Genome sequencing accuracy by RCA-seq versus long PCR template cloning and sequencing in identification of human papillomavirus type 58
© Wang et al.; licensee BioMed Central Ltd. 2014
Received: 2 September 2013
Accepted: 16 December 2013
Published: 13 January 2014
Genome variations in human papillomaviruses (HPVs) are common and have been widely investigated in the past two decades. HPV genotyping depends on the finding of the viral genome variations in the L1 ORF. Other parts of the viral genome variations have also been implicated as a possible genetic factor in viral pathogenesis and/or oncogenicity.
In this study, the HPV58 genome in cervical lesions was completely sequenced both by rolling-circle amplification of total cell DNA and deep sequencing (RCA-seq) and by long PCR template cloning and sequencing. By comparison of three HPV58 genome sequences decoded from three clinical samples to reference HPV-58, we demonstrated that RCA-seq is much more accurate than long-PCR template cloning and sequencing in decoding HPV58 genome. Three HPV58 genomes decoded by RCA-seq displayed a total of 52 nucleotide substitutions from reference HPV58, which could be verified by long PCR template cloning and sequencing. However, the long PCR template cloning and sequencing led to additional nucleotide substitutions, insertions, and deletions from an authentic HPV58 genome in a clinical sample, which vary from one cloned sequence to another. Because the inherited error-prone nature of Tgo DNA polymerase used in preparation of the long PCR templates of HPV58 genome from the clinical samples, the measurable error rate in incorporation of nucleotide into an elongating DNA template was about 0.149% ±0.038% in our studies.
Since PCR template cloning and sequencing is widely used in identification of single nucleotide polymorphism (SNP), our data indicate that a serious caution should be taken in finding of true SNPs in various genetic studies.
Human papillomaviruses (HPVs) are a group of more than 200 genotypes of small DNA tumor viruses (http://pave.niaid.nih.gov/#home) and can be grouped clinically as high-risk (oncogenic) types, which are frequently associated with invasive cervical cancer, and low-risk (non-oncogenic) types, which are found mainly in genital warts. To date, fifteen HPV types has been classified as high-risk HPVs, including HPV16, 18, 31, 33, 35, 39, 45, 51, 52, 56, 58, 59, 68, 73, and 82[1, 2]. Among the high-risk HPVs, HPV16 and HPV18 are the principal causes of cervical cancer, with a combined, worldwide contribution to ~70% of invasive cervical cancer[3, 4]. HPV58 has been found to be more prevalent than HPV18 in cervical intraepithelial neoplasia (CIN) lesions and appears almost equal frequency as HPV18 in cervical cancers in Zhejiang province, China and other Asian countries[6, 7]. We recently characterized HPV58 genome variations and RNA expression in women with cervical lesions.
HPV genome in size of ~7.9 kb encodes eight open reading frames (ORFs, E6, E7, E1, E2, E4, E5, L2 and L1) from one strand of its double stranded, circular genome in one direction and a long control region (LCR) between L1 and E6. The classification of papillomaviruses by genotyping depends on the most conserved L1 ORF. A new type of papillomavirus is granted when its DNA sequence of L1 ORF differs by more than 10% from the closest known HPV type. A subtype indicates the difference between 2% and 10% and a variant represents less than 2% difference[10–12]. Nucleotide variations in the LCR region are often used to describe intratype diversity or variant lineage[12–15]. Despite that HPV genome is viewed as a stable genome, its genome sequence variations in a given genotype appear different from one laboratory to another and from one geological region to another[14–22]. The reported variations have been found not only in the L1 ORF and the LCR region, but also in other parts of the viral genome and were identified mostly, if not all, by PCR amplicon sequencing. Although it is always questionable whether an authentic variation (s) does exist in the reported HPV genotype because of the use of error-prone Taq polymerase in PCR amplification, many attempts have been made to correlate such genome variations to HPV pathogenesis and viral carcinogenicity[23–26]. Thus, a more reliable method is urgently needed to study HPV genome variation and its possible biological role in HPV infection. Recently, next-generation sequencing (NGS) techniques are emerging as a promising application for HPV genotyping and identification of rare single nucleotide polymorphism (SNP)[8, 27–30].
