Vitamin D-Dependent Hereditary Rickets Type I in a Cat

Feline precision medicine using whole-exome sequencing identifies a novel frameshift mutation for vitamin D-dependent rickets type 2

OBJECTIVES

A 14-week-old female domestic longhair kitten presented with shifting lameness and disproportionately smaller size compared with a co-housed littermate.

METHODS

Hematology and serum biochemical testing were conducted to investigate causes for delayed growth, and radiographs of the appendicular skeleton were obtained.

RESULTS

The afflicted kitten had marked hypocalcemia, mild hypophosphatemia and substantial elevations in alkaline phosphatase activity, as well as pathognomonic radiographic findings consistent with rickets. Skeletal changes and hypocalcemia prompted testing of concentrations of parathyroid hormone (PTH) and vitamin D metabolites. Endocrine testing demonstrated significant increases in serum concentrations of PTH and 1,25-dihydroxycholecalciferol (calcitriol), supporting a diagnosis of vitamin D-dependent rickets type 2. Provision of analgesia, supraphysiologic doses of calcitriol and calcium carbonate supplementation achieved normalization of the serum calcium concentration and restoration of normal growth, although some skeletal abnormalities persisted. Once skeletally mature, ongoing calcitriol supplementation was not required. Whole-exome sequencing (WES) was conducted to identify the underlying DNA variant. A cytosine deletion at cat chromosome position B4:76777621 in VDR (ENSFCAT00000029466:c.106delC) was identified and predicted to cause a stop codon in exon 2 (p.Arg36Glufs*18), disrupting >90% of the receptor. The variant was unique and homozygous in this patient and absent in the sibling and approximately 400 other cats for which whole-genome and whole-exome data were available.

CONCLUSIONS AND RELEVANCE

A unique, heritable form of rickets was diagnosed in a domestic longhair cat. WES identified a novel frameshift mutation affecting the gene coding for the vitamin D3 receptor, determining the likely causal genetic variant. Precision medicine techniques, including whole-exome and whole-genome sequencing, can be a standard of care in cats to identify disease etiologies, and to target therapeutics and personalize treatment.

Vitamin D metabolism and disorders in dogs and cats

Vitamin D plays an important role in regulating calcium metabolism and in the development and maintenance of skeletal health of companion animals. There is also a growing interest in understanding the role vitamin D plays in non-skeletal health in both human and veterinary patients. This review provides an update of our current understanding of vitamin D biology in dogs and cats and gives an overview of how vitamin D metabolism can be assessed in companion animals. Congenital and acquired vitamin D disorders are then summarised before the review concludes with a summary of recent studies which have explored the role of vitamin D in the development and outcomes of non-skeletal diseases of dogs and cats.

Rickets in a Thoroughbred-cross foal: case report and review of the literature

Rickets is a metabolic bone disease associated with failure of endochondral ossification and impaired osteoid mineralization in growing animals. As a consequence, affected individuals can develop gross and microscopic bone malformations. The most common causes of rickets in domestic species include vitamin D and phosphorus deficiency. Rickets has been described in multiple species; however, comprehensive postmortem characterizations with confirmatory histopathology in equids have not been published. A 6-mo-old, Thoroughbred-cross foal was diagnosed with rickets based on gross autopsy findings and microscopic examination of the ribs and long bones. Grossly, all costochondral junctions of the ribs were enlarged with a "rachitic rosary" appearance, and there were multiple fracture calluses in the rib bodies. Epiphyses and metaphyses of the long bones appeared widened on sagittal section, and their physes were irregularly thickened. Histologically, there were poorly organized columns of hypertrophic chondrocytes within the physes of affected bones, islands of chondrocytes embedded within the primary and secondary spongiosa, and faintly eosinophilic seams of poorly mineralized osteoid within the bone trabeculae. Areas of focally increased osteoclastic activity were observed in some of the sections, perhaps pointing to a more complex metabolic bone disease in a growing animal. Low serum concentrations of calcium and 25-hydroxyvitamin D were detected in an antemortem sample. The pathogenesis of these imbalances was not definitively established, but lack of sunlight exposure, low concentration of vitamin D precursors in the diet (perhaps secondary to malnutrition), or both, were suspected; a genetic basis cannot be ruled out.

