Epigenetics

BackgroundPregnancy in women with multiple sclerosis (MS) is associated with a reduction of long-term disability progression. The mechanism that drives this effect is unknown, but converging evidence suggests a role for epigenetic mechanisms altering immune and/or central nervous system function. ObjectivesWe aimed to identify whole blood and immune cell-specific DNA methylation patterns associated with parity in relapse-onset multiple sclerosis. MethodsWe compared whole-blood methylation patterns between 96 matched pairs of nulligravida and parous females with MS (n=192). Parity was defined as at least one term or pre-term birth, and nulligravida was defined as no prior pregnancies. Methylation was measured with Illumina EPIC arrays, and data was pre-processed and statistically analysed using the ChAMP package. Cell-type proportions were estimated using the EpiDISH package, and cell-specific analysis conducted using linear regression. Gene-set enrichment analysis (GSEA) was performed with ToppGene API and GOmeth. Methylation age was calculated with the methyAge package. Methylation age acceleration (MAA) was calculated by regressing methylation age on chronological age. FDR<0.05 was used to assess significance. ResultsThe median time from last pregnancy to blood collection was 16.66 years (range = 1.45 - 44.42 years). We identified 903 differentially methylated positions (DMPs) in whole blood; 365 were hypomethylated and 528 were hypermethylated in parous women. We further identified two differentially methylated regions (DMRs) in CRYGN on Chromosome 7 and an intergenic region on Chromosome 15. There were four and eight cell type specific DMPs in CD4+ and CD8+ cells, respectively. Differentially methylated genes were enriched in neuronal plasticity pathways. Parity was associated with reduced MAA by a mean of 1.44 to 2.27 years using the PhenoAge (p = 0.002) and GrimAge (p = 0.005) algorithms. ConclusionWhole-blood methylation patterns are associated with birth history in females with relapse-onset multiple sclerosis. We found enrichment of differentially methylated genes encoding neuronal processes and reduced MAA in parous women. These methylation changes could mediate the long-term benefit of pregnancy for disease progression in multiple sclerosis.

The underlying mechanism of the HIV-1 reservoir, a major barrier to an HIV cure, is largely unknown. The integration of HIV-1 DNA and immune defense mechanisms can disrupt the host epigenetic landscape, potentially silencing HIV-1 replication. Using bisulfite capture DNA methylation sequencing, we profiled approximately 3.2 million CpG sites in CD4+ T cells isolated from the blood of 427 virally suppressed women with HIV. The average total CD4+ T cell HIV-1 Reservoir (HRCD4) size was 1,409 copies per million cells. Most proviruses were defective with only a small proportion being intact. We found 245 differentially methylated positions (CpG sites) and 85 methylated regions associated with the total HRCD4 size. Notably, 52% of significant methylation sites were in intronic regions. HRCD4-associated genes were involved in viral replication (e.g., ISG15), HIV-1 latency (e.g., MBD2), and cell growth and apoptosis (e.g., IRF9). A subset of the identified genes with aberrant methylation was an established target of HIV-1 integration (e.g., NFIA, SPPL3, DLEU2, ELMSAN1). Overall, HRCD4 size was inversely associated with DNA methylation of interferon signaling genes and positively associated with methylation at established HIV-1 integration sites. HRCD4-associated genes were enriched in pathways including immune defense against the virus (i.e., interferon- response and interferon-{gamma} response), DNA binding transcription repression, and host-virus interaction such as Tau protein binding. Together, our results show that epigenomic alterations in CD4+ T cells are associated with total HIV-1 reservoir size, offering new insights into HIV-1 latency and may provide potential molecular targets for future HIV-1 eradication strategies.

DNA methylation plays a critical role in the dynamics of gene expression regulation and the development of various disorders. Whole-genome bisulfite sequencing can provide single base resolution of CpG methylation levels and is the "gold standard" for DNA methylation quantification, but it also has a high cost. In contrast, targeted sequencing is optimal when focusing on specific candidate regions, while providing sufficient sequencing depth. Here, we present a targeted bisulfite sequencing approach to study the methylation status of regions of interest. We amplify selected regions from bisulfite-treated DNA and sequence them using Nanopore sequencing. In this work, we applied this workflow to candidate gene promoters for severe premature labor in a Latin American population. We successfully amplified fragments over 1 Kb in length using long PCR conditions for 12 genes that were barcoded per sample and pooled to be sequenced on MinION flow cells. This approach achieved high sequencing depths, ensuring reliable DNAm estimation. We found significant hypomethylation of the MIR155HG gene promoter in severe preterm birth samples, which is concordant with reported gene expression changes. We demonstrate that combining bisulfite DNA treatment with pooled long-read sequencing is a cost- and time-effective method to evaluate DNAm in several targeted regions and several samples in parallel. This study provides proof-of-concept for larger studies, demonstrating the applicability and high scalability of our assay to any locus of interest. Our experience suggests that this approach can be easily transferred to other diagnostic questions.

