- Email: [email protected]
Fanconi anemia: join the club!
190
News & Comment
TRENDS in Cell Biology Vol.11 No.5 May 2001
Journal Club
Mice come unstuck Knock out a protein involved in cell adhesion and you’d expect a loss of cell adhesion, right? In the case of α-catenin, you get much more. Normally a component of adherens junctions, α-catenin links the actin cytoskeleton to E-cadherin through β-catenin. Although β-catenin has long been known to play an essential role in Wingless signalling as well as adhesion, α-catenin was thought only to be important in adhesion. As α-catenin is required for early development, Vasloukhin et al.1 used a conditional mouse knockout system to remove α-catenin only from epithelial tissues. The α-catenin-null epithelia showed several interesting phenotypes. First, α-catenin-null cells showed a general loss of adhesion, with disrupted epithelial sheets. The cells also had defects in cell polarity, which supports previous work in Drosophila showing that disruption of adherens junctions causes cells to lose their ability to divide correctly in the epithelial plane2. This possibly explains some of the defects seen. The epithelial morphology also closely resembled that of pre-cancerous squamous cell carcinomas.
Second, the cells showed an increase in proliferation, with many multinucleate cells and dividing cells visible in normally undividing layers, suggesting unregulated mitosis. This was further highlighted in extracted α-catenin-null keratinocytes, which showed rapid growth and poor contact inhibition. The authors suggest that this increased proliferation is not simply due to lack of adhesion because conditional desmoplakin knockouts (which disrupt desmosome-based adhesion) show similar loss of adhesion phenotypes but no extra proliferation. Furthermore, this excess proliferation still occurs in α-catenin-null keratinocytes even in low Ca2+ conditions where desmosomes and adherens junctions cannot form between cells. In pursuing this excess proliferation, the authors discovered that the α-catenin-null keratinocytes show increased sensitivity to insulin and insulin-like growth factor 1 (IGF-1) and showed increased Ras–mitogen-activated protein kinase (MAPK) activity. They also show that, in α-catenin-null keratinocytes, E-cadherin interacts with
insulin receptor substrate 1 (IRS-1), a phenomenon that has also been observed in certain colon cancer cell lines3. The authors suggest that this could account for the increased Ras–MAPK activation seen. Exactly how this increased insulin/IGF-1 sensitivity occurs remains unclear, especially as the authors found no increase in receptor tyrosine kinase activity. If this E-cadhein–IRS-1 complex is somehow rerouting a signalling cascade, it will no doubt be the subject of future study and could prove to be important for models of colon cancer. 1 Vasloukhin, V. et al. (2001) Hyperproliferation and defects in epithelial polarity upon conditional ablation of α-catenin in skin. Cell 104, 605–617 2 Lu, B. et al. (2001) Adherens junctions inhibit asymmetric cell division in Drosophila epithelium. Nature 409, 522–525 3 Playford, M.P. et al. (2000) Insulin-like growth factor 1 regulates the location, stability, and transcriptional activity of β-catenin. Proc. Natl. Acad. Sci. U. S. A. 97, 12103–12108
Adam Cliffe [email protected]
Fanconi anemia: join the club! Fanconi anemia (FA) syndrome is a member of the family of chromosomal instability syndromes, which includes ataxia telangiectasia (AT), Bloom syndrome (BS) and Nijmegen breakage syndrome (NBS). These syndromes are characterized by spontaneous chromosomal breakage, and accumulating data demonstrate the anticipated role in DNA maintenance of the genes causing these syndromes. For FA, direct evidence for this was lacking. Although five FA genes have been cloned so far from the seven known FA complementation groups, the lack of apparent homology of FA proteins to anything in the database or even themselves leaves their biochemical function enigmatic. Molecular evidence that all FA proteins function in a common pathway comes from the observation that the FA-A, -C, -G and -F proteins form a complex that is disrupted in cells of other FA complementation groups, including -B and -E.
Recent cloning1 and subsequent analysis2 of the FA-D2 gene has mapped its function downstream of the other FA genes and has provided an important lead to the long-sought function of the FA proteins. Although the FAD2 protein sequence again revealed no functional clues, Garcia-Higuera et al.2 discovered that FAD2 can be found in a mono-ubiquitinated form under some circumstances – but only when the other FA proteins are functional. For instance, FAD2 ubiquitination is induced by DNA damage, which drives the protein from a diffuse nuclear distribution to profound nuclear spots. FAD2 spots colocalize with BRCA1 in so-called ‘ionizing-radiation-inducible foci’ that contain other proteins implicated in DNA repair such as Rad51 or the Rad50–Mre11–NBS complex. The observation is substantiated by specific co-immunoprecipitation of the ubiquitinated form of FAD2 by BRCA1, and the fact that formation of FAD2 foci and
DNA-damage-induced ubiquitination of FAD2 are impaired in BRCA1–/– cells. Does this mean that FA proteins are finally taken up in the realm of DNA repair proteins? It might appear so, but it remains to be seen what exactly it is they do and how they intermingle with BRCA1-associated DNA repair and recombination events. The fact that FAD2 spots are also seen during S phase and that FAD2 localizes to synaptonemal complexes in meiotic cells suggests a more general function for FA proteins than mere DNA repair. 1 Timmers, C. et al. (2001) Positional cloning of a novel Fanconi anemia gene, FANCD2. Mol. Cell 7, 241–248 2 Garcia-Higuera, I. et al. (2001) Interaction of the Fanconi anemia proteins and BRCA1 in a common pathway. Mol. Cell 7, 249–262
Jan de Boer [email protected]
http://tcb.trends.com 0962-8924/01/$ – see front matter © 2001 Elsevier Science Ltd. All rights reserved.
