Crosslinking of collagen in a heritable disorder of connective tissue: Ehlers-Danlos syndrome

Protective effects of riboflavin-UVA-mediated posterior sclera collagen cross-linking in a guinea pig model of form-deprived myopia

AIM

To evaluate the effect of posterior sclera collagen cross-linking induced by riboflavin-ultraviolet A (UVA) on form-deprived myopia in guinea pigs.

METHODES

Twenty-five pigmented guinea pigs of 3-week-old were randomly assigned into 4 groups that included normal control (NOR, n=7), form-deprived (FDM, n=7), normal with riboflavin-UVA cross-linking (NOR+CL, n=5) and form-deprived with cross-linking (FDM+CL, n=6). The NOR+CL group and the FDM+CL group received the riboflavin-UVA induced cross-linking at day 0. FDM was induced by monocularly deprived with facemask in the right eyes. The refraction, axial length and corneal curvature were measured by retinoscopy, A-scan and keratometer respectively in scheduled time points (day 0 and 1, 2, 3, 4wk after form-deprivation). At the end of 4 weeks' experiment, stress-strain tests of sclera were measured and morphological changes of sclera and retina were examined.

RESULTS

After 4wk, the interocular difference of refractive error were -0.11±0.67, -2.93±0.56, 1.10±0.58, and -1.63±0.41 D in the NOR, FDM, NOR+CL, and FDM+CL groups respectively. Mixed-effect linear model revealed significant effect of FDM (P<0.01) and CL (P<0.001). Also, after 4wk, the interocular difference of axial length were 0.01±0.04, 0.29±0.07, -0.13±0.06, and 0.11±0.05 mm in the NOR, FDM, NOR+CL, and FDM+CL group. Mixed-effect linear model revealed significant effect of FDM (P<0.001) and CL (P<0.01). As for corneal curvature, significant interocular difference have not found between any of the two groups. At the end of this experiment, the ultimate stress and elastic modulus were found significantly increased in both CL groups. But no difference was found in the groups without cross-linked. There was no abnormality observed in the retina and RPE cells of the treated eyes.

CONCLUSION

The posterior sclera collagen cross-linking induced by riboflavin-UVA can slow down the progress of myopia and increase the sclera biomechanical strength in the guinea pig model of form-deprived myopia.

Scleral Cross-linking Using Riboflavin and Ultraviolet-A Radiation for Prevention of Axial Myopia in a Rabbit Model

Myopic individuals, especially those with severe myopia, are at higher-than-normal risk of cataract, glaucoma, retinal detachment and chorioretinal abnormalities. In addition, pathological myopia is a common irreversible cause of visual impairment and blindness. Our study demonstrates the effect of scleral crosslinking using riboflavin and ultraviolet-A radiation on the development of axial myopia in a rabbit model. The axial length of the eyeball was measured by A-scan ultrasound in New Zealand white rabbits aged 13 days (male and female). The eye then underwent 360° conjunctival peritomy with scleral crosslinking, followed by tarsorrhaphy. Axial elongation was induced in 13 day-old New Zealand rabbits by suturing their right eye eyelids (tarsorrhaphy). The eyes were divided into quadrants, and every quadrant had two scleral irradiation zones, each with an area of 0.2 cm² and a radius of 4 mm. Crosslinking was performed by dropping 0.1% dextran-free riboflavin-5-phosphate onto the irradiation zones 20 sec before ultraviolet-A irradiation and every 20 sec during the 200 sec irradiation time. UVA radiation (370 nm) was applied perpendicular to the sclera at 57 mW/cm² (total UVA light dose, 57 J/cm²). Tarsorrhaphies were removed on day 55, followed by repeated axial length measurements. This study demonstrates that scleral crosslinking with riboflavin and ultraviolet-A radiation effectively prevents occlusion-induced axial elongation in a rabbit model.

Scleral cross-linking using riboflavin and ultraviolet-a radiation for prevention of progressive myopia in a rabbit model

