Bioinformatics Advance Access originally published online on August 4, 2008
Bioinformatics 2008 24(19):2165-2171; doi:10.1093/bioinformatics/btn414
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Model-based prediction of sequence alignment quality
1Biotechnology and Food Research, MTT Agrifood Research Finland, FI-31600 Jokioinen, 2Department of Statistics, 3Department of Mathematics, FI-20014, University of Turku, 4Institute of Medical Technology, FI-33014, University of Tampere and 5Tampere University Hospital, FI-33520 Tampere, Finland
*To whom correspondence should be addressed.
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ABSTRACT |
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 TOP ABSTRACT 1 INTRODUCTION2 METHODS3 RESULTS4 DISCUSSION AND CONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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Motivation: Multiple sequence alignment (MSA) is an essential prerequisite for many sequence analysis methods and valuable tool itself for describing relationships between protein sequences. Since the success of the sequence analysis is highly dependent on the reliability of alignments, measures for assessing the quality of alignments are highly requisite.
Results: We present a statistical model-based alignment quality score. Unlike other quality scores, it does not require several parallel alignments for the same set of sequences or additional structural information. Our quality score is based on measuring the conservation level of reference alignments in Homstrad. Reference sequences were realigned with the Mafft, Muscle and Probcons alignment programs, and a sum-of-pairs (SP) score was used to measure the quality of the realignments. Statistical modelling of the SP score as a function of conservation level and other alignment characteristics makes it possible to predict the SP score for any global MSA. The predicted SP scores are highly correlated with the correct SP scores, when tested on the Homstrad and SABmark databases. The results are comparable to that of multiple overlap score (MOS) and better than those of normalized mean distance (NorMD) and normalized iRMSD (NiRMSD) alignment quality criteria. Furthermore, the predicted SP score is able to detect alignments with badly aligned or unrelated sequences.
Availability: The method is freely available at http://www.mtt.fi/AlignmentQuality/
Contact: virpi.ahola{at}mtt.fi
Supplementary information: Supplementary data are available at Bioinformatics online.
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1 INTRODUCTION |
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TOPABSTRACT1 INTRODUCTION2 METHODS3 RESULTS4 DISCUSSION AND CONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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Biological nucleotide or amino acid sequence analysis is typically based on comparing a sequence of interest with homologous sequences. As more and more sequence data from different species is becoming available, comparative genomics using multiple sequence alignment (MSA) has increased its importance (Notredame, 2002). The quality of alignments is a focal limiting factor in sequence analysis (Landan and Graur, 2008). MSA programs are not always successful in constructing biologically plausible alignments, and the use of poor alignment results may lead, for instance, to incorrect structural model and the error can cumulate in the subsequent structure predictions (Domingues et al., 2000).
The alignment quality measures are important, e.g. for benchmarking the MSA programs and assessing the quality of a given alignment. The MSA programs have been validated and compared using reference sequence alignment databases such as Balibase, Homstrad, Prefab and SABmark (Bahr et al., 2001; Mizuguchi et al., 1998; Edgar, 2004; Walle et al., 2005). The benchmarking databases have usually been constructed using protein 3D structural alignments, and hence they are independent of sequence alignment methods (Edgar and Batzoglou, 2006). The quality of the MSA programs has typically been assessed by the sum-of-pairs (SP) or the column score (CS), that measure the proportion of correctly aligned residue pairs or alignment positions, respectively, in the alignment (Thompson et al., 1999). The lack of proper alignment validation methods, which could be used without reference alignments, is a disadvantage in benchmarking the MSA programs. The current approaches prohibit the analysis of any set of sequences and force to trust the structure-based reference alignment databases, although it is known that different structure alignment methods may produce different alignments (Goldsmith-Fischman and Honig, 2003; Lackner et al., 2000 Notredame, 2007).
Assessing the quality of a single alignment is even more complicated than the validation of the MSA methods because no reference alignment is available. The only way is to use validation methods, which do not need a reference alignment. Traditionally, alignment quality measures have been formulated on the basis of column-based conservation measures, which are then summed over the entire length of the alignment. The NorMD is a normalized mean distance (MD) score that measures the mean distance between similarities of residues at each alignment position (Thompson et al., 2001). Other column-based methods, such as IC or AL2CO, use entropy or the sum of pairs measures (Hertz and Stormo, 1999; Pei and Grishin, 2001). Beiko et al. (2005) presented a word-oriented objective function (WOOF) for alignment validation. The method is not a column-based, but relies on scoring conserved amino acid regions between pairs of sequences.
