Molecular determinants of HIV-1 intersubtype recombination potential

Sequence differences in the dimerization initiation signal (DIS) affect the rate of recombination between subtype B and subtype C HIV-1. To test the hypothesis that DIS sequences can be used to predict intersubtype recombination potentials, we measured the recombination rate between CRF01_A/E (AE) and B, which contain mismatches in the DIS, and between AE and C, which have an identical DIS. Compared with the intrasubtype recombination rate, the recombination rate between AE and subtype B virus was 9-fold lower, and the rate between AE and subtype C virus was 2-fold lower. Thus, DIS sequences can be used to predict the recombination potential between HIV-1 subtypes. Further analyses revealed that the 2-fold lower recombination rate between AE and C viruses can be restored to the intrasubtype recombination rate by matching a part of the LTR and a portion of the viral genome. Therefore, the lower intersubtype recombination rate between AE and C is not caused by a given region but is a cumulative effect by more than one region.     

核质置换技术治疗线粒体疾病的研究进展

线粒体疾病是线粒体基因组发生基因突变所导致的一类遗传疾病,目前尚无治愈方法。近年核质置换技术的出现为线粒体疾病的治疗提供了新思路。现有的核质置换技术包括生发泡移植、纺锤体移植、极体移植和原核移植技术,其中生发泡移植涉及的体外成熟可影响卵母细胞的发育潜能;纺锤体移植最大的挑战是纺锤体组装对机械刺激敏感,缺乏核膜包绕,操作过程可能对纺锤体造成损伤;极体移植违背了自然选择过程,其远期影响尚不明了;原核移植操作方便、基础研究证据充分,具有短期内向临床转化的潜能,但使用已经受精的合子作为胞质供体面临的伦理争议较大。本文就4种核质置换技术治疗线粒体疾病的应用现状进行综述。     

Replication-independent reduction in the number and diversity of recombinant progeny viruses in chickens vaccinated with an attenuated infectious laryngotracheitis vaccine

Recombination is closely linked with virus replication and is an important mechanism that contributes to genome diversification and evolution in alphaherpesviruses. Infectious laryngotracheitis (ILTV; Gallid alphaherpesvirus 1) is an alphaherpesvirus that causes respiratory disease in poultry. In the past, natural (field) recombination events between different strains of ILTV generated virulent recombinant viruses that have caused severe disease and economic loss in poultry industries. In this study, chickens were vaccinated with attenuated ILTV vaccines to examine the effect of vaccination on viral recombination and diversity following subsequent co-inoculation with two field strains of ILTV. Two of the vaccines (SA2 and A20) prevented ILTV replication in the trachea after challenge, but the level of viral replication after co-infection in birds that received the Serva ILTV vaccine strain did not differ from that of the mock-vaccinated (control) birds. Even though the levels of viral replication were similar in the two groups, the number of recombinant progeny viruses and the level of viral diversity were significantly lower in the Serva-vaccinated birds than in mock-vaccinated birds. In both the mock-vaccinated and Serva-vaccinated groups, a high proportion of recombinant viruses were detected in naïve in-contact chickens that were housed with the co-inoculated birds. Our results indicate that vaccination can limit the number and diversity of recombinant progeny viruses in a manner that is independent of the level of virus replication. It is possible that immune responses induced by vaccination can select for virus genotypes that replicate well under the pressure of the host immune response.     

Identification and characterization of human Rad51 inhibitors by screening of an existing drug library

Homologous Recombination (HR) plays an essential role in cellular proliferation and in maintaining genomic stability by repairing DNA double-stranded breaks that appear during replication. Rad51, a key protein of HR in eukaryotes, can have an elevated expression level in tumor cells, which correlates with their resistance to anticancer therapies. Therefore, targeted inhibition of Rad51 through inhibitor may improve the tumor response to these therapies. In order to identify small molecules that inhibit Rad51 activity, we screened the Prestwick Library (1120 molecules) for their effect on the strand exchange reaction catalyzed by Rad51. We found that Chicago Sky Blue (CSB) is a potent inhibitor of Rad51, showing IC₅₀ values in the low nanomolar range (400 nM). Biochemical analysis demonstrated that the inhibitory mechanism probably occurs by disrupting the Rad51 association with the single-stranded DNA, which prevents the nucleoprotein filament formation, the first step of the protein activity. Structure Activity Relationship analysis with a number of compounds that shared structure homology with CSB was also performed. The sensitivity of Rad51 inhibition to CSB modifications suggests specific interactions between the molecule and Rad51 nucleofilament. CSB and some of its analogs open up new perspectives in the search for agents capable of potentiating chemo- and radio-therapy treatments for cancer. Moreover, these compounds may be excellent tools to analyze Rad51 cellular functions. Our study also highlights how CSB and its analogs, which are frequently used in colorants, stains and markers, could be responsible of unwanted side effects by perturbing the DNA repair process.     

Circulation of genotype-I hepatitis B virus in the primitive tribes of Arunachal Pradesh in early sixties and molecular evolution of genotype-I

Retrospective serologic screening of 1077 serum samples collected from the primitive tribe from north-eastern India in 1963 revealed high prevalence of HBV (15% HBsAg carrier rate) and HCV (7% anti-HCV positivity) and co-circulation of multiple HBV genotypes-A, C, D and G. Full genome sequencing classified all the G-genotype samples as genotype-I. Comparison of genotype-I-HBV full-genome sequences representing 1963 (n = 5, this study) and 2005 (reported earlier) showed identical recombination break-points of genotypes-A/G/C. Genotype-C and genotype-C-fragment of I-genotype circulating in 1963 were distinctly different. The data demonstrates that the recombination events were not recent. Molecular clock analysis predicted existence of genotype-I in this tribe during 1920s.     

