The progressive ankylosis allele (ank) is a spontaneous recessive mutation that causes a severe phenotype of joint calcification and degeneration. Few homozygotes survive beyond 5 months of age, although the adults can breed. Generally no more than 2 litters are produced by homozygotes of either sex. At 4 or 5 weeks of age, homozygotes can be identified by their inability to grasp a wire cage lid with their front paws when suspended above it. The joints swell with a milky fluid, and undergo a process proceeding through mononuclear inflammatory infiltration, hydroxyapatite deposition and increased calcification, hyperplasia, and fibrous and bony ankylosis. (for details see Mahowald et al., 1989; Hakim et al., 1984; Sweet and Green, 1981.) The front feet are affected before the hind feet and the phenotype is more severe in the most distal joints of the limbs. Hyperplasia and degeneration of the joint tissues, and ankylosis occur progressively, decreasing mobility and resulting in a rigid, crouched posture in the adult. Their gate becomes slow, halting, and flatfooted. The morphological changes underlying the stiffening of the vertebral column and resulting thoracic kyphosis are detailed by Sampson, 1988a and 1988b, and Sampson et al., 1991.
There is an immune cell involvement in the joints secondary to the central defect. Treatment of ank/ank mice with hydrocortisone beginning at 5 weeks of age resulted in reduced synovial and subsynovial hyperplasia, reduced growth of cartilaginous and bony bridges, and increased accumulation of intra-articular apatite possibly due to the reduction in macrophage activity (Hakim et al., 1986.). Reconstitution of wild type mice with bone marrow or spleen cells from ank/ank mice does not transfer disease and reconstitution of ank/ank mice with wild type bone marrow or spleen cells does not prevent disease (Krug et al., 1997). Additionally, there is an immune cell defect. There is a reduction in the response of spleen cells to phytohaemagglutinin or concanavalin A although the response to LPS appears normal. Spleen cells or macrophages from ank/ank mice do not suppress normal spleen cell responses to phytohaemagglutinin and wild type spleen cells or macrophages do not restore phytohaemagglutinin responsiveness to spleen cells from ank/ank mice (Krug et al., 1989). Fibroblasts from ank/ank mice are Hyperproliferative in response to transforming growth factor beta 1 (Krug, 1998). No impact was found on the phenotype of ank/+ or ank/ank mice when transgenic HLA-B27 was co-expressed (Krug and Taurog, 2000).In addition to progressive ankylosis and calcification of the peripheral joints and axial skeleton, homozygotes have a smaller overall body size, and some develop balanitis, priapism, and scaling skin lesions on the plantar surface of the paws. The hypermineralization phenotype is associated with abnormal inorganic pyrophophate levels and the downregulation of osteopontin (Harmey et al., 2004).

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