Cystic fibrosis precision therapeutics: Emerging considerations

Small molecules that address fundamental defects underlying cystic fibrosis (CF), including modulators such as the approved drugs ivacaftor, lumacaftor, tezacaftor, and elexacaftor, have advanced dramatically over the past few years and are transforming care and prognosis among individuals with this disease. The new treatment strategies are predicated on established scientific insight concerning pathogenesis, and applying "personalized" or "precision" interventions for specific abnormalities of the cystic fibrosis transmembrane conductance regulator (CFTR). Even with the advent of highly effective triple drug combinations-which hold great promise for the majority of patients with CF worldwide-barriers to precision therapy remain. These include refractory CFTR variants (premature truncation codons, splice defects, large indels, severe missense mutations, and others) not addressed by available modulators, and access to leading-edge therapeutic compounds for patients with ultrarare forms of CF. In addition to describing the remarkable progress that has occurred regarding CF precision medicine, this review outlines some of the remaining challenges. The CF experience is emblematic of many conditions for which personalized interventions are actively being sought.

Non-obstructive vas deferens and epididymis loss in cystic fibrosis rats

This study utilizes morphological and mechanistic endpoints to characterize the onset of bilateral atresia of the vas deferens in a recently derived cystic fibrosis (CF) rat model. Embryonic reproductive structures, including Wolffian (mesonephric) duct, Mullerian (paramesonephric) duct, mesonephric tubules, and gonad, were shown to mature normally through late embryogenesis, with involution of the vas deferens and/or epididymis typically occurring between birth and postnatal day 4 (P4), although timing and degree of atresia varied. No evidence of mucus obstruction, which is associated with pathology in other CF-affected tissues, was observed at any embryological or postnatal time point. Reduced epididymal coiling was noted post-partum and appeared to coincide with, or predate, loss of more distal vas deferens structure. Remarkably, α smooth muscle actin expression in cells surrounding duct epithelia was markedly diminished in CF animals by P2.5 when compared to wild type counterparts, indicating reduced muscle development. RNA-seq and immunohistochemical analysis of affected tissues showed disruption of developmental signaling by Wnt and related pathways. The findings have relevance to vas deferens loss in humans with CF, where timing of ductular damage is not well characterized and underlying mechanisms are not understood. If vas deferens atresia in humans begins in late gestation and continues through early postnatal life, emerging modulator therapies given perinatally might preserve and enhance integrity of the reproductive tract, which is otherwise absent or deficient in 97% of males with cystic fibrosis.

Phase I dose-escalating trial of Escherichia coli purine nucleoside phosphorylase and fludarabine gene therapy for advanced solid tumors

BACKGROUND

The use of Escherichia coli purine nucleoside phosphorylase (PNP) to activate fludarabine has demonstrated safety and antitumor activity during preclinical analysis and has been approved for clinical investigation.

PATIENTS AND METHODS

A first-in-human phase I clinical trial (NCT 01310179; IND 14271) was initiated to evaluate safety and efficacy of an intratumoral injection of adenoviral vector expressing E. coli PNP in combination with intravenous fludarabine for the treatment of solid tumors. The study was designed with escalating doses of fludarabine in the first three cohorts (15, 45, and 75 mg/m(2)) and escalating virus in the fourth (10(11)-10(12) viral particles, VP).

RESULTS

All 12 study subjects completed therapy without dose-limiting toxicity. Tumor size change from baseline to final measurement demonstrated a dose-dependent response, with 5 of 6 patients in cohorts 3 and 4 achieving significant tumor regression compared with 0 responsive subjects in cohorts 1 and 2. The overall adverse event rate was not dose-dependent. Most common adverse events included pain at the viral injection site (92%), drainage/itching/burning (50%), fatigue (50%), and fever/chills/influenza-like symptoms (42%). Analysis of serum confirmed the lack of systemic exposure to fluoroadenine. Antibody response to adenovirus was detected in two patients, suggesting that neutralizing immune response is not a barrier to efficacy.

CONCLUSIONS

This first-in-human clinical trial found that localized generation of fluoroadenine within tumor tissues using E. coli PNP and fludarabine is safe and effective. The pronounced effect on tumor volume after a single treatment cycle suggests that phase II studies are warranted.

CLINICALTRIALSGOV IDENTIFIER

NCT01310179.

DeltaF508 CFTR processing correction and activity in polarized airway and non-airway cell monolayers

We examined the activity of DeltaF508 cystic fibrosis transmembrane conductance regulator (CFTR) stably expressed in polarized cystic fibrosis bronchial epithelial cells (CFBE41o(-)) human airway cells and Fisher Rat Thyroid (FRT) cells following treatment with low temperature and a panel of small molecule correctors of DeltaF508 CFTR misprocessing. Corr-4a increased DeltaF508 CFTR-dependent Cl(-) conductance in both cell types, whereas treatment with VRT-325 or VRT-640 increased activity only in FRT cells. Total currents stimulated by forskolin and genistein demonstrated similar dose/response effects to Corr-4a treatment in each cell type. When examining the relative contribution of forskolin and genistein to total stimulated current, CFBE41o(-) cells had smaller forskolin-stimulated I(sc) following either low temperature or corr-4a treatment (10-30% of the total I(sc) produced by the combination of both CFTR agonists). In contrast, forskolin consistently contributed greater than 40% of total I(sc) in DeltaF508 CFTR-expressing FRT cells corrected with low temperature, and corr-4a treatment preferentially enhanced forskolin dependent currents only in FRT cells (60% of total I(sc)). DeltaF508 CFTR cDNA transcript levels, DeltaF508 CFTR C band levels, or cAMP signaling did not account for the reduced forskolin response in CFBE41o(-) cells. Treatment with non-specific inhibitors of phosphodiesterases (papaverine) or phosphatases (endothall) did not restore DeltaF508 CFTR activation by forskolin in CFBE41o(-) cells, indicating that the Cl(-) transport defect in airway cells is distal to cAMP or its metabolism. The results identify important differences in DeltaF508 CFTR activation in polarizing epithelial models of CF, and have important implications regarding detection of rescued of DeltaF508 CFTR in vivo.

Synthesis and biological activity of 2-fluoro adenine and 6-methyl purine nucleoside analogs as prodrugs for suicide gene therapy of cancer

A novel series of 6-methylpurine nucleoside derivatives with substitutions at 5-position have been synthesised These compounds bear a 5'-heterocycle such as triazole or a imidazole with a two carbon chain, and an ether, thio ether or amine. To extend the SAR study of 2-fluoroadenine and 6-methyl purine nucleosides, their corresponding alpha-linker nucleosides with L-xylose and L-lyxose were also synthesized. All of these compounds have been evaluated for their substrate activity with E. coli PNP.

Activators of viral gene expression in polarized epithelial monolayers identified by rapid-throughput drug screening

Epithelial polarity and tight junction formation limit the ability of adenovirus, retrovirus and adeno-associated virus (AAV) to deliver and express virally encoded genes. Using an extended half-life luciferase assay and high-throughput luminometry, we screened 23 000 compounds and natural product extracts as potentiators to overcome this barrier. Seven strong activators were discovered (up to several hundred fold above control) and two of these exhibited spectrum of activity in multiple cell types (HeLa (human cervical carcinoma), cystic fibrosis bronchial epithelial (human bronchial), HT29 (human colonic carcinoma), Calu3 (airway serous glandular)). Enhanced transduction by unrelated gene transfer vectors (adenovirus, lentivirus, AAV, liposomal) was also observed. These results establish a strategy for identifying compounds that improve viral gene transfer to resistant cell types, and provide new tools for examining epithelial defense against viral infection. The compounds should have broad usefulness in experimental therapies for cancer and genetic diseases.