When the structures for at least two homologues are available, one can use the normalized iRMSD score (NiRMSD), which measures the root mean squared distance (RMSD) of all possible residue pairs within a certain sphere of the radius between two sequences (Armougom et al., 2006). The drawback of the NiRMSD score is that it can only be used to compare relative accuracy of two alignments. Another possibility is to use structural similarity scores (Pei and Grishin, 2006, Pei and Grishin, 2007). The quality assessments of the structure-based measures are, however, based only on the sequences whose structure is available. Yet another approach is based on aligning the same set of sequences with several MSA programs and comparing the results (Morgenstern et al., 2003). Such consistency based scores assume that the residue pairs coinciding in most of the alignments are biologically correct (Gotoh, 1990; Vingron and Argos, 1991). The multiple overlap score (MOS) determines alignment quality as the proportion of similarly aligned residue pairs among all pairs of aligned residues (Lassmann and Sonnhammer, 2005b). Their score is essentially similar to alignment consistency of the overall residue evaluation (alCORE) index in TCoffee alignment program (Notredame and Abergel, 2003). Landan and Graur (2007, Landan and Graur, 2008) realigned several suboptimal alignments and the same sequences in reversed residue order. Their reliability measure was obtained by calculating the proportion of alignments that reproduce the pairing of the residue pair at one alignment position and averaging the proportion over all the residue pairs and columns of the alignment.
We present a model-based alignment quality scoring method, which combines the information available on the reference align-ments with the conservation level and other observed properties of the MSA of interest. We used the Homstrad database as the source of reference information and aligned the reference sequences with Mafft (L-ins-I), Muscle and Probcons (Do et al., 2005; Edgar, 2004; Katoh et al., 2002; Katoh et al., 2005). The divergence between the reference and realignments was evaluated by calculating the SP score, which was chosen as the representative alignment quality score (Thompson et al., 1999). The correct SP score for the Homstrad alignment was statistically modelled in terms of the conservation level by taking the identity, number of sequences and the alignment length into account. The conservation level of the alignment was calculated using our statistical conservation score (Ahola et al., 2006), which defines the proportion of conserved amino acids in the alignment. As a final outcome, the estimated model parameters can be used to predict the quality of any global MSA. We tested the novel quality score on the structure-based alignments in Homstrad and SABmark databases by comparing the predicted SP scores with the correct SP scores. Finally, we compared the performance of the quality score with previous scores, MOS, NiRMSD and NorMD, in the same benchmarking databases.
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2 METHODS |
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TOPABSTRACT1 INTRODUCTION2 METHODS3 RESULTS4 DISCUSSION AND CONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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2.1 Alignment quality assessment
The Homstrad (Mizuguchi et al., 1998) database was used as the reference for constructing the alignment quality score. A set of training sequences in the Homstrad database were aligned using the Mafft with L-INS-i mode (Katoh et al., 2002, Katoh et al., 2005) and Muscle (Edgar, 2004) and Probcons (Do et al., 2005) with default parameters (Fig. 1, stage 1). These programs were chosen, because they are fast and they are among the most accurate alignment programs (Ahola et al., 2006; Golubchik et al., 2007; Nuin et al., 2006). The SP scores, i.e. the proportion of correctly aligned residue pairs in an alignment, were calculated between the reference and realignments to measure the quality of the automatically generated MSAs (Thompson et al., 1999). Furthermore, the conservation-level ConsAA (Ahola et al., 2006) and other variables describing the alignment, such as the sequence identity, alignment length and the number of sequences in the alignment, were calculated for the Mafft, Muscle and Probcons alignments. Beta regression model was fitted for the SP score using the ConsAA and other descriptive variables as predictors. The predicted SP score value of a new alignment could then be calculated by using the parameter values of the fitted model (Fig. 1, stage 2).
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The generalizability of the quality score was tested using unseen sequences of the SABmark and independent test sequences of the Homstrad databases. The predicted SP score values and their prediction intervals were calculated for the test alignments of the Homstrad and SABmark databases. The performance of the predicted SP scores was assessed by comparing them with the CS and correct SP scores in the Homstrad and with median fd and fm scores in the SABmark databases. Additionally, other alignment quality measures, MOS, NiRMSD and NorMD scores, were calculated for the Mafft, Muscle and Probcons alignments and compared with the correct alignment quality measures and the predicted SP scores. The ability of the quality scores to distinguish alignments with badly aligned or unrelated sequences from correct alignments was tested by adding random sequences to the reference alignments.
