Cell-type heterogeneity can be a major source of confounding and reverse causation in epigenome-wide association studies (EWAS). Adjustment for cell-type composition is therefore critical for an improved interpretation and understanding of EWAS.
For a given study, the best choice of cell-type deconvolution algorithm depends not only on the tissue and phenotype of interest but also on the presence of other confounders and the desired output.
Most variation in DNA methylation (DNAm) is driven by genetic factors and cell-type heterogeneity, with corresponding features — methylation quantitative trait loci (mQTLs) and cell-type-specific differentially methylated cytosines (DMCs) — readily identifiable using linear modelling.
Identification and interpretation of DNAm changes that accrue with age or exposure to environmental disease risk factors may benefit from differential variance statistics.
Analysing patterns of covariation in DNAm at regulatory elements can help to identify disrupted regulatory networks and gene modules in disease.
The inverse association between DNAm at regulatory elements and transcription factor binding can be exploited to elucidate the functional role of non-coding genome-wide association study (GWAS) single-nucleotide polymorphisms (SNPs) or functional effects caused by exposure to environmental disease risk factors.
Mendelian randomization can help to clarify the role of DNAm as a causal mediator between exposure to risk factors and disease.
Epigenetics plays a key role in cellular development and function. Alterations to the epigenome are thought to capture and mediate the effects of genetic and environmental risk factors on complex disease. Currently, DNA methylation is the only epigenetic mark that can be measured reliably and genome-wide in large numbers of samples. This Review discusses some of the key statistical challenges and algorithms associated with drawing inferences from DNA methylation data, including cell-type heterogeneity, feature selection, reverse causation and system-level analyses that require integration with other data types such as gene expression, genotype, transcription factor binding and other epigenetic information.
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The authors declare no competing financial interests.
- Bisulfite conversion
A technique in which DNA is treated with bisulfite, resulting in modification (upon amplification) of unmethylated cytosines into thymines, whereas methylated cytosines are protected from modification.
- Epigenome-wide-association studies
(EWAS). A study design that seeks associations between DNA methylation at many sites across the genome and an exposure, trait or disease of interest.
- Intra-sample normalization
The procedure of adjusting the raw data profile of a biological sample for technical biases and artefacts. This is often followed by inter-sample normalization, in which adjustments are made to the data for technical and biological factors that otherwise cause unwanted (and often confounding) data variation across samples.
When the relationship between an exposure and an outcome is not causal but is due to the effects of a third variable (the confounder) on the exposure and the outcome. White blood cell heterogeneity can act as a confounder in many epigenetic studies.
- Feature selection
The statistical procedure of identifying features which, in some broad sense, correlate with an exposure or phenotype of interest (POI).
- Differentially methylated cytosines
(DMCs). Cytosines (usually in a CpG context) that exhibit a statistically significant difference in DNA methylation between two groups of samples, according to some statistical test.
- Condition number
In the context of reference-based cell-type deconvolution, the condition number of a reference matrix represents an index of the numerical stability of the inference. Formally, it measures the sensitivity of the regression parameters (also known as cell weights) to small perturbations or errors in the reference matrix.
- Constrained projection
(CP). Also known as quadratic programming (QP). A widely used technique for performing multivariate linear regression with constraints (such as non-negativity and normalization) imposed on the regression coefficients. In the context of cell-type deconvolution, the coefficients correspond to cell-type proportions in a sample. By definition, these proportions are non-negative, and their sum must be ≤1.
- Beta distributions
The distributions of beta values. The beta value is a statistical term used to describe the quantification of DNA methylation at a given cytosine, as the ratio of methylated alleles to the total number of alleles (methylated + unmethylated), a number that by definition must lie between 0 (fully unmethylated) and 1 (fully methylated).
- Surrogate variable analysis
(SVA). A widely used technique for selecting features associated with a factor of interest, which is not confounded by other factors. SVA uses a model to identify the data variation that is orthogonal to the factor of interest and subsequently uses principal component analysis (PCA) on this orthogonal variation matrix to construct 'surrogate variables', which in theory should capture confounding sources of variation.
- Phenotype of interest
(POI). The factor or variable of interest in an epigenome-wide association study (EWAS). This factor is often binary, representing case–control status, but could also represent an ordinal variable (for example, genotype) or be continuous (for example, age).
- Blind source separation
(BSS). The problem of inferring the sources of variation gives rise to a data matrix without using any prior information ('blind'). Algorithms that can achieve this are called BSS algorithms, of which independent component analysis (ICA) is one example.
