Clustering of Circular Consensus Sequences (C3S) for Long Amplicon Analysis of PacBio Data
A manuscript on C3S-LAA is in review for publication. A preprint is avialble via bioRXiv: https://www.biorxiv.org/content/early/2017/12/20/236893
To improve and extend the functionality of the long amplicon analysis (LAA) module from PacBio, we restructured the clustering approach for LAA based on the high quality circular consensus sequence data, grouping these reads based on the primer sequences used to amplify the DNA and the molecular barcodes used to track individuals. This directs error correction and consensus sequence analysis to be performed on sequences that should be the same, from a given sample, leading to improved accuracy of the outputted data. In addition, integration of Minimus (Sommer et al. BMC Bioinformatics, 2007 8:64) for automated assembly of any overlapping amplicon consensus sequences allows for efficient processing of tiled amplicon resequence data.
Running C3S-LAA involves five steps. These are outlined below.
Run the reads of insert protocol in SMRT Portal to generate circular consensus sequence (CCS) reads.
The required dependencies and input files along with the output directory need to be specified. The parameters file is used by the cluster.py script (step 3) and includes information about the location of the Minimus assembler, the location of output from step 1 (fofn and ccs files), the location of a barcode (optional) and a primer pairs file (required), specific run parameters, and settings information for automated generation of a torque script.
Example of parameters.py file:
#########################################
### User input parameters for C3S-LAA ###
#########################################
### Required dependencies and input files
# path for the AMOS package that contains minimus assembler
amos_path = "/usr/local/amos/bin/"
# path to the primer pair info file
primer_info_file = "primer_pairs_info.txt"
# path to the barcode info file
barcode_info_file = "barcode_pairs_info.txt"
# path to PacBio file of file names (fofn) directing to the raw reads
fofn = "/path/m160901_060459_42157_c101086112550000001823264003091775_s1_p0.bas.h5"
# path to ccs reads
ccs = "/path/reads_of_insert.fasta"
# directory where the consensus files will be saved
consensus_output = "./output/"
### C3S-LAA parameters
# number of bases corresponding to padding + barcode that need to be trimmed from the amplicon consensus
trim_bp = 21
# 1: yes; 0: no
barcode_subset = 0
# reads >= "min_read_length" will be searched for the presence of primer sequences
min_read_length = 0
# 1: filter; 0: no filter
min_read_len_filter = 1
# searches for the primer sequence within n bases from the read terminals
primer_search_space = 100
# Maximum barcode seq length
max_barcode_length = 0
# Maximum padding seq length
max_padding_length = 5
### torque script settings
# walltime for consensus calling
walltime = 190
# node no./name for consensus calling
node = "1"
# no. of processors for consensus calling
processors = 12
# consensus sequences generated from >= "no_reads_threshold" will be used for assembly
no_reads_threshold = 100
Example of barcode_info_file (tab delimited text file):
f_barcode_name f_barcode_sequence r_barcode_name r_barcode_sequence
Sample1_BC.F1 CTATACATGACTCTGC Sample1_BC.R1 GCAGAGTCATGTATAG
Sample2_BC.F1 CTATACATGACTCTGC Sample2_BC.R2 CATGTACTGATACACA
Sample3_BC.F1 CTATACATGACTCTGC Sample3_BC.R3 GAGAGACGATCACATA
Sample4_BC.F1 CTATACATGACTCTGC Sample4_BC.R4 CTGATATGTAGTCGTA
Example of primers_info_file (tab delimited text file):
f_primer_name r_primer_name f_primer_sequence r_primer_sequence
TA_1_25390617_27_F TA_1_25395472_24_R AAACATTGGTGTGGAAAGCAACTGAAG AGGGTCACAGCACAGGACAGATTC
TA_1_25391952_24_F TA_1_25396540_27_R AGGGACAACGTAGGGAGCCTTTGG CGTCGACCACCGAATCAAGCAAGCATG
TA_2_37562840_25_F TA_2_37567441_24_R GGGTGTTGTTCGGTCACCTCCTTTG ATCCTTTGAGTGACTGAGGGTGTG
TA_2_37564580_25_F TA_2_37569533_24_R TACGAGGTGTTTGGTTTGGTGAACG CATGCATGCACACCTTCCAAGCTC
TA_3_33503980_28_F TA_3_33507309_27_R TGTCCACCGACGACATCATTGGAGAGTG TGCACAGTGCACATATGCTGCTTGGTG
