Tropism of AAV-2 vectors for neurons of the globus pallidus

Selective targeting of striatal parvalbumin-expressing interneurons for transgene delivery

PV^Cre mice--> combined with AAV-FLEX vectors allowed efficient and specific targeting of PV⁺ interneurons in the striatum. However, diffusion of viral particles to the globus pallidus caused massive transduction of PV⁺ projection neurons and subsequent anterograde transport of the transgene product to the subthalamic nucleus and the substantia nigra pars reticulata. Different AAV serotypes (1 and 9) and promoters (CBA and human synapsin) were evaluated. The combination of AAV1, a moderate expression level (human synapsin promoter) and a precise adjustment of the stereotaxic coordinates in the anterior and dorsolateral part of the striatum were necessary to avoid transduction of PV⁺ GP projection neurons. Even in the absence of direct transduction due to diffusion of viral particles, GP PV⁺ projection neurons could be retrogradely transduced via their terminals present in the dorsal striatum. However, in the absence of diffusion, GP-Str PV⁺ projection neurons were poorly or not transduced suggesting that retrograde transduction did not significantly impair the selective targeting of striatal PV⁺ neurons. Finally, a prominent reduction of the number of striatal PV⁺ interneurons (about 50 %) was evidenced in the presence of the Cre recombinase suggesting that functional effects of AAV-mediated transgene expression in PV⁺ striatal interneurons in PV^Cre mice should be analyzed with caution.

Efficient Early and Sustained Transduction of Human Fetal Mesencephalon Using Adeno-Associated Virus Type 2 Vectors

The success of transplantation of human fetal mesencephalic tissue into the putamen of patients with Parkinson's disease (PD) is still limited by the poor survival of the graft. In animal models of fetal transplantation for PD, antiapoptotic agents, such as growth factors or caspase inhibitors, or agents counteracting oxidative stress enhance the survival and reinnervation potential of the graft. Genetic modification of the transplant could allow a local and continuous delivery of these factors at physiologically relevant doses. The major challenge remains the development of strategies to achieve both early and sustained gene delivery in the absence of vector-mediated toxicity. We recently reported that E14 rat fetal mesencephalon could be efficiently tranduced by adeno-associated virus type 2 (AAV2) vectors and that gene expression was maintained until at least 3 months after transplantation in the adult rat striatum. Here we report that an AAV2 vector can mediate the expression of the EGFP reporter gene under the control of a CMV promoter in organotypic cultures of freshly explanted solid fragments of human fetal mesencephalic tissue as early as 3 days to at least 6 weeks postinfection. These results suggest that AAV2 vectors could be used to genetically modify the human fetal tissue prior to transplantation to Parkinson's patients to promote graft survival and integration.

A next step in adeno-associated virus-mediated gene therapy for neurological diseases: regulation and targeting

Recombinant adeno-associated virus (rAAV) vectors mediating long term transgene expression are excellent gene therapy tools for chronic neurological diseases. While rAAV2 was the first serotype tested in the clinics, more efficient vectors derived from the rh10 serotype are currently being evaluated and other serotypes are likely to be tested in the near future. In addition, aside from the currently used stereotaxy-guided intraparenchymal delivery, new techniques for global brain transduction (by intravenous or intra-cerebrospinal injections) are very promising. Various strategies for therapeutic gene delivery to the central nervous system have been explored in human clinical trials in the past decade. Canavan disease, a genetic disease caused by an enzymatic deficiency, was the first to be approved. Three gene transfer paradigms for Parkinson's disease have been explored: converting L-dopa into dopamine through AADC gene delivery in the putamen; synthesizing GABA through GAD gene delivery in the overactive subthalamic nucleus and providing neurotrophic support through neurturin gene delivery in the nigro-striatal pathway. These pioneer clinical trials demonstrated the safety and tolerability of rAAV delivery in the human brain at moderate doses. Therapeutic effects however, were modest, emphasizing the need for higher doses of the therapeutic transgene product which could be achieved using more efficient vectors or expression cassettes. This will require re-addressing pharmacological aspects, with attention to which cases require either localized and cell-type specific expression or efficient brain-wide transgene expression, and when it is necessary to modulate or terminate the administration of transgene product. The ongoing development of targeted and regulated rAAV vectors is described.