Our study demonstrates the effect of scleral cross-linking using riboflavin and ultraviolet-A radiation on the development of axial myopia in a rabbit model. Axial length of the eyeball was measured by A-scan ultrasound in 22 New Zealand white rabbits aged 13 days. The right eyes then underwent 360-degree conjunctival peritomy with (experimental group, n = 11) or without (control group, n = 11) scleral cross-linking, followed by tarsorrhaphy. The left eyes served as a control eye. In the experimental group, the right eyeballs were divided into quadrants, and every quadrant had either 2 (n = 8) or 6 (n = 3) scleral irradiation zones, each with an area of 0.2 cm² and radius of 4 mm. Cross-linking was performed by dropping 0.1% dextran-free riboflavin-5-phosphate onto the irradiation zones at 20 s before ultraviolet-A irradiation and every 20 s during the 200-s irradiation time. UVA radiation (370 nm) was applied perpendicular to the sclera at 57 mW/cm² (total UVA light dose, 57 J/cm²). Tarsorrhaphies were removed on day 55, followed by repeated axial-length measurement. In the control group, mean axial length in the right eyes increased from 10.50 ± 0.67 mm at baseline to 15.69 ± 0.39 mm 55 days later, for a mean change of 5.19 ± 0.85 mm. In the experimental group, corresponding values were 10.68 ± 0.74 mm and 14.29 ± 0.3 mm, for a mean change of 3.61 ± 0.76 mm. The between-group difference in the change in mean axial length was statistically significant (p < 0.001, Mann-Whitney nonparametric test). The present manuscript demonstrates that scleral cross-linking with riboflavin and ultraviolet-A radiation effectively prevents occlusion-induced axial elongation in a rabbit model.

Effect of cross-linking with riboflavin and ultraviolet A on the chemical bonds and ultrastructure of human sclera

This study examined the effect of the cross-linking with riboflavin-ultraviolet A (UVA) irradiation on the chemical bonds and ultrastructural changes of human sclera tissues using Raman spectroscopy and atomic force microscopy (AFM). Raman spectroscopy of the normal and cross-linked human sclera tissue revealed different types of the riboflavin-UVA and collagen interactions, which could be identified from their unique peaks, intensity, and shape. Raman spectroscopy can prove to be a powerful tool for examining the chemical bond of collagenous tissues at the molecular level. After riboflavin-UVA treatment, unlike a regular parallel arrangement of normal collagen fibrils, the AFM image revealed interlocking arrangements of collagen fibrils. The observed changes in the surface topography of the collagen fibrils, as well as in their chemical bonds in the sclera tissue, support the formation of interfibrilar cross-links in sclera tissues.

Long-term biomechanical properties of rabbit sclera after collagen crosslinking using riboflavin and ultraviolet A (UVA)

PURPOSE

Scleral crosslinking by the photosensitizer riboflavin and ultraviolet A (UVA) has been shown to increase significantly the scleral biomechanical rigidity and might therefore become a possible sclera-based treatment modality for progressive myopia. In the present study, the long-term effect of the new crosslinking method on biomechanical properties was investigated in the rabbit sclera.

METHODS

A 10 x 10 mm sector of the equatorial sclera of nine Chinchilla rabbit eyes was treated in vivo using a UVA double diode of 370 nm with a surface irradiance of 3 mW/cm(2) and application of 0.1% riboflavin-5-phosphate drops as photosensitizer for 30 min. Three days, 4 months and 8 months postoperatively, biomechanical stress-strain measurements of the treated scleral strips were performed and compared to contralateral control sclera using a microcomputer-controlled biomaterial tester. In addition, routine histological controls were performed.

RESULTS

Following the crosslinking treatment, Young's modulus was increased by 320% after 3 days, 277% after 4 months and 502% after 8 months, and ultimate stress by 341% after 3 days, 131% after 4 months and 213.8% after 8 months versus the controls. The decrease in ultimate strain was between 24% and 44.8%. On histology, no tissue damage was detected.

CONCLUSION

Our new method of scleral collagen crosslinking proved very effective and constant over a time interval of up to 8 months in increasing the scleral biomechanical strength. Therefore, the new treatment might become an option for strengthening scleral tissue in progressive myopia and other conditions associated with weakened sclera. There were no side-effects on the retina or retinal pigment epithelium. The new crosslinking treatment could now be tested in a suitable myopia model (like the tree shrew) and finally in human eyes.

Long-term biomechanical properties after collagen crosslinking of sclera using glyceraldehyde

PURPOSE

Chemical crosslinking by glyceraldehyde has been shown to increase significantly the biomechanical rigidity of sclera. It might therefore become an option for a sclera-based treatment of progressive myopia. The present pilot study was designed to test the long-term biomechanical efficiency of the new crosslinking method.

METHODS

Six Chinchilla rabbits were treated with sequential sub-Tenon's injections of 0.15 ml 0.5 m glyceraldehyde, which were given in the supero-nasal quadrant of the right eye (OD) five times over 14 days. The rabbits were killed 4 months and 8 months after crosslinking treatment, respectively. Biomechanical stress-strain measurements of scleral strips from the treatment area were performed and compared to non-treated contralateral control sclera using a microcomputer-controlled biomaterial testing device. In addition, the eyes were examined histologically by light microscopy to evaluate possible side-effects.

