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					  58                          Gene Therapy
                              John S. Lazo and Jennifer Rubin Grandis

    Most drugs used today are designed to treat symp-          tient in a phase I gene therapy protocol. This death has
toms rather than cure the underlying disease. Notable          stimulated a substantial review of the oversight mecha-
exceptions include cytotoxic chemotherapeutic agents,          nisms in human gene transfer research. One of the first
as described in Chapter 56, and agents that restore or         successes of gene therapy was reported in 2000, when
modulate hormone function, as outlined in Chapter 57.          three infants with a fatal form of severe combined im-
However, increased understanding of the molecular              munodeficiency syndrome (SCID) received ex vivo
and genetic etiology of diseases may permit permanent          gene therapy with a recombinant mouse leukemia viral
modification of organ function by drug-oriented meth-           vector encoding the C receptor gene. After 10 months,
ods. The first disease-associated gene, -globin, was              C transgene expression in T- and NK cells was de-
cloned over 25 years ago. It is now theoretically possible     tected and T-, B-, and NK-cell counts and function were
to isolate, sequence, and analyze genes causally associ-       comparable to those of age-matched controls.
ated with many heritable and acquired human diseases,              Although numerous obstacles must be overcome
including cystic fibrosis, Duchenne’s muscular dystro-          before gene therapy will be routinely employed, a rig-
phy, and Gaucher’s disease. Moreover, with the com-            orous approach to investigating the safety and efficacy
plete sequencing of the human genome, many of the es-          of gene transfer will ensure that clinical strategies em-
timated 100,000 human genes may become candidates              ploying genetic manipulation are rationally incorpo-
for genetic manipulations. Thus, it is now possible to         rated into the therapeutic armamentarium.
propose molecular pharmacological and genetic ap-
proaches to therapy . Many of these approaches fall un-
der the general rubric of gene therapy.
    Germ cell gene therapy will require considerable dis-
                                                               GENE THERAPY: DEFINITION
cussion about ethical issues and extensive information
                                                               AND GOALS
before it can be applied to humans, but somatic cell gene      The broadest definition of human gene therapy includes
therapy in humans is now being extensively explored.           the in vivo (direct administration of the gene therapy
During the past 5 years in the United States alone, more       formulation) and ex vivo (transfection of cells in tissue
than 500 human gene therapy clinical trials aimed at           culture by gene therapy followed by administration of
treating conditions ranging from inherited disorders           the transfected material into the patient) transfer of de-
such as cystic fibrosis to cancer and AIDS, have been ap-       fined genetic material to cells of patients. Principles of
proved by the Office of Biotechnology Activities (OBA,          gene therapy include transfer of one or more transgenes
formerly the Recombinant DNA Advisory Committee)               to prevent a disease, prevent an adverse consequence of
of the National Institutes of Health. Nearly 3500 patients     a disease, or facilitate recovery from the consequence.
have been enrolled in these studies (Fig. 58.1).               Although most of the controversy and excitement have
    With few exceptions, gene therapy was considered           centered on the transfer of functional genes, the thera-
safe if not particularly effective until the death of an 18-   peutic potential of genes that abrogate aberrant func-
year-old man in 1999, the first fatal outcome for a pa-         tion (e.g. antisense and ribonucleic acid–based strate-

                                                                58 Gene Therapy                                                 667

                                                                         the attributes and problems of conventional endocrine
                         Diagnostic markers 8%                           or antimicrobial therapy with respect to efficiency of
        diseases 8%                 Other diseases 2%                    targeting and the duration of effect. A second approach
                                                                         seeks permanent alteration of the genotype of the cell,
  Infectious                                                             leading to a modified phenotype that prevents or alters
  disease 7%                                                             a disease state. In this setting, gene therapy will perma-
                                                                         nently modify organ function.
                                                                             Theoretically, mutated or nonfunctional genes could
                                                                         be excised and replaced, and new genes with desired
  Monogenic                                                              functions could be permanently inserted into the
  diseases 12%                                                           genome. Stable integration of an antisense DNA might
                                                                         also be desirable in some circumstances. Because of the
                                        Cancer 63%                       technical difficulties associated with the delivery of nu-
                                                                         cleic acid–based products selectively to specific target
                                                                         cells in vivo, more experimental information is available
                                                                         for ex vivo human gene therapy.
Proposed uses or targets of human gene therapy trials.
During the past 5 years in the United States more than 500
human gene therapy clinical trials have been approved by
the Office of Biotechnology Activities of the National                    ANTISENSE
Institutes of Health.
                                                                         The antisense approach is use of nucleic acids to reduce
                                                                         the expression of a specific target gene. As shown in
gies) should also be considered. Two fundamental ap-                     Figure 58.2, a small piece of DNA, an oligodeoxynu-
proaches underlie the basis of gene therapy. In the first,                cleotide that is in the reverse orientation (antisense) to a
genetic material is introduced into cells to alter the cel-              portion of a target messenger RNA (mRNA) species, is
lular phenotype but not the genotype. This is typified by                 introduced into a cell and a DNA–RNA duplex is formed
the transfer of unintegrated DNA, antisense oligomers,                   by complementary Watson-Crick base pairing. Cessation
and ribozymes. In this regard, gene therapy has many of                  of protein synthesis then may result from the rapid

