Periodic hypokalaemic polymyopathy in Burmese and closely related cats: a review including the latest genetic data

Genetic Testing: practical dos and don'ts for cats

PRACTICAL RELEVANCE

A significant number of genetic variants are known for domestic cats and their breeds. Several DNA variants are causal for inherited diseases and most of the variants for phenotypic traits have been discovered. Genetic testing for these variants can support breeding decisions for both health and aesthetics. Genetic testing can also be used to monitor for the health of, or provide targeted therapy for, an individual cat and, more widely, can progress scientific discovery. Technological improvements have led to the development of large panel genetic testing, which can provide many DNA results for a low cost.

CLINICAL CHALLENGES

With the development of large panel genetic testing has come companies that can carry out this service, but which company is best to use may not always be clear - more tests are not necessarily better. Usage and interpretation of genetic data and how the results are presented by commercial laboratories may also be confusing for veterinary practitioners and owners, leading to misinterpretations for healthcare, improper genetic counseling, and poor breed and population management.

EVIDENCE BASE

The information provided in this review draws on scientific articles reporting the discovery, and discussing the meaning and implications, of DNA variants, as well as information from the Online Mendelian Inheritance in Animals (OMIA) website, which documents all the DNA variant discoveries. The author also provides suggestions and recommendations based on her personal experience and expertise in feline genetics.

AUDIENCE

This review is aimed at general practitioners and discusses the genetic tests that can be performed, what to consider when choosing a testing laboratory and provides genetic testing counseling advice. Practitioners with a high proportion of cat breeder clientele will especially benefit from this review and all veterinarians should realize that genetic testing and genomic medicine should be part of diagnostic plans and healthcare for their cat clients.

Reversible positioning head tilt observed in 14 cats with hypokalaemic myopathy

CASE SERIES SUMMARY

Positioning head tilt (PHT) is a dynamic neurological sign in which the head tilts to the opposite side to which it is moving. This sign is triggered in response to head movement and is thought to be due to the lack of inhibition of vestibular nuclei by the cerebellar nodulus and uvula (NU). The occurrence of PHT in animals has been suggested to be an indicator of NU dysfunction. Here, we describe the acute onset of PHT in 14 cats. All the cats were diagnosed with hypokalaemic myopathy caused by a range of pathologies. The PHT resolved along with other signs related to myopathy, such as cervical flexion and generalised weakness, after electrolyte correction in all cats.

RELEVANCE AND NOVEL INFORMATION

Hypokalaemic myopathy was the likely cause of PHT in the present feline cases.

Absence of 2899C<T Mutation in the WNK4 Gene in a Free-Ranging Lion (Panthera leo) with Polymyopathy

Simple Summary

Samples from an African lion cub in the Greater Kruger National Park area (South Africa), which could not walk, were tested for a gene mutation that causes one type of muscle weakness in domestic cats. The cause of the muscle weakness is believed to be genetic, but our study showed that the mutation that is found in similarly affected domestic cats was not present in the cub. Genetic diseases are more common in inbred animal populations, so this condition needs to be further evaluated to assist in the conservation of these magnificent creatures.

Abstract

Polyphasic skeletal muscle degeneration, necrosis and mineralization of skeletal muscle was diagnosed in eight juvenile free-ranging lions (Panthera leo), from five different litters in the Greater Kruger National Park area that were unable to walk properly. A detailed investigation was not possible in free-ranging lions, so the cause could not be determined. The cases resembled hypokalemic polymyopathy in domestic cats with muscle weakness. A candidate-gene approach previously identified a nonsense mutation in the gene coding for the enzyme lysine-deficient 4 protein kinase (WNK4) associated with the disease in Burmese and Tonkinese cats. In this study, we sequenced all 19 exons of the gene in one case, and two control samples, to identify possible mutations that may be associated with polymyopathy in free-ranging lions. Here, no mutations were detected in any of the exons sequenced. Our findings indicate that the WNK4 gene is not a major contributor to the condition in these lions. Further studies into the pathogenesis of this condition are needed to inform conservation policies for this vulnerable, iconic African species.

