Version Compatibility

Reference examples tested with: eCLIP pipeline (Yeo lab), chimeric eCLIP analysis scripts (Manakov 2022), HEAP pipeline (Li 2020), Hyb pipeline (Travis 2014), TargetScanHuman 8.0, miRDB 6.0, samtools 1.19+, bedtools 2.31+, pyHyb 0.4+.

Before using code patterns, verify installed versions match. If versions differ:

• Python: pip show <package> then help(module.function) to check signatures
• CLI: <tool> --version then <tool> --help to confirm flags

If code throws unexpected errors, introspect the installed package and adapt the example to match the actual API rather than retrying.

AGO-CLIP and miRNA Target Identification

"Identify direct miRNA-target interactions experimentally" -> Use Argonaute (AGO1-4) CLIP-seq variants to map miRNA-binding sites on mRNAs, then resolve which miRNA pairs with each site. Three approaches: (a) standard AGO-CLIP recovers AGO-bound sites but cannot say which miRNA; (b) chimeric methods (CLEAR-CLIP, chimeric eCLIP / miR-eCLIP) ligate the miRNA to its target during library prep, producing miRNA-mRNA chimeric reads that unambiguously assign miRNA-target pairs; (c) HEAP uses HaloTag-Ago2 for in vivo profiling. The chimeric methods are the gold standard for direct miRNA-target identification; standard AGO-CLIP must be combined with computational seed-matching (TargetScan, miRDB) to infer miRNA pairing. Resolution: chimeric reads pinpoint single miRNA-target pairs; AGO-only CLIP identifies "AGO-binding sites" of which a subset are miRNA targets.

• CLI (chimeric eCLIP / miR-eCLIP processing): custom pipeline starting from eCLIP-style preprocessing + chimeric-read identification + miRNA-mRNA junction extraction
• CLI (CLEAR-CLIP custom Moore 2015 pipeline): Hyb (Travis 2014) for chimera analysis
• CLI (Hyb pipeline): hyb run_hyb peaks.bam mature_miRNA.fa human.tab.gz to find miRNA-mRNA chimeras
• CLI (HEAP analysis): standard HITS-CLIP processing pipeline + Halo-Ago2 capture details
• Python (seed-pairing analysis on AGO CLIP peaks): scan peaks for canonical 7mer-m8, 7mer-1A, 8mer, 6mer seeds + 3' UTR position + miRNA expression filter

The Yeo lab miR-eCLIP / chimeric eCLIP (Manakov 2022) is the modern depth-improved version of chimeric AGO-CLIP, enriching for chimeras of specific miRNAs of interest (30-175x enrichment via PCR or on-bead probe capture). For comprehensive miRNA-target mapping, miR-eCLIP combined with eCLIP-seq-style normalization is the state-of-the-art.

Methods Taxonomy

Methodology evolves; verify the Manakov 2022 / Bracken 2024 papers for current chimeric eCLIP best practice. As of 2024, miR-eCLIP is the canonical approach for deep miRNA-target profiling.

Critical Choice: Chimeric vs Computational miRNA-Target Pairing

Two fundamentally different strategies:

Chimeric methods (CLEAR-CLIP, chimeric eCLIP / miR-eCLIP, CLASH): During library prep, a ligation step covalently joins the miRNA to its target mRNA, producing chimeric reads (miRNA at 5' + target mRNA at 3'). The miRNA-target pair is read directly from the sequence. Pro: direct evidence of binding interaction; no inference. Con: chimera rate is 1-5% of library by default (enriched to 30-175x with miR-eCLIP for specific miRNAs); deep sequencing or enrichment needed.

Computational pairing (HITS-CLIP / PAR-CLIP + seed-matching): Standard AGO CLIP identifies AGO-bound peaks; downstream computational scanning matches each peak against canonical miRNA seeds (7mer-m8, 7mer-1A, 8mer) from TargetScan, miRDB, or DIANA databases. Pro: any AGO CLIP data can be analyzed; no special library prep. Con: indirect; assigns miRNAs based on canonical seed rules, missing non-canonical interactions (3' compensatory, central pairing).

The Bartel lab CLEAR-CLIP analysis revealed substantial 3' auxiliary pairing beyond canonical seeds: ~50% of miRNA-target interactions have weak or non-canonical seed matching but strong 3' supplementary pairing. Chimeric methods recover these; computational seed-only inference misses them.

miRNA-Target Pairing Rules

Computational seed-matching against TargetScan / miRDB requires understanding the canonical miRNA-target pairing rules:

For TargetScan integration: download the TargetScanHuman 8.0 conserved-site predictions; filter for 7mer-8mer (drop 6mer if too noisy); cross-reference with the CLIP peak BED of the analysis.

Chimeric eCLIP / miR-eCLIP Workflow

Goal: Recover miRNA-mRNA chimeras from AGO chimeric eCLIP / miR-eCLIP libraries and produce a per-miRNA target list suitable for direct biological interpretation.

Approach: Apply eCLIP-style preprocessing, then run Hyb in chimera (type=mim) mode with bowtie2 alignment (required for short 21-23 nt miRNA sequences), filter chimeras to human mRNA targets, intersect with miRNA-expression atlas (filter > 100 TPM in matched cell type), and validate top targets against TargetScan conserved 7mer-m8 / 8mer predictions.

