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Pipeline: Two-Step Classifier

Overview

The two-step classifier approach identifies a minimal gene panel that distinguishes tolerant from sensitive phenotypes through:

  1. Step 1: Rank genes based on reproducibility, directionality consistency, and heterogeneity across datasets
  2. Step 2: Use logistic regression to find the minimum gene set that achieves good classification

Status: ✅ Validated - Produced the working 6-gene classifier panel

When to Use This Approach

✅ Use when:

  • You want a minimal gene panel for practical application (< 10 genes)
  • You have multiple independent datasets to assess reproducibility
  • You need cross-study validation (LOSO)
  • You don't want to exclude innate biomarkers (constitutively different)

✅ Advantages over stepwise:

  • Doesn't require control groups
  • Preserves innate biomarkers
  • Reproducibility-focused (prioritizes genes consistent across studies)
  • Designed for minimal feature sets (logistic regression handles small gene counts)

The Pipeline

Prerequisites

  • Multiple RNA-seq datasets with comparable phenotype labels (tolerant/sensitive)
  • Gene count matrices for each dataset
  • Metadata with phenotype labels
  • Shared gene annotation across datasets

Input Data Structure

This approach uses post-data integration:

  • Analyze each dataset independently first
  • Compare results across datasets
  • Identify reproducible signals
Dataset 1: [counts, metadata with phenotype]
Dataset 2: [counts, metadata with phenotype]
Dataset 3: [counts, metadata with phenotype]
...

Important: Do NOT pool datasets before analysis (see Problem Framing - Big Lesson #1)

Step 1: Reproducibility-Based Gene Ranking

Goal

Identify genes that consistently differentiate phenotypes across multiple studies

Criteria

For each gene, assess:

  1. Reproducibility: Gene is differentially expressed in multiple datasets
  2. Directionality consistency: Direction of fold change is same across datasets (e.g., always upregulated in tolerant)
  3. Heterogeneity: Variance in expression within phenotype groups

Implementation Approach

# Pseudocode for Step 1

for each_dataset in datasets:
    # Run differential expression
    deg_results[dataset] = DESeq2(
        counts=dataset.counts,
        design="~ phenotype"
    )

# Score genes across datasets
gene_scores = {}
for gene in all_genes:
    # Count how many datasets show significant DE
    reproducibility = count_significant_across_datasets(gene, deg_results)

    # Check directionality consistency
    fold_changes = [deg_results[ds][gene].log2FC for ds in datasets]
    directionality = 1.0 if all_same_sign(fold_changes) else 0.0

    # Assess heterogeneity
    heterogeneity = calculate_within_group_variance(gene, datasets)

    # Combined score
    gene_scores[gene] = (
        reproducibility * directionality / heterogeneity
    )

# Rank genes by score
ranked_genes = sort_descending(gene_scores)

Output

Ranked list of candidate genes, prioritizing those that:

  • Appear as DEGs in multiple datasets
  • Show consistent direction of change
  • Have low within-group variance (high between-group separation)

Step 2: Logistic Regression for Minimal Panel

Goal

Find the smallest set of genes that achieves good classification accuracy

Method

Use logistic regression with regularization (LASSO or elastic net) to select features:

# Pseudocode for Step 2

# Start with top N genes from Step 1 (e.g., top 50-100)
candidate_genes = ranked_genes[:100]

# Prepare training data (combined datasets)
X_train = combined_expression_matrix[candidate_genes]
y_train = phenotype_labels  # tolerant vs. sensitive

# Fit logistic regression with LASSO regularization
from sklearn.linear_model import LogisticRegressionCV

clf = LogisticRegressionCV(
    penalty='l1',  # LASSO for feature selection
    solver='liblinear',
    cv=5,  # cross-validation to find optimal regularization
    max_iter=1000
)
clf.fit(X_train, y_train)

# Extract selected genes (non-zero coefficients)
selected_genes = candidate_genes[clf.coef_[0] != 0]

print(f"Minimal gene panel: {len(selected_genes)} genes")
print(selected_genes)

Regularization Parameter Selection

  • Use cross-validation to find optimal L1 penalty
  • Balance between:
  • Fewer genes (higher penalty) → simpler assay, may sacrifice accuracy
  • More genes (lower penalty) → better accuracy, more complex assay

