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From the Departments of 1 Pathology and 2 Clinical Ophthalmology, Institute of Ophthalmology, University College London; and Moorfields Eye Hospital, London, England.
| Abstract |
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METHODS. The eyes of an SFD donor with a confirmed TIMP-3 mutation were examined using histologic techniques demonstrating connective tissue, calcium, and lipid. Immunohistochemical analyses were performed using antibodies against TIMP-3, collagen type IV, V, and VI, laminin, fibronectin, elastin, and fibrillin. Electron microscopy also was used.
RESULTS. A subretinal pigment epithelium (sub-RPE) deposit similar to that previously described was seen. A morphologically similar but different deposit was present internal to the nonpigmented ciliary epithelium (NPCE). Both deposits contained collagens, elastin, glycosaminoglycans, lipids, and calcium. Immunolabeling of TIMP-3 was found in the basement membrane of the NPCE, Bruchs membrane, and choroidal vessels in normal control subjects. In SFD, immunolabeling of TIMP-3 also was present in the sub-RPE deposit and in the inner portion of the ciliary body deposit. TIMP-3 immunoreactivity was more extensive in the SFD eye. The pattern of elastin immunoreactivity was remarkably similar to that of TIMP-3. Electron microscopy revealed a morphologically altered elastic layer of the Bruchs membrane.
CONCLUSIONS. Sub-RPE TIMP-3 immunoreactivity appears more extensive in SFD than in control subjects. There is also a correspondence between TIMP-3 and elastin immunoreactivies, which invites speculation as to a link between the SFD TIMP-3 mutation and altered elastin processing. The accumulation of abnormal material in SFD is more widespread than previously reported. In view of this, SFD might be better termed Sorsbys ocular epitheliopathy.
| Introduction |
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There are two previous histologic reports of the condition. Ashton and Sorsby9 reported two sisters with clinical features similar to SFD. There was, however, no family history and they were not among the original family pedigrees. It has been suggested that these two sisters might have had either a complication of AMD or dominant drusen rather than SFD. Capon and colleagues10 reported the light and electron microscopy findings of a descendant of one of the original pedigrees of SFD. They described a layer of lipid-positive floccular deposit that was up to 30 µm in thickness, on the inner aspect of Bruchs membrane. The composition of this deposit remains unknown. The anterior segments of these eyes were not described.
In this article, we report the histologic and immunohistochemical findings in both the anterior and posterior segments of the eye of an SFD patient with a confirmed TIMP-3 mutation. The localization of TIMP-3 in both normal and SFD eyes also was examined.
| Materials and Methods |
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Fixation and Processing
The right eye was fixed in 0.25% glutaraldehyde and 4%
paraformaldehyde in phosphate-buffered saline, and the left eye was
fixed in 10% formol saline for 48 hours. Specimens for light
microscopy and immunohistochemistry were processed through ascending
concentrations of alcohol into xylene and infiltrated with paraffin
wax. Five-micron-thick sections were dewaxed and rehydrated before use.
Specimens for lipid analyses were frozen in liquid nitrogencooled
isopentane. Ten-micron-thick cryostat sections were cut. Specimens for
electron microscopy were postfixed in osmium tetroxide, processed
through ascending concentrations of ethanol, and finally infiltrated
with Araldite resin.
Staining Techniques
Hematoxylin and eosin was used for general morphology. Mowry and
Marand Alcian blue/periodic acidSchiff (PAS), Gomori aldehyde
fuchsin, Fullmer and Lillie oxidation aldehyde fuchsin, and Verhoeff
techniques were used for demonstrating different classes of
glycosaminoglycans and elastic fibers at different stages of maturity.
McGeeRussel alizarin red S technique and the Pizzolato silver method
were used to demonstrate calcium. Congo red staining was used to
exclude amyloid. Oil red O was used on frozen sections to demonstrate
lipid.
The distribution of TIMP-3 (Triple Point Biologics, Forest Grove, OR), type IV collagen (Dako Ltd., High Wycombe, England), type V collagen (Chemicon Ltd, Harrow, UK), type VI collagen (Chemicon Ltd.), fibrillin (Chemicon Ltd.), fibronectin (Dako Ltd.), laminin (Dako Ltd.), and elastin (Elastin Company, MO) were investigated using a standard biotinstreptavidin biotin, alkaline phosphatase complex method (Dako Ltd.). The alkaline phosphatase label was visualized as a red final reaction product (Vector Ltd., Peterborough, England). Nuclei were weakly stained with Mayers hematoxylin. TIMP-3, elastin, and fibronectin antibodies had been raised in rabbit, and the others were mouse monoclonals.
