1. hCDC4 functions as part of an E3 ligase in the ubiquitin-mediated degradation of proteins.
Homologues: FBW7 (H. sapiens); Archipelago (D. melanogaster); SEL-10 (C. elegans); Fbw7 (M. musculus)
First identified and named by Lee Hartwell in budding yeast (S. cerevisiae): hCdc4
The three isoforms are all made by alternative gene splicing of the same gene locus on chromosome 4q32 (humans)
-α (nucleoplasm)
-β (cytoplasm)
-γ (nucleolus)
-all isoforms have F-box motif and WD40 motif in their C-terminus, and these are both highly evolutionarily conserved and used to bind other proteins
-F-box binds to other proteins in the E3 ligase complex
-WD40 repeats are used to recognize and bind to specific phosphorylated residues on the target proteins
hCdc4/FBW7 is the subunit of the E3 ligase complex in the UPS (ubiquitin-proteasome system) used for target specificity.
The F-box motif and WD40 motif regions of hCDC4/FBW7 are integral to its function.
-FBW7/hCDC4 recruits the E2 complex needed and holds the target protein, forcing the two to interact for ubiquitination.
-F box: 50 amino acid motif for protein-protein interactions
-WD40: 40 amino acid motif that terminates with tryptophan (W) and aspartic acid (D) that recognizes substrate proteins that are phosphorylated at multiple serine and threonine sites within the ‘phosphodegron sequence’
-in this way, it determines target specificity through recognition and binding of target proteins for ubiquitination and degradation; the substrates have to be phosphorylated at multiple Serine and Threonine sites within the ‘phosphodegron sequence’
 helps make sure the process of ubiquitin-mediated protein degradation is HIGHLY regulated.
Wang et al. (Dec. 2011)

2. Many substrates of hCDC4/FBW7 are dominant proto-oncogenes.
The top proteins are involved in upstream regulation of FBW7. Those along the bottom are substrates and normal downstream targets of FBW7 and affected by its mutations or disregulation; notice that most FBW7 substrate proteins are dominant proto-oncogenes that are mutated in a multitude of human cancers- ones we have talked about extensively in class like cyclin E and Myc.
Wang et al. (2012)

3. FBW7 knockout mouse models support its tumor suppressor role in vivo: the gene is deleted in 30% of human cancers.
The homozygous mutant mouse developed was embryonic lethal at day 10.5, suggesting that though it was not immediately necessary for cell division and differentiation, it was critical at some point in the cell cycle.
The heterozygous mutant mouse lived and had relatively normal embryonic development, though these mice did have shortened lifespans compared to the wild-type mice and were predisposed to certain tumors.
Conditional knockouts led to different phenotypes and tumorigenesis in related tissues.
They did knockouts in T-cells, bone marrow, intestines, livers, brain, breast, and mouse embryonic fibroblast tissues, all of which resulted in the upregulation of oncogenic proteins and tumorigenesis or death.
Cyclin E  Leads to genomic instability
Myc  Gene amplification through DNA over-replication
Notch  Upregulates cell proliferation; deregulates apoptosis
Chromosome region 4q32 is deleted in ~30% of human cancers
Mutations in 6% of primary human tumors
30%: cholangiocarcinoma (bile duct cancer)
30%: T-cell acute lymphoblastic leukemia
4-15%: pancreatic, gastric, and colon carcinomas
4-15%: prostate and endometrial cancers
Often, this mutation is coupled with mutations in other proteins acting upstream/downstream, like p53 and Ras. Example- FBW7 is a target protein of p53, so if there is a LOF mutation in p53, FBW7 has reduced expression and it ends up upregulating the proto-oncogenic targets anyway.
Wang et al. (2012)