The benefits of breastfeeding for the health and wellbeing of both infants and mothers are well documented, yet global breastfeeding rates are low. One factor associated with low breastfeeding is maternal body mass index (BMI), which is used as a measure of obesity. The negative relationship between maternal obesity and breastfeeding is likely caused by a variety of social, psychological, and physiological factors. Maternal obesity may also have a direct biological association with breastfeeding through changes in maternal DNA methylation. Here, we investigate this potential biological association using data from a UK-based cohort study, the Avon Longitudinal Study of Parents and Children (ALSPAC). We find that pre-pregnancy body mass index (BMI) is associated with lower initiation to breastfeed and shorter breastfeeding duration. We conduct epigenome-wide association studies (EWAS) of maternal BMI and breastfeeding outcomes and candidate-gene analysis of methylation sites associated with BMI identified via previous meta-EWAS. We find that DNA methylation at cg11453712, annotated to PHTP1, is associated with maternal BMI. From our results, neither this association nor those at candidate-gene sites are likely to mediate the link between maternal BMI and breastfeeding.

BackgroundOsteoporosis is a prevalent bone metabolic disorder characterized by reduced bone mass, disruption of bone microarchitecture, and increased bone fragility, leading to a heightened risk of fracture. This condition significantly impairs patients quality of life and increases mortality risk. Emerging evidence suggests that DNA methylation may play a crucial role in regulating the expression of genes related to bone metabolism, thereby influencing the development of osteoporosis. However, the precise relationship between DNA methylation and osteoporosis remains unclear and warrants further investigation. ResultsOur study revealed significant differences in both the quantity and ratio of DNA methylation between individuals with osteoporosis and non-osteoporosis controls, with differences predominantly occurring in CpG islands. GO/KEGG enrichment analyses highlighted distinct osteoporosis-related gene pathways. Notably, we identified six genes, MSX1, HOXD4, AXIN2, WNT5A, TGFB1, and STAT3, respectively, that are potentially involved in the pathogenesis of osteoporosis and are broadly involved in various diseases and biological processes. ConclusionsThese findings indicate distinct methylation patterns between osteoporosis patients and healthy individuals, with differential methylation levels in genes associated with osteoporosis. This research offers new insights into the epigenetic mechanisms underlying osteoporosis.

Changes in the sperm epigenome induced by age and/or stressors often follow common unexplained patterns affecting genes responsible for embryonic development and neurodevelopment. The stochastic epigenetic variation (SEV) hypothesis proposes that in response to stressors naturally variable methylation regions (VMRs) associated with morphogenic genes increase in methylation variation to diversify phenotypes and improve chances of survival of the genetic lineage. Here, we test predictions from the SEV hypothesis using mouse and rat sperm DNA methylation and other -omics data. We demonstrate that the context of DNA regions determines the response of sperm methylome to various factors rather than the stressors and/or timing of these factors. We propose a model explaining age/stress-dependent shifts in methylation in VMRs by an asymmetric increase in methylation variation of these regions. Because methylation variation in VMRs increases with age, sperm methylome response to stressors may be characterized as an acceleration of epigenetic aging.

Infinium Methylation BeadChip arrays remain one of the most popular platforms for epigenome-wide association studies, but tools for downstream pathway analysis have their limitations. Functional class scoring (FCS) is a group of pathway enrichment techniques that involve the ranking of genes and evaluation of their collective regulation in biological systems, but the implementations described for Infinium methylation array data do not retain direction information, which is important for mechanistic understanding of genomic regulation. Here, we evaluate several candidate FCS methods that retain directional information. According to simulation results, the best-performing method involves the mean aggregation of probe limma t-statistics by gene followed by a rank-ANOVA enrichment test using the mitch package. This method, which we call "LAM", outperformed an existing over-representation analysis method in simulations, and showed higher sensitivity and robustness in an analysis of real lung tumour-normal paired datasets. Using matched RNA-seq data we examine the relationship of methylation differences at promoters and gene bodies with RNA expression at the level of pathways in lung cancer. To demonstrate the utility of our approach, we apply it to three other contexts where public data were available. Firstly, we examine differential pathway methylation associated with chronological age. Secondly, we investigate pathway methylation differences in infants conceived with in vitro fertilisation. Lastly, we analyse differential pathway methylation in 19 disease states, identifying hundreds of novel associations. These results show LAM is a powerful method for the detection of differential pathway methylation as compared to existing methods. A reproducible vignette is provided to illustrate how to implement this method.