Reversible neurochemical changes mediated by delayed intrastriatal glial cell line-derived neurotrophic factor gene delivery in a partial Parkinson's disease rat model

BACKGROUND

Efficient protection of dopaminergic neurons against a subsequent 6-hydroxydopamine lesion by glial cell line-derived neurotrophic factor (GDNF) gene delivery has been demonstrated. By contrast, the neurorestorative effects of GDNF administered several weeks after the toxin have been less characterized. In particular, whether these were permanent or dependent on the continuous presence of GDNF remains elusive.

METHODS

A tetracycline-inducible adeno-associated virus (AAV)-1 vector expressing human GDNF cDNA was administered unilaterally in the rat striatum 5 weeks after 6-hydroxydopamine. Rats were treated with doxycycline (dox) or untreated from the day of vector injection until sacrifice (4 or 14 weeks). A sub-group was dox-treated for 7 weeks then untreated until 14 weeks. The motor behavior was assessed by amphetamine-induced rotations and spontaneous forelimb asymmetry. The amounts of tyrosine hydroxylase (TH), serine-40-phosphorylated TH (S40-TH) and aromatic amino acid decarboxylase (AADC) proteins were compared by western blotting and the dopamine levels quantified by high-performance liquid chromatography.

RESULTS

Dox-dependent behavioral improvements were demonstrated 4 weeks post-vector injection. At later time points, spontaneous partial recovery was observed in all rats, but no further improvement was found in dox-treated animals. TH levels were significantly increased in dox-treated rats at all time points. By contrast, striatal dopamine and S40-TH were increased at 4 weeks, but not 14 weeks, and AADC remained unchanged. Dox withdrawal after 7 weeks, resulted in TH levels comparable to the controls at 14 weeks.

CONCLUSIONS

Delayed GDNF gene delivery only transiently improved dopaminergic function. Over the long term, TH was more abundant, but not functional, and the increase was lost when GDNF gene expression was switched off.

Intrabody gene therapy ameliorates motor, cognitive, and neuropathological symptoms in multiple mouse models of Huntington's disease

Huntington's disease (HD) is an autosomal dominant neurodegenerative disease resulting from the expansion of a glutamine repeat in the huntingtin (Htt) protein. Current therapies are directed at managing symptoms such as chorea and psychiatric disturbances. In an effort to develop a therapy directed at disease prevention we investigated the utility of highly specific, anti-Htt intracellular antibodies (intrabodies). We previously showed that V(L)12.3, an intrabody recognizing the N terminus of Htt, and Happ1, an intrabody recognizing the proline-rich domain of Htt, both reduce mHtt-induced toxicity and aggregation in cell culture and brain slice models of HD. Due to the different mechanisms of action of these two intrabodies, we then tested both in the brains of five mouse models of HD using a chimeric adeno-associated virus 2/1 (AAV2/1) vector with a modified CMV enhancer/chicken beta-actin promoter. V(L)12.3 treatment, while beneficial in a lentiviral model of HD, has no effect on the YAC128 HD model and actually increases severity of phenotype and mortality in the R6/2 HD model. In contrast, Happ1 treatment confers significant beneficial effects in a variety of assays of motor and cognitive deficits. Happ1 also strongly ameliorates the neuropathology found in the lentiviral, R6/2, N171-82Q, YAC128, and BACHD models of HD. Moreover, Happ1 significantly prolongs the life span of N171-82Q mice. These results indicate that increasing the turnover of mHtt using AAV-Happ1 gene therapy represents a highly specific and effective treatment in diverse mouse models of HD.

