Spinal Muscular Atrophy: More than a Disease of Motor Neurons?

Spinal muscular atrophy (SMA) is the most common genetically inherited neurodegenerative disease resulting in infant mortality. SMA is caused by genetic deletion or mutation in the survival of motor neuron 1 (SMN1) gene, which results in reduced levels of the survival of motor neuron (SMN) protein. SMN protein deficiency preferentially affects α- motor neurons, leading to their degeneration and subsequent atrophy of limb and trunk muscles, progressing to death in severe forms of the disease. More recent studies have shown that SMN protein depletion is detrimental to the functioning of other tissues including skeletal muscle, heart, autonomic and enteric nervous systems, metabolic/endocrine (e.g. pancreas), lymphatic, bone and reproductive system. In this review, we summarize studies discussing SMN protein's function in various cell and tissue types and their involvement in the context of SMA disease etiology. Taken together, these studies indicate that SMA is a multi-organ disease, which suggests that truly effective disease intervention may require body-wide correction of SMN protein levels.

Expression of the fusogenic p14 FAST protein from a replication-defective adenovirus vector does not provide a therapeutic benefit in an immunocompetent mouse model of cancer.

When injected directly into a tumor mass, adenovirus (Ad) vectors only transduce cells immediately along the injection tract. Expression of fusogenic proteins from the Ad vector can lead to syncytium formation, which efficiently spreads the therapeutic effect. Fusogenic proteins can also cause cancer cell death directly, and enhance the release of exosome-like particles containing tumor-associated antigens, which boosts the anti-tumor immune response. In this study, we have examined whether delivery of an early region 1 (E1)-deleted, replication-defective Ad vector encoding the reptilian reovirus p14 fusion-associated small transmembrane (FAST) protein can provide therapeutic efficacy in an immunocompetent mouse tumor model. A high multiplicity of infection of AdFAST is required to induce cell fusion in mouse mammary carcinoma 4T1 cells in vitro, and FAST protein expression caused a modest reduction in cell membrane integrity and metabolic activity compared with cells infected with a control vector. Cells expressing FAST protein released significantly higher quantities of exosomes. In immunocompetent Balb/C mice harboring subcutaneous 4T1 tumors, AdFAST did not induce detectable cancer cell fusion, promote tumor regression or prolong mouse survival compared with untreated mice. This study suggests that in the context of the 4T1 model, Ad-mediated FAST protein expression did not elicit a therapeutic effect.

Retinal transduction profiles by high-capacity viral vectors.

Retinal gene therapy with adeno-associated viral (AAV) vectors is safe and effective in humans. However, the limited cargo capacity of AAV prevents their use for therapy of those inherited retinopathies (IRs) due to mutations in large (>5 kb) genes. Viral vectors derived from adenovirus (Ad), lentivirus (LV) and herpes virus (HV) can package large DNA sequences, but do not target efficiently retinal photoreceptors (PRs) where the majority of genes responsible for IRs are expressed. Here, we have evaluated the mouse retinal transduction profiles of vectors derived from 16 different Ad serotypes, 7 LV pseudotypes and from a bovine HV. Most of the vectors tested transduced efficiently the retinal pigment epithelium. We found that LV-GP64 tends to transduce more PRs than the canonical LV-VSVG, albeit this was restricted to a narrow region. We observed more extensive PR transduction with HdAd1, 2 and 5/F35++ than with LV, although none of them outperformed the canonical HdAd5 or matched the extension of PR transduction achieved with AAV2/8.

At the "junction" of spinal muscular atrophy pathogenesis: the role of neuromuscular junction dysfunction in SMA disease progression.

Spinal muscular atrophy (SMA) is caused by mutations that reduce the level of the survival motor neuron protein (SMN) resulting in death of alpha-motor neurons, yet it is unclear why these cells are preferentially affected by a reduction in this ubiquitously-expressed protein. In mouse models of SMA, one of the earliest events detected is defects at the neuromuscular junction (NMJ). Although NMJs are established at a normal frequency, there are structural as well as functional perturbations and a lack of maturation of the primitive synapse. These early defects are followed by loss of the NMJ, denervation of the muscle and onset of muscle atrophy. In this review, we discuss our current understanding of the contribution of NMJ dysfunction in SMA disease pathogenesis, and also provide an overview of therapies currently under preclinical and clinical development for treatment of SMA.