- Independent component analysis
(ICA). An unsupervised dimensionality reduction algorithm that decomposes the data matrix into a sum of linear components of variation, which are as statistically independent from each other as possible. Statistical independence is a stronger condition than the linear uncorrelatedness of principal component analysis (PCA) components, allowing improved modelling of sources of variation in complex data.
- Principal component analysis
(PCA). An unsupervised dimensionality reduction algorithm that decomposes the data matrix into a sum of linear principal components (PCs) of variation, ranked by decreased variance and uncorrelated to each other.
- Latent components
Components or sources of data variation that are 'hidden' (or latent) and that are inferred from the data using an unsupervised algorithm.
Of statistical inferences, using the phenotype of interest from the outset, for instance, when identifying features correlating with a phenotype.
- Variably methylated cytosines
(VMCs). Cytosines (usually in a CpG context) that exhibit a significant amount of variance in DNA methylation, as assessed across independent samples and relative to other CpG sites.
Of a statistical distribution or of a random sample thereof, the expected variance, or spread, being dependent on the mean.
- Logit transformation
A mathematical transformation that takes values defined on the unit interval (0,1) (for example, beta values (β)) into values defined on the open interval (−∞,+∞), termed M-values. Mathematically, M = log2[β/(1 − β)].
- Methylation quantitative trait loci
(mQTLs). CpG sites whose DNA methylation level is correlated with a single-nucleotide polymorphism (SNP). If the SNP occurs close to the CpG (for instance, within a 10 kb window), it is called cis-mQTL, otherwise trans-mQTL.
- Differentially variable cytosines
(DVCs). Cytosines (usually in a CpG context) that exhibit a statistically significant difference in the variance of DNA methylation between two groups of samples, according to some statistical test.
- Field defects
Genetic or epigenetic alterations that are thought to predate the development of cancer and that are usually seen in the normal tissue found adjacent to cancer.
- Type 1 error rate
The probability of erroneously calling the result of a test significant (positive) when the underlying true hypothesis is the null. It corresponds to the fraction of true negatives that are called positive, also known as the false-positive rate.
- Variably methylated regions
(VMRs). Contiguous genomic regions where DNA methylation is highly variable relative to a normal 'ground state'. A VMR can be defined for one given sample.
- Differentially variable regions
(DVRs). Contiguous genomic regions containing a statistically significant number of differentially variable cytosines (DVCs). This is different from a variably methylated region (VMR) in that a DVR is derived by comparing a fairly large number of cases and controls.
- Gene set enrichment analysis
(GSEA). A widely used statistical procedure to assess whether a derived gene list of interest is enriched for specific biological terms, usually including gene ontologies, signalling pathways, specific transcriptomic signatures or targets of gene regulators.
- System epigenomics
An emerging field whereby cellular phenotypes in normal development and disease are modelled as complex systems, using tools from complexity science (for example, dynamical system theory or statistical physics) to understand them.
A phenomenon that occurs when a genetic variant is associated with multiple traits. Vertical pleiotropy occurs where the traits are all on the same pathway (and is generally less of a problem), whereas horizontal pleiotropy exists where a genetic variant is associated with multiple traits via separate pathways.
- Expression quantitative trait loci
(eQTLs). Genes whose expression levels are correlated with single-nucleotide polymorphisms (SNPs). If the SNP occurs near (definitions vary, but it could range from 10 kb to a 1 Mb window centred on the transcription start site) the gene, it is called a cis-eQTL; otherwise, it is a trans-eQTL.
- TF hubs
In the context of a regulatory network where edges represent regulatory interactions between transcription factors (TFs) and target genes, those TFs with the largest number of interactions.
- Expression quantitative trait methylation loci
(eQTMs). Genes whose expression levels are correlated with the DNA methylation level of a CpG. If the CpG occurs close to the gene (within a 250 kb window), it is called a cis-eQTM.
A multi-dimensional array with the number of dimensions often called the 'order' or 'rank' of the tensor and for which linear decomposition algorithms are available, analogous to linear matrix factorization algorithms for data matrices. Scalars, vectors and matrices are tensors of order 0, 1 and 2, respectively.
- Mendelian randomization
A technique to estimate the effect of an exposure on an outcome using genetic variants and instrumental variables for the exposure. This approach can also be applied to assessing mediation.
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Teschendorff, A., Relton, C. Statistical and integrative system-level analysis of DNA methylation data. Nat Rev Genet 19, 129–147 (2018). https://doi.org/10.1038/nrg.2017.86
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