spatialSimGP

Introduction

spatialSimGP is a simulation tool that generates spatial transcriptomics data. The purpose of this package is to use a Gaussian Process for each gene to simulate data with spatial variation. We use the Poisson distribution to simulate the values on the raw counts scale. The mean and variance are tied together in the Poisson distribution, so we simulate the mean-variance relationship with our function. The mean-variance relationship is a bias in real spatial transcriptomics data, so we must make sure it is a feature of in silico data as well. spatialSimGP provides the option to simulate data with a fixed or unique length scale for each gene. The simulated data can be used to evaluate the performance of spatial transcriptomics analysis methods.

Bioconductor houses the infrastructure to store and analyze spatially resolved transcriptomics data for R users, including many SVG detection methods. This simulation framework can be used to benchmark SVG detection methods and to develop new methods for spatially resolved transcriptomics data. Additionally, this package interfaces with the widely used SpatialExperiment class from Bioconductor.

Installation

The following code will install the latest release version of the spatialSimGP package from Bioconductor. Additional details are shown on the Bioconductor page.

install.packages("BiocManager")
BiocManager::install("spatialSimGP")

The latest development version can also be installed from the devel version of Bioconductor or from GitHub.

Simulation Framework

The simulation framework is as follows:

$$\boldsymbol{c(s)}|\lambda(\boldsymbol{s}) \sim Poisson (\lambda(\boldsymbol{s})); \lambda(\boldsymbol{s})= exp(\boldsymbol{\beta} + \boldsymbol{C}(\sigma^2))$$

$\boldsymbol{s}$: spatial locations
$\boldsymbol{\beta}$: vector of means per gene
$\sigma^2$: spatial component of variance
$\boldsymbol{C}$: covariance function using a Matern kernel with squared exponential distance

The exponential covariance function is as follows:

$$(C_{ij}(\boldsymbol{\theta})) = \sigma^2\exp(\frac{-||\boldsymbol{s_i}-\boldsymbol{s_j}||}{l})$$

$\boldsymbol{\theta} = (\sigma^2, l)$
$l$: length scale parameter
- sets how quickly spatial correlation decays with distance
$||\boldsymbol{s_i}-\boldsymbol{s_j}||$: Euclidean distance between spatial locations

We calculate the covariance matrix using the exponential covariance function. Using mean $\boldsymbol{0}$ and covariance $C(\boldsymbol{\theta})$ in the multivariate Normal distribution, we simulate a Gaussian Process per gene. We use the Gaussian process and $\beta$ to calculate $\lambda$ and then use the Poisson distribution to simulate the gene expression levels for each spot.

Tutorial

Load packages and data

library(MASS)
library(SpatialExperiment)
library(STexampleData)
library(ggplot2)

Simulating Data with Prior Coordinates Matrix

One way to simulate data is to provide a matrix of coordinates. In this example, we use a subset of spots from STexampleData::Visium_humanDLPFC(), which is available from Bioconductor.

spe_demo <- Visium_humanDLPFC()

colData(spe_demo)$subset <- ifelse(colData(spe_demo)$array_row > 20 & colData(spe_demo)$array_row < 65 & colData(spe_demo)$array_col > 20 & colData(spe_demo)$array_col < 65, 1, 0)
spe_demo <- spe_demo[, colData(spe_demo)$subset == 1]

coords <- spatialCoords(spe_demo)

We also have to define our remaining parameters before simulating the data.

n_genes is the total number of genes to simulate. In this example, we simulate 10 genes.
proportion is the proportion of genes that will have no spatially varying patterns. In other words, these genes will just have random noise. In this example, 50% of the genes will have no spatial patterns.
range_sigma.sq is the range of the spatial variance parameter. In this example, the spatial variance parameter will range from 0.2 to 3.
range_beta is the range of the mean expression value. In this example, the mean parameter will range from 0.5 to 9.

n_genes <- 5
proportion <- 0.4
range_sigma.sq <- c(1.5, 3)
range_beta <- c(3, 7)

(A) Simulating Data with Fixed Length Scale

We first simulate 5 genes with a fixed length scale parameter. The length scale parameter determines how quickly the correlation decays with distance. Larger length scale parameters simulate larger spatial patterns. The simulate function returns a SpatialExperiment object with the simulated data. Remember to set the seed for reproducibility.

length_scale <- 60

set.seed(16)
spe <- spatial_simulate(n_genes, proportion, coords, range_sigma.sq, range_beta, length_scale, length_scale_option = "fixed")

(B) Simulating Data with Unique Length Scale

We can also simulate data with a unique length scale for each gene. This process is slower than simulating data with a fixed length scale, but it allows for more flexibility in the spatial patterns of each gene. Each gene has a unique length scale parameter, so the Gaussian Process kernel must be calculated for each gene, slowing down the simulation process.

length_scale <- c(60, 40, 20, 80, 100)

set.seed(1)
spe <- spatial_simulate(n_genes, proportion, coords, range_sigma.sq, range_beta, length_scale, length_scale_option = "unique")

Simulating Data with User-Created Coordinates Matrix

If you have your own coordinates matrix, you can use that to simulate data. We have included an example below.

# 10 spots per side
n_side <- 20

# x and y coordinates for the grid
x_coords <- rep(1:n_side, each = n_side)
y_coords <- rep(1:n_side, times = n_side)

# combine into a matrix
coords <- cbind(x_coords, y_coords)
colnames(coords) <- c("pxl_col_in_fullres", "pxl_row_in_fullres")

# run the simulation
set.seed(1)
length_scale <- 60
spe <- spatial_simulate(n_genes, proportion, coords, range_sigma.sq, range_beta, length_scale, length_scale_option = "fixed")

Note: If you want to have complete control over each simulated gene, you can set n_genes = 1, proportion = 0, range_sigma.sq = c(a,a), and range_beta = c(b,b). This will allow you to simulate one gene at a time at the exact spatial variance and mean expression level desired. You could loop through this process to simulate multiple genes with different parameters.

set.seed(123) 
n_genes <- 1 
proportion <- 0 
range_sigma.sq <- c(1, 1)
range_beta <- c(3, 3)
length_scale <- 60

spe <- spatial_simulate(n_genes, proportion, coords, range_sigma.sq, range_beta, length_scale, length_scale_option = "fixed")

Session Info