Ocular findings in Quarter Horses with hereditary equine regional dermal asthenia

Connective Tissue Disorders in Domestic Animals

Though soft tissue disorders have been recognized and described to some detail in several types of domestic animals and small mammals for some years, they remain uncommon. Because of their low prevalence, not much progress has been made not only in improved diagnosis but also in our understanding of the biochemical basis and pathogenesis of these diseases in animals. Ehlers-Danlos syndrome (EDS) described in dogs already in 1943 and later in cats has only minor impact on the well-being of the dog as its effects on skin of these animals are rather limited. The involved skin is thin and hyperextensible with easily inflicted injuries resulting in hemorrhagic wounds and atrophic scars. Joint laxity and dislocation common in people are less frequently found in dogs. No systemic complications, such as organ rupture or cardiovascular problems which have devastating consequences in people have been described in cats and dogs. The diagnosis is based on clinical presentation and on light or electron microscopic features of disorganized and fragmented collagen fibrils. Several case of bovine and ovine dermatosparaxis analogous to human Ehlers-Danlos syndrome type VIIC were found to be caused by mutations in the procollagen I N-proteinase (pnPI) or ADAMTS2 gene, though mutations in other sites are likely responsible for other types of dermatosparaxis. Cattle suffering from a form of Marfan syndrome (MFS) were described to have aortic dilatation and aneurysm together with ocular abnormalities and skeletal involvement. As in people, mutations at different sites of bovine FBN1 may be responsible for Marfan phenotype. Hereditary equine regional dermal asthenia (HERDA), or hyperelastosis cutis, has been recognized in several horse breeds as affecting primarily skin, and, occasionally, tendons. A mutation in cyclophilin B, a chaperon involved in proper folding of collagens, has been identified in some cases. Warmblood fragile foal syndrome (WFFS) is another Ehlers-Danlos-like disorder in horses, affecting primarily Warmbloods who present with skin fragility and joint hyperextensibility. Degenerative suspensory ligament desmitis (DSLD) affects primarily tendons and ligaments of certain horse breeds. Data from our laboratory showed excessive accumulation of proteoglycans in organs with high content of connective tissues. We have identified increased presence of bone morphogenetic protein 2 (BMP2) in active foci of DSLD and an abnormal form of decorin in proteoglycan deposits. Our most recent data obtained from next generation sequencing showed disturbances in expression of genes for numerous proteoglycans and collagens.

Evaluation of the Filum Terminale in Hereditary Equine Regional Dermal Asthenia

The objectives of this study were to describe the anatomy, histology, and ultrastructure of the equine filum terminale (FT) and to describe the FT in hereditary equine regional dermal asthenia (HERDA), a model of human Ehlers-Danlos syndromes (EDS). Those humans suffer from tethered cord syndrome (TCS) caused by an abnormally structured FT wherein its attachment at the base of the vertebral column leads to long-term stretch-induced injury to the spinal cord. The pathophysiology of TCS in EDS is poorly understood, and there is a need for an animal model of the condition. Histopathologic and ultrastructural examinations were performed on FT from HERDA (n = 4) and control horses (n = 5) and were compared to FT from human TCS patients with and without EDS. Adipose, fibrous tissue, and neuronal elements were assessed. CD3 and CD20 immunohistochemistry was performed to clarify cell types (HERDA n = 2; control n = 5). Collagen fibrils were assessed in cross-section for fibril diameter and shape, and in longitudinal section for fibril disorganization, swelling, and fragmentation. The equine and human FT were similar, with both containing fibrous tissue, ependyma, neuropil, and nerve twigs. Hypervascularity was observed in both HERDA horses and human EDS-TCS patients and was not observed in equine or human controls. Moderate to severe abnormalities in collagen fibril orientation and architecture were observed in all HERDA horses and were similar to those observed in human EDS-TCS patients.

