Enzymological characterization of a putative canine analogue of primary hyperoxaluria type 1

Calcium oxalate urolithiasis in juvenile dogs

BACKGROUND

The features of juvenile-onset calcium oxalate urolithiasis in dogs have not been previously reported.

METHODS

Calcium oxalate urolith submissions to the Minnesota Urolith Center between 2012 and 2016 were analyzed to identify those originating from juvenile (≤2 years, n = 510) or mature (7-9 years, n = 39,093) dogs. Breed, sex, urolith salt type and urolith location were compared between groups. Breeds represented in both groups were also compared with respect to sex, urolith salt type and urolith location.

RESULTS

French (odds ratios [OR] = 14.7, p < 0.001) and English (OR = 14.3, p < 0.001) Bulldogs were overrepresented in juvenile submissions. All juvenile French and English Bulldogs were male. Across all breeds, juvenile dogs were more likely to be male (89%, p < 0.001) than mature dogs (79%). Juvenile dogs were also more likely to form dihydrate stones compared to mature dogs (33% versus 14%, respectively; p < 0.001). Breed differences were discovered in sex, urolith salt type and stone location.

CONCLUSIONS

French and English Bulldogs comprise a greater proportion of juvenile calcium oxalate urolith submissions than expected based on their rarity in mature submissions. Inherited risk factors, particularly X chromosome variants, should be investigated due to the strong breed and sex predispositions identified.

Animal models of naturally occurring stone disease

The prevalence of urolithiasis in humans is increasing worldwide; however, non-surgical treatment and prevention options remain limited despite decades of investigation. Most existing laboratory animal models for urolithiasis rely on highly artificial methods of stone induction and, as a result, might not be fully applicable to the study of natural stone initiation and growth. Animal models that naturally and spontaneously form uroliths are an underused resource in the study of human stone disease and offer many potential opportunities for improving insight into stone pathogenesis. These models include domestic dogs and cats, as well as a variety of other captive and wild species, such as otters, dolphins and ferrets, that form calcium oxalate, struvite, uric acid, cystine and other stone types. Improved collaboration between urologists, basic scientists and veterinarians is warranted to further our understanding of how stones form and to consider possible new preventive and therapeutic treatment options.

Pathology and Epidemiology of Oxalate Nephrosis in Cheetahs

To investigate cases of acute oxalate nephrosis without evidence of ethylene glycol exposure, archived data and tissues from cheetahs ( Acinonyx jubatus) from North America ( n = 297), southern Africa ( n = 257), and France ( n = 40) were evaluated. Renal and gastrointestinal tract lesions were characterized in a subset of animals with ( n = 100) and without ( n = 165) oxalate crystals at death. Crystals were confirmed as calcium oxalate by Raman spectroscopy in 45 of 47 cheetahs tested. Crystals were present in cheetahs from 3.7 months to 15.9 years old. Cheetahs younger than 1.5 years were less likely to have oxalates than older cheetahs ( P = .034), but young cheetahs with oxalates had more oxalate crystals than older cheetahs ( P < .001). Cheetahs with oxalate crystals were more likely to have renal amyloidosis, interstitial nephritis, or colitis and less likely to have glomerular loop thickening or gastritis than those without oxalates. Crystal number was positively associated with renal tubular necrosis ( P ≤ .001), regeneration ( P = .015), and casts ( P ≤ .001) but inversely associated with glomerulosclerosis, renal amyloidosis, and interstitial nephritis. Crystal number was unrelated to the presence or absence of colitis and was lower in southern African than American and European animals ( P = .01). This study found no evidence that coexisting chronic renal disease (amyloidosis, interstitial nephritis, or glomerulosclerosis), veno-occlusive disease, gastritis, or enterocolitis contributed significantly to oxalate nephrosis. Oxalate-related renal disease should be considered as a potential cause of acute renal failure, especially in young captive cheetahs. The role of location, diet, stress, and genetic predisposition in the pathogenesis of oxalate nephrosis in cheetahs warrants further study.

