Gene therapy is a recently developed concept for which a wide range of applications can be and have been envisaged.
In gene therapy a molecule carrying genetic information is introduced into some or all cells of a host, as a result of which the genetic information is added to the host in a functional format.
The genetic information added may be a gene or a derivative of a gene, such as a cDNA, which encodes a protein. In this case the functional format means that protein can be expressed by the machinery of the host cell.
The genetic information can also be a sequence of nucleotides complementary to a sequence of nucleotides (be it DNA or RNA) present in the host cell. The functional format in this case is that the added DNA (nucleic acid) molecule or copies made thereof in situ are capable of base pairing with the complementary sequence present in the host cell.
Applications include the treatment of genetic disorders by supplementing a protein or other substance which is, through said genetic disorder, not present or at least present in insufficient amounts in the host, the treatment of tumors and (other) acquired diseases such as (auto)immune diseases or infections, etc.
As may be clear from the above, there are basically three different approaches in gene therapy, one directed towards compensating a deficiency present in a (mammalian) host; the second directed towards the removal or elimination of unwanted substances (organisms or cells) and the third towards application of a recombinant vaccine (tumors or foreign micro-organisms).
For the purpose of gene therapy, adenoviruses carrying deletions have been proposed as suitable vehicle. Adenoviruses are non-enveloped DNA viruses. Gene-transfer vectors derived from adenoviruses (so-called adenoviral vectors) have a number of features that make them particularly useful for gene transfer for such purposes. Eg. the biology of the adenoviruses is characterized in detail, the adenovirus is not associated with severe human pathology, the virus is extremely efficient in introducing its DNA into the host cell, the virus can infect a wide variety of cells and has a broad host-range, the virus can be produced in large quantities with relative ease, and the virus can be rendered replication defective by deletions in the early-region 1 (E1) of the viral genome.
The adenovirus genome is a linear double-stranded DNA molecule of approximately 36000 base pairs with the 55-kDa terminal protein covalently bound to the 5'terminus of each strand. The Ad DNA contains identical Inverted Terminal Repeats (ITR) of about 100 base pairs with the exact length depending on the serotype. The viral origins of replication are located within the ITRs exactly at the genome ends. DNA synthesis occurs in two stages. First, the replication proceeds by strand displacement, generating a daughter duplex molecule and a parental displaced strand. The displaced strand is single stranded and can form a so-called "panhandle" intermediate, which allows replication initiation and generation of a daughter duplex molecule. Alternatively, replication may proceed from both ends of the genome simultaneously, obviating the requirement to form the panhandle structure. The replication is summarized in FIG. 14 adapted from (Lechner and Kelly, 1977).
During the productive infection cycle, the viral genes are expressed in two phases: the early phase, which is the period upto viral DNA replication, and the late phase, which coincides with the initiation of viral DNA replication. During the early phase only the early gene products, encoded by regions E1, E2, E3 and E4, are expressed, which carry out a number of functions that prepare the cell for synthesis of viral structural proteins (Berk, 1986). During the late phase the late viral gene products are expressed in addition to the early gene products and host cell DNA and protein synthesis are shut off. Consequently, the cell becomes dedicated to the production of viral DNA and of viral structural proteins (Tooze, 1981).
The E1 region of adenovirus is the first region of adenovirus expressed after infection of the target cell. This region consists of two transcriptional units, the E1A and E1B genes, which both are required for oncogenic transformation of primary (embryonal) rodent cultures. The main functions of the E1A gene products are:
i) to induce quiescent cells to enter the cell cycle and resume cellular DNA synthesis, and PA1 ii) to transcriptionally activate the E1B gene and the other early regions (E2, E3, E4). Transfection of primary cells with the E1A gene alone can induce unlimited proliferation (immortalization), but does not result in complete transformation. However, expression of E1A in most cases results in induction of programmed cell death (apoptosis), and only occasionally immortalization is obtained (Jochemsen et al., 1987). Co-expression of the E1B gene is required to prevent induction of apoptosis and for complete morphological transformation to occur. In established immortal cell lines, high level expression of E1A can cause complete transformation in the absence of E1B (Roberts et al., 1985). PA1 a. E1 sequences, to allow replication of E1/E2 defective adenoviruses, and PA1 b. E2A sequences, containing the hr mutation and the ts 125 mutation, named ts400 (Brough