FSGS-associated alpha-actinin-4 (K256E) impairs cytoskeletal dynamics in podocytes.

Mutations in the ACTN4 gene, encoding the actin crosslinking protein alpha-actinin-4, are associated with a familial form of focal segmental glomerulosclerosis (FSGS). Mice with podocyte-specific expression of K256E alpha-actinin-4 develop foot process effacement and glomerulosclerosis, highlighting the importance of the cytoskeleton in podocyte structure and function. K256E alpha-actinin-4 exhibits increased affinity for F-actin. However, the downstream effects of this aberrant binding on podocyte dynamics remain unclear. Wild-type and K256E alpha-actinin-4 were expressed in cultured podocytes via adenoviral infection to determine the effect of the mutation on alpha-actinin-4 subcellular localization and on cytoskeletal-dependent processes such as adhesion, spreading, migration, and formation of foot process-like peripheral projections. Wild-type alpha-actinin-4 was detected primarily in the Triton-soluble fraction of podocyte lysates and localized to membrane-associated cortical actin and focal adhesions, with some expression along stress fibers. Conversely, K256E alpha-actinin-4 was detected predominantly in the Triton-insoluble fraction, was excluded from cortical actin, and localized almost exclusively along stress fibers. Both wild-type and K256E alpha-actinin-4-expressing podocytes adhered equally to an extracellular matrix (collagen-I). However, podocytes expressing K256E alpha-actinin-4 showed a reduced ability to spread and migrate on collagen-I. Lastly, K256E alpha-actinin-4 expression reduced the mean number of actin-rich peripheral projections. Our data suggest that aberrant sequestering of K256E alpha-actinin-4 impairs podocyte spreading, motility, and reduces the number of peripheral projections. Such intrinsic cytoskeletal derangements may underlie initial podocyte damage and foot process effacement encountered in ACTN4-associated FSGS.

Disruption of MAP kinase activation and nuclear factor binding to the IL-12 p40 promoter in HIV-infected myeloid cells.

Progressive immunodeficiency in HIV infection is paralleled by a decrease in IL-12 production, a cytokine crucial for cellular immune function. Here we examine the molecular mechanisms by which HIV infection suppresses IL-12 p40 expression. HIV infection of THP-1 myeloid cells resulted in decreased LPS-induced nuclear factor binding to the NF-kappaB, AP-1, and Sp1 sites of the IL-12 p40 promoter. By site-directed mutagenesis we determined that each of these sites was necessary for transcriptional activation of the IL-12 p40 promoter. Binding of NF-kappaB p50, c-Rel, p65, Sp1, Sp3, c-Fos, and c-Jun proteins to their cognate nuclear factor binding sites was somewhat impaired by HV infection, although a role for other as yet unidentified factors cannot be dismissed. The cellular levels of these transcription factors were unaffected by HIV infection, with the exception of a decrease in expression of NF-kappaB p65, consistent with the observed decrease in its binding to the IL-12 p40 promoter following HIV infection. Analysis of regulation of upstream LPS-induced MAP kinases demonstrated impaired phosphorylation of JNK and p38 MAPK, and suppressed phosphorylation and degradation of IkappaBalpha following HIV infection. These results suggest that alterations in nuclear factor binding to numerous sites in the IL-12 p40 promoter, together may contribute to the suppression in IL-12 p40 transcription previously reported. These effects on nuclear factor binding may be a direct effect of HIV infection on the IL-12 p40 promoter, or may occur indirectly as a consequence of altered MAP kinase activation.

Development of a size-restricted pIX-deleted helper virus for amplification of helper-dependent adenovirus vectors.