BackgroundResearching the murine epigenome in disease models has been hampered by the lack of an appropriate and cost-effective DNA methylation array. Until recently, investigators have been limited to the relatively expensive and analysis intensive bisulphite sequencing methods. Here, we performed a comprehensive, comparative analysis between the new Mouse Methylation BeadChip (MMB) and reduced representation bisulphite sequencing (RRBS) in two murine models of colorectal carcinogenesis, providing insight into the utility to each platforms in a real world environment. ResultsWe captured 1.47x106 CpGs by RRBS and 2.64x105 CpGs by MMB, mapping to 13,778 and 13,365 CpG islands, respectively. RRBS captured significantly more CpGs per island (median 41 for RRBS versus 2 for MMB). We found that 64.4% of intra-island CpG methylation variability can be captured by measuring approximately one quarter of CpG island (CGI) CpGs. MMB was more precise in measuring DNA methylation, especially at sites that had low RRBS coverage. This impacted differential methylation analysis, with more statistically significantly differentially methylated CpG sites identified by MMB in all experimental conditions, however the difference was minute when appropriate thresholding for the magnitude of methylation change (0.2 beta value difference) was applied, providing confidence that both techniques can identify similar differential DNA methylation. Gene ontology enrichment analysis of differentially hypermethylated gene promoters identified similar biological processes and pathways by both RRBS and MMB across two murine model systems. ConclusionMMB is an effective tool for profiling the murine methylome that performs comparably to RRBS, identifying similar differentially methylated pathways. Although MMB captures a similar proportion of CpG islands, it does so with fewer CpGs per island. We show that subsampling informative CpGs from CpG islands is an appropriate strategy to capture whole island variation. Choice of technology is experiment dependent and will be predicated on the underlying biology being probed.

BackgroundAccumulating evidence suggests prenatal air pollution exposure alters DNA methylation (DNAm), which could go on to affect long-term health. However, it remains unclear whether prenatal DNAm alterations persist through early life. Identifying DNAm changes that persist from birth into childhood would provide greater insight into the molecular mechanisms that most likely contribute to the association of prenatal air pollution exposure with health outcomes such as atopic disease. ObjectivesThis study investigated the persistence of DNAm changes associated with prenatal NO2 exposure (a surrogate measure of traffic-related air pollution) at age one to begin characterizing which DNAm changes most likely to contribute to atopic disease. MethodsWe used an atopy-enriched subset of CHILD study participants (N=145) to identify individual and regional cord blood DNAm differences associated with prenatal NO2, followed by an investigation of persistence in age one peripheral blood. As we had repeated DNAm measures, we also isolated postnatal-specific DNAm changes and examined their association with NO2 exposure in the first year of life. MANOVA tests were used to examine the association between DNAm changes associated with NO2 and child wheeze and atopy. ResultsWe identified 24 regions of altered cord blood DNAm, with several annotated to HOX genes. Two regions annotated to MPDU1 and C5orf63 were significantly associated with age one wheeze. Further, we found the effect of prenatal NO2 exposure across CpGs within all altered regions remained similar at age one. A single region of postnatal-specific DNAm annotated to HOXB6 was associated with year one NO2 and age one atopy. DiscussionRegional cord blood DNAm changes associated with prenatal NO2 exposure persist through at least the first year of life, and some of these changes are associated with age one wheeze. The early-postnatal period remains a sensitive window to DNAm perturbations that may also influence child health.