4. Endometrial adenocarcinomas
Endometrial adenocarcinoma is a cancer of the epithelial cells of the endometrium, which is the inner tissue lining the uterus. In most cases in these types of tumors where hCDC4 was mutated, there was a single base pair insertion of an adenine residue that introduced a premature stop codon (fig b normal vs tumor), resulting in a truncated gene product (shown above, fig. c). In most studied cases, the deleted portion coded for the WD40 motif, so it impaired the protein’s ability to recognize protein targets and degrade them as part of the ubiquitin ligase system.
Specific to endometrial cancer, this mutation impacted its ability to recognize cyclin E and target it for destruction in early S phase. Because the F-box protein part of the E3 ligase could not recognize the phosphorylated cyclin E, it resulted in deregulated cyclin E-associated kinase activity that led to tumorigenesis (this was not necessarily the result of the accumulated cyclin E protein).
Spruck et al. (2002)

5. T-cell Acute Lymphoblastic Leukemia
Conditional depletion of Fbw7 in mouse bone marrow, along with the conferred inhibition of p53 function, led to the development of T-cell acute lymphoblastic leukemia (T-ALL).
T-ALL is a cancer of the blood and bone marrow. The FBW7 mutations resulted in Notch pathway activation through the inhibition of Notch degradation. The FBW7-/- leukemic cells both had high expression and accumulation of Notch 1 and c-Myc proteins, both of which possess strong oncogenic activity and are substrates of hCdc4/FBW7.
Whereas in normal cells, hematopoetic stem cells become quiescent, cells with mutated FBW7 will cause the stem cells to keep dividing. With WT p53 activity there will be a program triggered to induce either apoptosis or senescence; however; these mutations are usually coupled with loss of p53 activity, so the stem cells keep expanding continuously and T-ALL is developed.
Wang et al. (Dec. 2011)

6. Summary: hCDC4/FBW7 acts as a tumor suppressor and is mutated / deleted in many human cancers.
Substrate proteins become oncogenes
LOF in
hCDC4/FBW7
Target proteins not degraded
other mutations, like p53
Disregulation of DNA replication, cell division
Tumorigenesis and CANCER

7. Therapies have been developed to target the substrate proteins of FBW7, as the loss of FBW7 has been linked to drug resistance.
A correlation has been found in multiple studies between the loss of the FBW7 tumor suppressor and drug resistance; loss of function of FBW7 can sensitize cells to certain drugs, but they also have resistance to certain chemotherapies- a very common method used to treat cancers. Since it is linked to drug resistance, scientists think that the FBW7 gene might be good to target to increase cancer cells’ drug sensitivity to certain conventional chemotherapies. Resistance to anti-tubulin chemotherapies resulted from increased Mcl-1 pro-survival factor levels (that resulted from loss of FBW7), and scientists found that using a drug to inhibit these Mcl-1 levels largely restored sensitivities to those chemotherapies. For example, without the FBW7 gene, the drug sorafenib effectively inhibits Mcl-1 and restores apoptotic activation. Some successful therapies have been developed to inhibit the substrate proteins of FBW7, inactivating proteins like Myc and Notch 1 and effectively restoring some tumor suppressor function.
Basic idea: therapy directed against FBW7 and its substrates (source: Welcker and Clurman)
Therapies that target FBW7 directly
Therapies that target its oncogenic protein substrates
Successes:
Myc has been targeted in murine sarcomas (source: Jain)
Notch has been targeted in human T-ALL (source: Grabher)
Wang et al. (Aug. 2011)

8. References
“Tumor suppressor functions of FBW7 in cancer development and progression.” Wang et al., 2012
“Emerging roles of the FBW7 tumour suppressor in stem cell differentiation.” Wang et al., Dec. 2011
“The two faces of FBW7 in cancer drug resistance.” Wang et al., Aug. 2011
“hCDC4 gene mutations in endometrial cancer.” Spruck et al., 2002
“FBW7 ubiquitin ligase: a tumour suppressor at the crossroads of cell division, growth, and differentiation.” Welcker and Clurman, 2008
“Inactivation of hCDC4 can cause chromosomal instability.” Rajagopalan et al., 2004
“The ubiquitous nature of cancer: the role of the SCF-Fbw7 complex in development and transformation.” Crusio et al., 2010