpoint controlled?

• Mediated by chk1 kinase as well as P53

• Chk1 inactivates cdc25 in response to unreplicated or damaged DNA

• Without cdc25, CDK1/cycB will remain inactive and cells will arrest in G2

• Some cancers end up over expressing

What is cdc25?

• Phosphatase which dephosphorylates CDK1 in order to activate it

• When DNA damage is repaired, inhibitory signal is turned off and cell cycle progression continues

How does cdc25 bypass G2?

Active cdc25 needs to dephosphorylate inhibitory phosphates on CDK1/cycB complex

What happens in S phase?

• Prophase separates all the replicated material out

• Disassembly of material and condensing of chromosome's

• Start to see chromosomal material and get kinetochores and spindles aligning at the different poles of the cell

• Microtubules come out and catch onto chromosomes at the centromere

• Alignments at the checkpoint and then the cell goes through anaphase very quickly and reforms the envelopes and cytokinesis

How are levels of MPF/M-phase CDK complex controlled?

• It is made much earlier and is activated by CAK160 but also deactivated by WEE1

• Passing G2 starts cascade - Phosphorylating cdc25 phosphatase

• cdc25 Dephosphorylates inhibitory phosphate groups on CDK1 making it active and traversing into M-phase

• Positive feedback ensures full movement into M phase

• Complex being active triggers degradation of cyclin

• G2: Chk1 mediated phosphorylation inactivates cdc25

• M-phase: CDK1 mediated phosphorylation activates cdc25

What happens if chromosome s aren’t attached during M phase?

• Unattached kinetochores inhibit APC/C and therefore inhibit anaphase initiation

  • If chromosome is only attached at one side

• This activates and recruits MAD2

• RIT1 is a Ras-related GTPase which interacts with MAD2 to inhibit APC/C

• CDK1 inhibits RIT1/MAD2 complex

  • Allows APC/C to initiate anaphase

• Spindle tension is also detected by cell and if wrong, attachments are destabilised to try again

How is chromatid separation controlled?

• Cohesion complex binds sister chromatids together

• Cohesions need to be cleaved by separase before the chromatids separate in anaphase

• Separase kept inactive by securin until degraded by proteolysis via APC/C

• When cell is ready to proceed, M-CDK phosphorylates APC/C, facilitating cdc20 binding which activates complex

What happens when cdc20 binds APC

• Complex releases inactive separase from securin by ubiquitinating securin

• Once active, separase can now break down cohesins to allow movement from metaphase to anaphase

• CDC20 triggers cyc B destruction

What 3 things happen when cell cycle control is lost?

1. Alterations in cell proliferation

2. Alterations in DNA damage response

3. Alterations in cell growth

2 things that lead to loss of control

1. p53 no longer acting as caretaker - no blocking of cell cycle

2. Rb can be permanently hyperphosphorylated and will release E2F to drive proliferation

What 2 genes are traditionally distinguished?

1. Oncogenes

2. Tumour suppressors

Explain Oncogenes

• Overactive form of normal cellular genes

  • Example of proto-oncogene: Ras turns into an abnormal form that is permanently switched on so cell thinks it is permanently stimulated by growth factors

• Can also enter cell as part of virus e.g. HPV - inserts oncogene into cells which causes predisposition to mutation

• Oncogenes more associated with spontaneous, somatic cancers

  • Myc, Ras, C-Fos

Explain Tumour suppressors

• Genes that usually inhibit cell proliferation and tumour development

• In tumours, these are lost or inactivated

• Requires two mutational events

What does oncogene being dominant mean?

Mutation only needs to occur in one of the chromosomes

What are 3 ways proto-oncogene can be made overactive = Oncogene?

1. Mutation in coding sequence - deletion or point mutation

2. Gene amplification- overproduce

3. Chromosome rearrangement

Cancer cells display abnormalities in chromosome structure in……

• Translocations

• Duplications

• Deletions

What is the philadelphia chromosome (translocation)?

• Reciprocal chromosomal translocation event

• Found in most chronic myeloid leukaemia pts.

• ABL1 gene from chr.9 and BCR gene from chr.22

  • ABL codes for TKase

  • BCR responsible for neutrophil function

• Codes hybrid constitutively active TKase which results in uncontrolled division and genome instability through various signalling pathways

What are the function of proto-oncogenes?

• Growth factors

• Growth factor receptors

• Elements of intracellular signalling pathways

  • Regulatory GTPases e.g. Ras

  • Cytoplasmic kinases e.g. Raf, CDKs

  • Anti-apoptosis factors e.g. Bcl-2

• Transcription factors e.g. Myc

G:C to T:A mutational hotspots

• Ras has a G to T transversion in codon 12 causing impaired GTPase function leaving Ras constantly on

• Causes cell division irrespective of growth factor signalling

Why is it harder to lose tumour suppressor genes?

• Recessive mutation

• Associated with inherited cancers as pts. are born with one mutation and second is lost during lifetime

• Because of this proliferation of cells always switched on

3 types of tumour suppressor genes

1. Gatekeepers: Monitor cell division and induce apoptosis e.g. Rb, CKI’s, apoptosis genes

2. Caretakers: Promote genome stability and oppose mutation rates via checkpoints e.g. P53, BRCA, MMR

3. Landscapers: Control cellular microenvironment e.g. cadherins, integrins

   1. Prevents metastasis of cancer cells by keeping them connected with their environment

   2. When mutated, contribute to neoplastic growth

Caretakers and the mutator phenotype hypothesis

• Mutation occurs and inactivates DNA repair gene so the mutation can have further mutations

• This eventually leads to proto-oncogene → oncogene

• CANCER

What are BRCA 1 and 2?

Both caretaker tumour suppressor genes

What are BRCA mutations?

• BRCA gene is tumour suppressor which creates protein to fix double stranded breaks in DNA from replication stress

• Repairing them means cycle continues

• BRCA mutations mean that the protein repairing DNA changes shape and is non-functional

Accurate Repair

• Main pathway with best outcome

• Uses homologous DNA template to fix DSB with exact nucleotides

Deletion pathways

• Other pathways which aren’t as favourable but you can still repair if you’ve lost BRCA 2

• Deletes large parts leading to chromosomal rearrangement but temporarily fixes DSB

Error prone repair

• Loss of BRCA 1 and 2

• Try join ends of DNA with what it can, uses incorrect nucleotides which can cause mutations

What are cytoskeletons?

• Immediate filaments inside cells

• Without which cell wouldn’t have shape and organisation or motility, contraction, tensile strength