Illumina MiSeq sequencing disfavours a sequence motif in the GFP reporter gene

The SSV-Seq 2.0 PCR-Free Method Improves the Sequencing of Adeno-Associated Viral Vector Genomes Containing GC-Rich Regions and Homopolymers

Adeno-associated viral vectors (AAV) are efficient engineered tools for delivering genetic material into host cells. The commercialization of AAV-based drugs must be accompanied by the development of appropriate quality control (QC) assays. Given the potential risk of co-transfer of oncogenic or immunogenic sequences with therapeutic vectors, accurate methods to assess the level of residual DNA in AAV vector stocks are particularly important. An assay based on high-throughput sequencing (HTS) to identify and quantify DNA species in recombinant AAV batches is developed. Here, it is shown that PCR amplification of regions that have a local GC content >90% and include successive mononucleotide stretches, such as the CAG promoter, can introduce bias during DNA library preparation, leading to drops in sequencing coverage. To circumvent this problem, SSV-Seq 2.0, a PCR-free protocol for sequencing AAV vector genomes containing such sequences, is developed. The PCR-free protocol improves the evenness of the rAAV genome coverage and consequently leads to a more accurate relative quantification of residual DNA. HTS-based assays provide a more comprehensive assessment of DNA impurities and AAV vector genome integrity than conventional QC tests based on real-time PCR and are useful methods to improve the safety and efficacy of these viral vectors.

Genome investigations show host adaptation and transmission of LA-MRSA CC398 from pigs into Danish healthcare institutions

Over the last decade, an increasing number of infections with livestock-associated methicillin-resistant Staphylococcus aureus of clonal complex 398 (LA-MRSA CC398) in persons without contact to livestock has been registered in Denmark. These infections have been suggested to be the result of repeated spillover of random isolates from livestock into the community. However, other studies also found emerging sub-lineages spreading among humans. Based on genome-wide SNPs and genome-wide association studies (GWAS), we assessed the population structure and genomic content of Danish LA-MRSA CC398 isolates from healthcare-associated infections from 2014 to 2016 (n = 73) and compared these to isolates from pigs in Denmark from 2014 (n = 183). Phylogenetic analyses showed that most human isolates were closely related to and scattered among pig isolates showing that the majority of healthcare-associated infections are the result of repeated spillover from pig farms, even though cases of human-to-human transmission also were identified. GWAS revealed frequent loss of antimicrobial resistance genes and acquisition of human-specific virulence genes in the human isolates showing adaptation in response to changes in selective pressures in different host environments, which over time could lead to the emergence of LA-MRSA CC398 lineages more adapted to human colonization and transmission.

Mycobiome Sequencing and Analysis Applied to Fungal Community Profiling of the Lower Respiratory Tract During Fungal Pathogenesis

Invasive fungal infections are an increasingly important cause of human morbidity and mortality. We generated a next-generation sequencing (NGS)-based method designed to detect a wide range of fungi and applied it to analysis of the fungal microbiome (mycobiome) of the lung during fungal infection. Internal transcribed spacer 1 (ITS1) amplicon sequencing and a custom analysis pipeline detected 96% of species from three mock communities comprised of potential fungal lung pathogens with good recapitulation of the expected species distributions (Pearson correlation coefficients r = 0.63, p = 0.004; r = 0.71, p < 0.001; r = 0.62, p = 0.002). We used this pipeline to analyze mycobiomes of bronchoalveolar lavage (BAL) specimens classified as culture-negative (n = 50) or culture-positive (n = 39) for Blastomyces dermatitidis/gilchristii, the causative agent of North America blastomycosis. Detected in 91.4% of the culture-positive samples, Blastomyces dominated (>50% relative abundance) the mycobiome in 68.6% of these culture-positive samples but was absent in culture-negative samples. To overcome any bias in relative abundance due to between-sample variation in fungal biomass, an abundance-weighting calculation was used to normalize the data by accounting for sample-specific PCR cycle number and PCR product concentration data utilized during sample preparation. After normalization, there was a statistically significant greater overall abundance of ITS1 amplicon in the Blastomyces-culture-positive samples versus culture-negative samples. Moreover, the normalization revealed a greater biomass of yeast and environmental fungi in several Blastomyces-culture-positive samples than in the culture-negative samples. Successful detection of Coccidioides, Scedosporium, Phaeoacremonium, and Aspergillus in 6 additional culture-positive BALs by ITS1 amplicon sequencing demonstrates the ability of this method to detect a broad range of fungi from clinical specimens, suggesting that it may be a potentially useful adjunct to traditional fungal microbiological testing for the diagnosis of respiratory mycoses.

