Ryunosuke Kanamaru

Understanding the function–structure and function–mutation relationships of p53 tumor suppressor protein by high-resolution missense mutation analysis

Inactivation of the tumor suppressor p53 by missense mutations is the most frequent genetic alteration in human cancers. The common missense mutations in the TP53 gene disrupt the ability of p53 to bind to DNA and consequently to transactivate downstream genes. However, it is still not fully understood how a large number of the remaining mutations affect p53 structure and function. Here, we used a comprehensive site-directed mutagenesis technique and a yeast-based functional assay to construct, express, and evaluate 2,314 p53 mutants representing all possible amino acid substitutions caused by a point mutation throughout the protein (5.9 substitutions per residue), and correlated p53 function with structure- and tumor-derived mutations. This high-resolution mutation analysis allows evaluation of previous predictions and hypotheses through interrelation of function, structure and mutation.

Differential Contributions of BRCA1 and BRCA2 to Early-Onset Breast Cancer

BACKGROUND Germ-line mutations in the BRCA1 and BRCA2 genes predispose women to breast cancer. BRCA1 mutations are found in approximately 12 percent of women with breast cancer of early onset, and the specific mutation causing a deletion of adenine and guanine (185delAG), which is present in 1 percent of the Ashkenazi Jewish population, contributes to 21 percent of breast cancers among young Jewish women. The contribution of BRCA2 mutations to breast cancer of early onset is unknown. METHODS Lymphocyte specimens from 73 women with breast cancer diagnosed by the age of 32 were studied for heterozygous mutations of BRCA2 by a complementary-DNA-based protein-truncation assay, followed by automated nucleotide sequencing. In addition, specimens from 39 Jewish women with breast cancer diagnosed by the age of 40 were tested for specific mutations by an allele-specific polymerase chain reaction. RESULTS Definite BRCA2 mutations were found in 2 of the 73 women with early-onset breast cancer (2.7 percent; 95 percent confidence interval, 0.4 to 9.6 percent), suggesting that BRCA2 is associated with fewer cases than BRCA1 (P=0.03). The specific BRCA2 mutation causing a deletion of thymine (6174delT), which is found in 1.3 percent of the Ashkenazi Jewish population, was observed in 1 of the 39 young Jewish women with breast cancer (2.6 percent; 95 percent confidence interval, 0.09 to 13.5 percent), indicating that it has a small role as a risk factor for early-onset breast cancer. Among young women with breast cancer, there are BRCA2 mutations that cause truncation of the extreme C terminus of the protein and that may be functionally silent, along with definite truncating mutations. CONCLUSIONS Germ-line mutations in BRCA2 contribute to fewer cases of breast cancer among young women than do mutations in BRCA1. Carriers of BRCA2 mutations may have a smaller increase in the risk of early-onset breast cancer.

Screening the p53 status of human cell lines using a yeast functional assay

We have screened the p53 status of 156 human cell lines, including 142 tumor cell lines from 27 different tumor types and 14 cell lines from normal tissues by using functional analysis of separated alleles in yeast. This assay enables us to score wild‐type p53 expression on the basis of the ability of expressed p53 to transactivate the reporter gene HIS3 via the p53‐responsive GAL1 promoter in Saccharomyces cerevisiae. Of 142 tumor cell lines, at least 104 lines (73.2%) were found to express the mutated p53 gene: 94 lines (66.2%) were mutated in both alleles, three lines (2.1%) were heterozygous, and no p53 cDNA was amplified from seven lines (4.9%). Of the 14 cell lines originating from normal tissues, all the transformed or immortalized cell lines expressed mutant p53 only. Yeast cells expressing mutant p53 derived from 94 cell lines were analyzed for temperature‐sensitive growth. p53 cDNA from eight cell lines showed p53‐dependent temperature‐sensitive growth, growing at 30°C but not at 37°C. Four temperature‐sensitive p53 mutations were isolated: CAT→CGT at codon 214 (H214R), TAC→TGC at codon 234 (Y234C), GTG→ATG at codon 272 (V272M), and GAG→AAG (E285K). Functionally wild‐type p53 was detected in 38 tumor cell lines (26.8%) and all of the diploid fibroblasts at early and late population doubling levels. These results strongly support the previous findings that p53 inactivation is one of the most frequent genetic events that occurs during carcinogenesis and immortalization. Mol. Carcinog. 19:243–253, 1997.