Pathogenesis of calcium oxalate urinary stone disease: species comparison of humans, dogs, and cats

Idiopathic calcium oxalate nephrolithiasis is a highly recurrent disease that is increasing in prevalence. Decades of research have not identified effective methods to consistently prevent the formation of nephroliths or induce medical dissolution. Idiopathic calcium oxalate nephroliths form in association with renal papillary subepithelial calcium phosphate deposits called Randall's plaques (RPs). Rodent models are commonly used to experimentally induce calcium oxalate crystal and stone formation, but a rodent model that conclusively forms RPs has not been identified. Both dogs and cats form calcium oxalate uroliths that can be recurrent, but the etiopathologic mechanisms of stone formation, especially renal pathologic findings, are a relatively unexploited area of study. A large animal model that shares a similar environment to humans, along with a shorter lifespan and thus shorter time to recurrence, might provide an excellent means to study preventative and therapeutic measures, along with enhancing the concepts of the one health initiative. This review article summarizes and compares important known features of idiopathic calcium oxalate stone disease in humans, dogs, and cats, and emphasizes important knowledge gaps and areas for future study in the quest to discover a naturally occurring animal model of idiopathic calcium oxalate stone disease.

Pathological Features of Oxalate Nephrosis in a Population of Koalas (Phascolarctos cinereus) in South Australia

The wild and captive koala population of the Mt Lofty Ranges in South Australia has a high level of renal dysfunction in which crystals consistent with calcium oxalate have been observed in the kidneys. This study aimed to describe the pathological features of the renal disease in this population, confirm the composition of renal crystals as calcium oxalate, and determine whether any age or sex predispositions exist for this disease. A total of 51 koalas (28 wild rescues, 23 captive) were examined at necropsy, of which 28 (55%) were found to have gross and/or histological evidence of oxalate nephrosis. Histopathological features included intratubular and interstitial inflammation, tubule dilation, glomerular atrophy, tubule loss, and cortical fibrosis. Calcium oxalate crystals were demonstrated using a combination of polarization microscopy, alizarin red S staining, infrared spectroscopy, and energy-dispersive X-ray analysis with scanning electron microscopy. Uric acid and phosphate deposits were also shown to be present but were associated with minimal histopathological changes. No significant differences were found between the numbers of affected captive and wild rescued koalas; also, there were no sex or age predispositions identified, but it was found that oxalate nephrosis may affect koalas <2 years of age. The findings of this study suggest that oxalate nephrosis is a leading disease in this koala population. Possible causes of this disease are currently under investigation.

Chronic kidney disease with three cases of oxalate-like nephrosis in Ragdoll cats

Two unrelated Ragdoll cat mothers in Norway were found dead from renal disease. The histopathology was consistent with oxalate nephrosis with chronic or acute-on-chronic underlying kidney disease. Both cats had offspring and relatives with signs of urinary tract disease, including a kitten dead with urethral gravel. Eleven living Ragdoll cats, including nine relatives of the dead cats and the male father of a litter with similarly affected animals, were tested for primary hyperoxaluria (PH) type 1 and 2 by urine oxalate and liver enzyme analysis. Renal ultrasound revealed abnormalities in five living cats. One of these was azotaemic at the time of examination and developed terminal kidney disease 9 months later. A diagnosis of PH was excluded in 11 cats tested. The inheritance and aetiological background of the renal disease present in the breed remains unresolved at this point in time.

End-Stage Kidney Disease Probably due to Reflux Nephropathy with Segmental Hypoplasia (Ask-Upmark Kidney) in Young Boxer Dogs in Norway. A Retrospective Study

