---
title: "Find Batch-biased SVGs"
package: "BatchSVG"
author:
  - name: "Kinnary Shah"
    affiliation: Department of Biostatistics, Johns Hopkins University
    email: kinnaryshahh@gmail.com
  - name: "Christine Hou"
    affiliation: Department of Biostatistics, Johns Hopkins University
    email: chris2018hou@gmail.com
output: BiocStyle::html_document
vignette: >
  %\VignetteIndexEntry{Tutorial for spe object}
  %\VignetteEncoding{UTF-8}
  %\VignetteEngine{knitr::rmarkdown}
editor_options: 
  markdown: 
    wrap: 72
---

```{r, include = FALSE}
knitr::opts_chunk$set(
    collapse = TRUE,
    comment = "#>"
)
```

# Introduction

A standard task in the analysis of spatially resolved transcriptomics (SRT) data is to identify spatially variable genes (SVGs). This is most commonly done within one tissue section at a time because the spatial relationships between the tissue sections are typically unknown. One challenge is how to identify and remove SVGs that are associated with a known bias or technical artifact, such as the slide or capture area, which can lead to poor performance in downstream analyses, such as spatial domain detection. Here, we introduce `BatchSVG`, a tool to identify biased features associated with batch effect(s)
(e.g. sample, slide, and sex) in a set of SVGs. Our approach compares the rank of per-gene deviance under a binomial model (i) with and (ii) without including a covariate in the model that is associated with the known bias or technical artifact. If the rank of a feature changes significantly between these, then we infer that this gene is likely associated with the bias or technical artifact and should be removed from the downstream analysis. The package is compatible with [SpatialExperiment](https://github.com/drighelli/SpatialExperiment) objects.

# Installation

```{r install bioc, eval=FALSE}
if (!requireNamespace("BiocManager")) {
    install.packages("BiocManager")
}
BiocManager::install("BatchSVG")
```

To install the development version from
[GitHub](https://christinehou11.github.io/BatchSVG) instead:

```{r install github, eval = FALSE}
remotes::install("christinehou11/BatchSVG")
```

# Biased Feature Identification

In this section, we will demonstrate the workflow for using
`BatchSVG` and show how the method helps to detect and visualize batch-biased SVGs. We will use an SRT dataset from
the [WeberDivechaLCdata](https://bioconductor.org/packages/release/data/experiment/html/WeberDivechaLCdata.html) package.


```{r library, message=FALSE}
library(BatchSVG)
library(WeberDivechaLCdata)
library(cowplot)
```

```{r load data, comment=NA, warning=FALSE, message=FALSE}
spe <- WeberDivechaLCdata_Visium()
spe
```

We will use an SVG set that was previously generated using the
[nnSVG]((https://www.nature.com/articles/s41467-023-39748-z)) package.

```{r load nnsvg, comment=NA, warning=FALSE, message=FALSE}
svgs <- read.csv("svgs_LC.csv", check.names = FALSE)
```

## Perform Feature Selection using `featureSelect()`

This function performs feature selection on a chosen subset of SRT
data (*input*) using a predefined set of SVGs
(*VGs*).

To assess the impact of the batch variable in the SRT data, we fit a binomial model per gene (i) with and (ii) without including a covariate in the model that is associated with the batch effect or unwanted variation. Using this model output, we define $d_{i, \text{ default}}$ and $d_{i, \text{ batch name}}$ as the residual deviance for gene $i$ using a binomial model with and without the batch effect, respectively. We calculate a per-gene relative change in deviance (RCD) as 
${RCD}_i = \frac{d_{i, \text{ default}} - d_{i, \text{ batch name}}}{d_{i, \text{ batch name}}}$. 
Generally, a higher per-gene deviance $d_i$ suggests that the gene's expression is more likely to be biologically meaningful. Therefore, a reduction in deviance after accounting for the batch covariate indicates that the batch explains a portion of the variation in gene expression that was previously attributed to biological differences.

