HIV-1 nucleocapsid traps reverse transcriptase on nucleic acid substrates

Conversion of the genomic RNA of human immunodeficiency virus (HIV) into full-length viral DNA is a complex multistep reaction catalyzed by the reverse transcriptase (RT). Numerous studies have shown that the viral nucleocapsid (NC) protein has a vital impact on various steps during reverse transcription, which is crucial for virus infection. However, the exact molecular details are poorly defined. Here, we analyzed the effect of NC on RT-catalyzed single-turnover, single-nucleotide incorporation using different nucleic acid substrates. In the presence of NC, we observed an increase in the amplitude of primer extension of up to 3-fold, whereas the transient rate of nucleotide incorporation ( k pol) dropped by up to 50-fold. To unravel the underlying molecular mechanism, we carefully analyzed the effect of NC on RT-nucleic acid substrate dissociation. The studies revealed that NC considerably enhances the stability of RT-substrate complexes by reducing the observed dissociation rate constants, which more than compensates for the observed drop in k pol. In conclusion, our data strongly support the concept that NC not only indirectly assists the reverse transcription process by its nucleic acid chaperoning activity but also positively affects the RT-catalyzed nucleotide incorporation reaction by increasing polymerase processivity presumably via a physical interaction of the two viral proteins.

Proton-induced fixed positive charge at the Si(100)-SiO2 interface

Positively charged defects induced by protons at the Si(100)-SiO2 interface are studied through density-functional calculations and realistic interface models. Protons generally preserve the bonding network, but cause the spontaneous breaking of strained bonds leading to threefold-coordinated Si(3)(+) and O(3)(+). Defect energies fall within a band of approximately 0.5 eV, which is stabilized by approximately 0.3 eV at the interface. Only the O(3)(+) at approximately 1 eV lower energies stand out as deep defects. This description is consistent with several experimental observations and supports the O(3)(+) as the origin of the fixed positive charge generated during silicon oxidation, in accord with a previous suggestion inferred from electrical data.

Proton diffusion mechanism in amorphous SiO2

We study proton diffusion in amorphous SiO2 from the atomic scale to the long-range percolative regime. Ab initio molecular dynamics suggest that the dominant atomic process consists in cross-ring interoxygen hopping assisted by network vibrations. A statistical analysis accounting for the disorder in amorphous SiO2 yields relations between transition energies and interoxygen distances for both cross-ring and nearest-neighbor hopping. The percolative regime is then addressed through large-size model systems reproducing these relations. Cross-ring hopping is confirmed as the dominant diffusion mechanism and supported by a good agreement with experiment for the activation energy.

During the early phase of HIV-1 DNA synthesis, nucleocapsid protein directs hybridization of the TAR complementary sequences via the ends of their double-stranded stem

Reverse transcription of HIV-1 genomic RNA requires two obligatory strand transfers. During the first strand transfer reaction, the minus strand strong-stop DNA (ss-cDNA) is transferred by hybridization of complementary sequences located at the 3' ends of the ss-cDNA and genomic template, respectively. In HIV-1, the major components of ss-cDNA transfer are the terminally redundant structured TAR elements and the nucleocapsid protein NCp7, which actively chaperones the hybridization of cTAR DNA to TAR. In the present study, we investigated the annealing kinetics of TAR with fluorescently labelled cTAR derivatives both in the absence and in the presence of NC(12-55), a peptide that contains the finger and C-terminal domains of NCp7. The annealing of TAR with cTAR involves two second-order kinetic components that are activated by at least two orders of magnitude by NC(12-55). The NC-promoted activation of cTAR-TAR annealing was correlated with its ability to destabilize the lower half of TAR stem, in order to generate the single-stranded complementary regions for nucleating the duplex structures. The two kinetics components have been assigned to two different pathways. The rapid one does not lead to extended duplex formation but is associated with a limited annealing of the terminal bases of cTAR to TAR. On the other hand, extended duplex formation follows a slower pathway that is limited kinetically by the nucleation of residues located mainly within the central double-stranded segment of both cTAR and TAR stems. An alternative mechanism involving an interaction through TAR and cTAR loops has been observed but is a minor pathway in the present conditions.

