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name: bio-crispr-screens-hit-calling description: Statistical methods for calling hits in CRISPR screens. Covers MAGeCK, BAGEL2, drugZ, and custom approaches for identifying essential and resistance genes. Use when identifying significant genes from screen count data after QC passes. tool_type: mixed primary_tool: bagel2

CRISPR Screen Hit Calling

BAGEL2 Analysis

Copy# BAGEL2 for Bayesian gene essentiality
# Uses reference essential/non-essential genes

# Calculate fold changes
bagel2 fc \
    -i counts.txt \
    -o foldchange.txt \
    -c Control1,Control2 \
    -t Treatment1,Treatment2

# Calculate Bayes Factor
bagel2 bf \
    -i foldchange.txt \
    -o bayes_factor.txt \
    -e essential_genes.txt \
    -n nonessential_genes.txt \
    -c 1  # Number of bootstrap iterations

# Precision-recall analysis
bagel2 pr \
    -i bayes_factor.txt \
    -o precision_recall.txt \
    -e essential_genes.txt \
    -n nonessential_genes.txt

DrugZ Analysis

Copy# DrugZ for drug screens (synergy/resistance)
drugz.py \
    -i counts.txt \
    -o drugz_output.txt \
    -c Control1,Control2 \
    -x Treatment1,Treatment2 \
    --remove-genes Control_genes.txt

# Output columns:
# Gene, sumZ (combined z-score), normZ, pval_synth (synthetic lethal), pval_supp (suppressor)

Custom Hit Calling in Python

Copyimport pandas as pd
import numpy as np
from scipy import stats

# Load counts
counts = pd.read_csv('counts.txt', sep='\t', index_col=0)
genes = counts['Gene']
ctrl_cols = ['Control1', 'Control2']
treat_cols = ['Treatment1', 'Treatment2']

# Normalize (reads per million)
def rpm_normalize(df):
    return df / df.sum() * 1e6

ctrl_rpm = rpm_normalize(counts[ctrl_cols])
treat_rpm = rpm_normalize(counts[treat_cols])

# Log2 fold change per sgRNA
lfc = np.log2((treat_rpm.mean(axis=1) + 1) / (ctrl_rpm.mean(axis=1) + 1))

# Aggregate to gene level
gene_lfc = pd.DataFrame({'Gene': genes, 'LFC': lfc}).groupby('Gene')['LFC'].agg(['mean', 'std', 'count'])
gene_lfc.columns = ['mean_lfc', 'std_lfc', 'n_sgrnas']

# Z-score based on null distribution (non-targeting controls or all genes)
null_mean = gene_lfc['mean_lfc'].median()
null_std = gene_lfc['mean_lfc'].std()
gene_lfc['z_score'] = (gene_lfc['mean_lfc'] - null_mean) / null_std
gene_lfc['pvalue'] = 2 * stats.norm.sf(abs(gene_lfc['z_score']))
gene_lfc['fdr'] = stats.false_discovery_control(gene_lfc['pvalue'])

# Call hits
essential = gene_lfc[(gene_lfc['z_score'] < -2) & (gene_lfc['fdr'] < 0.1)]
resistance = gene_lfc[(gene_lfc['z_score'] > 2) & (gene_lfc['fdr'] < 0.1)]

print(f'Essential genes: {len(essential)}')
print(f'Resistance genes: {len(resistance)}')

Robust Rank Aggregation (MAGeCK-style)

Copyfrom scipy.stats import rankdata, norm
import numpy as np

def rra_score(ranks, n_total):
    '''Calculate RRA score for a set of ranks'''
    k = len(ranks)
    sorted_ranks = np.sort(ranks)
    rho = sorted_ranks / n_total

    # Beta distribution p-values
    from scipy.stats import beta
    pvals = [beta.cdf(rho[i], i + 1, k - i) for i in range(k)]

    # Return minimum p-value (most significant)
    return min(pvals)

# Apply to each gene
def calculate_gene_rra(sgrna_pvals, genes, n_total):
    results = []
    for gene in genes.unique():
        gene_pvals = sgrna_pvals[genes == gene]
        gene_ranks = rankdata(gene_pvals)
        rra = rra_score(gene_ranks, len(gene_pvals))
        results.append({'gene': gene, 'rra_score': rra, 'n_sgrnas': len(gene_pvals)})
    return pd.DataFrame(results)

