Glutaryl-CoA dehydrogenase mutations in glutaric acidemia (type I): review and report of thirty novel mutations

Glutaric acidemia type I (GA1) is caused by mutations in the gene encoding the enzyme glutaryl-CoA dehydrogenase (GCD). Sixty-three pathogenic mutations identified by several laboratories are presented, 30 of them for the first time, together with data on expression in Escherichia coli and relationship to the clinical and biochemical phenotype. In brief, many GCD mutations cause GA1, but none is common. There is little if any relationship between genotype and clinical phenotype, but some mutations, even when heterozygous, seem especially common in patients with normal or only minimally elevated urine glutaric acid.

Alphavirus vectors for gene expression and vaccines

Alphavirus expression vectors are finding novel uses in research. They are showing increasing promise as vaccines and are being developed for diagnostic assays of other viruses. Some highlights over the past couple of years include improvements in packaging of replicons, targeting of Sindbis virus replicons, stable cell lines that can be induced to produce replicons, and the isolation of noncytopathic variants of Sindbis virus replicons. Reports that alphavirus vectors can efficiently infect neurons in rat hippocampal slices should increase their use in neurobiological studies.

Recombinant Semliki Forest virus and Sindbis virus efficiently infect neurons in hippocampal slice cultures

Gene transfer into nervous tissue is a powerful tool for the analysis of gene function. By using a rat hippocampal slice culture preparation, we show here that Semliki Forest virus (SFV) and Sindbis virus (SIN) vectors are useful for the effective infection of neurons. The stratum pyramidale and/or the granular cell layer were injected with recombinant virus encoding beta-galactosidase (LacZ) or green fluorescent protein (GFP). By using low concentrations of injected SFV-LacZ or SIN-LacZ, we detected LacZ staining of pyramidal cells, interneurons, and granule cells. About 60% of the infected cells showed clear neuronal morphology; thus, relatively few glial cells expressed the transgene. Expression of GFP from SFV and SIN vectors gave similar results, with an even higher percentage (>90%) of the GFP-positive cells identified as neurons. Infected pyramidal cells were readily recognized in living slices, displaying GFP fluorescence in dendrites of up to fourth order and in dendritic spines. They appeared morphologically normal and viable at 1-5 days postinfection. We conclude that both SFV and SIN vectors efficiently transfer genes into neurons in hippocampal slice cultures. In combination with the GFP reporter, SFV and SIN vectors will allow the physiological examination of identified neurons that have been modified by overexpression or suppression of a specific gene product.

Selection of RNA replicons capable of persistent noncytopathic replication in mammalian cells

The natural life cycle of alphaviruses, a group of plus-strand RNA viruses, involves transmission to vertebrate hosts via mosquitoes. Chronic infections are established in mosquitoes (and usually in mosquito cell cultures), but infection of susceptible vertebrate cells typically results in rapid shutoff of host mRNA translation and cell death. Using engineered Sindbis virus RNA replicons expressing puromycin acetyltransferase as a dominant selectable marker, we identified mutations allowing persistent, noncytopathic replication in BHK-21 cells. Two of these adaptive mutations involved single-amino-acid substitutions in the C-terminal portion of nsP2, the viral helicase-protease. At one of these loci, nsP2 position 726, numerous substitution mutations were created and characterized in the context of RNA replicons and infectious virus. Our results suggest a direct correlation between the level of viral RNA replication and cytopathogenicity. This work also provides a series of alphavirus replicons for noncytopathic gene expression studies (E. V. Agapov, I. Frolov, B. D. Lindenbach, B. M. Prágai, S. Schlesinger, and C. M. Rice, Proc. Natl. Acad. Sci. USA 95:12989-12994, 1998) and a general strategy for selecting RNA viral mutants adapted to different cellular environments.