BackgroundMigration from one environment to another often causes marked changes in developmental conditions. Here we compare epigenetic ageing and stability of the epigenetic maintenance system among British-Bangladeshi women who grew up in Bangladesh (adult migrants), where there are higher pathogen loads and poorer health care, to second-generation Bangladeshis who grew up in the UK. In our previous studies of these migrants, those who spent their childhoods in Bangladesh also had lower levels of reproductive hormones and a shorter reproductive lifespan compared to those who grew up in the UK, suggesting life history trade-offs during development. In the present study, we hypothesised that women who grew up in Bangladesh would have i) an older epigenetic/biological age compared to the women with a childhood in the UK and ii) that differences in the pace of epigenetic ageing might also be reflected by altered stability of DNA methylation marks. ResultsIllumina EPIC array methylation data from buccal tissue was used to establish epigenetic age estimates from 15 adult migrants and 11 second-generation migrants, aged 18-35 years. Using residuals from linear regression of DNA methylation-based biological age (DNAm age) on the chronological age, the results showed significant differences (p=0.016) in epigenetic age estimates: women whose childhood was in Bangladesh are on average 6.02 ({+/-} 2.34) years older, than those who grew up in London. We further investigated the efficiency of the epigenetic maintenance system which purportedly is reflected by epigenetic clocks. Methylation states of CpGs at the LHCGR/LHR locus, which contributes to Horvaths multi tissue epigenetic clock were evaluated. Based on the Ratio of Concordance Preference (RCP) approach that uses double-stranded methylation data, we find that maintenance of epigenetic information is more stable in women who grew up in Bangladesh. ConclusionsThe work supports earlier findings that adverse childhood environments lead to phenotypic life history trade-offs. The data indicate that childhood environments can induce subtle changes to the epigenetic maintenance system that are detectable long after exposure occurred. The implication of such a finding warrants further investigation as it implies that a less flexible epigenetic memory system established early in life could reduce the capacity to respond to different environmental conditions in adult life.

Non-syndromic cleft lip/palate (NSCLP), the most common human craniofacial malformations, is a complex disorder given its genetic heterogeneity and multifactorial component revealed by genetic, epidemiological and epigenetic findings. Association of epigenetic variations with NSCLP has been made, however still of little functional investigation. Here we combined a reanalysis of NSCLP methylome data with genetic analysis and used both in vitro and in vivo approaches to dissect the functional effects of epigenetic changes. We found a frequent differentially methylated region in mir152, hypomethylated in NSCLP cohorts (21-26%), leading to mir152 overexpression. In vivo analysis using zebrafish embryos revealed that mir152 upregulation leads to craniofacial impairment analogue to palatal defects. Also, we demonstrated that zebrafish embryonic hypoxia leads to mir152 upregulation combined with mir152 hypomethylation and also analogue palatal alterations. We therefore suggest mir152 hypomethylation, potentially induced by hypoxia in early development, as a novel and frequent predisposing factor to NSCLP.

The prevalence of many diseases differs by sex, potentially due to sex-specific patterns in DNA methylation. Autosomal sex-specific differences in DNA methylation have been observed in cord blood and placental tissue, but are not well studied in saliva or in diverse populations. We sought to characterize sex-specific DNA methylation on autosomal chromosomes in saliva samples from children in the Fragile Families and Child Wellbeing Study, a multi-ethnic prospective birth cohort containing an oversampling of Black, Hispanic and low-income families. DNA methylation from saliva samples were analyzed on 796 children at both ages 9 and 15 with DNA methylation measured using the Illumina HumanMethylation 450k array. An epigenome-wide association analysis of the age 9 samples identified 8,430 sex-differentiated autosomal DNA methylation sites at age 9 (P < 2.4x10-7), of which 76.2% had higher DNA methylation in female children. The strongest sex-difference was in the cg26921482 probe, in the AMDHD2 gene, with 30.6% higher DNA methylation in female compared to male children (P < 1x10-300). Treating the age 15 samples as an internal replication set, we observed highly consistent results between the age 9 and age 15 measurements, indicating stable and replicable sex-differentiation. Further, we directly compared our results to previously published DNA methylation sex differences in both cord blood and saliva and again found strong consistency. Our findings support widespread and robust sex-differential DNA methylation across age, human tissues, and populations. These findings help inform our understanding of potential biological processes contributing to sex differences in human physiology and disease.

Preeclampsia (PE) is a severe pregnancy complication that threatens maternal and neonatal health. Previous epigenome-wide association studies (EWAS) on PE have produced inconsistent results, possibly due to inadequate adjustment for confounders. Here, we analyzed DNA methylation changes in cord blood from newborns affected by PE, using a multi-ethnic cohort from Hawaii. We comprehensively adjusted for clinical variables (maternal age, BMI, parity) and estimated cell proportions. Additionally, we re-analyzed two public datasets with similar adjustments and conducted a meta-analysis combining all three datasets to increase statistical power. To further address confounding by gestational age, we also included idiopathic preterm samples as controls. After adjusting for cell type proportions and clinical characteristics, all previously reported significant CpG methylation changes associated with severe PE disappeared across our data, the two public datasets, and the meta-analysis. This result remained even after including idiopathic preterm samples. Instead, severe PE was associated with shifts in CD8T and natural killer (NK) cell proportions. We validated this lack of CpG changes using multiple published cord blood methylation datasets. Moreover, we observed that gestational progression itself is accompanied by significant changes in granulocyte, nRBC, CD8T, and B cell proportions. In summary, our study demonstrates that many previously reported DNA methylation changes in severe PE are artifacts caused by confounding factors such as cell type heterogeneity and gestational age. Severe PE is associated with changes in cell proportions rather than direct methylation alterations. These findings emphasize the importance of rigorous confounder adjustment in EWAS.

