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Mutat Res. Author manuscript; available in PMC 2017 May 01.
Published in final edited form as:
Mutat Res. 2016 May ; 787: 43–53. doi:10.1016/j.mrfmmm.2016.02.009.
Radiation-induced changes in DNA methylation of repetitive
elements in the mouse heart
Igor Koturbasha,*, Isabelle R. Mioussea, Vijayalakshmi Sridharanb, Etienne
Nzabarushimanaa, Charles M. Skinnera, Stepan B. Melnykc, Oleksandra Pavlivc, Martin
Hauer-Jensenb,d, Gregory A. Nelsone, and Marjan Boermab
aDepartment
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of Environmental and Occupational Health, University of Arkansas for Medical
Sciences, Little Rock, Arkansas 72205
bDivision
of Radiation Health, Department of Pharmaceutical Sciences, University of Arkansas for
Medical Sciences, Little Rock, Arkansas 72205
cDepartment
of Pediatrics, University of Arkansas for Medical Sciences, Little Rock, Arkansas
72205
dSurgical
Service, Central Arkansas Veterans Healthcare System, Little Rock, AR 72205
eDepartments
of Basic Sciences and Radiation Medicine, Loma Linda University, Loma Linda,
California 92354
Abstract
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DNA methylation is a key epigenetic mechanism, needed for proper control over the expression of
genetic information and silencing of repetitive elements. Exposure to ionizing radiation, aside
from its strong genotoxic potential, may also affect the methylation of DNA, within the repetitive
elements, in particular. In this study, we exposed C57BL/6J male mice to low absorbed mean
doses of two types of space radiation – proton (0.1 Gy, 150 MeV, dose rate 0.53±0.08 Gy/min),
and heavy iron ions (56Fe) (0.5 Gy, 600 MeV/n, dose rate 0.38±0.06 Gy/min). Radiation-induced
changes in cardiac DNA methylation associated with repetitive elements were detected.
Specifically, modest hypomethylation of retrotransposon LINE-1 was observed at day 7 after
irradiation with either protons or 56Fe. This was followed by LINE-1, and other retrotransposons,
ERV2 and SINE B1, as well as major satellite DNA hypermethylation at day 90 after irradiation
with 56Fe. These changes in DNA methylation were accompanied by alterations in the expression
of DNA methylation machinery and affected the one-carbon metabolism pathway. Furthermore,
loss of transposable elements expression was detected in the cardiac tissue at the 90-day timepoint, paralleled by substantial accumulation of mRNA transcripts, associated with major
satellites. Given that the one-carbon metabolism pathway can be modulated by dietary
*
Corresponding author: Igor Koturbash, MD, PhD, ikoturbash@uams.edu.
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Declaration of interests
The authors declare no conflict of interests.
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modifications, these findings suggest a potential strategy for the mitigation and, possibly,
prevention of the negative effects exerted by ionizing radiation on the cardiovascular system.
Additionally, we show that the methylation status and expression of repetitive elements may serve
as early biomarkers of exposure to space radiation.
Keywords
space radiation; heart; one-carbon metabolism pathway; DNA methylation; repetitive elements
1. Introduction
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DNA methylation is a key epigenetic mechanism that plays a critical role during the
development and in the maintenance of cellular homeostasis [1]. It is involved in the
regulation of proper expression of genetic information in a sex-, tissue-, and cell typedependent manner, as well as in the silencing of repetitive elements. In turn, alterations in
methylation of DNA may lead to the development of pathological states, including heart
disease. For instance, dilated cardiomyopathy has been characterized by genome-wide
alterations in DNA methylation, including hypo- and hypermethylation of genes that play
functional roles in the pathways linked to heart failure [2,3]. Changes in gene-specific
methylation have also been described as molecular signatures of arrhythmia [4].
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Alterations in DNA methylation are not limited to specific genes but can also be detected at
repetitive elements, which comprise half to two-thirds of mammalian genomes [5,6]. They
are the most highly methylated sequences in mammalian genomes, and are represented as
transposable elements and satellite repeats [1,7]. Retrotransposons, such as Long
Interspersed Nucleotide Element 1 (LINE-1), Endogenous Retroviruses 1 and 2 (ERV1 and
ERV2) and Short Interspersed Nucleotide Elements B1 and B2 (SINE B1 and SINE B2), as
well as the transposons Mariner and Charlie are among the most abundant transposable
elements in mammalian genomes and together comprise ~40% of the mouse genome [8].