TA_3_33506037_24_F TA_3_33509162_25_R CAAACCGCAGAGGATAGAGATCGC GGGTTCTCATCAACATTTGGACCTC
TA_6_7045710_25_F TA_6_7050495_28_R TAGGGAGAGGTGGGAATATAATGGG CCATCAAGTACAACAACGCATGATCATC
TA_6_7047707_27_F TA_6_7052049_28_R CAGCATGCGTATAAAGAAGGCGAGCTC CCCGATGTGCGACGCCGTAACAAATCTC
** NOTE: Primers must be named using the following convention: Name_Chromosome_StartPos_PrimerLength_Direction
python cluster.py
The CCS reads are used to cluster the data based on the presence of barcodes (optional) and both the forward and reverse primer sequences for each amplicon (the pipeline considers the sense and antisense primer sequences). This will generate a list of CCS identifiers belonging to each primer pair cluster. The list is used to link the corresponding raw reads, using the whitelist option in LAA, to carry out Quiver-based consensus calling using only the raw reads belonging to an amplicon-specific cluster. Executing cluster.py will compile the required information on barcode and amplicon-specific read clusters and output a submission script for running Quiver using the TORQUE resource management system (https://en.wikipedia.org/wiki/TORQUE). If you don't use TORQUE, you can execute cluster.py and edit the TORQUE related elements of the submission script for your specific platform.
qsub consensus_calling.sh
The submission script, consensus_calling.sh, is generated by "cluster.py" (step 3). For users running this submission script using TORQUE, the standard qsub parameters are written to the first 6 lines of the script, based on the user input parameters. This may be removed/modified. Executing the script will produce barcode and amplicon-specific consensus sequences (from Quiver) as separate fasta files.
#!/bin/sh
#PBS -N consensus_calling
#PBS -r n
#PBS -l walltime=190:00:00
#PBS -l nodes=1:ppn=12
#PBS -d /path/
<remainder of script [not shown] will include
information for running Quiver on
barcode and amplicon-specific clusters>
python consensus_assembly.py
If your sequences are expected to overlap, you can use consensus_assembly.py for assembly. This script first generates a multi-fasta file (merged_reads.fasta) containing the error corrected consensus sequences from all the amplicons for a given sample. Using each multi-fasta file, the script then carries out assembly to generate an output file (c3slaa_assembly.fasta) that contains the assembly results. The assembled contigs are named C3SLAA_1, C3SLAA_2, C3SLAA_3, etc.
** consensus_assembly.py requires amos package to be installed (http://amos.sourceforge.net/wiki/index.php/AMOS).
When multiple PacBio SMRT Cells are used, the sequence data are stored in separate directories. C3S-LAA can be still be used, but the fofn file (*bas.h5 file specified in the parameters file) cannot be a *bas.h5 file. Instead, a custom fofn text file needs to be created, which lists the paths to each of the three *bax.h5 files per SMRT Cell directory. The path to the custom fofn file needs to be specified in the parameters file.
Example of the contents of the fofn.txt file, where xx_s1_p0.x.bax.h5 files are from lane 1 and xx_s2_p0.x.bax.h5 files are from lane 2:
/absolute_path_to_SMRTCell1data/m170410_233007_42157_c101187522550000001823244205011702_s1_p0.1.bax.h5
/absolute_path_to_SMRTCell1data/m170410_233007_42157_c101187522550000001823244205011702_s1_p0.2.bax.h5
/absolute_path_to_SMRTCell1data/m170410_233007_42157_c101187522550000001823244205011702_s1_p0.3.bax.h5
/absolute_path_to_SMRTCell2data/m170410_233007_42157_c101187522550000001823244205011702_s2_p0.1.bax.h5
/absolute_path_to_SMRTCell2data/m170410_233007_42157_c101187522550000001823244205011702_s2_p0.2.bax.h5
/absolute_path_to_SMRTCell2data/m170410_233007_42157_c101187522550000001823244205011702_s2_p0.3.bax.h5
C3S-LAA can be executed via Python. This requires installation of the following components. Other versions of these components have not been tested.
C3S-LAA is released under an MIT open source license. The source code is available on GitHub: https://github.com/drmaize/C3S-LAA