familial colon cancer - model of carcinogenesis - lecture notes - julia

Question 1

what does normal colon look like histologically?

Answer 1

• straight tubular structures with no lateral branching and a structured pattern of epithelial growth, differentiation and death
• tubular structures lined by cells
• some inflammatory cells around (not abnormal)
• number of crypts should be uniform
• cells turn over every 3-4 days

Question 2

what does tubular adenoma look like histologically?

Answer 2

• benign colon polyp
• lack of organization to crypts

Question 3

what does dysplasia look like histologically?

Answer 3

• cells crowded and there’s some loss of cell polarity with nuceli present at the cell surface
• loss of cytoplasmic differentiation (reduced mucin production)
• nuclei are enlarged and the chromatin is dense and irregularly distributed
• abnormal mitotic figures
• colonic mucosa no longer a single layer - stratified

Question 4

what does adenocarcinoma look like histologically?

Answer 4

• invading into muscularis propria

Question 5

what causes FAP? how does it relate to sporadic colon cancer?

Answer 5

• genetic defect in adenomatous polyposis coli gene - has regulatory roles in colonic cells
• over 80% of sporadic polyps share the genetic abnormality seen in patients with FAP

Question 6

what causes hereditary non-polyposis colonic cancer (HNPCC)?

Answer 6

• familial form of colon cancer
• due to defects in DNA mismatch repair (MMR)
• results in accelerated tumor progression - accelerated growth and increased mutational events

Question 7

what does APC do?

Answer 7

• regulates transcription and cell proliferation through its regulation of beta catenin
• in abscense of Wnt ligand, beta-catenin is sequestered in a multiprotein degradation complex of axin, APC, CKI and GSK3beta
• upon phosphorylation, beta catenin is ubiquitinated by b-TrCP-E3 ligase complex
• B-catenin is degraded and there’s no transcription of Wnt genes
• so APC involved in squestering beta catenin and therefore preventing wnt target genes from being transcribed
• when wnt ligand binds, beta catenin released, activates TCF which results in wnt target genes being activated

Question 8

what does APC regulate? (4)

Answer 8

1. the amount of beta catenin - free beta catenin in the nucleus can initiate cell division
2. APC regulates microtubule assembly and has a role in cytoskeletal maintenance
3. APC regulates AXIN independently of beta-catenin
4. plays regulatory role in certain apoptosis pathways

Question 9

what is the role of beta catenin?

Answer 9

• acts at progression of G1 and S into cell cycle
• abscense of beta catenin in the process ultimately favors apoptosis
• anchors cadherin to actin at adherens cell junctions - attach cells to each other and basement membrane
• promotion of beta catenin promotes adherence and cell to cell connections

Question 10

where is beta catenin located in normal colonic cells? in cancerous or abnormal cells?

Answer 10

• normally has membranous localization
• most part of cell junctions
• in polyp cells, gets more localized to the cytoplasm and nucleus - implies more wnt signaling
• also free in cytoplasm in stem cells

Question 11

how is mutated APC in colon cancer functionally different from normal APC?

Answer 11

• mutated = 1 allele has germ line trunctaion mutation
• trunctated => protein that can’t bind to beta catenin - allows free beta catenin to accumulate in cells - beta catenin can’t bind actin => increased cell proliferation
• may also not be able to bind microtubules => effected cytoskeletal and mitotic spindle cell functions

Question 12

what is the normal process of crypt development?

Answer 12

• crypt ordered so that cell proliferation occurs toward the base of the crypt
• cells migrate toward the surface and differentiate
• eventually die and are shed into the lumen
• occurs continously over 3-4 days

Question 13

what is the role of wnt and notch in development of colonic crypts?

Answer 13

• together may play role in maintaining stem cell compartment
• changes in notch associated with differentiating cells in transit amplifying compartments
• differentiated cells lose wnt activation
• processes have strict spacial relationships in the crypt

Question 14

what are the consequences of mutating APC? (5)

Answer 14

1. altered interactions with microtubles/F-actin => decreased cell migration => inappropriate accumulation of cells
2. altered interactions with beta-catenin => decreased differentiation => maintains stem cell or transit amplifying cell character
3. changes interactions with beta-catenin and microtubules => changes proliferation levels => maintains stem cell or transit amplifying cell character, but faulty and less efficient mitosis
4. changes in interactions with microtubules => decreased genetic stability => chromosomal aberrations
5. changes in interactions with microtubules and beta catenin => decreased apoptosis => inappropriate survival of damaged cells

Question 15

what does normal crypt fission look like histologically?

Answer 15

• crypts normally form through an orderly process of crypt fission
• crypt numbers are normally held fairly constant
• indented region on right = almost completed crypt fission

Question 16

how does mutated APC affect crypt fission? what will it look like histologically?

Answer 16

• increased crypt fission
• altered stem cell dynamics
• abnormal crypt fission
• enhanced unstable stem cell population
• crypts will be incrased in number, longer in length, increased branching
• cells more crowded, especially in the lower parts of the crypt

Question 17

what is going on in this picture?

Answer 17

• top left corner has early adenomatous (dysplastic) crypt = begining of tubular adenoma
• rest has aberrant crypt fission, hyperplastic crypts
• due to both germline and acquired APC mutations

Question 18

what will catenin expression look like in early adenoma? how will this change in late adenoma

Answer 18

• membranous, cytoplasmic and nuclear localization
• illustrates increase in free beta-catenin in the cytoplasm and transcriptional activation by beta catenin in the nucleus
• in late, will only be cytoplasmic and nuclear
• as cells lose junctional integrity, concentration of membranous catenin (junction associated) markedly diminshes
• d= early, f= late

Question 19

how can NSAIDs be used theraputically for patients with colon cancer?

Answer 19

• have been found to prevent initiation of colon cancer/slow progression in pateints wtih FAP (and also with normal people)
• ligand for PPAR delta is blocked by NSAIDS => inhibits cell prolifferation effect of free beta catenin, permits cell death through apoptosis
• get reduction in polyps

Question 20

how are prostaglandins related to colon cancer?

Answer 20

• elevated levels promote colon cancer
• if you give animals a mutation that increases PG levels, you get worse polyp formation
• 15-PGDH gene is often lost in FAP pateints
• 15 PGDH catalyzes NAD+-linked oxidation of 15 (S)-hydroxyl group of prostaglandins and lipoxins and is the key enzyme responsible for the biological inactivation of these eicosanoids.

Question 21

what is celecoxib? how is it used?

Answer 21

• cox-inhibitor - reduces adenoma burden in retained rectum of patients after colectomy
• may reduce diminutive duodenal adenomas
• effects prostaglandin levels - inhibits cox-2, which normally increases PG levels