Sex reversal syndrome in the horse: Four new cases of feminization in individuals carrying a 64,XY SRY negative chromosomal complement

The Use of Genomic Screening for the Detection of Chromosomal Abnormalities in the Domestic Horse: Five New Cases of 65,XXY Syndrome in the Pura Raza Español Breed

Sex chromosomal abnormalities are a well-established cause of reproductive failure in domestic horses. Because of its difficult diagnosis, the Pura Raza Español breeding program established a routine screening for chromosomal abnormalities in all the horses prior to enrolling in the studbook. This genomic procedure combines an initial assessment based on the results from Short Tandem Repeat (STR) parentage testing followed by a Single-Nucleotide Polymorphism (SNP) based copy number aberration (CNA) confirmative analysis in positive cases. Using this methodology, we identified five new individuals carrying a 65,XXY chromosomal number aberration (CNA) among 27,330 foals enrolled over the past two reproductive seasons. The animals were initially flagged as CNA candidates due to abnormal results in STR testing. Subsequent analysis genotyping using an STR sex-linked dedicated panel and a medium-density SNP array in ECAX and ECAY confirmed the diagnosis as 65,XXY carriers. Four cases (upon sample availability) underwent further analysis using in situ fluorescent hybridization with ECAX and ECAY probes, showing identical results. Phenotypic analysis revealed abnormal gonad development in one of the cases, showing that the remaining four had a normal reproductive morphology. To our knowledge, this represents the largest number of horses exhibiting the equine form of Klinefelter syndrome (65,XXY) reported to date. Our study highlights the importance of genomic screening in the accurate detection of chromosomal abnormalities in horses.

Molecular cytogenetic screening of sex chromosome abnormalities in young horse populations

BACKGROUND

Chromosomal abnormalities occur in the equine population at a rate of approximately 2%. The use of molecular cytogenetic techniques allows a more accurate identification of chromosomal abnormalities, especially those with a low rate of abnormal metaphases, demonstrating that the actual incidence in equine populations is higher.

OBJECTIVES

Estimation of the number of carriers of karyotypic abnormalities in a sample from a population of young horses of various breeds, using molecular cytogenetic techniques.

STUDY DESIGN

Cross-sectional.

METHODS

Venous blood samples were collected from 500 young horses representing 5 breeds (Purebred Arabian, Hucul, Polish primitive horse [Konik], Małopolska, Coldblood, Silesian). Chromosomes and DNA were obtained from blood lymphocytes and evaluated by fluorescence in situ hybridisation (FISH) and PCR, using probes and markers for the sex chromosomes and select autosomes.

RESULTS

Nineteen horses, 18 mares and 1 stallion, were diagnosed with different chromosomal abnormalities: 17 cases of mosaic forms of sex chromosome aneuploidies with a very low incidence (0.6%-4.7%), one case of a SRY-negative 64,XY sex reversal mare, and one mare with X-autosome translocation. The percentage of sex chromosomal aberrations was established as 3.8% in the whole population, 6.08% in females and 0.49% in males.

MAIN LIMITATIONS

Limited sample size, confined to horses from Poland.

CONCLUSIONS

The rate of sex chromosomal abnormalities we identified was almost double that reported in previous population studies that used classical chromosome staining techniques. FISH allowed the detection of aneuploid cell lines which had a very low incidence. The FISH technique is a faster and more precise method for karyotype examination; however, it is usually focused on only one or two chromosomes while banding karyotyping includes the entire chromosome set.

Prevalence of Sex-Related Chromosomal Abnormalities in a Large Cohort of Spanish Purebred Horses

Chromosomal abnormalities are largely associated with fertility impairments in the domestic horse. To date, over 600 cases of individuals carrying abnormal chromosome complements have been reported, making the domestic horse the species with the highest prevalence. However, studies analyzing the prevalence of chromosomal diseases in whole populations are scarce. We, therefore, employed a two-step molecular tool to screen and diagnose chromosomal abnormalities in a large population of 25,237 Pura Raza Español horses. Individuals were first screened using short tandem repeats parentage testing results and phenotypic evaluations. Those animals showing results suggesting chromosomal abnormalities were re-tested using a single nucleotide polymorphism (SNP)-based diagnostic methodology to accurately determine the chromosomal complements. Thirteen individuals showed a positive screening, all of which were diagnosed as chromosomally abnormal, including five 64,XY mares with sex development disorders (DSD) and four cases of blood chimerism (two male/female and two female/female cases). In addition, we detected one Turner and one Klinefelter syndrome and two individuals carrying complex karyotypes. The overall prevalence in the entire population was ~0.05%, with the prevalence of 64,XY DSD and blood chimerism ~0.02% and ~0.016%, respectively. However, the overall results should be taken with caution since the individuals carrying Turner syndrome (in full (63,X) or mosaic (mos 63,X/64,XX) forms) cannot be detected due to limitations in the methodology employed. Finally, the lack of agreement between populational studies performed using karyotyping or molecular methods is discussed. To our knowledge, this is the largest populational study performed evaluating the prevalence of the most common chromosomal abnormalities in the domestic horse.

