Performance of individual
Accurate and comprehensive transcriptome assemblies lay the foundation for a range of analyses, such as differential gene expression analysis, metabolic pathway reconstruction, novel gene discovery, or metabolic flux analysis. With the arrival of next-generation sequencing technologies, it has become possible to acquire the whole transcriptome data rapidly even from non-model organisms. However, the problem of accurately assembling the transcriptome for any given sample remains extremely challenging, especially in species with a high prevalence of recent gene or genome duplications, those with alternative splicing of transcripts, or those whose genomes are not well studied. In this chapter, we provided a detailed overview of the strategies used for transcriptome assembly. We reviewed the different statistics available for measuring the quality of transcriptome assemblies with the emphasis on the types of errors each statistic does and does not detect. We also reviewed simulation protocols to computationally generate RNAseq data that present biologically realistic problems such as gene expression bias and alternative splicing. Using such simulated RNAseq data, we presented a comparison of the accuracy, strengths, and weaknesses of nine representative transcriptome assemblers including de novo, genome-guided, and ensemble methods.
- de novo
- ensemble approach
Transcriptome assembly from high-throughput sequencing of mRNA (RNAseq) is a powerful tool for detecting variations in gene expression and sequences between conditions, tissues, or strains/species for both model and non-model organisms [1, 2]. However, the ability to accurately perform such analyses is crucially dependent on the quality of the underlying assembly . Especially for the detection of sequence variations, but also for isoform detection and transcript quantification, mis-assembly of genes of interest can increase both the false positive and false negative rates, depending on the nature of the mis-assembly . These problems are exacerbated in non-model organisms where genomic sequences that can be used as the references, if available at all, are sufficiently different than those from the individuals sequenced .
Transcripts can be mis-assembled in several ways . Two of the most drastic assembly errors are fragmentation, where a single transcript is assembled as one or more smaller contigs, and chimeras, where a contig is assembled using part or all of more than one transcript. Fragmentation errors tend to result from fluctuations in the read coverage along a transcript, with the breaks in the transcript sequence occurring in regions that have lower coverage. By contrast, chimera errors often occur because of ambiguous overlaps within the reads, coupled with algorithms that choose the longest possible contig represented by the data, or by adjacent genes on the genome being merged. Both of these types of errors can have major impacts especially on gene identification. Small (single or few) nucleotide alterations to the contig sequence also happen as mis-assemblies. Sequence mistakes are often the result of mis-sequenced reads, but can also result from ambiguity for highly similar reads e.g. from heterozygous genes and from duplicated genes. In some cases, these errors can shift the reading frame for the contig, which can have significant impacts on the translated protein sequence. Finally, transcripts can be mis-assembled when alternative transcripts are collapsed into a single contig .
In the following sections, we will first review strategies used for transcriptome assembly as well as how their performance can be assessed. We then compare the performance of representative transcriptome assembly methods using a simulated human transcriptome and RNAseq. Finally we discuss a possible strategy to improve transcriptome assembly accuracy.
2. Transcriptome assembly strategies
De novo assemblers
Examples of popularly used
2.2. Genome-guided assemblers
Genome-guided assemblers avoid the ambiguity of kmer decompositions used in de Bruijn graphs by mapping the RNAseq data to the reference genome. In order to account of introns, mapping of the reads for genome-guided assembly needs to allow them to be split, where the first part of the read maps to one location (an exon), and the other half maps to a downstream location (another exon). This mapping is done by split-read mappers such as TopHat , STAR , HISAT , or HPG-aligner . Each of these methods maps the reads slightly differently, which may impact the quality of subsequent assembly.
This read mapping greatly reduces the complexity of transcript assembly by clustering the reads based on genomic location rather than relying solely on overlapping sequences within the reads themselves . However, this approach still has some major drawbacks. The most obvious drawback is that genome-guided assemblers require a reference genome, which is not available for all organisms. The quality of the reference genome, if it is available, also impacts the quality of the read mapping and, by extension, the assembly. This impact is particularly noteworthy when genes of interest contain gaps in the genome assembly, preventing the reads necessary to assemble those genes from mapping to part or all of the transcript sequence. Ambiguity occurs also when reads map to multiple places within a genome. How the specific algorithm handles choosing which potential location a read should map to can have a large impact on the final transcripts predicted . This problem is expounded when working with organisms different from the reference, where not all of reads map to the reference without gaps or mismatches.