A(2) adenosine receptors regulate CFTR through PKA and PLA(2)

We investigated adenosine (Ado) activation of the cystic fibrosis transmembrane conductance regulator (CFTR) in vitro and in vivo. A(2B) Ado receptors were identified in Calu-3, IB-3-1, COS-7, and primary human airway cells. Ado elevated cAMP in Calu-3, IB-3-1, and COS-7 cells and activated protein kinase A-dependent halide efflux in Calu-3 cells. Ado promoted arachidonic acid release from Calu-3 cells, and phospholipase A(2) (PLA(2)) inhibition blocked Ado-activated halide efflux in Calu-3 and COS-7 cells expressing CFTR. Forskolin- and beta(2)-adrenergic receptor-stimulated efflux were not affected by the same treatment. Cytoplasmic PLA(2) (cPLA(2)) was identified in Calu-3, IB-3-1, and COS-7 cells, but cPLA(2) inhibition did not affect Ado-stimulated cAMP concentrations. In cftr(+) and cftr(-/-) mice, Ado stimulated nasal Cl(-) secretion that was CFTR dependent and sensitive to A(2) receptor and PLA(2) blockade. In COS-7 cells transiently expressing DeltaF508 CFTR, Ado activated halide efflux. Ado also activated G551D CFTR-dependent halide efflux when combined with arachidonic acid and phosphodiesterase inhibition. In conclusion, PLA(2) and protein kinase A both contribute to A(2) receptor activation of CFTR, and components of this signaling pathway can augment wild-type and mutant CFTR activity.

Chlorzoxazone or 1-EBIO increases Na(+) absorption across cystic fibrosis airway epithelial cells

Previous studies demonstrated that chlorzoxazone or 1-ethyl-2-benzimidazolinone (1-EBIO) enhances transepithelial Cl(-) secretion by increasing basolateral K(+) conductance (G(K)) (Singh AK, Devor DC, Gerlach AC, Gondor M, Pilewski JM, and Bridges RJ. J Pharmacol Exp Ther 292: 778-787, 2000). Hence these compounds may be useful to treat cystic fibrosis (CF) airway disease. The goal of the present study was to determine whether chlorzoxazone or 1-EBIO altered ion transport across Delta F508-CF transmembrane conductance regulator homozygous CFT1 airway cells. CFT1 monolayers exhibited a basal short-circuit current that was abolished by apical amiloride (inhibition constant 320 nM) as expected for Na(+) absorption. The addition of chlorzoxazone (400 microM) or 1-EBIO (2 mM) increased the amiloride-sensitive I(sc) approximately 2.5-fold. This overlapping specificity may preclude use of these compounds as CF therapeutics. Assaying for changes in the basolateral G(K) with a K(+) gradient plus the pore-forming antibiotic amphotericin B revealed that chlorzoxazone or 1-EBIO evoked an approximately 10-fold increase in clotrimazole-sensitive G(K). In contrast, chlorzoxazone did not alter epithelial Na(+) channel-mediated currents across basolateral-permeabilized monolayers or in Xenopus oocytes. These data further suggest that alterations in basolateral G(K) alone can modulate epithelial Na(+) transport.

Evidence that systemic gentamicin suppresses premature stop mutations in patients with cystic fibrosis

Here we report the effects of gentamicin treatment on cystic fibrosis transmembrane regulator (CFTR) production and function in CF airway cells and patients with CF with premature stop mutations. Using immunocytochemical and functional [6-methoxy-N- (3-sulfopropyl) quinolinium (SPQ)-based] techniques, ex vivo exposure of airway cells from stop mutation CF patients led to the identification of surface-localized CFTR in a dose-dependent fashion. Next, five patients with CF with stop mutations and five CF control subjects were treated with parenteral gentamicin for 1 wk, and underwent repeated in vivo measures of CFTR function (nasal potential difference [PD] measurements and sweat chloride [Cl(-)] testing). During the treatment period, the number of nasal PD readings in the direction of Cl(-) secretion was increased approximately 3-fold in the stop mutation patient group compared with controls (p < 0.001), and four of five stop mutation patients with CF had at least one reading during gentamicin treatment with a Cl(-) secretory response of more than -5 mV (hyperpolarized). A response of this magnitude was not seen in any of the CF control subjects (p < 0.05). In an independent series of experiments designed to test the ability of repeat nasal PDs to detect wild-type CFTR function, evidence of Cl(-) secretion was seen in 88% of control (non-CF) nasal PDs, and 71% were more than -5 mV hyperpolarized. Together, these results suggest that gentamicin treatment can suppress premature stop mutations in airway cells from patients with CF, and produce small increases in CFTR Cl(-) conductance (as measured by the nasal PD) in vivo.

A clinical inflammatory syndrome attributable to aerosolized lipid-DNA administration in cystic fibrosis

Immunologic reactivity to lipid-DNA conjugates has traditionally been viewed as less of an issue than with viral vectors. We performed a dose escalation safety trial of aerosolized cystic fibrosis transmembrane conductance regulator (CFTR) cDNA to the lower airways of eight adult cystic fibrosis patients, and monitored expression by RT-PCR. The cDNA was complexed to a cationic lipid amphiphile (GL-67) consisting of a cholesterol anchor linked to a spermine head group. CFTR transgene was detected in three patients at 2-7 days after gene administration. Four of the eight patients developed a pronounced clinical syndrome of fever (maximum of 103.3EF), myalgias, and arthralgia beginning within 6 hr of gene administration. Serum IL-6 but not levels of IL-8, IL-1, TNF-alpha, or IFN-gamma became elevated within 1-3 hr of gene administration. No antibodies to the cationic liposome or plasmid DNA were detected. We found that plasmid DNA by itself elicited minimal proliferation of peripheral blood mononuclear cells taken from study patients, but led to brisk immune cell proliferation when complexed to a cationic lipid. Lipid and DNA were synergistic in causing this response. Cellular proliferation was also seen with eukaryotic DNA, suggesting that at least part of the immunologic response to lipid-DNA conjugates is independent of unmethylated (E. coli-derived) CpG sequences that have previously been associated with innate inflammatory changes in the lung.

In vivo sensitization of ovarian tumors to chemotherapy by expression of E. coli purine nucleoside phosphorylase in a small fraction of cells

This report examines a major barrier to suicide gene therapy in cancer and other diseases: namely, bystander cell killing. Existing vectors for in vivo gene delivery are inefficient and often transduce or transfect less than 1% of target cells. The E. coli PNP gene brings about cellular necrosis under conditions when 1 in 100 to 1 in 1000 cells express the gene product in vitro. In vivo bystander killing at or near this magnitude has not been reported previously. In the present experiments, transfection of cells with the E. coli PNP gene controlled by a SV40 promoter resulted in 30 nmol 6-methyl purine deoxyriboside (MeP-dR) converted per milligram tumor cell extract per hour (or conversion units (CU)). This level of expression led to elimination of entire populations of tumor cells in vitro after treatment with MeP-dR. Much earlier killing was observed using a tat transactivated E. coli PNP vector (approximately seven-fold higher activity, 230 CU). In vivo effects on tumor growth were next examined. Human ovarian tumors transfected with E. coli PNP were excised 5 days after i.p. implantation from the peritoneal cavities of mice in order to determine both E. coli PNP enzymatic activity and the fraction of cells expressing the gene. PNP activity at 5 days after gene transfer was approximately 170 CU and was expressed in approximately 0.1% of the tumor cells as judged by in situ hybridization. The expression of E. coli PNP at this level produced a 30% increase in life span (P < 0.001) and 49% reduction in tumor size (P < 0.005) after MeP-dR treatment, as compared with control tumors. Our observations lead to the conclusion that pronounced bystander killing by E. coli PNP is conferred in vivo, and that vectors capable of transgene expression in as few as one in 1000 cells can produce substantial antitumor effects if expression on a per cell basis is very high.

