Equine Turner Syndrome
Turner syndrome in horses is a chromosomal disorder in which affected mares only have one X chromosome (XO karyotype) instead of the usual two (XX in mares). Typical characteristics are underdeveloped ovaries (gonadal dysgenesis) and infertility. Other symptoms may include smaller body size and poorly developed secondary sexual characteristics. Until now, the diagnosis could only been made by karyotyping, in which the missing X chromosome has been detected in a laborious and time-consuming process. With the availability of SNP array-based testing, the examination can now be carried out simply as a DNA test.
Horses with Turner syndrome can usually lead a normal life, but are severely restricted in their ability to reproduce. The disease is caused by errors in cell division and is not hereditary. Early diagnosis helps to manage expectations with regard to growth and suitability for breeding; supportive measures can improve the quality of life of affected animals, should this be limited.
Genetics of the equine Turner Syndrome
Normally developed horses have 32 pairs of chromosomes. 31 pairs are 'autosomes', plus the sex chromosomes ('gonosomes'). Normal females have 2 X chromosomes as gonosomes (karyotype 64, XX). Males have 1 X chromosome and 1 Y chromosome (karyotype 64, XY).
In equine Turner syndrome, one of the X chromosomes or at least the essential sections of it are missing. Such a horse has the karyotype 63,X0 (X-monosomy).
The cause is an incomplete division of the chromosome pairs either during meiosis as part of the maturation division (formation of germ cells) or during mitotic cell divisions in embryonic development.
In humans, the frequency is 1 in 2,500 females, but no studies have yet been carried out on the occurrence in horses. However, it is assumed that eTS of the X chromosome is an underestimated cause of infertility in mares due to a lack of simple diagnostic options.
Effects on the Phenotype
External appearance:
Mostly normal external genitalia and normal coat, occasionally slightly smaller stature.
Reproductive organs:
Small, infantile ovaries (< 1 cm), indistinct or immature uterus, no normal oviductal structures (fallopian tubes).
Behaviour:
Frequently anoestrus (no cycle) or irregular oestrus; individual passive oestrus phenomena are possible.
Fertility:
Predominantly sterile. Rarely limited fertility in low-grade mosaic forms.
Clinical Diagnostics
Anamnese
Absence of cyclicity and chronic infertility with normal external appearance.
Examination
Palpation per rectum and/or transrectal ultrasound to assess ovarian size and tubal/uterus formation.
Blood tests/hormone analyses
Frequently low oestrogen and progesterone levels, elevated LH/FSH levels are possible.
Differential diagnostis
Small ovaries are non-specific findings and are observed in seasonal anoestria, prepubertal mares or hormonal dysregulation. The differentiation is made by anamnesis, seasonal classification, hormonal diagnostics and genetic analyses.
Chromosome analysis - DNA testing
Chromosome analysis (karyotyping):
After a cell culture has been created, the macrostructure of the chromosomes is visualised. The karyogram produced shows the number of chromosomes and their structure up to a certain resolution.
This analysis is complex, consequently expensive and can only be carried out by specialised laboratories.
Molecular genetic analysis:
The SNP-array-based DNA test for equine Turner syndrome is a simple diagnostic method that can be carried out in solitary or as a supplement to other DNA-based tests from a given hair root sample.
Applications of the DNA test for eTS
- Exclusion of the X0 karyotype in the purchase examinations of foals
- Supplementary diagnostics for mare infertility
Literature
Prakash, S., Guo, D., Maslen, C. et al. Single-nucleotide polymorphism array genotyping is equivalent to metaphase cytogenetics for diagnosis of Turner syndrome. Genet Med 16, 53–59 (2014). https://doi.org/10.1038/gim.2013.77