                         DNA Insert

                                               Vector                                       Antisense Oligomer

                      into genome                       hnRNA          Sense mRNA

                                          hnRNA                        Antisense
                                                                                                          DNA RNA
                                     Nucleus                                                              duplex
                                                                                    RNA RNA

                                                                                                         Ribonuclease H


Translation arrest or nuclease digestion by exogenously applied antisense oligonucleotides or by
antisense mRNA produced from DNA delivered by a plasmid. Heterogeneous nuclear RNA is
668                                                 VI CHEMOTHERAPY

degradation of the mRNA species due to activation of ri-       tion (Fig. 58.4). Transcription factor decoys that are du-
bonuclease H or disruption of translation. Cells and or-       plexes designed to bind to a particular transcription fac-
ganisms protect themselves against foreign DNA and             tor and prevent its normal function are another ap-
RNA by producing nucleases that degrade phosphodi-             proach examined in the context of NF B blockade.
ester bonds in oligodeoxynucleotides. Chemical modifi-          These strategies, like antisense itself, do not require in-
cation of the phosphodiester moiety can produce nucle-         tegration into the genome, and thus they share the phar-
ase-resistant oligomers. In the two most common                macological problems of absorption, distribution, me-
chemical analogues, the backbone phosphate is replaced         tabolism, and elimination of any traditional drug not
either with a methyl group to form a methyl phosphonate        based on nucleic acid.
or with a sulfur group to form a phosphorothioate (Fig.
58.3). These modifications grant extra stability to the
oligonucleotides, allowing for a longer half-life in vivo.
                                                               GENE EXCISION AND REPLACEMENT
    The antisense RNA can also be generated within
cells after delivery via a plasmid or attenuated virus         Diseases at a genetic level can result from several
containing a suitable promoter that controls expression        causes, including (1) mutation in a gene, (2) loss of ex-
of the antisense strand using methods of gene insertion        pression of a gene, (3) elevated expression of a gene, or
described later (Fig. 58.2). In addition to the strict anti-   (4) expression of a pathogenic viral or foreign gene. In
sense strategies, several related approaches have been         each case, gene replacement or excision therapy might
considered. Catalytic RNA, catalytic DNA, or ribo-             be desirable. Theoretically, the disease gene could be re-
zymes capable of degrading complementary mRNA                  placed through a homologous recombination event.
may decrease translation of targeted sequences. Oli-           Depending on the design of the replacement gene, it
gomers designed to interact with genes directly via            also would be possible to engineer stop codons or non-
Hoogsteen hydrogen binding in a triplex formation              sense sequences into the internal domains of a gene to
have been suggested as a means of disrupting transcrip-        ensure loss of protein production. Excision of an entire

                            O         Base

                        O         O
                        O         O          n


                            O         Base

                        O         O
                        S         O          n
                     Phosphorothioate                                      Definition of symbols:

                                                                                  Triplex           Decoy

                            O         Base

                        O         O
                                                                                  DNA               Transcription
                         CH3      O          n

FIGURE 58.3                                                    FIGURE 58.4
Chemical structures of oligodeoxynucleotides and the           Theoretical mechanism of transcription disruption by
analogues used in gene therapy.                                oligomers.
                                                      58 Gene Therapy                                              669

gene also is feasible. This strategy, however, requires ex-     GENE ADDITION
tremely efficient and specific homologous recombina-
tion events in the target cell population. Such strategies      A more practical approach has been to permit the in-
have allowed for the development of knockout animals,           troduced genes to integrate into the genome in a site-
but to date have not been practical for human somatic           nonspecific manner. The newly added gene could then
cell gene therapy. Ongoing investigations are exploring         function to provide a missing or mutated gene product
the feasibility of inducible vectors, use of the cre-lox sys-   (Fig. 58.5A). This is the approach of most current gene
tem, or cell type specific promoters to optimize gene ex-        therapy protocols and is exemplified by the development
pression in target cells.                                       of clinical trials for adenosine deaminase (ADA) defi-