Satellite repeats are the centromere-associated repetitive sequences and are represented in
the mouse as major satellites (6 Mb of 234 bp units located primarily at the pericentromeric
regions) and minor satellites (~600 Kb of 120 bp units, located at centromeres) [9].
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Previously considered as junk DNA, repetitive elements are now accepted as important
regulators of stability and proper function of the genome, including expression of genetic
information and chromatin structure [5,6]. Loss of epigenetic control over repetitive
elements can result in unwanted alterations in their expression and the expression of genes
within their regulatory network, and has been reported in numerous pathological states,
including cardiac ischemic injury [10].
It is becoming increasingly evident that exposure to ionizing radiation, aside from its
genotoxic potential, may also affect the cellular epigenome [11–13]. Of particular interest
are the effects of space radiation, such as protons and heavy ions, since in some cases,
epigenetic alterations represent the only long-term signatures of exposure. For instance,
exposure to low mean absorbed doses of 56Fe (600 MeV, dose range 0.1 – 0.4 Gy), did not
lead to increased production of reactive oxygen species, DNA damage or alterations in
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cellular senescence and apoptosis in the murine hematopoietic progenitor and stem cells
(HPSCs). At the same time, exposure to the leukemogenic dose of 0.4 Gy of 56Fe has led to
altered global and repetitive elements-associated methylation of DNA and DNA methylation
machinery that were detectable in HSPCs for at least 5 months after irradiation [14].
Accumulating evidence indicates that alterations in DNA methylation is the general feature
of space radiation and that these alterations are primary attributable to repetitive elements.
Indeed, exposures to low absorbed mean doses of protons and 56Fe ions relevant to the space
environment lead to significant alterations in DNA methylation and expression of repetitive
elements in the bone marrow, liver and lung tissues [14–17]. As these changes in repetitive
elements may persist for a long time post exposure, this suggests their potentially causative
role in radiation-induced late tissue damage and disease development and progression.
Furthermore, such epigenetic parameters may potentially serve as universal biomarkers of
exposure.
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Epidemiological studies indicate that exposures even to low doses of radiation may result in
the development of heart diseases [18–20]. Recently, concerns were also raised in regards to
the effects of space radiation on the cardiovascular system [21]. This particular study, the
first of this kind to our knowledge, addressed the effects of protons and 56Fe ions on
repetitive elements-associated DNA methylation and expression in the mouse heart.
2. Materials and Methods
2.1 Animals and irradiation
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Male C57BL/6J mice, at 10 weeks of age (Jackson Laboratory, Bar Harbor, ME) were
shipped to Brookhaven National Laboratories (BNL) in Upton, NY. After a one-week
acclimation period, the mice were randomly assigned to experimental groups and were
exposed to sham-irradiation (n=8), 0.1 Gy protons (150 MeV, dose rate 0.53±0.08 Gy/min)
(n=9), or 0.5 Gy 56Fe (600 MeV/n, dose rate 0.38±0.06 Gy/min) (n=10). The dose of
protons was chosen as likely during a solar particle event (SPE). The energy of 150 MeV is
commonly used in a therapeutic setting and also represents energy near the maximum
abundance of protons expected in most SPEs [21]. The dose of 56Fe was selected as likely to
be encountered by astronauts due to the galactic cosmic rays during deep space exploration
[22] and within the 56Fe dose range used in the previous studies [15,23,24]. At the selected
energy of 600 MeV/n, thorough penetration of the animals with a relatively flat Bragg peak
entrance region is expected. Dosimetry was performed by the NASA Space Radiation
Laboratory (NSRL) physics dosimetry group at BNL to ensure the quality of exposure. For
each exposure, animals were individually placed into clear Lucite cubes (3 in × 1½ in × 1½
in) with breathing holes. The focused beam of high-energy 56Fe particles was generated by
the Booster accelerator at BNL and transferred to the experimental beam line at the NSRL
facility. Dose calibration was performed with three parallel plate ion chambers that were
positioned upstream of the target and a NIST traceable Far West thimble chamber. The
values of the thimble chamber were then compared with the upstream ion chambers so that
the desired dose could be delivered to the samples based on upstream ion chamber
measurements. Sham irradiated mice served as controls and were placed into the same
enclosures and for the same amount of time, since previous studies report no effect of sham
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irradiation on molecular end-points [14]. During the entire experiment, sham-irradiated mice
were not housed together with irradiated mice. After irradiation, the mice were shipped to
Loma Linda University (LLU) under climate-controlled conditions and were housed at LLU
under a constant 12 h light:dark cycle. Regular chow and water were provided ad libitum.