et al., 1985; Rice and Klessig, 1985) to prevent cell death by E2A overexpression, and/or PA1 c. E2A sequences, must containing the hr mutation, under the control of an inducible promoter, and/or PA1 d. E2A sequences, containing the hr mutation and the ts 125 mutation (ts400), under the control of an inducible promoter PA1 1. a novel packaging cell line derived from diploid human embryonic retinoblasts (HER) that harbors nt. 80-5788 of the Ad5 genome. This cell line, named 911; deposited under no 95062101 at the ECACC, has many characteristics that make it superior to the commonly used 293 cells (Fallaux et al., 1996). PA1 2. novel packaging cell lines that express just E1A genes and not E1B genes. Established cell lines (and not human diploid cells of which 293 and 911 cells are derived) are able to express E1A to high levels without undergoing apoptotic cell death, as occurs in human diploid cells that express E1A in the absence of E1B. Such cell lines are able to trans-complement E1B-defective recombinant adenoviruses, because viruses mutated for E1B 21 kD protein are able to complete viral replication even faster than wild-type adenoviruses (Telling et al., 1994). The constructs are described in detail below, and graphically represented in FIGS. 1-5. The constructs are transfected into the different established cell lines and are selected for high expression of E1A. This is done by operatively linking a selectable marker gene (e.g. NEO gene) directly to the E1B promoter. The E1B promoter is transcriptionally activated by the E1A gene product and therefore resistance to the selective agent (e.g. G418 in the case NEO is used as the selection marker) results in direct selection for desired expression of the E1A gene PA1 3. Packaging constructs that are mutated or deleted for E1B 21 kD, but just express the 55 kD protein. PA1 4. Packaging constructs to be used for generation of complementing packaging cell lines from diploid cells (not exclusively of human origin) without the need of selection with marker genes. These cells are immortalized by expression of E1A. However, in this particular case expression of E1B is essential to prevent apoptosis induced by E1A proteins. Selection of E1 expressing cells is achieved by selection for focus formation (immortalization), as described for 293 cells (Graham et al., 1977) and 911 calls (Fallaux et al, 1996), that are E1-transformed human embryonic kidney (HEK) cells and human embryonic retinoblasts (HER), respectively. PA1 5. After transfection of HER cells with construct pIG.E1B (FIG. 4), seven independent cell lines could be established. These cell lines were designated PER.C1, PER.C3, PER.C4, PER.C5, PER.C6, PER.C8 and PER.C9. PER denotes PGK-E1-Retinoblasts. These cell lines express E1A and E1B proteins, are stable (e.g. PER.C6 for more than 57 passages) and complement E1 defective adenovirus vectors. Yields of recombinant adenovirus obtained on PER cells are a little higher than obtained on 293 cells. One of these cell lines (PER.C6) has been deposited at the ECACC under number 96022940. PA1 6. New adenovirus vectors with extended E1 deletions (deletion nt.459-3510). Those viral vectors lack sequences homologous to E1 sequences in said packaging cell lines. These adenoviral vectors contain pIX promoter sequences and the pIX gene, as pIX (from its natural promoter sequences) can only b expressed from the vector and not by packaging cells (Matsui et al, 1986, Hoeben and Fallaux, pers.comm.; Imler et al., 1996). PA1 7. E2A expressing packaging cell lines preferably based on either E1A expressing established cell lines or E1A-E1B expressing diploid cells (see under 2-4). E2A expression is either under the control of an inducible promoter or the E2A ts125 mutant is driven by either an inducible or a constitutive promoter. PA1 8. Recombinant adenovirus vectors as described before (see 6) but carrying an additional deletion of E2A sequences. PA1 9. Adenovirus packaging cells from monkey origin that are able to trans-complement E1-defective recombinant adenoviruses. They are preferably co-transfected with pIG.E1AE1B and pIG.NEO, and selected for NEO resistance. Such cells expressing E1A and E1B are able to transcomplement E1 defective recombinant human adenoviruses, but will do so inefficiently because of a block of the synthesis of late adenovirus proteins in cells of monkey origin (Klessig and Grodzicker, 1979). To overcome this problem, we generate recombinant adenoviruses that harbor a host-range mutation in the E2A gene, allowing human adenoviruses to replicate in monkey cells. Such viruses are generated as described in FIG. 12, except DNA from a hr-mutant is used for homologous recombination. PA1 10. Adenovirus packaging cells from monkey origin as described under 9, except that they will also be co-transfected with E2A sequences harboring the hr-mutation. This allows replication of human adenoviruses lacking E1 and E2A (see under 8). E2A in these cell lines is either under the control of an inducible promoter or the tsE2A mutant is used. In the latter case, the E2A gene will thus carry both the ts mutation and the hr mutation (derived from ts400). Replication competent human adenoviruses have been described that harbor both mutations (Brough et al., 1985; Rice and Klessig, 1985).
