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Contract Study:
Frequency of PSSM1 Mutation in
GVHS Accessions
In order to obtain an estimate of the frequency of the PSSM1 mutant allele in horses registered with the Gypsy Vanner Horse Society, 100 hair samples were selected at random from the GVHS database, spanning the years 2003-2015. Samples were tested using the TaqMan assay system for the G>A SNP in exon 6 of the GYS1 gene.
Of the 100 samples, 76 had a normal genotype of N/N and 24 tested as heterozygous N/PSSM1. This translates into 176 normal alleles and 24 mutant alleles, giving an allele frequency of 88% normal and 12% mutant. We did not find any samples that were homozygous for the mutant allele.
Using these observed values of allele frequency, our Hardy-Weinberg calculation of the EXPECTED genotype frequencies in the total population are as follows:
Expected genotype frequency for N/N: 77.44%
Expected genotype frequency for N/PSSM1: 21.12%
Expected genotype frequency for PSSM1/PSSM1: 1.44%
Therefore approximately 1 in 5 horses can be expected to be heterozygous for the mutation, and 1-2 horses per 100 horses can be expected to be homozygous for the mutation.
Kathryn T. Graves, PhD
Director
University of Kentucky Animal Genetic Testing and Research Lab
Upon reviewing the results of this study, the GVHS has contacted the University of Minnesota Equine Center Neuromuscular Diagnostic Laboratory, for their direction on next steps to conduct the scientific research to fully understand what this means for the Gypsy Vanner Breed. More information will be forthcoming once the particulars of this further study have been finalized. GVHS management is also working on finalizing an agreement with the University of Kentucky to offer PSSM1 testing to GVHS owners as part of specially priced testing "bundles". More information on this initiative will also follow in future editions of the Vanner Banner.
Until more study is completed this caution from Dr. Graves still applies:
"Please advise your breeders and owners to wait until a scientific estimate of frequency is available and work together to decide how PSSM1 will be dealt with in your breed before valuable breeding stock is lost."
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