Antigen retrieval was utilized in all cases. Sections for TIMP-3 and fibrillin required pressure cooking. Sections for fibronectin, laminin, and elastin demonstration were exposed to 0.1% trypsin for 15 minutes at 37°C, whereas types IV, V, and VI collagen sections received 0.4% pepsin treatment for 60 minutes at 37°C. Electron microscopic semithin sections were stained with toluidine blue, and ultrathin sections were stained using lead citrate and uranyl acetate.
Special stains, immunohistochemistry, and electron microscopy also were carried out in a normal donor eye from a 72-year-old normal female donor for comparison. This eye was fixed in 10% formol saline and was treated exactly as the SFD eye as described above. The son of the SFD donor, aged 57 years, also was affected; his right fundal picture showed widespread, drusenlike structures at the posterior pole (Fig. 1A ), whereas his left fundal picture showed a disciform macular scar (Fig. 1B) .
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| Results |
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Light Microscopy
The appearances of the ciliary body and posterior pole were
similar in both eyes. A single-cell, thick, patchy epiretinal membrane
was present. There was retinal gliosis with marked thinning of the
inner nuclear layer and near total loss of photoreceptor cells. There
was patchy loss and disorganization of the RPE. In many situations, an
up to 30 µm thick sub-RPE layer of eosinophilic material was seen.
This extended to just posterior to the ora serrata and was granular in
appearance (Fig. 1C)
. In places, striations oriented perpendicular to
Bruchs membrane were seen. Foci of looser, eosinophilic material
separated the deposits from Bruchs membrane, which itself was
discontinuous. Clumps of punctate mineralization also lay external to
the deposit.
In the anterior segment, a moderately uniform layer of amorphous, eosinophilic material up to 50 µm in thickness lay internal to the nonpigmented ciliary body epithelium (Fig. 1D) . It extended onto the anterior portion of the pars plana but was not present more posteriorly. There was no deposit on the iris.
Special Stains
The main sub-RPE deposit was PAS-positive but Alcian
bluenegative. A rim of positive staining with aldehyde fuchsin was
seen along portions of the outer aspect of the sub-RPE deposit, and
much of the deposit was positive with oxidation aldehyde fuchsin. The
elastin layer of Bruchs membrane was stained by the Verhoeff
technique. It was, however, disrupted in many places. Staining with
Congo red revealed no evidence of amyloid. The sub-RPE deposit was
shown to contain calcium with McGeeRussel alizarin red S method but
was negative with the Pizzolato silver technique for calcium oxalate.
It was lipid-positive with Oil red O staining.
Restricted neovascularization was present external to or within the sub-RPE deposit. The choroid was atrophic, with total loss of the choriocapillaris in some areas, especially where the RPE was absent. Some choroidal vessels displayed diffusely thickened walls. The sclera was unremarkable, and there was atrophy of the optic nerve.
The inner portion of the deposit was positive with aldehyde fuchsin, and the outer portion of the ciliary body was positive with oxidation aldehyde fuchsin. Staining with the other special stains was quantitatively similar to that beneath the RPE. However, both the calcium and lipid staining were less intense.
TIMP-3 Immunohistochemistry
In the normal control subject, Bruchs membrane, the
elastic layer of some choroidal vessels (Fig. 2A
), and the basement membrane of the nonpigmented ciliary epithelium
(Fig. 2B)
were immunoreactive with TIMP-3. In the SFD eye,
immunostaining for TIMP-3 was present in the entire sub-RPE deposit,
and it was more intense in areas where overlying RPE cells were present
(Fig. 2C) . In the ciliary body deposit, however, only the inner portion
was positively stained (Fig. 2D)
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In the control and SFD eyes, an antibody to type VI collagen highlighted retinal and choroidal vessels. In the SFD eye, the outer edge of the sub-RPE deposit was immunoreactive (Fig. 3A ). No positive staining was observed in the ciliary body deposit. In the control and SFD eyes, immunoreactivity for type IV collagen was present at the inner limiting membrane, the basement membrane of blood vessels, and Bruchs membrane. In addition, immunoreactivity of the inner and outer margins of the sub-RPE deposit (Fig. 3B) and a rim of ciliary body deposit just adjacent to the basement membrane was present in the SFD eye.