The Role of Vitamin D in Small Animal Bone Metabolism

Dogs and cats have differences in vitamin D metabolism compared to other mammalian species, as they are unable to perform vitamin D cutaneous synthesis through sun exposure. Therefore, they are dependent on the dietary intake of this nutrient. The classic functions of vitamin D are to stimulate intestinal calcium and phosphate absorption, renal calcium and phosphate reabsorption and regulate bone mineral metabolism. Thus, it is an important nutrient for calcium and phosphorus homeostasis. This review highlights the evidence of the direct and indirect actions of vitamin D on bone mineral metabolism, the consequences of nutritional imbalances of this nutrient in small animals, as well as differences in vitamin D metabolism between different size dogs.

Vitamin D Metabolism and Profiling in Veterinary Species

The demand for vitamin D analysis in veterinary species is increasing with the growing knowledge of the extra-skeletal role vitamin D plays in health and disease. The circulating 25-hydroxyvitamin-D (25(OH)D) metabolite is used to assess vitamin D status, and the benefits of analysing other metabolites in the complex vitamin D pathway are being discovered in humans. Profiling of the vitamin D pathway by liquid chromatography tandem mass spectrometry (LC-MS/MS) facilitates simultaneous analysis of multiple metabolites in a single sample and over wide dynamic ranges, and this method is now considered the gold-standard for quantifying vitamin D metabolites. However, very few studies report using LC-MS/MS for the analysis of vitamin D metabolites in veterinary species. Given the complexity of the vitamin D pathway and the similarities in the roles of vitamin D in health and disease between humans and companion animals, there is a clear need to establish a comprehensive, reliable method for veterinary analysis that is comparable to that used in human clinical practice. In this review, we highlight the differences in vitamin D metabolism between veterinary species and the benefits of measuring vitamin D metabolites beyond 25(OH)D. Finally, we discuss the analytical challenges in profiling vitamin D in veterinary species with a focus on LC-MS/MS methods.

Unusual presentation of vitamin D3-dependent rickets type II in a kitten

Case summary

A 15-week-old kitten presented with a 1-month history of intermittent generalised tremors and abdominal distension. Hypocalcaemia associated with increased 1,25-vitamin D₃ was consistent with vitamin D₃-dependent rickets type II. The bone appearance on CT scan was most consistent with the changes typically seen with nutritional secondary hyperparathyroidism and less typical of the changes seen with rickets. Our patient had a positive response to high vitamin D₃ therapy as it remained normocalcaemic 16 months after diagnosis, supporting the diagnosis of rickets.

Relevance and novel information

This case report is an unusual and interesting presentation of rickets in a kitten. Despite the characteristic vitamin D₃ disturbance for rickets type II, the atypical radiographic changes have not been previously reported. In the literature, a positive response to treatment is not commonly seen or follow-up is short. Our case responded well to treatment and was followed for 16 months from the time of diagnosis. This emphasises that the pathophysiology of the condition is not well understood, and that different types of vitamin D₃-dependent rickets type II may exist. Although the genetic defects responsible for some cases of rickets type I have been identified, this has still not been determined for rickets type II.

Vitamin D metabolism in dogs and cats and its relation to diseases not associated with bone metabolism

Due to the presence of receptors in the cells of numerous body tissues, vitamin D is associated with several physiological functions that go beyond calcium and phosphorus homoeostasis and control of bone metabolism in the body. In humans, several studies have associated lower vitamin D concentrations with numerous diseases, such as cancer, heart disease, autoimmune diseases and infectious diseases, and also with an increase in the total mortality rate of the population. Recently, this nutrient started to gain importance in veterinary medicine, and several articles have shown a correlation between low vitamin D status and diseases unrelated to bone metabolism. The present review aims to highlight the recent publications that investigated this relationship, bringing the evidence that exists so far in dogs and cats.

A genetic variant of CYP2R1 identified in a cat with type 1B vitamin D-dependent rickets: a case report

Background

Vitamin D-dependent rickets is rare in animals and humans. Several types of this condition are associated with genetic variants related to vitamin D metabolism. This is the first report of type 1B vitamin D-dependent rickets in a cat.

Case presentation

Here, we describe the case of a 3-month-old female domestic short-haired cat previously fed on commercial kitten food that presented at our clinic with seizures, lethargy, and generalized pain. Serum and ionized calcium concentrations and 1,25-dihydroxycholecalciferol in this cat were low, and radiographs showed skeletal demineralization and abnormally wide growth plates on the long bones. Initially, simple vitamin D deficiency was suspected; however, the cat’s profile, which included fed a well-balanced commercial diet, together with the findings of additional laboratory tests and the cat’s unresponsiveness to various treatments, raised the suspicion of vitamin D-dependent rickets. Examination of the DNA sequences of CYP2R1 and CYP27B1 genes, which are genes linked with vitamin D metabolism, showed a CYP2R1 frameshift mutation in exon 5 (where T is deleted at position c.1386). This mutation alters the amino acid sequence from position 462, while the stop codon introduced at position 481 prematurely truncates the 501 amino acid full-length protein. With this knowledge, a new treatment regime based on a standard dose of calcitriol was started and this markedly improved the cat’s condition.

