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From 1 The Childrens Hospital and 2 The Massachusetts Eye and Ear Infirmary, Harvard Medical School, Boston, Massachusetts.
| Abstract |
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METHODS. Naked plasmid DNA was injected under pressure into the cornea of mice. The expression of genes coding for beta galactosidase (ß-gal), enhanced green fluorescent protein (EGFP), vascular endothelial growth factor (VEGF), and soluble Flt-1 (s-Flt) was recorded and measured with regard to dose, time course, and bioactivity.
RESULTS. LacZ gene expression of the protein ß-gal was demonstrated as early as 1 hour, with expression persisting for 10 days. Plasmid-injected corneas remained clear and free of inflammation. EGFP was bicistronically expressed with VEGF to demonstrate the practicality of simultaneous in vivo analysis of gene expression and growth factor bioactivity. Corneal injection of a plasmid containing VEGF cDNA induced corneal and anterior chamber neovascularization. Moreover, corneal injection of plasmid containing the cDNA for the soluble form of the VEGF receptor Flt-1 effectively prevented corneal neovascularization.
CONCLUSIONS. The cornea is readily accessible for gene therapy in the laboratory and in the clinic. The method described is safe, effective, titratable, and easily monitored. Naked DNA delivery to the cornea has the potential to alter the treatment of a wide variety of corneal and anterior segment diseases.
| Introduction |
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An attractive feature of focal naked plasmid gene therapy is the ability to express locally a protein with reduced chance of systemic exposure and inflammation. However, the limitations of naked plasmid gene therapy as described include limited accessibility to tissue, poor transfection efficiency, invasive monitoring techniques, and the inability to control levels of gene expression. Ocular tissues, by contrast, are easily anesthetized, readily accessible, and quickly and noninvasively monitored by way of slit-lamp biomicroscopy. Prior attempts to deliver DNA to ocular tissues have met with limited success. Gold microbeads coated with naked plasmid and delivered via a jet of air to the cornea introduce foreign material into the cornea and do not have high transfection efficiency.4 The repeatability of such a method in clear cornea is also a concern. Lipid transfection reagents, electoporation, and viral techniques can induce inflammation or cell death, effects that may not be tolerable in the delicate transparent tissues of the eye.5 6 7
The technique of stromal hydration, that is, forcing saline solution into the corneal stroma, is used routinely at the end of cataract surgery to temporarily swell the cornea and ensure a watertight wound.8 We used stromal hydration to deliver naked plasmid to the cornea and discovered a straightforward and efficient method of safely transfecting cornea in vivo. The level of gene expression is remarkably titratable and is sufficient to induce biological effects not only in the cornea, but also the anterior and posterior segments of the eye.
| Materials and Methods |
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Murine VEGF164 cDNA was cloned into the vector pCMMP to make pCMMPmuVEGF. pCMMP contains a multiple cloning site flanked by a CMV promoter and the bovine growth hormone polyadenylation signal. The plasmid was sequenced to ensure fidelity and the correct orientation of mouse VEGF164.
Murine Flt-1 (13) was PCR amplified from Flt-1 cDNA (gift of Shay Soker, Childrens Hospital, Harvard Medical School, Boston, MA) with appropriate primers and cloned into pHIHGAdd2 to make pHIHGAdd2Flt. An EcoRI-SalI fragment containing murine Flt-1 (13) with a C-terminal His tag was cloned into the vector pHIHGAdd2 containing a multiple cloning site flanked by a CMV promoter and SV40 polyadenylation signal.
Animals and Anesthesia
All animal experiments were approved by the Childrens Hospital
Animal Care and Use Committee and conformed to the ARVO Statement for
the Use of Animals in Ophthalmic and Vision Research. Before all
experimental manipulations, CD-1 mice were anesthetized with 7080
mg/kg intraperitoneal Nembutal sodium solution (Abbott Laboratories,
North Chicago, IL).
