Victoria Ryder and Dr. Robert Seegmiller, Zoology

Mice heterozygous for the semi-dominant mutation disproportionate micromelia (Dmm) express mild chondrodysplasia by three weeks postnatally. This phenotype is attributed to a deletion of three contiguous nucleotides within the Col2A1 gene on mouse chromosome 15. The phenotype of Dmm/+ mice resembles that of human Stickler syndrome, which can be caused by mutations in the human COL2A1 gene. Because Stickler patients develop premature osteoarthritis in addition to the mild chondrodysplasia , we suspected that Dmm/+ mice develop premature osteoarthritis as well. The demonstration of a mouse model of osteoarthritis caused by a type II collagen gene mutation provides an opportunity to study the progression of osteoarthritic cartilage degeneration at the cellular and molecular levels in addition to providing a model in which potential therapies could be tested. The goal in the present project, therefore, was to determine whether Dmm/+ mice do, in fact, exhibit premature degeneration of the articular cartilage and can thus serve as a useful animal model for osteoarthritis.

In the present study, wild-type (+/+) and heterozygous (Dmm/+) mice were compared to determine whether heterozygotes exhibit a predisposition for early-onset osteoarthritis. A histological analysis of the articular cartilage was performed on 6-month-old mice. Excised knee joints were fixed in 4% paraformaldehyde, decalcified in 10% EDTA, embedded in paraffin, and sectioned at 10 μ. Serial frontal sections were stained with safranin-o and fast green, counter stained with hematoxylin, and examined under the light microscope. An ocular micrometer grid was used to measure the regional (superficial, transitional, and deep zones) and overall thickness of the articular cartilage, and to obtain information regarding the degree of cellularity (number of cells per unit area) within each region.

Relative to the control, the mean overall thickness of the articular cartilage in the mutant was decreased by approximately 30%. In contrast, the overall cellularity was increased by 50% in most tissue sections. The increased cellularity was most evident in the transitional and deep zones, where extracellular matrix normally accounts for the majority of the tissue volume. The marked increase in the number of cells per unit area within these regions suggests a mitotic response to the cartilage degradation.

These observations suggest that degenerative changes in the articular cartilage occur in the Dmm heterozygote as early as six months, supporting the hypothesis that these mice can serve as a useful animal model of osteoarthritis. These mice should prove beneficial because understanding the mechanism(s) by which mutations in the cartilage collagen genes lead to osteoarthritis will facilitate the development of drug and gene therapies for this chronically disabling skeletal disease.