2.2 Benchmarking databases
The Homstrad database provides homologous structure-based alignments for all known protein structures (Mizuguchi et al., 1998). The structures have been first clustered into homologous families and then the representative sequences of each family have been structure aligned and additional sequences have been added into the alignment by ClustalW. We used all the Homstrad families which contained at least two known structures, alignment had five or more sequences and was more than 50 residues long. Four database versions (Homs10, Homs20, Homs50 and Homs100) were build. The Homs10 includes alignments with 5 to 10 sequences, the Homs20 consists of alignments with 11 to 20 sequences, the Homs50 includes alignments with 20 to 50 sequences and the Homs100 the alignments with 50 to 100 sequences. In each of the rebuilt databases, each alignment includes all the representative sequences and, if needed, additional sequences. The additional sequences were randomly chosen and identical entries were excluded from the alignment. We used the combined set of four Homstrad datasets. The alignments were divided into training and test sets: two third (2390) of the alignments were included into the training and one third (1189) into the test set. The SP scores were calculated using the Bali_score program (Thompson et al., 1999). To study the effect of badly aligned or unrelated sequences on the quality scorings, we added 1 to 4 random sequences to each of the 284 test alignments of the Homs10 database and referred these datasets as Homs10+R1, Homs10+R2, Homs10+R3 and Homs10+R4.
The SABmark database consists of two alignment sets (Walle et al., 2005). Superfamily set includes alignments with low to intermediate sequence identity (
50%), while the alignments in the twilight zone set have very low to low identity (25%). We used the combined set of those superfamily and twilight zone alignments which had at least five sequences of more than 50 residues (375). The results for the superfamily and twilight zone alignments are provided separately in the Supplementary Material. The SABmark database does not provide multiple reference alignments, but pairwise alignments of all sequences within a group are available. Therefore, since the SP scores could not be calculated for the SABmark database, we used instead the developer's viewpoint score fd, that describes which part of the structural alignment is correctly represented in the sequence alignment, and modeller's viewpoint score fm that measures the proportion of correctly aligned residues in the sequence alignment (Sauder et al., 2000). The fd and fm values were calculated for the pairwise alignments and the median values were used to estimate the SP score. Script supplied with the SABmark database was used to calculate the fd and fm scores. The pairwise alignments in which no residues were correctly aligned were excluded from the analysis similar to Lassmann and Sonnhammer (2005b).
2.3 Positional conservation score
Let us assume that the amino acid frequencies n=(n1, n2,..., n20) in one position of an alignment follow a multinomial distribution with a parameter vector β=(β1, β2,..., β20) being the true probabilities of the 20 amino acids. We denote the maximum likelihood (ML) estimates of the amino acid probabilities in one alignment position as b=(b1, b2, ..., b20) and in the whole alignment as β0=(β
, β
,..., β
). Here, the latter ones were used as background probabilities. For the calculation of the positional conservation score, we defined the maximum Z-score statistic (Ahola et al., 2006) as
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denotes the i-th row of the amino acid scoring matrix C. The denominator |
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ij is the Kronecker's delta function (jj=1 and ij=0 for all i
j), and n=
nj is the actual number of residues at the alignment position.
The final positional conservation was determined by calculating the significance of the maxZ score using an importance sampling (IS) method (Rubin, 1988) and correcting the p-values by controlling the false discovery rate (FDR). The aim was to test the null hypothesis H0: βj=β
against the alternative HA: βj
β, where j=1,2,... 20 and at least one inequality is proper. This means that the alignment positions where the amino acid frequencies diverge from the background distribution are considered as conserved. We used as the IS distribution a mixture of multinomial distribution for 20 amino acids with parameter values 
=0.4 and 
=0.8. The detailed description of the IS distribution, the choice of the parameter values and the entire IS procedure have been presented in Ahola et al. (2006). The p-values were adjusted for making multiple test simultaneously by applying the step-up procedure by Benjamini and Yekutieli (2001). In the step-up procedure, we used 0.05 as the q-value. Further details of the step-up procedure have been presented in Ahola et al. (2006).