Stable alphavirus packaging cell lines for Sindbis virus and Semliki Forest virus-derived vectors

Alphavirus vectors are being developed for possible human vaccine and gene therapy applications. We have sought to advance this field by devising DNA-based vectors and approaches for the production of recombinant vector particles. In this work, we generated a panel of alphavirus vector packaging cell lines (PCLs). These cell lines were stably transformed with expression cassettes that constitutively produced RNA transcripts encoding the Sindbis virus structural proteins under the regulation of their native subgenomic RNA promoter. As such, translation of the structural proteins was highly inducible and was detected only after synthesis of an authentic subgenomic mRNA by the vector-encoded replicase proteins. Efficient production of biologically active vector particles occurred after introduction of Sindbis virus vectors into the PCLs. In one configuration, the capsid and envelope glycoproteins were separated into distinct cassettes, resulting in vector packaging levels of 10(7) infectious units/ml, but reducing the generation of contaminating replication-competent virus below the limit of detection. Vector particle seed stocks could be amplified after low multiplicity of infection of PCLs, again without generating replication-competent virus, suggesting utility for production of large-scale vector preparations. Furthermore, both Sindbis virus-based and Semliki Forest virus-based vectors could be packaged with similar efficiency, indicating the possibility of developing a single PCL for use with multiple alphavirus-derived vectors.

Regulated expression of a Sindbis virus replicon by herpesvirus promoters

We describe the use of herpesvirus promoters to regulate the expression of a Sindbis virus replicon (SINrep/LacZ). We isolated cell lines that contain the cDNA of SINrep/LacZ under the control of a promoter from a herpesvirus early gene which requires regulatory proteins encoded by immediate-early genes for expression. Wild-type Sindbis virus and replicons derived from this virus cause death of most vertebrate cells, but the cells discussed here grew normally and expressed the replicon and beta-galactosidase only after infection with a herpesvirus. Vero cell lines in which the expression of SINrep/LacZ was regulated by the herpes simplex virus type 1 (HSV-1) infected-cell protein 8 promoter were generated. One Vero cell line (V3-45N) contained, in addition to the SINrep/LacZ cDNA, a Sindbis virus-defective helper cDNA which provides the structural proteins for packaging the replicon. Infection of V3-45N cells with HSV-1 resulted in the production of packaged SINrep/LacZ replicons. HSV-1 induction of the Sindbis virus replicon and packaging and spread of the replicon led to enhanced expression of the reporter gene, suggesting that this type of cell could be used to develop sensitive assays to detect herpesviruses. We also isolated a mink lung cell line that was transformed with SINrep/LacZ cDNA under the control of the promoter from the human cytomegalovirus (HCMV) early gene UL45. HCMV carries out an abortive infection in mink lung cells, but it was able to induce the SINrep/LacZ replicon. These results, and those obtained with an HSV-1 mutant, demonstrate that this type of signal amplification system could be valuable for detecting herpesviruses for which a permissive cell culture system is not available.

Influence of valproate monotherapy on platelet activation and hematologic values

PURPOSE

Valproate (VPA) has been linked to coagulation disturbances, with both impaired and exaggerated clotting, which has been attributed to an effect of VPA on platelets or hemostatic proteins. Additional thrombocytic function testing may help to identify patients at risk of increased bleeding caused by platelet dysfunction.

METHODS

We evaluated the influence of VPA on hematologic routine values and platelet activation by using immunostaining and flow cytometry in 30 patients receiving long-term VPA therapy and in 30 controls.

RESULTS

The fraction of activated platelets was similar in both groups; however, the general extent of platelet activation was significantly lower in the patient group, with considerable interindividual variability. In addition, patients had a significantly lower platelet count, prolonged thrombin time, and higher mean corpuscular hemoglobin.

CONCLUSIONS

Our data confirm the previously reported hematologic changes caused by VPA and additionally suggest that VPA impairs procoagulatory thrombocytic function, which is reflected by reduced platelet activation and increased thrombin time. Possible mechanisms of VPA-platelet interaction are discussed.