In addition to the residual deviance itself, we also consider the ranks of the residual deviances where top-ranked genes have the largest residual deviance and are considered more important. Here, an increase in rank (e.g. from a rank of 1 to a rank of 500) when including the batch variable indicates that the relative importance of the feature is diminished once the batch variable is accounted for. Therefore, in addition to RCD, we also evaluated the rank deviance (RD), which is defined as 
${RD}_i = r_{i, \text{ batch name}} - r_{i, \text{ default}}$, where $r_{i, \text{ default}}$ and $r_{i, \text{ batch name}}$ are the per-gene rank when the binomial deviance model is run with and without the batch variable, respectively.


The `featureSelect()` function enables feature selection while
accounting for multiple batch effects. It returns a **list** of data
frames, where each batch effect is associated with a corresponding data
frame containing key results, including:

-   Relative change in deviance before and after batch effect adjustment (RCD)

-   Rank differences before and after batch effect adjustment (RD)

-   Number of standard deviations (nSD) for both relative change in
    deviance and rank difference

We use apply `featureSelect()` to the example
dataset while adjusting for the batch effect of *sample_id*.

```{r feature select, comment = NA, warning=FALSE}
list_batch_df <- featureSelect(input = spe, 
    batch_effect = "sample_id", VGs = svgs$gene_id)
```

```{r, eval=FALSE}
list_batch_df <- featureSelect(input = spe, 
    batch_effect = "sample_id", VGs = svgs$gene_id, verbose = FALSE)
```

```{r feature select class, comment = NA, warning=FALSE}
class(list_batch_df)
```

```{r feature select print, comment = NA, warning=FALSE}
head(list_batch_df$sample_id)
```

## Visualize SVG Selection Using `svg_nSD()`

The `svg_nSD()` function generates visualizations to assess the impact of the batch
variable(s) on the SVGs. It produces bar charts
of the distribution of SVGs based on relative change in deviance
and rank difference, with colors representing different nSD intervals.
Additionally, the scatterplots compare deviance and rank values with and
without batch effects. Using these plots, we can determine appropriate nSD thresholds
for filtering batch-biased SVGs.

The SVGs in red and very far off the line are most likely to be batch-biased SVGs since their deviance and/or rank values are more changed compared to the other SVGs.

```{r svg, comment=NA, warning=FALSE, message=FALSE}
plots <- svg_nSD(list_batch_df = list_batch_df, 
            sd_interval_dev = 5, sd_interval_rank = 5)
```

*Figure 1. Visualizations of nSD_dev and nSD_rank threshold selection*

```{r figure 1, warning=FALSE, message=FALSE, fig.width=10, fig.height=8}
plots$sample_id
```

## Identify Biased Genes Using `biasDetect()`

The function `biasDetect()` is designed to identify and filter out
batch-biased SVGs across different batch effects. Using threshold values
selected from the visualization results generated by `svg_nSD()`, this
function detects outliers that exceed a specified number
of standard deviation (nSD) threshold in either relative change in deviance, rank difference, or both.

The function outputs visualizations comparing deviance and rank values
with and without batch effects. Genes with high deviations, highlighted
in color, are identified as potentially biased and can be excluded based
on the selected nSD thresholds.

The function offers flexibility in customizing plot aesthetics,
allowing users to adjust the point size (**plot_point_size**),
shape (**plot_point_shape**), annotated text size (**plot_text_size**),
and color palette (**plot_palette**). Default values are
provided for these parameters if not specified. Users should refer to the
[ggplot2](https://ggplot2.tidyverse.org/index.html) aesthetic guidelines
to ensure appropriate values are assigned for each parameter.

We will use `nSD_dev = 10` and `nSD_rank = 8` for the example. Users
should adjust these values based on their datasets. We have chosen relatively high thresholds in this example to be more conservative with the SVGs we will filter out of the SVG list.

**Usage of Different Threshold Options**

-   `threshold = "dev"`: Filters batch-biased SVGs based only on the relative
    change in deviance. Genes with deviance changes exceeding the
    specified `nSD_dev` threshold are identified as batch-biased and
    can be removed.

```{r bias detect dev, comment = NA, message=FALSE, warning=FALSE}
bias_dev <- biasDetect(list_batch_df = list_batch_df, 
    threshold = "dev", nSD_dev = 10)
```

*Table 1. Outlier Genes defined by nSD_dev only*

```{r table 1, comment = NA, message=FALSE, warning=FALSE}
head(bias_dev$sample_id$Table)
```