Helper-dependent adenovirus vectors (hdAd), which are deleted of all viral protein-coding sequences, can mediate long-term expression of a therapeutic transgene and lead to life-long, phenotypic correction in animal models of genetic disease. Here, we describe a new system for the generation of hdAd, which utilizes the DNA size restrictions imposed on an Ad virion deleted of protein IX (pIX): such virions are reported to package up to only approximately 35 kb of viral DNA. A pIX(-) helper virus (approximately 37.3 kb) was easily grown on complementing 293pIX cells. Upon infection of noncomplementing cells, this virus was not capable of forming infectious virions, but provided replicative and packaging functions for propagation of a 30-kb hdAd. The pIX(-) helper virus was effective in amplifying an hdAd and, in combination with Cre-mediated excision in the viral-packaging signal, resulted in a 1000-fold reduction in helper virus contamination in hdAd stocks compared to Cre/lox alone, as determined by plaque assay. However, through slot blot analysis of DNA isolated from virions, we determined that the ratio of hdAd to helper DNA was 500:1, similar to the ratio observed when using Cre/lox alone. Surprisingly, a large amount of the 37.3-kb helper DNA was being packaged into the pIX-deleted virions, but these virions were incapable of establishing productive infections in plaque assays, for reasons which are still unclear. Nevertheless, the pIX(-) hdAd generated in this system infected cells and expressed a transgene at levels similar to those obtained with a pIX(+) hdAd. These data suggest that, although further studies are necessary to characterize the nature of the defective helper virions formed in this system, deletion of pIX from the helper virus genome does provide an effective method to prevent recovery of functional helper virus during hdAd amplification.

Toxicity associated with repeated administration of first-generation adenovirus vectors does not occur with a helper-dependent vector.

BACKGROUND: Certain gene therapy protocols may require multiple administrations of vectors to achieve therapeutic benefit to the patient. This may be especially relevant for vectors such as adenoviral vectors that do not integrate into the host chromosome. Because immunocompetent animal models used for gene transfer studies develop neutralizing antibodies to adenoviral vectors after a single administration, little is known about how repeat administrations of vectors might affect transgene expression and vector toxicity. MATERIALS AND METHODS: We used mice deficient in the membrane spanning region of immunoglobulin (IgM), which do not develop antibodies, to evaluate the effect of repeated intravenous administration of first-generation and helper-dependent adenoviral vectors expressing human alpha 1-antitrypsin (hAAT). The duration and levels of transgene expression were evaluated after repeated administration of vectors. Toxicity was assessed by measuring the level of liver enzymes in the serum and the degrees of hepatocyte hypertrophy and proliferation. RESULTS: We found that previous administration of first-generation adenoviral vectors can alter the response to subsequent doses. These alterations included an increase in transgene expression early (within 1 and 3 days), followed by a rapid drop in expression by day 7. In addition, previous administrations of first-generation vectors led to an increase in toxicity of subsequent doses, as indicated by a rise in liver enzymes and an increase in hepatocyte proliferation. In contrast to first-generation vectors, use of the helper-dependent adenovirus vector, Ad-STK109, which contained no viral coding regions, did not lead to increased toxicity after multiple administrations. CONCLUSIONS: We conclude that the response of the host to adenoviral vectors can be altered after repeated administration, compared with the response after the initial vector dose. In addition, these experiments provide further evidence for the relative safety of helper-dependent adenoviral vectors for gene therapy, compared with first-generation vectors.

Helper-dependent adenovirus vectors: their use as a gene delivery system to neurons.

Recombinant adenovirus vectors have provided a major advance in gene delivery systems for post-mitotic neurons. However, the use of these first generation vectors has been limited due to the onset of virally mediated effects on cellular function and viability. In the present study we have used primary cultures of cerebellar granule neurons to examine the efficacy and cytotoxic effects of a helper-dependent adenovirus vector (hdAd) in comparison with a first generation vector. Our results demonstrate that the hdAd system provides equally efficient infectivity with significantly reduced toxicity in comparison to first generation vectors. Neurons transduced with a high titre of a first generation vector exhibited a time-dependent shut down in global protein synthesis and impaired physiological function as demonstrated by a loss of glutamate receptor responsiveness. This was followed by an increase in the fraction of TUNEL-positive cells and a loss of neuronal survival. In contrast, hdAds could be used at titres that transduce >85% of neurons with little cytotoxic effect: cellular glutamate receptor responses and rates of protein synthesis were indistinguishable from uninfected controls. Furthermore, cell viability was not significantly affected for at least 7 days after infection. At excessive viral titres, however, infection with hdAd did cause moderate but significant changes in cell function and viability in primary neuronal cultures. Thus, while these vectors are remarkably improved over first generation vectors, these also have limitations with respect to viral effects on cellular function and viability. Gene Therapy (2000) 7, 1200-1209.

