A novel point mutation in the β1-tubulin gene in asymptomatic macrothrombocytopenic Norfolk and Cairn Terriers

BackgroundAn asymptomatic macrothrombocytopenia, phenotypically similar to asymptomatic inherited macrothrombocytopenia in Cavalier King Charles Spaniels, was described in a group of Norfolk Terriers (NT) from Northern Italy, and isolated cases were also reported in Cairn Terriers (CT). ObjectivesThe purpose of this work was to evaluate for the presence of a genetic defect in the 1-tubulin gene in macrothrombocytopenic NT and CT. MethodsSamples from 20 healthy dogs (13 NT and 7 CT) were collected at different institutions in Italy (n=8), United Kingdom (n=3), and United States (n=9). Genomic DNA was harvested from EDTA-anticoagulated blood and all coding areas and exon-intron splice sites in the gene encoding 1-tubulin were amplified and sequenced. ResultsTwelve dogs (9 NT and 3 CT) showed a single nucleotide polymorphism (SNP) in exon 1 at nucleotide position 5 (G5A) that would result in the change of an arginine to a histidine at amino acid position 2 (R2H). Four dogs (3 NT and one Cairn Terrier) were heterozygous for the SNP, and 4 dogs (one Norfolk Terrier and 3 CT) matched the normal canine genome. Homozygous dogs for the SNP were macrothrombocytopenic with platelet counts ranging from 19,000 to 110,000/L. Heterozygous and normal dogs had normal platelet counts and morphology. None had the CKCS point mutation. ConclusionsThe 1-tubulin N-terminal amino acids form the nucleotide-binding domain and thus this mutation could affect GTP binding enough to influence platelet formation in homozygous but not in heterozygous dogs. The presence of macrothrombocytopenia only in homozygous affected dogs reveals an association between the SNP and the phenotype.