News & Comment
TRENDS in Cell Biology Vol.11 No.5 May 2001
Journal Club
Mice come unstuck Knock out a protein involved in cell adhesion and you’d expect a loss of cell adhesion, right? In the case of α-catenin, you get much more. Normally a component of adherens junctions, α-catenin links the actin cytoskeleton to E-cadherin through β-catenin. Although β-catenin has long been known to play an essential role in Wingless signalling as well as adhesion, α-catenin was thought only to be important in adhesion. As α-catenin is required for early development, Vasloukhin et al.1 used a conditional mouse knockout system to remove α-catenin only from epithelial tissues. The α-catenin-null epithelia showed several interesting phenotypes. First, α-catenin-null cells showed a general loss of adhesion, with disrupted epithelial sheets. The cells also had defects in cell polarity, which supports previous work in Drosophila showing that disruption of adherens junctions causes cells to lose their ability to divide correctly in the epithelial plane2. This possibly explains some of the defects seen. The epithelial morphology also closely resembled that of pre-cancerous squamous cell carcinomas.
Second, the cells showed an increase in proliferation, with many multinucleate cells and dividing cells visible in normally undividing layers, suggesting unregulated mitosis. This was further highlighted in extracted α-catenin-null keratinocytes, which showed rapid growth and poor contact inhibition. The authors suggest that this increased proliferation is not simply due to lack of adhesion because conditional desmoplakin knockouts (which disrupt desmosome-based adhesion) show similar loss of adhesion phenotypes but no extra proliferation. Furthermore, this excess proliferation still occurs in α-catenin-null keratinocytes even in low Ca2+ conditions where desmosomes and adherens junctions cannot form between cells. In pursuing this excess proliferation, the authors discovered that the α-catenin-null keratinocytes show increased sensitivity to insulin and insulin-like growth factor 1 (IGF-1) and showed increased Ras–mitogen-activated protein kinase (MAPK) activity. They also show that, in α-catenin-null keratinocytes, E-cadherin interacts with
insulin receptor substrate 1 (IRS-1), a phenomenon that has also been observed in certain colon cancer cell lines3. The authors suggest that this could account for the increased Ras–MAPK activation seen. Exactly how this increased insulin/IGF-1 sensitivity occurs remains unclear, especially as the authors found no increase in receptor tyrosine kinase activity. If this E-cadhein–IRS-1 complex is somehow rerouting a signalling cascade, it will no doubt be the subject of future study and could prove to be important for models of colon cancer. 1 Vasloukhin, V. et al. (2001) Hyperproliferation and defects in epithelial polarity upon conditional ablation of α-catenin in skin. Cell 104, 605–617 2 Lu, B. et al. (2001) Adherens junctions inhibit asymmetric cell division in Drosophila epithelium. Nature 409, 522–525 3 Playford, M.P. et al. (2000) Insulin-like growth factor 1 regulates the location, stability, and transcriptional activity of β-catenin. Proc. Natl. Acad. Sci. U. S. A. 97, 12103–12108
Adam Cliffe [email protected]
Fanconi anemia: join the club! Fanconi anemia (FA) syndrome is a member of the family of chromosomal instability syndromes, which includes ataxia telangiectasia (AT), Bloom syndrome (BS) and Nijmegen breakage syndrome (NBS). These syndromes are characterized by spontaneous chromosomal breakage, and accumulating data demonstrate the anticipated role in DNA maintenance of the genes causing these syndromes. For FA, direct evidence for this was lacking. Although five FA genes have been cloned so far from the seven known FA complementation groups, the lack of apparent homology of FA proteins to anything in the database or even themselves leaves their biochemical function enigmatic. Molecular evidence that all FA proteins function in a common pathway comes from the observation that the FA-A, -C, -G and -F proteins form a complex that is disrupted in cells of other FA complementation groups, including -B and -E.
Recent cloning1 and subsequent analysis2 of the FA-D2 gene has mapped its function downstream of the other FA genes and has provided an important lead to the long-sought function of the FA proteins. Although the FAD2 protein sequence again revealed no functional clues, Garcia-Higuera et al.2 discovered that FAD2 can be found in a mono-ubiquitinated form under some circumstances – but only when the other FA proteins are functional. For instance, FAD2 ubiquitination is induced by DNA damage, which drives the protein from a diffuse nuclear distribution to profound nuclear spots. FAD2 spots colocalize with BRCA1 in so-called ‘ionizing-radiation-inducible foci’ that contain other proteins implicated in DNA repair such as Rad51 or the Rad50–Mre11–NBS complex. The observation is substantiated by specific co-immunoprecipitation of the ubiquitinated form of FAD2 by BRCA1, and the fact that formation of FAD2 foci and
DNA-damage-induced ubiquitination of FAD2 are impaired in BRCA1–/– cells. Does this mean that FA proteins are finally taken up in the realm of DNA repair proteins? It might appear so, but it remains to be seen what exactly it is they do and how they intermingle with BRCA1-associated DNA repair and recombination events. The fact that FAD2 spots are also seen during S phase and that FAD2 localizes to synaptonemal complexes in meiotic cells suggests a more general function for FA proteins than mere DNA repair. 1 Timmers, C. et al. (2001) Positional cloning of a novel Fanconi anemia gene, FANCD2. Mol. Cell 7, 241–248 2 Garcia-Higuera, I. et al. (2001) Interaction of the Fanconi anemia proteins and BRCA1 in a common pathway. Mol. Cell 7, 249–262
Jan de Boer [email protected]
http://tcb.trends.com 0962-8924/01/$ – see front matter © 2001 Elsevier Science Ltd. All rights reserved.