PGSUS: Partitioning Genetic Scores Using Siblings

The Partitioning Genetic Scores Using Siblings (PGSUS, pronounced "Pegasus") method decomposes the variance of a polygenic score (PGS) into its various sources. Specifically, PGSUS uses paired population-level GWAS statistics and sibling GWAS statistics to determine the proportion of variance in a PGS attributable to direct effects and to Stratification, Assortative mating, and Indirect parental (Dynastic) effects, collectively referred to as "SAD effects." PGSUS is implemented in Python and runs in a conda environment. Applying PGSUS helps identify and interpret the performance of a PGS with respect to a particular target cohort by performing a two-step decomposition of variance in a PGS.

Directory Contents

In addition to the files listed below, you can find files containing summary statistics, preprocessing steps, and actual commands for each analysis on the Harpak Lab Website Data Tab. Additionally, the code and necessary input to generate each of the figures contained withing the manuscript can be found on the Harpak Lab Website Data Tab.

• simulations: contains scripts to generate simulated summary statistics and perform the PGSUS decomposition while varying parameters of interest. Detailed discussion of implementation and different flags can be found in this directory.
• example: contains input files needed to run the example commands in this repo.

Creating and activating a conda environment

Before you can analyze your polygenic score, you first need to create the appropriate environment using conda. Once you have downloaded the requirements.txt file and have installed conda on your computer, the follwoing can be run to generate a conda environment with the necessary packages to apply PGSUS.

conda create --name pgsus --file requirements.txt

Once the environment is successfully created activate it using

source activate pgsus

The PGSUS software uses pysnptools to manipulate genetic data. It is most easily installed through pip using the command below while the environment is active.

pip install pysnptools

Formatting Summary Statistics

The first step in applying PGSUS is properly formatting the standard and sibling GWAS summary statistics. Use the munge_sumstats.py script with the following flags. Please download the necessary support files from the link above.

python munge_sumstats.py --pop-gwas-file example/example.pop.stats.linear \
--anc-data example/example.alt.alleles.txt.gz \
--sib-perm-file example/example.sib.stats.linear \
--preselected-snps example/example.snp.ids.txt \
--outdir example/ \
--outlabel pgsus_height_example \
--snp-id ID 

The possible flags that can be used to munge different input file formats are enumerated below:

• --pop-gwas-file a file containing summary statistics from a population GWAS. In this example, a compressed file from plink's implementation is used.

• --sib-perm-file a file containing summary statistics from a sibling GWAS.

• --preselected-snps a file with a single column containing the set of SNP IDs to be used in the PGSUS decomposition.

• --outdir directory path where results should be written.

• --outlabel file prefix for output files produced during the data munging.

• --snp-id label of then column containing SNP IDs formatted as chromsome:basepair. Defaults to "SNP".

• --chr label of the column containing the chromosome of each SNP. Default is "CHR".

• --bfile path to the plink formatted bed file with the target sample. Only required if the clumping of the loci has not been previously been performed and identified with the preselected SNPs flag.

• --anc-data file containing ancestral and derived alleles for each locus. Defaults to the file created using the 1000 Genomes data contained in the support files directory.

• --pos label of the base pair position of each locus. Defaults to "POS".

• --standard-beta label of the column in the population GWAS file that contains the effect size estimates. Defaults to "BETA".

• --sib-beta label of the column in the sibling GWAS file that contains the effect size estimates. Defaults to "BETA".

• --pval label of the column containing the population GWAS association p-values. Defaults to "P".

• --a1 label of the column containing the alternate allele counted in the GWAS for each locus. Defaults to "A1".

• --p-is-log presence of this flag indicates that p-values have been negative log 10 transformed.

• --logp-col label of the column containing the negative log 10 transformed p-values. Only necessary if the p-value has undergone transformation.

Once the script is run, there should be two files produced both with specified outlabel above: one with the suffix sib.preproc.txt and another with the the suffix standard.preproc.txt each containing the formatted summary statistics from the population and sibling GWAS. Note that there will be a new column entitled beta.altconsensus that contains the same effect size estimates, but some will have been multiplied by -1 in order to ensure the correct polarity of the effect allele.

Estimation of SAD variance using the PGSUS software

With the preprocessed data in hand the PGSUS software can now be run. There are a number of possible flags intended to help incorporate different sources of data. An example command is given below and each possible flag is described as well.

python pgsus.py --genetic-file 1kg.example.bed \
--pop-gwas example/pegasus_height_decomposition.standard.preproc.txt \
--sib-gwas example/pegasus_height_decomposition.sib.preproc.txt \
--chrom-pos SNP \
--pvalue 1 \
--pval-col P \
--pop-effect beta.altconsensus \
--pop-se se \
--sib-effect beta.altconsensus \
--sib-se EMP_SE \
--ascertainment-set gwas \
--outfile-label pegasus_height_decomposition \
--out example/  \
--chrom CHR \
--pos BP \
--permutation-test

Possible flags include:

• --genetic-file the plink formatted gentoype file for the prediction sample. This should contain the same SNPs that are in the ".preproc.txt" files. If not, the overlap will be found.

• --pop-gwas-file the preprocessed file containing the population GWAS summary statistics.

• --sib-gwas-file the preprocessed file containing the sibling GWAS summary statistics.

• --pvalue the label of the column containing the population GWAS association pvalues in the pop-gwas-file.

• --ascertainment-set either needs to be set to "gwas" or "sibs". Determines which GWAS to use for SNP selection given a particular threshold. Default is "gwas".

• --chrom label of the column containing the chromosome of each SNP. Default is "CHR".

• --pos label of the base pair position of each locus. Defaults to "POS".

• --pop-effect label of the column in the population GWAS file that contains the effect size estimates. Defaults to "BETA".

• --pop-se label of the column in the population GWAS file that contains the standard errors. Defaults to "se".

• --sib-effect label of the column in the sibling GWAS file that contains the effect size estimates. Defaults to "BETA".

• --sib-se label of the column in the sibling GWAS file that contains the standard errors. Defaults to "se".

• --pval-col label of the column containing the population GWAS association p-values. Defaults to "P".

• --nboots the number of bootstraps to perform in estimating the standard errror of the isotropic inflation factor using the Deming regression framework.

• --eigenvals file that contains the eigenvalues of the prediction sample genotype matrix saved as an ".npy" formatted file. If this flag is absent then a numpy implementation of PCA will be berformed on the plink binary file specified using bfile.

• --eigenvecs file that contains the eigenvectors of the prediction sample genotype matrix saved as an ".npy" formatted file.

• --permutation-test the presence of this flag indicates that the PC-wise permutation test should be performed. If absent, only the isotropic inflation factor will be estimated.

• --perm-pcs the number of top PCs to be tested for the PC-wise permutation procedure. Defaults to the first 100 PCs.

• --nperm the number of permutations to perform for each PC-wise decomposition in constructing the permutation based null. Defaults to 1000 permutations.

• --outfile-label file prefix for output files produced during the data munging.

• --out directory path where results should be written.