Eukaryotic Genome Annotation Pipeline

BLAST

See the BLAST chapter.

Compart algorithm

A compartment is defined as a sequence of compatible hits. Two BLAST hits are said to be compatible if they follow the natural flow of the target sequence. On a given strand, the relative position of the hits should be the same on both the query sequence and the genome. Compatible hits may overlap but may not be contained within one another. This definition of compatibility is transitive.

The Compart algorithm finds all non-overlapping compact compartments on the genome for a given query using a maximal coverage algorithm. Each compartment is assigned coverage, ${\mathrm{\Phi }}^c$, which is a measure of how well it represents the target sequence:

${\mathrm{\Phi }}^c=\sum _h{w}^h{L}_{\text{eff}}^h$

In this equation $L_{\text{eff}}^h$ is the effective length of the hit $h$. It is usually the hit length, but if the hit overlaps with a neighbor hit, its effective length is decreased by a half of the overlap.

For cDNA alignments, where most useful hits are of very high identity, the weight $w^h$ equals the identity of the hit and the coverage ${\mathrm{\Phi }}^c$ is the number of matches. For protein alignments, the weight is a constant equal 1. In this case the coverage ${\mathrm{\Phi }}^c$ is simply the target sequence length covered by the hits.

When there is more than one compartment, the query sequence is covered multiple times, and to a certain extent finding all compartments is equivalent to maximization of the total coverage. In the case of exon duplication events, the additional hits should be ignored rather than turned into additional compartments. Since typically only a relatively small portion of the gene is duplicated we introduce a penalty $P_{\text{new}}$ for an additional compartment. This penalty ensures that a new compartment is created only if there is enough gene material for it. The value of this parameter is usually 25%–40% of the target sequence length. So our maximal coverage algorithm finds the compartments configuration that maximizes the following total coverage:

$\mathrm{\Phi }=\sum _c({\mathrm{\Phi }}^c-P_{\text{new}})$

The process of optimization is performed very effectively using the dynamic programming algorithm.

Splign – Transcript alignment

Splign (1) is a tool for aligning spliced cDNA sequences against their genomic counterparts using pre-computed compartments. The program produces accurate spliced alignments via solving a score $S$ optimization problem formulated specifically to account for splice signals and introns.

$S=B_{\text{m}}{N}_{\text{m}}-P_{\text{mis}}{N}_{\text{mis}}-\sum _{\text{gaps}}\left(P_{\text{gopen}}+P_{\text{gextend}}l\right)-\sum _{\text{introns}}\left(P_{\text{iopen}}+P_{\text{iextend}}l\right)$

In this formula $B_{\text{m}}$ and $N_{\text{m}}$ are the bonus for a match and the number of matches, $P_{\text{mis}}$ and $N_{\text{mis}}$ , are the penalty for a mismatch and the number of mismatches, $P_{\text{gopen}}$ and $P_{\text{gextend}}$ , are the penalties for opening and extending a gap. These parameters are similar to the ones used in Blastn. The introns are accounted for by introduction of a special type of gap with $P_{\text{iopen}}$ and $P_{\text{iextend}}$ as the penalties for an opening and extending an intron. The formulation discriminates between the most frequent consensus (GT/AG), less frequent consensus (GC/AG, AT/AC), and not consensus donor/acceptor sites by giving different values to $P_{\text{iopen}}$.

Since the complexity of solving the global sequence alignment problem is proportional to the product of lengths of the sequences, the hits are arranged into compartments as described above and the dynamic programming matrix split into smaller blocks by seeding the global alignment with the high identity portion of the hits (Figure 1).

For each compartment, its genomic search space is expanded by the length of query cDNA ends not covered by local alignments. This allows detecting the end exons if they are missed by the local alignment tool for reasons such as the alignment length being shorter than the word size or the exon residing in a masked region. Each hit may correspond to an exon, a part of an exon, or even a number of exons. Therefore, it is important to be conservative when using local alignments for alignment seeding. Within each compartment, parts of alignments that overlap on the query are dropped. From the remaining alignments, the longest perfectly matching diagonals are extracted, and the cores are used to seed the global alignment.

Hits comprising compartments determine whether the query and the subject sequence align on the same strand. Most mRNA sequences have natural biological order and positive strand can be assumed when aligning them. On the contrary, EST and frequently RNA-Seq sequences are not oriented, so both the original sequence and its reverse complimentary have to be aligned and the strand is determined by comparing the resulting alignments.