Distribution of the Warmblood Fragile Foal Syndrome Type 1 Mutation (PLOD1 c.2032G>A) in Different Horse Breeds from Europe and the United States

Warmblood fragile foal syndrome (WFFS) is an autosomal recessive disorder caused by a single nucleotide variant in the procollagen-lysine-2-oxoglutarate-5-dioxygenase 1 gene (PLOD1:c.2032G>A, p.Gly678Arg). Homozygosity for the PLOD1 variant causes an Ehler-Danlos-like syndrome, which has to date only been reported in warmblood breeds but the WFFS allele has been also detected in the Thoroughbred. To investigate the breed distribution of the WFFS allele, 4081 horses belonging to 38 different breeds were screened. In total, 4.9% of the horses representing 21 breeds carried the WFFS allele. The affected breeds were mainly warmbloods, with carrier frequency as high as 17% in the Hanoverian and Danish Warmblood. The WFFS allele was not detected in most non-warmblood breeds. Exceptions include WFFS carriers in the Thoroughbred (17/716), Haflinger (2/48), American Sport Pony (1/12), and Knabstrupper (3/46). The origin of the WFFS allele remains unknown. The Arabian breed and specifically the stallion Bairactar Or. Ar. (1813), whose offspring were reported to have a similar phenotype in the 19th century, were hypothesized as the origin. DNA from a museum sample of Bairactar Or. Ar. showed that he did not carry the mutated allele. This result, together with the genotypes of 302 Arabians, all homozygous for the reference allele, does not support an Arabian origin of the WFFS allele. Our extensive survey shows the WFFS allele to be of moderate frequency and concern in warmbloods and also in breeds where it may not be expected.

Evaluation of the Structure of Myodural Bridges in an Equine Model of Ehlers-Danlos Syndromes

Myodural bridges have been described in various species as connective tissue structures “bridging” small cranio-cervical muscles to the dura. Myodural bridges are thought to stabilize the dural sac during head and neck movements and promote cerebrospinal fluid motion; however, their role in neurological diseases has not yet been established. We report ultrasonographic visualization, necropsy, histopathologic and ultrastructural findings of myodural bridges in horses with hereditary equine regional dermal asthenia (HERDA), an equine model of Ehlers-Danlos syndromes. Five HERDA and 5 control horses were studied. Post-mortem examination and ultrasonographic studies (3 HERDA and 4 controls) demonstrated that the atlanto-occipital and atlanto-axial myodural bridges are dynamic structures “moving” the dura. En block resection of the myodural bridges (4 HERDA and 5 controls) was accomplished and histopathology showed myofiber degeneration in 3 HERDA horses and 1 control. Ultrastructural examination revealed loosely packed collagen fibrils with abnormal orientation in all HERDA horses compared to mild abnormalities in 2 controls. Our study provides necropsy and ultrasonographic evidence of the dynamic aspect of the myodural bridges as dural sac stabilizers. Myodural bridges may be pathologically altered in connective tissue disease as evidenced by the ultrastructural morphology in the HERDA myodural bridge.

A 2-Year-Old Filly With Hereditary Equine Regional Dermal Asthenia: The First Case Report From China

Hereditary equine regional dermal asthenia (HERDA) is an autosomal recessive inheritable disorder described in the Quarter Horses and related breeds. In this case report, a 2-year-old Quarter Horse filly was diagnosed with HERDA based on clinical findings and genetic testing. The observed clinical signs were stretchy, loose and thin skin, and open wounds on the upper body. Skin biopsy results were consistent with the common findings previously described in the literature. This is the first HERDA case report in China (and in Asia). Genetic testing protocols should be implemented for breeding farms to prevent the disease.

Ocular Manifestations of Systemic Disease in the Horse

Many systemic diseases have ocular manifestations. In some cases, ocular abnormalities are the most obvious or first recognized sign of disease that prompts veterinary evaluation. In other cases, the systemic disease leads to secondary ocular changes that might lead to loss of vision or globe if not addressed. Therefore, recognition of ocular abnormalities that might result from systemic diseases is an essential skill for the equine practitioner. This article provides practitioners with information regarding the most common systemic diseases of horses in North America that have ocular manifestations, organized by ocular signs.

