Disorder of Sexual Development in a Mare with an Unusual Tentative Mosaic Karyotype: 63,X/64,Xdel(Y)

Abstract

The present report describes a 4-year-old Trakehner mare which was referred to the clinic for a breeding soundness evaluation. Clinical, histological, and postmortem examination revealed an underdeveloped genital tract, the absence of a cervix uteri, and small inactive ovaries without male gonadal tissue. Blood lymphocyte analysis revealed an unusual mosaic karyotype consisting of 2 cell lines. For the majority of cells (70%), monosomy X (63,X) was observed. The remaining cells (30%) contained 64 chromosomes including one X chromosome and a small rudimentary Y chromosome consisting mostly of heterochromatin. The centromere was retained, but its full functionality was questionable. PCR analysis revealed that the entire male-specific region of Y (Yq14), including the SRY gene, was deleted. It remained unclear if the pseudoautosomal region (Yq15) and parts of the heterochromatic region (Yq13) were affected by this deletion. The phenotype of the mare with this disorder of sex development associated with sex chromosome abnormalities is genetically comparable to 63,X monosomy which fully explains the clinical findings.

Molecular cytogenetic screening of sex chromosome abnormalities in young horse populations

“…In our study, the XY SRY‐negative karyotype was diagnosed in a mare whose body structure and external reproductive organs did not differ from those of healthy animals, but with undeveloped internal organs and the presence of testicular tissue. This is consistent with reports describing other cases of mares that were 64,XY Sry negative 12,16,19,25,48 …”

“…This is consistent with reports describing other cases of mares that were 64,XY Sry negative. 12,16,19,25,48 In our study, we identified, for the first time, a balanced reciprocal translocation between chromosome 1 and the X chromosome. Unlike in other livestock species, translocations in horses are rare, but as in other species, sustainable translocations cause infertility in both mares and stallions due to recurrent early embryo loss (REEL), which is the loss of embryo between conception and 40-60 days of gestation.…”

“…9 The most commonly reported karyotype aberration is non-mosaic X chromosome monosomy (63,X) 5,6,[8][9][10][11] or mosaic form 63,X/64,XX in mares [3][4][5][6][12][13][14][15] or 63,X/64, XY in males. 12,16,17 X chromosome monosomy has also been described as mosaic with a second abnormal cell line, for example, 63,X/65,XYY 18 or 63,X/64,X(del)Y 19 or as a mosaic of several cell lines. [20][21][22][23] The second most common chromosomal disorder in horses is karyotype 64,XY Sry negative, identified in animals with a mare-like phenotype, which belongs to a group collectively known as Disorders of Sex Development (DSD).…”

“…The most commonly reported karyotype aberration is non‐mosaic X chromosome monosomy (63,X) 5,6,8–11 or mosaic form 63,X/64,XX in mares 3–6,12–15 or 63,X/64,XY in males 12,16,17 . X chromosome monosomy has also been described as mosaic with a second abnormal cell line, for example, 63,X/65,XYY 18 or 63,X/64,X(del)Y 19 or as a mosaic of several cell lines 20–23 …”

Cytogenetic Analysis of a Mare and Her Foal with Suspected Genetic Causes of Disability

“…Later, B u g n o -P o n i e w i e r s k a et al [5] reported about a case of sex chromosome mosaicism 64, XX/65, XXY/66, XXYY in a mare. The main difference between mosaicism and chimerism is that mosaicism is a condition when cell lines are derived from one embryo and are produced by unbalanced sister chromosome velopment -DSD -with sex reversal) [16]. Animals phenotypically appear female, but the karyotype corresponds to males [15].…”