Calculate mutation frequency — calculate

Calculates mutation frequencies for arbitrary groupings and creates a new dataframe with the results. Mutation frequency is calculated by dividing the sum of mutations by the sum of the total_depth for a given group (mutations/bp). The operation is run using both the minimum and maximum independent mutation counting methods.

Usage

calculate_mf(
  mutation_data,
  cols_to_group = "sample",
  subtype_resolution = "none",
  variant_types = c("snv", "deletion", "insertion", "complex", "mnv", "sv", "ambiguous",
    "uncategorized"),
  calculate_depth = TRUE,
  correct_depth = TRUE,
  correct_depth_by_indel_priority = FALSE,
  precalc_depth_data = NULL,
  d_sep = "\t",
  summary = TRUE,
  retain_metadata_cols = NULL
)

Arguments

mutation_data: The data frame (or GRanges) to be processed containing mutation data. Required columns are listed in details.
cols_to_group: A vector of grouping variables. This should be the groups of interest that you want to calculate a frequency for. For instance, getting the frequency by "sample". Other options might include an experimental group Ex. "dose" or a locus Ex. c("sample", "locus"). All listed variables must be a column in the mutation_data. Do not include mutation subtype columns in this field. Please refer to subtype_resolution to group by subtype as the calculation will differ.
subtype_resolution: The degree at which to resolve the mutation subtypes when calculating frequencies. Mutation frequency will be calculated across all col_to_groups for each mutation subtype given the desired resolution. Subtype proportions will also be calculated. Options are "none", "type", "base_6", "base_12", "base_96", and "base_192". See details for definitions.
variant_types: Use this parameter to choose which variation types to include in the mutation counts. Provide a character vector of the variation types that you want to include. Alternatively, provide a character vector of the variation types that you want to exclude preceded by "-". Options are: "snv", "complex", "deletion", "insertion", "mnv", "sv", "ambiguous", "uncategorized". Ex. inclusion: "snv", exclusion: "-snv". Default includes all variants. For calculate_depth = TRUE: Regardless of whether or not a variant is included in the mutation counts, the total_depth for that position will be counted.
calculate_depth: A logical variable, whether to calculate the per-group total_depth from the mutation data. If set to TRUE, the mutation data must contain a total_depth value for every sequenced base (including variants AND no-variant calls). If set to FALSE, pre-calculated per-group total_depth values may be supplied at the desired subtype_resolution using the precalc_depth_data parameter. Alternatively, if no per-group total_depth is available, per-group mutation counts will be calculated, but mutation frequency will not. In such cases, mutation subtype proportions will not be normalized to the total_depth.
correct_depth: A logical value. If TRUE, the function will correct the total_depth column in mutation_data in order to prevent double-counting the total_depth values for the same genomic position. For rows with the same sample, contig, and start values, the total_depth will be retained for only one row. All other rows in the group will have their total_depth set to 0. The default is TRUE.
correct_depth_by_indel_priority: A logical value. If TRUE, during depth correction, should there be different total_depth values within a group of rows with the same sample, contig, and start values, the total_depth value for the row with the highest priority variation_type will be retained, while the other rows will have their total_depth set to 0. variation_type priority order is: deletion, complex, insertion, snv, mnv, sv, uncategorised, ambiguous, no_variant. If FALSE, the total_depth value for the first row in the group will be retained, while the other rows will have their total_depth set to 0. The default is FALSE.
precalc_depth_data: A data frame or a file path to a text file containing pre-calculated per-group total_depth values. This data frame should contain the columns for the desired grouping variable(s) and the reference context at the desired subtype resolution (if applicable). The precalculated total_depth column(s) should be called one of group_depth and subtype_depth. group_depth is used for subtype resolutions of "none", "type", and all non-snv mutations in "base_6", "base_12", "base_96", and "base_192". subtype_depth is used for snv mutations in "base_6", "base_12", "base_96", and "base_192". You can access a list of context values for each subtype resolution using MutSeqR::context_list$your_subtype_resolution.
d_sep: The delimiter used in the precalc_depth_data, if applicable. Default is tab-delimited.
summary: A logical variable, whether to return a summary table (i.e., where only relevant columns for frequencies and groupings are returned). Setting this to false returns all columns in the original mutation_data, which might make plotting more difficult, but may provide additional flexibility to power users.
retain_metadata_cols: a character vector that contains the names of the metadata columns that you would like to retain in the summary table. This may be useful for plotting your summary data. Ex. retain the "dose" column when summarising by "sample".