All procedures were approved by the Institutional Animal Care and Use Committees of LLU
and BNL.
2.2. Tissue harvesting
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At 7 days and 90 days after irradiation, groups of animals were sacrificed, and the heart was
obtained and snap-frozen in liquid nitrogen. Frozen tissues were shipped on dry ice to the
University of Arkansas for Medical Sciences for further molecular analyses and analysis of
components of methionine metabolism with HPLC-EC. The researchers were blinded
throughout all phases of the experiments; decoding only occurred after the final analyses
were performed.
2.3. Nucleic Acids Extraction
RNA and DNA were extracted simultaneously from flash-frozen cardiac tissue using the
AllPrep DNA/RNA extraction kit (Qiagen, Valencia, CA, USA) according to the
manufacturer’s protocol. DNA concentrations and integrity were analyzed by the Nanodrop
2000 (Thermo Scientific, Waltham, MA) and 1% agarose gel.
2.4. Analysis of methylation status of DNA repetitive elements by methylation-sensitive
quantitative PCR
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The methylation status of repetitive elements (LINE-1, ERV1, ERV2, SINE B1, SINE B2,
Charlie, Mariner, major and minor satellites) was determined by methylation-sensitive
McrBC-quantitative PCR (qPCR) as described before [14]. Primer sets are listed in
Supplementary Table 2 of the Supplementary Material.
2.5. Analysis of methylation status of the long interspersed nuclear element-1 repetitive
element by pyrosequencing
To determine the methylation status of LINE-1, total DNA was extracted, as described
above. Genomic DNA (1 μg) underwent bisulfite conversion, and pyrosequencing analysis
of LINE-1 ORF1 was performed on a PyroMark 96ID instrument.
2.6. Quantitative reverse transcription polymerase chain reaction (qRT-PCR)
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Total cardiac RNA was extracted as described above. Only RNA samples with the 260/280
ratios between 1.95 and 2.05 and the 260/230 ratios above 1.5 were considered for further
molecular analyses. cDNA was synthesized using random primers and a High Capacity
cDNA Reverse Transcription Kit (Applied Biosystems, Foster City, CA) according to the
manufacturer’s protocol (Life Technologies). The levels of gene transcripts were determined
by quantitative Real Time PCR (qRT-PCR) using TaqMan Gene Expression Assays (Life
Technologies). Assays for determination of mRNA abundance are provided in
Supplementary Table 1. Assays for determination of expression of repetitive elements are
provided in Supplementary Table 2. Each plate contained one experimental gene and a
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housekeeping gene. The cycle threshold (Ct) for each sample was determined from the linear
region of the amplification plot. The ΔCt values for all genes were determined relative to the
control gene Gapdh (Mm 99999915_g1, Life Technologies). The ΔΔCt were calculated
using each exposed group means relative to control group means. The fold change data were
calculated from the ΔΔCt values. All qRT-PCR reactions were conducted in triplicate and
repeated twice.
2.7. Determination of analytical components of methionine metabolism
Snap-frozen heart samples were used to determine levels of S-adenosylhomocysteine (SAH),
S-adenosylmethionine (SAM), methionine and adenosine using an HPLC-EC method.
Details of this method were previously published elsewhere [25].
2.8. Statistical analysis
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All data are presented as mean ± standard error of mean(s). All assessed parameters were
measured within the same batch of animals. For each repetitive element and time-point
combination, we analyzed DNA methylation and mRNA abundance in a one-way ANOVA
comparing among types of radiation. We compared control to protons and 56Fe with a t-test
in the ANOVA framework. We used Dunnett’s method to control Type I error 0.05 for the
two comparisons. Statistical analyses were performed using GraphPad Prism 6 (GraphPad
Software Inc. LaJolla, CA).
3. Results
3.1. Effects of protons and 56Fe irradiation on LINE-1 DNA methylation
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Retrotransposon LINE-1 is the most abundant repetitive element in the mammalian
genomes, comprising 20% and 21% of human and mouse genomes, respectively [8]. Its
genomic abundance and presence of a large number of CpG sites (sites that may be
potentially methylated) within its sequence have led to recognition of LINE-1 methylation
status as a surrogate biomarker in the evaluation of global DNA methylation levels [26].
