Final Exam: Pathology: Tales From The Crypt

Front 1

What was found in the biopsy of a 27 year old male with colon cancer?

Back 1

- Hundreds of colonic polyps
- Tubular Adenoma
- Dysplasia
- Adenocarcinoma

Front 2

How would you characterize the crypts of the colon?

Back 2

Microscopic appearance of normal colonic crypts - straight tubular structures with no lateral branching and a structured pattern of epithelial growth, differentiation and death.

Front 3

How would you describe the TUBULAR ADENOMA?

Back 3

Tubular adenoma - benign colon polyp - one of thousands of polyps from this patient

This polyp is similar to the sporadic polyps we see every day in patients without FAP.

Front 4

How would you describe the DYSPLASIA?

Back 4

The cytologic changes associated with progressive mutations in colonic cells - dysplasia
The cells are crowded and there is some loss of cell polarity with nuclei present at the cell surface. There is loss of cytoplasmic differentiation (eg, reduced mucin production)
The nuclei are enlarged and the chromatin is dense and irregularly distributed
An abnormal mitotic figure is present.

Front 5

What are the cellular characteristics of ADENOCARCINOMA?

Back 5

One of several adenocarcinomas (the largest) identified in the colon. It is invasive and has 2 patterns of differentiation.

To the left a tubular glandular pattern, to the right a solid pattern that by additional studies shoed an endocrine pattern of differentiation.

Both of these patterns mimic normal enterocyte phenotypes found in the colon.

Front 6

What is the pathological diagnosis for FAMILIAL ADENOMATOUS POLYPOSIS SYNDROME (FAPS)?

Back 6

A. Adenocarcinoma of colon with mixed glandular and neuroendocrine differentiation

B. Multifocal adenocarcinoma

C. Tubular adenomas (> 500)

Front 7

What is the difference between HEREDITARY colon cancer and NON-HEREDITARY colon cancer?

Back 7

INHERITED colon cancer is an autosomal dominant disease affecting an individual in the 3rd or 4th decade of life.

Front 8

What are the causes of colon cancer?

Back 8

According to the pie chart, most of the time, it is SPORADIC COLORECTAL CANCER, followed by FAMILIAL COLORECTAL CANCER, and then HEREDITARY COLORECTAL CANCER SYNDROMES, mutations in FAP and HNPCC

Front 9

What are 2 major variants of HEREDITARY COLON CANCER?

Back 9

1) FAP (familial adenomatous polyposis)
2) HNPCC (hereditary non-polyposis colon cancer)

Front 10

What is the difference between FAP and HNPCC?

Back 10

1) FAP has an ACCELERATED tumor initiation process, and NORMAL tumor progression, whereas

2) HNPCC has NORMAL tumor initiation and ACCELERATED tumor progression

Front 11

What are the steps in the MULTISTEP PATHWAY OF COLONIC CARCINOGENESIS?

Back 11

* (See slides!)

Front 12

What mutations do you find in FAP and HNPCC?

Back 12

1) FAP has germline mutations in APC (Adenomatous Polyposis Coli)

2) HNPCC has germiline mutations in mismatch repair genes

Front 13

What is APC?

Back 13

It is a molecule with a variety of gatekeeper roles in colonic epithelium.

Front 14

What is the Wnt PATHWAY and what does APC have to do with it?

Back 14

The Wnt signaling pathway is a critical component in the dynamicity of the crypt.

Front 15

What happens in the absence of Wnt ligand?

Back 15

In the absence of Wnt ligand, b-catenin is sequestered in a multiprotein degradation complex containing the scaffold protein Axin, the tumour suppressor gene product APC, as well as the kinases CKI and GSK3!, among others.

Upon sequential phosphorylation, b-catenin is ubiquitinated by the !-TrCP–E3-ligase complex and subsequently degraded by the proteasome machinery. There is no transcription of Wnt target genes.

Front 16

What does Wnt ligand associate with, and what are the consequences of it binding?

Back 16

Wnt ligand associates with Fz and LRP5/6 co-receptors. This in turn can lead to translocation of Axin (and perhaps the whole multiprotein complex) to the plasma membrane through direct interaction with LRP5/6 and Dsh/Fz.

