Alakazam: Gene usage analysis

Example data

A small example AIRR database, ExampleDb, is included in the alakazam package. For details about the AIRR format, visit the AIRR Community documentation site.

Gene usage analysis requires only the following columns:

• v_call
• d_call
• j_call

However, the optional clonal clustering (clone_id) and duplicate count (duplicate_count) columns may be used to quantify usage by different abundance criteria.

# Load required packages
library(alakazam)
library(dplyr)
library(scales)

# Subset example data
data(ExampleDb)

Tabulate V(D)J allele, gene or family usage by sample

The relative abundance of V(D)J alleles, genes or families within groups can be obtained with the function countGenes. To analyze differences in the V gene usage across different samples we will set gene="v_call" (the column containing gene data) and groups="sample_id" (the columns containing grouping variables). To quantify abundance at the gene level we set mode="gene":

# Quantify usage at the gene level
gene <- countGenes(ExampleDb, gene="v_call", groups="sample_id", mode="gene")
head(gene, n=4)

## # A tibble: 4 × 4
## # Groups:   sample_id [2]
##   sample_id gene     seq_count seq_freq
##   <chr>     <chr>        <int>    <dbl>
## 1 +7d       IGHV3-49       698    0.699
## 2 -1h       IGHV3-9         83    0.083
## 3 -1h       IGHV5-51        60    0.06 
## 4 -1h       IGHV3-30        58    0.058

In the resultant data.frame, the seq_count column is the number of raw sequences within each sample_id group for the given gene. seq_freq is the frequency of each gene within the given sample_id.

Below we plot only the IGHV1 abundance by filtering on the gene column to only rows containing IGHV1 family genes. We extract the family portion of the gene name using the getFamily function. Also, we take advantage of the sortGenes function to convert the gene column to a factor with gene name lexicographically ordered in the factor levels (method="name") for axis ordering using the ggplot2 package. Alternatively, we could have ordered the genes by genomic position by passing method="position" to sortGenes.

# Assign sorted levels and subset to IGHV1
ighv1 <- gene %>%
    mutate(gene=factor(gene, levels=sortGenes(unique(gene), method="name"))) %>%
    filter(getFamily(gene) == "IGHV1")

# Plot V gene usage in the IGHV1 family by sample
g1 <- ggplot(ighv1, aes(x=gene, y=seq_freq)) +
    theme_bw() +
    ggtitle("IGHV1 Usage") +
    theme(axis.text.x=element_text(angle=45, hjust=1, vjust=1)) +
    ylab("Percent of repertoire") +
    xlab("") +
    scale_y_continuous(labels=percent) +
    scale_color_brewer(palette="Set1") +
    geom_point(aes(color=sample_id), size=5, alpha=0.8)
plot(g1)

Alternatively, usage can be quantified at the allele (mode="allele") or family level (mode="family"):

# Quantify V family usage by sample
family <- countGenes(ExampleDb, gene="v_call", groups="sample_id", mode="family")

# Plot V family usage by sample
g2 <- ggplot(family, aes(x=gene, y=seq_freq)) +
    theme_bw() +
    ggtitle("Family Usage") +
    theme(axis.text.x=element_text(angle=45, hjust=1, vjust=1)) +
    ylab("Percent of repertoire") +
    xlab("") +
    scale_y_continuous(labels=percent) +
    scale_color_brewer(palette="Set1") +
    geom_point(aes(color=sample_id), size=5, alpha=0.8)
plot(g2)

Tabulating gene abundance using additional groupings

The groups argument to countGenes can accept multiple grouping columns and will calculate abundance within each unique combination. In the examples below, groupings will be perform by unique sample and isotype pairs (groups=c("sample_id", "c_call")). Furthermore, instead of quantifying abundance by sequence count, we will quantify it by clone count (each clone will be counted only once regardless of how many sequences the clone represents).
Clonal criteria are added by passing a value to the clone argument of countGenes (clone="clone_id"). For each clonal group, only the most common allele/gene/family will be considered for counting.

# Quantify V family clonal usage by sample and isotype
family <- countGenes(ExampleDb, gene="v_call", groups=c("sample_id", "c_call"), 
                     clone="clone_id", mode="family")
head(family, n=4)

## # A tibble: 4 × 5
## # Groups:   sample_id, c_call [3]
##   sample_id c_call gene  clone_count clone_freq
##   <chr>     <chr>  <chr>       <int>      <dbl>
## 1 +7d       IGHA   IGHV5           1    0.0172 
## 2 +7d       IGHA   IGHV6           1    0.0172 
## 3 +7d       IGHD   IGHV6           1    0.0213 
## 4 +7d       IGHG   IGHV5           1    0.00971

The output data.frame contains the additional grouping column (c_call) along with the clone_count and clone_freq columns that represent the count of clones for each V family and the frequencies within the given sample_id and c_call pair, respectively.

# Subset to IGHM and IGHG for plotting
family <- filter(family, c_call %in% c("IGHM", "IGHG"))
# Plot V family clonal usage by sample and isotype
g3 <- ggplot(family, aes(x=gene, y=clone_freq)) +
    theme_bw() +
    ggtitle("Clonal Usage") +
    theme(axis.text.x=element_text(angle=45, hjust=1, vjust=1)) +
    ylab("Percent of repertoire") +
    xlab("") +
    scale_y_continuous(labels=percent) +
    scale_color_brewer(palette="Set1") +
    geom_point(aes(color=sample_id), size=5, alpha=0.8) +
    facet_grid(. ~ c_call)
plot(g3)

Instead of calculating abundance by sequence or clone count, abundance can be calculated using copy numbers for the individual sequences. This is accomplished by passing a copy number column to the copy argument (copy="duplicate_count"). Specifying both clone and copy arguments is not meaningful and will result in the clone argument being ignored.

# Calculate V family copy numbers by sample and isotype
family <- countGenes(ExampleDb, gene="v_call", groups=c("sample_id", "c_call"), 
                     mode="family", copy="duplicate_count")
head(family, n=4)

## # A tibble: 4 × 7
## # Groups:   sample_id, c_call [3]
##   sample_id c_call gene  seq_count copy_count seq_freq copy_freq
##   <chr>     <chr>  <chr>     <int>      <dbl>    <dbl>     <dbl>
## 1 +7d       IGHG   IGHV3       516       1587    0.977     0.984
## 2 +7d       IGHA   IGHV3       240       1224    0.902     0.935
## 3 -1h       IGHM   IGHV3       237        250    0.421     0.386
## 4 -1h       IGHM   IGHV4       110        162    0.195     0.25

The output data.frame includes the seq_count and seq_freq columns as previously defined, as well as the additional copy number columns copy_count and copy_freq reflected the summed copy number (duplicate_count) for each sequence within the given gene, sample_id and c_call.

# Subset to IGHM and IGHG for plotting
family <- filter(family, c_call %in% c("IGHM", "IGHG"))
# Plot V family copy abundance by sample and isotype
g4 <- ggplot(family, aes(x=gene, y=copy_freq)) +
    theme_bw() +
    ggtitle("Copy Number") +
    theme(axis.text.x=element_text(angle=45, hjust=1, vjust=1)) +
    ylab("Percent of repertoire") +
    xlab("") +
    scale_y_continuous(labels=percent) +
    scale_color_brewer(palette="Set1") +
    geom_point(aes(color=sample_id), size=5, alpha=0.8) +
    facet_grid(. ~ c_call)
plot(g4)