Senescence and entrenchment in evolution of amino acid sites

Amino acid propensities at a site change in the course of protein evolution. This may happen for two reasons. Changes may be triggered by substitutions at epistatically interacting sites elsewhere in the genome. Alternatively, they may arise due to environmental changes that are external to the genome. Here, we design a framework for distinguishing between these alternatives. Using analytical modelling and simulations, we show that they cause opposite dynamics of the fitness of the allele currently occupying the site: it tends to increase with the time since its origin due to epistasis ("entrenchment"), but to decrease due to random environmental fluctuations ("senescence"). By analysing the genomes of vertebrates and insects, we show that the amino acids originating at negatively selected sites experience strong entrenchment. By contrast, the amino acids originating at positively selected sites experience senescence. We propose that senescence of the current allele is a cause of adaptive evolution.

[Resistance to protease inhibitors and efficiency of antiviral therapy in patients with chronic hepatitis C]

AIM

To clarify the role of virus resistance in the efficiency of antiviral therapy with protease inhibitors (PIs) chronic hepatitis C (CHC) patients, with moderate sensitivity to interferon-α.

MATERIALS AND METHODS

Eight Caucasian patients (4 men and 4 women) aged 21 to 65 years (median 52.5 years) with genotype 1b hepatitis C virus (HCV) infection were included in the study. Two patients were diagnosed with liver cirrhosis; 4 had been ineffectively treated with peginterferon in combination with ribavirin. None of the patients had obesity and/or insulin resistance. All the 8 patients received triple therapy with PIs (boceprevir (n=3), telaprevir (n=4), and simeprevir (n=1)) and as a result failed to achieve a sustained virologic response. All the participants were studied to identify mutations in HCV NS3/4A region.

RESULTS

Five of the 8 patients were found to have mutations in HCV NS3/4A region (substantially reducing drug susceptibility in 3 cases).

CONCLUSION

In CHC patients who are moderately sensitive to interferon-α and receive therapy with PIs, resistance to the latter is critically important for the efficiency of therapy and the timely identification of resistance mutations can contribute to the choice of an optimal treatment policy.

[EDAS, databases of alternatively spliced human genes]

EDAS, a database of alternatively spliced human genes, contains data on the alignment of proteins, mRNAs, and EST. It contains information on all exons and introns observed, as well as elementary alternatives formed from them. The database makes it possible to filter the output data by changing the cut-off threshold by the significance level. The database is accessible at http://www.gene-bee.msu.ru/edas/.

Alternative splicing and protein function

Background

Alternative splicing is a major mechanism of generating protein diversity in higher eukaryotes. Although at least half, and probably more, of mammalian genes are alternatively spliced, it was not clear, whether the frequency of alternative splicing is the same in different functional categories. The problem is obscured by uneven coverage of genes by ESTs and a large number of artifacts in the EST data.

Results

We have developed a method that generates possible mRNA isoforms for human genes contained in the EDAS database, taking into account the effects of nonsense-mediated decay and translation initiation rules, and a procedure for offsetting the effects of uneven EST coverage. Then we computed the number of mRNA isoforms for genes from different functional categories. Genes encoding ribosomal proteins and genes in the category "Small GTPase-mediated signal transduction" tend to have fewer isoforms than the average, whereas the genes in the category "DNA replication and chromosome cycle" have more isoforms than the average. Genes encoding proteins involved in protein-protein interactions tend to be alternatively spliced more often than genes encoding non-interacting proteins, although there is no significant difference in the number of isoforms of alternatively spliced genes.

Conclusion

Filtering for functional isoforms satisfying biological constraints and accountung for uneven EST coverage allowed us to describe differences in alternative splicing of genes from different functional categories. The observations seem to be consistent with expectations based on current biological knowledge: less isoforms for ribosomal and signal transduction proteins, and more alternative splicing of interacting and cell cycle proteins.