Preprocessing and harmonization of multi-omics data before integration. Covers normalization, batch correction, feature alignment, and missing value handling across data types. Use when preparing multi-omics datasets for integration analysis.
Reference examples tested with: MultiAssayExperiment 1.36+, SummarizedExperiment 1.40+, sva 3.50+.
Before using code patterns, verify installed versions match. If versions differ:
packageVersion('<pkg>') then ?function_name to verify parametersIf code throws ImportError, AttributeError, or TypeError, introspect the installed package and adapt the example to match the actual API rather than retrying.
Deep per-omic normalization (DESeq2 VST internals, methylation noob/BMIQ, proteomics VSN) is owned by the per-omic categories and pinned to their own tool versions; this skill calls the cross-omic container and batch tools, so MultiAssayExperiment and sva are the binding versions here.
"Get my omics onto a common footing for integration" -> Transform each block to a comparable scale, equalize block contribution, decide a single batch strategy, and triage missingness - because the integrator sees one stacked matrix of numbers and spends its first factors on whichever block has the largest raw variance, which is a property of the assay, not the biology.
MultiAssayExperiment to coordinate assays, sample map, and colDataScope: cross-omic harmonization of already-normalized blocks - container assembly, transform choice, per-view scaling, batch strategy, missing-data triage. Deep single-omic normalization -> differential-expression, methylation-analysis, proteomics, metabolomics. The method-selection decision -> integration-design. Fusion math -> mofa-integration, mixomics-analysis. Single-omic RNA batch mechanics -> differential-expression/batch-correction.
A factor model sees one stacked matrix and spends its leading factors on whichever block has the largest raw variance and the most non-Gaussian structure - a property of the measurement scale, not the biology. Harmonization is the act of making the variance budget mean something biological before the model gets to it. Every choice is therefore a silent vote on which omic dominates the shared latent space and which signal is allowed to appear:
The deliverable is not "the harmonized data" - it is a documented chain of transform -> scaling -> batch -> missing-value triage, each justified, because each silently determines the integration result.
| Omic | Raw-data pathology | Transform for integration | Why |
|------|--------------------|---------------------------|-----|
| RNA-seq (bulk) | NB counts; variance grows with mean; loads on high-count genes | VST or rlog (DESeq2), or log-CPM (voom) | variance-stabilize to approximately homoscedastic and Gaussian (Love 2014); blind=TRUE for unsupervised integration |
| Methylation | beta in [0,1]; variance compressed near 0 and 1 | M-value = log2(beta/(1-beta)) for modeling; beta for interpretation | logit un-compresses the extremes -> homoscedastic (Du 2010) |
| Proteomics / metabolomics | positive, right-skewed, multiplicative error; MNAR below detection | log2, then MNAR-aware imputation | log makes multiplicative error additive and Gaussian (Lazar 2016) |
| Microbiome / compositional | simplex, fixed-sum; spurious negative correlation | CLR after zero replacement | maps the simplex to Euclidean space; raw proportions invalid for L2 methods (Gloor 2017) -> metagenomics/abundance-estimation |
| ALL, after the above | blocks differ in total variance and feature count | per-VIEW scaling (MFA singular-value weighting; MOFA scale_views) | equalize block contribution without inflating individual features |
The transform is per-omic (owned by the per-omic categories); the scaling is cross-block (owned here). They are sequential and both required - scaling a heteroscedastic block does not make it homoscedastic.
| Tool | Citation | Role | When | |------|----------|------|------| | MultiAssayExperiment | Ramos 2017 Cancer Res 77:e39 | container: assays + sampleMap + colData | always - makes sample linkage a structural invariant, not string-munging | | sva ComBat | Johnson 2007 Biostatistics 8:118 | empirical-Bayes batch adjustment (transformed data) | scrub batch PER OMIC when the integrator needs clean input | | ComBat-seq | Zhang 2020 NAR Genom Bioinform 2:lqaa078 | batch adjustment on RNA-seq COUNTS | batch-correct counts before VST, not after | | limma removeBatchEffect | Ritchie 2015 Nucleic Acids Res 43:e47 | regress out batch for visualization | ordination/QC plots only - never feed to an inferential test | | sva / RUV | Leek 2007 PLoS Genet 3:e161; Risso 2014 Nat Biotechnol 32:896 | estimate hidden/unwanted variation as covariates | batch is unknown or driven by control genes | | imputeLCMD / DEP | Lazar 2016 J Proteome Res 15:1116 | QRILC/MinProb (MNAR) + kNN (MAR) imputation | below-detection proteomics/metabolomics gaps |
| Scenario | Recommended | Why |
|----------|-------------|-----|
| Blocks on different scales going into MOFA/PLS | per-omic transform then per-view scaling | the integrator assumes comparable, homoscedastic inputs |
| One omic has far more features than another | per-view scaling and/or filter the wider view harder | feature count buys variance votes; equalize block contribution |
| Batch differs across omics, integrator needs clean input | ComBat PER OMIC (ComBat-seq for counts) | scrub once per modality; never stack blocks and ComBat together |
| Downstream step is an inferential test | model batch as a covariate (~ batch + condition) | scrub-then-test exaggerates confidence (Nygaard 2016) |
| Batch correlates with the condition | do NOT correct; redesign or report the confound | confounded batch is irrecoverable; correcting deletes biology |
| Proteomics missing below detection (MNAR) | QRILC / MinProb (imputeLCMD) | MAR imputation biases low-abundance proteins upward |
| Sporadic within-feature gaps (MAR) | kNN / missForest | local borrowing is valid when missingness is random |
| Some samples missing a whole omic (mosaic) | -> mofa-integration (models the missingness) | do not impute a whole block; intersecting loses scarce n |
| Need the method decision, not the prep | -> integration-design | which integration paradigm fits the question |
Goal: Make sample linkage across omics a structural invariant and quantify how mosaic the cohort is, so the impute-versus-model-the-missingness decision is explicit.
