A Non-Reciprocal Autosomal Translocation 64,XX, t(4;10)(q21;p15) in an Arabian Mare with Repeated Early Embryonic Loss

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.

Lethal variants of equine pregnancy: is it the placenta or foetus leading the conceptus in the wrong direction?

Embryonic and foetal loss remain one of the greatest challenges in equine reproductive health with 5-10% of established day 15 pregnancies and a further 5-10% of day 70 pregnancies failing to produce a viable foal. The underlying reason for these losses is variable but ultimately most cases will be attributed to pathologies of the environment of the developing embryo and later foetus, or a defect intrinsic to the embryo itself that leads to lethality at any stage of gestation right up to birth. Historically, much research has focused on the maternal endometrium, endocrine and immune responses in pregnancy and pregnancy loss, as well as infectious agents such as pathogens, and until recently very little was known about the both small and large genetic variants associated with reduced foetal viability in the horse. In this review, we first introduce key aspects of equine placental and foetal development. We then discuss incidence, risk factors and causes of pregnancy loss, with the latter focusing on genetic variants described to date that can impact equine foetal viability.

Multivariable analysis to determine risk factors associated with abortion in mares

Risk factors associated with equine reproductive efficiency have been identified along with those associated specifically with early pregnancy loss (EPL). In contrast, no studies have reported risk factors associated with abortion (loss between day 70 and 300 post-cover). Given the causes of abortion differ to those of EPL, likely too will the risk factors. A retrospective cohort study was carried out to identify risk factors associated with abortion in UK and Irish based Thoroughbreds, collecting data on 20 exposure variables over a five-year period. A generalized linear mixed model was utilized to evaluate the associations between exposure variables and abortion, with clustering of observations accounted for at the mare and farm level. Variables with a likelihood ratio test (LRT) p value <0.2 were entered into the model in a forward stepwise approach. Pregnancy outcome was available on 4,439 pregnancies from 2,510 mares. Having had two or more prior abortions (odds ratio (OR) 7.91, 95% confidence interval (CI) 2.86, 21.88), conceiving on the second or subsequent covered estrous cycle (OR 1.84, 95% CI 1.22, 2.78) and conceiving multiple conceptuses (OR 1.68, 95% CI 1.02, 2.76) were associated with an increased risk of abortion compared to null parous, first estrous cycle covers and singleton conceptions respectively. Increasing paternal age (OR 0.95, 95% CI 0.90, 0.99) was associated with a decreasing risk of abortion. Mare and farm variance were not significant in the final model, LRT p=0.43. These findings provide evidence-based data to inform Thoroughbred breeding management practices to help mitigate abortion risk.

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.

Two Novel Cases of Autosomal Translocations in the Horse: Warmblood Family Segregating t(4;30) and a Cloned Arabian with a de novo t(12;25)

We report 2 novel autosomal translocations in the horse. In Case 1, a breeding stallion with a balanced t(4p;30) had produced normal foals and those with congenital abnormalities. Of his 9 phenotypically normal offspring, 4 had normal karyotypes, 4 had balanced t(4p;30), and 1 carried an unbalanced translocation with tertiary trisomy of 4p. We argue that unbalanced forms of t(4p;30) are more tolerated and result in viable congenital abnormalities, without causing embryonic death like all other known equine autosomal translocations. In Case 2, two stallions produced by somatic cell nuclear transfer from the same donor were karyotyped because of fertility issues. A balanced translocation t(12q;25) was found in one, but not in the other clone. The findings underscore the importance of routine cytogenetic screening of breeding animals and animals produced by assisted reproductive technologies. These cases will contribute to molecular studies of translocation breakpoints and their genetic consequences in the horse.

Whole genome analysis reveals aneuploidies in early pregnancy loss in the horse

The first 8 weeks of pregnancy is a critical time, with the majority of pregnancy losses occurring during this period. Abnormal chromosome number (aneuploidy) is a common finding in human miscarriage, yet is rarely reported in domestic animals. Equine early pregnancy loss (EPL) has no diagnosis in over 80% of cases. The aim of this study was to characterise aneuploidies associated with equine EPL. Genomic DNA from clinical cases of spontaneous miscarriage (EPLs; 14-65 days of gestation) and healthy control placentae (various gestational ages) were assessed using a high density genotyping array. Aneuploidy was detected in 12/55 EPLs (21.8%), and 0/15 healthy control placentae. Whole genome sequencing (30X) and digital droplet PCR (ddPCR) validated results. The majority of these aneuploidies have never been reported in live born equines, supporting their embryonic/fetal lethality. Aneuploidies were detected in both placental and fetal compartments. Rodents are currently used to study how maternal ageing impacts aneuploidy risk, however the differences in reproductive biology is a limitation of this model. We present the first evidence of aneuploidy in naturally occurring equine EPLs at a similar rate to human miscarriage. We therefore suggest the horse as an alternative to rodent models to study mechanisms resulting in aneuploid pregnancies.

