ABSTRACT
The ubiquitin-proteasome system is a major regulatory pathway of protein degradation and plays an important role in cellular division. Fbxw7 (or hCdc4), a member of the F-box family of proteins, which are substrate recognition components of the multisubunit ubiquitin ligase SCF (Skp1-Cdc53/Cullin-F-box-protein), has been shown to mediate the ubiquitin-dependent proteolysis of several oncoproteins including cyclin E1, c-Myc, c-Jun, and Notch. The oncogenic potential of Fbxw7 substrates, frequent allelic loss in human cancers, and demonstration that mutation of FBXW7 cooperates with p53 in mouse tumorigenesis have suggested that Fbxw7 could function as a tumor suppressor in human cancer. Here, we carry out an extensive genetic screen of primary tumors to evaluate the role of FBXW7 as a tumor suppressor in human tumorigenesis. Our results indicate that FBXW7 is inactivated by mutation in diverse human cancer types with an overall mutation frequency of approximately 6%. The highest mutation frequencies were found in tumors of the bile duct (cholangiocarcinomas, 35%), blood (T-cell acute lymphocytic leukemia, 31%), endometrium (9%), colon (9%), and stomach (6%). Approximately 43% of all mutations occur at two mutational "hotspots," which alter Arg residues (Arg465 and Arg479) that are critical for substrate recognition. Furthermore, we show that Fbxw7Arg465 hotspot mutant can abrogate wild-type Fbxw7 function through a dominant negative mechanism. Our study is the first comprehensive screen of FBXW7 mutations in various human malignancies and shows that FBXW7 is a general tumor suppressor in human cancer.
Subject(s)
Cell Cycle Proteins/genetics , F-Box Proteins/genetics , Genes, Tumor Suppressor , Neoplasms/genetics , Ubiquitin-Protein Ligases/genetics , 5-Methylcytosine/metabolism , Amination , Cell Cycle Proteins/metabolism , DNA Methylation , Dinucleotide Repeats , F-Box Proteins/metabolism , F-Box-WD Repeat-Containing Protein 7 , Gene Expression Regulation, Neoplastic , Gene Silencing , Humans , Models, Molecular , Mutation , Neoplasms/metabolism , Protein Isoforms , Substrate Specificity , Ubiquitin-Protein Ligases/metabolismABSTRACT
BACKGROUND: Germ-line mutations of RET proto-oncogene are the known cause of hereditary medullary thyroid carcinoma (MTC), which account for approximately 25% of all MTC cases and occur as multiple endocrine neoplasia type 2 syndromes. Here, we present the first comprehensive genetic screening and analysis of MTC among Iranian families. METHODS: A total of 55 patients with MTC (male to female ratio=1:1.6; average age of disease onset = 33 ± 13 years) from 53 independent families participated in this study. All of the patients had undergone total thyroidectomy between 1999 and 2006, and 51 of them were clinically characterized as apparently sporadic cases. Genomic DNA samples were obtained and following highly-specific polymerase chain reaction amplification of the 6 RET key exons (10, 11, 13, 14, 15, and 16) were subjected to direct DNA sequencing without a requirement for a purification step. RESULTS: Sequence analysis revealed that 9 (17.6%) of the apparently sporadic cases (from 8 kindreds) carried an RET germ-line mutation. Of the seven different mutations identified among all of the families studied, five were in the cysteine codons, with Cys634Arg having the highest prevalence (45.5%) among the afflicted families. Mutation carriers have an earlier age of onset (21 ± 6) versus the sporadic cases (37 ± 12). CONCLUSIONS: This is the first comprehensive genetic screening and analysis of MTC among Iranian families. The results further confirm the need and advantages of DNA sequencing for identification of hereditary MTC cases. There does not seem to be a meaningful correlation between single nucleotide polymorphism patterns and the average age of disease onset. Geographical distribution of the sporadic cases, however, shows a significant concentration toward the Northern regions of the country, noticeably the provinces situated directly to the south of the Caspian Sea.
Subject(s)
Carcinoma, Medullary/genetics , Multiple Endocrine Neoplasia Type 2a/genetics , Proto-Oncogene Proteins c-ret/genetics , Adolescent , Adult , Aged , Carcinoma, Neuroendocrine , Child , Exons/genetics , Female , Genetic Counseling , Germ-Line Mutation , Humans , Iran , Male , Middle Aged , Polymerase Chain Reaction/methods , Proto-Oncogene Mas , Thyroid Neoplasms/geneticsABSTRACT
BACKGROUND AND PURPOSE: Erb-B1 (epidermal growth factor receptor, EGFR) and Erb-B2 (HER-2) are two of the best-characterized members in the EGFR pathway. In many tumor types, overexpression of these proteins is associated with enhanced malignant potential. The aim of this study was to determine the prognostic impact of EGFR and HER-2 protein expression on colorectal cancer. METHOD: Immunohistochemistry was carried out in paraffin-embedded specimens of 115 colorectal carcinomas for the assessment of EGFR and HER-2 expression. Immunostaining for EGFR was graded negative, weak or strong according to extension and staining intensity. The results were correlated with traditional clinicopathologic parameters and patients' outcome. RESULTS: The mean survival time was 64 (range 9-78) months in the EGFR-negative group, 166 (range 2-293) months in the group with a low EGFR expression, and 51 (range 4-71) months in the group with a high EGFR expression. The median survival time was 31 (range 2-114) months in the HER-2 negative group, and 30 (range 4-293) months in the HER-2 positive group. None of the clinicopathologic parameters or patient prognoses had statistically significant association with EGFR or HER-2 expression. CONCLUSION: Conventional immunohistochemistry was unable to reveal any association between EGFR or HER-2 expression and outcome predicted by the biologic role of EGFR in tumor behavior and the established prognostic role of HER-2 in breast cancer.