Differential transgene expression profiles in rat brain, using rAAV2/1 vectors with tetracycline-inducible and cytomegalovirus promoters

The biodistribution of transgene expression in the CNS after localized stereotaxic vector delivery is an important issue for the safety of gene therapy for neurological diseases. The cellular specificity of transgene expression from rAAV2/1 vectors (recombinant adeno-associated viral vectors pseudotyped with viral capsids from serotype 1) using the tetracycline-inducible (TetON) expression cassette in comparison with the cytomegalovirus (CMV) promoter was investigated in the rat nigrostriatal pathway. After intrastriatal injection, although green fluorescent protein (GFP) was expressed mainly in neurons with both vectors, the relative proportions of DARPP-32-positive projection neurons and parvalbumin-positive interneurons were, respectively, 13:1 and 2:1 for the CMV and TetON vectors. DARP32-positive neurons projecting to the globus pallidus were strongly GFP positive with both vectors, whereas those projecting to the substantia nigra pars reticulata (SNpr) were efficiently labeled by the CMV vector but poorly by the TetON vector. Numerous GFP-positive cells were evidenced in the subventricular zone with both vectors. However, in the olfactory bulb (OB), GFP-positive neurons were observed with the CMV vector but not the TetON vector. We conclude that the absence of significant amounts of transgene product in distant regions (SN and OB) constitutes a safety advantage of the AAV2/1-TetON vector for striatal gene therapy. Midbrain injections resulted in selective GFP expression in tyrosine hydroxylase-positive neurons by the TetON vector whereas with the CMV vector, GFP-positive cells covered a widespread area of the midbrain. The biodistribution of GFP protein corresponded to that of the transcripts and not of the viral genomes. We conclude that the rAAV2/1-TetON vector constitutes an interesting tool for specific transgene expression in midbrain dopaminergic neurons.

Adeno-associated virus-mediated gene delivery approaches for the treatment of CNS disorders

Over the last few years, a large number of preclinical and clinical studies have demonstrated the potential of gene therapy applications using adeno-associated viral (AAV) vectors. Gene transfer via AAV vectors has been particularly successful for the treatment or adjunct therapy of several CNS disorders. The present review summarizes the progress on AAV gene delivery models for three different CNS disorders. In particular, we discuss advances in AAV-mediated gene transfer strategies in animal models of Parkinson's disease, Alzheimer's disease and spinal cord trauma and summarize the results from the first clinical studies using AAV systems.

Controlled delivery of glial cell line-derived neurotrophic factor by a single tetracycline-inducible AAV vector

An autoregulated tetracycline-inducible recombinant adeno-associated viral vector (rAAV-pTet(bidi)ON) utilizing the rtTAM2 reverse tetracycline transactivator (rAAV-rtTAM2) was used to conditionally express the human GDNF cDNA. Doxycycline, a tetracycline analog, induced a time- and dose-dependent release of GDNF in vitro in human glioma cells infected with rAAV-rtTAM2 serotype 2 virus. Introducing the Woodchuck hepatitis virus posttranscriptional regulatory element (WPRE) downstream to the rtTAM2 coding sequence, resulted in a more rapid induction and a higher basal expression level. In vivo, 8 weeks after a single injection of the rAAV-rtTAM2-GDNF vector encapsidated into AAV serotype 1 capsids in the rat striatum, the GDNF protein level was 60 pg/mg tissue in doxycycline-treated animals whereas in untreated animals, it was undistinguishable from the endogenous level ( approximately 4 pg/mg tissue). However, a residual GDNF expression in the uninduced animals was evidenced by a sensitive immunohistochemical staining. As compared to rAAV1-rtTAM2-GDNF, the rAAV1-rtTAM2-WPRE-GDNF vector expressed a similar concentration of GDNF in the induced state (with doxycycline) but a basal level (without doxycycline) approximately 2.5-fold higher than the endogenous striatal level. As a proof for biological activity, for both vectors, downregulation of tyrosine hydroxylase was evidenced in dopaminergic terminals of doxycycline-treated but not untreated animals. In conclusion, the rAAV1-rtTAM2 vector which expressed biologically relevant doses of GDNF in the striatum in response to doxycycline with a basal level undistinguishable from the endogenous striatal level, as measured by quantitative ELISA assay, constitutes an interesting tool for local conditional transgenesis.

