DNA polymerase β promoter mutations and transcriptional activity in esophageal squamous cell carcinoma

TRIP12 governs DNA Polymerase β involvement in DNA damage response and repair

The multitude of DNA lesion types, and the nuclear dynamic context in which they occur, present a challenge for genome integrity maintenance as this requires the engagement of different DNA repair pathways. Specific 'repair controllers' that facilitate DNA repair pathway crosstalk between double strand break (DSB) repair and base excision repair (BER), and regulate BER protein trafficking at lesion sites, have yet to be identified. We find that DNA polymerase β (Polβ), crucial for BER, is ubiquitylated in a BER complex-dependent manner by TRIP12, an E3 ligase that partners with UBR5 and restrains DSB repair signaling. Here we find that, TRIP12, but not UBR5, controls cellular levels and chromatin loading of Polβ. Required for Polβ foci formation, TRIP12 regulates Polβ involvement after DNA damage. Notably, excessive TRIP12-mediated shuttling of Polβ affects DSB formation and radiation sensitivity, underscoring its precedence for BER. We conclude that the herein discovered trafficking function at the nexus of DNA repair signaling pathways, towards Polβ-directed BER, optimizes DNA repair pathway choice at complex lesion sites.

Polymorphic mutations in the polb gene promoter and their impact on transcriptional activity

Background

DNA polymerase β is one of the key enzymes involved in DNA damage repair and its proper expression is strictly controlled within different cells. We previously reported that three genetic mutations in the promoter region of the polb gene are prevalent in the Chinese Han population and two types of mutation are associated with thymic hyperplasia. The purpose of this study was to explore whether other mutated sites exist within the promoter region of the polb gene.

Methods

Genomic DNAs of 421 healthy Chinese Han individuals were extracted from whole blood samples and used for gene amplification of the promoter region of the polb gene. After gel purification, PCR amplicons were sequenced by the Sanger sequencing method and used for sequence alignment with the Lasergene program. PCR products with novel mutations were then subcloned into luciferase reporter plasmid pGL4.10 and transfected into 293T cells for dual‐luciferase activity analysis.

Results

In total, 11 mutated sites were detected in the Chinese Han population and eight of these were reported for the first time. Using a dual luciferase reporter system, it was found that one novel mutation −142 C > G could decrease the transcription activity of the polb gene, whereas two novel mutations, −152_−151insC and −218 C > G, could significantly increase the transcription activity of the polb gene.

Conclusions

High polymorphic sites could be found in the promoter region of polb gene and approximately half of them could influence its transcription activity.

Two polymorphic mutations in promoter region of DNA polymerase β in relatively higher percentage of thymic hyperplasia patients

Background

DNA polymerase β is one of the key enzymes involved in the repair of DNA damage, and its high or low expression is closely related to tumorigenesis. In a previous study on lung cancer, we found three genetic mutations in the promoter region of the Polb gene could be detected in the Han Chinese population. The purpose of this study was to explore the relationship between these mutations and thymic hyperplasia.

Methods

Genomic DNA was extracted from 59 thymic hyperplasia patients by the salting out method and used for amplification of the promoter region of the Polb gene. The Polb gene mutation and its frequency were analyzed systematically by comparing them with the deposited wild‐type gene sequence in the NCBI database. The three typical mutated sequences in the promoter region of Polb gene, ‐196G > T, ‐168C > A and ‐188_‐187insCGCCC, were then amplified and ligated to pGL4.10 vector, so as to get the vectors used for the infection of 293T cells to explore their transcription activities by dual‐luciferase reporter system.

Results

Two types of mutations, ‐168C>A and‐188_‐187insCGCCC, were found in a significantly higher percentage in patients with thymic hyperplasia than in normal healthy people after sequencing analysis of 59 patients and 60 healthy controls. These results suggest that the two mutations may be closely related to thymic hyperplasia. in vitro functional experiments showed that‐168C>A could significantly increase promoter activity, whereas ‐188_‐187insCGCCC could significantly reduce promoter activity, suggesting that these two mutations may affect the expression level of the Polb gene in cells.

Conclusions

Two types of mutations in the promoter region of the Polb gene, ‐168C>A and‐188_‐187insCGCCC, are associated with thymic hyperplasia and may become a new risk factor for this disease.

Key points

Significant findings of the study

Genetic mutations in the Polb gene are reported to be associated with different kinds of cancers. However, their relationship with thymic hyperplasia is still unclear.

What this study adds

For the first time, we report that two nucleotide mutations in the promoter region of the Polb gene are closely related with thymic hyperplasia after sequencing 59 patients and 60 healthy controls in the Han Chinese population.

[Genetic Mutation Screening of DNA Polymerase in Human Lung Cancer]

BACKGROUND

DNA polymerase β is one of the key enzymes for DNA repair and it was reported that about 30 percent of different types of cancers carried mutations in its coding gene Polb. However, it is still controversial whether it is true or false because of the small sample size in these studies. In current study, we performed genetic screening of promoter and coding regions of Polb gene in 69 Chinese lung cancer patients using Sanger sequencing method, so as to elucidate real mutation frequency of Polb mutations in Chinese Han population.

METHODS

Salting out extraction method was used to get the genome DNAs from tumor and normal matched tissues of 69 lung cancer patients. The promoter and 14 coding regions of Polb gene were then amplified using these DNAs as the template. After purification, amplicons were sequenced and aligned to the wild type Polb gene in NCBI database, in order to find out the mutated sites of Polb gene in Chinese lung cancer patients.