Skin malformations in a neonatal foal tested homozygous positive for Warmblood Fragile Foal Syndrome

Background

Skin malformations that resembled manifestations of Ehlers-Danlos-Syndrome were described in a variety of domestic animals during the last century as cutis hyperelastica, hyperelastosis cutis, dermatosparaxis, dermal/collagen dysplasia, dermal/cutaneous asthenia or Ehlers-Danlos-like syndrome/s. In 2007, the mutation responsible for Hereditary Equine Regional Dermal Asthenia (HERDA) in Quarter Horses was discovered. Several case reports are available for similar malformations in other breeds than Quarter Horses (Draught Horses, Arabians, and Thoroughbreds) including four case reports for Warmblood horses. Since 2013, a genetic test for the Warmblood Fragile Foal Syndrome Type 1 (WFFS), interrogating the causative point mutation in the equine procollagen-lysine, 2-oxoglutarate 5-dioxygenase 1 (PLOD1, or lysyl hydroxylase 1) gene, has become available. Only limited data are available on the occurrence rate and clinical characteristics of this newly detected genetic disease in horses. In humans mutations in this gene are associated with Ehlers-Danlos Syndrome Type VI (kyphoscoliotic form).

Case presentation

This is the first report describing the clinical and histopathological findings in a foal confirmed to be homozygous positive for WFFS. The Warmblood filly was born with very thin, friable skin, skin lesions on the legs and the head, and an open abdomen. These abnormalities required euthanasia just after delivery. Histologic examination revealed abnormally thin dermis, markedly reduced amounts of dermal collagen bundles, with loosely orientation and abnormally large spaces between deep dermal fibers.

Conclusion

WFFS is a novel genetic disease in horses and should be considered in cases of abortion, stillbirth, skin lesions and malformations of the skin in neonatal foals. Genetic testing of suspicious cases will contribute to evaluate the frequency of occurrence of clinical WFFS cases and its relevance for the horse population.

Ocular dimensions, corneal thickness, and corneal curvature in quarter horses with hereditary equine regional dermal asthenia

OBJECTIVES

The aim of this study was to compare ocular dimensions, corneal curvature, and corneal thickness between horses affected with hereditary equine regional dermal asthenia (HERDA) and unaffected horses.

ANIMALS

Five HERDA-affected quarter horses and five healthy control quarter horses were used.

METHODS

Schirmer's tear test, tonometry, and corneal diameter measurements were performed in both eyes of all horses prior to ophthalmologic examinations. Ultrasonic pachymetry was performed to measure the central, temporal, nasal, dorsal, and ventral corneal thicknesses in all horses. B-mode ultrasound scanning was performed on both eyes of each horse to determine the dimensions of the ocular structures and to calculate the corneal curvature.

RESULTS

Each corneal region examined in this study was thinner in the affected group compared with the healthy control group. However, significant differences in corneal thickness were only observed for the central and dorsal regions. HERDA-affected horses exhibited significant increases in corneal curvature and corneal diameter compared with unaffected animals. The ophthalmologic examinations revealed mild corneal opacity in one eye of one affected horse and in both eyes of three affected horses. No significant between-group differences were observed for Schirmer's tear test, intraocular pressure, or ocular dimensions.

CONCLUSIONS

Hereditary equine regional dermal asthenia-affected horses exhibit decreased corneal thickness in several regions of the cornea, increased corneal curvature, increased corneal diameter, and mild corneal opacity. Additional research is required to determine whether the increased corneal curvature significantly impacts the visual accuracy of horses with HERDA.

Dermatological and morphological findings in quarter horses with hereditary equine regional dermal asthenia

BACKGROUND

Hereditary equine regional dermal asthenia (HERDA) is an autosomal recessive disorder affecting quarter horses (QHs); affected horses exhibit characteristic skin abnormalities related to abnormal collagen biosynthesis.

HYPOTHESIS/OBJECTIVES

To characterize the thickness and morphological abnormalities of the skin of HERDA-affected horses and to determine the interobserver agreement and the diagnostic accuracy of histopathological examination of skin biopsies from horses with HERDA.

ANIMALS

Six affected QHs, confirmed by DNA testing, from a research herd and five unaffected QHs from a stud farm.

METHODS

The skin thickness in 25 distinct body regions was measured on both sides in all affected and unaffected horses. Histopathological and ultrastructural evaluation of skin biopsies was performed.

