Model per-gene variance in expression

Overview

This repository contains functions to model the per-gene expression from a gene-by-cell matrix of (log-transformed) expression values. Genes with high variance are considered to be more interesting and are prioritized for further analyses. The code itself was originally derived from the scran R package, factored out into a separate C++ library for easier re-use.

Quick start

Given a tatami::Matrix of log-expression values for each gene in each cell, we can compute the per-gene variances and model the trend with respect to the mean across genes:

#include "scran_variances/scran_variances.hpp"
 
std::shared_ptr<tatami::Matrix<double, int> > mat = some_data_source();
 
scran_variances::ModelGeneVariancesOptions opt;
auto res = scran_variances::model_gene_variances(*mat, opt);
 
res.means; // vector of means across genes.
res.variances; // vector of variances across genes.
res.fitted; // vector of fitted values of the mean-variance trend for each gene.
res.residuals; // vector of residuals from the trend.

Typically, the residuals are used for feature selection, as these account for non-trivial mean-variance trends in transformed count data.

scran_variances::ChooseHighlyVariableGenesOptions copt;
copt.top = 5000;
auto chosen = scran_variances::choose_highly_variable_genes_index(
    res.residuals.size(), 
    res.residuals.data(), 
    copt
);
 
// Create the HVG submatrix for downstream analysis.
auto hvg_subset = tatami::make_DelayedSubset(mat, chosen, /* by_row = */ true);

Users can also fit a trend directly to their own statistics.

scran_variances::FitVarianceTrendOptions fopt;
fopt.span = 0.5;
fopt.minimum_mean = 1;
auto fit = scran_variances::fit_variance_trend(100, means, variances, fopt);
fit.fitted; // fitted values for all genes.
fit.residuals; // residuals values for all genes.

Check out the reference documentation for more details.

Building projects

CMake with FetchContent

If you're using CMake, you just need to add something like this to your CMakeLists.txt:

include(FetchContent)
 
FetchContent_Declare(
  scran_variances
  GIT_REPOSITORY https://github.com/libscran/scran_variances
  GIT_TAG master # or any version of interest
)
 
FetchContent_MakeAvailable(scran_variances)

Then you can link to scran_variances to make the headers available during compilation:

# For executables:
target_link_libraries(myexe libscran::scran_variances)
 
# For libaries
target_link_libraries(mylib INTERFACE libscran::scran_variances)

CMake with find_package()

find_package(libscran_scran_variances CONFIG REQUIRED)
target_link_libraries(mylib INTERFACE libscran::scran_variances)

To install the library, use:

mkdir build && cd build
cmake .. -DSCRAN_VARIANCES_TESTS=OFF
cmake --build . --target install

By default, this will use FetchContent to fetch all external dependencies. If you want to install them manually, use -DSCRAN_VARIANCES_FETCH_EXTERN=OFF. See the tags in extern/CMakeLists.txt to find compatible versions of each dependency.

Manual

If you're not using CMake, the simple approach is to just copy the files in include/ - either directly or with Git submodules - and include their path during compilation with, e.g., GCC's -I. This requires the external dependencies listed in extern/CMakeLists.txt, which also need to be made available during compilation.