A locus for autosomal dominant colobomatous microphthalmia maps to chromosome 15q12-q15

Congenital microphthalmia is a common developmental ocular disorder characterized by shortened axial length. Isolated microphthalmia is clinically and genetically heterogeneous and may be inherited in an autosomal dominant, autosomal recessive, or X-linked manner. Here, we studied a five-generation family of Sephardic Jewish origin that included 38 members, of whom 7 have either unilateral or bilateral microphthalmia of variable severity inherited as an autosomal dominant trait with incomplete penetrance. After exclusion of several candidate loci, we performed a genome-scan study and demonstrated linkage to chromosome 15q12-q15. Positive LOD scores were obtained with a maximum at the D15S1007 locus (maximum LOD score 3.77, at recombination fraction 0.00). Haplotype analyses supported the location of the disease-causing gene in a 13.8-cM interval between loci D15S1002 and D15S1040.

Non-erythroid genes inserted on either side of human HS-40 impair the activation of its natural alpha -globin gene targets without being themselves preferentially activated

The human alpha-globin gene complex includes three functional globin genes (5'-zeta2-alpha2-alpha1-3') regulated by a common positive regulatory element named HS-40 displaying strong erythroid-specific enhancer activity. How this enhancer activity can be shared between different promoters present at different positions in the same complex is poorly understood. To address this question, we used homologous recombination to target the insertion of marker genes driven by cytomegalovirus or long terminal repeat promoters in both possible orientations either upstream or downstream from the HS-40 region into the single human alpha-globin gene locus present in hybrid mouse erythroleukemia cells. We also used CRE recombinase-mediated cassette exchange to target the insertion of a tagged alpha-globin gene at the same position downstream from HS-40. All these insertions led to a similar decrease in the HS-40-dependent transcription of downstream human alpha-globin genes in differentiated cells. Interestingly, this decrease is associated with the strong activation of the proximal newly inserted alpha-globin gene, whereas in marked contrast, the transcription of the non-erythroid marker genes remains insensitive to HS-40. Taken together, these results indicate that the enhancer activity of HS-40 can be trapped by non-erythroid promoters in both upstream and downstream directions without necessarily leading to their own activation.

A novel C202F mutation in the connexin26 gene (GJB2) associated with autosomal dominant isolated hearing loss

Mutations in the GJB2 gene encoding connexin26 (CX26) account for up to 50% of cases of autosomal recessive hearing loss. In contrast, only one GJB2 mutation has been reported to date in an autosomal dominant form of isolated prelingual hearing loss. We report here a novel heterozygous 605G-->T mutation in GJB2 in all affected members of a large family with late childhood onset of autosomal dominant isolated hearing loss. The resulting C202F substitution, which lies in the fourth (M4) transmembrane domain of CX26, may impair connexin oligomerisation. Finally, our study suggests that GJB2 should be screened for heterozygous mutations in patients with autosomal dominant isolated hearing impairment, whatever the severity of the disease.

Wagner vitreoretinal degeneration with genetic linkage refinement on chromosome 5q13-q14

BACKGROUND

It has been previously described that Wagner disease is linked to chromosome 5q13-q14. This study was carried out to describe the ophthalmological aspects and report the results of genetic linkage analysis in a large pedigree affected by Wagner disease.

METHODS

Fourty members of one same family agreed to be examined.

RESULTS

Twenty patients presented vitreoretinal degeneration in both eyes without any extra-ocular abnormalities. In young patients, visual acuity was usually normal after correction of frequent mild myopia. Presenile cataracts progressed by the third decade and required removal for visual rehabilitation. The primary disorder involved an abnormal vitreous. A few avascular vitreous bands were usually the only optical feature in the mostly empty vitreous cavity. A circumferential vitreous condensation formed in contact with the retina on many spots. Less common retinal findings included retinal detachment, abnormal retinal pigmentation, progressive atrophy of the RPE simulating choroideremia and lattice degeneration. Genetic analysis revealed a highly significant linkage (lod score >5.0) between the disease and 10 markers of the chromosome 5q13-q14 region. Two recombination events allowed us to refine the linked interval to 20 cM between the D5S650 and D5S618 markers.