Previous studies clearly demonstrated that exposure to low doses of both low- and high-LET
irradiation may affect the methylation status of LINE-1 in different cells and tissues [15,27–
30]. Therefore, we first addressed the methylation of LINE-1 Open Reading Frame 2
(ORF2) that encodes the reverse transcriptase needed for LINE-1 retrotransposition.
Methylation-sensitive McrBC-qPCR did not reveal significant alterations in DNA
methylation of LINE-1 ORF2 7 days after exposure (Figure 1A). We then addressed the
methylation status of LINE-1 ORF1 that encodes ORF1p, a nucleic acid chaperone, using
pyrosequencing, the most sensitive and robust technique in the analyses of locus-specific
DNA methylation. Analysis of 5 CpG sites within the ORF1 revealed minor, although
significant, hypomethylation after exposure to protons and 56Fe at the 7-day time-point
(−1.1% and −0.9%, respectively; p<0.01 for both exposure regimens).
At the 90-day time-point, hypermethylation was observed in the LINE-1 ORF2 after
exposure to 56Fe (although insignificant) and in LINE-1 ORF1 after exposure to protons
and 56Fe (+0.8% and +0.82%, respectively; p<0.05 for both exposure regimens) (Figure 1B).
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Studies indicate that high-LET radiation may affect the DNA methylation of selective
LINE-1 elements (Lima et al., 2014). Taking this into consideration, we further assessed the
DNA methylation in three LINE-1 elements that belong to A-type promoter-carriers –
LINE-1 elements that maintain their retrotransposition activity in mice – L1MdA_I (0.21
MYR), L1MdA_II (1.62 MYR) and L1MdA_VI (4.62 MYR). While we did not identify
significant changes in the DNA methylation in any of those LINE-1 elements at the 7-day
time-point, significant DNA hypermethylation was detected in the evolutionary young
L1MdA_I (2.4-fold increase; p<0.05 after 56Fe exposure) and L1MdA_II (7.1-fold increase;
p<0.05 after protons or 56Fe) at the 90-day time-point (Figure 2A). No significant changes in
DNA methylation of the evolutionary old L1MdA_VI LINE-1 element were identified at
any time-point (Supplementary Figure 1).
3.2. Effects of protons and 56Fe irradiation on other repetitive elements
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Recent studies demonstrated that alterations in the methylation status of repetitive elements
in response to environmental stressors are often non-unidirectional, and are rather
characterized by loci of hypo- and hypermethylation (reviewed in [8]. Therefore, next we
addressed the methylation of a panel of repetitive elements that belong to different classes –
autonomous retrotransposons ERV1 and ERV2, non-autonomous retrotransposons SINE B1
and SINE B2, DNA transposons Charlie and Mariner and satellite DNA – major and minor
satellites.
No significant changes were identified in the DNA methylation of ERV1 at both time-points
and a strong DNA hypermethylation was observed in ERV2 elements 90 days after exposure
(6.2-fold, p<0.001 and 4.3-fold, p<0.01 after protons and 56Fe, respectively) (Figure 2B).
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The 7-day time-point was characterized by a 1.7-fold increase in the methylation of the
SINE B1 retrotransposon in response to protons irradiation, although insignificant (p-0.061)
(Figure 2C). Methylation of the other elements was affected to a lesser extent. At the 90-day
time-point, a 1.4-fold (p<0.05) and 1.9-fold (p<0.01) hypermethylation in response to 56Fe
irradiation was observed in SINE B1 and major satellites, respectively (Figure 2C and
Figure 3B, respectively). Exposure to protons did not affect methylation of repetitive
elements substantially.
3.3. Effects of protons and 56Fe irradiation on the expression of repetitive elements
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Ionizing radiation has been shown to affect the expression of repetitive elements, including
exposure to low absorbed mean doses of protons and 56Fe ions (Miousse et al., 2014b;
Nzabarushimana et al., 2014; Nzabarushimana et al., 2015). At the 7-day time-point, a
significant 2.2-fold loss of LINE-1 ORF2 expression was detected after exposure to 56Fe
(p<0.05), in congruence with its modest hypomethylation (Figure 4A). Exposure to protons
resulted in an insignificant 1.8-fold decrease in the expression of LINE-1 (p-0.069). At the
90-day time-point, a 1.45-fold insignificant decrease in the expression of LINE-1 ORF2 was
observed in the heart after exposure to 56Fe (p=0.074).