ProSplign – Protein alignment

Protein alignments are produced by ProSplign. Similarly to Splign, ProSplign is a global protein-to-genome alignment tool that produces accurate spliced alignments from pre-computed compartments. ProSplign uses a modified Needleman Wunsch type (7) global alignment algorithm for aligning. ProSplign scores the target protein sequence against translation of the genomic sequence using the following score:

$S=\sum _{\text{diag}}{S}_{\text{diag}}-\sum _{\text{gaps}}\left(P_{\text{gopen}}+P_{\text{gextend}}l\right)-\sum _{\text{introns}}\left(P_{\text{iopen}}+P_{\text{iextend}}l\right)$

where $S_{\text{diag}}$ is the score for an ungapped part of the alignment calculated using a BLOSUM62 matrix (8). Insertions and deletions for which the length is a multiple of three are scored with the default Blastp gap penalties $P_{\text{gopen}}$ and $P_{\text{gextend}}$. Gaps for which the length is not a multiple of three are frameshifts and have a much higher opening penalty $P_{\text{gopen}}$. The introns are scored as a special type of gap with a very small extension cost and an opening cost which is different between the most frequent consensus splices (GT/AG), less frequent consensus splices (GC/AG, AT/AC), and non-consensus splice sites.

Unlike Splign, ProSplign doesn’t use seeds because Blast hits for cross-species proteins do not give reliable information about seeds. Instead, ProSplign aligns the protein against a slightly extended genomic region identified by Compart as the compartment.

Not all parts of a protein are conserved well enough to provide a reliable alignment. In fact, some parts may not correspond to anything on the genome. The global alignment algorithm will align the whole protein, rendering a very low-identity alignment for the non-conserved portions of the protein. These unreliable and often misleading pieces of the alignment are filtered out by ProSplign during a post-processing step.

Chainer

Spliced alignments obtained using Splign and ProSplign are likely partial, either because the aligned sequences are partial or in the case of protein alignments because only conserved portions of the protein could be aligned. Chainer analyzes and assembles these partial alignments to provide longer gene models and additional information about alternative variants.

Because of their short length and high redundancy, RNA-Seq alignments with identical introns are first combined into single alignments with larger weights (Figure 2). Boundaries of these “micro-chains” don’t cross splices known from other alignments and their extension is limited to 20 bp.

These “micro-chains” are then combined by Chainer with cDNA and protein alignments based on their exon structure compatibility using a modified version of the Maximal Transcript Alignment algorithm (9) based on frame compatibility of the coding regions. For protein and annotated full-length cDNA alignments, the coding regions can be inferred. For other cDNA alignments, possible coding regions are predicted and scored using a 3-periodic fifth-order Markov model for coding propensity and Weight Matrix Method (WMM) models for splice signals and translation initiation and termination signals (10). All cDNAs with coding sequence (CDS) scores above a given threshold are marked as coding, and the CDS information is used when assembling chains. In many cases, this process determines the orientation of an EST if it was unknown before. RNA-Seq and some EST alignments are too short to score above the threshold and, if they are not spliced, their orientation is often also unknown. For these alignments, Chainer will consider that these sequences can be part of the 5’ end and harbor a start codon, or be part of the 3’ end and harbor a stop codon, or be internal to the CDS or to an untranslated region (UTR), and select the scenario that contributes to the longest CDS.

Afterward, UTRs are added if the necessary translation initiation or termination signals are present. There are no restrictions on the extension of a 5’-UTR other than the exon-intron structure compatibility.

The assembled full-length chains that share splices or CDS are combined into genes with alternative isoforms. Among the partial chains for a gene, the variant with the longest CDS is selected for extension by ab initio prediction.

HMM-based prediction

The core algorithm of the ab initio prediction capability of Gnomon is based on Genscan (5), which uses a 3-periodic fifth-order HMM for the coding propensity score and incorporates descriptions of the basic transcriptional, translational and splicing signals, as well as length distributions and compositional features of exons, introns and intergenic regions. The most important distinction of Gnomon from Genscan and other ab initio prediction programs is its ability to conform to the supplied alignments and extend and complement them when necessary.

Mathematically, an HMM-based ab initio prediction is a search in the gene configuration space for the gene that provides the maximal score. If all configurations that are not compatible with the available alignments are excluded from the search space, then the optimization process in the resulting collapsed space will yield a gene configuration that is possibly suboptimal from the ab initio point of view but exactly follows the experimental information available. This approach allows extension or connections of partial alignments (Figure 3). Untranslated regions, if present in the alignments, are also included in the gene model.

Gnomon recognizes as HMM states coding exons and introns on both strands and intergenic sequences. Translational and splice signals are described using WMM (10) and WAM (11) models. A 12-bp WMM model, beginning 6 bp prior to the initiation codon, is used for the translation initiation signal (12). A 6-bp first order WAM model starting at the stop codon is used for the translation termination signal. The donor splice signal is described by a 9-bp second order WAM model, and the acceptor splice signal is described by a 43-bp second order WAM model. Both donor and acceptor models include 3-bp of the coding exon. Coding portions of exons are modeled using an inhomogeneous 3-periodic fifth-order Markov model (13). The noncoding states are modeled using a homogeneous fifth-order Markov model.