Functional analysis of human MLH1 mutations in Saccharomyces cerevisiae

Hereditary non-polyposis colorectal cancer (HNPCC; OMIM 120435-6) is a cancer-susceptibility syndrome linked to inherited defects in human mismatch repair (MMR) genes. Germline missense human MLH1 (hMLH1) mutations are frequently detected in HNPCC (ref. 3), making functional characterization of mutations in hMLH1 critical to the development of genetic testing for HNPCC. Here, we describe a new method for detecting mutations in hMLH1 using a dominant mutator effect of hMLH1 cDNA expressed in Saccharomyces cerevisiae. The majority of hMLH1 missense mutations identified in HNPCC patients abolish the dominant mutator effect. Furthermore, PCR amplification of hMLH1 cDNA from mRNA from a HNPCC patient, followed by in vivo recombination into a gap expression vector, allowed detection of a heterozygous loss-of-function missense mutation in hMLH1 using this method. This functional assay offers a simple method for detecting and evaluating pathogenic mutations in hMLH1.

An Early Phase Ii Study of a 3-hour Infusion of Paclitaxel for Advanced Gastric Cancer

The purpose of this study was to evaluate the feasibility and efficacy of 3-hour infusional paclitaxel for the treatment of advanced gastric cancer with measurable metastatic diseases. Eligibility criteria included no more than one regimen of prior chemotherapy. Paclitaxel was administered as an intravenous infusion over 3 hours at a dose of 210 mg/m2 every 3 weeks. Premedication of dexamethazone, ranitidine, and diphenhydramine were given to all patients. Sixteen patients were registered in the study. One patient did not receive paclitaxel because of gastrointestinal bleeding before the initiation of drugs administration. Thirteen of the 15 patients had a prior history of chemotherapy. Although 10 patients (67%) developed grade 4 neutropenia, no serious infections occurred during the study. Nonhematologic toxicities were generally mild. Three (20%) patients who showed evidences of resistance to the previous intensive regimen achieved a partial response. In conclusion, a 3-hour infusion of paclitaxel is a safe and promising treatment for advanced gastric cancer. Paclitaxel appears to be non-cross resistant to other agents that are commonly used for gastric cancer. A large-scale phase II study is now underway.

Restriction fragment length polymorphism of the human CYP2E1 (cytochrome P450IIE1) gene and susceptibility to lung cancer: possible relevance to low smoking exposure.

Polymorphic metabolism of certain chemical carcinogens may result in differences in susceptibility to cancers. Human CYP2E1 (cytochrome P450IIE1) is an enzyme involved in the metabolic activation of precarcinogens such as nitrosamines. We detected a restriction fragment length polymorphism (RFLP) of the human CYP2E1 gene for the restriction endonuclease Dra I. The distribution of this polymorphism was examined among lung cancer patients (n = 91), patients with cancer of the digestive tract (n = 45) and controls (n = 76). A significant difference in the distribution was observed between lung cancer patients and controls (chi 2 = 11.4 with 2 df; p < 0.005). On the other hand, there was no significant difference between patients between cancer of the digestive tract and controls (chi 2 = 4.87 with 2 df; NS). This finding suggests that the Dra I polymorphism of the CYP2E1 gene is associated with susceptibility to lung cancer. In addition, an association was found between the amount of lifelong smoking exposure and the distribution of the genotypes of the RFLP among lung cancer patients. The distribution pattern seemed deviated from that of controls especially in the population of low smoking exposure. Our Northern blot analysis data using RNA from human liver autopsy samples suggest that the Dra I polymorphism might be associated with the gene expression of CYP2E1 at mRNA level.

Selective suppression of stress-activated protein kinase pathway by protein phosphatase 2C in mammalian cells

Protein phosphatase 2Cα (PP2Cα) or PP2Cβ‐1 expressed in COS7 cells suppressed anisomycin‐ and NaCl‐enhanced phosphorylations of p38 co‐expressed in the cells. PP2Cα or PP2Cβ‐1 expression also suppressed both basal and stress‐enhanced phosphorylations of MKK3b and MKK6b, which are upstream protein kinases of p38, and of MKK4, which is one of the major upstream protein kinases of JNK. Basal activity of MKK7, another upstream protein kinase of JNK, was also suppressed by PP2Cα or PP2Cβ‐1 expression. However, basal as well as serum‐activated phosphorylation of MKK1a, an upstream protein kinase of ERKs, was not affected by PP2Cβ or PP2Cβ‐1. A catalytically inactive mutant of PP2Cβ‐1 further enhanced the NaCl‐stimulated phosphorylations of MMK3b, MKK4 and MKK6b, suggesting that this mutant PP2Cβ‐1 works as a dominant negative form. These results suggest that PP2C selectively inhibits the SAPK pathways through suppression of MKK3b, MKK4, MKK6b and MKK7 activities in mammalian cells.