                                                                                prodrug        active drug




Possible mechanisms by which inserted therapeutic genes may alter cellular function. A. Gene
addition with return to a normal phenotype. B. Dominant-negative or phenotype deletion.
C. Gene addition to a unique phenotype, such as an enzyme that activates a prodrug.
670                                                   VI CHEMOTHERAPY

ciency, which is an example of inherited SCID. ADA is               in the presence of systemic administration of a nontoxic
a reasonable target for these reasons: (1) It is an auto-           prodrug. The transfected enzyme in the tumor cells con-
somal recessive disorder in which a defect in a single              verts the prodrug, such as ganciclovir, to an active cyto-
gene produces absence of or diminished ADA activity                 toxic compound. Theoretically, such an approach selec-
with fatal combined immunodeficiency. (2) ADA ex-                    tively kills tumor cells and is nontoxic to untransfected
pression is characteristic of a normal maintenance gene             cells. Clinical trials to assess the safety and efficacy of
with considerable variation in the normal ADA levels,               enzyme–prodrug cancer therapy are under way.
suggesting that stringent regulation of expression is un-
necessary. (3) A significant level of expression is not re-
quired to correct the phenotype. (4) Ex vivo gene trans-
                                                                    DELIVERY SYSTEMS
fer studies can be conducted. (5) Replacement of ADA
may reduce the production of toxic DNA metabolites                  In many cell types it is feasible to deliver nucleic acids
and thus provide a growth advantage for transfected                 and genes by a variety of methods when the cells are
cells.                                                              grown in tissue culture (Table 58.1). Nonetheless, some
    For ethical reasons, children enrolled in these clini-          cells, such as pneumocytes and neurons, are not readily
cal trials have also received standard therapy of enzyme            isolated from humans and do not grow well in vitro.
infusions, so the results of these studies have been diffi-          Furthermore, for many diseases it is essential to alter the
cult to interpret and are controversial. Nevertheless,              phenotype of a significant proportion of the total cell
there is some evidence that the ex vivo gene transfer ap-           population, making ex vivo gene therapy of limited use.
proach may evoke a biological response relevant to the                  There is general agreement that no ideal delivery
treatment of ADA deficiency. Such interpretations have               system is available for in vivo gene therapy. Direct or in-
stimulated efforts to use the ex vivo strategy for other            tratumoral injection of plasmid DNA or antisense
monogenic disorders, such as familial hypercholes-                  oligomers without a viral vector has been attempted.
terolemia, hemophilia B, and Gaucher’s disease.                     Expression of genes using traditional nonviral vectors
    Alternatively, the introduced gene could generate a             has been low compared to viral strategies. Nonetheless,
protein that acts to block or suppress the function of an-          recent breakthroughs in nonviral delivery systems, in-
other undesirable protein in a dominant-negative man-               cluding the gene gun, electroporation and naked DNA,
ner (Fig. 58.5B). Last, the introduced gene could result in         suggest that nonviral gene therapy can achieve local ex-
the production of an entirely new and unique protein                pression of therapeutic genes at levels equivalent to
that provides the recipient cell with a desirable pheno-            those of viral vectors.
type (Fig. 58.5C). In theory, an enzyme required for the                Although the mechanism remains undetermined, the
metabolic activation of a prodrug could be expressed,               injection of naked DNA into skeletal muscle has demon-
leading to the desired pharmacological activity near the            strated relatively high transfection efficiency. In this set-
genetically altered cell. This approach is used in cancer           ting, DNA is precipitated onto the surface of microscopic
gene therapy in which tumor cells are transfected with a            metal beads (e.g., gold) and the microprojectiles are ac-
gene encoding for an enzyme such as thymidine kinase                celerated and penetrate intact tissue to several cell layers.

