Impact of medical imaging on the epigenome - low-dose exposure in the course of computed tomography does not induce detectable changes of DNA-methylation profiles in peripheral blood cells

BACKGROUND

Computed tomography (CT) is a main contributor to artificial low-dose exposure. Understanding the biological effects induced by CT exposure and their dependency on the characteristics of photon spectra is essential for knowledge-driven risk assessment. In a previous gene expression study, we have identified upregulation of AEN, BAX, DDB2, EDA2R and FDXR after ex vivo exposure with single-energy CT and dual-energy CT (DECT). In this study, we focused on CT-induced changes of DNA methylation. This epigenetic modification of DNA is a central regulator of gene expression and instrumental in preserving genome integrity. Previous studies reported focal hypermethylation and global hypomethylation after exposure with doses above 100 mSv, however, the effect of low dose exposure on DNA methylation is hardly explored.

MATERIALS AND METHODS

DNA was isolated from peripheral blood of three healthy individuals 6 h after ex vivo exposition to single-energy (80 kV and 150 kV) and DECT (80 kV/Sn150 kV) with a calculated effective dose of 7.0 ± 0.08 mSv. The experimental setting was identical to the one used in our previous gene expression study enabling a direct comparison of gene expression results with changes of DNA methylation identified in this study. DNA methylation was analyzed by high-throughput sequencing of bisulfite-treated DNA targeted methylation sequencing.

RESULTS

Unsupervised hierarchical clustering based on DNA methylation profiles of all samples created three distinct clusters. Formation of these three clusters was solely determined by the origin of samples, indicating the absence of prominent irradiation-associated changes of DNA methylation. In line with this observation, inter-individual comparison of non-irradiated samples revealed 1163, 1224 and 4550 significant differentially methylated regions (DMRs), respectively, whereas the pairwise comparison of irradiated and non-irradiated samples failed to identify irradiation-induced DMRs in any of the three probands. This even applied to the genomic regions harboring AEN, BAX, DDB2, EDA2R and FDXR, the five genes known to be upregulated by CT exposure.

CONCLUSIONS

CT exposure with various photon spectra did not result in detectable changes of DNA methylation. However, minor effects in a subpopulation of irradiated cells cannot be ruled out. Thus, future studies with extended observation intervals are needed to investigate DNA methylation changes that are induced by indirect effects at later points of time or become detectable by clonal expansion of affected cells. Moreover, our data suggest that DNA methylation analysis is less sensitive in detecting immediate effects of low-dose irradiation when compared to gene expression analysis.

Gene expression changes and DNA damage after ex vivo exposure of peripheral blood cells to various CT photon spectra

Dual-energy CT provides enhanced diagnostic power with similar or even reduced radiation dose as compared to single-energy CT. Its principle is based on the distinct physical properties of low and high energetic photons, which, however, may also affect the biological effectiveness and hence the extent of CT-induced cellular damage. Therefore, a comparative analysis of biological effectiveness of dual- and single-energy CT scans with focus on early gene regulation and frequency of radiation-induced DNA double strand breaks (DSBs) was performed. Blood samples from three healthy individuals were irradiated ex vivo with single-energy (80 kV and 150 kV) and dual-energy tube voltages (80 kV/Sn150kV) employing a modern dual source CT scanner resulting in Volume Computed Tomography Dose Index (CTDIvol) of 15.79-18.26 mGy and dose length product (DLP) of 606.7-613.8 mGy*cm. Non-irradiated samples served as a control. Differential gene expression in peripheral blood mononuclear cells was analyzed 6 h after irradiation using whole transcriptome sequencing. DSB frequency was studied by 53BP1 + γH2AX co-immunostaining and microscopic evaluation of their focal accumulation at DSBs. Neither the analysis of gene expression nor DSB frequency provided any evidence for significantly increased biological effectiveness of dual-energy CT in comparison to samples irradiated with particular single-energy CT spectra. Relative to control, irradiated samples were characterized by a significantly higher rate of DSBs (p < 0.001) and the shared upregulation of five genes, AEN, BAX, DDB2, FDXR and EDA2R, which have already been suggested as radiation-induced biomarkers in previous studies. Despite steadily decreasing doses, CT diagnostics remain a genotoxic stressor with impact on gene regulation and DNA integrity. However, no evidence was found that varying X-ray spectra of CT impact the extent of cellular damage.

Prevalence of adverse reactions to food in Germany - a population study

OBJECTIVE

A population study was performed to identify the prevalence of all kinds of adverse reactions to food.

METHODS

In a representative cross-sectional survey performed in 1999 and 2000 in Berlin, 13 300 inhabitants of all ages were addressed by questionnaire. This questionnaire was answered by 4093 persons. All respondents mentioning any sign of food intolerance or the existence of allergic diseases (n = 2298) were followed up by telephone and, in case food intolerance could not be ruled out by patient history, were invited to attend to the clinic for personal investigation including double-blind, placebo-controlled food challenge tests (DBPCFC).

RESULTS

The self-reported lifetime prevalence of any adverse reaction to food in the Berlin population (mean age 41 years) was 34.9%. Eight hundred and fourteen individuals were personally investigated according to the guidelines. The point prevalence of adverse reactions to food confirmed by DBPCFC tests in the Berlin population as a mean of all age groups was 3.6% (95% confidence interval [3.0-4.2%]) and 3.7% in the adult population (18-79 years, 95% confidence interval [3.1-4.4.%]). Two and a half percent were IgE-mediated and 1.1% non-IgE-mediated, females were more frequently affected (60.6%). Based on a statistical comparison with available data of adults from the nationwide German Health Survey from 1998, adverse reactions to food in the adult population of Germany (age 18-79) were calculated with 2.6% [2.1-3.2%]).

CONCLUSIONS

The study gives for the first time information about the point prevalence of both immunological and nonimmunological adverse reactions to food and underlines the relevance of this issue in public health. The data also show that an individualized stepwise approach including provocation tests is mandatory to confirm the diagnosis.

Alterations in ganglioside expression during the differentiation of human mast cells

Gangliosides are physiological components of the outer cell membrane. In the present study, the role of ganglioside expression during differentiation of human mast cells was evaluated. After 11 days of culture in medium known to induce mast cell differentiation, 70% of peripheral blood mononuclear cells (PBMC) showed positive staining for the high affinity IgE receptor and tryptase on immunocytochemistry and an associated 20-fold increase of ganglioside GM3 expression. Furthermore, exogenous addition of GM3 during cultivation of PBMC in medium containing low levels of growth factors induced an increase of mast cell specific tryptase. The association of ganglioside expression with mast cell differentiation was confirmed by experiments with the human mast cell line HMC-1. FcepsilonRI-positive cultured cells enriched with immunobeads exhibited a 3-fold higher expression of GM3, compared to FcepsilonRI negative HMC-1 cells. Furthermore, measurable amounts of the gangliosides GM2, GM1 and GD1a were found only in the FcepsilonRI positive cells. A corresponding transient increase of mRNA for GalNAcT, the key enzyme in the production of these latter gangliosides, could be detected preceding the expression of these gangliosides and the FcepsilonRI by RT-PCR. Taken together, these data point to a functional role of gangliosides in the differentiation of human mast cells.