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Analysis of structure and activation of some receptor-type tyrosine kinase oncogenes
M Shibuya1, H Matsushime, H Yamazaki
1Institute of Medical Science, University of Tokyo, Japan.
Abstract:
We first describe the characterization of proto-oncogene of v-ros in chicken and human genomes. v-ros sequence of UR2 avian sarcoma virus carries a hydrophobic short stretch upstream of the kinase domain, suggesting that its proto-oncogene encodes for a receptor molecule. Using v-ros DNA as a probe we isolated chicken and human c-ros proto-oncogenes. These c-ros DNAs contained a tyrosine kinase domain, transmembrane domain and a part of an extracellular domain carrying an N glycosylation site which was not acquired by UR2 sarcoma virus. These results strongly suggest that proto-oncogene c-ros encodes for a receptor of cell growth or differentiation factor(s) and that the v-ros sequence is a truncated form of this receptor molecule. Structural alteration and overexpression under the control of viral promoter may be crucial for transforming activity of v-ros gene. We then report another example where a receptor-type oncogene is qualitatively and quantitatively activated. By screening with various onc probes we detected two human glioblastomas which have amplification of structurally altered c-erbB1 (epidermal growth factor (EGF) receptor) gene. c-erbB1 gene in these tumors bears a small deletion within the extracellular domain, and the gene product 140 kd protein shorter than the normal 170 kd EGF receptor was heavily phosphorylated on tyrosine residue even without ligand in in vitro phosphorylation reaction. Thus, these mutated EGF receptors seem to be fixed as a "switch-on" form in signal transduction for cell growth and might be involved in the transformation of glial cells.
Insights
Proto-oncogene c-ros encodes a cell growth receptor, with v-ros being a truncated form. Mutated epidermal growth factor (EGF) receptors in glioblastomas may drive cell growth and transformation.
Area of Science:
- Molecular Biology
- Oncology
- Genetics
Background:
- Proto-oncogenes encode proteins regulating cell growth and differentiation.
- Viral oncogenes can arise from cellular proto-oncogenes through mutation or truncation.
- Receptor tyrosine kinases play critical roles in cell signaling pathways.
Purpose of the Study:
- To characterize the chicken and human c-ros proto-oncogenes.
- To investigate the structural and functional alterations of the c-erbB1 (EGF receptor) gene in human glioblastomas.
- To understand the molecular mechanisms underlying oncogene activation in cancer.
Main Methods:
- DNA hybridization using v-ros as a probe to isolate c-ros genes.
- Sequence analysis to identify functional domains (tyrosine kinase, transmembrane, extracellular).
- Screening of glioblastoma DNA with onc probes to detect gene amplification and alterations.
- In vitro phosphorylation assays to assess kinase activity of mutated EGF receptors.
Main Results:
- Chicken and human c-ros proto-oncogenes encode receptor molecules with tyrosine kinase, transmembrane, and extracellular domains.
- The v-ros oncogene is a truncated form of the c-ros receptor, lacking parts of the extracellular domain.
- Two human glioblastomas showed amplification of a structurally altered c-erbB1 gene with a deletion in the extracellular domain.
- Mutated EGF receptors were constitutively phosphorylated, suggesting a "switch-on" state for signal transduction.
Conclusions:
- Proto-oncogene c-ros encodes a receptor for cell growth factors; v-ros is a truncated, potentially oncogenic form.
- Structural alterations and overexpression of c-ros may contribute to its transforming activity.
- Mutated and amplified EGF receptors in glioblastomas are likely involved in uncontrolled cell growth and glial cell transformation.