R-domain interactions with distal regions of CFTR lead to phosphorylation and activation

Cystic fibrosis is caused by the aberrant function of the cystic fibrosis transmembrane conductance regulator (CFTR) protein. We examined whether intramolecular binding interactions involving the regulatory (R) domain contributed to CFTR regulation and function. When the R-domain (amino acids 596-836) was coexpressed with Delta1-836 CFTR (a carboxyl hemi-CFTR beginning immediately after the R-domain), strong binding between the two polypeptides was exhibited. The R-domain that co-immunoprecipitated with Delta1-836 exhibited a slower mobility on SDS-PAGE that resulted from phosphorylation of the protein. A larger CFTR polypeptide that included the R-domain (M837X) also exhibited a phosphorylation-dependent mobility shift when coexpressed with Delta1-836. Moreover, coexpression of M837X and Delta1-836 led to enhanced halide permeability in living cells. The activity, unlike in full-length CFTR, was present without forskolin activation, but still sensitive to the PKA inhibitor, Rp-8-CPT-cAMPS. This PKA inhibition of activity was found to be dependent on the carboxy region of the R-domain, amino acids 723-836. Our results indicate that the R-domain binds CFTR residues after amino acid 836 and that this binding facilitates phosphorylation and CFTR activation. We have also characterized a subdomain within CFTR (residues 723-837) that is necessary for PKA-dependent constitutive activation. Finally, these experiments demonstrate that constitutive CFTR activity can be accomplished by at least two mechanisms: (1) direct modulation of the R-domain to abrogate PKA regulation and (2) modifications that increase R-domain susceptibility to steady-state phosphorylation through PKA.

Epithelial P2X purinergic receptor channel expression and function

P2X purinergic receptor (P2XR) channels bind ATP and mediate Ca(2+) influx--2 signals that stimulate secretory Cl(-) transport across epithelia. We tested the hypotheses that P2XR channels are expressed by epithelia and that P2XRs transduce extracellular ATP signals into stimulation of Cl(-) transport across epithelia. Electrophysiological data and mRNA analysis of human and mouse pulmonary epithelia and other epithelial cells indicate that multiple P2XRs are broadly expressed in these tissues and that they are active on both apical and basolateral surfaces. Because P2X-selective agonists bind multiple P2XR subtypes, and because P2X agonists stimulate Cl(-) transport across nasal mucosa of cystic fibrosis (CF) patients as well as across non-CF nasal mucosa, P2XRs may provide novel targets for extracellular nucleotide therapy of CF.

CFTR involvement in chloride, bicarbonate, and liquid secretion by airway submucosal glands

Previous studies demonstrated that ACh-induced liquid secretion by porcine bronchi is driven by active Cl(-) and HCO(-)(3) secretion. The present study was undertaken to determine whether this process was localized to submucosal glands and mediated by the cystic fibrosis transmembrane conductance regulator (CFTR). When excised, cannulated, and treated with ACh, porcine bronchi secreted 15.6 +/- 0.6 microliter. cm(-2). h(-1). Removal of the surface epithelium did not significantly affect the rate of secretion, indicating that the source of the liquid was the submucosal glands. Pretreatment with diphenylamine-2-carboxylate, a relatively nonselective Cl(-)-channel blocker, significantly reduced liquid secretion by 86%, whereas pretreatment with DIDS, which inhibits a variety of Cl(-) channels but not CFTR, had no effect. When bronchi were pretreated with glibenclamide or 5-nitro-2-(3-phenylpropylamino)benzoic acid (both inhibitors of CFTR), the rate of ACh-induced liquid secretion was significantly reduced by 39 and 91%, respectively, compared with controls. Agents that blocked liquid secretion also caused disproportionate reductions in HCO(-)(3) secretion. Polyclonal antibodies to the CFTR bound preferentially to submucosal gland ducts and the surface epithelium, suggesting that this channel was localized to these sites. These data suggest that ACh-induced gland liquid secretion by porcine bronchi is driven by active secretion of both Cl(-) and HCO(-)(3) and is mediated by the CFTR.

Components of human papillomavirus that activate transcription and support plasmid replication in human airway cells

Human papillomaviruses (HPVs) such as types 6 and 11 can establish lifelong infections in airway epithelial cells in patients, and long-term infection can lead to pulmonary involvement and death. The mechanisms underlying this persistence depend on both the transcriptional activity of the viral enhancers and promoters and the ability of this virus to maintain its double-stranded circular DNA genome in infected tissues. We investigated the transcription and replication properties of HPV sequence elements and protein products in a human airway cell line. We showed that incorporation of the upstream regulatory region and cotransfection with expression vectors of two virus-encoded proteins, E1 and E2, conferred approximately 5,000-fold stimulation of reporter gene expression. Transient plasmid replication in transfected human airway cells and lungs of FVB/N-C57BL/6 mice was demonstrated by a modified transient replication assay. These results have important implications for viral pathogenesis in airway cells and the potential of HPV-based replicons for gene transfer into airway epithelium.

Gene therapy of cancer: activation of nucleoside prodrugs with E. coli purine nucleoside phosphorylase

During the last few years, many gene therapy strategies have been developed for various disease targets. The development of anticancer gene therapy strategies to selectively generate cytotoxic nucleoside or nucleotide analogs is an attractive goal. One such approach involves the delivery of herpes simplex virus thymidine kinase followed by the acyclic nucleoside analog ganciclovir. We have developed another gene therapy methodology for the treatment of cancer that has several significant attributes. Specifically, our approach involves the delivery of E. coli purine nucleoside phosphorylase, followed by treatment with a relatively non-toxic nucleoside prodrug that is cleaved by the enzyme to a toxic compound. This presentation describes the concept, details our search for suitable prodrugs, and summarizes the current biological data.

Adenosine and its nucleotides activate wild-type and R117H CFTR through an A2B receptor-coupled pathway

ATP and its metabolites stimulate Cl- secretion in human epithelium in vitro and in vivo. The specific purinergic receptor subtypes that govern these effects have been difficult to separate, in part due to multiple parallel pathways for Cl- secretion in respiratory and intestinal epithelia. In a simplified model using COS-7 cells, we demonstrate acquisition of an ATP-, ADP-, AMP-, and adenosine (ADO)-regulated halide permeability specifically following expression of wild-type (wt) cystic fibrosis transmembrane conductance regulator (CFTR). This halide permeability is blocked by the P1 purinergic receptor antagonist 8-phenyl theophylline, sensitive to the protein kinase A inhibitor H-89, and associated with a modest, dose-dependent increase in cellular cAMP concentration. Phorbol esters poorly activate halide permeability compared with ADO, and ADO-stimulated efflux was not affected by treatment with the protein kinase C inhibitor bisindolylmaleimide I. The A2 ADO receptor (AR) agonists 5'-N-ethylcarboxamide adenosine and ADO were strong activators, whereas the A1 AR agonist R-phenylisopropyladenosine failed to activate halide permeability. Metabolic conversion of ADO nucleotides by surface ecto-5'-nucleotidase to more active (less phosphorylated) forms contributes to anion transport activation in these cells. Immunoprecipitation with anti-A2B AR antibody identified a 31-kDa protein in both COS-7 and human bronchial epithelial cells. Together, these findings indicate that ADO and its nucleotides are capable of activating wtCFTR-dependent halide permeability through A2B AR and that this AR subtype is present in human bronchial epithelium. We also present data showing that this pathway can activate clinically significant mutant CFTR molecules such as R117H.