Examples of popularly used genome-guided assemblers include Bayesembler , Cufflinks , and StringTie . While each of these methods uses the mapped reads to create a graph representing the splice junctions of the transcripts, how they select which splice junctions are real differs fundamentally. Cufflinks constructs transcripts based on using the fewest number of transcripts to cover the highest percentage of mapped reads. StringTie uses the number of reads that span each splice junction to construct a flow graph, constructing the transcripts based in order of the highest flow. Bayesembler constructs all viable transcripts for each splice junction and uses a Bayesian likelihood estimation based on the read coverage of each potential transcript to determine which combination of transcripts is most likely. Due to these fundamentally different approaches, each of these tools produces different sets of transcripts from the same set of reads. An example of assemblies produced by these methods and how the assembled contigs differ is described in Section 4.3.
2.3. Ensemble approach
While a core set of transcripts are expected to be assembled correctly by many different assemblers, many transcripts will be missed by any individual tool  (also see Section 4). Through combining the assemblies produced by multiple methods, ensemble assemblers such as EvidentialGene  and Concatenation  attempt to address the limitations of individual assemblers, ideally keeping contigs that are more likely to be correctly assembled and discarding the rest. Both of EvidentialGene and Concatenation filter the contigs obtained from multiple assemblers (usually
These approaches greatly reduce the number of contigs by removing redundant and highly similar sequences. However, there is no guarantee that the correct representative sequence is kept for a given cluster or that each cluster represents one unique gene. Because they require multiple assemblies to merge, they also come at a far greater computational cost. An example of how these ensemble assembly strategies perform compared to individual
2.4. Third generation sequencing
All of the methods described so far primarily use short but highly accurate reads from Illumina sequencing for assembly, with or without a reference. With the rise of third-generation sequencing technologies from Pacific Biosciences (PacBio SMRT) and Oxford Nanopore Technologies (ONT MinION), it is becoming possible to sequence entire mRNA molecules as one very long read, though with a high error rate . The ability to sequence the entire mRNA molecule is especially beneficial for detecting alternative splice forms, which remain a challenge for short-read only assembly, and potentially for more accurate transcript quantification if there is no bias in the mRNA molecules sequenced.
While many tools exist to perform genome assemblies using either these long reads alone or by combining long reads and Illumina reads, at present no short read transcriptome assemblers take advantage of long-reads in transcriptome assembly. If these long reads can be sufficiently error-corrected (e.g. [28, 29]), they can be used for a snapshot of the expressed transcriptome, without requiring assembly or external references [30, 31]. Alternatively, after an independent
3. Performance metrics used for transcriptome assembly
In this section, we will discuss commonly used metrics to assess the quality of transcriptome assemblies.
3.1. Metrics based on contig count and lengths
The most straightforward assembly metrics are those based on the number and lengths of the sequences produced . The number of sequences can be presented either or both of:
the number of contigs
the number of scaffolds
where for contigs no further joining of the sequences is performed after assembly, and for scaffold contigs that have some support for being from the same original sequence are combined together with a certain number of gaps between them.
Several different statistics are available for presenting the lengths of the sequences (either contigs or scaffolds). The most commonly reported metrics are:
minimum length (bp): the length of the shortest sequence produced
maximum length (bp): the length of the longest sequence produced
mean length (bp): the average length of the sequences produced
median length (bp): the length where half of the sequences are shorter, and half of the sequences are longer
N50 (bp): a weighted median where the sum of the lengths of all sequences longer than the N50 is at least half of the total length of the assembly
L50: the smallest number of sequences whose combined length is longer than the N50
Additional metrics similar to N50 (e.g. N90) based on different thresholds are also used.
For genome assemblies where the target number of sequences is known (one circular genome plus any smaller plasmids for prokaryotic organisms and the number of chromosomes for eukaryotic organisms), these metrics provide an estimate for the thoroughness of the assembly . For instance, in prokaryotic assemblies, the vast majority of the sequence is expected to be in one long sequence, and having many shorter sequences indicates fragmentation of the assembly . In this context, longer sequences (e.g. larger N50) tend to indicate higher quality assemblies. For transcriptome assemblies, however, the length of the assembled contigs varies depending on the lengths of the transcripts being assembled. For the human transcriptome, for example, while the longest transcript (for the gene coding the Titin protein) is over 100 kb, the shortest is only 186 bp, with a median length of 2787 bp . Emphasizing longer contigs also rewards assemblers that over-assemble sequences, either by including additional sequence incorrectly within a gene, or by joining multiple genes together to form chimeric contigs. Therefore, for transcriptome assembly, metrics based on contig lengths do not necessarily reflect its quality.