In this report, we analyzed and compared three HPV58 genome sequences from three cervical lesion samples decoded by an RCA-seq (rolling-circle amplification and deep sequencing)[8, 31], a technique that does not require prior knowledge of the underlying genome, and by long PCR template cloning and Sanger sequencing. We demonstrated that RCA-seq is more reliable method in identification of an authentic nucleotide variation in an episomal viral genome.
To have the RCA products being deep-sequenced using an Illumina HiSeq-2000 platform, we subsequently debranched the RCA products and prepared a paired-end library from each RCA sample[8, 31]. More than 104 millions of paired-end reads from the sample 9, 161 millions from the sample 10, and 120 millions from the sample 13 were obtained using the RCA-seq. Among those reads, 3453 in the sample 9, 13249 in the sample 10, and 1.4 million in the sample 13 could be mapped to the reference HPV58 genome (GI: 222386), giving a complete coverage of a full-length HPV58 genome. Detailed analyses of the nucleotide sequence at each position against the reference HPV58 genome showed that the HPV 58 genomes in three clinical samples contain nucleotide substitutions being verifiable either in other samples or in the same sample by the second RCA-seq.
The list of plasmids constructed and used in the study
RCA product from HPV58 CNZJ-3 (sample 9) was amplifiedwith Pr3506 (oYL35) and Pr7036 (oXHW262), gel purified andcloned into pCR-XL-TOPO vector. Insertion was verified bysequencing.
RCA product from HPV58 CNZJ-2 (sample 10) was amplifiedwith Pr3506 (oYL35) and Pr7036 (oXHW262), gel purified andcloned into pCR-XL-TOPO vector. Insertion was verified by sequencing.
RCA product from HPV58 CNZJ-1 (sample 13) was amplified with Pr3506 (oYL35) and Pr7036 (oXHW262), gel purified and cloned into pCR-XL-TOPO vector. Insertion was verified by sequencing.
RCA product from HPV58 CNZJ-3 (sample 9) was amplified with Pr6906 (oXHW263) and Pr3694 (oXHW264), gel purified and cloned into pCR-XL-TOPO vector. Insertion was verified by sequencing.
RCA product from HPV58 CNZJ-2 (sample 10) was amplified with Pr6906 (oXHW263) and Pr3694 (oXHW264), gel purified and cloned into pCR-XL-TOPO vector. Insertion was verified by sequencing.
RCA product from HPV58 CNZJ-1 (sample 13) was amplified with Pr6906 (oXHW263) and Pr3694 (oXHW264), gel purified and cloned into pCR-XL-TOPO vector. Insertion was verified by sequencing.
Nucleotide insertion and deletion created by long-PCR amplification in HPV58 isolates
Nucleotide substitutions by long-PCR amplification in HPV58 isolates create or inactivate restriction enzyme digestion sites
Notably, these additional nucleotide variations from the long PCR template cloning and sequencing were random and not duplicable each other between two cloned viral genome sequences derived from the same clinical samples. In addition, we found that the insertions or deletions identified from the cloned HPV58 genome fragment always happened at a run of multiple A, T, or C. Together, a total of additional 70 nucleotide variations were identified from six cloned HPV58 genome sequences which differ from the RCA-seq-defined HPV58 genome sequences. We concluded that these variations were derived from long PCR template amplification with a less efficient proof-reading Tgo DNA polymerase. By calculation, the Tgo DNA polymerase exhibited an error rate of ~0.149% ±0.038% in our long PCR template preparation per HPV58 genome in size of 7824 nts.
Rolling-circle amplification (RCA) has been developed as a powerful tool to amplify a whole genome in an episomal form for microbial genome organization and phylogenetic analyses. In this study, HPV58 genome in cervical lesions was decoded, respectively, by RCA-seq and by long PCR template cloning and sequencing. We demonstrated that the RCA-seq is better than the long PCR template cloning and sequencing in providing more accurate HPV58 genome sequence. This conclusion was drawn by comparing the same genome sequence decoded by the two techniques. All nucleotide variations to the reference HPV58 genome identified from each clinical sample by RCA-seq could be verified by the long PCR template cloning and sequencing, but not vice versa. In addition, the long PCR template cloning and sequencing technique creates additional sequence variations, which were random and not verifiable from one cloned sequence to another derived from the same sample.