The mechanism underlying cystic fibrosis transmembrane conductance regulator transport from the endoplasmic reticulum to the proteasome includes Sec61beta and a cytosolic, deglycosylated intermediary

Endoplasmic reticulum (ER) degradation pathways can selectively route proteins away from folding and maturation. Both soluble and integral membrane proteins can be targeted from the ER to proteasomal degradation in this fashion. The cystic fibrosis transmembrane conductance regulator (CFTR) is an integral, multidomain membrane protein localized to the apical surface of epithelial cells that functions to facilitate Cl- transport. CFTR was among the first membrane proteins for which a role of the proteasome in ER-related degradation was described. However, the signals that route CFTR to ubiquitination and subsequent degradation are not known. Moreover, limited information is available concerning the subcellular localization of polyubiquitinated CFTR or mechanisms underlying retrograde dislocation of CFTR from the ER membrane to the proteasome either before or after ubiquitination. In the present study, we show that proteasome inhibition with clasto-lactacystin beta-lactone (4 microM, 1 h) stabilizes the presence of a deglycosylated CFTR intermediate for up to 5 h without increasing the core glycosylated (band B) form of CFTR. Deglycosylated CFTR is present under the same conditions that result in accumulation of polyubiquitinated CFTR. Moreover, the deglycosylated form of both wild type and DeltaF508 CFTR can be found in the cytosolic fraction. Both the level and stability of cytosolic, deglycosylated CFTR are increased by proteasome blockade. During retrograde translocation from the ER to the cytosol, CFTR associates with the Sec61 trimeric complex. Sec61 is the key component of the mammalian co-translational protein translocation system and has been proposed to function as a two way channel that transports proteins both into the ER and back to the cytosol for degradation. We show that the level of the Sec61.CFTR complexes are highest when CFTR degradation proceeds at the greatest rate (approximately 90 min after pulse labeling). Quantities of Sec61.CFTR complexes are also increased by inhibition of the proteasome. Based on these results, we propose a model in which complex membrane proteins such as CFTR are transported through the Sec61 trimeric complex back to the cytosol, escorted by the beta subunit of Sec61, and degraded by the proteasome or by other proteolytic systems.

Cystic fibrosis transmembrane conductance regulator (CFTR) nucleotide-binding domain 1 (NBD-1) and CFTR truncated within NBD-1 target to the epithelial plasma membrane and increase anion permeability

The cystic fibrosis transmembrane conductance regulator (CFTR) is a member of the traffic ATPase family that includes multiple proteins characterized by (1) ATP binding, (2) conserved transmembrane (TM) motifs and nucleotide binding domains (NBDs), and (3) molecular transport of small molecules across the cell membrane. While CFTR NBD-1 mediates ATP binding and hydrolysis, the membrane topology and function of this domain in living eukaryotic cells remains uncertain. In these studies, we have expressed wild-type CFTR NBD-1 (amino acids 433-586) or NBD-1 containing the DeltaF508 mutation transiently in COS-7 cells and established that the domain is situated across the plasma membrane by four independent assays; namely, extracellular chymotrypsin digestion, surface protein biotinylation, confocal immunofluorescent microscopy, and functional measurements of cell membrane anion permeability. Functional studies indicate that basal halide permeability is enhanced above control conditions following wild-type or DeltaF508 NBD-1 expression in three different epithelial cell lines. Furthermore, when clinically relevant CFTR proteins truncated within NBD-1 (R553X or G542X) are expressed, surface localization and enhanced halide permeability are again established. Together, these findings suggest that isolated CFTR NBD-1 (with or without the DeltaF508 mutation) is capable of targeting the epithelial cell membrane and enhancing cellular halide permeability. Furthermore, CFTR truncated at position 553 or 542 and possessing the majority of NBD-1 demonstrates surface localization and also confers increased halide permeability. These findings indicate that targeting to the plasma membrane and assumption of a transmembrane configuration are innate properties of the CFTR NBD-1. The results also support the notion that components of the halide-selective pore of CFTR reside within NBD-1.

Activation of DeltaF508 CFTR in an epithelial monolayer

The DeltaF508 mutation leads to retention of cystic fibrosis transmembrane conductance regulator (CFTR) in the endoplasmic reticulum and rapid degradation by the proteasome and other proteolytic systems. In stably transfected LLC-PK1 (porcine kidney) epithelial cells, DeltaF508 CFTR conforms to this paradigm and is not present at the plasma membrane. When LLC-PK1 cells or human nasal polyp cells derived from a DeltaF508 homozygous patient are grown on plastic dishes and treated with an epithelial differentiating agent (DMSO, 2% for 4 days) or when LLC-PK1 cells are grown as polarized monolayers on permeable supports, plasma membrane DeltaF508 CFTR is significantly increased. Moreover, when confluent LLC-PK1 cells expressing DeltaF508 CFTR were treated with DMSO and mounted in an Ussing chamber, a further increase in cAMP-activated short-circuit current (i.e., approximately 7 microA/cm2; P < 0.00025 compared with untreated controls) was observed. No plasma membrane CFTR was detected after DMSO treatment in nonepithelial cells (mouse L cells) expressing DeltaF508 CFTR. The experiments describe a way to augment DeltaF508 CFTR maturation in epithelial cells that appears to act through a novel mechanism and allows insertion of functional DeltaF508 CFTR in the plasma membranes of transporting cell monolayers. The results raise the possibility that increased epithelial differentiation might increase the delivery of DeltaF508 CFTR from the endoplasmic reticulum to the Golgi, where the DeltaF508 protein is shielded from degradative pathways such as the proteasome and allowed to mature.

Cell to cell contact is not required for bystander cell killing by Escherichia coli purine nucleoside phosphorylase

Expression of Escherichia coli purine nucleoside phosphorylase (PNP) activates prodrugs and kills entire populations of mammalian cells, even when as few as 1% of the cells express this gene. This phenomenon of bystander killing has been previously investigated for herpes simplex virus-thymidine kinase (HSV-TK) and has been shown to require cell to cell contact. Using silicon rings to separate E. coli PNP expressing cells from non-expressing cells sharing the same medium, we demonstrate that bystander cell killing by E. coli PNP does not require cell-cell contact. Initially, cells expressing E. coli PNP convert the non-toxic prodrug, 6-methylpurine-2'-deoxyriboside (MeP-dR) to the highly toxic membrane permeable toxin, 6-methylpurine (MeP). As the expressing cells die, E. coli PNP is released into the culture medium, retains activity, and continues precursor conversion extracellularly (as determined by reverse phase high performance liquid chromatography of both prodrug and toxin). Bystander killing can also be observed in the absence of extracellular E. coli PNP by removing the MeP-dR prior to death of the expressing cells. In this case, 100% of cultured cells die when as few as 3% of the cells of a population express E. coli PNP. Blocking nucleoside transport with nitrobenzylthioinosine reduces MeP-dR mediated cell killing but not MeP cell killing. These mechanisms differ fundamentally from those previously reported for the HSV-TK gene.

Suppression of a CFTR premature stop mutation in a bronchial epithelial cell line

Cystic fibrosis (CF) is caused by mutations in the CF transmembrane conductance regulator (CFTR) protein. While 70% of CF chromosomes carry a deletion of the phenylalanine residue 508 (deltaF508) of CFTR, roughly 5% of all CF chromosomes carry a premature stop mutation. We reported that the aminoglycoside antibiotics G-418 and gentamicin can suppress two premature stop mutations [a stop codon in place of glycine residue 542 (G542X) and arginine residue 553 (R553X)] when expressed from a CFTR cDNA in HeLa cells. Suppression resulted in the synthesis of full-length CFTR protein and the appearance of a cAMP-activated anion conductance characteristic of CFTR function. However, it was unclear whether this approach could restore CFTR function in cells expressing mutant forms of CFTR from the nuclear genome. We now report that G-418 and gentamicin are also capable of restoring CFTR expression in a CF bronchial epithelial cell line carrying the CFTR W1282X premature stop mutation (a stop codon in place of tryptophan residue 1282). This conclusion is based on the reappearance of cAMP-activated chloride currents, the restoration of CFTR protein at the apical plasma membrane, and an increase in the abundance of CFTR mRNA levels from the W1282X allele.

Purification, characterization, and expression of CFTR nucleotide-binding domains

The nucleotide binding domains (NBDs) within CFTR were initially predicted to lie in the cell cytoplasm, and to gate anion permeability through a pore that was present in membrane spanning alpha helices of the overall polypeptide. Our studies designed to characterize CFTR suggest several important features of the isolated nucleotide binding domain. NBD-1 appears to bind nucleotides with similar affinity to the full-length CFTR protein. In solution, the domain contains a high beta sheet content and self-associates into ordered polymers with molecular mass greater than 300,000 Daltons. The domain is very lipophilic, disrupts liposomes, and readily enters the planar lipid bilayer. Clinically important mutations in the domain may disrupt the nucleotide binding capabilities of the protein, either through a direct effect on the nucleotide binding site, or through effects that influence the overall folding of the domain in vitro. Finally, after expression in human epithelial cells (including epithelial cells from a CF patient), the first nucleotide binding domain targets the plasma membrane even in the absence of other constituents of full-length CFTR and mediates anion permeability in these cells.