3.2. Metrics based on coded protein similarity
Rather than focusing on the number or length of the sequences produced by the assembly, performing similarity searches with the assembled sequences can provide an estimate of the quality of the contigs or scaffolds [24, 38]. Typically, the process consists of either similarity searches against well annotated databases (such as the protein datasets of related genomes or targeted orthologs, the BLAST non-redundant protein database  or the UniProt/Swiss-Prot database ), conserved domain search within the contig sequence that determines the potential function of the gene (such as PFAM or Panther [41, 42]), or a search against a lineage specific conserved single-copy protein database (such as BUSCO ). These similarity searches are usually performed on the predicted protein sequences for the contigs (e.g. using GeneMarkS ), but can also be performed directly from the assembled nucleotide sequences using BLASTX where translated nucleotide sequences are used to search against a protein database . If the organism being sequenced is closely related to a model organism with a well-defined transcriptome, nearly all of the contigs that are not erroneously assembled and code proteins should have identifiable potential homologs in the database. If a large percentage of the contigs do not have similar proteins identified in the database, there is a high probability that the sequences are incorrectly assembled, regardless of the length of the sequences. By performing similarity searches, over-assemblies or chimera contigs (those covering more than one gene) can be also detected as large gaps in the alignment between the query and the hits. As protein sequence annotations are necessary for most downstream analyses, they also provide a convenient metric without the need for additional, otherwise unnecessary analyses.
Despite these advantages, there are some limitations to using protein-similarity based metrics for assembler performance. First, the more divergent the organism being sequenced is from the sequences in the database searched and the more species-specific genes in the transcriptome, the lower the percentage of contigs with hits will be. This can result in some organisms appearing to have a lower quality assembly solely due to their divergence from those well represented in the databases. By extension, assemblies that recover more transcripts whose coded proteins have few similar sequences in the database will appear worse than assemblies that only recover conserved genes. This limitation can be somewhat mitigated by comparing only genes that are universally single-copy across different species, which are more likely to be conserved and similar enough to be identified. This is the strategy used in BUSCO . However, this comparison at best uses only a subset of the assembled contigs. Second, and more problematic, this metric rewards assemblies that artificially duplicate conserved genes with only small differences in the nucleotide sequence. In the extreme, this can result in several times as many contigs in the assembly than were present in the actual transcriptome, but with nearly all of the contigs coding conserved protein sequences. This is particularly an issue when the analysis depends on identifying the gene copy numbers in the assembly. It also has a large impact on the accuracy of contig quantification and differential expression analyses .
3.3. Assembly metrics based on benchmark transcriptomes
The only way to overcome the limitations of the metrics described in the previous sections is to compare the assembly output against a benchmark transcriptome where correct sequences of all transcripts are known. When an RNAseq data generated from a well-established model organism is used for assembly, many of correctly assembled contigs can be identified. However, variability in the transcriptome among e.g. cell types limits the amount of information that can be gained for incorrectly assembled contigs. It is also not possible to determine whether sequences from the reference that are missing from the assembled transcriptome are due to assembly errors, or whether they were not expressed in the library sequenced. Transcriptome sequences may also vary between the individual under study and the reference. Such variations can mask assembly errors that affect the contig sequences. Although this limitation can be mitigated by sequencing an individual that is genetically identical to the reference, it severely limits the types of organisms that can be used for the benchmark.
To comprehensively assess all of the assembly errors, we need to obtain RNAseq data from a transcriptome where all transcript sequences and expression patterns are known. Ideally, such a benchmark transcriptome would be synthetically produced and sequenced using standard protocols. However, currently no such synthetic mRNA library exists. An alternative approach is to simulate the sequencing of a given benchmark transcriptome. There are several tools that can generate simulated reads modeling short Illumina reads [46, 47] and/or long third-generation sequencing reads such as PacBio SMRT and ONT MinION [48, 49]. These tools typically either focus on identifying the statistical distribution of reads across the sequences and errors within the reads, as is the case for RSEM , PBSIM , and Nanosim , or attempt to reconstruct each step of the library preparation and sequencing pipeline, mimicking the errors and biases introduced at each step, as is the case for Flux Simulator .