Intrastromal Corneal Injections
Under direct microscopic observation, a nick in the
epithelium and anterior stroma of a CD-1 mouse cornea was made in the
midperiphery with a 1/2-inch 30-gauge needle (Becton Dickinson,
Franklin Lakes, NJ). A 1/2-inch 33-gauge needle with a 30°
bevel on a 10-µl gas tight syringe (Hamilton, Reno, NV) was
introduced into the corneal stroma and advanced 1.5 mm to the corneal
center. Two microliters of plasmid solution was forcibly injected into
the stroma to separate corneal lamellae and disperse the plasmid.
Analysis of ß-Galactosidase Production
Eyes injected with pLacZ were enucleated at various time points,
fixed, and stained with X-gal (Gibco BRL, Rockville, MD). Corneas were
removed after staining, and flat mounts were photographed with a MZ
FLII fluorescence stereomicroscope (Leica, Heerbrugg, Switzerland)
using a CCD digital camera (Dage, Michigan City, IN). The corneal area
stained with X-gal was quantified with Improvision Openlab v.2 software
(Coventry, England). A masked observer established threshold levels of
hue and saturation, and all areas of the cornea above the threshold
levels were marked as stained and quantified. The area of staining was
divided by the total area of the cornea, as determined by the limbal
vascular arcade. Selected corneas were paraffin embedded, sectioned,
and stained with hematoxylin and eosin.
Enhanced Green Fluorescent Protein Expression
Mice injected with pIRES2-enhanced green fluorescent protein
(EGFP) were examined and photographed under general anesthesia using
the Leica microscope equipped with the Dage CCD digital camera and
Improvision Openlab software.
Analysis of Angiogenesis Induction and Inhibition
Mice were perfused with fluorescein isothiocyanate
(FITC)-coupled lectin to label the vasculature. Under deep anesthesia,
the chest cavity was carefully opened, and a 16-gauge perfusion cannula
was introduced into the left ventricle. Drainage was achieved using a
16-gauge needle placed in the right atrium. After PBS perfusion,
fixation with 1% paraformaldehyde and 0.5% glutaraldehyde was
achieved at physiologic pressure followed by perfusion with
FITC-coupled Con A lectin (20 µg/ml in PBS, pH 7.4, 5 mg/kg BW;
Vector Laboratories, Burlingame, CA). The eyes were then enucleated,
and the corneas were flat mounted. Using a fluorescent microscope,
threshold levels of green saturation and intensity were established by
a masked observer and used to quantify all areas of neovascularization
within the limbal vascular arcade. The masked observer, using the Leica
microscope and Improvision Openlab software image analysis,
measured the area of neovascularization. The area of neovascularization
was divided by the total corneal area and expressed as a percentage.
VEGF ELISA
Each cornea was placed into 200 µl of lysis buffer (20 mM
imidazole HCl, 10 mM KCl, 1 mM MgCl2, 10 mM EGTA,
1% Triton, 10 mM NaF, 1 mM Na molybdate, 1 mM EDTA, pH 6.8)
supplemented with a protease inhibitor cocktail (Boehringer Mannheim,
Indianapolis, IN) followed by sonication. The lysate was cleared of
debris by centrifugation at 14,000 rpm for 15 minutes (4°C), and the
supernatant was assayed. Total protein was determined using a
commercial assay (BCA kit; Bio-Rad, Hercules, CA). Supernatant VEGF
levels were determined using a sandwich enzyme-linked immunosorbent
assay (ELISA) according to the manufacturers instructions (R&D
Systems, Minneapolis, MN) and normalized to total protein.
VEGF Pellet Construction
The technique for mouse corneal pocket pellet construction and
placement has been well described.9
Briefly, mouse
VEGF164 (R&D Systems) was mixed with Hydron
(Interferon Sciences, New Brunswick, NJ) and sucralfate (Sigma-Aldrich,
St. Louis, MO) so that each pellet contained 50 µg of
VEGF164 protein. Pellets were implanted into the
temporal quadrant of the cornea 24 hours after the area was injected
with pIRES2-EGFP or pHIHGAdd2Flt.