CONCLUSION

Ophthalmological aspects of Wagner's disease appear to progress with age. Regular ophthalmological examination is important for detecting retinal abnormalities. The gene involved in Wagner's disease lies in a 20 cM interval on chromosome 5q13-q14.

Unexpected and coordinated expression of Spi-1, Fli-1, and megakaryocytic genes in four Epo-dependent cell lines established from transgenic mice displaying erythroid-specific expression of a thermosensitive SV40 T antigen

Most erythroleukemic cell lines established in vitro coexpress erythrocytic and megakaryocytic markers that often are associated with expression of Spi-1 and/or Fli-1 transcription factors known as transactivators of megakaryocyte-specific promoters. In the present study, we examined the possibility of establishing new cell lines keeping strictly erythroid-specific properties in vitro through the targeted and conditional immortalization of erythrocytic progenitors. For that purpose, we established several lines of transgenic mice displaying erythroid-specific expression of a thermosensitive SV40 T antigen. As expected, these transgenic mice developed splenomegaly due to the massive amplification of Ter 119 positive erythroid nucleated cells expressing T antigen. Despite this drastic effect in vivo, the in vitro immortalization of erythropoietin-dependent erythroid progenitors unexpectedly occurred at low frequency, and all four cell lines established expressed both erythrocytic (globins) and megakaryocytic markers (glycoprotein IIb, platelet factor 4) as well as Spi-1 and Fli-1 transcripts at permissive temperature. Switching the cells to the nonpermissive temperature led to a marked increase in globin gene expression and concomitant decrease in expression of Spi-1, Fli-1, and megakaryocytic genes in an erythropoietin-dependent manner. Interestingly, enhanced expression of Spi-1 and Fli-1 genes already was detected in the Ter 119 positive cell population of transgenic mice spleen in vivo. However, like normal Ter 119 erythroid cells, these Ter 119 positive cells from transgenic mice still expressed high levels of beta-globin and very low or undetectable glycoprotein IIb and platelet factor 4 megakaryocytic transcripts. Taken together, these data indicate that the unexpected expression of megakaryocytic genes is a specific property of immortalized cells that cannot be explained only by enhanced expression of Spi-1 and/or Fli-1 genes.

Mutation in the zonadhesin-like domain of alpha-tectorin associated with autosomal dominant non-syndromic hearing loss

A gene responsible for autosomal dominant non-syndromic hearing impairment in two families (DFNA8 and DFNA12) has recently been identified as TECTA encoding alpha-tectorin, a major component of the tectorial membrane. In these families, missense mutations within the zona pellucida domain of alpha-tectorin were associated with stable severe mid-frequency hearing loss. The present study reports linkage to DFNA12 in a new family with autosomal dominant high frequency hearing loss progressing from mild to moderate severity. The candidate region refined to 3.8 cM still contained the TECTA gene. A missense mutation (C1619S) was identified in the zonadhesin-like domain. This mutation abolishes the first of the vicinal cysteines (1619Cys-Gly-Leu- 1622Cys) present in the D4 von Willebrand factor (vWf) type D repeat. These results further support the involvement of TECTA mutations in autosomal dominant hearing impairment, and suggest that vicinal cysteines are involved in tectorial membrane matrix assembly.

