Frieda Chen Ph.D., Ruolin Guo Ph.D., Shousaku Itoh D.D.S., Ph.D., Luisa Moreno Ph.D., Esther Rosenthal, Tanya Zappitelli, Ralph A. Zirngibl Ph.D., Ann Flenniken Ph.D., William Cole M.D., Marc Grynpas Ph.D., Lucy R. Osborne Ph.D., Wolfgang Vogel Ph.D., Lee Adamson Ph.D., Janet Rossant Ph.D., Jane E. Aubin Ph.D.
Using a genome-wide N-ethyl-N-nitrosourea (ENU)-induced dominant mutagenesis screen in mice, we identified a founder with low bone mineral density (BMD). Mapping and sequencing revealed a T to C transition in a splice donor of the collagen alpha1 type I (Col1a1) gene resulting in the skipping of exon 9 and a predicted 18 amino acid deletion within the N-terminal region of the triple helical domain of Col1a1. Col1a1Jrt/+ mice were smaller in size, had lower BMD associated with decreased bone volume/tissue volume (BV/TV) and reduced trabecular number and furthermore exhibited mechanically weak and brittle, fracture-prone bones, a hallmark of Osteogenesis Imperfecta (OI). Several markers of osteoblast differentiation were upregulated in mutant bone and histomorphometry showed that the proportion of trabecular bone surfaces covered by activated osteoblasts (Ob.S/BS and N.Ob/BS) was elevated but bone surfaces undergoing resorption (Oc.S/BS and N.Oc/BS) were not. The number of bone marrow stromal osteoprogenitors (CFU-ALP) was unaffected, but mineralization was decreased in cultures from young Col1a1Jrt/+ versus +/+ mice. Total collagen and type I collagen content of matrices deposited by Col1a1Jrt/+ dermal fibroblasts in culture was ∼40% and 30% respectively that of +/+ cells, suggesting that mutant collagen chains exerted a dominant negative effect on type I collagen biosynthesis. Mutant collagen fibrils were also markedly smaller in diameter than +/+ fibrils in bone, tendon and extracellular matrices deposited by dermal fibroblasts in vitro. Col1a1Jrt/+ mice also exhibited traits associated with Ehlers-Danlos syndrome (EDS): their skin had reduced tensile properties, tail tendon appeared more frayed and a third of the young adult mice had noticeable curvature of the spine. Col1a1Jrt/+ is the first reported model of combined OI/EDS and will be useful for exploring aspects of OI and EDS pathophysiology and treatment.