Conclusions

To the best of our knowledge, the present case is the first description of type 1B vitamin D-dependent rickets linked with a genetic variant of CYP2R1 in a cat.

Electronic supplementary material

The online version of this article (10.1186/s12917-019-1784-1) contains supplementary material, which is available to authorized users.

Metabolic bone disease and central retinal degeneration in a kitten due to nutritional inadequacy of an all-meat raw diet

A 5-month-old castrated male Sphynx kitten presented with left hindlimb lameness shortly after adoption. Prior to adoption, the breeder had fed the kitten an exclusively raw chicken diet. Radiographs revealed generalized osteopenia and a left tibia–fibula fracture. Ophthalmic examination revealed corneal vascularization and opacity in the right eye, and lesions suggestive of feline central retinal degeneration in the left eye. The patient’s diagnoses included metabolic bone disease and feline central retinal degeneration, which can result from taurine deficiency. The kitten’s nutritional diseases were managed with a complete and balanced canned diet designed for kitten growth and with taurine supplementation.

Feline hyperparathyroidism: pathophysiology, diagnosis and treatment of primary and secondary disease

PRACTICAL RELEVANCE

Hyperparathyroidism exists in primary and secondary forms. Primary hyperparathyroidism has typically been considered a disease that uncommonly affects cats, but this condition is more prevalent than previous diagnoses would suggest. Secondary hyperparathyroidism may be caused by either nutritional influences (ie, nutritional secondary hyperparathyroidism) or chronic kidney disease (ie, renal secondary hyperparathyroidism). Tertiary hyperparathyroidism has yet to be documented in veterinary medicine, but it is possible that this condition occurs in some cats following longstanding renal secondary hyperparathyroidism.

CLINICAL CHALLENGES

Diagnosis of this group of calcium metabolic disorders presents a number of challenges for the clinician. For example, clinical signs can be non-specific and, especially in the case of primary hyperparathyroidism, there is often a low index of suspicion for the disease; careful sample handling is required for testing of parathyroid hormone (PTH) and ionized calcium levels; and there is currently no feline-specific assay for PTH, which has implications for test sensitivity and interpretation of results.

AIMS

This article briefly outlines PTH and calcium physiology by way of introduction to a review of PTH measurement and interpretation. Various forms of feline hyperparathyroidism are then described, encompassing diagnosis and treatment options.

DNA mutations of the cat: the good, the bad and the ugly

PRACTICAL RELEVANCE

The health of the cat is a complex interaction between its environment (nurture) and its genetics (nature). Over 70 genetic mutations (variants) have been defined in the cat, many involving diseases, structural abnormalities and clinically relevant health concerns. As more of the cat's genome is deciphered, less commonly will the term 'idiopathic' be used regarding the diagnosis of diseases and unique health conditions. State-of-the-art health care will include DNA profiling of the individual cat, and perhaps its tumor, to establish the best treatment approaches. Genetic testing and eventually whole genome sequencing should become routine diagnostics for feline health care.

GLOBAL IMPORTANCE

Cat breeds have disseminated around the world. Thus, practitioners should be aware of the breeds common to their region and the mutations found in those regional populations. Specific random-bred populations can also have defined genetic characteristics and mutations.

AUDIENCE

This review of 'the good, the bad and the ugly' DNA variants provides the current state of knowledge for genetic testing and genetic health management for cats. It is aimed at feline and general practitioners wanting to update and review the basics of genetics, what tests are available for cats and sources for genetic testing. The tables are intended to be used as references in the clinic. Practitioners with a high proportion of cat breeder clientele will especially benefit from the review.

EVIDENCE BASE

The data presented is extracted from peer-reviewed publications pertaining to mutation identification, and relevant articles concerning the heritable trait and/or disease. The author also draws upon personal experience and expertise in feline genetics.