Protection from the toxicity of diisopropylfluorophosphate by adeno-associated virus expressing acetylcholinesterase

Organophosphorus esters (OP) are highly toxic chemicals used as pesticides and nerve agents. Their acute toxicity is attributed to inhibition of acetylcholinesterase (AChE, EC 3.1.1.7) in nerve synapses. Our goal was to find a new therapeutic for protection against OP toxicity. We used a gene therapy vector, adeno-associated virus serotype 2 (AAV-2), to deliver murine AChE to AChE-/- mice that have no endogenous AChE activity. The vector encoded the most abundant form of AChE: exons 2, 3, 4, and 6. Two-day old animals, with an immature immune system, were injected. AChE delivered intravenously was expressed up to 5 months in plasma, liver, heart, and lung, at 5-15% of the level in untreated wild-type mice. A few mice formed antibodies, but antibodies did not block AChE activity. The plasma AChE was a mixture of dimers and tetramers. AChE delivered intramuscularly had 40-fold higher activity levels than in wild-type muscle. None of the AChE was collagen-tailed. No retrograde transport through the motor neurons to the central nervous system was detected. AChE delivered intrastriatally assembled into tetramers. In brain, the AAV-2 vector transduced neurons, but not astrocytes and microglia. Vector-treated AChE-/- mice lived longer than saline-treated controls. AChE-/- mice were protected from diisopropylfluorophosphate-induced respiratory failure when the vector was delivered intravenously, but not intrastriatally. Since vector-treated animals had no AChE activity in diaphragm muscle, protection from respiratory failure came from AChE in other tissues. We conclude that AChE scavenged OP and in this way protected the activity of butyrylcholinesterase (BChE, EC 3.1.1.8) in motor endplates.

The "perivascular pump" driven by arterial pulsation is a powerful mechanism for the distribution of therapeutic molecules within the brain

We investigated the movement of interstitially infused macromolecules within the central nervous system (CNS) in rats with high and low blood pressure (BP)/heart rate and in rats euthanized immediately before infusion (no heart action). Adeno-associated virus 2 (AAV2), fluorescent liposomes, or bovine serum albumin was infused into rat striatum (six hemispheres per group) by convection-enhanced delivery (CED). After infusion, distribution volumes were evaluated. The rats with high BP/heart rate displayed a significantly larger distribution of the infused molecules within the injected site and more extensive transport of those molecules to the globus pallidus. This difference was particularly apparent for AAV2, for which a 16.5-fold greater distribution of viral capsids was observed in the rats with high BP/heart rate than in the rats with no heartbeat. Similar results were observed for liposomes, despite their larger diameter. The distribution of all infused molecules in all rats that had low or no blood flow was confined to the space around brain blood vessels. These findings show that fluid circulation within the CNS through the perivascular space is the primary mechanism by which viral particles and other therapeutic agents administered by CED are spread within the brain and that cardiac contractions power this process.

Treatment of human disease by adeno-associated viral gene transfer

During the past decade, in vivo administration of viral gene transfer vectors for treatment of numerous human diseases has been brought from bench to bedside in the form of clinical trials, mostly aimed at establishing the safety of the protocol. In preclinical studies in animal models of human disease, adeno-associated viral (AAV) vectors have emerged as a favored gene transfer system for this approach. These vectors are derived from a replication-deficient, non-pathogenic parvovirus with a single-stranded DNA genome. Efficient gene transfer to numerous target cells and tissues has been described. AAV is particularly efficient in transduction of non-dividing cells, and the vector genome persists predominantly in episomal forms. Substantial correction, and in some instances complete cure, of genetic disease has been obtained in animal models of hemophilia, lysosomal storage disorders, retinal diseases, disorders of the central nervous system, and other diseases. Therapeutic expression often lasted for months to years. Treatments of genetic disorders, cancer, and other acquired diseases are summarized in this review. Vector development, results in animals, early clinical experience, as well as potential hurdles and challenges are discussed.