An enhanced system for construction of adenoviral vectors by the two-plasmid rescue method.

The two-plasmid rescue method of constructing Ad vectors, which relies on either homologous or Cre-mediated recombination between two plasmids cotransfected into 293 or 293Cre4 cells, respectively, offers advantages over other approaches because of its simplicity. We have improved the efficiency of vector construction by both homologous and Cre-mediated recombination by replacing the single ITR in the shuttle plasmid with a head-to-head ITR junction. We have also expanded the versatility of this method by incorporating a Cre expression cassette into the plasmids to permit high-efficiency Cre-mediated vector rescue using 293 cells, abrogating the need for Cre-expressing cell lines. This new system retains the simplicity of the original but results in an approximately 100-fold increase in the number of recombinant viruses produced, all of which contain the foreign DNA insert, and allows high-efficiency Cre-mediated vector isolation using any E1-complementing cell line.

A high-efficiency Cre/loxP-based system for construction of adenoviral vectors.

Adenovirus (Ad) vectors provide a highly efficient means of mammalian gene transfer and are widely used for high-level protein expression in mammalian cells, as recombinant vaccines and for gene therapy. A commonly used method for constructing Ad vectors relies on in vivo homologous recombination between two Ad DNA-containing bacterial plasmids cotransfected into 293 cells. While the utility of this two-plasmid approach is well established, its efficiency is low owing to the inefficiency of homologous recombination. To address this, we have developed an improved method for Ad vector construction based on Cre-mediated site-specific recombination between two bacterial plasmids, each bearing a loxP site. Ad vectors are generated as a result of Cre-mediated site-specific recombination between the two plasmids after their cotransfection into 293 cells expressing Cre recombinase. The frequency of Ad vector rescue by Cre-mediated site-specific recombination is significantly higher (approximately 30-fold) than by in vivo homologous recombination. The efficiency and reliability of this method should greatly simplify and expedite the construction of recombinant Ad vectors for mammalian gene transfer. Ad vectors are commonly constructed by homologous recombination between two plasmids cotransfected into 293 cells. This method has numerous advantages but results in low numbers of plaques owing to inefficient recombination. We have developed an improved method based on Cre-mediated site-specific recombination, which results in vector rescue at frequencies approximately 30-fold higher than by homologous recombination. This method should greatly simplify and expedite the construction of recombinant Ad vectors for mammalian gene transfer.

Administration of helper-dependent adenoviral vectors and sequential delivery of different vector serotype for long-term liver-directed gene transfer in baboons.

The efficiency of first-generation adenoviral vectors as gene delivery tools is often limited by the short duration of transgene expression, which can be related to immune responses and to toxic effects of viral proteins. In addition, readministration is usually ineffective unless the animals are immunocompromised or a different adenovirus serotype is used. Recently, adenoviral vectors devoid of all viral coding sequences (helper-dependent or gutless vectors) have been developed to avoid expression of viral proteins. In mice, liver-directed gene transfer with AdSTK109, a helper-dependent adenoviral (Ad) vector containing the human alpha(1)-antitrypsin (hAAT) gene, resulted in sustained expression for longer than 10 months with negligible toxicity to the liver. In the present report, we have examined the duration of expression of AdSTK109 in the liver of baboons and compared it to first-generation vectors expressing hAAT. Transgene expression was limited to approximately 3-5 months with the first-generation vectors. In contrast, administration of AdSTK109 resulted in transgene expression for longer than a year in two of three baboons. We have also investigated the feasibility of circumventing the humoral response to the virus by sequential administration of vectors of different serotypes. We found that the ineffectiveness of readministration due to the humoral response to an Ad5 first-generation vector was overcome by use of an Ad2-based vector expressing hAAT. These data suggest that long-term expression of transgenes should be possible by combining the reduced immunogenicity and toxicity of helper-dependent vectors with sequential delivery of vectors of different serotypes.

