A mutation in COL9A1 causes multiple epiphyseal dysplasia: further evidence for locus heterogeneity

Abstract

Multiple epiphyseal dysplasia (MED) is an autosomal dominantly inherited chondrodysplasia. It is clinically highly heterogeneous, partially because of its complex genetic background. Mutations in four genes, COL9A2, COL9A3, COMP, and MATR3, all coding for cartilage extracellular matrix components (i.e., the alpha2 and alpha 3 chains of collagen IX, cartilage oligomeric matrix protein, and matrilin-3), have been identified in this disease so far, but no mutations have yet been reported in the third collagen IX gene, COL9A1, which codes for the alpha1(IX) chain. MED with apparently recessive inheritance has been reported in some families. A homozygous R279W mutation was recently found in the diastrophic dysplasia sulfate transporter gene, DTDST, in a patient with MED who had a club foot and double-layered patella. The series consisted of 41 probands with MED, 16 of whom were familial and on 4 of whom linkage analyses were performed. Recombination was observed between COL9A1, COL9A2, COL9A3, and COMP and the MED phenotype in two of the families, and between COL9A2, COL9A3, and COMP and the phenotype in the other two families. Screening of COL9A1 for mutations in the two probands from the families in which this gene was not involved in the recombinations failed to identify any disease-causing mutations. The remaining 37 probands were screened for mutations in all three collagen IX genes and in the COMP gene. The probands with talipes deformities or multipartite patella were also screened for the R279W mutation in DTDST. The analysis resulted in identification of three mutations in COMP and one in COL9A1, but none in the other two collagen IX genes. Two of the probands with a multipartite patella had the homozygous DTDST mutation. The results show that mutations in COL9A1 can cause MED, but they also suggest that mutations in COL9A1, COL9A2, COL9A3, COMP, and DTDST are not the major causes of MED and that there exists at least one additional locus.

Figure 1

Radiographs of the knees (A, anteroposterior (AP) view; B, lateral view) and thoraco-lumbar spine (C, AP view; D, lateral view) of proband 14 at age 14 years (table 1), and of the hips (E) of the proband’s affected brother at age 11 years (individual R.B. in fig. 2). The knee radiographs show irregular and flat epiphyses and multipartite patella. Endplate irregularities are seen in the thoraco-lumbar spine. The radiographs of the hips show varus deformity, shortening of the femoral necks, enlarged and flat femoral heads, dysplastic acetabuli, and dislocation of the left hip.

Figure 2

Pedigrees of four families with MED. Blackened symbols denote affected individuals. The probands are indicated by arrows (↖). The family pedigree of proband 12 is shown in panel A, that of proband 14 in panel B, that of proband 20 in panel C, and that of proband 32 in panel D (table 1). Alleles of the markers are shown below the symbols.

Figure 3

CSGE analysis of the PCR products for exons 8, 12, and 16 of the COMP gene, and for exon 8 of the COL9A1 gene. “E” denotes exon, “P” denotes proband, and “C” denotes control.

Figure 4

Agarose gel electrophoresis of α1(IX) reverse-transcriptase PCR products. RNA was isolated from EBV-transformed lymphoblasts from proband 41 and was analyzed for splicing by PCR as indicated in the Subject and Methods section. The control sample (C) shows one product of about 570 bp, whereas the sample from the proband (RT) has two major products of about 570 and 430 bp. In addition, two small minor products can be seen in the proband’s sample. “Wt” denotes wild type, “Del” denotes deletion, and “M” denotes molecular weight marker.

Figure 5

Radiographs of the thoraco-lumbar spine (A, lateral view) and lumbar spine (B, lateral view) of proband 41 (table 1), and of the left foot (C, AP view), hips (D, AP view), and left knee (E, AP view) of the proband’s affected mother. Radiographs of the spine show small anterior osteophytes on several of the lower thoracic vertebrae and on L4. Radiographs of the proband's mother show widespread and severe OA in all joints.