Researchers at Baker Institute for Animal Health have identified a genetic variant linked to XX disorder of sex development (XX DSD), a rare genetic disorder that causes dogs with female chromosomes to develop testes and masculinized genitals.

The discovery is a first step in developing a test to identify purebred dogs that carry the variant, helping breeders avoid producing affected dogs.

The exact cause of the disorder, which affects at least 28 breeds of dogs, as well as horses, llamas, pigs and humans, is still unknown. But the study, published in PLOS ONE, provides a lead to identify it.  

“This variant is the only marker that anyone has found to be associated with the disorder,” said Vicki Meyers-Wallen, associate professor of biomedical sciences at Cornell’s College of Veterinary Medicine, and first author of the study. “If we figure out the cause in dogs, then we can use this information to reduce the occurrence of this disorder in dogs, and possibly humans.”

Dogs are an excellent model for studying XX DSD in humans, because the disorder causes a remarkably similar range of effects in both species. Siblings within the same family can be affected to a different degree – one female might have a combination of ovary and testis and may be fertile, while a sibling might have two testes and be sterile. Affected dogs that appear to have male genitalia on the outside have an intact uterus internally which can be susceptible to potentially deadly uterine infections. Males (XY) are healthy but are carriers of the disorder.

Meyers-Wallen’s studies of XX DSD were complicated by its complex inheritance pattern. Purebred dogs can develop abnormalities even when they carry a single copy of the variant, while mixed breed dogs tend to manifest the disorder only when they carry copies on both chromosomes. These different inheritance patterns suggest that the severity of the disorder relies on the genetic background of the animal. The researchers suspect that epigenetics, a type of inherited information that is superimposed upon the genome, may play a role in how the disorder develops.

In some breeds, such as the German shorthaired pointer, embryos that carry a copy of the mutation on both chromosomes do not survive, resulting in small litter sizes.

To find genetic variations shared by dogs with the disorder, the researchers performed a genome-wide association study (GWAS) and whole genome sequencing. They pinpointed a variant located on chromosome 9 near the gene called SOX9, that regulates genes related to sex determination.

Next, the researchers looked at which genes were turned on or off in the developing gonads of embryos that carried the genetic variant and compared them to unaffected dogs. They had expected to see genes related to testis formation turned way up, but instead saw genes in pathways related to ovary development were turned way down. One gene in particular, called RSPO1, important in female sex development, was expressed at extremely low levels compared to unaffected females.

Research on DSDs also helps scientists understand how gonad development occurs normally. “We all think we’ve figured out how gonads are supposed to develop, but clearly there’s more to it,” said Meyers-Wallen. “We need to understand both the ovary and testis development pathways much better.”

Additional researchers involved in the project include Adam Boyko, assistant professor in biomedical sciences and Jennifer Grenier, director of the RNA Sequencing Core at the College of Veterinary Medicine.