Effects of stuffer DNA on transgene expression from helper-dependent adenovirus vectors.

We have analyzed transgene (lacZ) expression from a first-generation adenovirus (Ad) vector in comparison to helper-dependent (hd) Ads deleted for various portions of the viral coding sequences and generated by using the Cre/loxP helper-dependent system (R. J. Parks et al., Proc. Natl. Acad. Sci. USA 93:13565-13570, 1996). An hd vector deleted for approximately 70% of the Ad genome (AdRP1001) provided levels and durations of transgene expression similar to those of a control first generation Ad vector containing an identical expression cassette. Deletion of all Ad sequences from the hdAd and replacement with a approximately 22-kb fragment of lambda DNA resulted in a decrease in the level and duration of lacZ expression which could not be reversed by the inclusion of a matrix attachment region. However, substitution of the lambda stuffer in the fully deleted hdAd with sequences from the human hypoxanthine-guanine phosphoribosyltransferase gene resulted in significantly improved transgene expression. In vitro assays for cytotoxic T lymphocytes (CTL) directed against putative peptides encoded by the vector backbone showed that, although CTL were generated against the vector containing the lambda DNA, no such CTL were generated against the vector containing the hypoxanthine-guanine phosphoribosyltransferase (HPRT) sequences. Surprisingly, the rate of loss of the HPRT- and lambda-containing vectors from mouse liver was similar, despite the differences in expression kinetics, indicating that the lambda stuffer-directed CTL were inefficient at eliminating the transduced cells. Thus, the nature of the DNA backbone of hdAds can have important effects on the functioning of the vector. Since most fully deleted vectors require "stuffer" DNA as part of the vector backbone to maintain optimum vector size, these observations must be taken into account in the design of hdAd vectors.

Use of a liver-specific promoter reduces immune response to the transgene in adenoviral vectors.

Previous studies using adenoviral (Ad) vectors expressing human alpha1-antitrypsin (hAAT) under the control of ubiquitous promoters (RSV, mPGK) elicited the production of antibodies to hAAT in some mouse strains (C3H/HeJ and BALB/c) but not in others (C57BL/6J). In contrast, when a helper-dependent Ad vector (AdSTK109) with all viral coding sequences deleted and expressing hAAT from human genomic DNA with the endogenous promoter was used, C3H/HeJ mice failed to develop antibodies and demonstrated long-term expression. These results suggested that promoter choice and/or properties of the vector itself might influence the host immune response to the transgene product. Direct comparison of first-generation vectors expressing the hAAT cDNA from a ubiquitous mouse PGK promoter rather than from a liver-specific mouse albumin promoter demonstrated that an antibody response to hAAT occurred with the mPGK promoter but not with the albumin promoter in C3H/HeJ mice. As expected, neither vector elicits an antibody response in C57BL/6J mice. Coinjection of the two first-generation vectors containing the mPGK and albumin promoter in C3H/HeJ mice induced an antibody response with resulting loss of detectable hAAT from the sera of the injected mice in 3-4 weeks. From these data, we conclude that under certain conditions, the choice of promoter with its associated liver-specific expression can modulate the host immune response to the transgene independent of viral backbone.

Site-specific integration mediated by a hybrid adenovirus/adeno-associated virus vector.

Adenovirus (Ad) and adeno-associated virus (AAV) have attractive and complementary properties that can be exploited for gene transfer purposes. Ad vectors are probably the most efficient vehicles to deliver foreign genes both in vitro and in vivo. AAV exhibits the unique ability to establish latency by efficiently integrating at a specific locus of human chromosome 19 (AAVS1). Two viral elements are necessary for the integration at AAVS1: Rep68/78 and the inverted terminal repeats (AAV-ITRs). In this study, we report the development of two helper-dependent adenoviral (HD) vectors, one carrying the Rep78 gene, the other an AAV-ITR-flanked transgene. Although Rep proteins have been demonstrated to interfere with Ad replication, HD Rep78 vector was successfully amplified on serial passages in 293CRE4 cells with a yield of 50-100 transducing units per cell. DNA integration at the AAVS1 site also was demonstrated in hepatoma cells coinfected with the HD-expressing Rep78 and with the second HD vector carrying a transgene flanked by AAV-ITRs. The high transduction efficiency, large cloning capacity, and high titer of the HD, combined with the site-specific integration machinery provided by AAV-derived components, make the Ad/AAV hybrid viruses a promising vehicle for gene therapy.