Mineral metabolism in growing cats: changes in the values of blood parameters with age

The purpose of this study was to describe changes in calcium, phosphorus, magnesium, parathyroid hormone, calcitriol and calcidiol in cats from 3 to 15 months of age. Fourteen European shorthair healthy cats of both sexes (seven males, seven females) belonging to a research colony were studied from 3 to 15 months of age. Plasma concentrations of total calcium, ionised calcium, albumin, phosphorus, magnesium, intact parathyroid hormone (I-PTH), whole parathyroid hormone (W-PTH), calcidiol and calcitriol were measured at 3, 6, 9, 12 and 15 months of age. From 3 months of age to adulthood cats showed a decrease in calcium (both total and ionised), phosphorus and magnesium. No major changes in PTH were evident, although the ratio of W-PTH:I-PTH decreased significantly with age. A reciprocal change in vitamin D metabolites (decrease in calcitriol and increase in calcidiol) was identified during the growing process. Our results, showing changes in most parameters of mineral metabolism during growth, reinforce the need to use adequate age-related reference values for diagnostic purposes.

Genetic testing in domestic cats

Varieties of genetic tests are currently available for the domestic cat that support veterinary health care, breed management, species identification, and forensic investigations. Approximately thirty-five genes contain over fifty mutations that cause feline health problems or alterations in the cat's appearance. Specific genes, such as sweet and drug receptors, have been knocked-out of Felidae during evolution and can be used along with mtDNA markers for species identification. Both STR and SNP panels differentiate cat race, breed, and individual identity, as well as gender-specific markers to determine sex of an individual. Cat genetic tests are common offerings for commercial laboratories, allowing both the veterinary clinician and the private owner to obtain DNA test results. This article will review the genetic tests for the domestic cat, and their various applications in different fields of science. Highlighted are genetic tests specific to the individual cat, which are a part of the cat's genome.

A novel CYP27B1 mutation causes a feline vitamin D-dependent rickets type IA

A 12-week-old domestic cat presented at a local veterinary clinic with hypocalcemia and skeletal abnormalities suggestive of rickets. Osteomalacia (rickets) is a disease caused by impaired bone mineralization leading to an increased prevalence of fractures and deformity. Described in a variety of species, rickets is most commonly caused by vitamin D or calcium deficiencies owing to both environmental and or genetic abnormalities. Vitamin D-dependent rickets type 1A (VDDR-1A) is a result of the enzymatic pathway defect caused by mutations in the 25-hydroxyvitamin D(3)-1-alpha-hydroxylase gene [cytochrome P27 B1 (CYP27B1)]. Calcitriol, the active form of vitamin D(3), regulates calcium homeostasis, which requires sufficient dietary calcium availability and correct hormonal function for proper bone growth and maintenance. Patient calcitriol concentrations were low while calcidiol levels were normal suggestive of VDDR-1A. The entire DNA coding sequencing of CYP27B1 was evaluated. The affected cat was wild type for previously identified VDDR-1A causative mutations. However, six novel mutations were identified, one of which was a nonsense mutation at G637T in exon 4. The exon 4 G637T nonsense mutation results in a premature protein truncation, changing a glutamic acid to a stop codon, E213X, likely causing the clinical presentation of rickets. The previously documented genetic mutation resulting in feline VDDR-1A rickets, as well as the case presented in this research, result from novel exon 4 CYP27B1 mutations, thus exon 4 should be the initial focus of future sequencing efforts.

Musculoskeletal oddities in the cat: an overview of some curious causes of lameness

PRACTICAL RELEVANCE

Cats, both young and old, can suffer a variety of weird and wonderful musculoskeletal conditions that are a cause of lameness. These include developmental, metabolic and nutritional bone diseases, ectopic mineralisation disorders, conditions that cause lameness or exercise intolerance and primarily or secondarily affect muscle, and lastly pad conditions.

CLINICAL CHALLENGES

These conditions are mostly rare and can be challenging to diagnose. The aim of this review is to bring these conditions to the attention of practitioners so that, if they are encountered, further research around the topic can be undertaken. Radiographic changes and diagnostic tests that can be used to try to confirm diagnoses are described.

EVIDENCE BASE

These unusual causes of lameness are the subject of multiple single case reports or small case series, many of which are relatively old. The evidence presented here is drawn from these articles. However, it is not possible within the scope of this review to discuss all the conditions in as much detail as they may warrant, or to make reference to every article relating to them.