Neuroprotective adeno-associated virus Bcl-xL gene transfer in models of motor neuron disease

Recent work implicates excitotoxicity-induced apoptosis as the mechanism triggering motor neuron death in amyotrophic lateral sclerosis (ALS). Our laboratory has previously utilized glutamate excitotoxicity in vitro to study this process. The present experiment tests whether overexpression of the gene for Bcl-xL can inhibit excitotoxicity in this model system. To track Bcl-xL expression, the gene for green fluorescent protein (GFP) was inserted in-frame, upstream of the Bcl-xL gene. The GFP-Bcl-xL gene was then cloned into an adeno-associated viral (AAV2) vector. GFP expression in both SH-SY5Y and embryonic day 15 (E15) motor neurons (MNs) peaked 48 hours after infection. Bcl-xL expression in SH-SY5Y cells significantly reduced terminal deoxy-UTP nick-end labeling (TUNEL)-positive cells and maintained cell density after glutamate exposure. Similarly, Bcl-xL expression inhibited the development of TUNEL staining in E15 MNs and supported cell density after glutamate exposure. These findings suggest that AAV-mediated expression of genes for antiapoptotic proteins may provide a means for ALS gene therapy.

Recombinant adeno-associated viral vectors in the nervous system

Recombinant adeno-associated virus 2 (rAAV2) has been extensively used as a gene delivery vector for the nervous system. It targets primarily neurons in the nervous system and results in sustained long-term expression of transgenes. New rAAV serotypes have been characterized and demonstrated to have improved transduction efficiencies in various regions of the brain and spinal cord. This review discusses some properties of rAAV that have been studied in the nervous system such as cell tropism, duration of transgene expression, and distribution of viral transduction, as well as immunity and regulation of transgene expression issues, all of which are important for optimization of the use of rAAV in the nervous system.

Preferential expression of an AAV-2 construct in NOS-positive interneurons following intrastriatal injection

Most CNS studies using recombinant adeno-associated virus type 2 (rAAV-2) vectors have focused on gene delivery for the purpose of gene therapy. In the present study, we examined the feasibility of using rAAV-2 vectors to study the regulation of preprotachykinin-A (PPT-A) promoter activity in striatal medium spiny projection neurons. An rAAV-2 vector incorporating a PPT promoter fragment (shown previously to confer some cell-specificity of expression in vitro) coupled to a green fluorescent protein (GFP) reporter gene was stereotaxically injected into the rat striatum. Since medium spiny projection neurons represent the predominant neuronal type (90-95%) in the striatum, we predicted that the vast majority of GFP-expressing cells would be of this phenotype. Surprisingly, the transgene was actually expressed in a similar number of medium spiny projection neurons and interneurons, while glial expression of GFP was not observed. A preponderance of GFP-expressing interneurons was immunoreactive for the marker neuronal nitric oxide synthase (nNOS). Our results suggest that viral vector-related events that occur during transduction are the determining factor in the pattern of transgene expression observed, while the influence of the transgene promoter appears to be secondary, at least under the conditions employed.

Recombinant AAV viral vectors pseudotyped with viral capsids from serotypes 1, 2, and 5 display differential efficiency and cell tropism after delivery to different regions of the central nervous system

Recombinant adeno-associated virus 2 (rAAV2) has been shown to deliver genes to neurons effectively in the brain, retina, and spinal cord. The characterization of new AAV serotypes has revealed that they have different patterns of transduction in diverse tissues. We have investigated the tropism and transduction frequency in the central nervous system (CNS) of three different rAAV vector serotypes. The vectors contained AAV2 terminal repeats flanking a green fluorescent protein expression cassette under the control of the synthetic CBA promoter, in AAV1, AAV2, or AAV5 capsids, producing the pseudotypes rAAV2/1, rAAV2/2, and rAAV2/5. Rats were injected with rAAV2/1, rAAV2/2, or rAAV2/5 into selected regions of the CNS, including the hippocampus (HPC), substantia nigra (SN), striatum, globus pallidus, and spinal cord. In all regions injected, the three vectors transduced neurons almost exclusively. All three vectors transduced the SN pars compacta with high efficiency, but rAAV2/1 and rAAV2/5 also transduced the pars reticulata. Moreover, rAAV2/1 showed widespread distribution throughout the entire midbrain. In the HPC, rAAV2/1 and rAAV2/5 targeted the pyramidal cell layers in the CA1-CA3 regions, whereas AAV2/2 primarily transduced the hilar region of the dentate gyrus. In general, rAAV2/1 and rAAV2/5 exhibited higher transduction frequencies than rAAV2/2 in all regions injected, although the differences were marginal in some regions. Retrograde transport of rAAV1 and rAAV5 was also observed in particular CNS areas. These results suggest that vectors based on distinct AAV serotypes can be chosen for specific applications in the nervous system.

