Polygenic Risk Score (PRS) Builder

PRS Calculation Formula

A polygenic risk score is calculated as a weighted sum across genetic variants:

PRS = Σ (dosage_i × effect_size_i)

Where:

• dosage_i: Number of effect alleles at SNP i (0, 1, or 2)
• effect_size_i: Beta coefficient or log(odds ratio) from GWAS

Standardization

Raw PRS is standardized to z-scores for interpretation:

z-score = (PRS - population_mean) / population_std

This allows comparison to population distribution and percentile calculation.

Significance Thresholds

• Genome-wide significance: p < 5×10⁻⁸ (default threshold)
• This corrects for ~1 million independent tests across the genome
• Relaxed thresholds (e.g., p < 1×10⁻⁵) can include more SNPs but may add noise

Effect Size Handling

• Continuous traits (e.g., height, BMI): Beta coefficient (units of trait per allele)
• Binary traits (e.g., disease): Odds ratio converted to log-odds (beta = ln(OR))
• Missing effect sizes or non-significant SNPs are excluded

Data Sources

This skill uses ToolUniverse GWAS tools to query:

1. GWAS Catalog (EMBL-EBI)

   • Curated GWAS associations
   • 5000+ studies, millions of variants
   • Tools: gwas_get_associations_for_trait, gwas_get_snp_by_id

2. Open Targets Genetics

   • Integrated genetics platform
   • Fine-mapped credible sets
   • Tools: OpenTargets_search_gwas_studies_by_disease, OpenTargets_get_variant_info

Key Concepts

Polygenic Risk Scores (PRS)

Polygenic risk scores aggregate the effects of many genetic variants to estimate an individual's genetic predisposition to a trait or disease. Unlike Mendelian diseases caused by single mutations, complex diseases involve hundreds to thousands of variants, each with small effects.

Key Properties:

• Continuous distribution: PRS forms a bell curve in populations
• Relative risk: Compares individual to population average
• Probabilistic: High PRS doesn't guarantee disease, low PRS doesn't guarantee protection
• Ancestry-specific: PRS accuracy depends on matching GWAS and target ancestry

GWAS (Genome-Wide Association Studies)

GWAS compare allele frequencies between cases and controls (or correlate with trait values) across millions of SNPs to identify disease-associated variants.

Study Design:

• Discovery cohort: Initial identification of associations
• Replication cohort: Validation in independent samples
• Sample size: Larger studies detect smaller effects (power ∝ √N)
• Multiple testing correction: Bonferroni-type correction for ~1M tests

Effect Sizes and Odds Ratios

• Beta (β): Change in trait per copy of effect allele
  • Example: β = 0.5 kg/m² means each allele increases BMI by 0.5 units
• Odds Ratio (OR): Multiplicative change in disease odds
  • OR = 1.5 means 50% increased odds per allele
  • Convert to beta: β = ln(OR)

Linkage Disequilibrium (LD) and Clumping

Nearby variants are often inherited together (LD). To avoid double-counting:

• LD clumping: Select independent variants (r² < 0.1 within 1 Mb windows)
• Fine-mapping: Statistical methods to identify causal variants
• This skill uses raw associations; production PRS should include LD pruning

Population Stratification

GWAS and PRS are most accurate when ancestries match:

• Population structure: Different ancestries have different allele frequencies
• Transferability: European-trained PRS perform worse in non-European populations
• Solution: Train PRS on diverse cohorts or use ancestry-matched references

Applications

Clinical Risk Assessment

PRS can stratify individuals for:

• Screening programs: Target high-risk individuals (e.g., mammography, colonoscopy)
• Prevention strategies: Lifestyle interventions for high genetic risk
• Drug response: Pharmacogenomics based on metabolism genes

Example: Khera et al. (2018) showed PRS identifies 3× more individuals at >3-fold coronary artery disease risk than monogenic mutations.