Value

A data frame with the mutation frequency calculated. If summary is set to TRUE, the data frame will be a summary table with the mutation frequency calculated for each group. If summary is set to FALSE, the mutation frequency will be appended to each row of the original mutation_data.

sum_min: The sum of all mutations within the group, calculated using the "min" method for mutation counting. All identical mutations within a samples are assumed to be the result of clonal expansion and are thus only counted once.
sum_max: The sum of all mutations within the group, calculated using the "max" method for mutaiton counting. All identical mutations within a sample are assumed to be idenpendant mutational evens and are included in the mutation frequency calculation.
group_depth: The total_depth summed across groups.
subtype_depth: The total_depth summed across groups for a given sequence context. Used for calculating subtype frequencies.
mf_min: The mutation frequency calculated using the "min" method for mutation counting. mf_min = sum_min / depth.
mf_max: The mutation frequency calculated using the "max" method for mutation counting. mf_max = sum_max / depth.
proportion_min: The proportion of each mutation subtype within the group, normalized to the depth. Calculated using the "min" method. This is only calculated if subtype_resolution is not "none". If no depth is calculated or provided, proportion is calculated without normalization to the depth.
proportion_max: The proportion of each mutation subtype within the group, normalized to its read depth. Calculated using the "max" method. This is only calculated if subtype_resolution is not "none". If no depth is calculated or provided, proportion is calculated without normalization to the depth.

Details

Required columns:

contig: (or seqnames) The reference sequence name.
start: 1-based start position of the feature.
alt_depth: The read depth supporting the alternate allele.
variation_type: The category to which this variant is assigned.
subtype_col: The column containing the mutation subtype. This column depends on the subtype_resolution parameter.
reference context: The column containing the referene base(s) for the mutation. This column depends on the subtype_resolution parameter.
cols to group: all columns across which you want to calculate the mutation frequency. Ex. c("tissue", "dose"). These columns should be listed in cols_to_group.

It is also required to include the total_depth column if you are calculating depth from the mutation data. If you are using precalculated depth data, the total_depth column is not required.

Subtype Resolutions:

"none" calculates mutation frequencies across all selected grouping columns.
"type" calculates mutation frequencies across all selected grouping columns for each variation_type seperately; snv, mnv, deletion, insertion, complex, sv, ambiguous, uncategorized.
"base_6" calculates mutation frequencies across all selected grouping columns for each variation_type with snv mutations separated by normalized_subtype; C>A, C>G, C>T, T>A, T>C, T>G. The reference context is normalized_ref.
"base_12" calculates mutation frequencies across all selected grouping columns for each variation_type with snv mutations separated by subtype; A>C, A>G, A>T, C>A, C>G, C>T, G>A, G>C, G>T, T>A, T>C, T>G. The reference context is short_ref.
"base_96" calculates mutation frequencies across all selected grouping columns for each variation_type with snv mutations separated by normalized_context_with_mutation, i.e. the 96-base trinucleotide context. Ex. A[C>T]A. The reference context is normalized_context.
"base_192" calculates mutation frequencies across all selected grouping columns for each variation_type with snv mutations separated by context_with_mutation, i.e. the 192-base trinucleotide context. Ex A[G>A]A. The reference context is context.

Subtype depth: For SNV subtypes, the total_depth is summed based on the sequence context in which the SNV subtype occurs. Ex. for base_6, the two possible reference bases are C or T; hence, the total_depth is summed seperately for C:G positions and T:A positions. The MF for C>T mutations is calculated as total # C>T mutations / total_depth for C>G positions (sum / subtype_depth). Non-SNV mutation types will be caluclated as their sum / group_depth, since they can occur in the context of any nucleotide.

retain_metadata_cols at subtype_resolution: The summary table uses a pre-defined list of possible subtypes for each resolution. If a particular subtype within a given group is not recorded in the mutation data, the summary table will have no frame of reference for populating the metadata_cols. Thus, for subtypes that do not occur in the mutation data for a given group, the corresponding metadata_col will be NA.

Variant filtering: Variants flagged as TRUE in the filter_mut column will be excluded from the mutation counts. However, the total_depth of these variants will be included in the group/subtype depths if calculating depth.

Depth correction is important for preventing double-counting of reads in mutation data when summing the total_depth across samples or other groups. Generally, when several mutations have been detected at the same genomic position, within a sample, the total_depth value will be the same for all of them. However, in some datasets, whenever a deletion is detected, the data may contain an additional row with the same genomic position calling a "no_variant". The total_depth will differ between the deletion and the no_variant. In these cases, correct_depth_by_indel_priority == TRUE will ensure that the total_depth value for the deletion is retained, while the total_depth value for the no_variant is removed.

Examples

if (requireNamespace("MutSeqRData", quietly = TRUE)) {
  # Example data consists of 24 mouse bone marrow DNA samples imported
  # using import_mut_data() and filtered with filter_mut as in Example 4.
  # Sequenced on TS Mouse Mutagenesis Panel. Example data is
  # retrieved from MutSeqRData, an ExperimentHub data package.
  library(ExperimentHub)
  eh <- ExperimentHub()
  example_data <- eh[["EH9861"]]

  # Calculate mutation frequency by sample.
  # Calculate depth from the mutation data (default)
  # Correct the Depth (default) with indel priority (set)
  mf_example <- calculate_mf(
    mutation_data = example_data,
    cols_to_group = "sample",
    correct_depth_by_indel_priority = TRUE
  )
}