Impact of index hopping and bias towards the reference allele on accuracy of genotype calls from low-coverage sequencing

BACKGROUND

Inherent sources of error and bias that affect the quality of sequence data include index hopping and bias towards the reference allele. The impact of these artefacts is likely greater for low-coverage data than for high-coverage data because low-coverage data has scant information and many standard tools for processing sequence data were designed for high-coverage data. With the proliferation of cost-effective low-coverage sequencing, there is a need to understand the impact of these errors and bias on resulting genotype calls from low-coverage sequencing.

RESULTS

We used a dataset of 26 pigs sequenced both at 2× with multiplexing and at 30× without multiplexing to show that index hopping and bias towards the reference allele due to alignment had little impact on genotype calls. However, pruning of alternative haplotypes supported by a number of reads below a predefined threshold, which is a default and desired step of some variant callers for removing potential sequencing errors in high-coverage data, introduced an unexpected bias towards the reference allele when applied to low-coverage sequence data. This bias reduced best-guess genotype concordance of low-coverage sequence data by 19.0 absolute percentage points.

CONCLUSIONS

We propose a simple pipeline to correct the preferential bias towards the reference allele that can occur during variant discovery and we recommend that users of low-coverage sequence data be wary of unexpected biases that may be produced by bioinformatic tools that were designed for high-coverage sequence data.

Characterization of H5N1 Influenza Virus Quasispecies with Adaptive Hemagglutinin Mutations from Single-Virus Infections of Human Airway Cells

Transmission of avian influenza (AI) viruses to mammals involves phylogenetic bottlenecks that select small numbers of variants for transmission to new host species. However, little is known about the AI virus quasispecies diversity that produces variants for virus adaptation to humans. Here, we analyzed the hemagglutinin (HA) genetic diversity produced during AI H5N1 single-virus infection of primary human airway cells and characterized the phenotypes of these variants. During single-virus infection, HA variants emerged with increased fitness to infect human cells. These variants generally had decreased HA thermostability, an indicator of decreased transmissibility, that appeared to compensate for their increase in α2,6-linked sialic acid (α2,6 Sia) binding specificity and/or in the membrane fusion pH threshold, each of which is an advantageous mutational change for viral infection of human airway epithelia. An HA variant with increased HA thermostability also emerged but could not outcompete variants with less HA thermostability. These results provided data on HA quasispecies diversity in human airway cells.IMPORTANCE The diversity of the influenza virus quasispecies that emerges from a single infection is the starting point for viral adaptation to new hosts. A few studies have investigated AI virus quasispecies diversity during human adaptation using clinical samples. However, those studies could be appreciably affected by individual variability and multifactorial respiratory factors, which complicate identification of quasispecies diversity produced by selective pressure for increased adaptation to infect human airway cells. Here, we found that detectable HA genetic diversity was produced by H5N1 single-virus infection of human airway cells. Most of the HA variants had increased fitness to infect human airway cells but incurred a fitness cost of less HA stability. To our knowledge, this is the first report to characterize the adaptive changes of AI virus quasispecies produced by infection of human airway cells. These results provide a better perspective on AI virus adaptation to infect humans.

Reassembling haplotypes in a mixture of pooled amplicons when the relative concentrations are known: A proof-of-concept study on the efficient design of next-generation sequencing strategies

Next-generation sequencing can be costly and labour intensive. Usually, the sequencing cost per sample is reduced by pooling amplified DNA = amplicons) derived from different individuals on the same sequencing lane. Barcodes unique to each amplicon permit short-read sequences to be assigned appropriately. However, the cost of the library preparation increases with the number of barcodes used. We propose an alternative to barcoding: by using different known proportions of individually-derived amplicons in a pooled sample, each is characterised a priori by an expected depth of coverage. We have developed a Hidden Markov Model that uses these expected proportions to reconstruct the input sequences. We apply this method to pools of mitochondrial DNA amplicons extracted from kangaroo meat, genus Macropus. Our experiments indicate that the sequence coverage can be efficiently used to index the short-reads and that we can reassemble the input haplotypes when secondary factors impacting the coverage are controlled. We therefore demonstrate that, by combining our approach with standard barcoding, the cost of the library preparation is reduced to a third.