Transduction profiles of recombinant adeno-associated virus vectors derived from serotypes 2 and 5 in the nigrostriatal system of rats

We compared the transduction efficiencies and tropisms of titer-matched recombinant adeno-associated viruses (rAAV) derived from serotypes 2 and 5 (rAAV-2 and rAAV-5, respectively) within the rat nigrostriatal system. The two serotypes (expressing enhanced green fluorescent protein [EGFP]) were delivered by stereotaxic surgery into the same animals but different hemispheres of the striatum (STR), the substantia nigra (SN), or the medial forebrain bundle (MFB). While both serotypes transduced neurons effectively within the STR, rAAV-5 resulted in a much larger EGFP-expressing area than did rAAV-2. However, neurons transduced with rAAV-2 vectors expressed higher levels of EGFP. Consistent with this result, EGFP-positive projections emanating from transduced striatal neurons covered a larger area of the SN pars reticulata (SNr) after striatal delivery of rAAV-5, but EGFP levels in fibers of the SNr were higher after striatal injection of rAAV-2. We also compared the potentials of the two vectors for retrograde transduction and found that striatal delivery of rAAV-5 resulted in significantly more transduced dopaminergic cell bodies within the SN pars compacta and ventral tegmental area. Similarly, EGFP-transduced striatal neurons were detected only after nigral delivery of rAAV-5. Furthermore, we demonstrate that after striatal AAV-5 vector delivery, the transduction profiles were stable for as long as 9 months. Finally, although we did not target the hippocampus directly, efficient and widespread transduction of hippocampal neurons was observed after delivery of rAAV-5, but not rAAV-2, into the MFB.

Potential of gene therapy for pediatric neurotransmitter diseases: lessons from Parkinson's disease

Gene therapy methods have continued to develop rapidly, and many initial limitations that hampered clinical application have been overcome. Thus serious consideration of clinical application of gene therapy is warranted for selected disorders in which the pathogenesis is well defined. Parkinson's disease has been the most extensively studied target of gene therapy for central nervous system disorders and shares many features with pediatric neurotransmitter diseases. Neurotransmitter replacement therapy using catecholamine-synthesizing genes and delivery of neurotrophic factors such as glial cell line-derived neurotrophic factors has been successful in animal models of Parkinson's disease. Application of gene therapy for pediatric neurotransmitter diseases will require delineating the optimal set of genes to correct the consequences of the deficiencies. The optimal anatomical targets and proper timing of the gene replacement must be understood. Safety of gene therapy vehicles and the ability to regulate gene expression will be essential for eventual clinical application.

Promoters and regulatory elements that improve adeno-associated virus transgene expression in the brain

Since the first demonstration of central nervous system (CNS) transduction with recombinant adeno-associated virus, improvements in vector production and promoter strength have lead to dramatic increases in the number of cells transduced and the level of expression within each cell. The improvements in promoter strength have resulted from a move away from the original cytomegalovirus (CMV) promoter toward the use of hybrid CMV-based promoters and constitutive cellular promoters. This review summarizes and compares different promoters and regulatory elements that have been used with rAAV as a reference toward achieving a high level of rAAV-mediated transgene expression in the CNS.

AAV2 vectors mediate efficient and sustained transduction of rat embryonic ventral mesencephalon

The success of transplantation of human embryonic mesencephalic tissue to treat parkinsonian patients is limited by the poor survival of the transplant. We show that an AAV2 vector mediates efficient expression of the egfp reporter gene in organotypic cultures of freshly explanted solid fragments of rat embryonic ventral mesencephalon (VM). We observed early and sustained transgene expression (4 days to > or = 6 weeks). Furthermore, rAAV-infected rat embryonic VM transplanted in the adult striatum continued to express EGFP for > or = 3 months. More than 95% of the transduced cells were neurons. Dopaminergic neurons were transduced at low frequency at earlier time points. This method of gene delivery could prove useful to achieve local, continuous secretion of neurotrophic factors at physiologically relevant doses to treat Parkinson's disease.