In vivo gene therapy of cancer with E. coli purine nucleoside phosphorylase

We have developed a new strategy for the gene therapy of cancer based on the activation of purine nucleoside analogs by transduced E. coli purine nucleoside phosphorylase (PNP, E.C. 2.4.2.1). The approach is designed to generate antimetabolites intracellularly that would be too toxic for systemic administration. To determine whether this strategy could be used to kill tumor cells without host toxicity, nude mice bearing human malignant D54MG glioma tumors expressing E. coli PNP (D54-PNP) were treated with either 6-methylpurine-2'-deoxyriboside (MeP-dR) or arabinofuranosyl-2-fluoroadenine monophosphate (F-araAMP, fludarabine, a precursor of F-araA). Both prodrugs exhibited significant antitumor activity against established D54-PNP tumors at doses that produced no discernible systemic toxicity. Significantly, MeP-dR was curative against this slow growing solid tumor after only 3 doses. The antitumor effects showed a dose dependence on both the amount of prodrug given and the level of E. coli PNP expression within tumor xenografts. These results indicated that a strategy using E. coli PNP to create highly toxic, membrane permeant compounds that kill both replicating and nonreplicating cells is feasible in vivo, further supporting development of this cancer gene therapy approach.

Adenoviral gene delivery elicits distinct pulmonary-associated T helper cell responses to the vector and to its transgene

Replication-deficient adenovirus (Ad) vectors are effective to specifically target the respiratory epithelium for either corrective gene therapy such as cystic fibrosis or for mucosal immunization. As a consequence of transducing the lower respiratory tract with an E1/E3 deleted Ad5 vector, host responses have been characterized by the duration of transgene expression and by the induction of CTL responses. However, limited emphasis has been devoted to understanding the contribution of CD4+ T cell responses to the Ad vector. Both CD4+ and CD8+ T cells migrate into the lung following sequential intratracheal Ad5 transgene instillations. Isolated CD3+ T lymphocytes from the lungs were predominantly of the Th2 type, and after cell sorting, the IL-4-producing T cells were largely CD4+, while IFN-gamma expression was associated with both CD4+ and CD8+ T cells. Ab responses to the Ad5 vector and to the expressed transgene beta-galactosidase (beta gal) revealed elevated bronchial and serum IgA and IgG Abs with low neutralization titers. Analysis of serum IgG subclass responses showed IgG1 and IgG2b with lower IgG2a Abs to Ad5 and IgG2a and IgG2b Ab responses to beta gal. Ad5-specifc CD4+ T cells produced both Th1 (IFN-gamma and IL-2)- and Th2 (IL-4, IL-5, IL-6)-type cytokines, while beta gal-specific CD4+ T cells secreted IFN-gamma and IL-6. This study provides direct evidence for the concomitant induction of Th2- with Th1-type responses in both the pulmonary systemic and mucosal immune compartments to the Ad5 vector as well as a Th1-dominant response to the transgene.

Adenoviral gene delivery elicits distinct pulmonary-associated T helper cell responses to the vector and to its transgene

Replication-deficient adenovirus (Ad) vectors are effective to specifically target the respiratory epithelium for either corrective gene therapy such as cystic fibrosis or for mucosal immunization. As a consequence of transducing the lower respiratory tract with an E1/E3 deleted Ad5 vector, host responses have been characterized by the duration of transgene expression and by the induction of CTL responses. However, limited emphasis has been devoted to understanding the contribution of CD4+ T cell responses to the Ad vector. Both CD4+ and CD8+ T cells migrate into the lung following sequential intratracheal Ad5 transgene instillations. Isolated CD3+ T lymphocytes from the lungs were predominantly of the Th2 type, and after cell sorting, the IL-4-producing T cells were largely CD4+, while IFN-gamma expression was associated with both CD4+ and CD8+ T cells. Ab responses to the Ad5 vector and to the expressed transgene beta-galactosidase (beta gal) revealed elevated bronchial and serum IgA and IgG Abs with low neutralization titers. Analysis of serum IgG subclass responses showed IgG1 and IgG2b with lower IgG2a Abs to Ad5 and IgG2a and IgG2b Ab responses to beta gal. Ad5-specifc CD4+ T cells produced both Th1 (IFN-gamma and IL-2)- and Th2 (IL-4, IL-5, IL-6)-type cytokines, while beta gal-specific CD4+ T cells secreted IFN-gamma and IL-6. This study provides direct evidence for the concomitant induction of Th2- with Th1-type responses in both the pulmonary systemic and mucosal immune compartments to the Ad5 vector as well as a Th1-dominant response to the transgene.

8-cyclopentyl-1,3-dipropylxanthine and other xanthines differentially bind to the wild-type and delta F508 first nucleotide binding fold (NBF-1) domains of the cystic fibrosis transmembrane conductance regulator

Cystic fibrosis is an autosomal recessive disorder affecting chloride transport in pancreas, lung, and other tissues, which is caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR). Certain alkyl xanthines such as CPX (8-cyclopentyl-1,3-dipropylxanthine) stimulate Cl- efflux from cells bearing the delta F508 genotype common to most cases of cystic fibrosis. We have hypothesized that the CFTR molecule itself might be the site for CPX action, perhaps in the region of the first nucleotide binding fold (NBF-1) domain. Therefore, to test this hypothesis directly we have used a rapid membrane filtration assay to measure the kinetics of association and dissociation of [3H]CPX to both recombinant NBF-1 and recombinant NBF-1 bearing the delta F508 mutation. We report that [3H]CPX binds with higher affinity to the delta F508-NBF-1 of CFTR (Kd = 1.0 nM) than to the wild-type NBF-1 of CFTR (Kd = 17.0 nM). These Kd values were calculated from direct measurements of the association and dissociation rate constants. The rate constants for the dissociation reaction of the wild-type NBF-1 and delta F508-NBF-1 of CFTR were not different from each other. However, the corresponding rate constants for the association reaction were k(+1) (NBF-1) = 4.7 +/- 0.9 x 10(4) M(-1) s(-1) and k(+1) (delta F508-NBF-1) = 1.6 +/- 0.3 x 10(5) M(-1) s(-1), respectively. These Kd values were corroborated by equilibrium-binding experiments, which gave very similar values. We have also measured the relative displacement of various xanthines from both wild-type NBF-1 and delta F508-NBF-1, in anticipation that the order of potencies for binding might parallel the action of the different xanthines on CF cells. For wild-type NBF-1, the rank order was DA-CPX > DAX > CPX > caffeine > adenosine >> IBMX > 2-thioCPX. For delta F508-NBF-1, the rank order was DAX > CPX > caffeine > DA-CPX > adenosine >> IBMX > 2-thioCPX. These relative potencies show close parallels with previously observed relative potencies of these drugs on CF cells, and thus lend strong support to the hypothesis that the mechanism of action on CF cells may involve direct interaction with the CFTR molecule itself.