2.4 Whole alignment conservation score
After determining which of the alignment positions were conserved, we defined a measure for conservation of the whole alignment (Ahola et al., 2006). Instead of using the proportion of conserved positions in the alignment, we used the proportion of conserved residues, because this measure takes gaps into account. The proportion of conserved residues, ConsAA, was obtained by dividing the number of residues in the conserved positions j
J* by the number of residues in the whole alignment
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denotes the set of conserved positions, ni the number of residues at position i and m the length of the multiple sequence alignment. The ConsAA obtains values between 0 and 1; the higher the ConsAA value the higher is the conservation of the alignment.
2.5 Predicted SP score
2.5.1 Beta regression model
In this section, we present how the quality of the alignment can be assessed using the predicted SP score. The SP score is measured on the bounded unit interval [0,1], but otherwise the true distribution of the score is unknown. Let us assume that the SP score can have (almost) any shape between 0 and 1. The natural choice in this case is to assume that the SP score follows the beta distribution. The use of the normal distribution would not have been possible, since the values of the SP score are concentrated on the upper part of the [0,1] interval, and hence the SP score would have been very difficult to transform to follow the normal distribution. Moreover, the assumption of homogeneity of variances would be violated. Since the beta distribution is defined on the open unit interval (0,1), the SP score values were transformed to open unit scale according to the formula (McMillian and Creelman, 2005)
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is assumed to follow a beta distribution |
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, 
, 
> 0 and 
(.) is the gamma function. Both of the parameters and of the beta distribution are shape parameters. To work with more convenient parameters for the regression model, that is, location and precision parameters, let us reparameterize the model such that 
and
= +, which yields = µ and = – µ. This implies the mean and variance of 
to be |
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as precision parameter, since decrease in results in increased variance values for fixed µ.
The beta regression model for location of 
for alignment i is
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=0, 1,...,k is a vector of unknown regression parameters and xi0,...,xik are observed predictors. In our model, xi0=grand mean, xi1=ConsAA, xi2 =identity, xi3 =number of sequences and xi4=alignment length. The link function g is chosen as logit link g(µ)=ln(µ/(1 – µ)). Hence, the predicted values for the location of 
can be written as
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is
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=0,...,t are the unknown precision parameters and wi0,...,wit denote the predictors of the precision model. Although the predictors of the precision model could be different from that of the location model, in our model, we define wij=xij, where j=0, ... t, and t=k. Because the precision parameters must always be positive, the log function is a suitable choice for a link function. According to Smithson and Verkuilen (2006), we added a negative sign to the formula (2) to make the interpretation easier; the negative sign turns the interpretation of from precision to dispersion. Hence, the predicted dispersion values for 
can be obtained by |
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2.5.2 Model estimation and prediction
We fitted the same beta regression model to the Mafft, Muscle and Probcons aligned training set of the Homstrad database (Fig. 1, stage 1). The models for the location and dispersion are given by
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We fitted the model using the Nlmixed procedure of the SAS software package (SAS® 9.1 for Windows). The Nlmixed procedure uses quasi-Newton optimization method for estimation of the model. After estimating the parameters of the model, the g(µi) was calculated by replacing the
0, 1,...,4 in the formula (3) with the estimated parameter values (Table 1), and the predictors with the values calculated from the alignment of interest. The predicted SP scores were obtained using the formula (1). The predicted SP scores obtain values between 0 and 1; the higher the predicted value, the better is the quality of the alignment.
2.5.3 Prediction intervals
Strictly speaking, the predicted SP score is a model-based estimate of the average SP score given ConsAA and other predictor values. To take into account the uncertainty of the predicted values, the prediction intervals can be calculated by the formula
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denotes the predicted SP scores, t1– is the 1–'s quantile of the Student's t distribution and 
is the standard error of 
. Parameter 
can be calculated using the delta method (Cox, 1998). We calculated the 95% prediction intervals for the test alignments of the Homstrad and SABmark databases using SAS/Nlmixed procedure.
2.6 Other alignment quality scores
The predicted SP scores were compared with other alignment quality scores, MOS, NiRMSD and NorMD scores. These particular scores were chosen because they represent different approaches in assessing the quality of MSAs and are known to produce good results (Armougom et al., 2006; Lassmann and Sonnhammer, 2005b; Thompson et al., 2001).