RESULTS

In this study, we totally found only 5 mutated sites in Polb gene. In detail, 3 mutations (-196G>T, -188_-187insCGCCC, -168C>A) were located in the promoter region; 2 mutations (587C>G, 612A>T) were found in coding regions. Specially, mutations of -188_-187insCGCCC and 587C>G (resulting to the amino acid substitution of Thr to Ser at position 196) had never been reported by other groups before. However, all these 5 mutated sites could be detected in both tumor and matched normal tissues, which inferred that they are not lung tumor specific mutations.

CONCLUSIONS

No lung tumor specific mutations of Polb gene could be found in Chinese lung cancer patients and Polb gene mutation might not be a molecular marker for Chinese lung cancer patients.

MiR-149 sensitizes esophageal cancer cell lines to cisplatin by targeting DNA polymerase β

Human DNA polymerase β (polβ) is a small, monomeric protein essential for short‐patch base excision repair (BER). polβ plays an important role in the regulation of chemotherapy sensitivity in tumour cells. In this study, we determined that the expression levels of polβ mRNA and miR‐149 in tumour tissues were significantly higher than in adjacent non‐tumour tissues. We also found that the expression level of miR‐149 in EC tumour tissues was inverse to that of polβ expression. Bioinformatics analysis and dual‐luciferase reporter assay predicted that miR‐149 negatively regulates polβ expression by directly binding to its 3′UTR. CCK‐8 assay indicated that miR‐149 could enhance the anti‐proliferative effects of cisplatin in EC1 and EC9706 cell lines. Flow cytometry, caspase 3/7 activity, and immunofluorescence microscopy results indicated that miR‐149 could enhance the apoptotic effects of cisplatin in EC1 and EC9706 cell lines. We also showed that the expression of polβ lacking the 3′UTR sequence could override the proliferative and apoptotic functions of miR‐149, suggesting that miR‐149 negatively regulates polβ expression by binding to its 3′UTR. Surface plasmon resonance results also showed that miR‐149 could bind with wild‐type polβ. In addition, we identified a new variant of polβ (C1134G). In conclusion, this study confirms that miR‐149 may enhance the sensitivity of EC cell lines to cisplatin by targeting polβ, and that miR‐149 may be unable to regulate the C1134G variant of polβ. Based on these findings, potential drugs could be developed with a focus on enhanced sensitivity of EC patients to chemotherapy.

Overexpression of A613T and G462T variants of DNA polymerase β weakens chemotherapy sensitivity in esophageal cancer cell lines

BACKGROUND

Human DNA polymerase β (polβ) is a small monomeric protein that is essential for short-patch base excision repair. It plays an important role in regulating the sensitivity of tumor cells to chemotherapy.

METHODS

We evaluated the mutation of polβ in a larger cohort of esophageal cancer (EC) patients by RT-PCR and sequencing analysis. The function of the mutation was evaluated by CCK-8, in vivo tumor growth, and flow cytometry assays.

RESULTS

There are 229 patients with the polβ mutation, 18 patients with A613T mutation, 12 patients with G462T mutation among 538 ECs. Analysis results of survival time showed that EC patients with A613T, G462T mutation had a shorter survival than the others (P < 0.05). CCK-8 and flow cytometry assays results showed the A613T, G462T EC9706 cells were less sensitive than WT cells to 5-FU and cisplatin (P < 0.05). Experiments results in vivo showed that the tumor sizes of A613T and G462T group were larger than WT and polβ^-/- groups (P < 0.05).

CONCLUSIONS

In this study, we discovered A to T point mutation at nucleotide 613 (A613T) and G to T point mutation at nucleotide 462 (G462T) in the polβ gene through 538 EC patients cohort study. A613T and G462T variant of DNA polymerase β weaken chemotherapy sensitivity of esophageal cancer.

G648C variant of DNA polymerase β sensitizes esophageal cancer to chemotherapy

Human DNA polymerase β (polβ) is a small monomeric protein that is essential for short-patch base excision repair. It plays an important role in regulating the sensitivity of tumor cells to chemotherapy. We have previously identified a G to C point mutation at nucleotide 648 (G648C) of polβ in esophageal cancer (EC). In this study, we evaluated the mutation of polβ in a larger cohort of EC patients by RT-PCR and sequencing analysis. The function of the mutation was evaluated by MTT, in vivo tumor growth, and flow cytometry assays. The G648C mutation occurred in 15 (3.45 %) of 435 EC patients. In addition, patients with this mutation had significantly longer survival time than those without, following postoperative chemotherapy. Cell lines with G648C mutation in polβ gene were more sensitive to treatment with 5-fluorouracil and cisplatin than those with wild-type polβ. These results suggest that polβ gene with G648C mutation in surgically resected esophagus may be clinically useful for predicting responsiveness to chemotherapy in patients with EC. The polβ gene alteration may serve as a prognostic biomarker for EC.

Enhancement of silencing DNA polymerase β on the radiotherapeutic sensitivity of human esophageal carcinoma cell lines

Human DNA polymerase β (DNA polymeraseβ (polβ)) is a small monomeric protein which is essential for short-patch base excision repair (BER). It plays an important role in regulating the radiation sensitivity of tumor cells in the course of tumor radiation therapy. In this study, qRT-PCR and Western blot assays were used to quantify polβ expression levels in esophageal carcinoma (EC) cells that were transfected with polβ small interfering RNA (siRNA). Cell counting Kit-8 (CCK-8), flow cytometry, and Hoechst/PI stain assays were conducted to evaluate the effects of silencing polβ on the radiotherapeutic sensitivity of EC cells. We found that the expression levels of polβ in EC cells were significantly decreased after transfection with polβ siRNA. Then, we found that polβ silencing increased the sensitivity of EC cells to radiation therapy. In conclusion, our study paves the way for a better understanding of the mechanism of the polβ gene in DNA repair, and we propose that RNA interference technology will have important applications in gene therapy of EC and other cancers in the future.