Antenatal corticosteroids (ACS) are provided to improve perinatal survival when there is risk of preterm birth. Though evidence suggests increased risk of developing neurobehavioural disorders in exposed offspring, the mechanisms that mediate this relationship remain largely unknown. Here, we investigated the DNA methylation patterns in the prefrontal cortex (PFC) of exposed offspring. We hypothesized that differential methylation will be evident at both newborn and juvenile ages. Pregnant guinea pigs were administered saline or betamethasone (1mg/kg) on gestational days 50/51 to mimic a single course of ACS. gDNA was isolated from the PFC of term-born offspring on postnatal day 1 (PND1) and PND14 to identify differentially methylated CpG sites (DMCs) using reduced representative bisulfite sequencing. In the PND1 PFC, 1521 DMCs, annotating to 145 genes were identified following ACS. Identified genes were involved in pathways regulating developmental cellular process. In the PND14 PFC, 776 DMCs representing 46 genes were identified, and were enriched in synaptic signalling pathways. Though no individual DMCs were identified at both PND1 and PND14, differential methylation was consistently observed at the binding sites of transcription factors PLAGL1, TFAP2C, ZNF263, and SP1 at both ages. In this study, we identified an altered DNA methylome in the PFC of ACS-exposed guinea pig offspring at both newborn and juvenile ages. Notably, a unique methylation signature was consistently observed at four key transcription factor binding sites at multiple post-natal time points, indicating a persistent change which may predispose the development of altered neurobehavioural phenotypes that have been described in exposed offspring.

BackgroundHuman DNA is known to exhibit an overall tendency toward demethylation with aging. However, assuming a simple linear relationship between DNA methylation and age does not align with the phenotype of human development and the aging process. This study aimed to investigate the existence of DNA methylation patterns with peaks or troughs at specific ages in addition to simple linear changes. MethodsA large-scale dataset of genome-wide DNA methylation data from 10,420 individuals was analyzed. Hierarchical multiple regression models were applied to detect patterns of the association between age and DNA methylation: linear increase, linear decrease, U-shaped curve, and inverse U-shaped curve. ResultsAmong the 864,627 CpG sites analyzed, 8.4% exhibited an increase in DNA methylation with age, 23.9% showed a decrease, and 5.5% were better explained by a quadratic model (P < 5.7815x10 ). Within the non-linear subset, inverse U-shaped CpG sites peaking in methylation during middle age were predominant. Genes exhibiting quadratic association patterns between DNA methylation and age, and those linked to diseases with common onset during middle age, were also detected. ConclusionsNon-linear age-related DNA methylation patterns, with peaks or troughs occurring at specific ages, were detected. This suggests that humans do not simply age linearly, but that programmed mechanisms or cascade-like processes may exist to promote or suppress the expression of specific genes at certain ages, contributing onset of certain diseases at specific timings.

Sepsis survivors frequently develop long-term immune dysfunction, but the epigenetic mechanisms underlying persistent myeloid suppression remain unclear. Myeloid-derived suppressor cells (MDSCs), whose function is shaped by host age and sex, are key contributors to post-sepsis immune dysregulation. Here, we present a high-resolution epigenetic map targeting gene promoters of MDSCs after sepsis using MAPit-FENGC, a single-molecule assay that simultaneously profiles DNA methylation and chromatin accessibility. In a clinically relevant murine model including young and older adult male and female mice, splenic MDSCs were isolated for MAPit-FENGC and single-cell RNA sequencing. Unsupervised clustering identified nine promoter classes reflecting chromatin dynamics: age- and sex-dependent sepsis-induced opening (Classes 1-4), persistent closure with varying levels of DNA methylation (Classes 5-7), and constitutive openness post-sepsis (Classes 8, 9). Transcriptomic profiling corroborated these promoter states, linking accessibility with gene expression. These findings establish how epigenetic reprogramming of MDSCs may shape age- and sex-specific immune trajectories in sepsis survivors.