Modest decreases in the expression of SINE B1, SINE B2, Mariner and Charlie were
observed at the 7-day time-point with somewhat more pronounced effects observed after
exposure to 56Fe (Figure 4B–D). At the same time, a nearly 2-fold reactivation of
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centromeric minor satellite repeats was detected, although insignificant (p=0.062). The 90day time-point was characterized by even more diverse changes in the expression of
repetitive elements. At this time, expression of all retrotransposons (SINE B1 and SINE B2)
and transposons (Mariner and Charlie) was significantly decreased in the heart tissue,
independently of radiation type. While the expression of major satellites was not affected
significantly, dramatic accumulation of mRNA transcripts associated with minor satellites
was observed (15.6- and 22.4-fold for proton and 56Fe, respectively; p=0.058 for protons
and p<0.05 for 56Fe).
3.4. Effects of 56Fe and protons on DNA methylation machinery
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Several mechanisms may contribute to radiation-induced alterations in DNA methylation
and expression of repetitive elements. Earlier studies reported that irradiation may affect the
mRNA levels and enzymatic activity of DNA methyltransferases, enzymes involved in the
regulation of normal patterns of DNA methylation [31]. In our study, at the 7-day time-point
very, subtle overexpression of Dnmt1 and Dnmt3a DNA methyltransferases was observed
(Figure 5). A more pronounced overexpression of the Uhrf1 gene, involved in recruiting
Dnmt1 to hemimethylated sites during replication, in response to protons exposure was
observed, although it was not statistically significant (2.94-fold, p=0.059). At the 90-day
time-point, a significant loss of Dnmt3a and Dnmt3b was observed after exposure to protons
(−1.7- and −1.8-fold, respectively, p<0.05), and 56Fe (−1.76- and −1.8-fold, with p<0.01
and <0.05, respectively).
3.5. Effects of 56Fe and protons on the one-carbon metabolism pathway
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Given that the aberrant expression of DNA methyltransferases could not explain the
observed alterations in the methylation of repetitive elements, next we addressed the tissue
levels of components involved in the one-carbon metabolism pathway, and metabolism of
methionine, in particular. The latter is needed for the synthesis of proteins and polyamines,
serves as a precursor for glutathione, and is needed for the synthesis of Sadenosylmethionine (SAM) – a major donor of methyl groups for DNA methylation.
Alterations in the one-carbon metabolism pathway may significantly affect DNA
methylation [32].
At the 7-day time point, we did not identify substantial changes in the tissue levels of
methionine or its down-stream product SAM. At the same time, exposure to both 56Fe and
protons were associated with modestly reduced levels of SAH and, subsequently, increased
SAM/SAH ratios (Table 1). These finding are in good agreement with the lack of changes in
DNA methylation at the 7-day time-point.
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At the 90-day time-point, the levels of both methionine and SAM were substantially
increased in cardiac tissue, suggesting that alterations in one-carbon metabolism pathway
are, at least in part, attributable to repetitive elements-associated DNA hypermethylation.
4. Discussion
A limited number of studies have examined the effects of space radiation on cardiac function
and structure in animal models. Exposure to protons (0.5 Gy, 1 GeV) and 56Fe (0.15 Gy, 1
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GeV/n) in a mouse model induced cardiac infiltration of CD68-positive cells, increased
DNA oxidation, myocardial fibrosis, and modified cardiac function, both at baseline and in
response to myocardial infarction, in a radiation-type specific manner [33,34]. Exposure
to 28Si (0.1–0.5 Gy, 300 MeV/n) caused prolonged apoptosis and increased expression of the
common pro-inflammatory cytokines IL-1β, IL-6, or TNF-α in a mouse model [35]. These
studies have convincingly demonstrated that exposure to low mean absorbed doses of space
radiation may significantly impact the cardiovascular system and lead to the development of
heart diseases.
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Epigenetic alterations are becoming increasingly recognized among the driving forces of
disease development and progression, including heart diseases. Genome-wide alterations in
DNA methylation are observed in dilated cardiomyopathy [2], and heart failure is
accompanied by altered methylation of genes important for cardiovascular function [3].
Alterations in the promoter methylation occurs in genes that are up-regulated in failing
hearts [36,37], providing evidence that epigenetic changes are involved in gene regulation in
cardiovascular disease.
