Canine Chondrodysplasia, test for Karelian Bear Dogs and Norwegian Elkhounds Grey

Chondrodysplasia is a genetic disorder of cartilage and bone development. It can occur in dogs as well as in humans. In general, there are three types of skeletal dysplasias that differ in their symptoms - osteodysplasia, chondrodysplasia and dysostosis. Osteodysplasia and chondrodysplasia affect the bone and cartilage tissue whereas dysostosis affects an individual bone or group of bones.  Osteodysplasia can be characterized not only by altered bone mineral density, but also by chondrodysplasia leading to abnormal endochondral ossification. This abnormal ossification inside the cartilage affects the linear growth of bones and results in disproportionate short stature of the affected dog.

During the endochondral ossification of long bones, progressive changes take place both on the surface and inside the cartilage tissue that result in formation of the individual cartilaginous growth plates that are subsequently replaced with bone tissue. These plates are at the end of the long bones and are responsible for the longitudinal growth of bones. They consist of ECM (extracellular matrix) and linear columns of differentiating chondrocytes.  The chondrocytes are organised into zones. In the proliferation zone, the mature cells are replaced with trabecular bone at the end of the growth period.

Today, the chondrodysplasia has been identified in two canine breeds - Norwegian Elkhound and Karelian Bear Dog.  It is probable that this disease was introduced to Karelian Bear Dog from Norwegian Bear Dog. The affected dogs show shortened limbs compared to healthy dogs. Further, it may occur skeletal abnormalities such as substantially shorter protruding toes on the outer side of the front paws, bowed forelimbs, shortened vertrebral bodies und substantially delayed ossification of carpal bones. The growth plate histology revealed unusual wide bars of ECM, disorganized columnar structure and atypical large chondrocytes with altered morphology.

The chondrodysplasia is caused by genes that codes for cartilage ECM proteins such as cartilage oligomeric matrix protein (COMP), several different collagens and proteoglycans  aggrecan and perlecan. Specifically, there occurs mutation c.2083C>T in exon 16 of gene that codes for integrin subunit alpha (α10β1). This subunit is responsible for correct proliferation of chondrocytes and accumulation of collagen fibrils around the ECM. The role of this α10β1subunit is fundamental for the correct process of endochondral ossification.

Chondrodysplasia is a recessively inherited disease. The disease develops in dogs which inherit the mutated gene from each parent.  These dogs are designated as P/P (positive/positive). The carriers of the mutated gene are designated as N/P (negative/positive). The carriers inherited the mutated gene from one parent only and are without clinical signs. However, they pass the disease on to their offspring.  When mating two heterozygotes (N/P), there will be theoretically 25% of the offspring healthy, 50% of the offspring will be carriers and 25% of the offspring will inherit the mutated gene from both parents and will be affected by Chondrodysplasia.  Mating one healthy dog (N/N) with a carrier of this mutation (N/P) will theoretically produce 50% carriers and 50% healthy offspring.  If a carrier (N/P) is mated with an affected dog (P/P), there will be theoretically 50% affected dogs and 50% carriers.

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References:

Kyöstilä, K., Lappalainen, A. K., & Lohi, H. (2013). Canine chondrodysplasia caused by a truncating mutation in collagen-binding integrin alpha subunit 10. PloS one, 8(9), e75621.

Kyöstilä, K. (2015). Identification of Novel Mutations and Molecular Pathways for Canine Neurodegeneration and Chondrodysplasia. Dissertationes Scholae Doctoralis Ad Sanitatem Investigandam Universitatis Helsinkiensis.