Implementation Example: 6-Gene Classifier

From Issue #44 and notebook post:

Datasets Used

  • Dataset 1: C. virginica with P. marinus exposure
  • Dataset 5: C. virginica with P. marinus exposure

Results

Step 1 output: Ranked ~1000s of genes by reproducibility

Step 2 output: 6-gene panel with strong phenotype separation

Performance

✅ Strong separation between tolerant and sensitive phenotypes
✅ Genes are reproducible across independent datasets
✅ Small enough for practical assay development

Related notebook: Two-script pipeline for gene classifier

Validation: Leave-One-Study-Out (LOSO)

Big Lesson #4: Avoid Training Set in Test Set

When evaluating cross-study performance:

  • Within-study validation: OK to include training samples if only evaluating performance within same study
  • Cross-study validation: Never include training study in test set → use LOSO

Including training data in test set leads to overfitting and falsely optimistic accuracy.

LOSO Procedure

For each dataset:

  1. Train classifier on all OTHER datasets
  2. Test on held-out dataset (never seen during training)
  3. Report accuracy, sensitivity, specificity
# LOSO Cross-Validation
for test_dataset in all_datasets:
    training_datasets = [ds for ds in all_datasets if ds != test_dataset]

    # Train on all except test_dataset
    X_train = combine_datasets(training_datasets, selected_genes)
    y_train = get_labels(training_datasets)
    clf.fit(X_train, y_train)

    # Test on held-out dataset
    X_test = test_dataset[selected_genes]
    y_test = test_dataset['phenotype']
    accuracy = clf.score(X_test, y_test)

    print(f"Test on {test_dataset.name}: Accuracy = {accuracy:.3f}")

Expected Performance

  • High accuracy (> 80%) across LOSO folds suggests robust, generalizable panel
  • Variable accuracy suggests study-specific effects; may need batch correction or dataset filtering

See Validation & Pitfalls for more on LOSO and other validation strategies.

Innate vs. Reactive Biomarkers

A key advantage of this approach: it captures innate biomarkers

Innate Biomarker Preservation

Unlike the stepwise approach, the classifier method does not filter based on stress response.

Genes that are constitutively different between resistant/sensitive oysters (even in controls) will be: - Identified in Step 1 if reproducible across datasets - Selected in Step 2 if predictive of phenotype

Analysis: Issue #53 and notebook

To characterize whether your biomarkers are innate or reactive:

  • Compare gene expression in controls only (resistant vs. sensitive)
  • If significant → innate biomarker
  • If not significant in controls but significant in treated → reactive biomarker

Workflow Summary

graph TD
    A[Multiple Datasets<br/>Independent DE Analysis] --> B[Step 1: Gene Scoring]
    B --> C[Reproducibility:<br/>Present in multiple datasets]
    B --> D[Directionality:<br/>Consistent fold change sign]
    B --> E[Heterogeneity:<br/>Low within-group variance]
    C --> F[Ranked Gene List]
    D --> F
    E --> F
    F --> G[Step 2: Logistic Regression<br/>with LASSO]
    G --> H[Minimal Gene Panel<br/>e.g., 6 genes]
    H --> I[LOSO Validation]
    I --> J{Accuracy > 80%<br/>across folds?}
    J -->|Yes| K[Validated Classifier]
    J -->|No| L[Refine: Add features,<br/>adjust penalty, filter datasets]

Code & Resources

Primary analysis repository:
Cvirg_Pmarinus_RNAseq/analyses/

Key notebook:
Two-script pipeline for gene-expression classifier

Related issues:

Additional exploration:

Practical Considerations

Minimum Dataset Requirements

  • At least 2 independent studies (for reproducibility assessment)
  • Comparable phenotype definitions (resistant/sensitive)
  • Sufficient sample size (n > 10 per group per study recommended)

Computational Requirements

  • Moderate: Can run on laptop/workstation
  • No need for HPC (unlike full nf-core pipelines)
  • Python/R with standard ML libraries (scikit-learn, glmnet)

Assay Development

Once you have a minimal panel:

  • 6-10 genes → feasible for qPCR assay
  • < 20 genes → feasible for targeted RNA-seq panel
  • > 20 genes → may need full RNA-seq (cost-prohibitive for routine screening)

Next: Learn about validation strategies and common pitfalls in Validation & Pitfalls