Statistics
Data are presented as mean ± SEM. Significance was tested
using the paired two-sided t-test. P values < 0.05 were deemed significant.
| Results |
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1800 ng produced a
maximal effect at 24 hours, with 40.5% ± 7.5% (n =
3) of the corneal surface staining for ß-gal (Figs. 3A
3B
3C
3D)
. Twelve hundred nanograms of pLacZ in 2 µl of saline was
injected into corneas that were stained at 1, 4, 8, 12, and 24 hours
and 2, 5, 10, and 12 days. The expression of ß-gal protein (Figs. 3E
3F
3G
3H)
was observed as early as 1 hour (1.8% ± 1.4%,
n = 3), peaked by 24 hours (50% ± 3.3%,
n = 3), and was negligible by 12 days (data not shown).
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To confirm the production of VEGF protein after pCMMPmuVEGF injection, corneal VEGF levels were assayed via ELISA. Seven days after 1200 ng pCMMPmuVEGF injection, 7.07 ± 1.53 pg VEGF/µg total corneal protein was detected. In contrast, the pIRES2-EGFPinjected corneas contained only 0.25 ± 0.14 pg VEGF/µg corneal protein (n = 6, P < 0.001).
To determine whether VEGF-induced corneal neovascularization could be inhibited, 1200 ng pHIHGAdd2Flt was injected into corneas. pHIHGAdd2Flt shows constitutive expression of a soluble fragment of the VEGF receptor Flt-1 (extracellular domains 13), an inhibitor of VEGF bioactivity. The contralateral control corneas received 1200 ng of pIRES2-EGFP. Twenty-four hours later, controlled-release pellets containing 50 µg of mouse VEGF164 were implanted in the corneas as described previously.9 10 Seven days later, the pHIHGAdd2Flt treated corneas showed greatly reduced levels of corneal neovascularization when compared with control corneas, 8.7% ± 3.0% (n = 10) vs. 32.3% ± 4.9% (n = 10), respectively (P < 0.04; Figs. 4D 4E 4F ).
| Discussion |
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Plasmid saline solutions induce little to no inflammation, as in the case of pLacZ, or appropriately cause inflammation and neovascularization, as with pIRES2-EGFPmusVEGF and pCMMPmuVEGF. Ongoing work in the larger rabbit eye has validated the results in the mouse eye described here (data not shown). Given the extraordinarily rapid expression of protein with this approach, it should prove useful in the treatment of acute corneal diseases. The inhibition of pathologic corneal vessels, via local gene injection, could theoretically be achieved. Anti-inflammatory gene products could be used to control postoperative inflammation in cataract surgery, without the attendant glaucoma risk associated with topical steroids. Control of immune cell responses with cytokine inhibitors expressed locally and transiently could limit rejection episodes in corneal transplants. Visually handicapping persistent epithelial defects might be effectively treated with plasmids encoding growth factors. Because standard ophthalmologic equipment, that is, the slit-lamp biomicroscope, allows one to visualize the cornea and surrounding tissues under high magnification and determine transfection success and safety, these methods can be readily transferred from the laboratory to the clinic. Protocols for anterior chamber surgery or manipulation often require only one anesthetic drop instilled into the conjunctival cul-de-sac. Quick, safe, and readily available anesthetic technique makes laboratory use and eventually clinical application of direct stromal injection attractive. How broadly these techniques are applied will be determined by ongoing work.
| Footnotes |
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Submitted for publication November 10, 2000; revised April 3, 2001; accepted April 19, 2001.
Commercial relationships policy: N.
The publication costs of this article were defrayed in part by page charge payment. This article must therefore be marked "advertisement" in accordance with 18 U.S.C.
1734 solely to indicate this fact.
Corresponding author: Anthony P. Adamis, Massachusetts Eye and Ear Infirmary, 243 Charles Street, Boston, MA 02114. tony_adamis{at}meei.harvard.edu
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