Isolation of temperature-sensitive p53 mutations from a comprehensive missense mutation library.

Temperature-sensitive (ts) mutations have been used as a genetic and molecular tool to study the functions of many gene products. Each ts mutant protein may contain a temperature-dependent intramolecular mechanism such as ts conformational change. To identify key ts structural elements controlling the protein function, we screened ts p53 mutants from a comprehensive mutation library consisting of 2,314 p53 missense mutations for their sequence-specific transactivity through p53-binding sequences in Saccharomyces cerevisiae. We isolated 142 ts p53 mutants, including 131 unreported ts mutants. These mutants clustered in β-strands in the DNA-binding domain, particularly in one of the two β-sheets of the protein, and 15 residues (Thr155, Arg158, Met160, Ala161, Val172, His214, Ser215, Pro223, Thr231, Thr253, Ile254, Thr256, Ser269, Glu271, and Glu285) were ts hot spots. Among the 142 mutants, 54 were examined further in human osteosarcoma Saos-2 cells, and it was confirmed that 89% of the mutants were also ts in mammalian cells. The ts mutants represented distinct ts transactivities for the p53 binding sequences and a distinct epitope expression pattern for conformation-specific anti-p53 antibodies. These results indicated that the intramolecular β-sheet in the core DNA-binding domain of p53 was a key structural element controlling the protein function and provided a clue for finding a molecular mechanism that enables the rescue of the mutant p53 function.

Phase II study of paclitaxel with 3-h infusion in patients with advanced gastric cancer

Abstract.Abstract.Background: To increase the options for agents for gastric cancer chemotherapy, we performed a phase II clinical trial on the use of a 3-h infusion of paclitaxel to confirm its efficacy and the feasibility of its use in patients with advanced gastric cancer.Methods: Thirty-two (32) patients with measurable metastatic gastric cancer were enrolled in this study. Seventeen patients (53%) had received prior chemotherapy for metastatic disease, 4 patients (13%) had adjuvant chemotherapy alone, and 11 patients (34%) were chemotherapy-naive.Paclitaxel was intravenously infused for 3 h, at a dose of 210 mg/m2, once every 3 weeks. To prevent hypersensitivity reactions, standard premedication was administered to all patients.Results: Nine (28%; 9/32 ) objective partial responses (PRs) were observed (95% confidence interval [CI], 14%–47%), and the remaining 23 patients showed stable (12 patients; 37.5%) and progressive disease (11 patients; 34.4%). The median time to response was 20 days (range, 14–38 days). The median response duration was 87 days (range, 50–103 days). The median survival of all patients was 234 days (range, 13–646+ days). The major adverse reactions were myelosuppression (grade 3/4 leukopenia and neutropenia were observed in 59% and 88% of the patients, respectively), alopecia, and peripheral neuropathy. Peripheral neuropathy was observed in 19 patients, however, most of the patients recovered after the completion of treatment.Conclusion: A 3-h infusion of paclitaxel is an effective therapy for advanced gastric cancer and is clinically well tolerated by the patients.

The inhibitory effects of 5-fluorouracil on the metabolism of preribosomal and ribosomal RNA in L-1210 cells in vitro

SummaryAddition of 5FU to the culture medium of mouse L-1210 cells resulted in inhibition of the maturation process of ribosomal RNA precursors in vitro. In the presence of 10-6M 5FU for 2 h, the 45S preribosomal RNA was processed to 32S preribosomal RNA, but 28S rRNA was not produced. The processing to 18S rRNA was intact at this drug concentration. Higher concentrations of 5FU for a longer incubation period affected the RNA processing more severely. At 10-5M of the drug for 24 h the processing to 18S rRNA was also inhibited, in addition to the processing to 18S rRNA and 32S preribosomal RNA. When the cells were labeled with 14C-UR for 2 h following 3H-5FU at 10-6M for 24 h, the radioactivities of newly synthesized RNA labeled with 14C-UR accumulated in the region of 45S and 32S preribosomal RNA, and no processing to 28S rRNA was observed. Radioactivity corresponding to 3H-5FU did not persist in the preribosomal RNA region, because further maturation proceeded in the condition of depletion of 5FU after the long incubation period. Thus, inhibition of the processing of preribosomal RNA to 28S rRNA was not brought about by the accumulation of 5FU-substituted 45S preribosomal RNA, but by some other, yet unknown, mechanism.