Modulation of adenovirus-mediated gene transfer by nitric oxide

We assessed the role of .NO in recombinant adenovirus-mediated gene transfer both in vitro and in vivo. NIH3T3 fibroblasts, stably transfected with the human inducible nitric oxide synthase, but lacking tetrahydrobiopterin (NIH3T3/iNOS [inducibile nitric oxide synthase]), were infected with replication-deficient adenovirus (E1-deleted), containing either the luciferase or the Lac Z reporter genes (AdCMV-Luc and AdCMV-Lac Z; 1-10 plaque forming units [pfu]/cell). Incubation of infected cells with sepiapterin (50 microM), a precursor of tetrahydrobiopterin, progressively increased nitrate/nitrite levels in the medium and decreased both luciferase and beta-galactosidase protein expression to approximately 60% of their corresponding control values, 24 h later. NIH3T3/iNOS cells had normal ATP (adenosine 5'-triphosphate) levels and did not release LDH(lactic dehydrogenase) into the medium. Pretreatment of these cells with N(G)-monomethyl-L-arginine (L-NMMA; 1 mM), an inhibitor of iNOS, prevented the sepiapterin-mediated induction of .NO and restored gene transfer to baseline values. Incubation of NIH3T3/iNOS with 8-bromo-cGMP (400 microM) in the absence of sepiapterin, or exposure of AdCMV-Luc to large concentrations of .NO, did not alter the efficacy of gene transfer. .NO produced by NIH3T3/iNOS cells also suppressed beta-galactosidase expression in NIH3T3 cocultured cells stably transfected with beta-galactosidase gene, suggesting .NO inhibited gene expression at either the transriptional or posttranscriptional levels. To investigate the effects of inhaled .NO on gene transfer in vivo, CD1 mice received an intratracheal instillation of AdCMV-Luc (4 x 10(9) pfu in 80 microl of saline) and exposed to .NO (25 ppm in room air) for 72 h. At that time, no significant degree of lung inflammation was detected by histological examination. However, lung luciferase activity decreased by 53% as compared with air breathing controls (P < 0.05; n > or = 8). We concluded that overproduction of .NO decreases the efficiency of adenovirus-mediated gene transfer in lung cells in the absence of cytotoxicity or inflammation.

Relationship between airway ion transport and a mild pulmonary disease mutation in CFTR

Patients with cystic fibrosis (CF) display defects in airway ion transport, but the influence of airway transport phenotype on improved prognosis is not known. We studied airway bioelectric properties in five CF patients with the rare A455E mutation that is associated with mild pulmonary disease. We also evaluated five patients possessing premature truncation mutations (G542X and R553X) for which an association with mild pulmonary disease has not been as well established. We found no evidence in vivo that a mild lung disease mutation in the CF transmembrane regulator gene (CFTR) led to correction or partial correction of: (1) unstimulated Cl- secretion; (2) beta-agonist-activated Cl- secretion; (3) basal sodium reabsorption; or (4) amiloride-sensitive airway sodium transport. Early phase therapeutic trials in CF, including human gene transfer trials, rely heavily on improvements in airway potential difference to identify promising interventions and an improved prognosis. Based on our findings in a naturally occurring group of CF patients with an improved pulmonary prognosis (A455E), one can argue that marked clinical benefit might be possible without any improvement whatsoever in airway bioelectric phenotype. Moreover, if genetic modifiers exist that influence the severity of a particular CFTR mutation (e.g., A455E), these may be independent of human airway Cl-secretion in vivo, since we detected minimal Cl--secretory responses in patients with A455E.

Efficiency of plasmid delivery and expression after lipid-mediated gene transfer to human cells in vitro

Cationic liposome-mediated gene transfer has become increasingly important in the development of experimental therapies for human diseases, such as melanoma, human immunodeficiency virus infection, cystic fibrosis and alpha-1 antitrypsin deficiency. However, very little is known about the mechanisms by which lipid-mediated gene transfer occurs. We studied the kinetics of plasmid delivery and expression by using this technique. Plasmid entry in the cystic fibrosis respiratory epithelial cell line 2CFSME0-1 as well as in two other cell lines (HeP 2g and HeLa) occurred in 95 to 100% of cells within 1 hr of the initiation of lipid-mediated gene transfer. In hepatic and respiratory cells, transcription of a construct containing the cystic fibrosis transmembrane conductance regulator was observed in more than 80% of the cell population; similarly high levels of plasmid utilization were obtained in studies of HLA-B7 expression in human melanoma cells. Studies directly relevant to current human trials of lipid-mediated gene transfer indicate that plasmid entry, transcription and translation are often surprisingly efficient, and may occur in nearly 100% of human cells in culture when sensitive methods for detection are used. Furthermore, conventional X-gal immunohistochemistry markedly underestimates transfection efficiency during transient gene expression. These studies point to a new mechanistic understanding of the features that limit expression by using cationic liposomes.

A method for the rapid detection of recombinant CFTR during gene therapy in cystic fibrosis

We developed an assay to detect wild-type CFTR in respiratory epithelial cells with the objective to evaluate the efficacy of DNA delivery during in vivo gene transfer. The method is based on the previous observation that the common delta F508-CFTR mutant does not reach the apical membrane as does the transgene product. We thus used a monoclonal antibody, MATG 1031, raised against the first extracellular loop sequence of the CFTR protein and an immunodetection protocol lacking premature fixation or permeabilization. Specificity of MATG 1031 for its epitope was controlled by immunoblotting. In HT29-19A, 184, CAPAN-1 human cell lines, and in respiratory primary cultures, staining with MATG 1031, examined by confocal scanning laser microscopy, appeared as small dots restricted to the apical surface. No such staining was observed in NIH-3T3 fibroblasts, in the cystic fibrosis cell line CFPAC-1 or in primary cultures from cystic fibrosis patients. Apical immunostaining with MATG 1031 was restored in CFPAC-1 cells cultured at a low temperature (30 degrees C) and in CFPAC-1 cells transfected with wild-type CFTR Recombinant CFTR was also recognized in CF respiratory cells lipotransfected with wild-type CFTR plasmid DNA MATG 1031 immunostaining was further investigated under blinded conditions in primary cultures derived from nasal curettage. In all the cell cultures examined, our protocol allowed discrimination between non-CF and CF cells. We propose that this method is convenient to detect apical CFTR and may be used to monitor in vivo gene transfer.

Development of drug targeting based on recombinant expression of the chicken avidin gene

The chemistry required for covalent biotinylation of drugs, radiopharmaceuticals and other ligands is highly developed, and a large number of biotinylated reagents can be readily synthesized. In order to investigate whether expression of avidin cDNA in mammalian cells might be useful as part of a drug targeting strategy, we transiently expressed the avidin gene in two human tumor cell lines (the cervical carcinoma cell line, HeLa, and the liver derived line, Hep G2). Avidin protein as detected by either immunohistochemistry or binding of streptavidin-biotin complexes was present and functional following transient expression. This result indicated that the mechanisms underlying avidin oligomerization which are necessary for proper protein folding are present within mammalian carcinoma cell lines. Next, we generated a producer cell line (derived from psi2) capable of releasing a recombinant retrovirus encoding chicken avidin, and a tumorigenic murine breast cancer cell line (16/C) with stable avidin expression. We show that these cell lines are suitable for conferring functional expression of avidin in vitro. These experiments establish a means by which avidin gene expression can be explored as a mechanism for targeted gene delivery of biotin-derivitized drugs in vitro, and have important implications for utilization of this strategy in vivo.

Interaction between cystic fibrosis transmembrane conductance regulator and outwardly rectified chloride channels

We have previously described a protocol for the simultaneous isolation and reconstitution of a protein kinase A (PKA)-sensitive outwardly rectified chloride channel (ORCC) and the cystic fibrosis transmembrane conductance regulator (CFTR) from bovine tracheal epithelium. Immunoprecipitation of CFTR from this preparation prevented PKA activation of the ORCC, suggesting that CFTR regulated the ORCC and that this regulatory relationship was preserved throughout the purification procedure. We now report the purification of CFTR from bovine tracheal epithelia and the purification of a CFTR conduction mutant (G551D CFTR) from retrovirally transduced mouse L cells using a combination of alkali stripping, Triton-X extraction, and immunoaffinity chromatography. Immunopurified CFTR proteins were reconstituted in the absence and presence of ORCC. To test the hypothesis that only functional CFTR can support activation of ORCC by PKA and ATP, we used an inhibitory anti-CFTR505-511 peptide antibody or G551D CFTR. When anti-CFTR505-511 peptide antibodies were present prior to the addition of PKA and ATP, activation of both the ORCC and CFTR was prevented. If the antibody was added after activation of the ORCC and CFTR Cl- channels by PKA and ATP, only the CFTR Cl- channel was inhibited. When ORCC and G551D CFTR were co-incorporated into planar bilayers, only the ORCC was recorded and this channel could not be further activated by the addition of PKA and ATP. Thus, functional CFTR is required for activation of the ORCC by PKA and ATP. We also tested the hypothesis that PKA activation of ORCC was dependent on the extracellular presence of ATP. We added ATP on the presumed extracellular side of the lipid bilayer under conditions where it was not possible to activate the ORCC, i.e. in the presence of inhibitory anti-CFTR505-511 antibody or G551D CFTR. In both cases the ORCC regained PKA sensitivity. Moreover, the addition of hexokinase + glucose to the extracellular side prevented activation of the ORCCs by PKA and ATP in the presence of CFTR. These experiments confirm that both the presence of CFTR as well as the presence of ATP on the extracellular side is required for activation of the ORCC by PKA and ATP.

