Introduction
This package is intended to provide tools for the quality assessment of data produced by Oxford Nanopore’s MinION sequencer. It includes a functions to generate a number plots for examining the statistics that we think will be useful for this task.
However, nanopore sequencing is an emerging and rapidly developing technology. It is not clear what will be most informative. We hope that IONiseR will provide a framework for visualisation of metrics that we haven’t thought of, and welcome feedback at mike.smith@cruk.cam.ac.uk.
Getting started
In order to get started we need to load the IONiseR library. In addition also load ggplot2 and gridExtra, which are useful for arranging the plots in this vignette, but they are not essential to using the package.
library(IONiseR)
library(ggplot2)
library(gridExtra)
Reading data
Once the libraries are loaded we need to read some data. We do this using the function readFast5Summary(). This function takes a vector containing the path names of fast5 files you’d like to read. The example below looks in a specific folder and selects all of the files whose name ends with “.fast5”. We then pass this list of files to the reading function.
You should replace “/path/to/data” with the location of your fast5 files.
fast5files <- list.files(path = "/path/to/data/", pattern = ".fast5$", full.names = TRUE)
example.summary <- readFast5Summary( fast5files )
Raw fast5 data isn’t distributed with this package, but example of the summarised format can found in the accompanying minionSummaryData package. The following command will load this data, giving us an object called s.typhi.rep1. If you have your own MinION data you wish to work with you should ignore this section of code and modify the example above to read your own files.
The data presented in this example are taken from the publication by Ashton et al (Ashton et al. 2015). You can obtain the original data from the European Nucleotide Archive here: http://www.ebi.ac.uk/ena/data/view/ERR668747
library(minionSummaryData)
data(s.typhi.rep1)
Typing the name of the resulting object will print a short summary of its contents to the screen.
s.typhi.rep1
## Object of class: Fast5Summary
## Contains information from:
## 9502 fast5 files
## |- 8209 template strands
## |- 4454 complement strands
## |- 3738 full 2D reads
## |- 2159 pass reads
The Fast5Summary class
The s.typhi.rep1 object is an example of a the Fast5Summary class.
The structure of the class tries to reflect the variety of data one can find in fast5 files. Depending upon the quality of the data basecalling may not be successful resulting in fast5 files that are essentially empty or files that contain event information but no called bases. More commonly, basecalling is successful to some extent, but there is still a range of possibilities including files with only a template strand called, files with template and complement strands of differening lengths, and ideally files with well matched template and complement strands, plus a consensus 2D read.
It is possible to read fast5 files that have not been basecalled, but since in most cases uploading to the Metrichor basecaller is performed automatically, it is unlikely that this will be a common usecase.
The Fast5Summary class has four slots, each of which is designed to store data relating to one level of processing described above. The data themselves are stored as either as a data.frame or a ShortReadQ, one per slot. The table below gives the names of the four slots, along with a description of stage within the base calling process this represents and the specific fields that are currently stored in the appropriate data.frame.
| readInfo - All fast5 files contain this level of information |
| File name, channel, mux, pass/fail status |
| rawData - If events were recorded this level is populated |
| Median signal, start time, duration, no. of events |
| baseCalled - Created if base calling succeeded. Separate entries for each strand |
| Start time, duration, no. of events, strand, 2D status |
| fastq - Up to three entries per file (template, complement and 2D reads) |
| Bases and qualities |
Subsetting
Since the the number of entries in each slot can be different between a specific reads, the id column is present in all entires corresponds to the fast5 file data was read from. Subsetting operations work relative to this id field, so all data from the selected files is retained. In the example below we select two files and can see that both the template and complement read information is retained in the baseCalled slot.
baseCalled(s.typhi.rep1[1:2])
## id num_events duration start_time strand full_2D
## 1: 1 10278 369.7328 4960.870 template TRUE
## 2: 2 5793 205.6166 6451.166 template TRUE
## 3: 1 8636 223.2206 5333.229 complement TRUE
## 4: 2 4796 168.4536 6659.120 complement TRUE
Summary plots
Data production
Once data have been read into a summary object IONiseR contains a number of functions for plotting the data. The first example below visualises the accumulation of reads over the run time. The second plot shows the how many channels were active (i.e. the number of molecules being read) during each minute of the experiment.
p1 <- plotReadAccumulation(s.typhi.rep1)
p2 <- plotActiveChannels(s.typhi.rep1)
grid.arrange(p1, p2, ncol = 2)
Read types
We may also be interested in the proportion of reads types that were generated. Ideally, Oxford Nanopore’s sequencing technology works by reading both the template and complement strands of a double-stranded DNA molecule. The readings from both strands are then combined to give a higher confidence consensus sequence for the whole fragment - referred to as a 2D read.
Given the nature of this process, there is a strict hierarchy to the data that can be found in a fast5 file. A full 2D read requires both a complement and template strand to have been read correctly. Similarly, a complement strand can only be present if the template was read successfully. Finally, you can encounter a file containing no called bases on either strand.