The MOS is based on comparing several test alignments build for the same sequence set, and calculating the proportion of alignments where all pairs of residues are similarly aligned (Lassmann and Sonnhammer, 2005b). The MOS has been implemented into the MUMSA program package. Even if we were only interested in the MOS scores of the Mafft (L-INS-i), Muscle (default) and Probcons (default) alignments, for each MUMSA run, several parallel alignments from the same set of sequences were needed. We used seven alignment programs, six of which were the same as those used in the original paper of MUMSA (Lassmann and Sonnhammer, 2005b). We run Clustal, Kalign, Muscle, Mummals, Poa and Probcons with default parameters (Do et al., 2005; Edgar, 2004; Grasso and Lee, 2004; Pei and Grishin, 2006; Thompson et al., 1997). Additionally, we run Muscle with two different parameter settings (-maxiters 1 and -maxiters 2), and Mafft with four different parameter settings [-Localpair, -Localpair -maxiterate 1000 (L-INS-i), -Globalpair, -Globalpair -maxiterate 1000] (Edgar, 2004; Katoh et al., 2002, 2005).
The NiRMSD score is a structure-based alignment quality score, which does not need a reference alignment, but assumes that the structures of at least two of the aligned sequences are known (Armougom et al., 2006). For the NiRMSD runs, template files including the PDB codes of the sequences were created. For some of the alignments, the NiRMSD failed to find the structure of the two sequences from the PDB database, and the NiRMSD score could not be calculated. The NiRMSD is integrated into the T_coffee package (Notredame et al., 2000).
The NorMD score is a conservation-based score. The NorMD measures the mean distance between similarities of residue pairs at each alignment column (Thompson et al., 2001). The column-wise scores were summed over the entire length of the alignment and normalized to obtain the final NorMD score. To calculate the NorMD score, we used the NorMD program with 1 and 0.1 as gap opening and gap extension penalties, respectively.
2.7 Comparison of the quality scores
The predicted SP, MOS, NiRMSD and NorMD scores were compared with the CS and correct SP scores in the independent test set of the Homstrad database and with the median fd and fm scores in the SABmark database. Additional comparisons were made between the tested quality scores. These comparisons were carried out using Spearman correlation coefficient (rs), which measures the strength of the relationship between the two scores. The agreement of the predicted SP and MOS scores with the correct SP score values and between each other were studied with the mean squared error (MSE) addressing the question of how far the actual quality score values are from each other.
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3 RESULTS |
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TOPABSTRACT1 INTRODUCTION2 METHODS3 RESULTS4 DISCUSSION AND CONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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3.1 Evaluation of the predicted SP score
We calculated the predicted SP scores for the Mafft, Muscle and Probcons alignments in the test set of the Homstrad database and in the whole SABmark database. First, the predicted SP scores were calculated for the alignments using the Mafft, Muscle and Probcons parameters, respectively. Second, the predicted SP scores were calculated for the same Mafft, Muscle and Probcons alignments, but using parameter set obtained by different alignment program. The purpose of this analysis was to study whether the parameter sets estimated by one algorithm could be used for assessing the quality of alignments obtained by another alignment algorithm. In the first analysis, the correlation coefficients indicate moderately strong relationships between the correct and predicted SP scores in the Mafft, Muscle and Probcons alignments (r=0.645, 0.651 and 0.663, respectively). The comparisons with the CS score showed similar results, the correlations being 5% lower than that for the correct SP score (Supplementary Table 1). The second analysis suggests that the quality assessment is independent from the chosen parameter set: the correlations between the correct and predicted SP scores differed at most 2.7 % (Supplementary Table 1). The results for the CS score had more variation between the parameter sets, the mean difference in correlation being 5.6 % between the parameter sets.
The agreement between the correct and predicted SP scores was evaluated by calculating the MSE between the two scores. The low MSEs of the Mafft, Muscle and Probcons alignments (MSE=0.008-0.009) showed on average <1% mean squared difference between the two scores, suggesting that the predicted SP scores could be used instead of the correct SP scores. Similar conclusion can be drawn from the comparisons of the individual scorings of the two methods showing that 68%, 63% and 69% of the Mafft, Muscle and Probcons alignments are within the 5% range from the correct SP value and 87%, 84%, 86% within the 10% range from the correct value, respectively. It should be noted that most of the alignments, in which the predicted SP scores diverged significantly from the correct values, were difficult alignment cases involving, for example, several insertions of different length.