Second-Best sgRNA Method

Copy# Conservative approach: use second-best sgRNA per gene
# Reduces false positives from single outlier sgRNAs

def second_best_lfc(lfc_series, genes):
    '''Return second-most extreme LFC per gene'''
    results = []
    for gene in genes.unique():
        gene_lfc = lfc_series[genes == gene].sort_values()
        if len(gene_lfc) >= 2:
            # For dropout, use second smallest (second most negative)
            results.append({'gene': gene, 'second_best_lfc': gene_lfc.iloc[1]})
        else:
            results.append({'gene': gene, 'second_best_lfc': gene_lfc.iloc[0]})
    return pd.DataFrame(results)

second_best = second_best_lfc(lfc, genes)

Compare Methods

Copy# Load results from different methods
mageck = pd.read_csv('mageck.gene_summary.txt', sep='\t')
bagel = pd.read_csv('bagel_bf.txt', sep='\t')
drugz = pd.read_csv('drugz_output.txt', sep='\t')

# Merge on gene
merged = mageck[['id', 'neg|fdr']].rename(columns={'id': 'gene', 'neg|fdr': 'mageck_fdr'})
merged = merged.merge(bagel[['Gene', 'BF']].rename(columns={'Gene': 'gene', 'BF': 'bagel_bf'}), on='gene')
merged = merged.merge(drugz[['GENE', 'fdr_synth']].rename(columns={'GENE': 'gene', 'fdr_synth': 'drugz_fdr'}), on='gene')

# Consensus hits
merged['mageck_hit'] = merged['mageck_fdr'] < 0.1
merged['bagel_hit'] = merged['bagel_bf'] > 5  # BF > 5 suggests essential
merged['drugz_hit'] = merged['drugz_fdr'] < 0.1

merged['consensus'] = merged['mageck_hit'].astype(int) + merged['bagel_hit'].astype(int) + merged['drugz_hit'].astype(int)

# High confidence hits called by 2+ methods
high_conf = merged[merged['consensus'] >= 2]
print(f'High confidence hits (2+ methods): {len(high_conf)}')

Time-Course Analysis

Copy# For screens with multiple timepoints
def time_course_hits(counts, timepoints, genes):
    '''Identify genes with consistent depletion over time'''
    lfc_by_time = {}

    for t in timepoints:
        t0_cols = [c for c in counts.columns if 'T0' in c]
        t_cols = [c for c in counts.columns if f'T{t}' in c]

        t0_mean = counts[t0_cols].mean(axis=1)
        t_mean = counts[t_cols].mean(axis=1)

        lfc_by_time[t] = np.log2((t_mean + 1) / (t0_mean + 1))

    # Aggregate and check for consistent direction
    lfc_df = pd.DataFrame(lfc_by_time)
    lfc_df['Gene'] = genes

    gene_summary = lfc_df.groupby('Gene').mean()
    gene_summary['all_negative'] = (gene_summary < 0).all(axis=1)
    gene_summary['trend'] = gene_summary.apply(lambda x: np.polyfit(range(len(timepoints)), x[:-1], 1)[0], axis=1)

    return gene_summary[gene_summary['all_negative']].sort_values('trend')

Visualize Results

Copyimport matplotlib.pyplot as plt

# Rank plot
fig, ax = plt.subplots(figsize=(10, 6))

results = pd.read_csv('mageck.gene_summary.txt', sep='\t')
results = results.sort_values('neg|score')
results['rank'] = range(1, len(results) + 1)

ax.scatter(results['rank'], -np.log10(results['neg|fdr']),
           c=['red' if fdr < 0.05 else 'gray' for fdr in results['neg|fdr']],
           alpha=0.5, s=10)

# Label top hits
top = results[results['neg|fdr'] < 0.01].head(10)
for _, row in top.iterrows():
    ax.annotate(row['id'], (row['rank'], -np.log10(row['neg|fdr'])))

ax.axhline(-np.log10(0.05), linestyle='--', color='black')
ax.set_xlabel('Gene Rank')
ax.set_ylabel('-log10(FDR)')
ax.set_title('CRISPR Screen Hits')
plt.savefig('hit_ranking.png', dpi=150)

Related Skills

• mageck-analysis - MAGeCK workflow
• screen-qc - QC before hit calling
• pathway-analysis - Functional analysis of hits