Although the viral DNA from each clinical sample was enriched by RCA using phi29 DNA polymerase[34, 35] which features only a minimal error rate of 1 in 106-107 and the reported sequencing accuracy for a NGS with Illumina HiSeq platform is 98%, we found that RCA-seq in this report displayed a more reliable approach, by calling consensus bases, in decoding the entire HPV58 genome in size of 7824 nts. This could be achieved by using a combination of both high fidelity phi29 DNA polymerase at the experimental side and appropriate sequencing data analysis at the computational side. First, we defined a position as identical if more than 95% of the bases obtained were identical to those of the reference genome. Second, nucleotide substitutions were identified if the majority of bases in the reads differ from the reference genome with read depth of 5 or more. Third, nucleotide positions with read depth less than 5 were treated as ambiguous sites since there is insufficient depth to make a high confidence call.
We determined that additional nucleotide variations observed in our long PCR template cloning and sequencing were derived from the proofreading error of Tgo DNA polymerase used in our long PCR template preparation. Although the enriched DNA chimeras in the RCA reaction which might carry a misincorporated base from phi29 DNA polymerase amplification were served, after debranching by AgeI- or DraII-specific digestion of HPV58 genome, for subsequent long PCR template preparation and cloning, this carry-over possibility appears to be unlikely because the phi29 DNA polymerase features only a minimal error rate of 1 in 106-107 in contrast to ~ 3 in 104 for PCR with Taq DNA polymerase. Other studies indicate that the majority of DNA sequence changes introduced during PCR are polymerase-mediated and PCR accumulates about one mutation per 400 bases after 30 cycles[38, 39]. The PCR-induced transitions are the major source of error in other NGS studies[40, 41]. The reported Tgo DNA polymerase from the Roche product manual has an overall error frequency of ~0.2% in nucleotide misincorporation during PCR amplification, very close to what we found in our study.
In summary, a more accurate RCA-seq superior to PCR cloning and sequencing has been developed in our lab for HPV genotyping and determination of SNPs in clinical HPV variants. Our protocol provides much higher numbers of specific HPV reads over other high-throughput NGS platforms/protocols applied to HPV genotyping[27–30]. Since PCR sequencing and PCR template cloning and sequencing are widely used in various epidemiology studies to identify natural variants of an HPV type, caution should be taken in evaluation of any new SNPs found by these assays as PCR amplification might introduce nucleotide misincorporation into the studying genotype (s).
Materials and methods
Sample DNA preparation
Three HPV58- positive cervical CIN2/3 tissues were collected from Women's Hospital, School of Medicine, Zhejiang University. The study was approved by the Institutional Review Board for clinical research of this hospital. Informed consent was obtained from each participant prior the study. DNA was isolated from each sample by using TRIzol Reagent (Invitrogen, Carlsbad, CA) according to the manufacturer's instructions.
RCA enrichment, RCA chimeras debranching, paired-end library preparation and deep sequencing
Rolling circle amplification (RCA) based on phi29 DNA polymerase used to enrich the HPV58 genome and RCA chimeras debranching, paired-end library preparation and deep sequencing with an Illumina HiSeq-2000 platform have described in our other publications[8, 31].