Spi-1/PU.1 is a positive regulator of the Fli-1 gene involved in inhibition of erythroid differentiation in friend erythroleukemic cell lines

Spi-1/PU.1 and Fli-1 are two members of the ETS family of transcription factors whose expression is deregulated by proviral insertion in most erythroleukemic cell lines induced by the spleen focus-forming virus (SFFV) and Friend murine leukemia virus (F-MuLV) components of the Friend viral complex, respectively. In this study, we present evidence that transcription of the Fli-1 gene is positively regulated by Spi-1/PU.1 in SFFV-transformed cell lines: (i) all SFFV-transformed cell lines expressing Spi-1/PU.1 are characterized by a specific pattern of Fli-1 gene transcripts initiated in the -200 region instead of position -400 as reported for F-MuLV-transformed cell lines; (ii) these Fli-1 transcripts initiated in the -200 region are downregulated in parallel with that of Spi-1/PU.1 during hexamethylenebisacetamide (HMBA) induced differentiation; and (iii) Fli-1 transcription is upregulated in SFFV cells lines following stable transfection of a Spi-1/PU.1 expression vector. Furthermore, we found by transient transfection assays that the -270/-41 region of the Fli-1 gene displays promoter activity which is transactivated by Spi-1/PU.1. This promoter is strictly dependent on the integrity of two highly conserved ETS DNA binding sites that bind the Spi-1/PU.1 protein in vitro. Finally, we show that transfection of constitutive or inducible Fli-1 expression vectors in SFFV-transformed cells inhibits their erythroid differentiation induced by HMBA. Overall, these data indicate that Fli-1 is a target gene of the Spi-1/PU.1 transcription factor in SFFV-transformed cell lines. We further suggest that deregulated synthesis of Fli-1 may trigger a common mechanism contributing to erythroleukemia induced by either SFFV or F-MuLV.

hsp27 as a switch between differentiation and apoptosis in murine embryonic stem cells

Small stress proteins are developmentally regulated and linked to cell growth and differentiation. The early phase of murine embryonic stem (ES) cell differentiation, characterized by a gradual growth arrest, is accompanied with hsp27 transient accumulation. This differentiation process also correlated with changes in hsp27 phosphorylation and oligomerization. The role of hsp27 was investigated in ES clones stably transfected with murine or human hsp27 genes, placed in sense or antisense orientation. Several clones were obtained that either underexpressed endogenous murine hsp27 or overexpressed murine or human hsp27. Maintained undifferentiated, these clones showed similar growth rates. We report here that hsp27 constitutive overexpression enhanced the differentiation-mediated decreased rate of ES cell proliferation but did not alter morphological changes. In contrast, hsp27 underexpression, which attenuated cell growth arrest, induced differentiation abortion because of an overall cell death by apoptosis. Recently, we showed that hsp27 interfered with cell death probably because of its ability to modulate intracellular glutathione. hsp27 accumulation during ES cell differentiation was also correlated with an increase in glutathione, which was attenuated by hsp27 down-expression. Hence, hsp27 transient expression seems essential for preventing differentiating ES cells from undergoing apoptosis, a switch that may be redox regulated.

Prelingual deafness: high prevalence of a 30delG mutation in the connexin 26 gene

Prelingual non-syndromic (isolated) deafness is the most frequent hereditary sensory defect. In >80% of the cases, the mode of transmission is autosomal recessive. To date, 14 loci have been identified for the recessive forms (DFNB loci). For two of them, DFNB1 and DFNB2, the genes responsible have been characterized; they encode connexin 26 and myosin VIIA, respectively. In order to evaluate the extent to which the connexin 26 gene (Cx26) contributes to prelingual deafness, we searched for mutations in this gene in 65 affected Caucasian families originating from various countries, mainly tunisia, France, New Zealand and the UK. Six of these families are consanguineous, and deafness was shown to be linked to the DFNB1 locus, 10 are small non consanguineous families in which the segregation of the trait has been found to be compatible with the involvement of DFNB1, and in the remaining 49 families no linkage analysis has been performed. A total of 62 mutant alleles in 39 families were identified. Therefore, mutations in Cx26 represent a major cause of recessively inherited prelingual deafness since according to the present results they would underlie approximately half of the cases. In addition, one specific mutation, 30delG, accounts for the majority (approximately 70%) of the Cx26 mutant alleles. It is therefore one of the most frequent disease mutations so far identified. Several lines of evidence indicate that the high prevalence of the 30delG mutation arises from a mutation hot spot rather than from a founder effect. Genetic counseling for prelingual deafness has been so far considerably impaired by the difficulty in distinguishing genetic and non genetic deafness in families presenting with a single deaf child. Based on the results presented here, the development of a simple molecular test could be designed which should be of considerable help.

