Medium-Chain Acyl-CoA Dehydrogenase Deficiency (MCADD) is one of the most commonly diagnosed inborn errors of metabolism in humans. It, however, had never been recognized as a disorder in animals until publication of a study documenting a gene variant of ACADM, the gene encoding MCAD in a neutered male Cavalier King Charles Spaniel (CKCS) (1). The affected Cavalier was homozygous for a insertion-deletion variant that resulted in a nonfunctional truncated protein. The variant was genotyped in a cohort of 162 CKCS that were unrelated to the index case. These samples were obtained from the Vetsuisse Biobank as part of a screening program for syringomyelia (SM) Figure 1.  The SM and MCADD phenotypes of the genotyped dogs is unknown.

Figure 1 – Genotype Distribution of the ACADM variant in 162 CKCS samples obtained from the Vetsuisse Biobank (1).

The frequency of the mutant allele was 23.5%. Clearly this variant has been in CKCS for many generations to be present in almost 40% of the samples. Why has this disorder not been recognized by CKCS breeders? The symptoms of MCADD overlap significantly with those of SM. Is it possible that MCADD is being misdiagnosed as SM? Without phenotypic information on the tested dogs this question cannot be answered. The ACKCSC Breeders Education Committee applied to the ACKCS Charitable Trust for funding to purchase 100 MCADD test swabs in order to genotype 100 healthy CKSC. We wanted to know the variant frequency in a phenotypical healthy population of Cavaliers. The working hypothesis was that we would find homozygous wild-type dogs and dogs heterozygous for the mutant allele but no homozygous mutant dogs. The swabs were collected at the 2024 ACKCSC National Dog Show in Mesquite, Texas. Phenotypic information was collected with each swab. The results were unexpected (Figure 2). Not only was the mutant allele frequency higher in our sample (30%) but we had 5 dogs that had no symptoms of MCADD that were homozygous for the mutant allele. The phenotypic information collected on these dogs indicated that they were all healthy dogs. One of the homozygous mutant dogs is a 5 year old bitch that has had two litters for a total of eight healthy puppies. During her gestation and lactation, she ate a high fat puppy diet with no symptoms of MCADD.

Figure 2 – Genotype Distribution of the ACADM variant in 100 CKCS samples obtained at the 2024 ACKCSC National.

The results of our testing suggest that MCADD is a polygenic disorder with the mutant allele displaying incomplete penetrance. Another gene suppresses the MCADD phenotype in some dogs with a homozygous mutant genotype. We currently do not know the proportion of homozygous mutant dogs that display the MCADD phenotype. Contrary to published recommendations, it is important to not remove asymptomatic homozygous mutant dogs from the breeding population. To do so is to risk losing the gene(s) that suppress the MCADD phenotype. Instead we need to attract and fund research to identify the additional genes. Comparing the genomes of asymptomatic dogs to symptomatic dogs has the potential to reveal other genes. Once the mode of inheritance is completely understood a breeding plan can be developed to effectively manage the disorder.

In addition to genotyping the Vetsuisse dogs for MCADD, the authors also measured the acylcarnitine C8/C10 and C8/C12 ratios in 5 dogs of each ACADM genotype (1). The specific C8/C12 and C8/C12 ratios were elevated in homozygous dogs.

Figure 3 – Acylcarnitine C8/C10 and C8/C12 ratios in the blood of Vetsuisse dogs with different ACADM genotypes (1).

The blood level of fatty acids in phenotypically asymptomatic Cavaliers should be compared to the levels in phenotypically symptomatic Cavaliers to determine if the difference in phenotype is due to a mechanism(s) that permits the asymptomatic dog to compensate for elevated blood levels of medium chain fatty acids. This is a study that breeders can conduct. MCADD is a new to the Cavalier community but our dogs have been living with it for a long time. We need to work together to better understand the genetics of this disorder. Our dogs are depending on us!

Reference: Christen, M.; Bongers, J.; Mathis, D.; Jagannathan, V.; Quintana, R. G.; and Leeb, T. ACADM Frameshift Variant in Cavalier King Charles Spaniels with Medium-Chain Acyl-CoA Dehydrogenase Deficiency. Genes 2022, 13,1847.https//doi.org/10.3390/genes13101847