Clinical Cytogenetics of the Dog: A Review

A rare finding of double Barr bodies and X-monosomy/X-trisomy mosaicism in a dog with presumed idiopathic epilepsy

A 4-year-old spayed female Border Collie dog presented to the Neurology and Neurosurgery service for an approximately five-month history of seizures. A complete neurodiagnostic workup was performed and did not reveal any significant abnormalities. The patient's seizures were well controlled with a combination of anticonvulsants. During a manual blood smear review at a follow-up appointment, double Barr bodies were identified in segmented neutrophils. Karyotyping revealed that the patient is mosaic for X-monosomy and X-trisomy, a finding that has never been reported in a dog and is rarely reported in people. This case demonstrates how the identification of abnormal neutrophil nuclear appendages may correlate with chromosomal abnormalities in dogs.

Molecular Cytogenetics in Domestic Bovids: A Review

The discovery of the Robertsonian translocation (rob) involving cattle chromosomes 1 and 29 and the demonstration of its deleterious effects on fertility focused the interest of many scientific groups on using chromosome banding techniques to reveal chromosome abnormalities and verify their effects on fertility in domestic animals. At the same time, comparative banding studies among various species of domestic or wild animals were found useful for delineating chromosome evolution among species. The advent of molecular cytogenetics, particularly the use of fluorescence in situ hybridization (FISH), has allowed a deeper investigation of the chromosomes of domestic animals through: (a) the physical mapping of specific DNA sequences on chromosome regions; (b) the use of specific chromosome markers for the identification of the chromosomes or chromosome regions involved in chromosome abnormalities, especially when poor banding patterns are produced; (c) better anchoring of radiation hybrid and genetic maps to specific chromosome regions; (d) better comparisons of related and unrelated species by comparative FISH mapping and/or Zoo-FISH techniques; (e) the study of meiotic segregation, especially by sperm-FISH, in some chromosome abnormalities; (f) better demonstration of conserved or lost DNA sequences in chromosome abnormalities; (g) the use of informatic and genomic reconstructions, in addition to CGH arrays, to predict conserved or lost chromosome regions in related species; and (h) the study of some chromosome abnormalities and genomic stability using PCR applications. This review summarizes the most important applications of molecular cytogenetics in domestic bovids, with an emphasis on FISH mapping applications.

Non-mosaic X monosomy (77,X) in a female dog with signs of virilization

A 14-month-old female Miniature Poodle dog with an enlarged clitoris and asymmetry in the placement of the teats was subjected to clinical, histopathological, and genetic studies. Macroscopically, the uterus and fallopian tubes appeared normal, while both ovaries were diffusely altered. At histology, the ovarian parenchyma was almost completely effaced by a diffuse hyperplasia of theca cells with atretic primary follicles. Chromosome analysis showed pure (non-mosaic) X monosomy (77,X). This finding was confirmed by the highly sensitive droplet digital PCR (ddPCR) approach. Despite the observed virilization, molecular analysis did not show the presence of Y-linked genes (SRY, ZFY, and TSPY1) in the blood cells or ovary tissue. To the best of our knowledge, this is the first case of X monosomy in a dog associated with virilization.

Cytogenetic and molecular insight into the genetic background of disorders of sex development in seventeen cats

The genetic background of feline disorders of sex development (DSDs) is poorly understood. We performed comprehensive cytogenetic, molecular, and histological studies of 17 cats with abnormal external genitalia, unusual behavior, or tricolor coats (atypical in males). The DSD phenotype of three cats was associated with sex chromosome abnormalities: X/Y translocation (38,XX^SRY+), 37,X/38,XY mosaicism, and XX/XY leukocyte chimerism. The remaining 14 affected cats were classified as XY DSD (SRY-positive). In this group and 38 normal males, we analyzed a priori selected candidate genes (SRY, TAC3, CYP11B1 and LHCGR). Only a previously reported nonpathogenic variant was found in SRY. Moreover, SRY gene copy number was determined, and three variants were observed: 6, 5 (modal), and 4 copies in a single DSD case. The known variants in TAC3 and CYP11B1, responsible for testicular hypoplasia, persistent primary dentition or congenital adrenal hyperplasia, were not found in the study group. Nine novel polymorphisms were identified in the LHCGR gene, one of which, a potentially regulatory indel variant in 5'UTR, was significantly associated (p = 0.0467) with XY DSD. Our report confirmed that abnormalities of sex chromosomes are important causes of feline DSDs. We also showed that the indel variant of LHCGR can be considered a promising marker associated with XY DSD phenotype.