DNA methylation (DNAm), capturing biological gestational age (GA) and epigenetic gestational age acceleration (EGAA), can be modified by environmental exposures. The Asthma&Allergy array is a new DNAm array developed with content focused on asthma and allergy loci. The association between content on the Asthma&Allergy array and chronological GA and EGAA has not been evaluated alone or in the context of perinatal exposures. We performed an epigenome wide association study(EWAS) based on chronological GA at single CpG sites and regions. We further constructed a multi-CpG site methylation model to predict chronological GA in cord blood from 391 newborn children from a Detroit-based birth cohort. Associations between perinatal environmental factors with GA, epigenetic gestational age (EGA), and EGAA were assessed. We identified 2,435 CpG sites associated with chronological GA. HLA class II (HLA-DRB1,HLA-DQB1,HLA-DRB6) were the most significantly associated with chronological GA. Our multi-CpG site model attained predictive accuracy (cross-validated Pearsons correlation=0.75) comparable to other EGA methods. Using genes implicated in region-based analyses (n=395 regions), the pathways most significantly enriched with chronological GA-associated CpGs included T helper 1(Th1) and 2(Th2) activation, macrophage classical activation, and IL10 signaling, which were also enriched in at least one of the other published epigenetic clocks. In multi-exposure models, prenatal indoor pet exposure and unplanned C-section were associated with EGA deceleration, while infants first-born status was associated with EGAA. Our findings highlight enrichment for T cell modulated pathways and antigen presentation as biological processes enriched in chronological GA, as well as novel perinatal factors that may impact EGAA.

DNA methylation within CpG islands is a key epigenetic mechanism regulating gene expression. SHANK3 encodes a synaptic scaffolding protein essential for neurodevelopment and synaptic function, and aberrant SHANK3 methylation has been implicated in neuropsychiatric disorders. To enable reliable locus-specific investigation of SHANK3 epigenetic regulation, we developed a high-resolution melting (HRM) assay. In silico screening identified a CpG-rich region upstream of exon 3 as the most suitable locus for assay design. Bisulfite-converted sequences representing fully methylated and unmethylated states were used to generate three primer sets, of which two successfully amplified the target region and produced distinct melting profiles discriminating methylated from unmethylated templates. The assay was optimized on two HRM platforms (LightCycler(R) 480 and CFX96), and conversion efficiency was confirmed with commercial control DNAs. This locus-specific HRM assay provides a methodological framework for qualitative SHANK3 methylation analysis and represents a promising tool for future validation studies and potential clinical investigations in neurodevelopmental and neuropsychiatric disorders.

The level and the biological significance of mitochondrial DNA (mtDNA) methylation in human cells is a controversial topic. Using long-read third-generation sequencing technology, mtDNA methylation can be detected directly from the sequencing data, which overcomes previously suggested biases, introduced by bisulfite treatment-dependent methods. We investigated mtDNA from whole blood-derived DNA and established a workflow to detect CpG methylation with Nanopolish. In order to obtain native mtDNA, we adjusted a whole-genome sequencing protocol and performed ligation library preparation and Nanopore sequencing. To validate the workflow, 897bp of methylated and unmethylated synthetic DNA samples at different dilution ratios were sequenced and CpG methylation was detected. Interestingly, we observed that reads with higher methylation in the synthetic DNA did not pass Guppy calling, possibly affecting conclusions about DNA methylation in Nanopore sequencing. We detected in all blood-derived samples overall low-level methylation across the mitochondrial genome, with exceptions at certain CpG sites. Our results suggest that Nanopore sequencing is capable of detecting low-level mtDNA methylation. However, further refinement of the bioinformatical pipelines including Guppy failed reads are recommended.

Rejuvenation of tissues in physiologically aging mice can be accomplished by long-term partial reprogramming via expression of reprogramming factors (Oct4, Sox2, Klf4 and c-Myc). To investigate the epigenetic determinants of partial reprogramming-mediated rejuvenation, we used whole genome bisulfite sequencing to carry out unbiased comprehensive profiling of DNA methylation changes in skin from mice subjected to partial reprogramming, as well as young and untreated old controls. We found a striking convergence of age- and rejuvenation-related epigenetic alterations on targets of the Polycomb repressive complex 2 (PRC2). These results are also supported by a likewise prominent enrichment of PRC2 targets in gene expression data, suggesting that PRC2 activity can modulate aging and mediate tissue rejuvenation.