The function plotReadCategories() will visualise the total number of fast5 files summarised in an Fast5Summary object, along with the counts of those containing template, complement and 2D calls. For an ideal dataset all four bars will be the same height, and the difference between them can reflect the quality of a dataset. These values are the same as those printed out when typing in the name of a summary data object.
It may also be interesting to examine the base quality scores for the reads in the three categories. The function plotReadCategoryQuals() allows one to do this, calculating the mean quality score for each sequence.
p1 <- plotReadCategoryCounts(s.typhi.rep1)
p2 <- plotReadCategoryQuals(s.typhi.rep1)
grid.arrange(p1, p2, ncol = 2)
Reading rates
We can also look the performance of the pores over time. For example, we may be interested in how rapidly events occur, which should be analagous to the rate at which molecules move through the nanopores. The function plotEventRate() visualises this.
In similar fashion, we can also look at the rate at which actual nucleotide bases are called changes over time using plotBaseProductionRate(). One would expect this to be closely related to the event rate (since each event should correspond to a base moving through the pore), however it is possible to envisage an scenario where events are recorded, but for some reason the base caller struggles to interpret them.
p1 <- plotEventRate(s.typhi.rep1)
p2 <- plotBaseProductionRate(s.typhi.rep1)
grid.arrange(p1, p2, ncol = 2)
Layout plots
When considering channel related metrics we can plot them as they are laid out on the flow cell. To create plots of this form we can use the function layoutPlot().
p1 <- layoutPlot(s.typhi.rep1, attribute = "nreads")
p2 <- layoutPlot(s.typhi.rep1, attribute = "kb")
grid.arrange(p1, p2, ncol = 2)
The attribute argument currently takes three possible values: “nreads”, “kb” or “signal” which will respectively plot the total number of reads, the cumulative number of bases read by a channel, and the median signal recorded by the pore.
Plotting alternative metrics
If you wish to map a different metric on to the channel layout you can use the fuction channelHeatmap(). This function requires a data.frame as input, with one column called ‘channel’. You can then use to the argument zValue to specify the intensity with which each channel is plotted.
To demonstrate this, in the example below we will plot both the number of full 2D reads produced by each channel, and the proportion of all reads from the channel that are 2D. When retrieving data from the Fast5Summary object, each slot is returned as a data.table, allowing us to use dplyr and its associated pipe paradigm to manipulate the data.
library(dplyr)
read_count_2D <- baseCalled(s.typhi.rep1) %>% ## start with base called reads
filter(strand == 'template') %>% ## keep template so we don't count things twice
left_join(readInfo(s.typhi.rep1), by = 'id') %>% ## channel stored in @readInfo slot, match by id column
group_by(channel) %>% ## group according to channel
summarise(d2_count = length(which(full_2D == TRUE)), ## count those with full 2D status
d2_prop = length(which(full_2D == TRUE)) / n()) ## divide by total count of reads from channel
## plot side-by-side
p1 <- channelHeatmap(read_count_2D, zValue = 'd2_count')
p2 <- channelHeatmap(read_count_2D, zValue = 'd2_prop')
grid.arrange(p1, p2, ncol = 2)
Plotting against channel and time
If we want to look at the patterns that affect specific channels, or all channels for specific periods of time, we can use the function channelActivityPlot(). By default this will plot a for every FAST5 file, sorted by channel on the y-axis and the time of the first and last recorded events on the x-axis.
If provided with a zScale argument each line will be colored according to the specified data. zScale expects to be passed a data.frame containing the id of a read, and a column containing the metric of interest. In the example below we extract the id and median_signal columns from a Fast5Summary object.
data(s.typhi.rep3, package = 'minionSummaryData')
## we will plot the median raw signal for each read on z-axis
z_scale = select(rawData(s.typhi.rep3), id, median_signal)
channelActivityPlot( s.typhi.rep3, zScale = z_scale )
Small numbers of reads with extreme values can compress the colours on the z-axis. To make time related patterns easier to pick out reads across all channels are grouped by the starting time, and the mean value for all reads in the group is calculated and shown along the bottom of the plot.
Pentamer content
Given that the sequence of a read is inferred from the recorded signal, one might wish to see if fluctuations in current over time are reflected in the base content of the reads produced. The function plotKmerFrequencyCorrelation() breaks reads into groups by the time the read was first entered a pore. The distribution of kmer (defaults to pentamers, but can be specified using the kmerLength argument) frequencies is then calculated for each window. The correlation between each window and all others is then plotted, allowing one to see if the kmer content alters during run time. The argument only2D switches between using the recorded template and complement strands, or the consensus 2D sequence.
plotKmerFrequencyCorrelation( s.typhi.rep3, only2D = FALSE )
The data included in minionSummaryData are very early in the life span of the MinION device, with a relatively smaller number of reads produced by only a few channels. The following plots are created using a more recent, but currently unpublished, set of data. Distinct waves in the average signal can be seen, although this doesn’t seem to greatly impact the pentamer content of the resulting reads.
p1 <- channelActivityPlot(dat, select(rawData(dat), id, median_signal))
p2 <- plotKmerFrequencyCorrelation(dat)
grid.arrange(p1, p2, ncol = 2)