Vitamin D-dependent non-type 1, non-type 2 rickets in a 3-month-old Cornish Rex kitten

CASE PRESENTATION AND ASSESSMENT: A 3-month-old female Cornish Rex kitten was found to have non-painful swelling of the carpal and tarsal regions when presented for routine neutering. The kitten was smaller in stature and less active than its siblings and, according to the owner, had a bunny-hopping gait, was reluctant to climb stairs and strained during defecation. Radiography of the affected limbs and a subsequent radiographic survey of the entire skeleton demonstrated features consistent with rickets. The three littermates were clinically and radiographically normal. As a nutritionally complete diet was being fed, it seemed most likely that the kitten had an inborn error related to vitamin D metabolism. Serum biochemistry demonstrated reduced total alkaline phosphatase activity and increased concentrations of parathyroid hormone. Concentrations of 1,25- and 25-hydroxycholecalciferol were markedly reduced, confirming the diagnosis of rickets.

TREATMENT

The kitten was treated with calcitriol, administered orally once daily, and improved rapidly both clinically and radiologically. Serial laboratory studies suggested that the error in vitamin D metabolism was transient, and, at the time of writing, as an adult, the cat appears to require no ongoing replacement calcitriol therapy.

CLINICAL RELEVANCE

This case emphasises the value of examining a full 'calcium profile' via a human or veterinary reference laboratory, and a favourable prognosis in some kittens with rickets makes such investigations worthwhile. Even when finances preclude detailed investigation, trial therapy using a nutritionally complete diet and physiological doses of calcitriol or cholecalciferol is inexpensive and can produce a good response.

Successful Therapy of Vitamin D-Dependant Rickets in a Kitten

A 7 mo old, 2.4 kg, intact female kitten was evaluated for an inability to walk after falling out of the owner's arms. Diagnostic testing abnormalities included hypocalcemia, low ionized calcium, and elevated intact parathyroid hormone concentration. The 25-hydroxyvitamin D level was normal. Radiographic abnormalities included generalized osteopenia, a nondisplaced, folding fracture of the proximal right fibula, and sclerosis with a compression fracture of the proximal right tibia. Based on these findings and response to calcium carbonate and calcitriol therapy, a diagnosis of vitamin D-dependent rickets was made. Reports of similar cases in veterinary medicine are sparse and no other reports to date document radiographic abnormalities with a successful therapeutic outcome.

Feline genetics: clinical applications and genetic testing

DNA testing for domestic cat diseases and appearance traits is a rapidly growing asset for veterinary medicine. Approximately 33 genes contain 50 mutations that cause feline health problems or alterations in the cat's appearance. A variety of commercial laboratories can now perform cat genetic diagnostics, allowing both the veterinary clinician and the private owner to obtain DNA test results. DNA is easily obtained from a cat via a buccal swab with a standard cotton bud or cytological brush, allowing DNA samples to be easily sent to any laboratory in the world. The DNA test results identify carriers of the traits, predict the incidence of traits from breeding programs, and influence medical prognoses and treatments. An overall goal of identifying these genetic mutations is the correction of the defect via gene therapies and designer drug therapies. Thus, genetic testing is an effective preventative medicine and a potential ultimate cure. However, genetic diagnostic tests may still be novel for many veterinary practitioners and their application in the clinical setting needs to have the same scrutiny as any other diagnostic procedure. This article will review the genetic tests for the domestic cat, potential sources of error for genetic testing, and the pros and cons of DNA results in veterinary medicine. Highlighted are genetic tests specific to the individual cat, which are a part of the cat's internal genome.

Vitamin D metabolism and rickets in domestic animals: a review

Rickets and osteomalacia are increasing in prevalence in people because of cultural practices, breast-feeding, decreased sun exposure, and increased sunscreen usage. Several hereditary forms of rickets owing to either renal phosphate wasting or defects in vitamin D metabolism are also reported in people. Rickets is well recognized in domestic animals, but published reports are not always supported by microscopic findings, and diagnoses based on clinical signs and radiology are unreliable. Most cases in domestic animals are caused by dietary deficiency of either vitamin D or phosphorus, but occasional inherited forms are reported in pigs, sheep, cats, and dogs. There is variation between species in susceptibility to dietary vitamin D and phosphorus deficiency and in the ability to manufacture vitamin D in their skin. A number of mouse models have been discovered or created to study human skeletal diseases and skeletal homeostasis. With the discovery that vitamin D is involved in not only calcium and phosphorus homeostasis but also in the immune system and cancer, there is great potential for new and existing animal models to generate valuable information about vitamin D and its many functions. This review presents an overview of vitamin D metabolism and rickets in domestic and laboratory animals and makes comparisons where appropriate with the disease in humans.