The Harry M. Zweig Memorial Fund for Equine Research

Genomic Profiling of Osteochondritis Dissecans Using an Equine Whole Transcript Exon Array

Dr. Alan J. Nixon

NixonOsteochondritis dissecans (OCD) remains a prominent and debilitating musculoskeletal syndrome among growing horses. In concert with other forms of developmental orthopedic disease (DOD), including physitis, angular limb deformity, cuboidal bone collapse, and wobbler syndrome, this complex continues to affect 35 to 40% of the annual weanling and yearling crop. For individual animals, the impact has largely been through joint and growth plate pain, deformity and mechanical dysfunction, and subsequent osteoarthritis.

The economic costs of lameness and cosmetic disfigurement, add to the morbidity associated with OCD, spinal ataxia, and angular limb diseases. Further, osteochondrosis (OC) seems to affect the more rapidly growing animals, making this group of related diseases a frustrating syndrome for owners and farm managers. Despite adoption of environmental controls such as trace mineral supplementation to mares and weanlings, overall growth rate restrictions, and consistent exercise regimens, the incidence of osteochondrosis (OC) and OCD remains unacceptably high. The predominant sites for occurrence vary between horsebreeds, but epiphysitis(physitis) and OCD are particularly common in Thoroughbreds, Standardbreds, Warmbloods, Arabians and Quarter Horses. In addition to the pain and debility of physitis and joint disease, there are serious mechanical consequences to OCD induced joint surface disruption. Fetlock and shoulder OCD frequently degenerate to osteoarthritis with permanent lameness. Other common OCD sites such as the hock and stifle can also be debilitating, and frequently require surgery or intraarticular medication for adequate resolution. Better longer-term control will only come through an understanding of the cause and cascade of events in OCD formation.

We consider OCD and other forms of DOD remain as unresolved key targets in equine health research. Defining the cause of OC at a molecular and genetic level is fundamental to reducing the incidence and consequences of DOD.

Despite corrections to pasture and water quality for mares, and improvements in the composition and volume of supplements for weanlings, DOD and particularly OCD remain at a high incidence plateau. Our pilot studies using global gene scans of cartilage specimens from OCD and normal animals identified several candidate causative genes which were subsequently analyzed in more detail. These studies determined that OCD cartilage has a prominent footprint of gene expression derangements, spanning from increases in growth factors and bioactive peptides important in the control of growth plate development and maturation, to changes in gene modulators and proteins controlling the cell cycle and cell developmental states. The prioritization and interaction of these developmentally important proteins in OCD remains essentially uncharted. This largely stems from the lack of global gene array tools. Although the equine genome project completed the horse gene sequence detection earlier in 2007, it is not clear that a full genome equine gene chip will ever be commercialized due to market limitations. Given these constraints our group developed a custom gene chip capable of interrogating the equine genome.

We hypothesize that OCD is the result of aberrations in the signaling processes of key regulatory proteins controlling cartilage maturation and formation into bone.

Objectives of this study include a complete gene expression profiling in OCD tissues using the Cornell equine gene chip, developed by our lab for this project, and providing interrogation of the full equine genome. Commercial versions will not be available for several years, if at all. This proposal expands previous work on 4 animals with early OCD lesions. Microarray techniques will be used for global gene expression mapping, and selected microarray and targeted gene data will be verified with precise gene quantitative assays, in situ definition of gene expression dysfunction on actual microscopic tissue sections, and laser microdissection to punch out microscopic islands of specific regions of the OCD flap to establish gene profiles of discrete zones within the diseased cartilage. Functional verification of the role of key molecules in OCD will use gene overexpression and gene knock down techniques to study the impact of differential gene regulation on cartilage matrix structure and competency. In so doing we will characterize whether the suspect genes actually perturb cartilage formation in the way we see in the natural disease.