A missense mutation in the mouse Col2a1 gene causes spondyloepiphyseal dysplasia congenita, hearing loss, and retinoschisis

Abstract

A missense mutation in the mouse Col2a1 gene has been discovered, resulting in a mouse phenotype with similarities to human spondyloepiphyseal dysplasia (SED) congenita. In addition, SED patients have been identified with a similar molecular mutation in human COL2A1. This mouse model offers a useful tool for molecular and biological studies of bone development and pathology.

Introduction: A new mouse autosomal recessive mutation has been discovered and named spondyloepiphyseal dysplasia congenita (gene symbol sedc).

Materials and methods: Homozygous sedc mice can be identified at birth by their small size and shortened trunk. Adults have shortened noses, dysplastic vertebrae, femora, and tibias, plus retinoschisis and hearing loss. The mutation was mapped to Chr15, and Col2a1 was identified as a candidate gene.

Results: Sequence analyses revealed that the affected gene is Col2a1, which has a missense mutation at exon 48 causing an amino acid change of arginine to cysteine at position 1417. Two human patients with spondyloepiphyseal dysplasia (SED) congenita have been reported with the same amino acid substitution at position 789 in the human COL2A1 gene.

Conclusions: Thus, sedc/sedc mice provide a valuable model of human SED congenita with molecular and phenotypic homology. Further biochemical analyses, molecular modeling, and cell culture studies using sedc/sedc mice could provide insight into mechanisms of skeletal development dependent on Col2a1 and its role in fibril formation and cartilage template organization.

FIG. 1

Comparison of the nucleotide sequences in exon 48 of the Col2a1 gene around position 4249, where a single base substitution from C to T is shown for the sedc allele. This novel mutation changes codon 1417 CGC to TGC and results in an amino acid change of Arg1417 to Cys in the Col2a protein in sedc/sedc mice. A BstF5I restriction enzyme site GGATG in the sedc-Col2a1 sequence is also indicated (*).

FIG. 2

Comparison of the thoracic and lumbar vertebrae of 16-week-old heterozygous (sedc/+) and homozygous (sedc/sedc) mice (magnification ×5). Vertebral bodies are shorter and wider in the sedc/sedc spine throughout both thoracic and lumbar regions, with increased mineralization.

FIG. 3

Bone parameters in sedc/sedc mice.1, vertebral height; 2, upper vertebral width; 3, middle vertebral width; 4, lower vertebral width; 5, transverse processes width; 6, femoral length; 7, femoral condylar width; 8, femoral midshaft width; 9, tibial length; 10, tibial condylar width; 11, tibial midshaft width; and 12, tibial malleolar width.

FIG. 4

(A–D) Histological sections stained with hematoxylin and eosin (H&E) of lumbar vertebrae from sedc/sedc and normal mice (magnification ×45). (A) Longitudinal section of spine and spinal cord from a 16-week-old normal mouse. Note the well-defined intervertebral disk (ID) in the center and the relatively straight growth plates (GP) to either side of it. The lumbar spinal cord (SC) is at the top of the section. (B) Comparable section from a 16-week-old sedc/sedc mouse. The entire spine is two to three times thicker than the control spine (A). The ID is enlarged and blends with the GPs on either side. The plates are irregularly thickened and project ventrally and dorsally. (C) Cross-section from the spinal cord and spine from a 4-month-old normal mouse. Note the vertebral body (VB) right of the lumbar spinal cord (SC). (D) Cross-section from the spinal cord and spine from a 1-year-old sedc/sedc mouse. The vertebral body (VB) is three times larger in diameter than in the normal mouse. Both mature bone and proliferative cartilage contribute to the enlargement. Note that the proliferative cartilage, which is also undergoing degeneration, projects into and deforms the spinal cord (SC).

FIG. 5

(A–C) Histological sections stained with H&E of distal femur and knee from sedc/sedc and normal mice (magnification, ×35). (A) Longitudinal section of the distal femur, knee joint, and patella from a 4-month-old control. The growth plate (GP) of the distal femur has a normally curved configuration, and its sides are parallel. The articular cartilage (AC) of the femur and patella are of uniform thickness. (B) Longitudinal section of distal femur and knee from a 4-month-old sedc/sedc mouse. The growth plate (GP) of the distal femur is two to three times thicker than the control plate and is irregularly thickened. The articular cartilage (AC) of the femur and patella are irregularly thickened compared with the control. (C) Longitudinal section of distal femur and knee from a 1-year-old sedc/sedc mouse. The thickened articular cartilage (AC) of femur and patella are undergoing degeneration. Superficial layers are separating from deeper layers. A cleft (C) is present in the growth plate that is quiescent in this older mouse.

FIG. 6

(A–D) Midfrontal sections of the proximal growth plate of femurs from (A) a 2-week-old heterozygous control, (B) a 2-week-old with sedc/sedc, (C) a 6-week-old heterozygous control, and (D) a 6-week-old sedc/sedc. The clear columnar arrangements of the chondrocytes in the proliferative (PC) and hypertrophic (HC) zones of (A and C) controls are nonexistent in (B and D) sedc/sedc mice. (Toluidine blue stain: A and B, magnification, ×100; C and D, magnification, ×200).