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

Hereditary diseases represent a serious problem in horses, especially in terms of sport use and breeding. Nowadays, we know the genetic basis of several breed-specific inherited diseases. In this study, we focused on the cytogenetic analysis of the clinical case of a healthy mare and her foal with numerous malformations in order to confirm or disprove the suspicion of genetic causes of a disability in this offspring. We used conventional metaphase staining to analyse chromosomal aberrations – breaks and gaps. In general, the number of breaks exceeding the norm (2—3 breaks/100 meta-phases) may indicate the influence of the external environment with a potential teratogenic effect on the offspring during its mother gravidity. Compared to the norm, we found a slightly increased percentage of chromosomal aberrations in both the mother and the foal. As another method, we used karyotyping to assess the number and morphology of chromosomes, where in addition to conventional staining, we also applied differential staining of metaphases (G-banding). Multiplication, loss or rearrangement of chromosome segments are almost always associated with pathology. In the karyotypes we constructed, we observed changes in both individuals, compared to the international standard; in the mare, we probably recorded the mosaic form of her karyotype. In the foal, we found 64, XX with a suspected morphological change which was probably related to autosomal chromosome pair 31. The cytogenetic analysis of suspected individuals is also very beneficial for horse owners and breeders. Thanks to the combination of cytogenetic and modern molecular-genetic methods, we were able to identify individuals unsuitable for breeding.

Copy Number Variation (CNV): A New Genomic Insight in Horses

Simple Summary

This study aimed to contribute to our knowledge of CNVs, a type of genomic marker in equines, by producing, for the first time, a fine-scale characterization of the CNV regions (CNVRs) in the Pura Raza Española horse breed. We found not only the existence of a unique pattern of genomic regions enriched in CNVs in the PRE in comparison with the data available from other breeds but also the incidence of CNVs across the entire genome. Since these regions could affect the structure and dose of the genes involved, we also performed a gene ontology analysis which revealed that most of the genes overlapping in CNVRs were related to the olfactory pathways and immune response.

Abstract

Copy number variations (CNVs) are a new-fangled source of genetic variation that can explain changes in the phenotypes in complex traits and diseases. In recent years, their study has increased in many livestock populations. However, the study and characterization of CNVs in equines is still very limited. Our study aimed to investigate the distribution pattern of CNVs, characterize CNV regions (CNVRs), and identify the biological pathways affected by CNVRs in the Pura Raza Española (PRE) breed. To achieve this, we analyzed high-density SNP genotyping data (670,804 markers) from a large cohort of 654 PRE horses. In total, we identified 19,902 CNV segments and 1007 CNV regions in the whole population. The length of the CNVs ranged from 1.024 kb to 4.55 Mb, while the percentage of the genome covered by CNVs was 4.4%. Interestingly, duplications were more abundant than deletions and mixed CNVRs. In addition, the distribution of CNVs across the chromosomes was not uniform, with ECA12 being the chromosome with the largest percentage of its genome covered (19.2%), while the highest numbers of CNVs were found in ECA20, ECA12, and ECA1. Our results showed that 71.4% of CNVRs contained genes involved in olfactory transduction, olfactory receptor activity, and immune response. Finally, 39.1% of the CNVs detected in our study were unique when compared with CNVRs identified in previous studies. To the best of our knowledge, this is the first attempt to reveal and characterize the CNV landscape in PRE horses, and it contributes to our knowledge of CNVs in equines, thus facilitating the understanding of genetic and phenotypic variations in the species. However, further research is still needed to confirm if the CNVs observed in the PRE are also linked to variations in the specific phenotypical differences in the breed.

Sex Reversal Syndrome in an Egyptian Arabian Horse Detected Using Genomic Data - A case report

A 4-year-old Straight Egyptian Arabian horse was evaluated in 2016 due to a malformation of external genitalia and male sexual behavior. On physical examination, small teats in the inguinal area and a rudimentary penis-like structure surrounded by a clitoral fossa could be seen. There was no evidence of vulva and vaginal canal. A stallion like behavior was observed, especially in the presence of mares in heat, when the animal was excited and aggressive and had erection of the penis-like structure. Blood samples were collected for two purposes: hormonal (testosterone and estradiol plasma concentration analyses) and genetic (cytogenetic and molecular analysis). The karyotype showed 32 pairs of chromosomes in all cells (2n = 64) including 14 and 18 pairs of metacentric and acrocentric chromosomes respectively, in agreement with a presumptive 64, XX complement. This result agree with STR and SNP molecular analysis, which also ruled out the possibility of hematopoietic chimerism. In addition, SNP genotyping showed no numerical chromosomal aberrations or large deletions or duplications, that can be linked to the phenotype in any autosome, nor numerical chromosomal abnormalities in the father and mother of the horse analyzed. In conclusion, we determined that the animal in the present study is a male pseudohermaphrodite.

