1 Division of Veterinary and Biomedical Sciences, Murdoch University, Murdoch, Australia, 2 Shigei Medical Research Institute, Okayama, Japan and 3 Division of Laboratory Medicine and 4 University of Melbourne Department of Medicine, Austin and Repatriation Medical Centre, Heidelberg, Australia
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Abstract |
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Methods. Affected dogs were identified by a urinary protein creatinine 0.3. A total of 10 affected adult Dalmatians and eight unaffected age- and sex-matched dogs from breeds other than Dalmatians were examined. In addition, kidneys from five Dalmatian fetuses from affected mothers were examined histologically and ultrastructurally.
Results. All affected dogs were purebred Dalmatians and had a common progenitor. Successive generations were affected, and males and females were affected equally often and equally severely, consistent with autosomal dominant inheritance. The median age at onset of renal failure was 18 months (range 8 months to 7 years). Affected dogs were not clinically deaf, and did not have the ocular abnormalities seen in human X-linked or autosomal recessive Alport syndrome. In addition, they did not have the leucocyte inclusions, low platelet counts or large platelets seen in autosomal dominant hereditary nephritis due to MYH9 mutations. The renal histology and ultrastructural appearance of the GBM appeared to be normal in utero. However, affected adult kidneys demonstrated segmental glomerular hyalinosis and sclerosis with tubulo-interstitial inflammation and fibrosis, and on ultrastructural examination the GBM was lamellated with subepithelial frilling, vacuolation and occasional intramembranous deposits. All 1(IV)
5(IV) type IV collagen chains were present in the affected GBM and Bowman's capsule.
Conclusions. Autosomal dominant Alport syndrome in Dalmatians resembles the disease in Bull terriers but has arisen independently. These models will enable us to determine how genetic mutations affect the corresponding proteins and overall membrane structure in autosomal dominant Alport syndrome.
Keywords: autosomal dominant Alport syndrome; Dalmatian dogs; glomerular basement membrane; hereditary nephritis
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Introduction |
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Autosomal dominant Alport syndrome is rare, and the clinical phenotype differs from that seen with X-linked and autosomal recessive inheritance. Haematuria is associated with both normal renal function and progressive renal impairment, hearing loss is common, but ocular abnormalities do not occur [8,9]. Again, the GBM is lamellated but the 3(IV)
5(IV) collagen chains are present in the affected GBM [10]. Mutations affect the COL4A3 or COL4A4 genes [8,9] which are also abnormal in autosomal recessive inheritance. Autosomal dominant Alport syndrome has to be distinguished from autosomal dominant hereditary nephritis with progressive renal failure, deafness, and leucocyte inclusions, low platelet counts and large platelets [11,12]. This is caused by mutations in the MYH9 gene which encodes a non-muscle myosin heavy chain [13,14].
The diagnosis of all forms of Alport syndrome depends on the presence of the typical clinical features together with a family history of the disease, or on the ultrastructural demonstration of a lamellated GBM. It is always caused by a mutation in one of the type IV collagen genes. Autosomal dominant inheritance is characterized by disease in successive generations, and males and females being affected equally often and equally severely. It is the only form of inheritance in which disease is transmitted from father to son.
Canine models of X-linked, autosomal recessive and dominant Alport syndrome in Samoyed and Navasota dogs, Cocker spaniels and Bull terriers, respectively [1519], have contributed enormously to our understanding of the pathogenesis and pathology of Alport syndrome. We describe here a novel canine model of autosomal dominant Alport syndrome in Dalmatians and compare its clinical, histological and ultrastructural features with those seen in the Bull terrier model and the human disease.
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Animals and methods |
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Dalmatian fetuses (n=5) were from litters with an affected mother and were aged 35 days (canine gestation is 6163 days). Normal fetuses (n=5) were also at about 35 days of gestation, and were from dog breeds where there was no known history of renal disease. Fetuses were obtained either at the time of routine hysterectomy or after euthanasia for medical reasons.
Clinical features
Random voided urine specimens were tested for haematuria and UPC. Haematuria was present if the urine was positive for blood on testing with Multistix (Ames Co., Miles Laboratories, Elkhart, IN, USA) or if any red cells were observed in a field at x600 magnification by light microscopy. UPC was estimated using a Cobas Mira autoanalyser (HoffmanLaRoche, Basel, Switzerland) and the results confirmed on a second specimen taken at least 1 month later.
Hearing was tested clinically at 4 weeks by a veterinarian. Eyes were dilated with topical 1.0% tropicamide and examined by direct ophthalmoscopy by an experienced observer for the oil droplet sign of anterior lenticonus and the dot-and-fleck retinopathy seen in human X-linked and autosomal recessive Alport syndrome. Blood films were made within 3 h of collection, stained with the May Grunwald Giemsa stain, and examined for neutrophil inclusions and platelet number and size.
Histological examination
Renal tissue obtained from biopsies, collected under general anaesthesia or after euthanasia for medical indications, was fixed immediately in neutral-buffered formalin and processed for light microscopy by standard methods. Three micron paraffin-embedded sections were stained with haematoxylin and eosin, periodic acid-Schiff and silver methenamine.