DNA ligase gene disruptions can depress viral growth and replication in poxvirus-infected cells.

Poxvirus-encoded DNA ligases are assumed to play a role in viral DNA replication; however mutational inactivation of vaccinia ligase has not been reported to affect viral growth rates in culture. This communication re-examines this surprising aspect of poxviral biology using both Shope fibroma virus (SFV) and vaccinia virus. SFV and vaccinia ligase deficiencies create essentially identical phenotypes. In particular, ligase-deficient SFV strains are mildly UV sensitive and etoposide resistant, phenotypes previously shown to characterize ligase-deficient vaccinia strains. Moreover, we find that ligase mutations can inhibit the growth of both SFV and vaccinia virus in vitro. The poor growth observed in the absence of a viral ligase is correlated with a two- to tenfold reduction in viral and extragenomic DNA synthesis. This phenotype is host dependent. No differences in viral growth or DNA yield were seen when vaccinia strains were cultured on rabbit (SIRC) cells, but ligase deficiencies reduced growth and DNA yields when vaccinia was plated on BSC-40 cells or SFV on SIRC cells. Despite these replicative defects, mutational inactivation of SFV ligase produced no detectable increase in the number of viral DNA breaks and had no effect on virus-catalyzed extragenomic DNA recombination or UV repair. We conclude that poxviral ligases do play a role in viral DNA replication, but the replicative defect is obscured in some cell lines.

An adenoviral vector deleted for all viral coding sequences results in enhanced safety and extended expression of a leptin transgene.

Adenoviral (Ad)-mediated in vivo gene transfer and expression are limited in part by cellular immune responses to viral-encoded proteins and/or transgene immunogenicity. In an attempt to diminish the former responses, we have previously developed and described helper-dependent (HD) Ad vectors in which the viral protein coding sequences are completely eliminated. These HD vectors have up to 37 kb insert capacity, are easily propagated in a Cre recombinase-based system, and can be produced to high concentration and purity (>99.9% helper-free vector). In this study, we compared safety and efficacy of leptin gene delivery mediated by an HD vector (HD-leptin) and a first-generation E1-deleted Ad vector (Ad-leptin) in normal lean and ob/ob (leptin-deficient) mice. In contrast to evidence of liver toxicity, inflammation, and cellular infiltration observed with Ad-leptin delivery in mice, HD-leptin delivery was associated with a significant improvement in associated safety/toxicity and resulted in efficient gene delivery, prolonged elevation of serum leptin levels, and associated weight loss. The greater safety, efficient gene delivery, and increased insert capacity of HD vectors are significant improvements over current Ad vectors and represent favorable features especially for clinical gene therapy applications.

Genomic DNA transfer with a high-capacity adenovirus vector results in improved in vivo gene expression and decreased toxicity.

Many applications for human gene therapy would be facilitated by high levels and long duration of physiologic gene expression. Adenoviral vectors are frequently used for gene transfer because of their high cellular transduction efficiency in vitro and in vivo. Expression of viral proteins and the low capacity for foreign DNA limits the clinical application of first- and second-generation adenoviral vectors. Adenoviral vectors with all viral coding sequences deleted offer the prospect of decreased host immune responses to viral proteins, decreased cellular toxicity of viral proteins and increased capacity to accommodate large regulatory DNA regions. Currently most vectors used in vivo for preclinical and clinical studies express cDNAs under the control of heterologous eukaryotic or viral promoters. Using an adenoviral vector with all viral coding sequences deleted and containing the complete human alpha1-antitrypsin (PI) locus, we observed tissue-specific transcriptional regulation in cell culture and in vivo; intravenous injection in mice resulted in high levels of very stable expression for more than ten months and decreased acute and chronic toxicity. These results indicate significant advantages of regulated gene expression using genomic DNA for gene transfer and of adenoviral gene transfer vectors devoid of all viral coding sequences.