Detection and quantitation of human respiratory syncytial virus (RSV) using minigenome cDNA and a Sindbis virus replicon: a prototype assay for negative-strand RNA viruses

We describe here a novel approach for detecting and quantitating human respiratory syncytial virus (RSV) based on expression of a reporter gene from an RSV minigenome. BHK cells were cytoplasmically transformed with a noncytopathic Sindbis virus replicon expressing T7 RNA polymerase. These cells were then cotransfected with T7 expression plasmids that contain the cDNA of an RSV minigenome and the genes for RSV nucleocapsid proteins N, P, and L. The minigenome contains a reporter gene such as lacZ or CAT flanked by cis-acting RSV transcription signals. Subsequent infection of these cells with RSV resulted in a high level of reporter gene expression which could be inhibited by ribavirin. Mock-infected cells exhibited background levels of expression. This assay can be used to quantitate RSV and titer neutralizing antibody and may be a valuable tool for screening compounds for anti-RSV activity. It serves as a prototype for other negative-strand RNA viruses.

Noncytopathic Sindbis virus RNA vectors for heterologous gene expression

Infection of vertebrate cells with alphaviruses normally leads to prodigious expression of virus-encoded genes and a dramatic inhibition of host protein synthesis. Recombinant Sindbis viruses and replicons have been useful as vectors for high level foreign gene expression, but the cytopathic effects of viral replication have limited their use to transient studies. We recently selected Sindbis replicons capable of persistent, noncytopathic growth in BHK cells and describe here a new generation of Sindbis vectors useful for long-term foreign gene expression based on such replicons. Foreign genes of interest as well as the dominant selectable marker puromycin N-acteyltransferase, which confers resistance to the drug puromycin, were expressed as subgenomic transcripts of noncytopathic replicons or defective-interfering genomes complemented in trans by a replicon. Based on these strategies, we developed vectors that can be initiated via either RNA or DNA transfection and analyzed them for their level and stability of foreign gene expression. Noncytopathic Sindbis vectors express reasonably high levels of protein in nearly every cell. These vectors should prove to be flexible tools for the rapid expression of heterologous genes under conditions in which cellular metabolism is not perturbed, and we illustrate their utility with a number of foreign proteins.

Sindbis virus replicons and Sindbis virus: assembly of chimeras and of particles deficient in virus RNA

Alphaviruses are a well-characterized group of positive-strand RNA viruses. The identification of cis-acting elements in their genomes and their replication strategy have made them useful as vectors for the expression of heterologous genes. In infected cells, the nonstructural proteins, required for replication and transcription of the viral genes, are translated from the genomic RNA; the structural proteins, the capsid protein that interacts with the RNA to form the nucleocapsid and the proteins embedded in the lipid envelope, are translated from a subgenomic mRNA and can be replaced by heterologous genes. Such modified genomes are self-replicating (replicons); they can be introduced into the cells by transfection and can also be packaged into extracellular particles with defective helper (DH) RNAs. The particular DH RNA determines how well it is replicated and to what extent it is packaged. One potential complication of this system has been that recombination between the replicon genome and the DH RNA may occur. The studies described here were designed to prevent recombination by expressing the capsid protein from one DH RNA and the virus membrane proteins from a second helper RNA. Recombination to yield a nonsegmented infectious virus genome would then require several independent crossover events. There is a translational enhancer located downstream of the initiating AUG in the RNA of the capsid gene that had to be conserved in the second helper to achieve high-level expression of the viral glycoproteins. For this reason, we modified the capsid protein gene in two ways: the first was to use the capsid protein gene from a different alphavirus, Ross River virus, and the second was to make deletions in that gene to maintain the translational enhancer in the RNA but to eliminate the positively charged region in the protein that should be essential for the specific and nonspecific interactions with RNA. Transfections with replicon RNA and the deleted chimeric DH RNA as the only helper resulted in the high-level production of particles that were almost completely devoid of RNA. The inclusion of a helper expressing an intact Sindbis virus capsid protein gene led to the production of high levels of packaged replicons. Recombinants were not detected even after several undiluted passages.