      TA B L E      5 8 . 1 Vectors Approved for Human Use by the U. S. Office of Biotechnology Activities

      Vector                           Advantages                                    Disadvantages

      Liposomes                        No replication risk, nonimmunogenic,          Limited efficiency
                                        useful for plasmids or viruses
      Naked or particle-mediated DNA   No replication risk                           Moderate efficiency, nonspecific cell targeting
      Retrovirus                       Efficient transfer, manufacturing easy,        Small DNA capacity (9 kb), random DNA inser-
                                         most commonly used                            tion, targets only dividing cells, replication risk
      Adenovirus                       Infects nonproliferating cells, noninte-      Immunogenic, small DNA capacity (7.5 kb),
                                         grating                                       replication risk, repeated injections required
                                                                                       for long-term expression
      Adeno-associated virus           Low immunogenicity, targets nonprolifer-      Difficult to manufacture, low titer
                                         ating cells, may have discrete genome
                                         insertion sites
      Herpesvirus                      Targets central nervous system, low im-       Difficult to manufacture, host toxicity
                                                     58 Gene Therapy                                                  671

In preclinical trials, efficiency remains low, but expression   tionality of protooncogenes in cancer, blocking immune
has been noted to last for several weeks, and there has        cell activity after kidney transplantation, treating rheu-
been no significant inflammatory response.                       matoid arthritis, or influencing autoimmune diseases.
    Some investigators have used electrical current            Studies to date have not reported marked clinical effi-
(electroporation) to improve DNA (or drug) entry into          cacy, which might be due to protein binding and poor
tumor cells with some preliminary success. Liposomes           entry into cells. Additional chemical modifications and
are attractive vehicles for gene delivery, since they can      possibly the use of carriers, such as liposomes, may im-
carry plasmid, antisense, or viral DNA. Compared with          prove drug delivery and utility.
viral approaches, however, liposomes remain relatively             A proportion of the human gene therapy trials ap-
inefficient at facilitating gene transfer, although their       proved by the OBA seek to correct a single-gene de-
safety profile remains more desirable. Some of the at-          fect, such as adenosine deaminase deficiency, gluco-
tributes and limitations of the nonviral methods are           cerebrosidase deficiency in Gaucher’s disease, or the
listed in Table 58.1.                                          mutated chloride transport gene in cystic fibrosis. The
    Because viruses can efficiently integrate into the          major difficulties limiting success have been immuno-
genome, many clinical trials are exploring the use of          genicity associated with the vector delivery system,
replication-defective recombinant viral vectors and de-        low transfection efficiency, and transient transgene ex-
livery systems. Retroviruses contain their genetic infor-      pression.
mation as a double-strand DNA genome that is tran-                 Most human gene therapy trials are designed to ex-
scribed, and the single-strand proviral DNA product is         press a new gene product that facilitates the correction of
stably integrated into the host genome. Recombinant            a disease process, such as cancer. Almost half of the cur-
DNA technology has been used to remove deleterious             rent gene therapy–based protocols in the United States
viral genes involved in replication, and the resulting         are aimed at boosting the immune response to tumor
vector is replication defective, nonpathogenic, and un-        antigens. Thus, there are attempts to express the lym-
able to produce infectious particles. Ideally, with a retro-   phokine interleukin-2 in tumor cells to stimulate a nat-
viral vector, only a single administration should be re-       ural immune response against the producing tumor cell
quired because the gene should be permanently                  and its malignant neighbors. In other types of studies,
retained and expressed. No clinical evidence of mutage-        malignant cells infected with a vector that encodes a tu-
nesis has emerged from the clinical trials performed to        mor suppressor gene, p53, lead to growth arrest, apo-
date, but the number of patients treated and the time of       ptosis or enhanced sensitivity to cytotoxic agents.
exposure has been limited.                                     Others have used vectors encoding the herpesvirus pro-
    Adenoviral vectors have also been used in human            tein thymidine kinase that target cells for killing when
trials. These vectors enter cells by either an adenovirus      exposed to the antiviral prodrug ganciclovir; this is
fiber–specific receptor or a surface integrin receptor.          known as suicide gene therapy. Similarly, attempts are
They efficiently transfer genes in nonreplicating and           being made to produce HIV-infected cells that express
replicating cells. Nonetheless, immunological responses        thymidine kinase or other enzymes that activate the
to viruses have been noted with adenoviral vectors.            nontoxic prodrugs to cytotoxic compounds. Disruption
Replication-selective adenovirus vectors have been in-         of viral functions with decoy molecules that compete
troduced to optimize infection of target cells and mini-       with, sequester, or cleave products produced by HIV
mize infection of normal cells. Over 200 cancer patients       also is being examined.
have been treated to date in more than 10 clinical trials          Most of these trials have been early phase I or II
with little evidence of toxicity reported. Replication,        studies that are designed to evaluate safety rather
however, has generally been transient ( 10 days), with         than efficacy of the gene therapy formulation. Results
limited efficacy observed when the gene therapy was             of ongoing and pending phase III studies will more
administered as a single agent. More encouraging anti-         precisely place the role of gene therapy in a clinical
tumor effects have been observed when the gene ther-           context. Although the feasibility of human gene trans-
apy was combined with cytotoxic chemotherapy.                  fer has been demonstrated in the completed clinical
Further modifications are likely to be required before          trials, there has been a paucity of evidence to support
there can be general application of adenoviral vectors         the efficacy and reliability of gene transfer ap-
for cancer therapy.                                            proaches. Future gene therapy studies will capitalize
                                                               on preclinical efforts to improve cellular targeting,
                                                               gene transfer efficiency, and sustained expression.
                                                               Regulation of the expression of the introduced trans-
                                                               gene would be desirable, and use of cell type–specific
                                                               promoters, such as the actin or surfactant promoter,
Antisense clinical trials, most with phosphorothioates,        or drug-controlled promoters, such as the tetracy-
have been directed toward blocking viral production in         cline promoter, are being examined in preclinical
patients with AIDS or genital warts, disrupting the func-      models.
672                                               VI CHEMOTHERAPY