Approach: Build a MultiAssayExperiment from per-omic SummarizedExperiments; the sampleMap (assay, primary, colname) links assay columns to subjects. intersectColumns gives complete cases (subjects), which is what integration wants; intersectRows aligns features and is a trap on heterogeneous omics.
library(MultiAssayExperiment)
rna <- SummarizedExperiment(assays=list(vst=vst_rna), colData=sample_info) # already VST-normalized
prot <- SummarizedExperiment(assays=list(log2=norm_prot), colData=sample_info) # already log2 + median-normalized
meth <- SummarizedExperiment(assays=list(mval=m_values), colData=sample_info) # already M-values
mae <- MultiAssayExperiment(experiments=ExperimentList(RNA=rna, Protein=prot, Methylation=meth),
colData=sample_info)
table(complete.cases(mae)) # subjects with every omic
paired <- intersectColumns(mae) # complete-case fallback; counts the n it costs
Goal: Stop the highest-variance or highest-dimensional omic from hijacking the shared factors without inflating noise features.
Approach: Filter near-constant features per block, then scale each block so its blocks contribute comparably. Per-view scaling divides a whole block by its total variance/first singular value; reserve per-feature unit-variance scaling for inside PLS and only after filtering low-variance features.
drop_constant <- function(mat, min_sd=1e-8) mat[apply(mat, 1, sd, na.rm=TRUE) > min_sd, ] # per-feature scaling blows up zero-variance features
scale_per_view <- function(mat) mat / sqrt(sum(apply(mat, 1, var, na.rm=TRUE))) # whole-block scaling: equalizes contribution, preserves within-block feature ratios
blocks <- lapply(list(RNA=vst_rna, Protein=norm_prot, Methylation=m_values), drop_constant)
blocks <- lapply(blocks, scale_per_view)
sapply(blocks, function(x) sum(apply(x, 1, var))) # each block now contributes comparably
Goal: Remove technical batch once, in one place, without deleting biology or double-correcting.
Approach: First cross-tabulate batch against the biological variable; if they are collinear, stop - the effect is unrecoverable. Otherwise correct PER OMIC (ComBat on transformed data, ComBat-seq on counts) when the integrator needs clean input, OR model batch as a covariate inside the downstream step - never both, and never on a stacked multi-omic matrix.
library(sva)
with(as.data.frame(colData(mae)), table(Batch, Condition)) # confounding gate: any empty cell = collinear -> do NOT correct
mod <- model.matrix(~ Condition, data=as.data.frame(colData(mae))) # protect biology
vst_rna_bc <- ComBat(dat=vst_rna, batch=colData(mae)$Batch, mod=mod, par.prior=TRUE) # ONE omic at a time
Stacking RNA, protein, and methylation into one matrix and running a single ComBat (with omic-type as the batch or a covariate) is a failure mode, not a recipe: it treats cross-omic differences as noise and erases the very signal the integration is meant to find. MOFA's multi-group group= is also not batch correction - it asks whether the same factors operate within each group, and does not regress batch out.
Goal: Match the imputation to the missingness mechanism, and never fabricate a whole assay.
Approach: Separate the three regimes - missing features (filter), missing values within a feature (impute by mechanism: MNAR below detection vs MAR sporadic), and whole missing samples (do not impute; use a missing-tolerant integrator). Proteomics/metabolomics missingness is largely MNAR (a peptide is absent because it is low), so MAR methods bias it upward.
keep <- rowMeans(is.na(norm_prot)) < 0.30 # drop features missing in >30% of samples before imputing
prot_f <- norm_prot[keep, ]
library(imputeLCMD)
prot_mnar <- impute.QRILC(prot_f)[[1]] # left-censored draw for below-detection (MNAR) gaps
For a mosaic cohort (a subject profiled for RNA and methylation but not proteomics), do not impute the missing proteomics profile - MOFA2 ignores missing entries in its likelihood and tolerates incomplete views natively, so route that case to mofa-integration rather than fabricating a block that would manufacture cross-omic correlation.