Identification of mutations in a Sindbis virus variant able to establish persistent infection in BHK cells: the importance of a mutation in the nsP2 gene

Sindbis virus is a positive strand RNA virus that has provided a valuable model for studying virus structure and replication. It is also being developed as a vector for the expression of heterologous proteins. Many studies with this virus are carried out in cultured BHK cells where infection is usually highly cytopathic and within 1 or 2 days after infection all of the cells are dead. Weiss et al. had established a persistently infected culture of BHK cells by infecting the cells with a virus preparation highly enriched in defective interfering (DI) particles and had isolated an attenuated virus, SIN-1 virus, from the culture [Weiss et al. (1980) J. Virol. 33, 463-474]. SIN-1 virus, free of DI particles, was able to establish a persistent infection in BHK cells. We initiated studies to determine what changes in the genome of the virus were responsible for this phenotype. We describe here the cDNA cloning and sequencing of the 5' terminus and the four nonstructural protein genes from SIN-1 virus. A single coding mutation in the nsP2 gene (a predicted change of Pro-726 --> Ser) produced a virus that was able to establish persistent infection in BHK cells. Additional mutations in the other genes were required to decrease the synthesis of viral RNA to a level similar to that found in cells infected with SIN-1 virus. Incorporation of the nsP2 mutation into a Sindbis virus expression vector led to a higher level of synthesis of the reporter protein, beta-galactosidase, than that obtained with the original Sindbis virus replicon.

Packaging signals in alphaviruses

Alphaviruses synthesize large amounts of both genomic and subgenomic RNA in infected cells, but usually only the genomic RNA is packaged. This implies the existence of an encapsidation or packaging signal which would be responsible for selectivity. Previously, we had identified a region of the Sindbis virus genome that interacts specifically with the viral capsid protein. This 132-nucleotide (nt) fragment lies within the coding region of the nsP1 gene (nt 945 to 1076). We proposed that the 132-mer is important for capsid recognition and initiates the formation of the viral nucleocapsid. To study the encapsidation of Sindbis virus RNAs in infected cells, we designed a new assay that uses the self-replicating Sindbis virus genomes (replicons) which lack the viral structural protein genes and contain heterologous sequences under the control of the subgenomic RNA promoter. These replicons can be packaged into viral particles by using defective helper RNAs that contain the structural protein genes (P. Bredenbeek, I. Frolov, C. M. Rice, and S. Schlesinger, J. Virol. 67:6439-6446, 1993). Insertion of the 132-mer into the subgenomic RNA significantly increased the packaging of this RNA into viral particles. We have used this assay and defective helpers that contain the structural protein genes of Ross River virus (RRV) to investigate the location of the encapsidation signal in the RRV genome. Our results show that there are several fragments that could act as packaging signals. They are all located in a different region of the genome than the signal for the Sindbis virus genome. For RRV, the strongest packaging signal lies between nt 2761 and 3062 in the nsP2 gene. This is the same region that was proposed to contain the packaging signal for Semliki Forest virus genomic RNA.

Alphavirus-based expression vectors: strategies and applications

Alphaviruses are positive-strand RNA viruses that can mediate efficient cytoplasmic gene expression in insect and vertebrate cells. Through recombinant DNA technology, the alphavirus RNA replication machinery has been engineered for high-level expression of heterologous RNAs and proteins. Amplification of replication-competent alpha-virus RNAs (replicons) can be initiated by RNA or DNA transfection and a variety of packaging systems have been developed for producing high titers of infectious viral particles. Although normally cytocidal for vertebrate cells, variants with adaptive mutations allowing noncytopathic replication have been isolated from persistently infected cultures or selected using a dominant selectable marker. Such mutations have been mapped and used to create new alphavirus vectors for noncytopathic gene expression in mammalian cells. These vectors allow long-term expression at moderate levels and complement previous vectors designed for short-term high-level expression. Besides their use for a growing number of basic research applications, recombinant alphavirus RNA replicons may also facilitate genetic vaccination and transient gene therapy.

