Haemophilia B in Rhodesian Ridgebacks is currently the most important canine haemophilia in Germany. The aim of this study was to define the underlying genetic defect. Genetic studies were performed including six phenotypically affected male dogs (factor IX activity: approximately 1%), four suspected carriers (factor IX activity 48-69%, one confirmed by affected offspring), and 12 healthy dogs. Comparison of the entire coding region of the canine factor IX DNA sequences and exon-intron junctions from affected dogs with the wild type canine factor IX DNA revealed a G-A missense mutation in exon 7. This mutation results in a glycine (GGA) to glutamic acid (GAA) exchange in the catalytic domain of the haemophilic factor IX. All affected dogs were hemizygous for the detected mutation and carriers were heterozygous, whereas none of the Rhodesian Ridgebacks with normal factor IX activity showed the mutation. No further alterations in the sequences between affected dogs and the healthy control group could be observed. None of the Rhodesian Ridgebacks with undefined haemophilia B status (n=30) and no individual of three other dog breeds (Doberman Pinscher: n=20; German Wire haired Pointer: n=20; Labrador: n=25) showed the presence of the mutation. Amino acid sequence alignment and protein structural modelling analysis indicate that the detected mutation causes a relevant functional defect. The results of this study suggest that the detected mutation is responsible for a severe form of haemophilia B in Rhodesian Ridgebacks.
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