      Study Questions

1. Severe combined immunodeficiency (SCID) syn-                 (A) Deletion of viral genes will reduce toxicity of
   dromes are excellent models for gene therapy be-            the viral vector to normal cells.
   cause of the genetic basis of these disorders and sig-      (B) Deletion of a p53 inhibitory protein will be se-
   nificant advances in the technology to transfer              lective for tumors that have lost p53 function.
   therapeutic genes into hematopoietic precursor              (C) Deletion of a key regulatory sequence will al-
   cells. For all these reasons, which of the following        low for induction of the therapeutic gene in tumor
   syndromes represents an ideal candidate for gene            cells.
   therapy?                                                    (D) Results of preclinical studies suggest that only
   (A) B-cell deficiency                                        tumor cells are affected by this treatment.
   (B) DiGeorge’s syndrome                                     (E) Clinical results support that only patients with
   (C) C Deficiency                                             p53 mutations in their tumors respond to the treat-
   (D) Adenine deaminase deficiency                             ment.
   (E) T-cell deficiency
2. All of the following are desirable characteristics in    ANSWERS
   the design of a gene therapy vector EXCEPT               1. C. SCID-X1 ( C deficiency) is an optimal model
   (A) Ability to produce at high titer on a commer-           for gene therapy because there is little C gene
   cial scale                                                  transcription regulation; C expression is ubiquitous
   (B) Ability to transfect both dividing and nondi-           and constitutive among different hematopoietic lin-
   viding cells                                                eages; and C exerts no autonomous function.
   (C) Ability to produce site-specific integration into     2. D. The vector should have no size limit to the ge-
   the chromosome of the target cell                           netic material it can deliver. The coding sequence of
   (D) Ability to limit size of genetic material it can        a therapeutic gene can vary from several hundred
   deliver                                                     base pairs to more than 10,000 base pairs. In addi-
   (E) Ability to deliver only certain cell types              tion, the requirement for appropriate regulatory se-
3. A patient with ornithine transcarbamylase (OTC)             quences may be required for efficient transduction
   deficiency is being treated in a gene therapy clinical       and expression of the therapeutic DNA. The ability
   trial. The gene therapy approach for this disease is        to produce a high titer on a commercial scale is es-
   primarily designed to                                       sential to carry out large-scale tests. It is necessary
   (A) Replace the enzyme ornithine transcarbamylase           to be able to transfer genes in nonreplicating and
   (B) Decrease the accumulation of ammonia                    replicating cells. It is also important to optimize de-
   (C) Eliminate the need for a modified diet                   livery to target cells and minimize delivery to nor-
   (D) Target a protooncogene                                  mal cells.
   (E) Enhance the immune system                            3. A. OTC is a metabolic enzyme required to break
4. A 25-year-old hemophiliac is interested in receiving        down ammonia. Total lack of this enzyme leads to
   gene therapy. He should contemplate all of the fol-         death shortly after birth owing to a buildup of am-
   lowing approaches EXCEPT                                    monia. The partial presence of OTC also leads to
   (A) Intravenous infusion of a retroviral vector ex-         accumulation of ammonia, which can be controlled
   pressing the B-domain-deleted factor VIII                   by drugs and dietary intake. The genetic cause of
   (B) Ex vivo transfection of autologous fibroblasts           this disease, its morbidity, and the need for rapid
   transfected with a plasmid encoding B-domain-               production of OTC by adenoviral vectors may ex-
   deleted factor VIII                                         tend the life span of OTC-deficient newborns to al-
   (C) Intravenous adenoviral-mediated delivery of             low for drug treatment and dietary manipulation.
   factor VIII                                                 Jesse Gelsinger, the 18-year-old patient who was the
   (D) Adeno-associated virus (AAV) vector deliv-              first patient to die on a phase I gene therapy trial,
   ered to skeletal muscle                                     had OTC deficiency.
   (E) Retroviral vector expressing B-domain deleted        4. C. Systemic administration of adenoviral vectors
   factor VIII transfected into dermal fibroblasts that         has not been used in the treatment of hemophilia
   are then reimplanted                                        because of the transient gene expression and im-
5. A patient with advanced inoperable squamous cell            munogenic consequences of adenoviral delivery. All
   carcinoma of the head and neck receives a replica-          of the other approaches are under investigation or
   tion-selective adenovirus on a gene therapy clinical        have been published in the literature on treatment
   trial. The rationale for the use of this treatment:         of hemophilia.
                                                     58 Gene Therapy                                                673

