First WNK4-Hypokalemia Animal Model Identified by Genome-Wide Association in Burmese Cats

Barbara Gandolfi,Richard Malik,Chris R. Helps,Boyd R. Jones,Alejandro Cortes,Leslie A. Lyons,Timothy J. Gruffydd-Jones,George Erhardt,Eva M. Prinzenberg,Yann Herault

doi:10.1371/journal.pone.0053173

Abstract

Burmese is an old and popular cat breed, however, several health concerns, such as hypokalemia and a craniofacial defect, are prevalent, endangering the general health of the breed. Hypokalemia, a subnormal serum potassium ion concentration ([K+]), most often occurs as a secondary problem but can occur as a primary problem, such as hypokalaemic periodic paralysis in humans, and as feline hypokalaemic periodic polymyopathy primarily in Burmese. The most characteristic clinical sign of hypokalemia in Burmese is a skeletal muscle weakness that is frequently episodic in nature, either generalized, or sometimes localized to the cervical and thoracic limb girdle muscles. Burmese hypokalemia is suspected to be a single locus autosomal recessive trait. A genome wide case-control study using the illumina Infinium Feline 63K iSelect DNA array was performed using 35 cases and 25 controls from the Burmese breed that identified a locus on chromosome E1 associated with hypokalemia. Within approximately 1.2 Mb of the highest associated SNP, two candidate genes were identified, KCNH4 and WNK4. Direct sequencing of the genes revealed a nonsense mutation, producing a premature stop codon within WNK4 (c.2899C>T), leading to a truncated protein that lacks the C-terminal coiled-coil domain and the highly conserved Akt1/SGK phosphorylation site. All cases were homozygous for the mutation. Although the exact mechanism causing hypokalemia has not been determined, extrapolation from the homologous human and mouse genes suggests the mechanism may involve a potassium-losing nephropathy. A genetic test to screen for the genetic defect within the active breeding population has been developed, which should lead to eradication of the mutation and improved general health within the breed. Moreover, the identified mutation may help clarify the role of the protein in K+ regulation and the cat represents the first animal model for WNK4-associated hypokalemia.

Highlights

Potassium is the most abundant cation in mammals [1,2]
Clinical Presentation Clinical presentation of cats diagnosed with hypokalemia included: episodic or incessant musculoskeletal weakness frequently characteristically by ventroflexion of the head and neck (Figure 1)
Periodic paralyses constitute a group of hereditary muscle disorders characterized by acute and reversible attacks of muscle weakness associated with decrease blood pressure [30] in humans and is reported in other species such as, cow [31], horse [32,33], rat and dog [34,35]

Summary

Introduction

The resting membrane potential of cells is affected by the relationship between intracellular and extracellular potassium concentrations and the resting potassium conductance [3]. Since the extracellular potassium greatly affects the tendency of cells to fire action potentials, potassium plays a crucial role in the function of nervous tissue and muscle (skeletal, cardiac and smooth) throughout the body [1,2], implying perturbations can be debilitating or even lifethreatening. Inherited hypokalemia has been discovered in a variety of mammals by genetic studies of individuals and families affected by clinical disease. Genetic studies have shown that in humans, HYPP is attributable to a channelopathy associated with abnormal sodium conductance, usually inherited as an autosomal dominant trait. Primary hypokalemic disorders have been documented in humans, manifesting as episodic weakness associated with low serum potassium [19]

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Conclusion