Using simulated RNAseq data with a known transcriptome as a benchmark gives the most detailed and close to true performance metric for assemblies. Specifically, this strategy allows the quantification of each of the following categories:
correctly assembled sequences (true positives or TPs)
sequences that are assembled with errors (false positives or FPs)
sequences in the reference that are missing from the assembly (false negatives or FNs)
“Correctness” and “incorrectness” (or error) can be defined using varying degrees of sequence similarities. Using the strictest threshold, a contig sequence is assembled “correctly” only if the entire nucleotide or coded protein sequence is identical to a reference transcript. All other contigs found in the assembly, including those whose sequences have no similarity in the reference transcriptome (missing contigs), are considered to be assembled “incorrectly” (FPs) regardless of the similarity against the reference sequences.
Note that true negatives (TNs) can be counted only if the assembly experiments are done including reads that are derived from transcripts that are not part of the reference transcriptome (negative transcripts). Using these categories, following assembly metrics can be calculated:
Sensitivity (or recall) =
F-measure (or F1 score) =
False discovery rate (FDR) =
Often in an RNAseq simulation, negative transcripts are not included; hence TN cannot be counted. In such cases, we can calculate an alternative metric as the accuracy:
Despite the added benefits of simulation for measuring the performance of assemblers, these metrics assume that the simulation accurately reflects the nature of real RNAseq data. Differences in the distribution of reads or errors between the simulations and real data can impact the relative performance of the assemblers. Assemblers that perform well on simulated data may perform poorly on real data if those assumptions are not met. Consequently, great care must be taken to ensure that the simulated data captures the features of real data as accurately as possible to best characterize the performance of different assembly strategies.
4. Performance analysis of transcriptome assemblers
In this section, as an example, we compare the performance of transcriptome assemblers using a simulated benchmark transcriptome dataset.
4.1. Benchmark transcriptome and simulated RNAseq
RNAseq datasets were generated by Flux Simulator  using the hg38 human genome (available at https://genome.ucsc.edu/cgi-bin/hgGateway?db=hg38) as the reference. The older hg19 human genome (available at http://genome.ucsc.edu/cgi-bin/hgGateway?db=hg19) was also used as an alternate reference genome to assess the impact of using a different reference with genome-guided assemblers. The gene expression profile was generated by Flux Simulator using the standard parameters from the hg38 reference genome and transcriptome model. Approximately 250 million pairs of reads were computationally generated with the given expression model with no PolyA tail. The simulated library construction was fragmented uniformly at random, with an average fragment size of 500 (±180) nucleotides (nt). Because reads overlapping within read pairs can cause problems for some assemblers, fragments shorter than 150 nt were removed. The simulated sequencing was performed using paired-end reads of length of 76 nt using the default error model based on the read quality of Illumina-HiSeq sequencers. Note that only reference transcripts with full coverage of RNAseq data were included in the benchmarking, as transcripts without full coverage cannot be correctly assembled as a single contig. This filtering removed 2700 transcripts expressed in the benchmark transcriptome, leaving 14,040 unique sequences derived from 8557 genes (5309 with no alternative splicing; on average 1.64, ranging up to 13, isoforms per gene).
The read pairs generated by Flux Simulator were quality filtered using Erne-filter version 2.0 . The reads were filtered using ultra-sensitive settings with a minimum average quality of q20 (representing a 99% probability that the nucleotide is correctly reported). The filtering was performed in paired-end mode to ensure that both reads of the pair were either kept or discarded concurrently to keep the pairs together. The remaining reads were normalized using Khmer  with a kmer size of 32 and an expected coverage of 50×. The normalization was also performed in paired-end mode to maintain pairs.
De novo assemblies
We compared the performance among four
Performance statistics for each assembler is given in Table 2. Precision is a measure of how likely an assembled contig is to be correct, and recall is a measure of how likely the assembler is to correctly assemble a contig. In these terms, for assemblers with high precision, the contigs produced are more likely to be correct, but the assembly may miss a large number of sequences present in the sample. Conversely, assemblers with high recall values correctly assemble more of the sequences present in the sample, but may do so at the cost of accumulating a large number of incorrectly assembled contigs. In these statistics, both the modified accuracy score (accuracy*; see Section 3.3) and the F1 score are a measure of the number of correctly assembled contigs relative to the number of missing and incorrectly assembled contigs. FDR is the proportion of assembled reads that are incorrect. Based on these statistics, Trinity is the best performing
In Table 1, the results from pooling (taking the union of) the outputs of multiple runs of each assembler across a range of kmer lengths are also shown. With these pooled assemblies, the proportion of correctly assembled transcripts in the benchmark for Trinity increased from 41 to 46%, and for rnaSPAdes from 36 to 47%. However, the pooling process also accumulated several times more unique incorrect sequences than additional correct sequences recovered. For Trinity, the C/I decreased from 0.8432 to 0.3470, and for rnaSPAdes this ratio decreased from 0.5900 to 0.0621.