Translation of Sindbis virus mRNA: analysis of sequences downstream of the initiating AUG codon that enhance translation

Alphaviruses, particularly Sinbis virus and Semliki Forest virus, are proving to be useful vectors for the expression of heterologous genes. In infected cells, these self-replicating vectors (replicons) transcribe a subgenomic mRNA that codes for a heterologous protein instead of the structural proteins. We reported recently that translation of the reporter gene lacZ is enhanced 10-fold when the coding sequences of this gene are fused downstream of and in frame with the 5' half of the capsid gene (I. Frolov and S. Schlesinger, J. Virol. 68:8111-8117, 1994). The enhancing sequences, located downstream of the AUG codon that initiates translation of the capsid protein, have a predicted hairpin structure. We have mutated this region by making changes in the codons which do not affect the protein sequence but should destabilize the putative hairpin structure. These changes caused a decrease in the accumulation of the capsid-beta-galactosidase fusion protein. When these alterations were inserted into the capsid gene in the context of the intact Sindbis virus genome, they led to a decrease in the rate of virus formation but did not affect the final yield. We also altered the original sequence to one that has 12 contiguous G.C base pairs and should form a stable hairpin. The new sequence was essentially as effective as the original had been in enhancement of translation and in the rate of virus formation. The position of the predicted hairpin structure is important for its function; an insertion of 9 nucleotides or a deletion of 9 nucleotides decreased the level of translation. The insertion of a hairpin structure at a particular location downstream of the initiating AUG appears to be a way that alphaviruses have evolved to enhance translation of their mRNA, and, as a consequence, they produce high levels of the structural proteins which are needed for virus assembly. This high level of translation requires an intracellular environment in which host cell protein synthesis is inhibited.

A plasmid-based self-amplifying Sindbis virus vector

Sindbis virus was used as a self-amplifying eukaryotic expression vector. A recombinant cDNA genome of this (+)-strand RNA virus was placed under the transcriptional control of a Rous sarcoma virus LTR (RSV) promoter. Transfection of this plasmid construct into mammalian cell lines (3T3, HepG2, and 293 cells) resulted in expression of the luciferase reporter gene. High-expression levels were also measured after transfection into primary rat myoblasts. In differentiated myotubes, expression levels generated by the Sindbis virus vector were up to 200 times higher than those obtained with a conventional RSV expression vector. In vivo expression was detected after injection of plasmid DNA into mouse quadriceps. In vivo expression was transient and undetectable by day 16. This self-amplifying expression vector can be used for generating high-level expression of transgenes in vitro and in vivo. Its transient nature in vivo could allow for safe, short-term delivery of gene products in gene therapy protocols. It should facilitate the study of Sindbis and other RNA viruses.

Mild expression of fibrodysplasia ossificans progressiva: a report of 3 cases

We describe 3 unusually mild cases of fibrodysplasia ossificans progressiva (FOP) in an 80-year-old man, a 44-year-old woman, and a 17-year-old woman. The man, whose daughter had classic features of FOP, lacked malformation of the great toes and experienced unusually slow progression of the disease. Both women displayed late onset heterotopic ossification. The older women displayed an unusually slow progression of the disease. All 3 patients remained ambulatory at the time of examination. Recognition of a mild form of FOP will influence diagnosis, counselling, and research in this rare condition.

RNA viruses as vectors for the expression of heterologous proteins

RNA viruses comprise a wide variety of infectious agents, some of which are the cause of disease in humans, animals, and plants. Recombinant DNA technology is now making it feasible to modify these genomes and engineer them to express heterologous proteins. Several different schemes are being employed that depend on the genome organization of the virus and on the strategy of replication of the particular virus. Several different examples are illustrated and potential uses as well as possible problems are discussed. In the future reverse genetics may convert some of these viruses from agents of disease to agents of cure.