This paper is a retrospective morphologic study of 7 young Boxer dogs, showing end-stage kidney lesions compatible with chronic pyelonephritis with severe segmental cortical atrophy and fibrosis, associated with chronic tubulointerstitial inflammation of varying degree. Azotemia was observed in 6 of the 7 cases. The gross kidney lesions were as follows: bilateral small kidneys with numerous segmental cortical scars causing depression of the renal cortical surface. Histologic examination revealed salient atrophy of nephrons, including paucity of glomeruli, glomerulocystic lesions, colloid-filled tubular microcysts, and a conspicuously increased occurrence of arteries with narrowed lumina caused by intimal thickening. These segmental abnormalities were accompanied by pronounced interstitial fibrosis. All but 1 dog showed salient tubulointerstitial lympho-plasmacytic infiltration, which in 3 cases also included diffuse infiltration of polymorphonuclear neutrophilic leukocyte (PMN)-cells and occurrence of tubular PMN-casts. Morphologic signs of abnormal metanephric differentiation (renal dysplasia) were observed in all cases in the form of atypical tubules or asynchronous nephronic development (immature glomeruli) or both. However, other morphologic primary dysplastic features were absent. Based on the morphologic features, it is concluded that the end-stage kidney disease in these young Boxer dogs was the result of chronic atrophic nonobstructive pyelonephritis, most probably caused by vesico-ureteral reflux, compatible with reflux nephropathy causing segmental hypoplasia (Ask-Upmark kidney) in man. It is proposed that atypical tubular epithelium in the form of adenomatoid proliferation of collecting duct epithelial cells should be considered an acquired compensatory lesion, rather than the result of disorganized metanephric development.

Inherited metabolic disease in companion animals: searching for nature's mistakes

Inborn errors of metabolism are caused by genetic defects in intermediary metabolic pathways. Although long considered to be the domain of human paediatric medicine, they are also recognised with increasing frequency in companion animals. The diagnosis of diseased animals can be achieved by searching for abnormal metabolites in body fluids, although such screening programmes have, until now, not been widely available to the small animal clinician. A comprehensive battery of analytical tools exists for screening for inborn metabolic diseases in humans which can be applied to animals and serve not only for the diagnosis of affected patients but also to detect asymptomatic carriers and further our understanding of metabolic pathways in dogs and cats. Moreover, naturally occurring animal models of inherited metabolic diseases provide a unique opportunity to study the biochemical and molecular pathogenesis of these disorders and to investigate possible therapeutic options.

Alanine glyoxylate aminotransferase deficiency: biochemical and molecular genetic lessons from the study of a human disease

The decision to treat a patient with primary hyperoxaluria type 1 (PHI) by combined liver and kidney transplantation, the former to correct the metabolic lesion which was then thought to be deficiency of cytoplasmic 2-oxoglutarate:glyoxylate carboligase, and the latter to replace the organ which is destroyed, provided an opportunity to investigate the disease by modern biochemical methods. It was shown that 2-oxoglutarate:glyoxylate carboligase (the first decarboxylating component of 2-oxoglutarate dehydrogenase) is entirely mitochondrial so that deficiency of a cytoplasmic form of this enzyme could not be the cause of PHI. The deficient enzyme proved to be hepatic peroxisomal alanine:glyoxylate aminotransferase (AGT). The disease can be diagnosed enzymologically on percutaneous liver biopsies and this is possible for the fetus in utero. There are four types of genetically determined heterogeneity in PHI:(1) responsiveness and non-responsiveness to pharmacological doses of pyridoxine, in terms of an effect on the rate of oxalate production; (2) the presence or absence of residual catalytic AGT activity; (3) CRM+ and CRM-variants; (4) locational variation by virtue of which the enzyme (AGT) is mitochondrial and not peroxisomal. About one third of patients with PHI have residual AGT activity and at least a large proportion of these have mitochondrial and not peroxisomal AGT. The molecular features which guide peroxisomal and mitochondrial enzymes from their sites of synthesis into the appropriate organelle are reviewed and the possibilities for genetic variation in the relevant parts of the AGT molecule are discussed. The gene directing the synthesis of AGT has been cloned and sequenced, as has the AGT cDNA from a patient with mitochondrial AGT. Three point mutations causing amino acid substitution in the predicted AGT protein sequence have been identified: proline----leucine at residue 11, glycine----arginine at residue 170 and isoleucine----methionine at residue 340. The present evidence based on screening PHI patients and control subjects suggest that the substitution at residue 11, which cosegregates with that at residue 340, generates an amphiphilic alpha-helix which resembles mitochondrial targeting sequences but that misrouting of all the newly synthesized AGT into mitochondria requires the substitution at residue 170 which may act by impeding the entry of the enzyme into peroxisomes. The recognition of enzyme locational heterogeneity in PHI due to mutations affecting leader sequences should encourage a search for similar metabolic lesions in other inborn errors of metabolism affecting peroxisomal and/or mitochondrial enzymes.