/bio-causal-genomics-pleiotropy-detection

name: bio-causal-genomics-pleiotropy-detection
description: Detect and correct for horizontal pleiotropy in Mendelian randomization analyses using MR-PRESSO for outlier removal, MR-Egger regression for directional pleiotropy, and Steiger filtering for variant directionality. Use when validating MR results, detecting pleiotropic instruments, or running sensitivity analyses for causal inference.
tool_type: r
primary_tool: MR-PRESSO

Version Compatibility

Reference examples tested with: MR-PRESSO 1.0+, TwoSampleMR 0.5+

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

• R: `packageVersion("<pkg>")` then `?function_name` to verify parameters

If code throws ImportError, AttributeError, or TypeError, introspect the installed package and adapt the example to match the actual API rather than retrying.

Pleiotropy Detection

**"Check my MR results for pleiotropic bias"** → Detect and correct for horizontal pleiotropy using outlier removal (MR-PRESSO), directional pleiotropy testing (MR-Egger intercept), and variant directionality filtering (Steiger) to validate causal inference results.

• R: `MRPRESSO::mr_presso()` for global and distortion tests
• R: `TwoSampleMR::mr_egger_regression()` for Egger intercept test

Overview

Horizontal pleiotropy violates the exclusion restriction assumption of MR: instruments affect the outcome through pathways other than the exposure. Detecting and correcting for pleiotropy is essential for valid causal inference.

Types of pleiotropy:

• **Vertical** (mediated): Instrument -> exposure -> outcome (valid, not a problem)
• **Horizontal** (direct): Instrument -> outcome bypassing exposure (violates MR assumptions)
• **Balanced**: Pleiotropic effects cancel out (IVW still valid, Egger intercept ~0)
• **Directional**: Pleiotropic effects are systematic (biases IVW, Egger detects this)

MR-PRESSO

**Goal:** Detect and remove pleiotropic outlier instruments from an MR analysis.

**Approach:** Run MR-PRESSO to test for global pleiotropy, identify individual outlier SNPs, test whether their removal changes the causal estimate (distortion test), and obtain a corrected estimate.

# install.packages('remotes')
# remotes::install_github('rondolab/MR-PRESSO')
library(MRPRESSO)

# Input: harmonized data from TwoSampleMR
# Columns needed: beta.exposure, beta.outcome, se.exposure, se.outcome
presso_input <- data.frame(
  bx = dat$beta.exposure,
  by = dat$beta.outcome,
  bxse = dat$se.exposure,
  byse = dat$se.outcome
)

# --- Run MR-PRESSO ---
# NbDistribution: Number of simulations for null distribution (minimum 1000)
# SignifThreshold: P-value threshold for outlier detection (0.05 standard)
presso_result <- mr_presso(
  BetaOutcome = 'by', BetaExposure = 'bx',
  SdOutcome = 'byse', SdExposure = 'bxse',
  OUTLIERtest = TRUE, DISTORTIONtest = TRUE,
  data = presso_input,
  NbDistribution = 5000,
  SignifThreshold = 0.05
)

# --- Global test ---
# Tests whether there is any pleiotropy among instruments
# Significant p-value: Evidence of horizontal pleiotropy
global_p <- presso_result$`MR-PRESSO results`$`Global Test`$Pvalue
cat('Global test p-value:', global_p, '\n')

# --- Outlier test ---
# Identifies individual pleiotropic SNPs
outliers <- presso_result$`MR-PRESSO results`$`Outlier Test`
cat('\nOutlier test results:\n')
print(outliers)

# Outlier SNPs (p < 0.05)
outlier_indices <- which(outliers$Pvalue < 0.05)
cat('Outlier SNPs:', length(outlier_indices), '\n')

# --- Distortion test ---
# Tests whether removing outliers significantly changes the causal estimate
# Significant: Outliers were meaningfully biasing the estimate
distortion_p <- presso_result$`MR-PRESSO results`$`Distortion Test`$Pvalue
cat('Distortion test p-value:', distortion_p, '\n')

# --- Corrected estimate ---
# MR estimate after removing outlier SNPs
main_results <- presso_result$`Main MR results`
cat('\nRaw IVW estimate:', main_results$`Causal Estimate`[1], '\n')
cat('Corrected IVW estimate:', main_results$`Causal Estimate`[2], '\n')

MR-Egger Diagnostics

**Goal:** Test for directional pleiotropy and obtain a pleiotropy-adjusted causal estimate.

**Approach:** Fit MR-Egger regression where the intercept estimates average pleiotropic bias, and check I-squared for instrument strength under the NOME assumption.

library(TwoSampleMR)

# MR-Egger regression allows for a non-zero intercept
# The intercept estimates the average pleiotropic effect
egger <- mr_egger_regression(dat$beta.exposure, dat$beta.outcome,
                              dat$se.exposure, dat$se.outcome)

# --- Egger intercept ---
# Significant intercept (p < 0.05): Directional pleiotropy present
# Non-significant: No evidence (but low power with < 10 SNPs)
cat('Egger intercept:', round(egger$b_i, 5), '\n')
cat('Intercept SE:', round(egger$se_i, 5), '\n')
cat('Intercept p-value:', format.pval(egger$pval_i), '\n')

# --- Egger slope ---
# Valid causal estimate EVEN with directional pleiotropy (InSIDE assumption)
cat('\nEgger causal estimate:', round(egger$b, 4), '\n')
cat('Egger SE:', round(egger$se, 4), '\n')
cat('Egger p-value:', format.pval(egger$pval), '\n')

# --- I-squared for Egger ---
# I^2 measures instrument strength for MR-Egger specifically
# I^2 > 0.9: Egger estimate reliable
# I^2 < 0.6: Egger has low power, interpret with caution (NOME violation)
isq <- Isq(dat$beta.exposure, dat$se.exposure)
cat('\nI-squared:', round(isq, 3), '\n')
if (isq < 0.9) cat('Warning: I-squared < 0.9; Egger estimate may be unreliable (NOME violation)\n')

Steiger Filtering

**Goal:** Verify that instruments act in the correct causal direction (exposure -> outcome, not reverse).

**Approach:** Apply the Steiger test to each instrument, remove those explaining more outcome variance than exposure variance, and re-run MR on filtered instruments.

library(TwoSampleMR)

# Steiger test: Verify each instrument explains more variance in
# the exposure than the outcome. Instruments fa