Last updated: 2022-07-25

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Before you begin:

  • Make sure that R is installed on your computer
  • For this lab, we will use the following R libraries:
require(data.table)
require(dplyr)
require(tidyr)
require(bigsnpr)
require(ggplot2)

The R template to do the exercises is here.

Note: if on the online server, set your working directory to your home directory using in R

setwd("home/<username>/")

The data files are in the folder /data/SISG2022M15/data/.

Population Structure Inference

Introduction

We will be working with a subset of the genotype data from the Human Genome Diversity Panel (HGDP) and HapMap.

The file “YRI_CEU_ASW_MEX_NAM.bed” is a binary file in PLINK BED format with accompanying BIM and FAM files. It contains genotype data at autosomal SNPs for:

  • Native American samples from HGDP
  • Four population samples from HapMap:
    • Yoruba in Ibadan, Nigeria (YRI)
    • Utah residents with ancestry from Northern and Western Europe (CEU)
    • Mexican Americans in Los Angeles, California (MXL)
    • African Americans from the south-western United States (ASW)

File with ancestry labels assignment for each sample: Population_Sample_Info.txt

Exercises

Here are some things to look at:

  1. Examine the dataset:
  • How many samples are present?
  • How many SNPs?
  • What is the number of samples in each population?
  1. Get the first 10 principal components (PCs) in PLINK using all SNPs. The basic command would look like
plink2 --bfile <plink_bed_prefix> --pca 10 --out <output_prefix>

This generates a file <output_prefix>.eigenvec containing the PCs (eigenvectors) as well as another file <output_prefix>.eigenval containing the top eigenvalues.

  • Make a scatterplot of the first two PCs with each point colored by population membership.
  • Interpret the first two PCs, what ancestries are they reflecting?
  • Make a scree plot of the eigenvalues for the first 10 PCs. Approximate the proportion of variance explained by the first two PCs.
  1. Now redo Question 2 above using the bigsnpr R package specifying a \(r^2\) threshold of 0.2 (i.e. LD pruning) as well as a minimum minor allele count (MAC) of 20. The basic command would look like
# run PCA
obj.bed <- bed(bedfile = <plink_bed_file>)
pc.out <- bed_autoSVD(
  obj.bed, 
  thr.r2 = <r2_threshold>, 
  k = <number_of_PCs>, 
  min.mac = <min_MAC>
)
# plot PC2 vs PC1
plot(pc.out, type = "scores", scores = 1:2)
# scree plot
plot(pc.out) 
# plot SNP loadings (should be centered at 0)
plot(pc.out, type = "loadings", scores = 1:<number_of_PCs>, coeff = 0.4)
  • Run PCA and make a scatter plot of the first two principal components (PCs) with each point colored according to population membership.
  • Does the plot change from the one in Question 2?
  • Check the SNP loadings for the first 10 PCs.

(Hint: This tutorial document from bigsnpr might be helpful)

  1. Predict the proportional Native American and European Ancestry for the HapMap MXL from the PCA output in Question 3 using one of the principal components. (Which PC is most appropriate for this analysis?) Assume that the HapMap MXL have negligible African Ancestry.

  2. Make a barplot of the proportional ancestry estimates from question 4.

Extra: 6. Check if there are samples related 2nd degree or closer. If so, run PCA as in Question 3 removing these samples then project the remaining samples onto the PC space. The basic command would look like

# check for 3rd degree relateds or closer
snp_plinkKINGQC(
  plink2.path = "/usr/bin/plink2", 
  bedfile.in = <plink_bed_prefix>, 
  thr.king = 2^-3.5,
  make.bed = FALSE
)

(Hint: This returns a data frame which contains all pairs of individuals related 3rd degree or closer. We can then remove them when calling bed_autoSVD() using the ind.row argument. Finally, you can use bed_projectSelfPCA() to project related samples on the PC space.)


sessionInfo()
R version 4.2.1 (2022-06-23)
Platform: x86_64-pc-linux-gnu (64-bit)
Running under: Ubuntu 22.04 LTS

Matrix products: default
BLAS:   /usr/lib/x86_64-linux-gnu/blas/libblas.so.3.10.0
LAPACK: /usr/lib/x86_64-linux-gnu/lapack/liblapack.so.3.10.0

locale:
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[10] LC_TELEPHONE=C         LC_MEASUREMENT=C.UTF-8 LC_IDENTIFICATION=C   

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base     

other attached packages:
[1] workflowr_1.7.0

loaded via a namespace (and not attached):
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[29] glue_1.6.2       R6_2.5.1         processx_3.7.0   fansi_1.0.3     
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