A complete high quality nanopore-only assembly of an XDR Mycobacterium tuberculosis Beijing lineage strain identifies novel variation in repetitive PE/PPE gene regions
Authors: Bainomugisa Arnold , Duarte Tania , Lavu Evelyn , Pandey Sushil , Coulter Chris , Marais Ben J. , Coin Lachlan 
Published: January 2018

AbstractA better understanding of the genomic changes that facilitate the emergence and spread of drug resistant M. tuberculosis strains is required. Short-read sequencing methods have limited capacity to identify long, repetitive genomic regions and gene duplications. We sequenced an extensively drug resistant (XDR) Beijing sub-lineage 2.2.1.1 “epidemic strain” from the Western Province of Papua New Guinea using long-read sequencing (Oxford Nanopore MinION®). With up to 274 fold coverage from a single flow-cell, we assembled a 4404947bp circular genome containing 3670 coding sequences that include the highly repetitive PE/PPE genes. Comparison with Illumina reads indicated a base-level accuracy of 99.95%. Mutations known to confer drug resistance to first and second line drugs were identified and concurred with phenotypic resistance assays. We identified mutations in efflux pump genes (Rv0194), transporters (secA1, glnQ, uspA), cell wall biosynthesis genes (pdk, mmpL, fadD) and virulence genes (mce-gene family, mycp1) that may contribute to the drug resistance phenotype and successful transmission of this strain. Using the newly assembled genome as reference to map raw Illumina reads from representative M. tuberculosis lineages, we detect large insertions relative to the reference genome. We provide a fully annotated genome of a transmissible XDR M. tuberculosis strain from Papua New Guinea using Oxford Nanopore MinION sequencing and provide insight into genomic mechanisms of resistance and virulence.Data SummarySample Illumina and MinION sequencing reads generated and analyzed are available in NCBI under project accession number PRJNA386696 (https://www.ncbi.nlm.nih.gov/sra/?term=PRJNA386696)The assembled complete genome and its annotations are available in NCBI under accession number CP022704.1 (https://www.ncbi.nlm.nih.gov/sra/?term=CP022704.1)Impact statementWe recently characterized a Modern Beijing lineage strain responsible for the drug resistance outbreaks in the Western province, Papua New Guinea. With some of the genomic markers responsible for its drug resistance and transmissibility are known, there is need to elucidate all molecular mechanisms that account for the resistance phenotype, virulence and transmission. Whole genome sequencing using short reads has widely been utilized to study MTB genome but it does not generally capture long repetitive regions as variants in these regions are eliminated using analysis. Illumina instruments are known to have a GC bias so that regions with high GC or AT rich are under sampled and this effect is exacerbated in MTB, which has approximately 65% GC content. In this study, we utilized Oxford Nanopore Technologies (ONT) MinION sequencing to assemble a high-quality complete genome of an extensively drug resistant strain of a modern Beijing lineage. We were able to able to assemble all PE/PPE (proline-glutamate/proline-proline-glutamate) gene families that have high GC content and repetitive in nature. We show the genomic utility of ONT in offering a more comprehensive understanding of genetic mechanisms that contribute to resistance, virulence and transmission. This is important for settings up predictive analytics platforms and services to support diagnostics and treatment.