Ionizing radiation is a potent epigenotoxic stressor that may significantly, and often for a
long-term or even permanently, affect the major epigenetic mechanisms of regulation –
methylation of DNA, histone modification and non-coding RNAs. In this study, we aimed to
examine the effects of low mean absorbed doses of two common types of space radiation
(0.1 Gy of protons, 150 MeV and 0.5 Gy of 56Fe, 600 MeV/n) on cardiac DNA methylation
in a mouse model.
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In this analysis, we included retrotransposons (capable of autonomous propagation –
LINE-1, ERV1, and ERV2 and non-autonomous elements SINE B1 and SINE B2), DNA
transposons (Charlie and Mariner), and satellite repeats (major and minor satellites). These
repetitive elements comprise about one third of the mouse genome and are usually heavily
methylated in order to prevent their aberrant activity [8].
First, we analyzed the methylation of LINE-1 ORF2 using the conventional methylationsensitive RT-PCR. We did not identify significant changes in the methylation of LINE-1
after exposure to protons or 56Fe at either time-point. Utilization of a more robust and
accurate approach for locus-specific DNA methylation – pyrosequencing – allowed us to
identify modest hypomethylation of the cardiac LINE-1 ORF1 at the 7-day time-point,
independently of the type of exposure. These findings are in good agreement with previous
studies that reported global and LINE-1-specific hypomethylation early after exposure to
ionizing radiation [11,29].
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This hypomethylation at the early time-point, was followed by hypermethylation of LINE-1
ORF1 at the 90-day time-point. This DNA hypermethylation stemmed from the evolutionary
young LINE-1 elements, while DNA methylation in evolutionary old elements remained
unchanged. Similar patterns were also observed in the recent study by Lima et al, where
time-dependent fluctuations in LINE-1 methylation were observed after exposure to 56Fe
within the same tissue [15]. Together, these studies provide evidence that space-radiation
induced changes in DNA methylation are dynamic and may vary depending on the time after
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irradiation. At the same time, based on accumulating evidence [16] and the results of the
current study, it seems that relatively early post-irradiation time-points are characterized by
global and repetitive elements-associated DNA hypomethylation, while later time points are
characterized by DNA hypermethylation.
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Recent studies clearly indicate that alterations in DNA methylation induced by
environmental stressors are not unidirectional and can be characterized by loci of both hypoand hypermethylation [38]. Therefore, next we addressed the DNA methylation status of
ERV1, ERV2, SINE B1, SINE B2, Charlie, Mariner, major and minor satellites in the
murine hearts. While minor and non-significant alterations were observed at the 7-day timepoint, modest hypermethylation, similar to hypermethylation of LINE-1, was observed in the
methylation of repetitive elements at the 90-day time-point. This DNA hypermethylation
was more pronounced after exposure to 56Fe, and affected primarily ERV2, SINE B1,
Mariner and major satellites. This finding is in good agreement with our previous
observation of delayed DNA hypermethylation in repetitive elements in the murine lung
after exposure to low mean absorbed doses of 56Fe [16].
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To investigate the mechanisms of radiation-induced changes in DNA methylation, we first
addressed the expression of a panel of genes, involved in the regulation and maintenance of
normal DNA methylation patterns. Although the 7-day time-point was characterized by
hypomethylation of repetitive elements, minor increases in the expression of Dnmt1 and
Dnmt3a methyltransferases, as well as Uhrf1 were observed. DNA methyltransferases and
Uhrf1 are involved in radiation-induced DNA repair [39] and may hence be up-regulated in
response to DNA damage. Interestingly, a recent study indicates that overexpression of
Uhrf1 drives DNA hypomethylation, suggesting another possible mechanism for the
observed LINE-1 hypomethylation at an early time point [40]. While significant
hypermethylation was observed in repetitive elements at the 90-day time-point, DNA
methyltransferases exhibited unidirectional trends towards their decreased expression.
Because similar trends were observed in the expression of Mecp2, the enzyme involved in
binding methyl groups to DNA, this may be potentially perceived as a compensatory
mechanism in response to DNA hypermethylation. Further studies will be clearly needed to
investigate this effect and its consequences.
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Because methionine is required for the biosynthesis of SAM, the major methyl donor used
by methyltransferases for DNA methylation, the methionine metabolism pathway is closely
linked to DNA methylation status. In our study, at the 7-day time-point, only a modest
decrease in the cardiac tissue levels of SAH were detected, in congruence with the lack of
substantial changes in DNA methylation and DNA methylation machinery. The 90-day timepoint was characterized by substantially increased levels of methionine and SAM, and, with
that, an associated increase in SAM/SAH ratio. These findings suggest that the delayed
effects of space irradiation are associated with the increased synthesis of the donors of
methyl groups which may explain the observed hypermethylation within the repetitive
elements.
