RATTACA 🐀

🚧 Under Construction 🚧

The RATTACA method uses SNP genotypes produced from low-coverage DNA sequencing to predict phenotypes in HS rats, then uses these trait predictions to assign for study animals with predicted extreme phenotypes. This package provides helper tools for fitting linear mixed models (LMM) for prediction, and assessing LMM performance under the goals of the RATTACA project.

Predictions are calculated by estimating marker effects using the 'RR-BLUP' method of the rrBLUP package. The primary output of this package - a set of phenotype predictions - can then be used to assign rats using the RATTACA assignment package.

Installation

RATTACA predictions are conducted in R. You will need to install R >= 4.1 with the following R packages:

• genio (>=1.1.2) to read/write PLINK genotype data
• rrBLUP (>=4.6.2) to fit prediction models
• roxygen2 (>=2.7.0) (optional: to build package documentaion)
• viridis (>=0.6.2) (for optional plotting)
• scales (>=1.2.0) (for optional plotting)

First, simply clone the repository:

git clone [email protected]:Palmer-Lab-UCSD/rattaca.git

To build the package documentation (which allows dynamic generation of RATTACA documentation in R), navigate into the cloned repository, start R, and from the R command line:

library(roxygen2)
roxygenize()

Then install from the command line. Assuming the cloned repository is in the current working directory:

R CMD INSTALL [-l library_path] rattaca

Next, exit R to build the package. Navigate outside the cloned repository. With the repo inside the current working directory, execute from the command line:

R CMD build rattaca

... which will generate a tar ball rattaca-<version_number>.tar.gz in the current working directory. Then start R and in the R command line:

install.packages('rattaca-<version_number>.tar.gz', repos=NULL)

... ensuring repos is set to NULL to install from a local package.

Download a tagged version

On this GitHub page, on the right-hand side, click Releases. Choose a release version, and click on the compressed source code link for .tar.gz. Given that this downloaded in the Downloads folder, you can install or build as above:

cd ~/Downloads
R CMD INSTALL [-l library_path] rattaca-<tagged_version_number>.tar.gz

Package Features

Here we introduce rattaca package features with a series of examples.

Example: fitting model to simulation data

Using the R command line we can generate arbitrary simulated biallelic genotypes

library(rattaca)

q_markers <- 100
n_samples <- 210

genotypes <- sample(c(0,1), q_markers * n_samples,
                    replace=TRUE)

genotypes <- matrix(genotype, ncol=q_markers, nrow=n_samples)

from these data we can generate a simulation closure to sample trait values given a trait heritability and genotypes

heritability <- 0.4

simulator <- rattaca::gen_sim_closure_r_sq(heritability, genotypes)

trait_data <- simulator(genotypes)

Using the genotype and trait pairs, we can fit the data using the rattaca wrapper for rrBLUP's rrBLUP::mixed.solve.

out_pars <- rattaca::fit(trait_data,
                         genotypes)

Then by using the ls function we see that the out_pars list contains

ls(out_pars)
[1] "beta"  "beta.SE"   "LL"  "pearson_corr"
[5] "r_sq"  "spearman_corr"    "u"
[9] "u.SE"  "Ve"  "Vu"

where the parameter definitions are as follows:

parameter	description
beta	Fixed effect size, i.e. the intercept term
beta.SE	Standard error of intercept
LL	log-likelihood of fit
pearson_corr	Pearson correlation between predictions and training data
r_sq	Coefficient of determination, R^2, between predictions and training data
spearman_corr	Spearman correlation between predictions and training data
u	BLUP for each marker random effect
u.SE	Standard error BLUP for each marker random effect
Ve	Variance of error term
Vu	Variance of random effects

Note, that fitting is computationally expensive for large amounts of fitting data. Test on your system prior to running to estimate time.

Loading genotypes and trait data from R

Suppose that our current working direcotry contained a directory data for which the plink and trait data files are located,

-data/
    |- genotype.bed
    |- genotype.bim
    |- genotype.fam
    |- traits.csv

Then to load the genotypes and trait data for mass where rfid is the sample id column,

genotypes <- rattaca::load_and_prepare_plink_data("data/genotypes")
trait <- rattaca::load_and_prepare_trait_data("data/traits.csv",
                "rfid", "mass")

RATTACA Scripts

rattaca scripts can be found in the inst directory of this repository.

Example: fitting marker BLUPs

To fit the model to data, you'll need to have on hand:

• genotype files
• trait measurements as csv
• rat_fit.R script

Note that the animal identifies in the genotype files must match that of the trait measurements. Let us assume that the current working directory

- rat_fit.R
- data/
    |- genotype.bed
    |- genotype.bim
    |- genotype.fam
    |- traits.csv

Then to run the script

$ Rscript rat_fit.R \
--plink_genotypes_prefix 'data/genotype' \
--trait_file 'data/traits.csv' \
--trait_rat_id_colname 'rfid' \
--trait 'mass' \
--out_dir 'results'

which will print the results to file results/mass.bpar.

Example: performing power analysis

Output specification

The .bpar file contains records separated by line. Records come in three flavors and are distinguishable by line prefex:

Prefix	Description
##	Meta data containing key value pairs delimited by =.
#	Header, defines the values for each variant record, comma delimited.
No prefix	Variant record, comma delimited.

The header includes

Column Name	Description
var_id	As specified in
random_effect	BLUP of the random effect for variant produced by rrBLUP:mixed.solve
random_effect_se	Standard error of BLUP produced by rrBLUP::mixed.solve

Contribute

Please clone or fork the repository. Submit changes as a new branch and open a Pull Request. Also, please add new unit tests and run them prior to submitting a recommended change. Recall that an R package can be tested by, assuming that the cloned rattaca package directory is in your current working directory,

R CMD check rattaca

This command will:

• print several helpful messages to the standard out,
• create directory rattaca.Rcheck with helpful log files,
• build documentation, and
• run unit-tests.

Citation

• Johnson, Benjamin B., et al. "RATTACA: Genetic predictions in Heterogeneous Stock rats offer a new tool for genetic correlation and experimental design." bioRxiv (2023): 2023-09.

References

• Endelman, J.B. 2011. Ridge regression and other kernels for genomic selection with R package rrBLUP. Plant Genome 4:250-255.