In the SABmark dataset, the result shows that the predicted SP score is highly correlated with the correct quality scores in the Mafft, Muscle and Probcons alignments (rfd=0.735, 0.737 and 0.720, and rfm=0.734, 0.734 and 0.694, respectively) (Supplementary Fig. 1). When repeating the analysis using different parameter sets, the correlations differed not more than 1.5% (Supplementary Table 1). The results for the twilight zone alignments showed on average about 20% decrease in the quality scores (Supplementary Table 3). Although there was still a moderately high correlation between the correct and predicted quality scores, this result indicates that distantly related protein families should be more emphasized when performing the quality scoring. Interestingly, all the alignments obtained the highest correlations when the Muscle parameters were used.
3.2 Comparison of the quality scores
To compare the performance of our quality score to those of the MOS, NiRMSD and NorMD scores, the correlation coefficients were calculated between the quality scores and the correct quality scores (Table 2). As the results were similar among the alignment programs, the following results are presented as the average correlation coefficients over the Mafft, Muscle and Probcons alignments. The only exception was the NorMD, for which the results were somewhat dependent on the alignment algorithm (Table 2). When analysing the Homstrad database, the MOS score showed especially high correlation with the correct SP score (rmean = 0.87). The correlation was somewhat decreased (rmean = 0.79) when compared with the CS score (Supplementary Table 2). These correlations were even higher than that of the predicted SP score. The relationships of the correct SP score for the NorMD and NiRMSD scores were substantially lower than that of the MOS and predicted SP scores (Table 2). The predicted SP and MOS scores were strongly related (rmean = 0.80), also the NorMD score showed moderate relationship between the both MOS and the predicted SP scores, although the result of the NorMD was dependent on the alignment programs. The NiRMSD showed negative correlation between all the other scores and was not related with the conservation-based scores NorMD and predicted SP. However, it was moderately correlated with the MOS.
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The MOS score obtained values within the bounded interval [0,1], while the NiRMSD and NorMD scores could obtain any real values. They could not be scaled because the maximum and minimum values are unknown. Hence, the agreement with the correct SP score can only be studied with the MOS and predicted SP scores. The MSE of the MOS from the correct SP (MSEmean = 0.004) and from the predicted SP scores (MSEmean = 0.005) showed only about 0.5% mean squared difference between the MOS and the correct and predicted SP scores, indicating very low divergence between all the three scores.
Although the identity of the SABmark alignments is 50% in the alignments obtained from the superfamily dataset and 25% in the alignments from the twilight zone dataset, all the benchmarked programs performed rather well (Table 2, Supplementary Fig. 1). The MOS, NiRMSD and NorMD were all highly correlated with the correct quality scores fd and fm (rmean=0.83, –0.85, 0.74, respectively for both fd and fm). When only twilight zone alignments were considered, the quality decreased on average 13%, 7%, 26% in the MOS, NiRMSD and NorMD scores, respectively (Supplementary Table 3). This indicates that the structure-based NiRMSD is well adapted for scoring distantly related sequences, while the scorings of the NorMD are strongly affected by the increased evolutionary distance. It should be noted that the MOS had very strong relationship with the predicted SP scores (rmean = 0.91). Also, the pairwise correlations between all the other tested alignment quality measures were relatively high (Table 2).
3.3 Sensitivity to adding random sequences
The ability of the alignment quality measures to distinguish between correct alignments and alignments with badly aligned or unrelated sequences was studied in the Homs10+R1, Homs10+R2, Homs10+R3 and Homs10+R4 databases. The predicted SP score correctly reacts to the subtle appearances of random sequences and could be useful in detecting badly aligned or unrelated sequences from the alignment (Supplementary Table 4). The MOS score, on the contrary, seems to overcorrect the influence of unrelated or badly aligned sequences. For more detailed evaluation of the sensitivity of the predicted SP, MOS, Nirmsd and NorMD quality scores to random sequences, see Supplementary Material.
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4 DISCUSSION AND CONCLUSIONS |
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TOPABSTRACT1 INTRODUCTION2 METHODS3 RESULTS4 DISCUSSION AND CONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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We introduced a statistical model-based score for the alignment quality assessment. Additionally, we presented a comparison of three alignment quality scores, MOS, NiRMSD and NorMD. The results indicate that the predicted SP scores have a moderate or strong relationship with the correct SP, CS, fd and fm scores in the Homstrad and SABmark test sets. This is true both in terms of correlation coefficient, measuring the strength of relationships between the correct and predicted scores, and of the MSE, measuring the agreement between the two scores. Thus, the predicted alignment quality scores can be used to estimate the correct SP scores. Importantly, the quality of our score decreases in the same proportion as the number of unrelated sequences increases. Our preliminary results showed that the predicted SP score may also be useful in identifying badly aligned or unrelated sequences from the alignment.
