PacBio Amplicon Analysis
PacBio Amplicon Analysis (pbaa) separates complex mixtures of amplicon targets from genomic samples. The pbaa application is designed to cluster and generate high-quality consensus sequences from HiFi reads. This application only works on HiFi amplicon data. There are several assumptions made within the code that will only support high quality reads (>QV20). This application will not work on CLR data. pbaa is reference aided method (pseudo de-novo).
Typical use cases involve multi-allelic samples where the sample-specific ploidy or copy number is unknown. pbaa can effectively separate alleles with one to many variants, including SNVs and large indels contained within the target region. pbaa has been optimized and tested for datasets with a moderate to high (<50) cluster counts.
The latest version can be installed via bioconda package pbaa.
Please refer to our official pbbioconda page for information on Installation, Support, License, Copyright, and Disclaimer.
pbaa has two executables, cluster and bam paint.
pbaa - PacBio HiFi Amplicon Analysis.
Usage:
pbaa <tool>
-h,--help Show this help and exit.
--version Show application version and exit.
Tools:
cluster Run clustering tool.
bampaint Add color tags to BAM records, based on pbaa clusters.
This tool runs the placement, clustering, and consensus algorithms.
pbaa cluster - Run clustering tool.
Usage:
pbaa cluster [options] <guide input> <read input> <prefix>
guide input FILE Guide sequence(s) in fasta format indexed with samtools faidx version 1.9 or
greater. A FOFN can be provided for multiple files.
read input FILE De-multiplexed HiFi reads in fastq format indexed with samtools fqidx version 1.9
or greater. A FOFN can be provided for multiple files.
prefix STR Output prefix for run.
Placement and Variant Options:
--filter INT Variants with coverage lower than filter will be ignored. [3]
--trim-ends INT Number of bases to trim from both sides of reads during graph construction and
variant detection. [5]
--pile-size INT The number of best alignments to keep for each read during error correction. [30]
--min-var-frequency FLOAT Minimum coverage frequency within a pile. [0.3]
--max-alignments-per-read INT The number of random alignments, for each read, within a guide grouping [1000]
Clustering Options:
--max-reads-per-guide INT The number randomly selected reads to use within a guide grouping. [500]
--iterations INT Number of iterations to run k-means. [9]
--seed INT Randomization seed. [1984]
Consensus Options:
--max-consensus-reads INT Maximum number of reads to use per cluster consensus. [100]
Filtering Options:
--max-amplicon-size INT Upper read length cutoff, longer reads will be skipped. [15000]
--min-read-qv FLOAT Low read QV cutoff. [20]
--off-target-groups STR Group names to exclude, i.e. these loci are off-target (not amplified).
--min-cluster-frequency FLOAT Low frequency cluster cutoff. [0.1]
--min-cluster-read-count INT Low read count cluster cutoff. [5]
--max-uchime-score FLOAT High UCHIME score cutoff. [1]
General Options:
-h,--help Show this help and exit.
--version Show application version and exit.
-j,--num-threads INT Number of threads to use, 0 means autodetection. [0]
--log-level STR Set log level. Valid choices: (TRACE, DEBUG, INFO, WARN, FATAL). [WARN]
--log-file FILE Log to a file, instead of stderr.
If you have a BAM file (mapped amplicon reads) this tool will add IGV tags for grouping (HP) and coloring (YC). It matches the read names in the clustering file, and the BAM file. The amplicon reads can be aligned against any reference.
pbaa bampaint - Add color tags to BAM records, based on pbaa clusters.
Usage:
pbaa bampaint [options] <read info file> <input bam> <output bam>
read info file FILE Read information file produced by pbaa cluster.
input bam FILE Bam file to add color tags.
output bam FILE Output bam file name.
Options:
-h,--help Show this help and exit.
--version Show application version and exit.
-j,--num-threads INT Number of threads to use, 0 means autodetection. [0]
--log-level STR Set log level. Valid choices: (TRACE, DEBUG, INFO, WARN, FATAL). [WARN]
--log-file FILE Log to a file, instead of stderr.
pbaa requires two input files, guide/reference sequences in fasta file format and HiFi de-multiplexed reads in fastq file format (indexed with fqidx). Guide/reference sequences should contain the amplified region, but not much more. Providing a chromosome as a guide, for example, will lead to issues during read placement.
pbaa supports batching of samples via the FOFN (file of file name[s]) format. A FOFN is a line separated file that contains the full paths to the input files. All input sequence files need to be indexed before running pbaa. Indexing can be achieved with samtools version 1.9 or greater.
Guide/reference sequence choice affects read grouping/placement. It is important to choose guides that are sufficiently divergent. If too many similar alleles are used for the same locus the fraction of un-placed reads will increase because the number of informative guide kmers decreases. Too few guides can also cause cluster dropout; it's the goldilocks problem.
Guide sequences should be grouped into locus assignments. For example if multiple HLA-A alleles are used in the guide sequence, they should be grouped, so clustering will be performed at the locus level.
Allele_1|HLA-A (sequence name | group name)
Allele_2|HLA-A (sequence name | group name)
In the example above, reads assigned to either allele (1,2) will be merged into a single dataset for clustering.
For more details on setting up guides see guide_reference.md
pbaa will generate three output files.