RESULTS

Following the crosslinking treatment, the ultimate stress was 10.2 +/- 2.3 MPa after 4 months and 8.5 +/- 2.2 MPa after 8 months versus 2.4 +/- 0.3 MPa in the controls (increases of 325% and 254.17%, respectively); Young's modulus was 104.6 +/- 13.7 MPa after 4 months and 53.2 +/- 5.2 MPa after 8 months versus 9.6 +/- 1.3 MPa in the controls (increases of 989.6% and 554.17%, respectively); and ultimate strain was 15.8 +/- 1.5% after 4 months and 24.1 +/- 0.7% after 8 months versus 38.4 +/- 4.6% in the controls (decreases of 58.84% and 37.24%, respectively). Histologically, no side-effects were found.

CONCLUSION

Our new method of scleral collagen crosslinking proved very efficient in increasing scleral biomechanical strength over a period of up to 8 months. Glyceraldehyde can be applied easily by sequential parabulbar injections. Before clinical application in myopic patients, a study in an animal myopia model is recommended.

Crosslinking of scleral collagen in the rabbit using glyceraldehyde

PURPOSE

To strengthen rabbit sclera in vivo using chemical crosslinking with glyceraldehyde for a scleral-based treatment of progressive myopia.

SETTING

Department of Ophthalmology, Martin-Luther-University, Halle, Germany.

METHODS

Five chinchilla rabbits were treated with sequential sub-Tenon injections of 0.15 mL 0.5 M glyceraldehyde into the superonasal quadrant of the right eye 5 times during 14 days. The rabbits were humanely killed and biomechanical stress-strain measurements of scleral strips from the treatment area were performed and compared with nontreated contralateral control sclera using a microcomputer-controlled biomaterial tester. The treated eyes were examined histologically by light microscopy to exclude possible adverse effects.

RESULTS

Following the crosslinking treatment, the ultimate stress was 15.8 MPa +/- 6.0 (SD) versus 3.1 +/- 0.3 MPa in the controls (increase of 409.7%; P<.02), the Young modulus was 129.6 +/- 53.7 MPa versus 11.5 +/- 1.8 MPa in the controls (increase of 1027%, P<.01), and ultimate strain was 19.8% +/- 2.6% MPA versus 38.2% +/- 5.1% MPA in the controls (decrease of 48.2% P<.05). Histologically, mild side effects were found in the peripheral cornea adjacent to the treatment area, with some inflammatory infiltrate and moderate loss of keratocytes.

CONCLUSIONS

Glyceraldehyde crosslinking of scleral collagen increased the scleral biomechanical rigidity efficiently. Glyceraldehyde can be easily applied by sequential parabulbar injections. There were no side effects on the retina, so the new method might become a treatment modality for strengthening scleral tissue to prevent progressive myopia.

Cross-linking of scleral collagen in the rabbit using riboflavin and UVA

PURPOSE

Scleral biomechanical weakness and thinning is known to be one of the main factors in the pathogenesis of progressive myopia. We tried to strengthen rabbit sclera by cross-linking scleral collagen using ultraviolet A (UVA) and the photosensitizer riboflavin.

METHODS

Circumscribed 10 x10 mm sectors of the posterior--equatorial sclera of six chinchilla rabbit eyes were treated in vivo using a UVA double diode with 4.2 mW/cm(2) UVA at 370 nm and applying 0.1% riboflavin-5-phosphate drops as photosensitizer for 30 min. 1 day postoperatively biomechanical stress--strain measurements of three treated scleral strips were performed using a microcomputer-controlled biomaterial testing device and compared to non-treated contralateral control sclera. In addition, three treated eyes were examined histologically by light microscopy, TUNEL staining and electron microscopy to evaluate side-effects.

RESULTS

Following the cross-linking treatment, the ultimate stress was 11.87+/-1.8 MPa versus 3.63+/-0.40 in the controls (increase of 227.9%, p=0.014), Young's modulus 27.67+/-4.16 MPa versus 4.9+/-.15 MPa in the controls (increase of 464.7%, p=0.021) and ultimate strain 92.2+/-7.43% versus 165.63+/-19.09% in the controls (decrease of 54.52%, p=0.012). Histologically, serious side-effects were found in the entire posterior globe with almost complete loss of the photoreceptors, the outer nuclear layer and the retinal pigment epithelium (RPE).

