Christmas disease in a Hovawart family resembling human hemophilia B Leyden is caused by a single nucleotide deletion in a highly conserved transcription factor binding site of the F9 gene promoter

Hemophilia B is a classical monogenic X-chromosomal recessively transmitted bleeding disorder caused by genetic variants within the coagulation factor IX gene (F9). Although hemophilia B has been described in 32 dog breeds hitherto, it has not yet been reported in the Hovawart. Here we describe the identification of a Hovawart family transmitting typical signs of an X-linked bleeding disorder. Five males had been reported to suffer from recurrent hemorrhagic episodes, four of them had to be euthanized finally and one died due to severe blood loss. A blood sample of one of these males with only 2% of the normal concentration of plasma factor IX (FIX) together with samples of seven relatives including the mother and grandmother were provided for further analysis. Next generation sequencing of DNA of the mother and grandmother revealed a single nucleotide deletion in the F9 promoter (NC_006621.3:g.109,501,492delC; CanFam3.1). Genotyping of the deletion in 1,298 dog specimens (83 different breeds) including 720 Hovawarts revealed that the mutation was only present in the aforementioned Hovawart family. The deletion is located 73 bp upstream of the F9 start codon in the highly conserved overlapping DNA binding sites of hepatocyte nuclear factor 4α (HNF4α) and androgen receptor (AR). The deletion only abolishes binding of HNF4α as demonstrated by electrophoretic mobility shift assay (EMSA) using purified recombinant human HNF4α and a transient overexpression lysate of human AR with double-stranded DNA probes encompassing the mutated promoter region. Luciferase reporter assays using wild type and mutated promoter fragment constructs transfected into Hep G2 cells showed a 65.3% reduction in expression of the mutated promoter. The data presented here provide evidence that the deletion identified in the Hovawart family caused a rare type of hemophilia B resembling human hemophilia B Leyden.