Biomedical subjects

Aleksey Y Ogurtsov

Publications and source records attributed to Aleksey Y Ogurtsov.

12 recordsLinked to original sources

PubMed2006

Distant conserved sequences flanking endothelial-specific promoters contain tissue-specific DNase-hypersensitive sites and over-represented motifs.

The transcriptional regulation of genes is a complex process, particularly for genes exhibiting a tissue-specific pattern of expression. We studied 28 genes that are expressed primarily in endothelial cells, another 28 genes that are expressed highly, but not exclusively, in cultured endothelial cells, and three control sets, consisting of genes not expressed in endothelium, genes expressed in neural tissues and housekeeping genes. For each gene, we identified conserved non-coding sequences (CNSs) of lengths 50 to >1000 nucleotides, located within the upstream intergenic region (from 500 to as far as 200 000 nucleotides upstream from the transcription start) or within the first intron. As a functional test, we assayed the CNSs from the set of endothelial cell-specific genes (EC-CNSs) for DNase hypersensitivity. Among 262 distant EC-CNSs, 33% are hypersensitive (HS) in endothelial cells, whereas only 16% are HS in control fibroblasts. A search for short sequence patterns revealed a number of motifs which are over-represented in EC-CNSs relative to CNSs from the control gene sets. In particular, the motif SAGGAAR is strongly and consistently over-represented among EC-CNSs, and is more over-represented in HS CNSs than in non-HS CNSs. CNSs which contain this motif are no closer to the promoter than an average CNS. This motif contains the core element of binding sites from the Ets family of transcription factors. Thus, one or several factors from this family may play a key role in the regulation of endothelial gene expression.

5' Flanking Region↗

PubMed2006

A periodic pattern of mRNA secondary structure created by the genetic code.

Single-stranded mRNA molecules form secondary structures through complementary self-interactions. Several hypotheses have been proposed on the relationship between the nucleotide sequence, encoded amino acid sequence and mRNA secondary structure. We performed the first transcriptome-wide in silico analysis of the human and mouse mRNA foldings and found a pronounced periodic pattern of nucleotide involvement in mRNA secondary structure. We show that this pattern is created by the structure of the genetic code, and the dinucleotide relative abundances are important for the maintenance of mRNA secondary structure. Although synonymous codon usage contributes to this pattern, it is intrinsic to the structure of the genetic code and manifests itself even in the absence of synonymous codon usage bias at the 4-fold degenerate sites. While all codon sites are important for the maintenance of mRNA secondary structure, degeneracy of the code allows regulation of stability and periodicity of mRNA secondary structure. We demonstrate that the third degenerate codon sites contribute most strongly to mRNA stability. These results convincingly support the hypothesis that redundancies in the genetic code allow transcripts to satisfy requirements for both protein structure and RNA structure. Our data show that selection may be operating on synonymous codons to maintain a more stable and ordered mRNA secondary structure, which is likely to be important for transcript stability and translation. We also demonstrate that functional domains of the mRNA [5'-untranslated region (5'-UTR), CDS and 3'-UTR] preferentially fold onto themselves, while the start codon and stop codon regions are characterized by relaxed secondary structures, which may facilitate initiation and termination of translation.

3' Untranslated Regions↗

PubMed2006

Analysis of internal loops within the RNA secondary structure in almost quadratic time.

MOTIVATION: Evaluating all possible internal loops is one of the key steps in predicting the optimal secondary structure of an RNA molecule. The best algorithm available runs in time O(L(3)), L is the length of the RNA. RESULTS: We propose a new algorithm for evaluating internal loops, its run-time is O(M(*)log(2)L), M < L(2) is a number of possible nucleotide pairings. We created a software tool Afold which predicts the optimal secondary structure of RNA molecules of lengths up to 28 000 nt, using a computer with 2 Gb RAM. We also propose algorithms constructing sets of conditionally optimal multi-branch loop free (MLF) structures, e.g. the set that for every possible pairing (x, y) contains an optimal MLF structure in which nucleotides x and y form a pair. All the algorithms have run-time O(M(*)log(2)L).

Algorithms↗

PubMed2006

Computational models with thermodynamic and composition features improve siRNA design.