Adeno-associated viral vectors penetrate human solid tumor tissue in vivo more effectively than adenoviral vectors

The transduction efficiencies of adeno-associated viral vectors (AAV, serotype 2) and adenovirus vectors (ADV, serotype 5) were examined in three different models of cancer. First, we used flow cytometry to quantitate AAV-GFP or ADV-GFP transduction in 13 cell lines derived from malignant tissue (6 gliomas, 6 mammary cancers, and 1 leukemia). These experiments showed variable transduction efficiency (0%-81%) between the cell lines, with ADV being more effective compared to AAV in 9 of 13 cell lines. Second, spheroids prepared from human glioblastomas were infected with ADV or AAV expressing GFP or lacZ cassettes, and after 2 weeks, uniform reporter gene expression was observed on the spheroid. Whereas AAV produced consistent transduction throughout the spheroids, ADV infection was mainly limited to the outer cell layers of the spheroids, suggesting that AAV were more efficient at penetrating solid tumor tissue. Third, human biopsies from glioblastoma multiforme patients were xenografted into nude rats and grown for 4 weeks followed by viral vector injection. Combined use of high-resolution magnetic resonance imaging (MRI) and histologic analysis allowed the identification of transduced cells and their spatial distribution within the tumors. AAV-mediated transgene expression was observed in cell clusters through the entire tumor, while ADV-mediated transduction was restricted to cells at the tumor periphery. Thus, while AAV and ADV vectors may infect tumor-derived cell lines to a similar degree, AAV penetrated glioblastoma spheroids and xenografts more efficiently compared to ADV vectors. These results suggest that AAV may be suitable for therapeutic gene delivery to malignant tumors.

Recombinant adeno-associated virus serotypes 2- and 5-mediated gene transfer in the mammalian brain: quantitative analysis of heparin co-infusion

Recombinant adeno-associated viruses (rAAVs) are among the most promising vectors for gene delivery into the CNS. However, a major hurdle for gene transfer to the mammalian brain is to achieve high transduction levels in target cells beyond the immediate injection site. Therefore, building upon the optimization of injection parameters on which we have recently reported, it is important to define additional methods to increase the volume of distribution. Here, we establish an optimal heparin concentration, and show that co-injection of heparin together with rAAV2 leads to a significantly higher and more homogeneous distribution of transduced cells. In contrast, the diffusion pattern of rAAV serotype 5 differs from that of rAAV2, in that its distribution is less homogeneous, more variable, and patchy. Furthermore, this study illustrates the influence of receptor binding on the expression pattern following injection of rAAV in the CNS. In addition to improvements in expression cassettes and viral titers and the use of very slow infusion rates, gene transfer studies in the CNS where the goal is to obtain widespread transduction should consider co-injecting the viral vector rAAV2 with heparin to maximize transduction efficiency and viral spread.

Gene therapy: recombinant adeno-associated virus vectors

Gene transfer using recombinant adeno-associated virus (rAAV) vectors shows great promise for human gene therapy. The broad host range, low level of immune response, and longevity of gene expression observed with these vectors in numerous disease paradigms has enabled the initiation of a number of clinical trials using this gene delivery system. This review presents an overview of the current developments in the field of AAV-mediated gene delivery. Such developments include the establishment of new production methods allowing the generation of high titer preparations, improved purification methods, the use of alternative AAV serotypes, and the generation of trans-splicing rAAV genomes. Together, these developments have improved results interpretation, host range, and the coding capacity of rAAV vectors. Furthermore, the recent identification of regions within the viral capsid that are amenable to modification has begun to address the issue of direct rAAV vector targeting, which could potentially allow targeted gene delivery to specific cell populations. The versatility shown by this vector has enabled new diseases to be realistically considered for therapeutic intervention and considerably broadened the scope of gene therapy.