We can change point size using **plot_point_size**.

```{r size change, message=FALSE, warning=FALSE}
# size default = 3
bias_dev_size <- biasDetect(list_batch_df = list_batch_df, 
    threshold = "dev", nSD_dev = 10, plot_point_size = 4)
```

*Figure 2. Customize point size*

```{r figure 2, warning=FALSE, message=FALSE, fig.width= 10, fig.height=4}
plot_grid(bias_dev$sample_id$Plot, bias_dev_size$sample_id$Plot)
```

-   `threshold = "rank"`: Identifies biased genes based only on rank
    difference. Genes with rank shifts exceeding `nSD_rank` are
    considered batch-biased.

```{r bias detect rank, comment = NA, message=FALSE, warning=FALSE}
bias_rank <- biasDetect(list_batch_df = list_batch_df, 
    threshold = "rank", nSD_rank = 8)
```

*Table 2. Outlier Genes defined by nSD_rank only*

```{r table 2, comment = NA, message=FALSE, warning=FALSE}
head(bias_rank$sample_id$Table)
```

We can change point shape using **plot_point_shape**.

*Figure 3. Customize point shape*

```{r figure 3, message=FALSE, warning=FALSE, fig.width= 10, fig.height=4}
# shape default = 16
bias_rank_shape <- biasDetect(list_batch_df = list_batch_df, 
    threshold = "rank", nSD_rank = 8, plot_point_shape = 2)

plot_grid(bias_rank$sample_id$Plot, bias_rank_shape$sample_id$Plot)
```

-   `threshold = "both"`: Detects biased genes based on both relative change in deviance and rank difference, providing a more stringent filtering
    approach.

```{r both, comment = NA, message=FALSE, warning=FALSE}
bias_both <- biasDetect(list_batch_df = list_batch_df, threshold = "both",
    nSD_dev = 10, nSD_rank = 8)
```

*Table 3. Outlier Genes defined by nSD_dev and nSD_rank*

```{r table 3, comment = NA, message=FALSE, warning=FALSE}
head(bias_both$sample_id$Table)
```

We can change point color using **plot_palette**. The color
palette
[here](https://r-graph-gallery.com/38-rcolorbrewers-palettes.html) can
be referenced as the function uses `RColorBrewer` to generate
colors.

*Figure 4. Customize the palette color*

```{r figure 4, message=FALSE, warning=FALSE, fig.width= 10, fig.height=8}
# color default = "YlOrRd"
bias_both_color <- biasDetect(list_batch_df = list_batch_df, 
    threshold = "both", nSD_dev = 10, nSD_rank = 8, plot_palette = "Greens")

plot_grid(bias_both$sample_id$Plot, bias_both_color$sample_id$Plot,nrow = 2)
```

We can change text size using **plot_text_size**. We can also specify unique color palettes for both batch effects.

*Figure 5. Customize text size and color palette*

```{r figure 5, message=FALSE, warning=FALSE, fig.width= 10, fig.height=8}
# text size default = 3
bias_both_color_text <- biasDetect(list_batch_df = list_batch_df, 
    threshold = "both", nSD_dev = 10, nSD_rank = 8, 
    plot_palette = c("Blues"), plot_text_size = 4)

plot_grid(bias_both$sample_id$Plot, bias_both_color_text$sample_id$Plot,nrow = 2)
```

## Refine SVGs by Removing Batch-Affected Outliers

Finally, we obtain a refined set of SVGs by
removing the batch-biased SVGs based on user-defined thresholds for
`nSD_dev` and `nSD_rank`.

Here, we use the results from bias_both, which applied
`threshold = "both"` to account for both deviance and rank differences.

```{r new svgs, comment = NA, message=FALSE, warning=FALSE}
bias_both_df <- bias_both$sample_id$Table
svgs_filt <- setdiff(svgs$gene_id, bias_both_df$gene_id)
svgs_filt_spe <- svgs[svgs$gene_id %in% svgs_filt, ]
nrow(svgs_filt_spe)
```

After obtaining the refined set of SVGs, these genes can be further
analyzed using established SRT clustering algorithms
to explore tissue layers and spatial organization.

# `R` session information {.unnumbered}

```{r session info}
## Session info
sessionInfo()
```