BACKGROUND: Small interfering RNAs (siRNAs) have become an important tool in cell and molecular biology. Reliable design of siRNA molecules is essential for the needs of large functional genomics projects. RESULTS: To improve the design of efficient siRNA molecules, we performed a comparative, thermodynamic and correlation analysis on a heterogeneous set of 653 siRNAs collected from the literature. We used this training set to select siRNA features and optimize computational models. We identified 18 parameters that correlate significantly with silencing efficiency. Some of these parameters characterize only the siRNA sequence, while others involve the whole mRNA. Most importantly, we derived an siRNA position-dependent consensus, and optimized the free-energy difference of the 5' and 3' terminal dinucleotides of the siRNA antisense strand. The position-dependent consensus is based on correlation and t-test analyses of the training set, and accounts for both significantly preferred and avoided nucleotides in all sequence positions. On the training set, the two parameters' correlation with silencing efficiency was 0.5 and 0.36, respectively. Among other features, a dinucleotide content index and the frequency of potential targets for siRNA in the mRNA added predictive power to our model (R = 0.55). We showed that our model is effective for predicting the efficiency of siRNAs at different concentrations. We optimized a neural network model on our training set using three parameters characterizing the siRNA sequence, and predicted efficiencies for the test siRNA dataset recently published by Novartis. On this validation set, the correlation coefficient between predicted and observed efficiency was 0.75. Using the same model, we performed a transcriptome-wide analysis of optimal siRNA targets for 22,600 human mRNAs. CONCLUSION: We demonstrated that the properties of the siRNAs themselves are essential for efficient RNA interference. The 5' ends of antisense strands of efficient siRNAs are U-rich and possess a content similarity to the pyrimidine-rich oligonucleotides interacting with the polypurine RNA tracks that are recognized by RNase H. The advantage of our method over similar methods is the small number of parameters. As a result, our method requires a much smaller training set to produce consistent results. Other mRNA features, though expensive to compute, can slightly improve our model.

Artificial Intelligence↗

PubMed2005

Selection in favor of nucleotides G and C diversifies evolution rates and levels of polymorphism at mammalian synonymous sites.

The impact of synonymous nucleotide substitutions on fitness in mammals remains controversial. Despite some indications of selective constraint, synonymous sites are often assumed to be neutral, and the rate of their evolution is used as a proxy for mutation rate. We subdivide all sites into four classes in terms of the mutable CpG context, nonCpG, postC, preG, and postCpreG, and compare four-fold synonymous sites and intron sites residing outside transposable elements. The distribution of the rate of evolution across all synonymous sites is trimodal. Rate of evolution at nonCpG synonymous sites, not preceded by C and not followed by G, is approximately 10% below that at such intron sites. In contrast, rate of evolution at postCpreG synonymous sites is approximately 30% above that at such intron sites. Finally, synonymous and intron postC and preG sites evolve at similar rates. The relationship between the levels of polymorphism at the corresponding synonymous and intron sites is very similar to that between their rates of evolution. Within every class, synonymous sites are occupied by G or C much more often than intron sites, whose nucleotide composition is consistent with neutral mutation-drift equilibrium. These patterns suggest that synonymous sites are under weak selection in favor of G and C, with the average coefficient s approximately 0.25/Ne approximately 10(-5), where Ne is the effective population size. Such selection decelerates evolution and reduces variability at sites with symmetric mutation, but has the opposite effects at sites where the favored nucleotides are more mutable. The amino-acid composition of proteins dictates that many synonymous sites are CpGprone, which causes them, on average, to evolve faster and to be more polymorphic than intron sites. An average genotype carries approximately 10(7) suboptimal nucleotides at synonymous sites, implying synergistic epistasis in selection against them.

Animals↗

PubMed2004

Positive selection at sites of multiple amino acid replacements since rat-mouse divergence.

New alleles become fixed owing to random drift of nearly neutral mutations or to positive selection of substantially advantageous mutations. After decades of debate, the fraction of fixations driven by selection remains uncertain. Within 9,390 genes, we analysed 28,196 codons at which rat and mouse differ from each other at two nucleotide sites and 1,982 codons with three differences. At codons where rat-mouse divergence involved two non-synonymous substitutions, both of them occurred in the same lineage, either rat or mouse, in 64% of cases; however, independent substitutions would occur in the same lineage with a probability of only 50%. All three non-synonymous substitutions occurred in the same lineage for 46% of codons, instead of the 25% expected. Furthermore, comparison of 12 pairs of prokaryotic genomes also shows clumping of multiple non-synonymous substitutions in the same lineage. This pattern cannot be explained by correlated mutation or episodes of relaxed negative selection, but instead indicates that positive selection acts at many sites of rapid, successive amino acid replacement.

Alleles↗

PubMed2004

Comparative analysis of orthologous eukaryotic mRNAs: potential hidden functional signals.

Sequencing of multiple, nearly complete eukaryotic genomes creates opportunities for detecting previously unnoticed, subtle functional signals in non-coding regions. A genome-wide comparative analysis of orthologous sets of mammalian and yeast mRNAs revealed distinct patterns of evolutionary conservation at the boundaries of the untranslated regions (UTRs) and the coding region (CDS). Elevated sequence conservation was detected in approximately 30 nt regions around the start codon. There seems to be a complementary relationship between sequence conservation in the approximately 30 nt regions of the 5'-UTR immediately upstream of the start codon and that in the synonymous positions of the 5'-terminal 30 nt of the CDS: in mammalian mRNAs, the 5'-UTR shows a greater conservation than the CDS, whereas the opposite trend holds for yeast mRNAs. Unexpectedly, a approximately 30 nt region downstream of the stop codon shows a substantially lower level of sequence conservation than the downstream portions of the 3'-UTRs. However, the sequence in this poorly conserved 30 nt portion of the 3'-UTR is non-random in that it has a higher GC content than the rest of the UTR. It is hypothesized that the elevated sequence conservation in the region immediately upstream of the start codon is related to the requirement for initiation factor binding during pre-initiation ribosomal scanning. In contrast, the poorly conserved region downstream of the stop codon could be involved in the post- termination scanning and dissociation of the ribosomes from the mRNA, which requires only the mRNA-ribosome interaction. Additionally, it was found that the choice of the stop codon in mammals, but not in yeasts, and the context in the immediate vicinity of the stop codons in both mammals and yeasts are subject to strong selection. Thus, genome-wide analysis of orthologous gene sets allows detection of previously unrecognized patterns of sequence conservation, which are likely to reflect hidden functional signals, such as ribosomal filters that could regulate translation by modulating the interaction between the mRNA and ribosomes.

