The biomechanical and molecular characteristics of equine hyperelastosis cutis (HC) are not fully known. This study sought to better characterize HC by analysis of ultimate tensile strength, modulus of elasticity, toughness, and thickness of skin from 23 affected and unaffected horses. In addition total soluble collagen and glycosaminoglycan concentrations of skin were analyzed from 26 affected and unaffected horses. Affected horses' skin proved to be significantly weaker at five of seven sample locations (p<=0.05). The modulus of elasticity proved to be significantly different at three of seven sample locations and toughness at two of seven locations (p<=0.05). No significant difference was proven to exist between HC affected and unaffected horses for skin thickness or total soluble collagen and GAG concentrations. Collectively this data demonstrates that HC animals' reduced skin tensile strength is not due to a deficit of either collagen or GAG, but likely a result of altered collagen micro-architecture.

Equine hyperelastosis cutis (HC or HERDA), a connective tissue disorder in American Quarter Horses, results in hyperelastic skin with poor wound healing. Similar conditions are found in many species and all forms display decreased skin tensile strength. Fibroblasts produce collagen and elastin fibers, forming networks, providing the dermis with strength, and elasticity. This study aims to carry out a 3-part evaluation between horse skin fibroblast (cells from horses affected with HERDA, cells from horses that are carriers of HERDA (recessive HERDA gene), and cells from horses that are normal (neither affected or carriers of HERDA); Studies include: 1. Cell proliferation assay 2. Apoptosis analysis of fibroblasts 3. Mechanobiology of stretched fibroblast. Studies have shown cellular deformation to have an overall effect on mechanical properties of healthy and unhealthy tissues. This investigation provides a micromechanical evaluation of HC/HERDA in an effort to quantify the cellular level differences between each condition.

Hereditary Equine Regional Derma Asthenia (HERDA) is a painful disfiguring autosomal recessive skin disorder of Quarter Horse lineages. Affected horses cannot be ridden and most are humanely destroyed. Five years following homozygosity mapping of a putative causal mutation responsible for HERDA, it remains unclear how this mutation causes the HERDA syndrome. HERDA horses have a missense mutation in peptidyl-prolyl cis-trans isomerase B (PPIB) which encodes cyclophilin B (CYPB) and alters folding and post-translational modifications of fibrillar collagen. Loss of function mutations in CYPB recognized in other species classically present as the debilitating bone disease, severe to lethal osteogenesis imperfect (OI). Objectives of this study were to develop a novel method for cryogenic clamping of tendons and ligaments of high tensile strength and validate its performance by ultimate tensile strength testing of normal equine deep digital flexor tendon. This validated method was then used to compare tendon and ligament of HERDA vs. control horses along with great vessels and skin. We hypothesized that all tissues of high fibrillar collagen content would have altered tensile properties due to the CYPB mutation affecting fibrous connective tissue globally within HERDA horses. Based on previous studies in our laboratory identifying reduced hydroxylysine content and altered collagen crosslink ratios in the skin of HERDA affected animals that implicate lysyl hydroxylase-1 (LH1) dysfunction, we hypothesized that the HERDA PPIB mutation modified an interaction between CYPB and LH1, interfering with hydroxylysine synthesis and its availability for collagen crosslink formation. In addition, we hypothesized that mutant CYPB may also lead to modifications of other known CYPB protein complexes, such as the CYPB, prolyl-3 hydroxylase-1 (P3H1) and cartilage associated protein (CRTAP) triplex. Goals of this study were to investigate the tensile properties of tissues with high fibrillar collagen content from HERDA homozygotes, to elucidate the mechanistic relationship of the HERDA CYPB mutation to the clinical disease, and to provide evidence to substantiate a heterozygote phenotype in HERDA which could be useful to explaining the correlation between lineages that carry the HERDA allele and performance outcomes in the discipline of western cutting competition.