CONCLUSIONS

Our new method of scleral collagen cross-linking proved very effective in increasing the scleral mechanical strength; the new treatment may represent an option for strengthening scleral tissue in progressive myopia. However, serious side-effects were observed in the outer retina. In future studies these side-effects could be avoided by reducing the irradiation dose below the cytotoxic level of the retina. Before its clinical application, the new method should be tested in a myopia animal model.

Collagen crosslinking of human and porcine sclera

PURPOSE

To develop methods of collagen crosslinking the sclera to increase its biomechanical strength for the treatment of progressive myopia.

SETTING

Department of Ophthalmology, Technical University of Dresden, Dresden, Germany.

METHODS

Sagitally oriented scleral strips of 4.0 mm x 8.0 mm were prepared from 5 human postmortem eyes and 50 porcine cadaver eyes and treated with various crosslinking methods including physical crosslinking by combined riboflavin-ultraviolet A (UVA) or rose bengal/white-light irradiation and chemical crosslinking by incubation with glucose, ribose, glyceraldehyde, and glutaraldehyde solutions. Parallel scleral strips from the same eye were used as untreated controls. After crosslinking, stress-strain measurements of the treated and control scleras were performed using a microcomputer-controlled biomaterial tester.

RESULTS

A statistically significant increase in scleral rigidity was found after crosslinking with riboflavin-UVA, with a rise in stress in treated porcine (157%) and human (29%) sclera, and after treatment with glyceraldehyde, with a rise in stress in treated porcine (487%) and human (34%) sclera, and with glutaraldehyde, with a rise in stress in treated porcine (817%) and human sclera (122%) at 8% strain. The other crosslinking methods proved ineffective. The untreated human sclera had a 4-fold higher stiffness than porcine sclera.

CONCLUSIONS

Collagen crosslinking induced by riboflavin-UVA, glyceraldehyde, and glutaraldehyde led to a significant increase in biomechanical strength in human and porcine sclera. Using these methods, collagen crosslinking might become a treatment possibility for progressive myopia. Future animal and clinical studies must determine the best application methods and the long-term effects of increased crosslinking on scleral rigidity and prevent potential toxicity or serious side effects.

The Ehlers-Danlos syndrome: an analysis of the structure of the collagen fibres of the skin

The structure of the collagen fibres of the skin in the Ehlers-Danlos syndrome (ED-S) was studied in eight patients with ED-S Type I, three patients with ED-S Type II and three patients with the X-linked Type V. The results show that the reducible cross-links are present and undergo the same maturation process to non-reducible cross-links as in normal skin. Transmission electron microscopy revealed a normal ultrastructure of the collagen fibrils. At a higher morphological level of organization scanning electron microscopy demonstrated progressive increase in fibre bundle disorder from the X-linked to mitis, to gravis, in which the fibres making up the large fibre bundles demonstrated a considerable inability to aggregate.

Purification and enzymatic properties of lysyl hydroxylase from fetal porcine skin

Lysyl hydroxylase from fetal porcine skin is shown to bind in a highly specific manner to aminoethyl-Sepharose 4B. When coupled to ammonium sulfate fractionation and DEAE-cellulose chromatography, chromatography of lysyl hydroxylase preparations on aminoethyl-Sepharose 4B has yielded a highly purified (greater than 95%) preparation of lysyl hydroxylase. The enzyme consists of two subunits with molecular weights of 70 000 and 115 000. The overall recovery of activity was 2.5%, yielding approximately to 3.5 mg of purified enzyme from 900 g of fetal porcine skin. The enzyme is more active at 30 degrees C than at 37 degrees C and has a pH optimum near 8.0. Both catalase and bovine serum albumin are required by the enzyme for maximum activity. The sulfhydryl reagents p-(chloromercuri)-benzoate, N-ethylmaleimide, and iodoacetamide are potent inhibitors of the enzyme, whereas dithiothreitol appears to be an activator.

