OA associated with familial osteochondrodysplasias caused by mutations in (mostly) structural cartilage matrix genes represents a subset of secondary OA. Studies of this form of OA, however, are important as they provide insights into the molecular processes that lead to articular cartilage destruction in all forms of OA.
Table summarizes genetic defects in structural macromolecules of cartilage that lead to alterations in chondrogenesis, skeletal malformations, reduced skeletal function, or predisposition to injury and OA. Mutations in type II collagen cause a spectrum of diseases known as 'type II collagenopathies'. The severity ranges from developmental lethality (achondrogenesis type II, hypochondrogenesis), to moderately severe dwarphism (SED, Kniest dysplasia), to normal stature with premature OA. In the case of lethal mutations with absence of type II collagen, some embryonic cartilage develops with type I collagen susbstituted for type II collagen, and bone is formed [
44]. Mutations that cause a moderately severe phenotype (SED, Kniest dysplasia) generally result from reduced secretion of typeII collagen or reduced content of this collagen in cartilage [
45-
47]. The mildest phenotype (Stickler syndrome, typeI) is caused by premature stop codons resulting in a null-allele mosaicism [
47,
48] and is associated with early-onset OA. Several mouse models of the human type II collagenopathies exist and resemble, depending on the type and position of the mutation in the COL2A1 gene, a lethal or a mild human phenotype [
49-
52].
| Table 2Structural gene mutations in cartilage that result in abnormal cartilage matrix |
Mutations in genes for type IX collagen result in MED. This clinically heterogeneous disorder is characterized by mild short stature and early-onset OA. In some families, splice-site mutations in COL9A2 and COL9A3 causing skipping of exon 3 in α2(IX) and α3(IX) transcripts, respectively [
53-
58], are associated with a phenotype characterized by normal to near-normal height, epiphyseal dysplasia of several joints during childhood, and OA of the knees in adulthood. A complex splicing defect of exon 8 and/or exon 10 in the COL9A1 gene has been identified in other families [
59]. Consistent with these findings, transgenic mice overexpressing a truncated α1(IX) chain (exerting a dominant negative effect on assembly of collagen IX molecules) exhibited mild chondrodysplasia and progressive OA [
60]. A similar phenotype was seen in mice carrying two null alleles for COL9A1 [
61]. Type IX collagen molecules are heterotrimers composed of three different polypeptide chains, α1(IX), α2(IX), and α3(IX), which are localized on the surface of collagen-II-containing fibrils, where they get cross-linked to residues within type II collagen molecules and may help stabilize the fibrillar network [
62]. The absence of α1(IX) chains or expression of a dominant negative form results in the absence or reduced amounts of collagen IX in cartilage, and this may destabilize the collagen network.
Mutations in the COL11A1 and COL11A2 genes, encoding two of the polypeptide subunits of heterotrimeric collagen XI molecules, give rise to the 'type XI collagenopathies'. COL11A1 mutations are associated with Marshall or Stickler syndrome, characterized by severe myopia, vitreoretinal degeneration, cleft palate, midfacial hypoplasia, early-onset OA, and sensorineuronal hearing loss [
63-
65]. Extensive genotype–phenotype correlations of patients with Stickler, Stickler-like, or Marshall syndrome have suggested that null-allele mutations in COL2A1, encoding the polypeptide chains of collagen II molecules as well as one of the chains in collagen XI, cause the typical Stickler phenotype (in which vitreoretinal degeneration is common and hearing loss is less common), while splicing mutations in COL11A1 are responsible for the Marshall syndrome (in which hearing loss is common and vitreoretinal degeneration is less common). Patients with glycine substitutions or small deletions in COL11A1 (dominant negative mutations) may have a mixed phenotype characteristic of both syndromes [
66]. Mutations in COL11A2 are associated with a nonocular Stickler-like syndrome, otospondylomegaepiphyseal dysplasia [
67-
70], Weissenbacher–Zweymuller syndrome [
69], or nonsyndromic forms of deafness called DFNA13 (deafness, autosomal dominant nonsyndromic sensorineural 13) [
71]. The explanation for the lack of an ocular phenotype in these syndromes is the presence of a unique form of type XI collagen in the vitreous. In cartilage, collagen XI molecules are heterotrimers of the products of COL11A1, COL11A2, and COL2A1, but in the vitreous the COL11A2 chain is replaced by a chain encoded by COL5A2 [
72].
The
cho/cho mouse, which is homozygous for a premature stop codon in the amino-terminal region of α1(XI) collagen [
73], has provided important insights into the role of collagen XI in skeletal development. Heterozygous animals are relatively unaffected; however, with age they develop ostearthritis. Homozygous animals die at birth, with short limbs, short snout, and cleft palate. Growth-plate cartilages show a disorganized structure with thick collagen fibrils. The presence of thick fibrils, providing direct evidence of a role of type XI collagen in regulating fibril diameters, leads to the formation of a large-pore network of fewer fibrils in
cho/cho cartilage than the small-pore network of thin fibrils found in wild-type cartilage. The changes in pore size causes the proteoglycan aggregates to be more loosely entrapped within the mutant matrix than in the wild type. Mice with Col11a2 null alleles are phenotypically comparable with patients who have otospondylomegaepiphyseal dysplasia, and their phenotype suggests that the α2(XI) chain may be required for correct fibril assembly or lateral association between individual collagen fibrils [
74].
Mutations that prevent glycosaminoglycan sulfation of aggrecan cause chondrodysplastic phenotypes associated with achondrogenesis, atelostogenesis, diastrophic dysplasia, and autosomal recessive MED [
75]. MED and pseudoachondroplasia can also be the result of mutations in COMP, a pentameric molecule belonging to the thrombospondin family of matrix molecules and which is localized in the pericellular, territorial matrix of chondrocytes. The protein contains several repeat domains, including eight calcium-binding, calmodulin-like repeats. Most COMP mutations identified in patients with MED or pseudoachondroplasia are amino acid substitutions that may disturb calcium binding [
76-
78]. Mutations in the gene encoding matrilin-3 [
79] can also give an MED clinical phenotype, a phenomenon which provides genetic evidence for a functional interaction between aggrecan, COMP, collagen IX, and matrilin-3 in cartilage [
80].
Mutations in a gene encoding the core protein of a proteoglycan associated with articular cartilage cause camptodactyly–arthropathy–coxa vara–pericarditis syndrome [
81]. This chondroitin sulfate proteoglycan, called superficial zone protein, lubricin, or proteoglycan 4, is produced by the superficial articular chondrocytes and synovial cells. It is responsible for lubrication of the cartilage surface [
82]. The synthesis is impaired in arthritic joints and downregulated by inflammatory cytokines such as IL-1.
Mutation at the progressive ankylosis (ank) locus in the mouse causes a progressive form of arthritis with deposition of apatite crystals, formation of bony outgrowths, joint destruction, and ankylosis [
83]. In humans, mutations in the ank gene, ANKH, have been linked to craniometaphyseal dysplasia [
84,
85]. In addition, analysis of two families from England and Argentina with familial calcium pyrophosphate deposition disease have identified mutations in ANKH [
86,
87]. An early-onset form of this disease with severe PGOA has been linked also to a region on chromosome 8q [
88].