Coactivation of human alpha1- and alpha2-globin genes in single induced MEL cells containing one human alpha-globin locus

We developed a reverse-transcription polymerase chain reaction assay, performed on single isolated cells, to demonstrate the coexpression of human alpha1- and alpha2-globin mRNA in induced mouse erythroleukemic cells containing a single human alpha-globin locus. These results indicate that both alpha1 and alpha2 genes are activated from the same alpha-globin gene locus implying that HS-40-dependent transcriptional activation is mediated, either by a simultaneous interaction of HS-40 with both a alpha1 and alpha2-globin gene promoters, or by a dynamic process characterized by alternative, but short-lived, interactions with each alpha-globin gene promoter.

A new alpha chain variant Hb Sallanches [alpha 2 104(G11) Cys-->Tyr] associated with HbH disease in one homozygous patient

We identified a new alpha-chain variant (alpha Sal) associated with haemolytic anaemia and low level of HbH in one homozygous patient. This new mutation is located in codon 104 (TGC-->TAC) of the alpha 2 globin gene and results in a Cys-->Tyr replacement. In vitro and in vivo biosynthetic studies suggest that the mechanism leading to HbH disease in this homozygous patient is mostly related to a significant instability of alpha Sal:beta dimers rather than to the hyperinstability of the alpha Sal chain itself only.

Targeted inactivation of the major positive regulatory element (HS-40) of the human alpha-globin gene locus

We have examined the role of the major positive upstream regulatory element of the human alpha-globin gene locus (HS-40) in its natural chromosomal context. Using homologous recombination, HS-40 was replaced by a neo marker gene in a mouse erythroleukemia hybrid cell line containing a single copy of human chromosome 16. In clones from which HS-40 had been deleted, human alpha-globin gene expression was severely reduced, although basal levels of alpha 1 and alpha 2-globin mRNA expression representing less than 3% of the level in control cell lines were detected. Deletion of the neo marker gene, by using FLP recombinase/FLP recombinase target system, proved that the phenotype observed was not caused by the regulatory elements of this marker gene. In the targeted clones, deletion of HS-40 apparently does not affect long-range or local chromatin structure at the alpha promoters. Therefore, these results indicate that, in the experimental system used, HS-40 behaves as a strong inducible enhancer of human alpha-globin gene expression.

A third locus for hereditary haemorrhagic telangiectasia maps to chromosome 12q

Hereditary haemorrhagic telangiectasia (HHT) or Rendu-Osler-Weber disease is an autosomal dominant vascular disorder which associates epistaxis, mucocutaneous and visceral telangiectases, and recurrent haemorrhage with chronic anaemia and visceral shuntings. Recently, the tumour growth factor (TGF)-beta binding protein endoglin localized to 9q33-34 was identified as responsible for HHT in several large kindreds with pulmonary arteriovenous malformations (PAVMs). Additional linkage studies demonstrated that HHT is a genetically heterogeneous disorder with families unlinked to this region of 9q. In the families in which HHT was not linked to chromosome 9, less PAVMs were present. Furthermore, in one of these families, HHT was found linked to 3p22, where the TGF-beta II receptor is located. In this linkage study, we have analysed DNA from two families, in which HHT was unlinked to chromosome 9q and 3p, and PAVMs were absent, with a series of genetic markers on the centromeric region of chromosome 12. Using two-point linkage analysis, a significant lod score of Zmax = 7.86 at theta = 0.05 was obtained with the D12S85 microsatellite marker.

