The role of structural genes in the pathogenesis of osteoarthritic disorders

The role of sequence variations within the genes encoding collagen II, IX and XI in non-syndromic, early-onset osteoarthritis

OBJECTIVE

We sought to determine whether sequence variations in cartilage collagen genes are associated with primary, early-onset osteoarthritis (OA).

METHODS

The cartilage collagen genes, COL2A1, COL9A1, COL9A2, COL9A3, COL11A1 and COL11A2, were screened for sequence variations in 72 Finnish probands and one US family with primary early-onset hip and/or knee OA. In addition, allelic association studies were performed using six to 12 common polymorphisms from each gene by genotyping 72 OA patients and 103 controls.

RESULTS

Altogether 239 sequence variations were found, of which 16 were not present in the controls. Seven of the unique variations, four in COL11A1, two in COL11A2 and one in COL2A1, were studied further, because they resulted in the substitution of conserved amino acids or were predicted to affect mRNA splicing. Co-segregation of a sequence variation and the phenotype was found in all four families available for study. Association analysis failed to identify any common predisposing alleles.

CONCLUSIONS

Early-onset OA demonstrates locus and allelic heterogeneity since the identified variations were in three different collagen genes and each of the six probands had a different mutation. It is also possible that some OA cases represent the mild end of the chondrodysplasia phenotypic spectrum. The major susceptibility alleles in this form of OA, however, remain to be identified.

Activation of the discoidin domain receptor 2 induces expression of matrix metalloproteinase 13 associated with osteoarthritis in mice

Human genetic studies indicate that mutations in type IX and XI collagens result in early-onset osteoarthritis (OA) with a wide spectrum of osteochondrodysplasia. However, a convincing causal chain of events underlying the role of these collagen mutations in the pathogenesis of OA has not been elucidated. Here we show that the expression of a cell surface collagen receptor, discoidin domain receptor 2 (DDR2), is increased in chondrocytes of the articular cartilage of knee joints in mice that develop OA as a result of a heterozygous mutation in type XI collagen. At the same time point, 6 months, we also found increased expression and activity of matrix metalloproteinase 13 (MMP-13) in the mutant mouse knee cartilage. The expression of both DDR2 and MMP-13 was increased in chondrocytes cultured on plates coated with native type II collagen but not on gelatin, and overexpression of DDR2, but not of a truncated form, was found to induce the expression of MMP-13 when chondrocytes were cultured on type II collagen but not on plastic. The DDR2-induced expression of MMP-13 appears to be specific, since we did not observe induction of MMP-1, MMP-3, MMP-8, ADAMTS-4, ADAMTS-5, and IL-1 transcripts in human chondrocytes or Mmp-3, Mmp-8, Adamts-4, Adamts-5, and Il-1 in mouse chondrocytes. Our data suggest that the defect in the cartilage matrix of mice that are heterozygous for a type XI collagen mutation (cho/+) permits activation and up-regulation of DDR2 in chondrocytes. This could be due to increased exposure of chondrocytes to type II collagen as a result of the decreased amount of type XI collagen in the mutant cartilage. The specific induction of MMP-13 by DDR2 in response to its cartilage-specific ligand, type II collagen, may contribute to cartilage damage in hereditary OA.

Estrogen receptor-alpha gene haplotype is associated with primary knee osteoarthritis in Korean population

Estrogen and estrogen receptors (ERs) are known to play important roles in the pathophysiology of osteoarthritis (OA). To investigate ER-alpha gene polymorphisms for its associations with primary knee OA, we conducted a case-control association study in patients with primary knee OA (n = 151) and healthy individuals (n = 397) in the Korean population. Haplotyping analysis was used to determine the relationship between three polymorphisms in the ER-alpha gene (intron 1 T/C, intron 1 A/G and exon 8 G/A) and primary knee OA. Genotypes of the ER-alpha gene polymorphism were determined by PCR followed by restriction enzyme digestion (PvuII for intron 1 T/C, XbaI for intron 1 A/G, and BtgI for exon 8 G/A polymorphism). There was no significant difference between primary knee OA patients and healthy control individuals in the distribution of any of the genotypes evaluated. However, we found that the allele frequency for the exon 8 G/A BtgI polymorphism (codon 594) was significantly different between primary knee OA patients and control individuals (odds ratio = 1.38, 95% confidence interval = 1.01-1.88; P = 0.044). In haplotype frequency estimation analysis, there was a significant difference between primary knee OA patients and control individuals (degrees of freedom = 7, chi2 = 21.48; P = 0.003). Although the number OA patients studied is small, the present study shows that ER-alpha gene haplotype may be associated with primary knee OA, and genetic variations in the ER-alpha gene may be involved in OA.

Increased presence of cells with multiple elongated processes in osteoarthritic femoral head cartilage

OBJECTIVE

This study examined the morphology of chondrocytes in articular cartilage from osteoarthritic (OA) and non-OA human femoral heads and in particular the appearance of a sub-population of cells with multiple elongated processes radiating up to 30 microm into the extracellular matrix.

METHODS

Cartilage explants were removed from 8 anatomical sites over the surface of OA (n=6) and non-OA (n=5) femoral heads. Cells were labeled for vimentin intermediate filaments and visualized using epi-fluorescence and confocal microscopy. The percentage of cells with elongated processes was correlated with macroscopic and histological indicators of osteoarthritis.

RESULTS

Cells with processes accounted for less than 10% of the total cell population in non-OA cartilage. By contrast, in the peripheral regions of the OA femoral head these cells accounted for 20-45% of the total cell population, the differences being statistically significant. These peripheral areas are habitually non-load bearing and were also the most likely to show gross fibrillation and pannus formation. A statistically significant correlation was demonstrated between the percentage of cells with processes and the histological extent of the OA degradation, quantified in terms of the Mankin score.

CONCLUSIONS

The extension of cell processes, which may be associated with localized breakdown of the pericellular matrix, will undoubtedly alter numerous aspects of cell function including phenotypic expression and mechanotransduction. Hence these significant changes in chondrocyte morphology are likely to have important implications for the aetiology of osteoarthritis and the development of potential treatment strategies.

Genomics of osteoarthritis

PURPOSE OF REVIEW

This review addresses the progress in three major fields of "genomics of osteoarthritis" over the past year: genetic alterations thought to be important for the initiation and progression of osteoarthritis, differential gene expression analysis, and functional genomics of osteoarthritis.

RECENT FINDINGS

Distinct genetic risk factors may predispose different joint sites to osteoarthritis, and although clear loci for susceptibility genes for common osteoarthritis have yet to emerge from the epidemiological studies, new approaches are narrowing down known loci. The search for specific genes using cDNA array technology has further demonstrated its potential in arthritis research as a powerful tool that could further provide biological insights into disease mechanisms, osteoarthritis polymorphic subtypes, the molecular validation of animal models, and the monitoring of drug activity on gene expression levels. Gene expression analysis has further characterized the striking shift in the gene expression pattern during "dedifferentiation" of chondrocytes in vitro as well as added depth to the phenotype of differentiated versus undifferentiated mesenchymal stem cells. Several new molecules potentially relevant to the disease process were identified, among them beta2-microglobulin (B2M), clusterin, and chitinase-like molecule 2.

SUMMARY

Functional genomic approaches will in the future allow to complement traditional biochemistry and molecular biology. Although there are limitations to cDNA array technology, "molecular portraits" of osteoarthritic chondrocytes in vivo and in vitro can be produced to analyze whole or large biologic systems rather than just single aspects of it. This will stimulate the testing of new markers, which are needed for the diagnosis and monitoring of osteoarthritis.