Genetics of Equine Reproductive Diseases

Reproductive disorders are genetically heterogeneous and complex; available genetic tests are limited to chromosome analysis and 1 susceptibility gene. Cytogenetic analysis should be the first test to confirm or rule out chromosomal aberrations. No causative genes/mutations are known. The only available genetic test for stallion subfertility is based on a susceptibility gene FKBP6. The ongoing progress in equine genomics will improve the status of genetic testing. However, because subfertile phenotypes do not facilitate collection of large numbers of samples or pedigrees, and clinical causes of many cases remain unknown, further progress requires constructive cross-talk between geneticists, clinicians, breeders, and owners.

Azoospermia and Y Chromosome-Autosome Translocation in a Friesian Stallion

This case report describes spermatogenic arrest and azoospermia in a stallion with a unique Y chromosome-autosome translocation. Clinical diagnosis of azoospermia was based on history of infertility and evaluation of ejaculates collected for artificial insemination. Clinical and ultrasonographic evaluation of the external and internal genitalia did not reveal any abnormalities except for smaller than normal testicular size. Azoospermia of testicular origin was confirmed by determining alkaline phosphatase concentration in semen. Histological evaluation of testicular tissue after castration confirmed early spermatogenic arrest. Cytogenetic evaluation showed the presence of translocation between the Y chromosome and chromosome 13. To the authors' knowledge, this is the first case of azoospermia with a cytogenetically detected Y chromosome abnormality, suggesting that the horse Y chromosome may carry sequences critical for normal spermatogenesis.

An Autosomal Translocation 73,XY,t(12;20)(q11;q11) in an Infertile Male Llama (Lama glama) With Teratozoospermia

Structural chromosome abnormalities, such as translocations and inversions occasionally occur in all livestock species and are typically associated with reproductive and developmental disorders. Curiously, only a few structural chromosome aberrations have been reported in camelids, and most involved sex chromosomes. This can be attributed to a high diploid number (2n = 74) and complex chromosome morphology, which makes unambiguous identification of camelid chromosomes difficult. Additionally, molecular tools for camelid cytogenetics are sparse and have become available only recently. Here we present a case report about an infertile male llama with teratozoospermia and abnormal chromosome number 2n = 73,XY. This llama carries an autosomal translocation of chromosomes 12 and 20, which is the likely cause of defective spermatogenesis and infertility in this individual. Our analysis underlines the power of molecular cytogenetics methods over conventional banding-based chromosome analysis for explicit identification of normal and aberrant chromosomes in camelid karyotypes. This is the first case of a translocation and the first autosomal aberration reported in any camelid species. It is proof of principle that, like in other mammalian species, structural chromosome abnormalities contribute to reproductive disorders in camelids.

Chromosome Aberrations and Fertility Disorders in Domestic Animals

The association between chromosomal abnormalities and reduced fertility in domestic animals is well recorded and has been studied for decades. Chromosome aberrations directly affect meiosis, gametogenesis, and the viability of zygotes and embryos. In some instances, balanced structural rearrangements can be transmitted, causing fertility problems in subsequent generations. Here, we aim to give a comprehensive overview of the current status and future prospects of clinical cytogenetics of animal reproduction by focusing on the advances in molecular cytogenetics during the genomics era. We describe how advancing knowledge about animal genomes has improved our understanding of connections between gross structural or molecular chromosome variations and reproductive disorders. Further, we expand on a key area of reproduction genetics: cytogenetics of animal gametes and embryos. Finally, we describe how traditional cytogenetics is interfacing with advanced genomics approaches, such as array technologies and next-generation sequencing, and speculate about the future prospects.

Fertility Assessment in Sorraia Stallions by Sperm-Fish and Fkbp6 Genotyping

The Sorraia, a critically endangered indigenous Iberian horse breed, is characterized by low genetic variability, high rate of inbreeding, bad sperm quality and subfertility. Here, we studied 11 phenotypically normal but subfertile Sorraia stallions by karyotyping, sex chromosome sperm-FISH and molecular analysis of FKBP6 - a susceptibility locus for impaired acrosome reaction (IAR). The stallions had normal sperm concentration (>300 million cells/ml), but the numbers of progressively motile sperm (21%) and morphologically normal sperm (28%) were invariably low. All stallions had a normal 64,XY karyotype. The majority of sperm (89%) had normal haploid sex chromosome content, although 11% of sperm carried various sex chromosome aneuploidies. No correlation was found between the percentage of sperm sex chromosome abnormalities and inbreeding, sperm morphology or stallion age. Direct sequencing of FKBP6 exon 4 for SNPs g.11040315G>A and g.11040379C>A revealed that none of the stallions had the susceptibility genotype (A/A-A/A) for IAR. Instead, all animals had a G/G-A/A genotype - a testimony of low genetic variability. The findings ruled out chromosomal abnormalities and genetic predisposition for IAR as contributing factors for subfertility. However, low fertility of the Sorraia stallions could be partly attributed to relatively higher rate of sex chromosome aneuploidies in the sperm.