3' Untranslated Regions↗

PubMed2004

Bioinformatical assay of human gene morbidity.

Only a fraction of eukaryotic genes affect the phenotype drastically. We compared 18 parameters in 1273 human morbid genes, known to cause diseases, and in the remaining 16 580 unambiguous human genes. Morbid genes evolve more slowly, have wider phylogenetic distributions, are more similar to essential genes of Drosophila melanogaster, code for longer proteins containing more alanine and glycine and less histidine, lysine and methionine, possess larger numbers of longer introns with more accurate splicing signals and have higher and broader expressions. These differences make it possible to classify as non-morbid 34% of human genes with unknown morbidity, when only 5% of known morbid genes are incorrectly classified as non-morbid. This classification can help to identify disease-causing genes among multiple candidates.

Computational Biology↗

PubMed2004

Indel-based evolutionary distance and mouse-human divergence.

We propose a method for estimating the evolutionary distance between DNA sequences in terms of insertions and deletions (indels), defined as the per site number of indels accumulated in the course of divergence of the two sequences. We derive a maximal likelihood estimate of this distance from differences between lengths of orthologous introns or other segments of sequences delimited by conservative markers. When indels accumulate, lengths of orthologous introns diverge only slightly slower than linearly, because long indels occur with substantial frequencies. Thus, saturation is not a major obstacle for estimating indel-based evolutionary distance. For introns of medium lengths, our method recovers the known evolutionary distance between rat and mouse, 0.014 indels per site, with good precision. We estimate that mouse-human divergence exceeds rat-mouse divergence by a factor of 4, so that mouse-human evolutionary distance in terms of selectively neutral indels is 0.056. Because in mammals, indels are approximately 14 times less frequent than nucleotide substitutions, mouse-human evolutionary distance in terms of selectively neutral substitutions is approximately 0.8.

Animals↗

PubMed2003

Patterns in interspecies similarity correlate with nucleotide composition in mammalian 3'UTRs.

Post-transcriptional regulation and the formation of mRNA 3' ends are crucial for gene expression in eukaryotes. Interspecies conservation of many sequences within 3'UTRs reveals selective constraint due to similar function. To study the pattern of conservation within 3'UTRs, we compiled and aligned 50 sets of complete orthologous 3'UTRs from four orders of mammals. We observed a mosaic pattern of conservation, with alternating regions of high (phylogenetic footprints) and low similarity. Conservation in 3'UTRs correlates with their base composition and also with the synonymous substitution rate in corresponding coding regions. The non-uniform distribution of conservation is more pronounced for 3'UTRs with a moderate or low level of overall conservation, where invariant nucleotides are more numerous, and their runs of lengths 4-7 occur more frequently than if conservation were random. Many runs of invariant nucleotides are AU-rich or pyrimidine-rich. Some of these runs coincide with known functional cis- elements of eukaryotic mRNAs, such as the U-rich upstream element, polyadenylation signal and DICE regulatory signal. More divergent regions of multiple alignments of 3'UTRs are often more G- and/or C-rich. Our results provide evidence on the importance of moderately conserved regions in 3'UTRs and suggest that regulatory functions of 3'UTRs might utilize gene-specific information in these regions.

3' Untranslated Regions↗

PubMed2002

A hierarchical approach to aligning collinear regions of genomes.

MOTIVATION: As a first approximation, similarity between two long orthologous regions of genomes can be represented by a chain of local similarities. Within such a chain, pairs of successive similarities are collinear (non-conflicting), i.e. segments involved in the nth similarity precede in both sequences segments involved in the (n+1)th similarity. However, when all similarities between two long sequences are considered, usually there are many conflicts between them. Although some conflicts can be avoided by masking transposons or low-complexity sequences, selecting only those similarities that reflect orthology and, thus, belong to the evolutionarily true chain is not trivial. RESULTS: We propose a simple, hierarchical algorithm of finding the true chain of local similarities. Starting from similarities with low P-values, we resolve each pairwise conflict by deleting a similarity with a higher P-value. This greedy approach constructs a chain of similarities faster than when a chain optimal with respect to some global criterion is sought, and makes more sense biologically.

Algorithms↗

PubMed2002

OWEN: aligning long collinear regions of genomes.

OWEN is an interactive tool for aligning two long DNA sequences that represents similarity between them by a chain of collinear local similarities. OWEN employs several methods for constructing and editing local similarities and for resolving conflicts between them. Alignments of sequences of lengths over 10(6) can often be produced in minutes. OWEN requires memory below 20 L, where L is the sum of lengths of the compared sequences.

Algorithms↗