RESULTS

The average skin thickness in all of the evaluated regions was thinner in the affected horses. A statistically significant difference between skin thickness of the affected and unaffected animals was observed only when the average magnitude of difference was ≥38.7% (P = 0.038). The interobserver agreement for the histopathological evaluation was fair to substantial. The histopathological sensitivity for the diagnosis of HERDA was dependent on the evaluator and ranged from 73 to 88%, whereas the specificity was affected by the region sampled and ranged from 35 to 75%.

CONCLUSIONS AND CLINICAL IMPORTANCE

Despite the regional pattern of the cutaneous signs, skin with decreased thickness was not regionally distributed in the HERDA-affected horses. Histopathological evaluation is informative but not conclusive for establishing the diagnosis. Samples of skin from the neck, croup or back are useful for diagnosis of HERDA. However, the final diagnosis must be confirmed using molecular testing.

Manual corneal thickness measurements of healthy equine eyes using a portable spectral-domain optical coherence tomography device

REASONS FOR PERFORMING STUDY

Corneal thickness measurements of the equine globe using spectral-domain optical coherence tomography (SD-OCT) have not been reported.

OBJECTIVES

To determine corneal thickness measurements and the intra- and interoperator reliability of a portable SD-OCT device in equine eyes.

STUDY DESIGN

Prospective observational study.

METHODS

Horses free of ocular disease were used for this study. Gentle manual restraint, in combination with detomidine hydrochloride and a head stand, were employed to ensure proper animal positioning. Corneal pachymetry measurements were obtained from both eyes of each animal 3 times by 2 operators in succession. A 6 mm corneal pachymetry protocol was performed using a portable SD-OCT device. All measurements were obtained manually by one operator (C.G.P.) using the integrated calliper function. Measurements included epithelial thickness, stromal thickness, Descemet's membrane thickness and total corneal thickness. All recorded measurements were analysed to determine both intra- and interoperator reliability.

RESULTS

Thirty horses with a mean age of 10.6 ± 6.4 years were examined. Mean epithelial, stromal, Descemet's membrane and total corneal thickness values obtained were, respectively, 174.7 ± 13.6, 599.2 ± 45.4, 38.4 ± 15.3 and 812.0 ± 44.1 μm for operator A and 175.9 ± 12.9, 599.2 ± 44.9, 38.4 ± 15.0 and 812.9 ± 42.9 μm for operator B. A positive correlation was found between Descemet's membrane thickness and age, whereby Descemet's membrane thickness increased by 2 μm/year (P<0.0001). The coefficients of variation for both operators were <4% for all measurements. Intraclass correlations ranged from 0.92 to 0.98.

CONCLUSIONS

Manual corneal thickness measurement using a portable SD-OCT device provides epithelial, stromal, Descemet's membrane and total corneal thickness measurements with clinically acceptable intra- and interoperator reliability in healthy equine eyes.

Mutation in cyclophilin B that causes hyperelastosis cutis in American Quarter Horse does not affect peptidylprolyl cis-trans isomerase activity but shows altered cyclophilin B-protein interactions and affects collagen folding

The rate-limiting step of folding of the collagen triple helix is catalyzed by cyclophilin B (CypB). The G6R mutation in cyclophilin B found in the American Quarter Horse leads to autosomal recessive hyperelastosis cutis, also known as hereditary equine regional dermal asthenia. The mutant protein shows small structural changes in the region of the mutation at the side opposite the catalytic domain of CypB. The peptidylprolyl cis-trans isomerase activity of the mutant CypB is normal when analyzed in vitro. However, the biosynthesis of type I collagen in affected horse fibroblasts shows a delay in folding and secretion and a decrease in hydroxylysine and glucosyl-galactosyl hydroxylysine. This leads to changes in the structure of collagen fibrils in tendon, similar to those observed in P3H1 null mice. In contrast to cyclophilin B null mice, where little 3-hydroxylation was found in type I collagen, 3-hydroxylation of type I collagen in affected horses is normal. The mutation disrupts the interaction of cyclophilin B with the P-domain of calreticulin, with lysyl hydroxylase 1, and probably other proteins, such as the formation of the P3H1·CypB·cartilage-associated protein complex, resulting in less effective catalysis of the rate-limiting step in collagen folding in the rough endoplasmic reticulum.