A genome-wide scan for tying-up syndrome in Japanese Thoroughbreds

Characterisation of phenotypic patterns in equine exercise-associated myopathies

BACKGROUND

Equine exercise-associated myopathies are prevalent, clinically heterogeneous, generally idiopathic disorders characterised by episodes of myofibre damage that occur in association with exercise. Episodes are intermittent and vary within and between affected horses and across breeds. The aetiopathogenesis is often unclear; there might be multiple causes. Poor phenotypic characterisation hinders genetic and other disease analyses.

OBJECTIVES

The aim of this study was to characterise phenotypic patterns across exercise-associated myopathies in horses.

STUDY DESIGN

Historical cross-sectional study, with subsequent masked case-control validation study.

METHODS

Historical clinical and histological features from muscle samples (n = 109) were used for k-means clustering and validated using principal components analysis and hierarchical clustering. For further validation, a blinded histological study (69 horses) was conducted comparing two phenotypic groups with selected controls and horses with histopathological features characterised by myofibrillar disruption.

RESULTS

We identified two distinct broad phenotypes: a non-classic exercise-associated myopathy syndrome (EAMS) subtype was associated with practitioner-described signs of apparent muscle pain (p < 0.001), reluctance to move (10.85, p = 0.001), abnormal gait (p < 0.001), ataxia (p = 0.001) and paresis (p = 0.001); while a non-specific classic RER subtype was not uniquely associated with any particular variables. No histological differences were identified between subtypes in the validation study, and no identifying histopathological features for other equine myopathies identified in either subtype.

MAIN LIMITATIONS

Lack of an independent validation population; small sample size of smaller identified subtypes; lack of positive control myofibrillar myopathy cases; case descriptions derived from multiple independent and unblinded practitioners.

CONCLUSIONS

This is the first study using computational clustering methods to identify phenotypic patterns in equine exercise-associated myopathies, and suggests that differences in patterns of presenting clinical signs support multiple disease subtypes, with EAMS a novel subtype not previously described. Routine muscle histopathology was not helpful in sub-categorising the phenotypes in our population.

Pathways of calcium regulation, electron transport, and mitochondrial protein translation are molecular signatures of susceptibility to recurrent exertional rhabdomyolysis in Thoroughbred racehorses

Recurrent exertional rhabdomyolysis (RER) is a chronic muscle disorder of unknown etiology in racehorses. A potential role of intramuscular calcium (Ca2+) dysregulation in RER has led to the use of dantrolene to prevent episodes of rhabdomyolysis. We examined differentially expressed proteins (DEP) and gene transcripts (DEG) in gluteal muscle of Thoroughbred race-trained mares after exercise among three groups of 5 horses each; 1) horses susceptible to, but not currently experiencing rhabdomyolysis, 2) healthy horses with no history of RER (control), 3) RER-susceptible horses treated with dantrolene pre-exercise (RER-D). Tandem mass tag LC/MS/MS quantitative proteomics and RNA-seq analysis (FDR <0.05) was followed by gene ontology (GO) and semantic similarity of enrichment terms. Of the 375 proteins expressed, 125 were DEP in RER-susceptible versus control, with 52 ↑DEP mainly involving Ca2+ regulation (N = 11) (e.g. RYR1, calmodulin, calsequestrin, calpain), protein degradation (N = 6), antioxidants (N = 4), plasma membranes (N = 3), glyco(geno)lysis (N = 3) and 21 DEP being blood-borne. ↓DEP (N = 73) were largely mitochondrial (N = 45) impacting the electron transport system (28), enzymes (6), heat shock proteins (4), and contractile proteins (12) including Ca2+ binding proteins. There were 812 DEG in RER-susceptible versus control involving the electron transfer system, the mitochondrial transcription/translational response and notably the pro-apoptotic Ca2+-activated mitochondrial membrane transition pore (SLC25A27, BAX, ATP5 subunits). Upregulated mitochondrial DEG frequently had downregulation of their encoded DEP with semantic similarities highlighting signaling mechanisms regulating mitochondrial protein translation. RER-susceptible horses treated with dantrolene, which slows sarcoplasmic reticulum Ca2+ release, showed no DEG compared to control horses. We conclude that RER-susceptibility is associated with alterations in proteins, genes and pathways impacting myoplasmic Ca2+ regulation, the mitochondrion and protein degradation with opposing effects on mitochondrial transcriptional/translational responses and mitochondrial protein content. RER could potentially arise from excessive sarcoplasmic reticulum Ca2+ release and subsequent mitochondrial buffering of excessive myoplasmic Ca2+.

Coding sequences of sarcoplasmic reticulum calcium ATPase regulatory peptides and expression of calcium regulatory genes in recurrent exertional rhabdomyolysis

Background

Sarcolipin (SLN), myoregulin (MRLN), and dwarf open reading frame (DWORF) are transmembrane regulators of the sarcoplasmic reticulum calcium transporting ATPase (SERCA) that we hypothesized played a role in recurrent exertional rhabdomyolysis (RER).

Objectives

Compare coding sequences of SLN, MRLN, DWORF across species and between RER and control horses. Compare expression of muscle Ca²⁺ regulatory genes between RER and control horses.

Animals

Twenty Thoroughbreds (TB), 5 Standardbreds (STD), 6 Quarter Horses (QH) with RER and 39 breed‐matched controls.