Alignment of known RefSeq transcripts

Since many of the known RefSeq sequences are curated (most notably for Vertebrates) and, as such, are high-value targets when annotating a genome, special attention is given to their proper placement. Masking may interfere with the alignment process, so RefSeq transcripts for which all alignments on the masked genome are under a coverage threshold may be re-aligned to the unmasked genome.

The alignments are ranked and filtered based on adjustable criteria (such as coverage, identity, rank) as well as location information contained in the RefSeq tracking database. Typically, only the best-placed alignment for a given query is selected for use in downstream steps.

Alignment of non-Refseq transcripts

INSDC mRNAs, ESTs and 454 sequences are first screened against a database of mitochondrial sequences, cloning vectors, adaptors, bacterial IS-elements and repetitive sequences, and excluded from further processing if a large portion of their sequence hits a contaminant. In addition, transcripts identified as low-quality by curation staff are screened out.

Following this initial screen, the sequences are aligned with BLAST and Splign, as explained above, and ranked and filtered. For a given transcript, typically only the best-placed alignment (rank 1) is selected. For sequences that cannot be oriented (e.g., unspliced ESTs), alignments to both strands are passed dowstream. If used, cross-species transcripts are aligned with more stringent criteria than same-species transcripts to insure that only the most-likely ortholog transcript is passed downstream.

Alignment of proteins

Similarly to transcripts, proteins are first screened against a database of repeats and the curated list of low-quality transcripts. Proteins are then aligned to the masked genome with BLAST and ProSplign. The alignments are further ranked and filtered and passed to the gene prediction step.

Alignment of short reads

Short reads (RNA-Seq) available in the SRA can be used for gene prediction. A specific dataflow was engineered to handle the large volume and short length of sequences produced by new generation sequencing technologies.

RNA-Seq data from so-called next-generation sequencing platforms present several challenges for use in gene prediction. First, the reads are substantially shorter than conventional transcript data such as ESTs and mRNAs, so an individual read contains relatively little information. For example, typically only 5-25% of reads from the Illumina platform span an intron, which is the most useful data for building gene models. Second, the reads are extremely numerous and redundant, with highly-expressed genes being represented by tens of millions of reads. This presents a challenge for throughput. And third, the depth of coverage results in apparent background expression in most of the genome that isn't desirable to represent in the final gene models.

The annotation pipeline addresses these issues in several ways to reduce the complexity of the RNA-Seq data and convert it to a form useful for gene predictions:

1.

Datasets and associated metadata are obtained from the SRA and BioSample databases, enabling robust tracking of evidence.

2.

The reads are "uniquified" so that 100% identical sequences are aligned only once.

3.

Unique reads are aligned, ranked, and filtered for high identity and coverage alignments.

4.

Alignments with the same splice structure and the same or similar start and end points are collapsed into a single representative alignment. The number of reads from each SRA run is tracked for each collapsed alignment.

5.

Alignments containing rare introns or that represent apparent noise or background are filtered from the dataset.

Taken together, these steps reduce the size and complexity of a typical RNA-Seq dataset by 100-1000x. The resulting collapsed alignments can be used by themselves or combined with transcript and/or protein alignments for the gene prediction step.

Models based on known and curated RefSeq

RefSeq transcripts are given precedence over overlapping Gnomon models with the same splice pattern. Alignments of known same-species RefSeq transcripts or curated genomic sequences are used directly to annotate the gene, RNA, and CDS features on the genome. Since the RefSeq sequence may not align perfectly or completely to the genomic sequence, a consequence of this rule is that the annotated product may differ from the conceptual translation of the genome.

Models based on Gnomon predictions

Gnomon predictions are included in the final set of annotations if they do not share all splice sites with a RefSeq transcript and if they meet certain quality thresholds including:

• Only fully- or partially-supported Gnomon predictions, or pure ab initio Gnomon predictions with high coverage hits to UniProtKB/SwissProt proteins are selected.
• When multiple fully-supported transcript variants are predicted for a gene, only the Gnomon predictions supported in their entirety by a single long alignment (e.g., a full-length mRNA) or by RNA-Seq reads from a single BioSample are selected.
• Poorly-supported Gnomon predictions conflicting with better-supported models annotated on the opposite strand are excluded from the final set of models.
• Gnomon predictions with high homology to transposable or retro-transposable elements are excluded from the final set of models.

Integrating RefSeq and Gnomon annotations

As a result of the model selection process, a gene may be represented by multiple splice variants, with some of them known RefSeq and others model RefSeq (originating from Gnomon predictions).

Gnomon predictions selected for the final annotation set are assigned model RefSeq accessions with XM_ or XR_ prefixes for protein-coding and non-coding transcripts, respectively, and XP_ prefixes for proteins to distinguish them from known RefSeq with NM_/NR_ and NP_ prefixes. Model RefSeq can be searched in Entrez with the query “srcdb_refseq_model[properties]” while known RefSeq sequences can be obtained with the query “srcdb_refseq_known[properties]”.