Myopathy with oval inclusions in a domestic shorthair cat

Case summary

An 18-month-old castrated male domestic shorthair cat was presented with a 2-month history of collapse and severe weakness, particularly affecting the pelvic limbs. A biceps femoris muscle biopsy revealed excessive variability in myofibre size, mild necrosis, minimal centronucleation and scattered 10 μm intracytoplasmic oval inclusions. The inclusions appeared amphophilic with haematoxylin and eosin, blue with Gomori trichrome and unstained with nicotinamide adenine dinucleotide dehydrogenase tetrazolium reductase staining. ATPase staining revealed a normal mosaic pattern and atrophy of both type 1 and 2 myofibres. The pathological diagnosis was a myopathy with inclusions. In contrast to previous feline myofibre inclusions previously reported in the literature, inclusions were not identified after immunohistochemistry using anti-desmin, tubulin, spectrin, laminin, LAMP and LC3 antibodies. After supportive care and corticosteroid treatment, clinical improvement was noted and the cat was discharged 10 days after initial presentation. Clinical and neurological re-examinations were performed at 1, 3, 6 and 9 months after discharge. Owner contact at both 10 and 30 months post-discharge confirmed that persistent muscular weakness was present.

Relevance and novel information

This case report describes a novel and slowly progressive feline myopathy associated with oval amphophilic inclusions unreactive to immunostaining, which have not been previously reported in feline myopathies.

A novel hypokalaemic polymyopathy and subsequent unrelated nutritional thiamine deficiency in a young Burmese cat

Case summary

An 8-month-old female spayed Burmese cat was referred for investigation of reduced appetite, reluctance to walk and jump and amaurosis. On serum biochemistry there was severe hypokalaemia and marked elevation of creatine kinase, suggestive of hypokalaemic polymyopathy. The neurological signs were consistent with thiamine deficiency. The cat was negative for the periodic hypokalaemic polymyopathy (PHP) of Burmese cats, and was ultimately diagnosed with a previously undescribed potassium wasting nephropathy requiring ongoing oral potassium supplementation. The response to treatment was excellent and the cat has remained clinically normal over a 12-month follow-up period.

Relevance and novel information

PHP in Burmese cats has been well described, but all cases to date have been shown to be secondary to a genetic mutation in WNK4, resulting in potassium wasting into the urine. This is the first case report of another potassium wasting nephropathy in a young Burmese cat, with subsequent development of nutritional thiamine deficiency.

Supernumerary maxillary fourth premolar teeth in five related Burmese cats

Case series summary

This article reports a case series of five Burmese cats with supernumerary teeth. All included cases displayed either unilateral or bilateral supernumerary maxillary fourth premolar teeth. Two cases also displayed supernumerary mandibular fourth premolar teeth, and in one case bilateral supernumerary maxillary third incisor teeth were found. Examination of the pedigrees of the affected cats revealed that all five cats were related with one common ancestor. All patients in this case series were successfully treated.

Relevance and novel information

Supernumerary maxillary fourth premolar teeth in cats have not been reported previously in the literature. As all five cats in this case series were shown to be related, supernumerary maxillary fourth premolar teeth may be yet another heritable health condition affecting the Burmese breed. Therefore, Burmese breeders should endeavour to reduce the incidence of supernumerary teeth within the breed by having animals examined by a veterinary dentist prior to breeding, and neutering affected, as well as related, individuals. Burmese cat owners must be made aware of the condition in order to institute early evaluation, diagnosis and treatment.

Case-based clinical reasoning in feline medicine: 1: Intuitive and analytical systems

AIM

This is Article 1 of a three-part series on clinical reasoning that encourages practitioners to explore and understand how they think and make case-based decisions. It is hoped that, in the process, they will learn to trust their intuition but, at the same time, put in place safeguards to diminish the impact of bias and misguided logic on their diagnostic decision-making.

SERIES OUTLINE

This first article discusses the relative merits and shortcomings of System 1 thinking (immediate and unconscious) and System 2 thinking (effortful and analytical). Articles 2 and 3, to appear in the March and May 2016 issues of JFMS, respectively, will examine managing cognitive error, and use of heuristics (mental short cuts) and illness scripts in diagnostic reasoning.