Screening and detection of chromosomal copy number alterations in the domestic horse using SNP-array genotyping data

Chromosomal abnormalities are a common cause of infertility in horses. However, they are difficult to detect using automated methods. Here, we propose a simple methodology based on single nucleotide polymorphism (SNP)-array data that allows us to detect the main chromosomal abnormalities in horses in a single procedure. As proof of concept, we were able to detect chromosomal abnormalities in 33 out of 268 individuals, including monosomies, chimerisms, and male and female sex-reversions, by analyzing the raw signal intensity produced by an SNP array-based genotyping platform. We also demonstrated that the procedure is not affected by the SNP density of the array employed or by the inbreeding level of the individuals. Finally, the methodology proposed in this study could be performed in an open bioinformatic environment, thus permitting its integration as a flexible screening tool in diagnostic laboratories and genomic breeding programs.

A Fast PCR Test for the Simultaneous Identification of Species and Gender in Horses, Donkeys, Mules and Hinnies

Having considered that the current methods are costly and time-consuming, we designed an only 3 pairs primer-based PCR test to accurately identify the species and gender in horses, donkeys, mules and hinnies. Through a thorough sequence comparison between horse and donkey's highly similar genomes, and a vast amount of preliminary confirmation, we found that three fragments, CNGB3 gene on an autosome, displacement loop region on mitochondrial DNA and SRY genes on chromosome Y, within these equine DNA, are enough to enable us achieving our goal. The PCR test described here would be an economical, fast and accurate alternative for the most commonly-used methods, polymerase chain reaction-restriction fragment length polymorphism, microsatellite assay, and sequencing.

Horse Clinical Cytogenetics: Recurrent Themes and Novel Findings

Clinical cytogenetic studies in horses have been ongoing for over half a century and clearly demonstrate that chromosomal disorders are among the most common non-infectious causes of decreased fertility, infertility, and congenital defects. Large-scale cytogenetic surveys show that almost 30% of horses with reproductive or developmental problems have chromosome aberrations, whereas abnormal karyotypes are found in only 2-5% of the general population. Among the many chromosome abnormalities reported in the horse, most are unique or rare. However, all surveys agree that there are two recurrent conditions: X-monosomy and SRY-negative XY male-to-female sex reversal, making up approximately 35% and 11% of all chromosome abnormalities, respectively. The two are signature conditions for the horse and rare or absent in other domestic species. The progress in equine genomics and the development of molecular tools, have qualitatively improved clinical cytogenetics today, allowing for refined characterization of aberrations and understanding the underlying molecular mechanisms. While cutting-edge genomics tools promise further improvements in chromosome analysis, they will not entirely replace traditional cytogenetics, which still is the most straightforward, cost-effective, and fastest approach for the initial evaluation of potential breeding animals and horses with reproductive or developmental disorders.

Impaired Reproductive Function in Equines: From Genetics to Genomics

Fertility is one of the key factors in the economic and productive success of the equine industry. Despite this, studies on the genetic causes affecting reproductive performance are scarce, especially in mares, where the genetic architecture of the reproductive traits is extremely complex. Today, with the increasing availability of new genomic methodologies for this species, we are presented with an interesting opportunity to understand the genetic basis of equine reproductive disorders. These include, among others, novel techniques for detecting chromosomal abnormalities, whose association with infertility in horses was established over 50 years ago; new sequencing technologies permitting an accurate detection of point mutations influencing fertility, as well as the study of inbreeding and molecular homozygosity, which has been widely suggested as one of the main causes of low reproductive performance in horses. Finally, over the last few years, reproductive performance has also been associated with copy number variants and candidate genes detected by genome-wide association studies on fertility traits. However, such studies are still scarce, probably because they depend on the existence of large and accurate phenotypic datasets of reproductive and/or fertility traits, which are still difficult to obtain in equines.

A new molecular screening tool for the detection of chromosomal abnormalities in donkeys

Chromosomal abnormalities are a major cause of infertility and reproductive problems in equids. Nowadays, their detection is rising due to the use of new diagnostic tools based on molecular markers instead of karyotyping. Reports of this kind of genetic aberrations in domestic donkeys (Equus asinus) are extremely scarce, despite their importance in human activities. In the present study, we analysed the implementation of a short-tandem-repeat (STR)-based molecular method initially developed for horses, as a diagnostic tool to detect chromosomal abnormalities in donkeys. The frequency of five X-linked (LEX003, LEX026, TKY38, TKY270 and UCEDQ502) and one Y-linked (ECAYM2) molecular markers and one Y-linked gene (sex-determining region Y, SRY) was characterized in 121 donkeys from two diverse breeds, the Spanish Andalusian and the African Moroccan breeds. The molecular panel showed 100% sensitivity and 99.67% specificity in detecting 10 different chromosomal abnormalities in the species. In conclusion, this methodology is a valid, rapid and low-cost tool for the detection and characterization of chromosomal abnormalities in domestic donkeys.