Research Applications

• Gene discovery: PRS-based phenome-wide association studies (PheWAS)
• Genetic correlation: Compare PRS across traits
• Causal inference: Mendelian randomization using PRS as instruments
• Simulation studies: Model polygenic architecture

Personal Genomics

Consumer genetic testing (23andMe, Ancestry DNA) provides raw genotypes. Users can:

• Calculate PRS for traits not reported
• Compare to published PRS models
• Understand genetic contribution vs. lifestyle factors

Caution: Personal PRS should not replace medical advice. Results may cause anxiety if not properly contextualized.

Limitations and Considerations

Scientific Limitations

1. Heritability Gap: PRS explains a fraction of genetic heritability

   • Type 2 diabetes: ~50% heritable, PRS explains ~10-20%
   • Rare variants, epistasis, and gene-environment interactions not captured

2. Ancestry Bias: Most GWAS are European ancestry

   • PRS accuracy drops in non-European populations
   • Need for diverse cohort recruitment

3. Winner's Curse: Discovery effect sizes often overestimated

   • Replication studies show smaller effects
   • Meta-analyses provide better estimates

4. Missing Heritability: Unexplained genetic contribution from:

   • Rare variants not captured by SNP arrays
   • Structural variants (CNVs, inversions)
   • Epigenetic factors

Clinical Limitations

1. Not Diagnostic: PRS is probabilistic, not deterministic

   • High PRS doesn't mean you will get disease
   • Low PRS doesn't mean you won't get disease

2. Environmental Factors: Many complex diseases are 50%+ environmental

   • Smoking, diet, exercise, stress, pollution
   • PRS doesn't account for these

3. Pleiotropy: Same variants affect multiple traits

   • Genetic correlation between diseases
   • Risk for one may protect against another

4. Actionability: Not all high-risk predictions have interventions

   • Alzheimer's PRS has limited actionability currently
   • Ethical considerations for testing

Ethical Considerations

1. Privacy: Genetic data is identifiable and permanent

   • Can't be changed like passwords
   • Familial implications (relatives share genetics)

2. Discrimination: Potential for genetic discrimination

   • GINA protects against health/employment discrimination (US)
   • Life insurance and long-term care not protected

3. Psychological Impact: Knowledge of high risk can cause anxiety

   • Need for genetic counseling
   • Risk communication training

4. Equity: Ancestry bias means unequal benefits

   • Europeans benefit most from current PRS
   • Exacerbates health disparities

References

Key Publications

1. Lambert et al. (2021): "The Polygenic Score Catalog as an open database for reproducibility and systematic evaluation"

   • PGS Catalog: https://www.pgscatalog.org/
   • Repository of published PRS models

2. Khera et al. (2018): "Genome-wide polygenic scores for common diseases identify individuals with risk equivalent to monogenic mutations"

   • Nature Genetics, 50:1219–1224
   • Demonstrated clinical utility of PRS

3. Torkamani et al. (2018): "The personal and clinical utility of polygenic risk scores"

   • Nature Reviews Genetics, 19:581–590
   • Comprehensive review of PRS applications

4. Martin et al. (2019): "Clinical use of current polygenic risk scores may exacerbate health disparities"

   • Nature Genetics, 51:584–591
   • Addresses ancestry bias and equity concerns

5. Choi et al. (2020): "Tutorial: a guide to performing polygenic risk score analyses"

   • Nature Protocols, 15:2759–2772
   • Practical guide to PRS calculation and evaluation

Resources

• PGS Catalog: https://www.pgscatalog.org/ - Published PRS models
• LD Hub: http://ldsc.broadinstitute.org/ - Genetic correlations
• PRSice: https://www.prsice.info/ - PRS calculation software
• GWAS Catalog: https://www.ebi.ac.uk/gwas/ - Association database

Workflow

1. Trait Selection

Identify the disease or trait of interest:

• Use standard terminology (e.g., "type 2 diabetes" not "T2D")
• Check GWAS Catalog for availability
• Verify sufficient GWAS studies exist (n > 10,000 samples ideal)

2. Association Collection

Query GWAS databases for genome-wide significant associations:

prs = build_polygenic_risk_score(
    trait="coronary artery disease",
    p_threshold=5e-8,  # Genome-wide significance
    max_snps=1000
)

Considerations:

• P-value threshold: 5e-8 is conservative, 1e-5 includes more variants
• LD clumping: Production systems should prune correlated SNPs
• Study quality: Prefer large meta-analyses over small studies

