Neoplasia lecture 7 Dr Heyam Awad FRCPath

Continuation of insensitivity to growth inhibitors

TGF beta pathway TGF beta is a potent inhibitor of cell proliferation TGF beta binds to receptors.. Receptors activated .. Transmit signal through SMAD proteins transcription activation of CDKIS. This results in Growth suppression It also causes Repression of MYC CDK2, CDK4 that encode cyclin A and E

Mutations affecting TGF beta signaling causes cancer These mutations involve TGF beta receptor or SMAD molecules that transduce antiproliferative signals from the receptor to the nucleus Mutation affecting type 2 receptor seen in colon, stomach and endometrial cancer SMAD4 mutated in pancreatic cancer

In some cancers mutations in TGF B happen downstream of signaling pathway. These mutations change other properties of TGF beta Causes Immune system suppression or promotion of angiogenesis so: facilitate tumor progression so Can prevent or promote tumor growth????

100% of pancreatic 83% of colon at least one component of is TGF b pathway is mutated

100% of pancreatic 83% of colon at least one component of is TGF b pathway is mutated

Contact inhibition Normally cells proliferate in organized fashion. Monolayers are formed and contact between adjacent cells inhibits further growth. This process is called contact inhibition. In cancer cells: contact inhibition is lost so cells pile upon each other.

Contact inhibition

Contact inhibition is mediated by cadherin molecules. E cadherin’s function is facilitated by NF2 protein If E cadherin is lost: no contact inhibition….. Cells proliferate in uncontrolled fashion.

NF2 gene’s protein product is neurofibromin 2=merlin facilitates contact inhibition Homozygotic loss of NF2 causes neural tumors ( neurofibromatosis syndrome)

APC ( adenomatous polyposis coli) gene APC gene is tumor suppressor gene Suppresses growth by regulating intracellular beta catenin levels Functions of beta catenin: binds and inhibits E cadherin to nucleus.. Activate cell proliferation

SO: Beta catenin stimulates growth by two ways: Inhibits contact inhibition by binding to E cadherin and stimulating TWIST and SLUG transcription regulators that decrease cadherin expression Stimulates growth by increasing transcription of growth promoting genes like cyclin D1 and MYC

APC suppresses growth by being part of a complex that destructs the beta catenin.

Beta catenin is an important component of WNT signaling WNT is a soluble factor that induces cell proliferation by binding to a receptor and transmit signals that prevent degradation of beta catenin Now beta catenin to nucleus .. Transcription activator in conjunction with another molecule TcF

Quiescent cells not exposed to WNT Quiescent cells not exposed to WNT .. Cytoplasmic b catenin is degraded by destruction complex ( of which APC is a main component) APC is an important component of these destruction complexes Loss of APC .. B catenin not degraded and WNT pathway activated without the WNT This leads to transcription of growth promoting genes cyclin D1 ,MYC and transcription regulators: TWIST AND SLUG that repress e cadherin and thus reduce contact inhibition

WNT pathway

APC adenomatous polyposis coli APC syndrome: one allel lost in germ line.. Patients have polyps (adenomatous polyps= adenoma).. Hundreds of adenomas 100% risk of malignancy Some polyps acquire another hit.. Malignant 70-80% of sporadic colon cancers have APC mutation Colonic cancers with normal APC have mutated b catenin making them undegradable by. APC

FAP syndrome: colon full of adenomas!

Third hallmark: limitless replicative potential Normal cells: limited capacity to duplicate ( usually 60 -70 doublings) After these doublings cells lose capacity to replicate and become senescent This is because of progressive shortening of telomeres

Cells avoid senescence by activating telomerase. Telomere length is maintained in all cancer cells.. Mainly by upregulation of telomerase but also by other mechanisms like DNA recombinations

If cells have short telomere and no telomerase… then shortened telomeres fuse and cells divide causing more DNA breaks ( this happens of course if the cell cycle checkpoints are disabled) This bridging, fusion, breakage cycle continues and ends in mitotic catastrophe unless the cell acquires telomerase activation

The presence of a time window during which cells have short telomeres, disabled checkpoints and low telomerase activity is beneficial to tumor cells because it causes genetic instability that allows the accumulation of mutations. This happens during progression of colonic adenomas to colonic carcinomas.

Adenoma- carcinoma sequence in colon cancer… note that telomerase are acquired late in the process