Genetically Engineering a Sheep Model of Hypophosphatasia

Abstract

Hypophosphatasia (HPP) is a rare inherited disorder that affects the development of bones and teeth. The disease is caused by mutations in the tissue‐nonspecific alkaline phosphatase (TNSALP) gene (ALPL) and accompanied by a highly variable clinical presentation. Although HPP patient studies have advanced our understanding of HPP, as well as documented disease severity, the rarity of the disease combined with a lack of sufficient animal models has significantly delayed mechanistic understanding and therapeutic development. Similar to other human disease models, current HPP models have been engineered virtually exclusively in rodents – specifically mice harboring null or loss‐of‐function mutations. Although useful for modeling some features of HPP, these murine models do not faithfully represent the broad spectrum of human HPP clinical bone, muscle and tooth (odonto‐HPP) phenotypes. However, in sheep both Haversian bone remodeling and tooth development are analogous to humans with sheep TNSALP amino acid sequence sharing 89% identity with human. Thus, our objective was to generate a sheep model of HPP and we posit that ALPL mutation‐specific replacements in the sheep genome using CRISPR/Cas9 will produce a model that accurately phenocopies the bone and tooth pathophysiology of HPP. To genetically engineer a sheep model of HPP, a total of 52 in vitro genetically manipulated embryos were generated targeting the Exon 10 c.1077G>A mutation using CRISPR/Cas9. Three embryos were implanted per recipient ewe and implantations were performed in a total of 17 recipient ewes. Nine ewe pregnancies were confirmed by the measurement of Pregnancy‐Associated Protein on post‐implantation day 35 and further validated by ultrasound. From 9 live newborn lambs, four were heterozygous for our specific point mutation (4/6), one compound heterozygous (1/6) and one homozygous (1/6) for a total mutation efficiency rate of 66.6% (6/9). Similar to HPP patients with the same mutation, mutant sheep have variable clinical expression accompanied with decreased bone formation and mineralization in tails observed by DXA as well as significantly and appropriately decreased serum alkaline phosphatase activity. Additionally, an apparent “shell tooth” phenotype is observed in mutant lambs, further suggesting a mutant exon 10 genotype/phenotype that more completely recapitulates human skeletal and odonto‐HPP.Support or Funding Information1. William Townsend Porter Pre‐doctoral Fellowship2. TAMU/Texas Heart Institute Center for Cell and Organ Biotechnology Innovation Kitchen GrantThis abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.