# Step 1: eCLIP-style preprocessing (see clip-seq/clip-preprocessing)
umi_tools extract --bc-pattern=NNNNNNNNNN \
    --stdin=R1.fq.gz --read2-in=R2.fq.gz \
    --stdout=R1.umi.fq.gz --read2-out=R2.umi.fq.gz

cutadapt -a AGATCGGAAGAGCACACGTCT -A AGATCGGAAGAGCGTCGTGTAGGGAAAGAGTGT \
    -q 6 -m 18 -o R1.trim.fq.gz -p R2.trim.fq.gz \
    R1.umi.fq.gz R2.umi.fq.gz

# Step 2: Chimera-specific alignment
# Chimeric reads have miRNA sequence (21-23 nt) at 5' followed by target mRNA
# Step 2a: Trim 5' for miRNA portion + align miRNA part
# Step 2b: Trim 3' for target mRNA portion + align target part
# Use custom chimeric-eCLIP pipeline OR Hyb (Travis 2014)

# Hyb pipeline approach (CLEAR-CLIP and chimeric methods)
hyb \
    in=R1.trim.fq.gz \
    db=miRNA_and_human_mRNA.fa \
    align=blastall \
    type=mim   # multimer (miRNA-target) chimera mode

# Output: .blast and .hyb files with miRNA-mRNA chimera coordinates

# Step 3: Filter chimeras by miRNA + target alignment quality
# Hyb reports each chimera as: miRNA_id  target_id  miRNA_alignment  target_alignment
# Filter for:
#   - miRNA portion 18-25 nt
#   - Target portion 18-50 nt
#   - miRNA-target seed match (7mer-m8 / 7mer-A1 / 8mer)
#   - Target alignment unique
awk '$5 == "human_mRNA"' chimeras.hyb > chimeras_human_mRNA.tsv

# Step 4: Aggregate chimeras into per-miRNA target list
# Each miRNA -> targets (with read counts as binding affinity proxy)
awk '{print $3, $4}' chimeras_human_mRNA.tsv | sort | uniq -c | sort -rn > mirna_target_counts.tsv

CLEAR-CLIP (Moore 2015) Analysis

CLEAR-CLIP was the first method to recover ~130k miRNA-target chimeras from mouse brain (Moore 2015). The analytical insight: AGO-CLIP reads ligated together during library prep produce chimeric reads at low rates that contain unambiguous miRNA-target pairs.

# CLEAR-CLIP analysis uses the Hyb pipeline (Travis 2014)
# Pre-requisite: AGO HITS-CLIP / PAR-CLIP BAM

# Extract candidate chimeric reads (reads that don't fully align to human mRNA)
samtools view -h dedup.bam | awk '$6 ~ /S/' | wc -l   # soft-clipped reads candidate chimeras

# Run Hyb in chimera mode
hyb \
    in=R1.trim.fq.gz \
    db=mature_miRNA_plus_human_mRNA.fa \
    align=blastall \
    type=mim

# Filter for canonical seed match
python analyze_chimeras.py \
    --chimeras chimeras.hyb \
    --mirna_db mature_human_miRNA.fa \
    --seed_types 7mer-m8 7mer-A1 8mer \
    --output validated_chimeras.tsv

miR-eCLIP Probe Enrichment (Manakov 2022)

To recover deep coverage of one or a few miRNAs' targets, miR-eCLIP uses probe-based or PCR-based enrichment to amplify chimeras containing specific miRNAs.

# After chimera identification, filter for specific miRNA
# Example: enrich for hsa-miR-21 targets
grep "hsa-miR-21" chimeras_human_mRNA.tsv > mir21_targets.tsv

# Count unique target sites per miRNA
awk '{print $3}' mir21_targets.tsv | sort -u | wc -l

# Cross-reference with TargetScan conserved predictions for validation
bedtools intersect -wa -wb \
    -a mir21_targets_3utr_coords.bed \
    -b targetscan_mir21_conserved_targets.bed > mir21_validated_targets.bed

Per-Method Failure Modes

Standard AGO-CLIP -- Cannot assign miRNA

Trigger: Standard AGO eCLIP / HITS-CLIP run; user wants per-miRNA target list.

Mechanism: Standard AGO-CLIP enriches for AGO-bound RNAs but does not retain miRNA identity. Computational seed-matching infers which miRNAs are likely bound but each peak gets matched to dozens of candidate miRNAs.

Symptom: Peak BED has 100k peaks; seed-matching assigns 10-50 candidate miRNAs per peak.

Fix: Switch to chimeric method for direct pairing, OR filter computational predictions by miRNA expression in the same cell type (only consider miRNAs > 100 TPM in matched small-RNA-seq).

Chimeric methods -- Low chimera rate

Trigger: Standard chimeric eCLIP without probe enrichment; expecting deep per-miRNA targets.

Mechanism: Chimeras are 1-5% of total reads in standard chimeric eCLIP. For a 30M-read library, only 300k-1.5M chimeras; distributed across 200+ miRNAs gives only ~5000-15000 per miRNA.