The E1B coded protein assist E1A in redirecting the cellular functions to allow viral replication. The E1B 55 kD and E4 33 kD proteins, which form a complex that is essentially localized in the nucleus, function in inhibiting the synthesis of host proteins and in facilitating the expression of viral genes. Their main influence is to establish selective transport of viral mRNAs from the nucleus to the cytoplasm, concomittantly with the onset of the late phase of infection. The E1B 21 kD protein is important for correct temporal control of the productive infection cycle, thereby preventing premature death of the host cell before the virus life cycle has been completed. Mutant viruses incapable of expressing the E1B 21 kD gene-product exhibit a shortened infection cycle that is accompanied by excessive degradation of host cell chromosomal DNA (deg-phenotype) and in an enhanced cytopathic effect (cyt-phenotype) (Telling et al., 1994). The deg and cyt phenotypes are suppressed when in addition the E1A gene is mutated, indicating that these phenotypes are a function of E1A (white et al., 1988). Furthermore, the E1B 21 kDa protein slows down the rate by which E1A switches on the other viral genes. It is not yet known through which mechanisms) E1B 21 kD quenches these E1A dependent functions.
Vectors derived from human adenoviruses, in which at least the E1 region has been deleted and replaced by a gene of interest, have been used extensively for gene therapy experiments in the pre-clinical and clinical phase.
As stated before all adenovirus vectors currently used in gene therapy have a deletion in the E1 region, where novel genetic information can be introduced. The E1 deletion renders the recombinant virus replication defective (Stratford-Perricaudet and Perricaudet, 1991). We have demonstrated that recombinant adenoviruses are able to efficiently transfer recombinant genes to the rat liver and airway epithelium of rhesus monkeys (Bout et al., 1994b; Bout et al., 1994a). In addition, we (Vincent et al., 1996a; Vincent et al., 1996b) and others (see e.g. Haddada et al., 1993) have observed a very efficient in vivo adenovirus mediated gene transfer to a variety of tumor cells in vitro and to solid tumors in animals models (lung tumors, glioma) and human xenografts in immunodeficient mice (lung) in vivo (reviewed by Blaese et al., 1995).
In contrast to for instance retroviruses, adenoviruses a) do not integrate into the host cell genome; b) are able to infect non-dividing cells and c) are able to efficiently transfer recombinant genes in vivo (Brody and Crystal, 1994). Those features make adenoviruses attractive candidates for in vivo gene transfer of, for instance, suicide or cytokine genes into tumor cells.
However, a problem associated with current recombinant adenovirus technology is the possibility of unwanted generation of replication competent adenovirus (RCA) during the production of recombinant adenovirus (Lochmuller et al., 1994; Imler et al., 1996). This is caused by homologous recombination between overlapping sequences from the recombinant vector and the adenovirus constructs present in the complementing cell line, such as the 293 cells (Graham et al., 1977). RCA in batches to be used in clinical trials is unwanted because RCA i) will replicate in an uncontrolled fashion; ii) can complement replication defective recombinant adenovirus, causing uncontrolled multiplication of the recombinant adenovirus and iii) batches containing RCA induce significant tissue damage and hence strong pathological side effects (Lockmuller et al., 1994). Therefore, batches to be used in clinical trials should be proven free of RCA (Ostrove, 1994). In one aspect of the invention this problem in virus production is solved in that we have developed packaging cells that have no overlapping sequences with a new basic vector and thus are suited for safe large scale production of recombinant adenoviruses one of the additional problems associated with the use of recombinant adenovirus vectors is the host-defence reaction against treatment with adenovirus.
Briefly, recombinant adenoviruses are deleted for the E1 region (see above). The adenovirus E1 products trigger the transcription of the other early genes (E2, E3, E4), which consequently activate expression of the late virus genes. Therefore, it was generally thought that E1 deleted vectors would not express any other adenovirus genes. However, recently it has been demonstrated that some cell types are able to express adenovirus genes in the absence of E1 sequences. This indicates, that some cell types possess the machinery to drive transcription of adenovirus genes. In particular, it was demonstrated that such cells synthesize E2A and late adenovirus proteins.
In a gene therapy setting, this means that transfer of the therapeutic recombinant gene to somatic cells not only results in expression of the therapeutic protein but may also result in the synthesis of viral proteins. Cells that express adenoviral proteins are recognized and killed by Cytotoxic T Lymphocytes, which thus a) eradicates the transduced cells and b) causes inflammations (Bout et al., 1994a; Engelhardt et al., 1993; Simon et al, 1993). As this adverse reaction is hampering gene therapy, several solutions to this problem have been suggested, such as a) using immunosuppressive agents after treatment; b) retainment of the adenovirus E3 region in the recombinant vector (see patent application EP 95202213) and c) and using ts mutants of human adenovirus, which have a point mutation in the E2A region (patent WO/28938).