Translation of Sindbis virus mRNA: effects of sequences downstream of the initiating codon

One incentive for developing the alphavirus Sindbis virus as a vector for the expression of heterologous proteins is the very high level of viral structural proteins that accumulates in infected cells. Although replacement of the structural protein genes by a heterologous gene should lead to an equivalent accumulation of the heterologous protein, the Sindbis virus capsid protein is produced at a level 10- to 20-fold higher than that of any foreign protein. Chimeric mRNAs which contain the first 275 nucleotides of the Sindbis virus 26S mRNA fused to the lacZ gene are also translated at the higher level. The enhancing sequences, located downstream of the AUG codon that initiates translation of the capsid protein, have a predicted hairpin-like structure; deletions in this region destroy the activity. These sequences enhance translation in infected cells but have the opposite effect in uninfected cells. Furthermore, translation of this RNA in infected cells is suppressed by a second viral RNA lacking the hairpin-like structure, but translation of the latter RNA is not affected. We propose that the hairpin-like structure presents a barrier to the movement of the ribosomes during translation of mRNA. In infected cells, under conditions in which this mRNA is essentially the only RNA being translated, a slowdown in the transit of the ribosomes gives factors present at low concentrations a chance to bind to the translation complex and permits a high level of functional complexes to be formed. In uninfected cells and in infected cells translating two different viral subgenomic mRNAs, a pause in the movement of the ribosomes along the RNA is no longer an advantage, because the required factors are now usurped by other translation complexes.

Clinical and pathological features of an autosomal dominant, adult-onset leukodystrophy simulating chronic progressive multiple sclerosis

OBJECTIVE

To study the clinical and pathological features of a kindred with an adult-onset autosomal dominant leukodystrophy.

PATIENTS

Five symptomatic and nine asymptomatic at-risk members of the kindred.

INTERVENTIONS

Subjects underwent detailed histories and general and neurologic examinations. Further evaluation included electroencephalography, evoked potentials, electromyography, autonomic testing, and analysis of serum, urine, and cerebrospinal fluid. One patient underwent sural nerve biopsy and analysis. Another, previously studied patient, underwent a limited autopsy.

RESULTS

Cerebellar and pyramidal dysfunction began in the fourth and fifth decades of life; subtle autonomic symptoms were often present years earlier. Frontal lobe dysfunction and abnormalities of the central visual pathways were mild and of late onset. Sensorineural hearing loss was common. The peripheral nervous system was spared. Autopsy results of one patient revealed severe degeneration of the white matter at all levels of the neuraxis, but most prominent in the frontoparietal and cerebellar regions, with sparing of the subcortical U fibers. Histological and ultrastructural examinations failed to show evidence of a specific pathogenetic mechanism or etiology.

CONCLUSION

This disorder seems to be a distinct type of hereditary leukodystrophy, but its exact nature remains unknown.

Interactions between Sindbis virus RNAs and a 68 amino acid derivative of the viral capsid protein further defines the capsid binding site

In previous studies of encapsidation of Sindbis virus RNA, we identified a 570nt fragment (nt 684-1253) from the 12 kb genome that binds to the viral capsid protein with specificity and is required for packaging of Sindbis virus defective interfering RNAs. We now show that the capsid binding activity resides in a highly structured 132nt fragment (nt 945-1076). We had also demonstrated that a 68 amino acid peptide derived from the capsid protein retained most of the binding activity of the original protein and have now developed an RNA mobility shift assay with this peptide fused to glutathione-S-transferase. We have used this assay in conjunction with the original assay in which the intact capsid protein was immobilized on nitrocellulose to analyze more extensive deletions in the 132-mer. All of the deletions led to a reduction in binding, but the binding of a 5' 67-mer was enhanced by the addition of nonspecific flanking sequences. This result suggests that the stability of a particular structure within the 132nt sequence may be important for capsid recognition.

