Epitheliogenesis Imperfecta (EI) is a mechanobullous disease that occurs in newborn American Saddlebred and Belgian Draft foals. Necropsy evaluations of two American Saddlebred foals revealed broad skin lesions, dental abnormalities and oral mucosa lesions. Construction of a partial pedigree showing occurrences of EI in American Saddlebred horses was consistent with a recessive pattern of inheritance. An allelic frequency of 0.04 was estimated for the EI gene. The pathological signs of EI were similar to a disease in humans known as Herlitz Junctional Epidermolysis Bullosa (HJEB). HJEB is caused by a defect in one of the three subunits of the laminin 5 protein (LAM 3, LAM 3 and LAM 2), which leads to a separation of the epidermis from the dermis. Transmission electron microscopy revealed a separation within the lamina lucida at the sites of epidermal/dermal splits in the skin of EI affected foals. This indicated that a defect in the laminin 5 protein was responsible for EI. Linkage disequilibrium (LD) between microsatellite markers and the EI disease locus was tested for in the American Saddlebred and Belgian Draft breeds. Genotyping of microsatellite alleles was used to determine fit to Hardy-Weinberg equilibrium for control and EI populations for both breeds using Chi square analysis. Two microsatellite loci (ASB14 and AHT3) were not in Hardy-Weinberg equilibrium in EI affected American Saddlebred horses. This suggested that the EI disease locus was located on ECA 8, the putative location of LAM 3. No evidence of LD between any of the tested microsatellite loci and the EI locus was observed in the Belgian Draft samples. A cDNA library was built from Thoroughbred horse skin to serve as a resource for sequencing equine skin gene transcripts. 313 ESTs were sequenced, of which 207 were putatively identified (66%) by database search. Examination of the pathology and ultrastructure of EI affected foals and comparison with HJEB indicated that laminin 5 was the responsible defective protein. The LD analysis suggested that LAM 3 was the EI disease locus in American Saddlebred horses.

Observed and expected heterozygosity (H) levels for ten electrophoretic blood marker loci and expected H for seven red blood cell (RBC) anitgen/antibody loci were examined for 20 years in American Standardbred and 30 years in American Saddlebred horses. Standardbreds were classed by gait, Trotter and Pacer, and evaluated separately in most analyses. 4,404 Trotters and 12,271 Pacers were found to have statistically highly significant losses of mean total observed H through time for the ten electrophoretic loci (P<0.005), although in Trotters the loss was more extreme (P<<0.001). Loss of H in 5984 Saddlebreds was not significant (P=0.259). Correlations of RBC expected H through time showed decreases in all three groups.