At the same time, other mechanisms of the observed radiation-induced alterations in DNA
methylation cannot be excluded. For instance, recent advances in understanding the role of
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5-hydroxymethylation and methyl-deoxygenases may provide further knowledge in the
process of DNA demethylation. In this study, we addressed the expression of Tet1, the major
deoxygenase involved in the conversion of 5-methylcytosine into 5-hydroxymethylcytosine.
While at the 7-day time-point non-significant increases in the expression of Tet1 were
observed in cardiac tissue, congruent with weak hypomethylation at this time, significant
decreases were observed at the 90-day time-point (−2.2-fold, p<0.01 for both, protons
and 56Fe) (Figure 6A), which was associated with hypermethylation of repetitive elements.
This finding clearly indicates that the mechanisms of radiation-induced alterations in DNA
methylation are complex and may involve multiple pathways.
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DNA methylation plays a critical role in the silencing of repetitive elements [1] and
observed alterations in their methylation status may result in the aberrant expression of
repetitive elements. In this study, loss of LINE-1 expression was detected after exposure to
protons or 56Fe despite its modest hypomethylation at the 7-day time-point and was still
detectable at the 90-day time-point. Furthermore, the mRNA levels of other transposable
elements, including SINE B1, SINE B2, Charlie, and Mariner were also decreased after
irradiation. This indicates that other mechanisms, such as histone modifications, may be
involved in their silencing. Indeed, the expression of the histone deacetylase Hdac2, one of
the major regulators of transposable elements expression [41], was decreased in response to
protons or 56Fe exposure, although insignificantly (Fig. 6B). The studies that will explore
further the interplay between DNA methylation and histone modifications in the regulation
of transposable elements expression are warranted.
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Observed alteration in the methylation and expression of transposable elements may
significantly affect the cellular epigenome, alter the expression of genetic information and
potentially lead to disease development. It has been shown that retrotransposons like LINE-1
and SINE can influence the expression of adjacent gene promoters [42–44]. Furthermore,
SINEs have been shown to function as insulators, facilitating the expression of numerous
genes [45]. Decreased expression of LINE-1 in cardiomyocytes is associated with reduced
oxidative stress [10].
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The most pronounced changes, however, were detected in the expression of repetitive
elements that are transcribed from the satellite repeats in centromeric (minor satellites) and
pericentromeric (major satellites) regions and are involved in the formation of
heterochromatin [46–48]. Increases in their expression may significantly impact the
chromatin status, centromeric cohesion and dissociation during chromosome segregation
[46,47]. A recent study has shown that a pronounced (up to 27-fold) increase in satellite
repeat transcripts was a common feature of all examined human failing hearts when
compared to normal hearts [49]. It remains unknown whether this satellite-associated
aberrant transcription is one of the mechanisms or consequences of the disease. Our data
indicate that transcriptional activation of satellite repeats may occur as an early response to
irradiation (7-day time-point), and may persist over time (90-day time-point), suggesting
that the accumulation of satellite DNA mRNA transcripts may serve as one of the driving
forces of the disease, rather than being merely a consequence.
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Koturbash et al.
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In conclusions, we report that exposure to low absorbed mean doses of protons and 56Fe ions
lead to non-unidirectional alterations in the methylation and expression of repetitive
elements. We further show that space radiation affects the one-carbon metabolism pathway,
and these changes, at least in part, contribute to alterations in DNA methylation. Given that
the one-carbon metabolism pathway can be modulated by dietary modifications, these
findings suggest a potential strategy for the mitigation and, possibly, prevention of the
negative effects exerted by space radiation on the cardiovascular system. Additionally, we
show that the methylation status and expression of repetitive elements may serve as early
biomarkers of exposure to space radiation. Further studies, especially those that investigate
the effects of combined exposures to low mean absorbed doses of protons and heavy ions
(conditions most relevant to the space environment) as well as time- and dose-dependent
correlations between space radiation and DNA methylation endpoints are clearly needed and
are being currently performed at our laboratories.
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Supplementary Material
Refer to Web version on PubMed Central for supplementary material.