Translocation results in Axin degradation and/or dissociation of the multiprotein complex. GSK3b also might be displaced from this complex through Dsh action. b-catenin is then released from the multiprotein complex, accumulates in the cytoplasm in a non-phosphorylated form, and subsequently translocates into the nucleus where by association with TCF/LEF factors it promotes transcription of Wnt target genes.”

*NOTE: Normal APC regulates transcription and cell proliferation through its regulation of beta-catenin.

Front 17

What 3 things does APC regulate?

Back 17

Regulates:
1 - The amount of free 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 - Not illustrated - it plays a regulatory role in certain apotosis pathways.

Front 18

What is the role of B- catenin in the cell cycle?

Back 18

Beta catenin also acts at progression of G1 to S in cell cycle

Note that the absence of beta catenin in this process ultimately favors apoptosis

Front 19

In non proliferating cells, where can you find B- catenin?

So what is another major role of B- catenin?

Back 19

It's found mostly located in the Adherens cell junctions

Another major role of beta catenin - anchoring cadherin to actin in cell junctions - these junctions securely attach adjoining cells and also play a role in anchoring the cell to the basement membrane. They are also a key differentiation component of colonocytes (all epithelium)

Front 20

What is the role of B- catenin in normal cells?

Back 20

Normal colon mucosa - membranous localization of beta catenin - the vast majority of beta catenin is part of cell junctions

Front 21

What is the difference between PROLIFERATIVE and NONPROLIFERATIVE enterocytes?

Back 21

*

Front 22

What happens in a mutated APC gene?

What about in patients with FAP?

Back 22

The mutated APC gene is truncated and results in the formation of a protein that cannot bind beta catenin thereby allowing free beta catenin to accumulate in cells.
There is also evidence that the mutated APC cannot bind microtubules as well the normal form thereby effecting cytoskeletal and mitotic spindle cell functions
In FAP patients, 1 allele shows a germ line truncation mutation.
In patients with sporadic polyps, a single acquired mutation in the APC gene occurs.

Front 23

Essentially, what does APC do?

Back 23

The mutated APC gene is truncated and results in the formation of a protein that cannot bind beta catenin thereby allowing free beta catenin to accumulate in cells.

Front 24

What does APC do under physiologic conditions?

Back 24

It has a key role in crypt structure and growth.

Front 25

How is the crypt developed and maintained?

Back 25

the role of APC in crypt development
The crypt is ordered so that cell proliferation occurs at the base of the crypt , cells migrate towards the surface and differentiate, eventually die (apotosis) and shed into the lumen. This process occurs continuously over 3-4 days and the number of crypts/mm of colon stays fairly constant in the mature colon. Spatial relationships and stromal interactions maintain a tight control on this process.

Front 26

What effect does a change in Notch and Wnt have on crypt development?

Back 26

Wnt and Notch expression together may play a role in maintaining the stem cell compartment in the crypt and changes in Notch expression appear to be associated with the differentiating cells in the transit amplifying compartments. Differentiated cells lose Wnt activation. These processes have strict spatial relationships in the crypt.

Front 27

See slide 27

Back 27

*

Front 28

How do crypts develop?

Back 28

Normal human crypts with one pair on the far right almost completing a crypt fission. In adults crypt numbers are held fairly constant although colonic mucosa is constantly regenerating.

Front 29

What did mouse experiments show when you have 1 mutation in 1 allele?

Back 29

Experimental evidence that with a germline APC mutation in 1 allele you get abnormal crypt branching - focus on the upper panel of this diagram. This upper panel reflects the genotypic and phenotypic state of our patient during childhood while the lower panel occurred during adolescence.

Front 30

What happens when you get a mutation in the other allele?

Back 30

an acquired mutation in the second allele causes further architectural abnormality in the experimental animals. This can be induced by giving a carcinogen and it can also occur spontaneously. This also occurs in our patient - hence the increased numbers of polyps.

Front 31

Mutations in what genes lead to tubular adenoma?

Back 31

Tubular adenoma - kRas, Smad4 mutations

Front 32

So how does knowledge of the pathway suggest ways to treat this cancer?

Back 32

chemoprevention is the key
NSAIDS have been shown to decrease and slow the formation of polyps in patients with FAP (and now with normal people). Mechanistically this can be explained through the observation that a ligand for PPAR delta is blocked by NSAIDS thereby inhibiting the cell proliferation effect of free beta catenin and permitting cell death through apotosis.