- {prefix}_passed_cluster_sequences.fasta
- {prefix}_failed_cluster_sequences.fasta
- {prefix}_read_info.txt
The headers entries in the consensus sequence output contain statistics about the clusters. These statistics are used for filtering (pass/fail criterion).
example of a passing sequence:
>sample-bc1001--bc1001_guide-HLA-A_cluster-1_ReadCount-22 uchime_score:-1 uchime_left_parent:N/A uchime_right_parent:N/A cluster_freq:0.44 diversity:0.188552 avg_quality:53.5878 duplicate_parent:N/A seq_length:3152 filters:none
example of a failing sequence:
>sample-bc1011--bc1011_guide-HLA-DRB1_cluster-3_ReadCount-6 uchime_score:0.03125 uchime_left_parent:bc1011--bc1011_HLA-DRB1_0 uchime_right_parent:bc1011--bc1011_HLA-DRB1_1 cluster_freq:0.12 diversity:0 avg_quality:42.2187 duplicate_parent:N/A seq_length:3706 filters:fail-low-frequency
The fields in the header are:
-
uchime_score The UCHIME score flags chimeric consensus sequences. The higher the score the more likely the sequence is chimeric. For more details see: Edgar, Robert C., et al. “UCHIME improves sensitivity and speed of chimera detection.” Bioinformatics 27.16 (2011): 2194-2200.
-
uchime_left_parent/uchime_right_parent The parent sequences of a chimeric sequence.
-
cluster_freq A measures of the clusters’ frequencies. The frequency is calculated by reads counts within groupings.
-
diversity A measure of the variability of variants within a cluster. Clusters with homogenous reads will have low diversity. A negative value indicates this metric was not calculated.
-
avg_quality The average PHRED quality of the reads within the cluster.
-
seq_length The sequence length of the cluster.
-
filters This is a space separated field enumerating the possible reasons a cluster was placed in the fail category.
One row per read, columns as follows:
- SeqName
- GuideName
- strand
- SecondBestGuideName
- Score
- FirstHighest/SecondHighest/UniqueHitSum
- Sample, input fastq
- Sequence length
- Average read quality
- Cluster ID
- Cluster Size
Example:
m64012_200712_164638/72090819/ccs HLA-DRB5 - HLA00622_DQB1_02-01-01_7480_bp|HLA-DQB1 0.714286 f:5/s:2/sum:8 /pbi/dept/appslab/projects/old/2020/jh_hla/2020-07-13_HGgendx/fastq_sqII/demultiplex.bc1099--bc1099.fastq 3125 58.6149 1 1
Demonstration data for testing pbaa can be found here. Dataset contains HiFi reads for 6 pooled HLA genes in FASTQ format. Outputs from running pbaa as well as validated genotypes are included.
Targeted Sequencing For Amplicons Document
We've optimized the default parameters to perform well on several datasets. In general use the defaults unless needed. If you discover an edge case, please share this experience.
Amplification can generate off target reads. Similarly, pbaa can accidentally place a few reads in the wrong grouping/locus. These reads may generate clusters that are off-target (not amplified). By providing a list of guide-names / group-names (one per line), pbaa will filter these out.
There are four hurestics to consider adjusting depending on the experiment (max-reads-per-guide, max-alignments-per-read, pile-size, min-var-frequency). In the above image, there are a total of max-reads-per-guide to consider for each HiFi (A-read). An A-read is randomly aligned to max-alignments-per-read. These alignments are sorted in decreasing sequence idenity. A total of pile-size reads are retained to correct the A-read. The min-var-frequency is the fraction of reads within a pile that support that the A-read is correct at any position along the A-read.
Advanced / Hidden Options
A number of heuristics and advanced options are hidden from the interface. They are documented here, but consider them experimental features. Changing these settings can have undesired effects.
--skip-consensus : Only run read placement and clustering. No consensus sequences will be generated.
--skip-chimera-detection : Skip chimera detection (UCHIME algorithm) step.
--kmer-size : Kmer size, not to exceed 31, length must be odd. This only affects read placement.
Why are reads missing : By default pbaa only uses 500 reads per locus/guide/grouping. Increasing --max-reads-per-guide will use more reads, at the expense of runtime/memory. If runtime is not a consideration use all the reads.
Considerations for pooled experiments: Pooled experiments are bounded by runtime/memory. The settings below increase the accuracy of the results at the expense of runtime memory.
--max-reads-per-guide : Increase the number of reads uses locus/guide/grouping.
--max-alignments-per-read : Increase the number of alignments per read. This setting can drastically increase computational expense.
--pile-size : The number of reads to use for error masking per read.
--min-var-frequency : The minimum variant frequency within a pile of reads used for error correction. Decreasing this value will increase sensitivity to low-frequency variants. This can lead to over clustering.
Extra false clusters: There are a number of reasons pbaa might generate false positive clusters. Chimeric reads, false variant calls, and clustering errors. Often the statistics in the fasta headers can provide clues. Changing filtering settings may reduce false positives.
Missing clusters: pbaa was tuned for sensitivity, favoring false positives over false negatives. However, false negatives do happen at a low rate. First check if the missing cluster is in the {prefix}_failed_cluster_sequences.fasta file. Then check that there is sufficient coverage over the guide sequences, by aligning reads to the guide sequences. For this application we recommend a minimum of 25x depth of coverage per allele. If there is coverage check for missed variants. If you believe pbaa failed to discover a variant resulting in a false negative, please share a small example. Variant discover is the most challenging step in pbaa.
Feedback or bug report: Please share your experiences via Github issue.
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- 1.0.3: September 1st, 2021
- Changes to error masking for indels.
- 1.0.2: June 22nd, 2021
- Minor bug fixes.
- 1.0.1: May the 4th be with you, 2021
- Decrease memory and runtime by refactoring how the reads are loaded.
- 1.0.0: April 23rd, 2021
- Major algorithm refactor, and many changes to CLI.
- For previous releases please see beta release documentation