Acknowledgments
We are thankful to Dr. Kristy Kutanzi for the critical reading and editing of the manuscript. This work was
supported by National Aeronautics and Space Administration [NNX10AD59G]; National Space Biomedical
Research Institute [RE03701 through NCC 9-58]; the National Institutes of Health [CA148679 to MB, R37
CA71382 to MHJ, 1P20GM109005, and Clinical and Translational Science Award UL1TR000039 and
KL2TR000063], US Veterans Administration, the Arkansas Biosciences Institute (ABI), ABI Grant for Core
Metabolomics Laboratory, and ACH Foundation Grants for studies of metabolic changes in children with Autism
and patients with Down Syndrome.
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Highlights
•
Radiation-induced dynamic changes in cardiac DNA methylation were detected
•
Early LINE-1 hypomethylation was followed by hypermethylation at a later
time-point
•
Radiation affected one-carbon metabolism in the heart tissue
•
Irradiation resulted in accumulation of satellite DNA mRNA transcripts
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Figure 1.
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Effects of space radiation on the DNA methylation status of retrotransposon LINE-1 in the
mouse heart. (A) Methylation of LINE-1 ORF2, as measured by methylation-sensitive realtime PCR. (B) Methylation of LINE-1 ORF1, as measured by pyrosequencing. Data are
presented as mean ± SE. *p≤0.05, **p≤0.01, ANOVA with Dunnett’s test.
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Figure 2.
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Effects of space radiation on the DNA methylation status of transposable elements in the
mouse heart, as measured by methylation-sensitive real-time PCR. (A) LINE-1 elements
L1MdA_I and L1MdA_VI; (B) autonomous retrotransposons ERV1 and ERV2; (C) nonautonomous retrotransposons SINE B1 and SINE B2. Data are presented as mean ± SE.
*p≤0.05, **p≤0.01, ***p≤0.001, ANOVA with Dunnett’s test.
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Figure 3.
Effects of space radiation on the DNA methylation status of transposable elements and
satellite DNA in the mouse heart, as measured by methylation-sensitive real-time PCR. (A)
DNA transposons Charlie and Mariner; (B) major and minor satellites. Data are presented as
mean ± SE. *p≤0.05, **p≤0.01, ANOVA with Dunnett’s test.
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Figure 4.
Effects of space radiation on the expression of repetitive elements in the mouse heart, as
measured by quantitative real-time PCR. (A) Retrotransposons LINE-1; (B) SINE B1, and
SINE B2; (C) DNA transposons Charlie and Mariner; (D) major and minor satellites. Data
are presented as mean ± SE. *p≤0.05, **p≤0.01, ANOVA with Dunnett’s test.
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Figure 5.
Effects of space radiation on the expression of DNA methylation machinery, as measured by
quantitative real-time PCR. Data are presented as mean ± SE. *p≤0.05, **p≤0.01, ANOVA
with Dunnett’s test.
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Figure 6.
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Effects of space radiation on the expression of (A) methyl-deoxygenase Tet1 and (B) histone
deacetylase Hdac2, as measured by quantitative real-time PCR. Data are presented as mean
± SE. *p≤0.05, **p≤0.01, ANOVA with Dunnett’s test.
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Table 1
7 days post-irradiation
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*
90 days post-irradiation
Metabolite
Control
Protons
56Fe
Control
Protons
56Fe
Methionine
0.45 ± 0.01
0.44 ± 0.027
0.52 ± 0.05
0.35 ± 0.11
0.48 ± 0.05
0.50 ± 0.04
SAM
1.02 ± 0.07
1.09 ± 0.07
1.14 ± 0.13
0.68 ± 0.09
0.85 ± 0.07*
0.93 ± 0.04*
SAH
0.43 ± 0.04
0.33 ± 0.01
0.30 ± 0.026*
0.30 ± 0.01
0.27 ± 0.01
0.30 ± 0.05
SAM/SAH
2.38 ± 0.16
3.30 ± 0.17*
3.82 ± 0.25*
2.30 ± 0.23
3.20 ± 0.34
3.40 ± 0.57
Adenosine
2.90 ± 0.49
1.76 ± 0.17
2.30 ± 0.36
0.71 ± 0.10
0.40 ± 0.05
0.51 ± 0.12
Koturbash et al.
Methionine metabolism in hearts of mice exposed to 0.1 Gy of protons, 0.5 Gy of 56Fe, or control treatment (nmol/mg protein, mean ± SEM).
p<0.05 compared to sham-irradiated control
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