Symptom: Per-miRNA target count is sparse; rare miRNAs have < 100 chimeras.

Fix: Use miR-eCLIP with probe enrichment for specific miRNAs of interest (30-175x boost). Or sequence ultra-deep (200M+ reads) for global chimera profiling.

Hyb -- BLAST sensitivity vs miRNA length

Trigger: miRNA sequences (21-23 nt) too short for BLAST default sensitivity.

Mechanism: BLAST defaults need >= 100 nt for reliable alignment. miRNA 21-23 nt hits below threshold; many true chimeras lost.

Symptom: Hyb returns few chimeras; rerun with align=bowtie2 gives more.

Fix: Use bowtie2 mode for miRNA alignment (hyb align=bowtie2 type=mim); short-read aligners are designed for short sequences.

Computational seed matching -- High false positive

Trigger: TargetScan predictions used as ground truth without CLIP validation.

Mechanism: TargetScan reports all potential 7mer-m8 / 8mer matches in 3' UTRs; many sites are not functional miRNA targets (no AGO binding observed).

Symptom: TargetScan predicts thousands of targets per miRNA; only a fraction are validated by CLIP.

Fix: Use CLIP overlap as the validation: TargetScan prediction AND AGO-CLIP peak = high-confidence target. Sites in TargetScan but not in CLIP = unfunctional predictions.

Non-canonical miRNA-target pairing missed

Trigger: Seed-matching only; 3' compensatory pairing missed.

Mechanism: ~50% of miRNA-target interactions have weak seeds but strong 3' UTR pairing (positions 12-17). Seed-only matching loses these.

Symptom: Chimeric methods find targets that TargetScan misses; these have weak seeds.

Fix: Accept chimeric method's targets even with weak seeds (the chimera IS the evidence). Or use TargetScan + RNAhybrid (full miRNA-target duplex prediction) for non-canonical sites.

HEAP -- Mouse-only

Trigger: Want HEAP-style in vivo AGO profiling in human tissue.

Mechanism: HEAP uses a transgenic mouse with Halo-Ago2 allele; not available in human or other species.

Symptom: Cannot replicate HEAP results in human.

Fix: Use eCLIP / chimeric eCLIP on human samples; HEAP is specifically for mouse tissue studies.

miRNA expression filter forgotten

Trigger: Computational miRNA-target assignment without filtering by miRNA expression.

Mechanism: Many miRNA databases include rare or developmental-specific miRNAs. If the miRNA is not expressed in the cell type, it cannot bind anything.

Symptom: Per-miRNA target lists include miRNAs at < 1 TPM expression - implausible binding.

Fix: Cross-reference with matched small-RNA-seq from the same cell type; filter for miRNAs > 100 TPM. ENCODE-validated cell types have published miRNA atlases.

Decision Tree by Use Case

Reconciliation: AGO-CLIP vs TargetScan vs Chimeric

Operational rule: For publication-grade miRNA-target list: (a) chimeric eCLIP / miR-eCLIP for direct pairing; (b) cross-reference with TargetScan conserved predictions; (c) filter by miRNA expression > 100 TPM in matched small-RNA-seq; (d) validate top targets with reporter assay (luciferase / GFP fusion with target 3' UTR).

Common Errors

References

• Chi SW et al 2009 Nature 460:479 (AGO HITS-CLIP)
• Hafner M et al 2010 Cell 141:129 (PAR-CLIP for AGO)
• Helwak A et al 2013 Cell 153:654 (CLASH; chimera method)
• Travis AJ et al 2014 Methods 65:263 (Hyb pipeline)
• Moore MJ et al 2015 Nat Commun 6:8864 (CLEAR-CLIP, 130k chimeras mouse brain)
• Bracken CP et al -- chimeric AGO-CLIP, targeted (consult current literature for verified venue/year; earlier "2016 Nat Methods 13:739" attribution could not be confirmed).
• Li K et al 2020 Mol Cell 80:1100 (HEAP, Halo-Ago2 in vivo mouse)
• Manakov SA et al 2022 bioRxiv 2022.02.13.480296 (chimeric eCLIP / miR-eCLIP, 30-175x enrichment)
• Agarwal V et al 2015 eLife 4:e05005 (TargetScan 7.0)
• Lewis BP et al 2003 Cell 115:787 (original 7mer/8mer seed rules)
• Bartel DP 2018 Cell 173:20 (miRNA target principles)
• McGeary SE et al 2019 Science 366:eaav1741 (TargetScan 8.0 / quantitative target prediction).

Related Skills

• clip-seq/clip-peak-calling - AGO CLIP peak calls
• clip-seq/binding-site-annotation - 3' UTR annotation
• clip-seq/clip-motif-analysis - Seed motif scan
• clip-seq/differential-clip - miRNA perturbation experiments
• clip-seq/m6a-clip - DART-seq uses similar APOBEC1 fusion
• small-rna-seq/target-prediction - TargetScan / miRDB / DIANA
• small-rna-seq/differential-mirna - miRNA expression
• small-rna-seq/mirdeep2-analysis - miRNA discovery