Ultrastructural examination
Renal tissue was fixed in 1.5% chilled glutaraldehyde in 0.1 M phosphate-buffered saline, post-fixed in 1% Dalton's osmium tetroxide and embedded in Epon 812 (TAAB Laboratories, TAAB Lab Equipment, Berkshire, UK). Thin sections were cut on a Reichert Ultra Cut E microtome, supported on 200 mesh copper grids, and stained with saturated uranyl acetate and lead citrate. After carbon coating, the grids were examined in a Philips 301 transmission electron microscope.
Basement membrane composition
Fixed, embedded blocks of kidney tissue were incubated in 0.2 M HCl at 110127°C for 6 min depending on the requirements of individual antibodies. Sections were then incubated with rat monoclonal antibodies against the 1(IV)
6(IV) collagen chains and the colour developed with the LSAB2 kit (DAKO, Glostrup, Denmark). These antibodies had been prepared by immunizing rats with synthetic peptides from the C-termini of the human
(IV) non-collagenous domains. Their use has been described previously [21].
This project had the approval of the Animal Ethics' Committee of Murdoch University.
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Results |
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Haematuria was present in four of the five affected dogs (80%) tested, and none of the normals. Four males and four females had renal failure. Their median age at onset of renal failure was 18 months (range 8 months to 7 years). None of the affected dogs was clinically deaf at 4 weeks of age (n=10) or subsequently demonstrated any features suggesting deafness to their owner-breeders, and none had the ocular abnormalities seen in human X-linked or autosomal recessive Alport syndrome (n=6). In addition, none had the leucocyte inclusions, thrombocytopenia or large platelets present in autosomal dominant hereditary nephritis with haematological abnormalities due to MYH9 mutations (n=6).
Histological and ultrastructural appearance
Adult kidneys
The light microscopic appearance was nearly normal in biopsies from dogs with early disease (n=3). Later, at post-mortem (n=4), the kidneys were macroscopically small with moderate pallor and firmness. Histologically there was segmental hyalinosis and glomerular sclerosis, tuft adhesions without an increase in tuft cellularity, mild through to marked tubulo-interstitial inflammation and fibrosis, and gross capillary wall thickening.
Ultrastructural examination of the GBM from biopsies from affected dogs early in the disease and later at post-mortem showed lamellation, subepithelial frilling and vacuolation (Figure 1). These abnormalities all increased the width of the affected GBM compared with normal. The frilling affected both the paramesangium and the peripheral capillary loops in individual glomeruli. Vacuolation and occasional intramembranous deposits were present. Foot process effacement overlying the abnormal GBM, and mesangial matrix expansion were noted. The ultrastructural abnormalities were, however, less prominent than in affected Bull terriers since some glomerular loops from each dog were only mildly abnormal, and GBM changes often affected only part of the circumference of the glomerular loop. The tubular basement membranes and Bowman's capsule were of normal thickness and appearance in the affected kidneys.
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Fetal kidneys
The light microscopic appearance was normal in all five fetuses from litters where one parent was affected, and ultrastructural examination showed that the GBM had none of the subepithelial frilling, lamellation or vacuolation seen in affected neonatal Bull terriers. The absence of ultrastructural GBM changes in these embryos did not, however, preclude in utero abnormalities in this model since it was not possible to confirm that any fetus was affected. The histological and ultrastructural appearances of the normal fetal kidneys (n=5) showed no significant abnormalities.
Basement membrane composition
Each of the 1(IV)
6(IV) collagen chains was demonstrated in the renal basement membranes from affected dogs. The
1(IV)
2(IV) chains were present in all the basement membranes, the
3(IV)
5(IV) chains in the GBM, Bowman's capsule and the distal TBM, and a tiny amount of the
6(IV) chain was demonstrated in Bowman's capsule. This distribution was identical to that seen in Bull terriers with hereditary nephritis and in normal dogs.
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Discussion |
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The clinical and histological phenotypes are nearly identical for both the Dalmatian and Bull terrier models of autosomal dominant Alport syndrome. The only distinguishing features are the possible absence of ultrastructural changes in utero in Dalmatians and their less prominent GBM abnormalities. All affected Dalmatians and Bull terriers have arisen from two independent progenitors in each breed within the past 50 years, and are likely to have two different mutations. The mutation in Bull terriers is also likely to affect the COL4A3 or COL4A4 genes and the minor differences in phenotype could be due to the small number of Dalmatians examined, differences in the underlying mutations or to environmental effects.
The abundance of canine models of Alport syndrome may reflect an increased mutation rate in the causative genes, the care with which inherited disease is monitored and investigated in pedigree dogs, or the sophistication of diagnostic techniques available to veterinary medicine. The Bull terrier model of autosomal dominant Alport syndrome in particular has contributed enormously to our understanding of a condition that is rare or infrequently recognized. The Bull terrier model has suggested that the abnormalities in autosomal dominant Alport syndrome begin in utero, that impaired renal function is principally due to tubulo-interstitial disease, and that extrarenal basement membranes are not affected [1821,23].
The Dalmatian and Bull terrier models of autosomal dominant Alport syndrome can be used to explore the relationship between genetic mutations, the corresponding biochemical defects, and the clinical and ultrastructural phenotypes.
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Acknowledgments |
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Notes |
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References |
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