3. Effect Size Extraction

Extract beta coefficients or odds ratios:

• Beta for continuous traits (direct use)
• OR for binary traits (convert to log-odds)
• Handle missing values (exclude or impute from meta-analysis)

4. SNP Filtering

Quality control filters:

• MAF filter: Exclude rare variants (MAF < 0.01) for robustness
• Genotype QC: Remove SNPs with high missingness (> 10%)
• Hardy-Weinberg: Exclude SNPs violating HWE (p < 1e-6)
• Ambiguous SNPs: Remove A/T and G/C SNPs (strand ambiguity)

5. Score Calculation

Calculate weighted sum of genotype dosages:

result = calculate_personal_prs(
    prs_weights=prs,
    genotypes=my_genotypes,
    population_mean=0.0,
    population_std=1.0
)

Genotype Sources:

• 23andMe raw data export
• Ancestry DNA raw data
• Whole genome sequencing (VCF files)
• SNP array data (Illumina, Affymetrix)

6. Risk Interpretation

Convert to percentiles and risk categories:

result = interpret_prs_percentile(result)
print(f"Percentile: {result.percentile:.1f}%")
print(f"Risk: {result.risk_category}")

Risk Categories:

• Low risk: < 20th percentile (genetic protection)
• Average risk: 20-80th percentile (typical genetic predisposition)
• Elevated risk: 80-95th percentile (moderately increased risk)
• High risk: > 95th percentile (substantially increased risk)

Clinical Interpretation:

• Percentiles assume normal distribution
• Relative risk vs. average (not absolute risk)
• Combine with family history, clinical risk factors
• PRS is NOT diagnostic - many high-risk individuals never develop disease

Best Practices

PRS Construction

1. Use validated PRS from PGS Catalog when available

   • Published models have been externally validated
   • Include LD clumping and ancestry-specific weights

2. Match ancestries between GWAS and target population

   • European GWAS for European individuals
   • Use multi-ancestry GWAS when available

3. Include as many SNPs as practical

   • More SNPs = better prediction (up to a point)
   • Balance between coverage and genotyping cost

4. Consider trait architecture

   • Highly polygenic traits (height, education): benefit from relaxed thresholds
   • Oligogenic traits (IBD, T1D): few large-effect variants, strict thresholds

Clinical Use

1. Combine with clinical risk scores

   • Add PRS to Framingham Risk Score, QRISK, etc.
   • Integrated models improve prediction

2. Stratify screening and prevention

   • Intensify surveillance for high PRS (e.g., earlier mammography)
   • Lifestyle interventions for modifiable risk

3. Provide genetic counseling

   • Explain probabilistic nature of PRS
   • Discuss limitations and uncertainty
   • Address psychological impact

4. Consider actionability

   • Is there an intervention for high risk?
   • Benefits vs. harms of knowing genetic risk

Research Use

1. Report methods transparently

   • Document SNP selection criteria
   • Report LD clumping parameters
   • Specify ancestry of GWAS and target

2. Validate in held-out cohorts

   • Split data: training vs. testing
   • Report out-of-sample prediction accuracy (R², AUC)

3. Compare to existing PRS

   • Benchmark against PGS Catalog models
   • Report incremental improvement

4. Test across ancestries

   • Evaluate transferability to non-European populations
   • Report performance stratified by ancestry

Disclaimer

This skill is for educational and research purposes only.

• Not for clinical diagnosis or treatment decisions
• Not validated for clinical use - use PGS Catalog models for clinical-grade PRS
• Requires genetic counseling - interpretation requires expertise
• Does not account for family history, environment, or lifestyle factors
• Ancestry-specific - accuracy depends on matching GWAS ancestry

For clinical genetic testing, consult:

• Genetic counselors (certified by ABGC/ABMGG)
• Medical geneticists
• Healthcare providers with genomics training

PRS is a rapidly evolving field. Guidelines and best practices will continue to change as research progresses.

Regulatory Status:

• FDA does not currently regulate PRS (as of 2024)
• Some countries restrict direct-to-consumer genetic risk reporting
• Check local regulations before clinical implementation