Long PCR template preparation, cloning, and sequencing
RCA products prepared separately from three clinical samples (sample 9, 10 and 13) without S1 digestion and DNA shearing were selectively linearized by AgeI or DraII digestion and then amplified with a primer pair Pr3506 (5′-GACAGTAGACCACGAGGA-3′, forward) and Pr7036 (5′-TACTCAGGATC/CGTCCCAAAGGAAACTGATC-3′, backward) for AgeI-digested RCA products, and with a primer pair Pr6906 (5′-TACATCGAATT/CTCCCAGGCTATTACTTGC-3′, forward) and Pr3694 (5′-CCAATGCCATGTGGATGAC-3′, backward) for DraII-digested RCA products, respectively, using the Expand Long Template PCR System (Roche, Cat No. 11681834001). In brief, 2 μl of digested RCA products were used for amplification in a 50-μl reaction together with 38.75 μl DEPC-treated water, 2.5 μl dNTP mix (10 mM each)), 0.5 μl of each forward and reverse primers (20 μM), 5 μL of 10x PCR buffer II with MgCl2, and 0.75 μl of the expand long template enzyme mix which contains high-fidelity thermostable Tgo DNA polymerase with proofreading activity. Individual long PCR product was gel-purified and cloned into pCR-XL-TOPO vector according to the manufacturer’s instruction (Invitrogen, Cat No K4700-10). In brief, the PCR products generated from AgeI-digested RCA products using primer pairs Pr3506 (forward) and Pr7036 (backward) and the PCR products generated from DraII-digested RCA products using primer pairs Pr6906 (forward) and Pr3694 (backward) were precipitated, purified by agarose gel electrophoresis using crystal violet and then isolated using S.N.A.P Gel Purification Kit (Invitrogen). Purified long PCR products were cloned into a pCR-XL-TOPO vector. Two clones from each plasmid (Table 1) were prepared and sequenced from two different directions. Sequencing results of each clone were aligned against the HPV58 reference genome to identify nucleotide substitutions, insertions, or deletions.
Restriction enzyme digestion
RCA product and plasmid pXHW57-1 from the clinical sample 9 were amplified with primer pair Pr743 (5′-CGTGTTGTTACACTTGTGAC-3′) and Pr1424 (5′-GTATTACAACTGTCTACATCCG-3′), and then digested with AfeI at 37°C overnight. RCA product and pXHW55-1 from the sample 10 were amplified with primer pair Pr4974 (5′-CATCTCCTCATAGACTTGTAAC-3′) and Pr5181 (5′-CGAGTACGAAGTGTAGCCT-3′), and then digested with EcoRV at 37°C overnight. RCA product and pXHW59-1 from the sample 13 were amplified with primer pair Pr2417 (5′-GTATGATAGATGATGTAACAGC-3′) and Pr2960 (5′-ACGCTTTAGTCTTTGATGCTA-3′), and then digested with FspI at 37°C overnight. Non-digested products without enzymes were used as controls.
Sequencing data analyses
The trimmed reads were aligned to human (hg19 assembly) from UCSC genome browser and HPV58 reference genome (GenBank accession number D90400 or GI:222386) from PaVe database simultaneously using the Burrows-Wheeler Alignment tool (BWA) with default setting. The output alignment files in SAM format were further processed using SAMtools. The genome coverage files (WIGGLE or BAM files) were loaded onto the Integrative Genomics Viewer (IGV,http://www.broadinstitute.org/igv) to visualize sequence alignments, genomic annotations and substitutions with defaulted settings. The cloning sequences were aligned to HPV58 reference genome using NCBI Blast tool.
Quantitative real-time PCR
Quantitative real-time PCR, with an HPV58-specific primer pair Pr 743 (5′-CGTGTTGTTACACTTGTGAC-3′) and Pr 854 (5′-CTAGGGCACACAATGGTACA-3′) using Power SYBR Green PCR Master Mix (Invitrogen) for high sensitivity and reproducibility, was performed on ~100 pg of samples DNA (sample #10 and #13) before and after RCA enrichment, of which the RCA products from clinical samples #10 and #13 without S1 digestion and DNA shearing were selectively linearized by AgeI or DraII digestion. A 10-fold serial dilution starting from 100 pg (~1.3 x 107 copies) of the plasmid pXW59 which contains an HPV58 DNA fragment from nt 6906 to 3695 was amplified using the same primer set by qPCR to create a standard curve. The threshold cycle (Ct) values of qPCR data from 2 repeats were calculated for copy number analysis. A primer pair optimized for routine SYBR Green real-time PCR assays which specifically amplifies a genomic region containing human GAPDH promoter was purchased from Diagenode (Denville, NJ) and was used an internal control.
This study was supported by the Intramural Research Programs of the NCI, Center for Cancer Research and the NHLBI, NIH, and the Natural Science Foundation of China (NSFC 81172475) and the Natural Science Foundation of Zhejiang Province of China (grant NO: LQ13H160003).
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