Defects in the biochemistry of collagen in diseases of connective tissue

The collagens are the major structural glycoproteins of connective tissues. A unique primary structure and a multiplicity of post-translational modification reactions are required for normal fibrillogenesis. The post-translational modifications include hydroxylation of prolyl and lysyl residues, glycosylation, folding of the molecule into triple-helical conformation, proteolytic conversion of precursor procollagen to collagen, and oxidative deamination of certain lysyl and hydroxylysyl residues. Any defect in the normal mechanisms responsible for the synthesis and secretion of collagen molecules or the deposition of these molecules into extracellular fibers could result in abnormal fibrillogenesis; such defects could result in a connective tissue disease. Recently, defects in the regulation of the types of collagen synthesized and in the enzymes involved in the post-translational modifications have been found in heritable diseases of connective tissue. Thus far, the primary heritable disorders of collagen metabolism in man include lysyl hydroxylase deficiency in Ehlers-Danlos syndrome type VI, p-collagen peptidase deficency in Ehlers-Danlos syndrome type VII, decreased synthesis of type III collagen in Ehlers-Danlos syndrome type IV, lysyl oxidase deficency in S-linked cutis laxa and Ehlers-Danlos syndrome type V, and decreased synthesis of type I collagen in osteogenesis imperfecta.

Clinical pathologic observations in pseudoxanthoma elasticum

Skin biopsy material obtained from 2 patients with pseudoxanthoma elasticum associated with congenital hyperphosphatasia was compared with skin biopsy material obtained from 3 patients with pseudoxanthoma elasticum unassociated with any other disease. Clinically normal skin showed calcification of normal-appearing elastin as the initial change in pseudoxanthoma elasticum. Elastin fibers become more ravelled as the disease progresses. A previously unreported early transient calcification of collagen was demonstrated. Other abnormal collagen forms may also be present. The Von Kossa stain appears to be the stain of choice for identifying these earliest histologic changes.

Cross-linking of collagen

The formation of collagen cross-links is attributable to the presence of two aldehyde-containing amino acids which react with other amino acids in collagen to generate difunctional, trifunctional, and tetrafunctional cross-links. A necessary prerequisite for the development of these cross-links is that the collagen molecules be assembled in the naturally occurring fibrous polymer. Once this condition is met, cross-linking occurs in a spontaneous, progressive fashion. The chemical structures of the cross-links dictate that very precise intermolecular alignments must occur in the collagen polymer. This seems to be a function of each specific collagen because the relative abundance of the different cross-links varies markedly, depending upon the tissue of origin of the collagen.

Lysyl-protocollagen hydroxylase deficiency in fibroblasts from siblings with hydroxylysine-deficient collagen

Cell culture studies were performed on members of a family in which two sisters, ages 9 and 12, have a similar disorder characterized clinically by severe scoliosis, joint laxity and recurrent dislocations, hyperextensible skin, and thin scars. The skin collagen from the sisters was markedly deficient in hydroxylysine, but other amino acids were present in normal amounts. Hydroxylysine in collagen from fascia and bone was reduced to a lesser extent. Since the most likely explanation for the hydroxylysine deficiency was a reduction in enzymatic hydroxylation of lysine residues in protocollagen, we measured the activity of lysyl-protocollagen hydroxylase in crude lysates of cultured skin fibroblasts. Enzyme activities in the two affected children were 14 and 10% of controls, whereas the activity was about 60% of normal in the mother, a pattern most consistent with autosomal recessive inheritance. The mutant enzyme demonstrated the same cofactor requirements as that from normal cells. Deficiency of lysyl-protocollagen hydroxylase is the first inborn error of human collagen metabolism to be defined at the biochemical level.

Reducible crosslinks in hydroxylysine-deficient collagens of a heritable disorder of connective tissue

Reducible compounds that participate in crosslinking were analyzed in hydroxylysine-deficient collagens of patients with a heritable disorder of connective tissue. After treatment with [(3)H]sodium borohydride, new compounds, as well as a totally different pattern of tritiated compounds, were found in hydroxylysine-deficient collagen from skin as compared with age-matched controls. The amount of desmosines detected indicated that more elastin was present in abnormal skin than in control skin.Bone collagen, which was not as deficient in hydroxylysine as skin collagen, had the same compounds as normal bone collagen, but their relative proportions were altered, consistent with a deficiency of hydroxylysine, a precursor of the crosslinks. Although the content of hydroxylysine in collagen of cartilage is essentially normal in these patients, analysis after reduction revealed a different pattern of reduced compounds from that of normal cartilage. It is speculated that Type II collagen, the major collagen component in cartilage, contains a normal amount of hydroxylysine, while Type I collagen, which is the major source of the crosslinks, is hydroxylysine-deficient. This distribution would explain the findings of an abnormal profile of reducible compounds despite an almost normal total hydroxylysine content. The finding that the deficiency of hydroxylysine in the collagen of these patients is accompanied by changes in number, chemical nature, and, probably, distribution of crosslinkages, and the previously reported alterations in the solubility characteristics, suggest that at least some skeletal and connective tissue abnormalities are directly related to underlying molecular pathology.