Although the four
4.3. Genome-guided assemblies
We next compared the transcriptome assembly performance among three genome-guided assemblers: Bayesembler (version 1.2.0) , Cufflinks (version 2.2.1) , and StringTie (version 1.0.4) . To demonstrate the impact of using different reference genomes on genome-guided transcriptome assemblies, we used both of the hg38 as well as hg19 genomes as the references. Assembly assessment was done against the hg38 benchmark transcriptome.
Table 3 shows the performance of each of these tools in the two scenarios (RNAseq data and the reference were derived from the same or different genomes). As observed with
As with the
4.4. Comparison of
de novo and genome-guided assemblers
While the overall statistics are comparable between the best
As with the individual assemblies, fewer incorrectly assembled contigs were produced by all of the tools, and most are assembler specific (Figure 3C and D). In particular, only 1387 incorrectly assembled contigs were produced by all of the
Overall, these results suggest that genome-guided assemblies provide relatively few correctly assembled contigs relative to performing multiple
4.5. Ensemble assemblies
We compared the two ensemble transcriptome assembly methods, EvidentialGene (version 2017.03.09)  and Concatenation (version 1)  using the simulated RNAseq data. The strategies for these assemblies followed the recommendations by each method. For EvidentialGene, the pooled results from all of the four
In addition to the two ensemble methods, we also included three “consensus” approaches taking the consensus of the pooled
The performance of these ensemble strategies is shown in Table 5. Both of EvidentialGene and Concatenation resulted in an over-estimation in the number of transcripts present. Interestingly, while Concatenation produced a larger total number of transcripts (19,767) than EvidentialGene (19,177), ~2300 of those sequences were redundant, leading to fewer unique sequences (17,497 by Concatenation). Additionally, Concatenation both kept more of the correctly assembled contigs from the individual
|Consensus 2||21,444||21,444||6711 (47.80)||14,433||0.4650|
|Consensus 3||12,600||12,600||6144 (43.76)||6456||0.9517|
|Consensus 4||9095||9095||5331 (37.97)||3764||1.416|
Consensus 2 produced the most correctly assembled contigs of any method (6711), but at the cost of more incorrectly assembled contigs than Concatenation (14,433). However, both Consensus 3 and Consensus 4 kept the majority of the correctly assembled contigs while reducing the number of incorrectly assembled contigs by roughly half or three quarters, respectively. Consensus 4 had the highest precision (0.5861) and lowest FDR (0.4139) of any method. However, the additional reduction in the number of correctly assembled contigs lead to Consensus 3 having slightly higher accuracy* (0.2998) and F1 score (0.4613).
In Figure 4 all individual methods (both
Transcriptome assembly can be approached from multiple different strategies. Historically, these approaches have revolved around assembling short but highly accurate Illumina reads with or without an existing genome assembly as a reference, referred to as genome-guided or
Genome-guided assemblers avoid the limitations of the de Bruijn graphs by mapping the reads to the reference genome. This mapping, however, introduces its own limitations and trade-offs. Reads that are ambiguous between splice forms in the same genomic locations or across multiple genomic locations create similar challenges to the de Bruijn graphs. These ambiguities are compounded when the mapping must take into account mismatches due to sequencing errors as well as biological variations.
The limitations of the individual tools can potentially be overcome by combining multiple different assemblies in ensemble. As each tool and set of parameters results in a different set of correctly assembled contigs, accurately selecting these correctly assembled contigs without selecting any redundant incorrectly assembled contigs would leverage the strengths of each methods without the weaknesses of any. However, currently available ensemble strategies cannot guarantee that the correct sequence is chosen, leading to ensemble assemblies that are less accurate than individual assemblies. As the selection criteria for ensemble methods improve, such as with the “Consensus” approach shown here, these methods can also leverage new assembly approaches that can better handle certain subsets of transcripts (e.g. alternative splice forms) that may have other weaknesses that prevent them from being competitive as a general transcript assembly tool.
Overall, as our results demonstrated, transcriptome assemblers can still be improved, regardless of the approach used. While the genome-guided assemblers generally perform best when the assembly is performed against the same reference sequence that the RNAseq data was generated from, this is not always possible. When these sequences differ, the genome-guided assemblers may have lower accuracy than the
This work was supported by a grant from the National Science Foundation to ENM (Award #: 1339385).
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