Comparative Fluorescence In Situ Hybridization (FISH) Mapping of Twenty-Three Endogenous Jaagsiekte Sheep Retrovirus (enJSRVs) in Sheep (Ovis aries) and River Buffalo (Bubalus bubalis) Chromosomes

Endogenous retroviruses (ERVs) are the remnants of ancient infections of host germline cells, thus representing key tools to study host and viral evolution. Homologous ERV sequences often map at the same genomic locus of different species, indicating that retroviral integration occurred in the genomes of the common ancestors of those species. The genome of domestic sheep (Ovis aries) harbors at least twenty-seven copies of ERVs related to the exogenous and pathogenic Jaagsiekte sheep retrovirus (JSRVs), thus referred to as enJSRVs. Some of these loci are unequally distributed between breeds and individuals of the host species due to polymorphic insertions, thereby representing invaluable tools to trace the evolutionary dynamics of virus populations within their hosts. In this study, we extend the cytogenetic physical maps of sheep and river buffalo by performing fluorescent in situ hybridization (FISH) mapping of twenty-three genetically characterized enJSRVs. Additionally, we report the first comparative FISH mapping of enJSRVs in domestic sheep (2n = 54) and river buffalo (Bubalus bubalis, 2n = 50). Finally, we demonstrate that enJSRV loci are conserved in the homologous chromosomes and chromosome bands of both species. Altogether, our results support the hypothesis that enJSRVs were present in the genomes of both species before they differentiated within the Bovidae family.

Case Report: Disorder of Sexual Development in a Chinese Crested Dog With XX/XY Leukocyte Chimerism and Mixed Cell Testicular Tumors

A 10-year-old intact female Chinese Crested dog was presented for evaluation and further diagnostics due to persistent symptoms of vulvar swelling, vaginal discharge, and an 8-year history of acyclicity. At presentation, generalized hyperpigmentation and truncal alopecia were identified, with no aberrations of the female phenotype. Vaginal cytology confirmed the influence of estrogen at multiple veterinary visits, and hormonal screening of progesterone and anti-Mullerian hormone indicated gonadal presence. Based on findings from abdominal laparotomy and gonadectomy, the tissue was submitted for histopathology. Histopathologic evaluation identified the gonads to be abnormal testes containing multiple Sertoli and interstitial (Leydig) cell tumors. The histopathologic diagnosis of testes and concurrent normal external female phenotype in the patient lead to a diagnosis of a disorder of sexual development (DSD). Karyotype evaluation by conventional and molecular analysis revealed a two cell line chimeric pattern of 78,XX (80%) and 78,XY (20%) among blood leukocytes, as well as a positive PCR test for the Y-linked SRY gene. Cytogenetic analysis of skin fibroblasts revealed the presence of 78,XX cells exclusively, and PCR tests for the Y-linked SRY gene were negative in the hair and skin samples. These results are consistent with an XX/XY blood chimerism. This is one of the few case reports of a canine with the diagnosis of leukocyte chimerism with normal female phenotypic external genitalia. This case illustrates a distinct presentation for hormonally active Sertoli cell tumorigenesis and demonstrates surgery as a curative treatment option for clinically affected patients.

Whole genome sequencing identifies a missense polymorphism in PADI6 associated with testicular/ovotesticular XX disorder of sex development in dogs

Disorders of sex development (DSDs) are congenital malformations defined as discrepancies between sex chromosomes and phenotypical sex. Testicular or ovotesticular XX DSDs are frequently observed in female dogs, while monogenic XY DSDs are less frequent. Here, we applied whole genome sequencing (WGS) to search for causative mutations in XX DSD females in French Bulldogs (FB) and American Staffordshire Terries (AST) and in XY DSD Yorkshire Terries (YT). The WGS results were validated by Sanger sequencing and ddPCR. It was shown that a missense SNP of the PADI6 gene, is significantly associated with the XX DSD (SRY-negative) phenotype in AST (P = 0.0051) and FB (P = 0.0306). On the contrary, we did not find any associated variant with XY DSD in YTs. Our study suggests that the genetic background of the XX DSD may be more complex and breed-specific.

The Second Case of Non-Mosaic Trisomy of Chromosome 26 with Homologous Fusion 26q;26q in the Horse

Simple Summary

We present chromosome and DNA analysis of a normal Thoroughbred mare and her abnormal foal born with neurologic defects. We show that the foal has an abnormal karyotype with three copies of chromosome 26 (trisomy chr26), instead of the normal two. However, two of the three chr26 have fused, forming an unusual derivative chromosome. Chromosomes of the dam are normal, suggesting that the chromosome abnormality found in the foal happened during egg or sperm formation or after fertilization. Analysis of the foal and the dam with chr26 DNA markers indicates that the extra chr26 in the foal is likely of maternal origin and that the unusual derivative chromosome resulted from the fusion of two parental chr26. We demonstrate that although conventional karyotype analysis can accurately identify chromosome abnormalities, determining the mechanism and parental origin of these abnormalities requires DNA analysis. Most curiously, this is the second case of trisomy chr26 with unusual derivative chromosome in the horse, whereas all other equine trisomies have three separate copies of the chromosome involved. Because horse chr26 shares genetic similarity with human chr21, which trisomy causes Down syndrome, common features between trisomies of horse chr26 and human chr21 are discussed.