Bystander killing of melanoma cells using the human tyrosinase promoter to express the Escherichia coli purine nucleoside phosphorylase gene

We used a gene transfer-based system to generate highly toxic purine bases in tumor cells transfected with the Escherichia coli purine nucleoside phosphorylase (PNP) gene. Because these toxic purines are membrane permeant, they mediate effective killing of neighboring cells that do not express E. coli PNP ("bystander" toxicity). In mixed cultures containing increasing percentages of cells with gene expression, 100% cancer cell growth arrest and total population killing was demonstrated when as few as 1-2% of cells expressed E. coli PNP. We used E. coli PNP to test bystander killing of human melanoma cells. A 529-bp region upstream of the human tyrosinase gene start site was shown to direct melanoma-specific expression in human cell lines. When this human tyrosinase regulatory region was used to control E. coli PNP expression, profound toxicity was observed in melanoma cells after treatment with the relatively nontoxic substrate 6-methylpurine-deoxyriboside, which is converted by E. coli PNP into the highly toxic purine base 6-methylpurine. Bystander toxicity was estimated as at least 100 cells killed for each cell expressing E. coli PNP, a level substantially higher than that of other tumor sensitization genes currently being used in clinical trails. These results suggest that the high bystander activity of the system could lead to significant antimelanoma responses in vivo.

Severe phenotype in mice with termination mutation in exon 2 of cystic fibrosis gene

Mice with a termination codon mutation in exon 2 of the cystic fibrosis (CF) gene were generated using homologous recombination in embryonic stem cells. Animals homozygous for the mutant allele display a severe intestinal phenotype similar to that previously reported for CF mutant mice. The null nature of this allele was demonstrated by the absence of detectable wild-type mRNA, by the absence of detectable CFTR in the serous gland collecting ducts of salivary tissues, and by the lack of cAMP-mediated short-circuit current responses in colonic epithelium of mutant animals.

Cystic fibrosis transmembrane conductance regulator mutations that disrupt nucleotide binding

Increasing evidence suggests heterogeneity in the molecular pathogenesis of cystic fibrosis (CF). Mutations such as deletion of phenylalanine at position 508 (delta F508) within the cystic fibrosis transmembrane conductance regulator (CFTR), for example, appear to cause disease by abrogating normal biosynthetic processing, a mechanism which results in retention and degradation of the mutant protein within the endoplasmic reticulum. Other mutations, such as the relatively common glycine-->aspartic acid replacement at CFTR position 551 (G551D) appear to be normally processed, and therefore must cause disease through some other mechanism. Because delta F508 and G551D both occur within a predicted nucleotide binding domain (NBD) of the CFTR, we tested the influence of these mutations on nucleotide binding by the protein. We found that G551D and the corresponding mutation in the CFTR second nucleotide binding domain, G1349D, led to decreased nucleotide binding by CFTR NBDs, while the delta F508 mutation did not alter nucleotide binding. These results implicate defective ATP binding as contributing to the pathogenic mechanism of a relatively common mutation leading to CF, and suggest that structural integrity of a highly conserved region present in over 30 prokaryotic and eukaryotic nucleotide binding domains may be critical for normal nucleotide binding.

Tumor cell bystander killing in colonic carcinoma utilizing the Escherichia coli DeoD gene to generate toxic purines

Inefficiency of gene delivery, together with inadequate bystander killing, represent two major hurdles in the development of a toxin-mediated gene therapy for human malignancy. The product of the Escherischia coli DeoD gene (purine nucleoside phosphorylase, PNP) differs from the mammalian enzyme in its substrate specificity and is capable of catalyzing the conversion of several non-toxic deoxyadenosine analogs to highly toxic adenine analogs. We have found that expression of E. coli PNP in < 1% of a human colonic carcinoma cell line leads to the death of virtually all bystander cells after treatment with 6-methyl-purine-2'-deoxyribonucleoside, a deoxyadenosine analog that is a substrate for E. coli PNP but not human PNP. Minimal toxicity was observed in non-transfected or E. coli LacZ transfected cells that were treated with this compound. These results establish a rational approach to achieve significant bystander killing, even after gene transfer to only a small fraction of tumor cells.

Expression and polarized targeting of a rab3 isoform in epithelial cells

Pathways of polarized membrane traffic in epithelial tissues serve a variety of functions, including the generation of epithelial polarity and the regulation of vectorial transport. We have identified a candidate regulator of polarized membrane traffic in epithelial cells (i.e., rab3B), which is a member of the rab family of membrane traffic regulators. Rab3B is highly homologous to a brain-specific rab3 isoform (rab3A) that targets in a polarized fashion to the presynaptic nerve terminal, where it probably regulates exocytosis. The coding region for human rab3B was cloned from epithelial mRNA using a reverse-transcription polymerase chain reaction strategy. This cDNA clone hybridized to a single mRNA species in Northern blots of poly(A)+ RNA isolated from epithelial cell lines. A rab3B-specific antibody that was raised against recombinant fusion protein recognized a 25-kD band in immunoblots of cell lysates prepared from cultured epithelial cells (e.g., T84 and HT29-CL19A), but not from a variety of nonepithelial cells (e.g., PC12 neuroendocrine cells). Immunofluorescence analysis confirmed that rab3B protein is preferentially expressed in cultured epithelial cells as well as in a number of native epithelial tissues, including liver, small intestine, colon, and distal nephron. Rab3B localized to the apical pole very near the tight junctions between adjacent epithelial cells within all of these cell lines and native epithelial tissues, as determined by immunofluorescence and immunoelectron microscopic analysis. Moreover, this pattern of intracellular targeting was regulated by cell contact; namely, rab3B was reversibly retrieved from the cell periphery as epithelial cell contact was inhibited by reducing the extracellular Ca2+ concentration. Our results indicate that neurons and epithelial cells express homologous rab3 isoforms that target in a polarized fashion within their respective tissues. The pattern and regulation of rab3B targeting in epithelial cells implicates this monomeric GTPase as a candidate regulator of apical and/or junctional protein traffic in epithelial tissues.

Strategy for achieving selective killing of carcinomas

Carcinomas are malignancies derived from epithelial cells that frequently respond poorly to conventional chemotherapy. Selective expression or transduction of toxin genes to carcinomas, i.e. molecular chemotherapy, may offer important advantages over conventional chemotherapy. As one approach to developing a means of selectively expressing toxin genes, the transcriptional regulatory sequences of a gene expressed in multiple carcinomas were used to direct expression of the herpes simplex virus thymidine kinase (HSVtk) coding sequences. The secretory leukoprotease inhibitor (SLPI) gene was found to be expressed in lung, breast, oropharyngeal, bladder, endometrial, ovarian and colorectal carcinomas. The tissue-specific transcriptional regulatory sequences were isolated and used to construct a chimeric gene in which the SLPI sequences directed HSVtk expression. SLPI-expressing carcinomas were reduced in number by transduction of the SLPI-directed toxin plasmid plus ganciclovir, but the same treatment had no effect on a cell line that did not express SLPI. These results suggest that SLPI-directed therapeutic genes could be used for directing toxicity to carcinoma tissues, especially if combined with other targeting strategies.