Evidence of substantial recombination among Trypanosoma cruzi II strains from Minas Gerais

Due to the scarcity of evidence of sexuality in Trypanosoma cruzi, the causative agent of Chagas disease, it has been general accepted that the parasite reproduction is essentially clonal with infrequent genetic recombination. This assumption is mainly supported by indirect evidence, such as Hardy–Weinberg imbalances, linkage disequilibrium and a strong correlation between independent sets of genetic markers of T. cruzi populations. However, because the analyzed populations are usually isolated from different geographic regions, the possibility of population substructuring as generating these genetic marker imbalances cannot be eliminated. To investigate this possibility, we firstly compared the allele frequencies and haplotype networks using seven different polymorphic loci (two from mitochondrial and five from different nuclear chromosomes) in two groups of TcII strains: one including isolates obtained from different regions in Latin America and the other including isolates obtained only from patients of the Minas Gerais State in Brazil. Our hypothesis was that if the population structure is essentially clonal, Hardy–Weinberg disequilibrium and a sharp association between the clusters generated by analyzing independent markers should be observed in both strain groups, independent of the geographic origin of the samples. The results demonstrated that the number of microsatellite loci in linkage disequilibrium decreased from 4 to 1 when only strains from Minas Gerais were analyzed. Moreover, we did not observed any correlation between the clusters when analyzing the nuclear and mitochondrial loci, suggesting independent inheritance of these markers among the Minas Gerais strains. Besides, using a second subset of five physically linked microsatellite loci and the Minas Gerais strains, we could also demonstrate evidence of homologous recombination roughly proportional to the relative distance among them. Taken together, our results do not support a clonal population structure for T. cruzi, particularly in TcII, which coexists in the same geographical area, suggesting that genetic exchanges among these strains may occur more frequently than initially expected.     

Prevalence analysis of different human bocavirus genotypes in pediatric patients revealed intra-genotype recombination

BackgroundHuman bocavirus (HBoV) genotypes 1–4 have been detected worldwide in respiratory samples and stool samples, and are increasingly associated with respiratory and intestinal infections of previously unknown etiology in young children. Several studies revealed evidence of extensive recombination among HBoV genotypes at the NP1 and VP1 gene boundary region. This study explored the prevalence of HBoV genotypes in pediatric patients in Beijing, and studied their phylogeny.MethodsA total of 4941 respiratory specimens and 1121 fecal specimens were collected from pediatric patients with respiratory infections from January 2006 to December 2013, or with acute diarrhea from October 2010 to December 2012. Conventional PCR was used to detect HBoV1–4 within these samples. Gene fragments at the NP1 and VP1 gene boundary were amplified from HBoV-positive specimens, sequenced, and their phylogenetic inferences constructed using MEGA 6.0 software. Recombination events were identified with SimPlot software.ResultsHuman bocavirus 1, 2, and 3 were detected in 9 (0.80%), 15 (1.33%), and 1 (0.08%) of 1121 stool samples, respectively. However, only HBoV1 (82, 1.65%) was detected in respiratory specimens. Phylogenetic analysis of gene fragments at the HBoV NP1 and VP1 gene boundary indicated that HBoV1 sequences obtained from fecal or respiratory specimens across 8 years were highly conserved (99–100%), while 15 HBoV2 sequences collected across 2 years in Beijing were more diverse with up to 4.40% variation. Of the 15 HBoV2 sequences, 14 clustered into a new lineage divergent from other HBoV2 sequences in GenBank. Five HBoV2 genomic sequences were analyzed for recombination, revealing intra-genotype recombination between HBoV2A and HBoV2B.ConclusionsMore HBoV1 were detected in children with respiratory tract diseases, and HBoV2 in patients with acute diarrhea. Phylogenetic analysis revealed a new cluster of HBoV2 was prevalent in China, which may be the result of intra-genotype recombination between HBoV2A and HBoV2B.     

Few colonies of the host Bombus terrestris disproportionately affect the genetic diversity of its parasite,Crithidia bombi

Sex and recombination have long been considered as necessary means for hosts to keep up with and resist to their faster reproducing parasites. On the other hand, comparatively little attention has been paid to potential benefits of recombination for the parasites. Using as model organisms the bumblebee Bombus terrestris and its genetically highly variable trypanosomatid parasite Crithidia bombi we analysed the infection dynamics as well as the relative frequency of parasite recombinants over time, in colonies that were either immune-challenged with heat-killed bacteria or sham-inoculated. In addition, we used infective cells from a given colony to infect workers from other, untreated colonies, to investigate whether recombinant parasite strains may have a competitive advantage over the parental strains to infect the surrounding host population. We show that in our experimental setup the host immune status does not influence the proportion of recombinant parasite cells in the infection. Neither do recombinant parasite strains have an advantage over the parental ones at infecting workers unrelated to the host colony the infection originally came from. However, we found that the prevalence of recombinants was highly variable among colonies, with one particular colony producing significantly more recombinant strains than others. As the successful infection of daughter queens – the only individuals surviving the winter to the next year – is proportional to the number of circulating parasite strains in the colony, we suggest that such “super-producing” colonies may be responsible for most of the infections happening in the next year.