Throughout this application, various publications are referred to by citations within parentheses and in the bibliographic description, immediately preceding the claims.  The disclosures of these publications are hereby incorporated by reference into the present disclosure to more fully describe the state of the art to which this invention pertains. 
Production of recombinant adenoviruses useful for gene therapy requires the use of a cell line capable of supplying in trans the gene products of the viral E1 region which are deleted in these recombinant viruses.  At present the only useful cell line available is the 293 cell line originally described by Graham et al.  in 1977.  293 cells contain approximately the left hand 12% (4.3 kb) of the adenovirus type 5 genome (Aiello (1979) and Spector (1983)). 
Adenoviral vectors currently being tested for gene therapy applications typically are deleted for Ad2 or Ad5 DNA extending from approximately 400 base pairs from the 5′ end of the viral genome to approximately 3.3 kb from the 5′ end, for a total E1 deletion of 2.9 kb.  Therefore, there exists a limited region of homology of approximately 1 kb between the DNA sequence of the recombinant virus and the Ad5 DNA within the cell line.  This homology defines a region of potential recombination between the viral and cellular adenovirus sequences.  Such a recombination results in a phenotypically wild-type virus bearing the Ad5 E1 region from the 293 cells.  This recombination event presumably accounts for the frequent detection of wild-type adenovirus in preparations of recombinant virus and has been directly demonstrated to be the cause of wild-type contamination of the Ad2 based recombinant virus Ad2/CFTR-1 (Rich et al. ( 1993)). 
Due to the high degree of sequence homology within the type C adenovirus subgroup such recombination is likely to occur if the vector is based on any group C adenovirus (types 1, 2, 5, 6). 
In small scale production of recombinant adenoviruses, generation of contaminating wild-type virus can be managed by a screening process which discards those preparations of virus found to be contaminated.  As the scale of virus production grows to meet expected demand for genetic therapeutics, the likelihood of any single lot being contaminated with a wild-type virus also will rise as well as the difficulty in providing non-contaminated recombinant preparations. 
There will be over one million new cases of cancer diagnosed this year, and half that number of cancer-related deaths (American Cancer Society, 1993).  p53 mutations are the most common genetic alteration associated with human cancers, occurring in 50-60% of human cancers (Hollstein et al. ( 1991); Bartek et al. ( 1991); Levine (1993)).  The goal of gene therapy in treating p53 deficient tumors, for example, is to reinstate a normal, functional copy of the wild-type p53 gene so that control of cellular proliferation is restored.  p53 plays a central role in cell cycle progression, arresting growth so that repair or apoptisis can occur in response to DNA damage.  Wild-type p53 has recently been identified as a necessary component for apoptosis induced by irradiation or treatment with some chemotherapeutic agents (Lowe et al. ( 1993) A and B).  Due to the high prevalence of p53 mutations in human tumors, it is possible that tumors which have become refractory to chemotherapy and irradiation treatments may have become so due in part to the lack of wild-type p53.  By resupplying functional p53 to these tumors, it is reasonable that they now are susceptible to apoptisis normally associated with the DNA damage induced by radiation and chemotherapy. 
One of the critical points in successful human tumor suppressor gene therapy is the ability to affect a significant fraction of the cancer cells.  The use of retroviral vectors has been largely explored for this purpose in a variety of tumor models.  For example, for the treatment of hepatic malignancies, retroviral vectors have been employed with little success because these vectors are not able to achieve the high level of gene transfer required for in vivo gene therapy (Huber, B. E. et al.,  1991; Caruso M. et al.,  1993). 
To achieve a more sustained source of virus production, researchers have attempted to overcome the problem associated with low level of gene transfer by direct injection of retroviral packaging cell lines into solid tumors (Caruso, M. et al.,  1993; Ezzidine, Z. D. et al.,  1991; Culver, K. W. et al.,  1992).  However, these methods are unsatisfactory for use in human patients because the method is troublesome and induces an inflammatory response against the packaging cell line in the patient.  Another disadvantage of retroviral vectors is that they require dividing cells to efficiently integrate and express the recombinant gene of interest (Huber, B. E. 1991).  Stable integration into an essential host gene can lead to the development or inheritance of pathogenic diseased states. 
Recombinant adenoviruses have distinct advantages over retroviral and other gene delivery methods (for review, see Siegfried (1993)).  Adenoviruses have never been shown to induce tumors in humans and have been safely used as live vaccines (Straus (1984)).  Replication deficient recombinant adenoviruses can be produced by replacing the E1 region necessary for replication with the target gene.  Adenovirus does not integrate into the human genome as a normal consequence of infection, thereby greatly reducing the risk of insertional mutagenesis possible with retrovirus or adeno-associated viral (AAV) vectors.  This lack of stable integration also leads to an additional safety feature in that the transferred gene effect will be transient, as the extrachromosomal DNA will be gradually lost with continued division of normal cells.  Stable, high titer recombinant adenovirus can be produced at levels not achievable with retrovirus or AAV, allowing enough material to be produced to treat a large patient population.  Moreover, adenovirus vectors are capable of highly efficient in vivo gene transfer into a broad range of tissue and tumor cell types.  For example, others have shown that adenovirus mediated gene delivery has a strong potential for gene therapy for diseases such as cystic fibrosis (Rosenfeld et al. ( 1992); Rich et al. ( 1993)) and α1-antitrypsin deficiency (Lemarchand et al. ( 1992)).  Although other alternatives for gene delivery, such as cationic liposome/DNA complexes, are also currently being explored, none as yet appear as effective as adenovirus mediated gene delivery. 
As with treating p53 deficient tumors, the goal of gene therapy for other tumors is to reinstate control of cellular proliferation.  In the case of p53, introduction of a functional gene reinstates cell cycle control allowing for apoptotic cell death induced by therapeutic agents.  Similarly, gene therapy is equally applicable to other tumor suppressor genes which can be used either alone or in combination with therapeutic agents to control cell cycle progression of tumor cells and/or induce cell death.  Moreover, genes which do not encode cell cycle regulatory proteins, but directly induce cell death such as suicide genes or, genes which are directly toxic to the cell can be used in gene therapy protocols to directly eliminate the cell cycle progression of tumor cells. 
Regardless of which gene is used to reinstate the control of cell cycle progression, the rationale and practical applicability of this approach is identical.  Namely, to achieve high efficiencies of gene transfer to express therapeutic quantities of the recombinant product.  The choice of which vector to use to enable high efficiency gene transfer with minimal risk to the patient is therefore important to the level of success of the gene therapy treatment. 
Thus, there exists a need for vectors and methods which provide high level gene transfer efficiencies and protein expression which provide safe and effective gene therapy treatments.  The present invention satisfies this need and provides related advantages as well.