Comparison of the effects of Sindbis virus and Sindbis virus replicons on host cell protein synthesis and cytopathogenicity in BHK cells

Infection of BHK cells by Sindbis virus leads to rapid inhibition of host cell protein synthesis and cytopathic effects (CPE). We have been studying these events to determine whether the expression of a specific viral gene is required and, in the present study, have focused our attention on the role of the structural proteins--the capsid protein and the two membrane glycoproteins. We tested a variety of Sindbis viruses and Sindbis virus replicons (virus particles containing an RNA that is self-replicating but with some or all of the viral structural protein genes deleted) for their abilities to inhibit host cell protein synthesis and cause CPE in infected BHK cells. Our results show that shutoff of host cell protein synthesis occurred in infected BHK cells when no viral structural proteins were synthesized and also under conditions in which the level of the viral subgenomic RNA was too low to be detected. These results support the conclusion that the early steps in viral gene expression are the ones required for the inhibition of host cell protein synthesis in BHK cells. In contrast, the Sindbis viruses and Sindbis virus replicons were clearly distinguished by the time at which CPE became evident. Viruses that synthesized high levels of the two membrane glycoproteins on the surface of the infected cells caused a rapid (12 to 16 h postinfection) appearance of CPE, and those that did not synthesize the glycoprotein spikes showed delayed (30 to 40 h) CPE.

A cell line that expresses a reporter gene in response to infection by Sindbis virus: a prototype for detection of positive strand RNA viruses

We describe a stably transformed cell line (BHKSINLuc2) that contains a defective Sindbis virus genome under the control of a Rous sarcoma virus promoter and the luciferase gene downstream of the viral subgenomic RNA promoter. This cell line expresses high levels of luciferase activity following infection with Sindbis virus and provides a sensitive assay for titering variants of Sindbis virus that lack the structural protein genes, in particular, Sindbis virus replicons that express heterologous proteins. Cell lines such as this may be of value for detection of positive-strand RNA viruses.

Recombination between Sindbis virus RNAs

The Sindbis virus RNA genome is divided into two modules--one coding for the nonstructural protein genes and the other coding for the structural protein genes. In our studies of recombination, the two parental RNAs were defective in different modules. Analysis of the recombinant RNAs demonstrated that the parental RNAs each contributed its intact module and that the crossovers occurred within the defective modules. The recombinational events giving rise to infectious virion RNAs could create deletions, rearrangements or insertions as long as they occurred outside of the functional module. These crossovers produced RNA genomes that contained two functional subgenomic RNA promoters.

Sindbis virus expression vectors: packaging of RNA replicons by using defective helper RNAs

Since the recovery of infectious RNA transcripts from full-length cDNA clones, alphavirus genome RNAs have been engineered to allow expression of heterologous RNAs and proteins. The highest levels of expression of heterologous products are achieved when the viral structural genes are replaced by the heterologous coding sequences. Such recombinant RNAs are self-replicating (replicons) and can be introduced into cells as naked RNA, but they require trans complementation to be packaged and released from cells as infectious virion particles. In this report, we describe a series of defective Sindbis virus helper RNAs which can be used for packaging Sindbis virus RNA replicons. The defective helper RNAs contain the cis-acting sequences required for replication as well as the subgenomic RNA promoter which drives expression of the structural protein genes. In cells cotransfected with both the replicon and defective helper RNAs, viral nonstructural proteins translated from the replicon RNA allow replication and transcription of the defective helper RNA to produce the virion structural proteins. A series of defective helper RNAs were compared for the ability to package the replicon RNA as well as for the ability to be replicated and packaged. One defective helper RNA not only packaged the replicon but also was itself encapsidated and would be useful under conditions in which extensive amplification is advantageous. Other defective helper RNAs were able to package the replicon efficiently but were packaged very poorly themselves. These helpers should be useful for applications in which expression of the viral structural proteins or virus spread is not desired.