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

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.

Congenital Malformations in River Buffalo (Bubalus bubalis)

Simple Summary

Congenital malformations (due to genetic causes) represent a hidden danger for animal production, above all when genetic selection is undertaken for production improvements. These malformations are responsible for economic losses either because they reduce the productivity of the farm, or because their spread in the population would decrease the total productivity of that species/breed. River buffalo is a species of increasing interest all over the world for its production abilities, as proved by the buffalo genome project and the genetic selection plans that are currently performed in different countries. The aim of this review is to provide a general view of different models of congenital malformations in buffalo and their world distribution. This would be useful either for those who performed buffalo genetic selection or for researchers in genetic diseases, which would be an advantage to their studies with respect to the knowledge of gene mutations and interactions in this species.

Abstract

The world buffalo population is about 168 million, and it is still growing, in India, China, Brazil, and Italy. In these countries, buffalo genetic breeding programs have been performed for many decades. The occurrence of congenital malformations has caused a slowing of the genetic progress and economic loss for the breeders, due to the death of animals, or damage to their reproductive ability or failing of milk production. Moreover, they cause animal welfare reduction because they can imply foetal dystocia and because the affected animals have a reduced fitness with little chances of survival. This review depicts, in the river buffalo (Bubalus bubalis) world population, the present status of the congenital malformations, due to genetic causes, to identify their frequency and distribution in order to develop genetic breeding plans able to improve the productive and reproductive performance, and avoid the spreading of detrimental gene variants. Congenital malformations most frequently reported in literature or signaled by breeders to the Department of Veterinary Medicine and Animal Production of the University Federico II (Naples, Italy) in river buffalo are: musculoskeletal defects (transverse hemimelia, arthrogryposis, umbilical hernia) and disorders of sexual development. In conclusion this review put in evidence that river buffalo have a great variety of malformations due to genetic causes, and TH and omphalocele are the most frequent and that several cases are still not reported, leading to an underestimation of the real weight of genetic diseases in this species.

Equine 5α-reductase activity and expression in epididymis

The 5α-reductase enzymes play an important role during male sexual differentiation, and in pregnant females, especially equine species where maintenance relies on 5α-reduced progesterone, 5α-dihydroprogesterone (DHP). Epididymis expresses 5α-reductases but was not studied elaborately in horses. Epididymis from younger and older postpubertal stallions was divided into caput, corpus and cauda and examined for 5α-reductase activity and expression of type 1 and 2 isoforms by quantitative real-time polymerase chain reaction (qPCR). Metabolism of progesterone and testosterone to DHP and dihydrotestosterone (DHT), respectively, by epididymal microsomal protein was examined by thin-layer chromatography and verified by liquid chromatography tandem mass spectrometry (LC-MS/MS). Relative inhibitory potencies of finasteride and dutasteride toward equine 5α-reductase activity were investigated. Pregnenolone was investigated as an additional potential substrate for 5α-reductase, suggested previously from in vivo studies in mares but never directly examined. No regional gradient of 5α-reductase expression was observed by either enzyme activity or transcript analysis. Results of PCR experiments suggested that type 1 isoform predominates in equine epididymis. Primers for the type 2 isoform were unable to amplify product from any samples examined. Progesterone and testosterone were readily reduced to DHP and DHT, and activity was effectively inhibited by both inhibitors. Using epididymis as an enzyme source, no experimental evidence was obtained supporting the notion that pregnenolone could be directly metabolized by equine 5α-reductases as has been suggested by previous investigators speculating on alternative metabolic pathways leading to DHP synthesis in placenta during equine pregnancies.

Sex Reversal in Non-Human Placental Mammals

Gonads are very peculiar organs given their bipotential competence. Indeed, early differentiating genital ridges evolve into either of 2 very distinct organs: the testis or the ovary. Accumulating evidence now demonstrates that both genetic pathways must repress the other in order for the organs to differentiate properly, meaning that if this repression is disrupted or attenuated, the other pathway may completely or partially be expressed, leading to disorders of sex development. Among these disorders are the cases of XY male-to-female and XX female-to-male sex reversals as well as true hermaphrodites, in which there is a discrepancy between the chromosomal and gonadal sex. Here, we review known cases of XY and XX sex reversals described in mammals, focusing mostly on domestic animals where sex reversal pathologies occur and on wild species in which deviations from the usual XX/XY system have been documented.