Methods

Sanger sequencing of SERCA regulatory genes with comparison of amino acid (AA) sequences among control, RER horses, human, mouse, and rabbit reference genomes. In RER and control gluteal muscle, quantitative real‐time polymerase chain reaction of SERCA regulatory peptides, the calcium release channel (RYR1), and its accessory proteins calsequestrin (CASQ1), and calstabin (FKBP1A).

Results

The SLN gene was the highest expressed horse SERCA regulatory gene with a uniquely truncated AA sequence (29 versus 31) versus other species. Coding sequences of SLN, MRLN, and DWORF were identical in RER and control horses. A sex‐by‐phenotype effect occurred with lower CASQ1 expression in RER males versus control males (P < .001) and RER females (P = .05) and higher FKBP1A (P = .01) expression in RER males versus control males.

Conclusions and Clinical Importance

The SLN gene encodes a uniquely truncated peptide in the horse versus other species. Variants in the coding sequence of SLN, MLRN, or DWORF were not associated with RER. Males with RER have differential gene expression that could reflect adaptations to stabilize RYR1.

Current status of myopathies affecting athletic horses

Muscular disorders rank among the most prevalent problems of horses competing in a broad variety of athletic disciplines, including track racing, dressage, endurance racing and Western riding disciplines. As described in this review, active scientific investigation is continuing to elucidate the different mechanisms underlying specific muscular disorders in horses, and is discovering and defining new disorders, and new methods of diagnosis, treatment and management. The flourishing field of equine rehabilitation and regenerative medicine is also driving the progressive application of a variety of modalities to the treatment and management of musculoskeletal conditions in horses. However, it is essential that this be accompanied by appropriate scientific investigation to verify the efficacy of recommended modalities and treatment protocols.

Heritability of Recurrent Exertional Rhabdomyolysis in Standardbred and Thoroughbred Racehorses Derived From SNP Genotyping Data

Recurrent exertional rhabdomyolysis (RER) in Thoroughbred and Standardbred racehorses is characterized by episodes of muscle rigidity and cell damage that often recur upon strenuous exercise. The objective was to evaluate the importance of genetic factors in RER by obtaining an unbiased estimate of heritability in cohorts of unrelated Thoroughbred and Standardbred racehorses. Four hundred ninety-one Thoroughbred and 196 Standardbred racehorses were genotyped with the 54K or 74K SNP genotyping arrays. Heritability was calculated from genome-wide SNP data with a mixed linear and Bayesian model, utilizing the standard genetic relationship matrix (GRM). Both the mixed linear and Bayesian models estimated heritability of RER in Thoroughbreds to be approximately 0.34 and in Standardbred racehorses to be approximately 0.45 after adjusting for disease prevalence and sex. To account for potential differences in the genetic architecture of the underlying causal variants, heritability estimates were adjusted based on linkage disequilibrium weighted kinship matrix, minor allele frequency and variant effect size, yielding heritability estimates that ranged between 0.41-0.46 (Thoroughbreds) and 0.39-0.49 (Standardbreds). In conclusion, between 34-46% and 39-49% of the variance in RER susceptibility in Thoroughbred and Standardbred racehorses, respectively, can be explained by the SNPs present on these 2 genotyping arrays, indicating that RER is moderately heritable. These data provide further rationale for the investigation of genetic mutations associated with RER susceptibility.

The genetics of skeletal muscle disorders in horses

Horses are remarkable athletes and a fascinating species in which to study the genetic bases of athletic performance, skeletal muscle biology, and neuromuscular disease. Genetic selection in horses has resulted in many breeds that possess anatomical, physiological, and metabolic variations linked to speed, power, and endurance that are beginning to be defined at the molecular level. Along with the concentration of positive traits, equine breeding programs have also inadvertently concentrated heritable muscle diseases for which mutations impacting electrical conduction, muscle contraction, and energy metabolism within and across breeds have been characterized. The study of heritable muscle diseases in horses has provided exciting insights into the normal structure and function of muscle and important diagnostic tools for veterinarians. Results empower breeders and breed associations to make difficult decisions about how to use this information to improve the overall health and well-being of horses.

Genetic mapping of recurrent exertional rhabdomyolysis in a population of North American Thoroughbreds

Recurrent exertional rhabdomyolysis is a heritable disorder that results in painful skeletal muscle cramping with exercise in up to 10% of all Thoroughbred racehorses. Here, we report a genome-wide association study with 48 282 SNPs analyzed among 48 case and 37 control Thoroughbreds. The most significant SNPs spanned approximately 13 Mb on ECA16, and the P-value of the most significant SNP after correcting for population structure was 8.0 × 10(-6) . This region on ECA16 was further evaluated by genotyping 247 SNPs in both the initial population and a second population of 34 case and 98 control Thoroughbreds. Several SNPs across the 13-Mb region on ECA16 showed significance when each population was analyzed separately; however, the exact positions of the most significant SNPs within this region on ECA16 varied between populations. This variability in location may be attributed to lack of power owing to insufficient sample sizes within each population individually, or to the relative distribution of long, conserved haplotypes, characteristic of the Thoroughbred breed. Future genome-wide association studies with additional horses would likely improve the power to resolve casual loci located on ECA16 and increase the likelihood of detecting any additional loci on other chromosomes contributing to disease susceptibility.