When comparing the predicted SP scores with the other alignment quality measures, MOS, NiRMSD and NorMD, only the MOS slightly outperformed the predicted SP score. Although the MOS score obtained more accurate results in comparison with the correct alignment quality values, the MOS score is computationally laborious, since it demands approximately 12 alignments per each test alignment. Furthermore, the method is not robust to the number of alignments or the choice of alignment programs and their parameters (Lassmann and Sonnhammer, 2005b). It should be noted, that the consistency based scores cannot distinct between related and unrelated sequences (Vingron, 1996). This can be seen in the Homs10+R1–4 databases with added random sequences, for which the MOS score penalized random sequences even 27% more than which would have been necessary. The high correlation and low MSE between the predicted SP and MOS scores indicate that our simple quality score could be used instead of the MOS to measure the quality of the alignments.
Although the NorMD and NiRMSD scores also had moderately strong relationships with the correct SP score, they cannot be directly used to estimate the SP score. This is because the range of their scoring values are not limited between zero and one, and the regression curves, SP=0+1NorMD (NiRMSD), do not lay in the y=x curve (Supplementary Fig. 1). After making a linear transformation, the NorMD and NiRMSD scores could be used to estimate the SP score, but not using the raw scores. The NorMD and NiRMSD scores do not measure the alignment quality in terms of sensitivity, as the SP score does. The NorMD measures the conservation of the sequence alignment, whereas the NiRMSD compares pairwise structural alignments. The advantage of the NiRMSD score is that the increased evolutionary distance between the sequences had no noticeable effect on the quality scorings (Supplementary Table 3). The drawback of the NiRMSD is that it can only be used when the structure is available for at least two of the aligned proteins. It determines the quality of the alignment based only on the sequences whose structure is known ignoring all the other sequences in the alignment. Moreover, since the NiRMSD can obtain any real values, only relative comparison between the other scoring methods is possible.
The advantage of the predicted SP is that it can be applied to any sequence alignment and it does not require structural information or several alignments from the same set of sequences. The prediction intervals can be calculated for the SP score in order to measure the uncertainty of prediction. We also studied the robustness of the method to the choice of the alignment methods using Mafft (L-INS-i), Muscle and Probcons alignment programs to estimate the beta regression model and then cross-validating the quality scorings of the alignments by applying three different parameter sets. Different parameter values caused only minor differences to the quality scorings indicating that any parameter set can be used. The differences arising from the alignment methods, such as the Muscle provided slightly worse results than the other two methods, were in agreement with the earlier results of benchmarking the MSA programs (Ahola et al., 2006; Nuin et al., 2006).
The use of the Homstrad database enabled us to incorporate information from the whole protein-fold space into the quality scoring method. However, the use of the reference alignments may also be a limitation, because of a lack of variety of sequence alignments in terms of sequence identities and numbers of sequences (Blackshields et al., 2006). The Homstrad database contains alignments with different numbers of sequences, but mostly with high sequence identity. Since the 3D structure of only some of the proteins is known and the rest of the sequences have been aligned with ClustalW, then alignments may not always be structurally correct in all details. The Homstrad also includes some difficult alignment cases which are not suitable for modelling global alignment quality. The performance of the SP score could be further improved by applying complementary reference alignment databases. However, already these results indicate that the quality of any global MSA can be evaluated using the statistical prediction model of the SP score.
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ACKNOWLEDGEMENTS |
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TOPABSTRACT1 INTRODUCTION2 METHODS3 RESULTS4 DISCUSSION AND CONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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The authors thank Pentti Riikonen for support with the Linux cluster, the Academy of Finland, the Graduate School in Computational Biology, Bioinformatics and Biometry (ComBi) and the Medical Research Fund of Tampere University Hospital.
Funding: Academy of Finland (120632 to V.A., 120569 to T.A.).
Conflict of Interest: none declared.
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FOOTNOTES |
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Associate Editor: Burkhard Rost
Received on May 20, 2008; revised on July 25, 2008; accepted on August 1, 2008
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REFERENCES |
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TOPABSTRACT1 INTRODUCTION2 METHODS3 RESULTS4 DISCUSSION AND CONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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