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Electron Microscopy
The sub-RPE deposit was present between the RPE basement membrane
and the elastic layer of the Bruchs membrane (Fig. 4A
). It appeared to be divided into three main zones. The principle
abnormality was the central electron-dense area, with a branching,
frondlike appearance reminiscent of some corals. Some of this material
exhibited a characteristic banding pattern typically seen in
wide-spaced material (also known as long-spaced collagen) (Fig. 4B)
.
This material was sandwiched between two layers of amorphous material
containing collagen fibers.
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The outer collagenous layer of Bruchs membrane and the choroidal endothelial basement membrane were relatively normal where the elastic layer was intact. However, it was invaded by the electron-dense deposit when it was broken. Glial cell processes, cellular debris, and new vessels also were present in the deposit.
The ciliary body deposit also could be divided into three main zones (Fig. 4D) . The layer adjacent to the ciliary epithelium was similar in appearance and composition to that of the loose collagen layer in the subretinal deposit. Oxytalan fibers, which correspond to the fibrous material positive with oxidation aldehyde fuchsin, were present. The middle zone consisted of a granular deposit with no definite structure. The innermost layer was more electron-dense and appeared to be composed of tightly packed material not dissimilar to that in the middle zone. Wide-spaced material, glial cell processes, cellular debris, and new vessels were not seen in any part of the ciliary body deposit.
| Discussion |
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Similar sub-RPE deposits have been observed in dominant, late-onset retinal degeneration,11 dominant drusen,11 dominant retinitis pigmentosa (RP),13 and hypobetalipoproteinemia with RP.13 In all these cases, the exact morphology and composition might be slightly different, but they all contain extracellular matrix material (including collagen, elastin, and glycoaminoglycans) with or without calcium and lipids.11 12 13 It is possible that the accumulation of these extracellular materials might be a nonspecific response to changes in Bruchs membrane.
TIMP-3 Mutation and the Deposits
TIMP-3 is an inhibitor of matrix metalloproteinases (MMPs),
and MMPs break down connective tissue material in the extracellular
matrix. It is, therefore, perhaps unexpected to find a mutation in
TIMP-3 associated with an increased accumulation of
extracellular material, unless the mutant TIMP-3 shows gain of
function. There is, however, evidence to suggest that mutant TIMP-3
forms a dimer,14
which might not be degraded as rapidly as
the normal TIMP-3, and may accumulate in Bruchs membrane. Because the
mutant TIMP-3 appears to be able to inhibit the activity of
MMPs14
it might lead to an exaggerated inhibition of
protease activity, which, in turn, to an increased accumulation of
extracellular material. As the mutant TIMP-3 is only one amino acid
different from the normal TIMP-3, it is believed that the antibody
against TIMP-3 was immunopositive for both the mutant and the
normal protein. Although immunohistochemistry is not a quantitative
method for the assessment of the amount of protein present, our
findings of increased extent of TIMP-3 immunoreactivity supports the
notion that mutant TIMP-3 may accumulate. There is also an interesting
parallel in that there is more TIMP-3 protein in eyes of AMD donors
than in those of age-matched control subjects.15
TIMP-3 Mutation and Elastin
The localization of TIMP-3 and elastin was similar. They were both
present in the elastic layer of Bruchs membrane, in the elastic layer
of choroidal vessels, and in the basement membrane of the nonpigmented
ciliary epithelium (zonules contain immature elastic fibers). This
suggests that TIMP-3 might play a role in the turnover of elastic
fibers. Although mutant TIMP-3 might inhibit protease activity, it is
not certain whether its actions are entirely normal. It is possible
that mutant TIMP-3 in some way fails to protect elastic fibers, leading
to the damage seen in SFD.
TIMP-3 Mutation and Choroidal Neovascularization
There is a potential paradox in that TIMP-3 has been
reported to have an inhibitory effect on angiogenesis,16
and yet in 2 of the 10 symptomatic patients with SFD in a family survey
has central visual loss due to choroidal
neovascularization.3
Mutant TIMP-3 may not exhibit
antiangiogenic properties. Alternatively, the breaks in the Bruchs
membrane might be too extensive to contain the extension of new
vessels despite extensive TIMP-3 accumulation in Bruchs membrane.
| Summary |
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| Footnotes |
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Submitted for publication February 17, 1998; revised August 20, 1998, November 12, 1998, June 10, 1999, and September 28, 1999; accepted September 29, 1999.
Commercial relationships policy: N.
Corresponding author: Victor Chong, Professorial Unit, Moorfields Eye Hospital, City Road, London, EC1V 2PD, England. v.chong{at}ucl.ac.uk
| References |
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