Structural characterization of the beta-globin gene cluster in an individual expressing a very low level of G gamma globin chains

We report the case of a normal individual displaying an extremely unbalanced G gamma/A gamma-globin ratio (G gamma-globin chains undetectable by urea/triton/ acrylamide gel electrophoresis and just reaching the threshold of detection by high performance liquid chromatography) associated with a very low level of G gamma-globin mRNA (at the most 5% of total gamma-mRNA after reverse transcriptase polymerase chain reaction determination). By DNA Southern blotting and sequencing, the very low level of G gamma-globin chains in this individual was found in association with subhaplotype [+ -----] (Hinc II 5' to epsilon, Xmn I 5' to G gamma, Hind III in G gamma and A gamma, Hinc II in and 3' to psi beta), with G gamma- and A gamma-globin gene sequences of the B type chromosome, and with a number of AT repeats in the locus control region hypersensitive site-2 site, similar to that reported to be associated with the Bantu beta S haplotype. These structural characteristics, described for the first time combined in the same individual, suggest that the G gamma/A gamma ratio in adults, is controlled by sequences distributed all along the beta-globin gene cluster.

Inducible expression of a neo gene integrated into the human alpha-globin gene cluster

We replaced the 3' flanking region of the human alpha 1-globin gene that binds in vitro the specific transcription factors GATA-1 and AP1/NF-E2, by a neo marker gene using homologous recombination in a MEL (mouse erythroleukemia line) hybrid cell line harbouring a single human chromosome 16. Using an improved method of the neo-positive and HSV-tk negative selection, one correctly targeted clone was obtained out of 164 clones analyzed. In contrast to non-targeted clones, the expression of teh neo gene in the targeted clone acquired the erythroid differentiation-dependent inducibility normally characteristic of the alpha-globin genes. No difference was observed in the expression of the human zeta, alpha 2, alpha 1, or theta-globin genes before and after induction of differentiation between the targeted clone and parental cells. These results indicate that, at least in the experimental system used, the 3' flanking region of the human alpha 1-globin gene can be replaced by an exogenous non-erythroid gene without affecting the regulation of the globin genes contained in the alpha-globin cluster.

Expression of human colony-stimulating factor-1 (CSF-1) receptor in murine pluripotent hematopoietic NFS-60 cells induces long-term proliferation in response to CSF-1 without loss of erythroid differentiation potential

NFS-60 and FDCP-Mix cells are interleukin-3--dependent multipotent hematopoietic cells that can differentiate in vitro into mature myeloid and erythroid cells. Retrovirus-mediated transfer of the human colony-stimulating factor-1 (CSF-1) receptor gene (c-fms) enabled NFS-60 cells but not FDCP-Mix cells to proliferate in response to CSF-1. The phenotype of NFS-60 cells expressing the human CSF-1 receptor (CSF-1R) grown in CSF-1 did not grossly differ from that of original NFS-60 as assessed by cytochemical and surface markers. Importantly, these cells retained their erythroid potentiality. In contrast, a CSF-1-dependent variant of NFS-60, strongly expressing murine CSF-1R, differentiated into monocyte/macrophages upon CSF-1 stimulation and almost totally lost its erythroid potentiality. We also observed that NFS-60 but not FDCP-Mix cells could grow in response to stem cell factor, (SCF), although both cell lines express relatively high amounts of SCF receptors. This suggests that SCF-R and CSF-1R signalling pathways share at least one component that may be missing or insufficiently expressed in FDCP-Mix cells. Taken together, these results suggest that human CSF-1R can use the SCF-R signalling pathway in murine multipotent cells and thereby favor self-renewal versus differentiation.

Functional analysis of the 4 bp deletion identified in the 5' untranslated region of one of the beta-globin genes from a Chinese beta-thalassaemic heterozygote

Two plasmids have been constructed in which a beta-galactosidase/phleomycin-resistance fusion gene reporter is placed under the control of the human beta-globin gene promoter and 5' untranslated region including or not including nucleotides 40-43 previously found deleted in one Chinese beta-thalassaemic allele. Transient expression assays of these two plasmids failed to reveal any significative effect of this 4 bp deletion either on the level of the beta-galactosidase activity produced in HeLa cells transfected in standard conditions, or on the rate of synthesis of the beta-galactosidase protein in transfected HeLa cells submitted to increasing osmotic shocks. These results suggest that this 4 bp deletion is not responsible for the beta-thalassaemic phenotype in vivo.

A method improving the efficiency of the positive-negative selection used to isolate homologous recombinants

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