Trigger: stacking raw counts, betas, and z-scores into one factorization. Mechanism: the model assumes comparable, homoscedastic, roughly-Gaussian features. Symptom: the leading factors are dominated by the highest-dynamic-range block; small blocks never surface. Fix: per-omic transform then per-view scaling, in that order.
Trigger: mixOmics scale=TRUE (its default) on unfiltered blocks. Mechanism: a near-constant feature's tiny SD is divided out, blowing it up to unit variance. Symptom: a factor built from technical jitter at the detection floor. Fix: filter near-constant features before any per-feature scaling; prefer per-view scaling for cross-block equalization.
Trigger: ComBat with batch correlated with (or equal to) the condition. Mechanism: the batch and biology terms are collinear, so the correction has no way to separate them. Symptom: the condition effect vanishes after correction. Fix: cross-tabulate batch x condition first; if collinear, do not correct - the design cannot be rescued post hoc.
Trigger: ComBat per omic AND a batch term inside the integration/DE step. Mechanism: the second correction operates on residuals already partly stripped. Symptom: over-shrunk effects and inflated confidence. Fix: correct once, in one place; prefer modeling for inferential steps, scrubbing only when the tool cannot accept a covariate.
Trigger: kNN/missForest on below-detection proteomics gaps. Mechanism: MAR methods borrow toward the observed mean, but the missing values are low by definition. Symptom: low-abundance proteins biased upward; the low-abundance biology is erased. Fix: QRILC/MinProb for MNAR entries, kNN/missForest only for sporadic MAR gaps.
Trigger: top-N HVG / variable-feature selection per omic applied without justification. Mechanism: an integration model can only place a feature on a latent factor if the feature is in the input, so the cutoff pre-commits which axes of variation can be discovered. Symptom: two analysts with different cutoffs fit different models and get different factors; a real low-variance signal silently cannot appear. Fix: treat the filter as a modeling choice - declare and justify it, filter the wider views harder to equalize dimensionality, and confirm the headline factors are stable across a sensible cutoff range.
Trigger: imputing an entire missing omic profile so DIABLO/CCA will run. Mechanism: the imputed block is reconstructed from the other omics. Symptom: "discovered" cross-omic correlation that the imputation manufactured. Fix: use MOFA2 (models the missingness) or restrict to overlapping samples; never fabricate a whole assay.
Trigger: exporting an MAE assay to Python, or joining omics on gene symbols. Mechanism: Bioconductor is samples-in-columns while AnnData/mixOmics are samples-in-rows; symbols are non-unique and IDs map one-to-many. Symptom: genes treated as samples (silent), or duplicated/lost rows after a merge. Fix: assert matrix shape after every cross-language hop; join on Ensembl/UniProt/RefMet, and decide the collapse rule for ambiguous mappings explicitly.
| Threshold | Source | Rationale | |-----------|--------|-----------| | Per-view scaling (not per-feature) for cross-block equalization | Escofier 1994 Comput Stat Data Anal 18:121 (MFA) | equalizes block contribution without inflating individual noise features | | Near-constant feature filter (sd > ~1e-8) before per-feature scaling | mixOmics docs | per-feature unit-variance scaling blows up zero-variance features | | Feature missingness filter ~30-50% before imputing | Lazar 2016 J Proteome Res 15:1116 | a feature missing in most samples cannot be imputed reliably | | Batch x condition cross-tab must have no empty cell before ComBat | Nygaard 2016 Biostatistics 17:29 | an empty cell means batch and biology are collinear and inseparable | | Model batch as covariate (not scrub) for inferential steps | Nygaard 2016 Biostatistics 17:29 | scrub-then-test understates variance and exaggerates significance | | M-values (not beta) for methylation modeling | Du 2010 BMC Bioinformatics 11:587 | beta is heteroscedastic; the logit is approximately homoscedastic |
| Error / symptom | Cause | Solution | |-----------------|-------|----------| | One omic dominates every shared factor | no per-view scaling / unequal feature counts | per-view scale; filter the wider view harder | | A factor is built from low-signal features | per-feature scaling without filtering | drop near-constant features before scaling | | Condition effect disappears after batch correction | batch confounded with condition | check the cross-tab; do not correct collinear designs | | Low-abundance proteins look unexpectedly high | MAR imputation on MNAR gaps | QRILC/MinProb for below-detection values | | Cross-omic correlation that does not replicate | whole-sample imputation | use MOFA missing-view handling; do not fabricate a block | | Genes appear as samples after export | orientation flip across languages | transpose; assert shape after every hop |
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