However, these strategies to circumvent the immune response have their limitations.
The use of ts mutant recombinant adenovirus diminishes the immune response to some extent, but was less effective in preventing pathological responses in the lungs (Engelhardt et al., 1994a).
The E2A protein may induce an immune response by itself and it plays a pivotal role in the switch to the synthesis of late adenovirus proteins. Therefore, it is attractive to make recombinant adenoviruses which are mutated in the E2 region, rendering it temperature sensitive (ts), as has been claimed in patent application WO/28938.
A major drawback of this system is the fact that, although the E2 protein is unstable at the non-permissive temperature, the immunogenic protein is still being synthesized. In addition, it is to be expected that the unstable protein does activate late gene expression, albeit to a low extent. ts125 mutant recombinant adenoviruses have been tested, and prolonged recombinant gene expression was reported (Yang et al., 1994b; Engelhardt et al., 1994a; Engelhardt et al., 1994b; Yang et al., 1995). However, pathology in the lungs of cotton rats was still high (Engelhardt et al., 1994a), indicating that the use of ts mutants results in only a partial improvement in recombinant adenovirus technology. Others (Fang et al., 1996) did not observe prolonged gene expression in mice and dogs using ts125 recombinant adenovirus. An additional difficulty associated with the use of ts125 mutant adenoviruses is that a high frequency of reversion is observed. These revertant are either real revertants or the result of second site mutations (Kruijer et al., 1983; Nicolas et al., 1981). Both types of revertants have an E2A protein that functions at normal temperature and have therefore similar toxicity as the wild-type virus.
In another aspect of the present invention we therefore delete E2A coding sequences from the recombinant adenovirus genome and transfect these E2A sequences into the (packaging) cell lines containing E1 sequences to complement recombinant adenovirus vectors.
Major hurdles in this approach are a) that E2A should be expressed to very high levels and b) that E2A protein is very toxic to cells.
The current invention in yet another aspect therefore discloses uses of the ts125 mutant E2A gene, which produces a protein that is not able to bind DNA sequences at the non permissive temperature. High levels of this protein may be maintained in the cells (because it is not toxic at this temperature) until the switch to the permissive temperature is made. This can be combined with placing the mutant E2A gene under the direction of an inducible promoter, such as for instance tet, methallothionein, steroid inducible promoter, retinoic acid .beta.-receptor or other inducible systems. However in yet another aspect of the invention, the use of an inducible promoter to control the moment of product of toxic wild-type E2A is disclosed.
Two salient additional advantages of E2A-deleted recombinant adenovirus are the increased capacity to harbor heterologous sequences and the permanent selection for cells that express the mutant E2A. This second advantage relates to the high frequency of reversion of ts125 mutation: when reversion occurs in a cell line harboring ts125 E2A, this will be lethal to the cell. Therefore, there is a permanent selection for those cells that express the ts125 mutant E2A protein. In addition, as we in one aspect of the invention generate E2A-deleted recombinant adenovirus, we will not have the problem of reversion in our adenoviruses.
In yet another aspect of the invention as a further improvement the use of non-human cell lines as packaging cell lines is disclosed.
For GMP production of clinical batches of recombinant viruses it is desirable to use a cell line that has been used widely for production of other biotechnology products. Most of the latter cell lines are from monkey origin, which have been used to produce e.g. vaccines. These cells can not be used directly for the production of recombinant human adenovirus, as human adenovirus can not or only to low levels replicate in cells of monkey origin. A block in the switch of early to late phase of adenovirus lytic cycle is underlying defective replication. However, host range mutations in the human adenovirus genome are described (hr400-404) which allow replication of human viruses in monkey cells. These mutations reside in the gene encoding E2A protein (Klessig and Grodzicker, 1979; Klessig et al., 1984; Rice and Klessig, 1985) (Klessig et al., 1984). Moreover, mutant viruses have been described that harbor both the hr and temperature-sensitive ts125 phenotype (Brough et al., 1985; Rice and Klessig, 1985).
We therefore generate packaging cell lines of monkey origin (e.g. VERO, CV1) that harbor:
Furthermore we disclose the construction of novel and improved combinations of (novel and improved) packaging cell lines and (novel and improved) recombinant adenovirus vectors. We provide:
A further aspect of the invention provides otherwise improved adenovirus vectors, as well as novel strategies for generation and application of such vectors and a method for the intracellular amplification of linear DNA fragments in mammalian cells.