5. B. dl1520 (Onyx-015) was the first adenovirus de-             Somia N and Verma IM. Gene therapy: Trials and
   veloped with deletion of a gene encoding a p53-in-               tribulations. Nature Rev Genet 2000;1:91–99.
   hibitory protein, E1B-55kD, theoretically making it          Friedman T, Noguchi P, and Mickelson C. The evolution
   selective for tumor cells that have lost p53 function.           of public review and oversight mechanisms in hu-
   Controversial data demonstrate that the mechanism                man gene transfer research: Joint roles of the FDA
   of selectivity is more complex than originally                   and NIH. Curr Opin Biotechnol 2001;12:304–307.
   thought. In addition, clinical results have demon-           Mannucci PM and Tuddenham EGD. The hemophilias:
   strated responses in patients whose tumors did not               From royal genes to gene therapy. N Engl J Med
   have mutant p53.                                                 2001;344:1773–1784.
                                                                Kirn D. Clinical research results with dl1520 (Onyz-
SUPPLEMENTAL READING                                                015), a replication-selective adenovirus for the
Huber BE and Lazo JS. (eds.). Gene therapy for neo-                 treatment of cancer: What have we learned? Gene
    plastic diseases. Ann N Y Acad Sci 1994;716;1–351.              Therapy 2001;8:89–98.
Fischer A et al. Gene therapy for human severe com-             McCormick F. Cancer gene therapy: Fringe or cutting
    bined immunodeficiencies. Immunity 2001;15:1–4.                  edge? Nature Rev Cancer 2001;1:130–141.

    Case        Study        Cystic Fibrosis and Gene Therapy

     K     ris Allen was diagnosed with cystic fibrosis
           (CF) shortly after birth. Genetic analysis re-
     vealed that he had the most common form of dys-
                                                                 ANSWER: Aerosol delivery of the CFTR gene. Both
                                                                 viruses and liposome–DNA complexes are capable
                                                                 of successful CFTR gene transfer to the nasal and
     function of the CF transmembrane conductance                airway epithelia of patients with CF. In fact, gene
     regulator gene (CFTR) leading to faulty processing          transfer to the airways is one of the few areas where
     and protein trafficking. His therapy to date has con-        liposome–DNA complexes match the expression
     sisted of palliative treatments, such as daily physio-      obtained using viral vectors without the viruses’ in-
     therapy to improve chest and lung function, pancre-         flammatory side effects. Current trials are aimed at
     atic enzyme replacement, and a high calorie diet.           optimizing gene delivery with reduced toxicity to
     Conventional treatment of his recurrent pulmonary           produce sustained correction of the epithelial trans-
     disease is less and less effective, and he is interested    port defect.
     in gene therapy. What would be a logical strategy
     for this patient?

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