Abstract

We present cytogenetic and genotyping analysis of a Thoroughbred foal with congenital neurologic disorders and its phenotypically normal dam. We show that the foal has non-mosaic trisomy for chromosome 26 (ECA26) but normal 2n = 64 diploid number because two copies of ECA26 form a metacentric derivative chromosome der(26q;26q). The dam has normal 64,XX karyotype indicating that der(26q;26q) in the foal originates from errors in parental meiosis or post-fertilization events. Genotyping ECA26 microsatellites in the foal and its dam suggests that trisomy ECA26 is likely of maternal origin and that der(26q;26q) resulted from Robertsonian fusion. We demonstrate that conventional and molecular cytogenetic approaches can accurately identify aneuploidy with a derivative chromosome but determining the mechanism and parental origin of the rearrangement requires genotyping with chromosome-specific polymorphic markers. Most curiously, this is the second case of trisomy ECA26 with der(26q;26q) in the horse, whereas all other equine autosomal trisomies are ‘traditional’ with three separate chromosomes. We discuss possible ECA26 instability as a contributing factor for the aberration and likely ECA26-specific genetic effects on the clinical phenotype. Finally, because ECA26 shares evolutionary homology with human chromosome 21, which trisomy causes Down syndrome, cytogenetic, molecular, and phenotypic similarities between trisomies ECA26 and HSA21 are discussed.

The Cytogenetics of the Water Buffalo: A Review

The water buffalo (Bubalus bubalis), also known as the Asian buffalo, is an essential domestic bovid. Indeed, although its world population (~209 million heads) is approximately one-ninth that of cattle, the management of this species involves a larger human population than that involved with raising cattle. Compared with cattle, water buffalo have been understudied for many years, but interest in this species has been increasing, especially considering that the world population of these bovids grows every year-particularly that of the river buffalo. There are two genera of buffalo worldwide: the Syncerus (from the African continent), and the Bubalus (from the southwest Asian continent, Mediterranean area, southern America, and Australia). All species belonging to these two genera have specific chromosome numbers and shapes. Because of such features, the study of chromosomes is a fascinating biological basis for differentiating various species (and hybrids) of buffaloes and characterizing their karyotypes in evolutionary, clinical, and molecular studies. In this review, we report an update on essential cytogenetic studies in which various buffalo species were described from evolutionary, clinical, and molecular perspectives-particularly considering the river buffalo (Bubalus bubalis 2n = 50). In addition, we show new data on swamp buffalo chromosomes.

Identification of aneuploidy in dogs screened by a SNP microarray

Microarray analysis is an efficient approach for screening and identifying cytogenetic imbalances in humans. SNP arrays, in particular, are a powerful way to identify copy-number gains and losses representing aneuploidy and aneusomy, but moreover, allow for the direct assessment of individual genotypes in known disease loci. Using these approaches, trisomies, monosomies, and mosaicism of whole chromosomes have been identified in human microarray studies. For canines, this approach is not widely used in clinical laboratory diagnostic practice. In our laboratory, we have implemented the use of a proprietary SNP array that represents approximately 650,000 loci across the domestic dog genome. During the validation of this microarray prior to clinical use, we identified three cases of aneuploidy after screening 2053 dogs of various breeds including monosomy X, trisomy X, and an apparent mosaic trisomy of canine chromosome 38 (CFA38). This study represents the first use of microarrays for copy-number evaluation to identify cytogenetic anomalies in canines. As microarray analysis becomes more routine in canine genetic testing, more cases of chromosome aneuploidy are likely to be uncovered.

Chromosome abnormalities in dogs with disorders of sex development (DSD)

Disorders of sex development (DSD) caused by chromosome abnormalities are rarely diagnosed in dogs. In this report, there is a focus on five DSD cases in which the dogs had abnormal karyotypes. All animals were recognized by owners as females, however, these dogs had a large number of reproductive defects. Among these were abnormal external genitalia such as an enlarged clitoris, abnormal development of the labia, abnormal location of the vulva and urethral orifice, and other abnormalities were observed in four dogs. Gonadal histology assessments were conducted on three dogs and there were diagnoses of the presence of an ovary, inactive testes, and ovotestis with calcification in ovarian follicles. Results from cytogenetic analysis indicated there were the following karyotypes: (a) X trisomy in a mosaic form (79,XXX/78,XX); (b) Robertsonian translocation in a mosaic form (77,XX,rob/78,XX); (c) nonmosaic X/autosome translocation (78,X,t(X;A)); (d) X/autosome translocation in a mosaic form (78,X,t(X;A)/78,XX); and (e) leukocyte chimerism (78,XX/78,XY). The findings in the present study, emphasize that cytogenetic analysis is essential for elucidating the pathogenesis of DSD in dogs.