Rapid purification of recombinant baculovirus using fluorescence-activated cell sorting

Expression of foreign proteins in the baculovirus-insect cell expression system has been limited by difficulties in rapid identification and purification of recombinant virus. Although the process of identifying recombinant virus has been greatly facilitated by the introduction of vectors that lead to insect cell co-expression of beta-galactosidase with foreign genes of interest, isolation of pure recombinant virus using plaque purification may still take several weeks to months to accomplish. Using a fluorescent beta-galactosidase substrate, we have established that insect cells harboring recombinant virus can be rapidly isolated using fluorescence-activated cell sorting. Pure recombinant virus can then be readily obtained using this cellular fraction, with a pure viral culture generally obtained within 2-3 weeks of insect cell transfection.

Regulation of plasma membrane recycling by CFTR

The gene that encodes the cystic fibrosis transmembrane conductance regulator (CFTR) is defective in patients with cystic fibrosis. Although the protein product of the CFTR gene has been proposed to function as a chloride ion channel, certain aspects of its function remain unclear. The role of CFTR in the adenosine 3',5'-monophosphate (cAMP)-dependent regulation of plasma membrane recycling was examined. Adenosine 3',5'-monophosphate is known to regulate endocytosis and exocytosis in chloride-secreting epithelial cells that express CFTR. However, mutant epithelial cells derived from a patient with cystic fibrosis exhibited no cAMP-dependent regulation of endocytosis and exocytosis until they were transfected with complementary DNA encoding wild-type CFTR. Thus, CFTR is critical for cAMP-dependent regulation of membrane recycling in epithelial tissues, and this function of CFTR could explain in part the pleiotropic nature of cystic fibrosis.

Intrinsic anion channel activity of the recombinant first nucleotide binding fold domain of the cystic fibrosis transmembrane regulator protein

The first nucleotide binding fold (NBF-1) from the cystic fibrosis transmembrane regulator (CFTR) has been expressed in bacteria and found to bind ATP and to express anion channel activity when reconstituted onto a planar lipid bilayer. This evidence suggests that the NBF forms the anion-selective portion of the CFTR channel. We also found that the recombinant NBF-1 anion channel is blocked by ATP (1 mM), under which condition it appears to have a minimal conductance of approximately 9 pS and an ohmic current-voltage relationship. We further found that the recombinant NBF-1 bearing the delta F508 mutation has nearly identical anion channel activity to that of the wild-type protein but can be distinguished from wild type under bianionic conditions with chloride and gluconate. We conclude from these data that the anion channel activity of the recombinant NBF-1 could represent all or part of the anion conductance mechanism of CFTR and that the role of the ATP binding by the NBF could be to modulate this anion channel activity.

Recombinant synthesis, purification, and nucleotide binding characteristics of the first nucleotide binding domain of the cystic fibrosis gene product

The majority of mutations which lead to clinical cystic fibrosis are located within the two predicted nucleotide binding domains of the cystic fibrosis gene product. We have used a prokaryotic expression system to synthesize and purify the first nucleotide binding domain (NBD-1, amino acids 426-588) with and without the most common mutation associated with the disease (the deletion of phenylalanine at position 508, delta F508). Both wild type and delta F508 NBD-1 bind ATP-agarose in a quantitatively comparable manner; this binding was inhibited by excess Na2ATP, trinitrophenol-ATP, or 8-azido-ATP. Irreversible NBD-1 labeling by an ATP analog was demonstrated using [32P]8-azido-ATP. This covalent labeling was inhibited by preincubation with Na2ATP, with half-maximal inhibition for Na2ATP occurring at approximately 5 mM for both the wild type and delta F508 nucleotide binding domain. These experiments are among the first to confirm the expectation that the cystic fibrosis transmembrane conductance regulator NBD-1 binds nucleotide. Since, under the conditions used in our study, NBD-1 without phenylalanine 508 displays very similar nucleotide binding characteristics to the wild type protein, our results support previous structural models which predict that the delta F508 mutation should not cause an alteration in ATP binding.

Identification of a membrane protein from T84 cells using antibodies made against a DIDS-binding peptide

The outwardly rectified chloride channel of secretory epithelial cells is inhibited by disulfonic stilbene (DS) compounds such as 4,4'-diisothiostilbene-2,2'-disulfonic acid (DIDS) [R. J. Bridges, R. T. Worrell, R. A. Frizzell, and D. J. Benos, Am. J. Physiol. 256 (Cell Physiol. 25): C902-C912, 1989]. A 13-amino acid peptide (P49) corresponding to the putative DS binding site region of the murine anion exchange protein was synthesized, and polyclonal antibodies were generated against it and then purified over a P49 affinity column. The resulting monospecific antibodies reacted on Western blots with a 95- to 100-kDa protein from human erythrocytes and a 55- to 60-kDa protein from the human colonic tumor cell line, T84. The reaction with T84 protein did not appear to represent recognition of an anion exchanger because anion efflux from T84 cells was independent of external Cl-. In addition, monoclonal antibodies raised against human band 3 recognized the band 3 protein in human red cell ghost preparations but recognized nothing in T84 cell membrane preparations. In T84 cells, DIDS protected the 60-kDa protein from antibody binding. The anti-P49 antibody blocked outwardly rectified Cl- channels incorporated into planar lipid bilayer membranes from rat colon. Immunocytochemical data reveal specific binding of the anti-P49 antibody to perinuclear cytoplasmic vesicles. Forskolin caused these antibody-labeled vesicles to migrate from the perinuclear region to the plasma membrane under conditions and with a time course identical to that seen for stimulation of Cl- transport in these cells. Our results suggest that the protein may be a part of a chloride channel complex of secretory epithelial cells.

Antisense oligodeoxynucleotide to the cystic fibrosis gene inhibits anion transport in normal cultured sweat duct cells

We have tested the hypothesis that the cystic fibrosis (CF) gene product, called the CF transmembrane conductance regulator (CFTR), mediates anion transport in normal human sweat duct cells. Sweat duct cells in primary culture were treated with oligodeoxynucleotides that were antisense to the CFTR gene transcript in order to block the expression of the wild-type CFTR. Anion transport in CFTR transcript antisense-treated cells was then assessed with a halide-specific dye, 6-methoxy-N-(3-sulfopropyl)quinolinium, and fluorescent digital imaging microscopy to monitor halide influx and efflux from single sweat duct cells. Antisense oligodeoxynucleotide treatment (3.9 or 1.3 microM) for 24 hr virtually abolished Cl- transport in sweat duct cells compared with untreated cells or control cells treated with sense oligodeoxynucleotides. Br- uptake into sweat duct cells was also blocked after a 24-hr CFTR transcript antisense treatment, but not after treatment for only 4 hr. Lower concentrations of antisense oligodeoxynucleotides were less effective at inhibiting Cl- transport. These results indicate that oligodeoxynucleotides that are antisense to CFTR transcript inhibit sweat duct Cl- permeability in both a time-dependent and dose-dependent manner. This approach provides evidence that inhibition of the expression of the wild-type CFTR gene in a normal, untransfected epithelial cell results in an inhibition of Cl- permeability.

Diagnosis of genetic disease by primer-specified restriction map modification, with application to cystic fibrosis and retinitis pigmentosa

Detection of small alterations or abnormalities in genomic DNA (eg, point mutations or small deletions) has become increasingly important in the diagnosis of genetic disease and polymorphism. When a mutation or polymorphism creates a new restriction endonuclease site, it can easily be identified by polymerase chain reaction (PCR) amplification of the DNA region of interest, followed by digestion with the restriction endonuclease. However, useful restriction sites are the exception, and a variety of specialised techniques have been developed to identify subtle DNA abnormalities. We have shown that where a DNA mutation does not create a useful novel restriction site, such a site can be introduced by PCR and specially chosen primers. The approach is simple and inexpensive and should be broadly applicable in the diagnosis of genetic polymorphism and mutation. The technique is illustrated here by the three base-pair deletion responsible for most cases of cystic fibrosis and by detection of the point mutation in the rhodopsin gene that has been associated with some cases of autosomal dominant retinitis pigmentosa.