Type II collagen, which consists of a large helical domain and telopeptides at each end, is the most abundant protein of cartilage matrix. The aim of this study was to develop a biochemical marker reflecting the degradation of the helical region of type II collagen and to evaluate its clinical performance in patients with osteoarthritis (OA) and rheumatoid arthritis (RA).
We developed a competitive polyclonal antibody-based enzyme-linked immunosorbent assay (ELISA) using the 622-632 peptide derived from the sequence of the alpha1 chain of human type II collagen (HELIX-II) as immunogen and standard. We measured urinary levels of HELIX-II peptide and C-terminal crosslinking telopeptide of type II collagen (CTX-II) in 90 patients with knee OA (73% women; mean +/- SD age 63.0 +/- 8.0 years, mean +/- SD disease duration 6.1 +/- 6.8 years), 89 patients with early RA (disease duration </=3 years) (79% women; mean +/- SD age 48.7 +/- 11.6 years), 25 patients with Paget's disease of bone (HELIX-II only), and 162 healthy controls. In RA patients, we investigated the relationships between baseline urinary HELIX-II and CTX-II levels and the progression of joint destruction as measured by the changes in the total Sharp score (average from 2 independent readers) over 1 year.
The intraassay and interassay variations of the HELIX-II ELISA were lower than 13% and 15%, respectively. The HELIX-II ELISA showed no significant cross-reactivity with human intact or denatured type II collagen, with the homologous peptides from human type I or type III collagens, or with HELIX-II peptides elongated (by 1 amino acid) or shortened (by 1 or 2 amino acids) at the C-terminal end, indicating that the HELIX-II ELISA recognized a neoepitope from the alpha1 chain of type II collagen. Median urinary HELIX-II levels were increased in patients with knee OA (by 56%; P < 0.0001) or early RA (by 123%; P < 0.0001) compared with those in age- and sex-matched healthy controls. Baseline urinary HELIX-II levels in the highest tertile were associated with an increased risk of radiographic progression in RA (increase in the total Sharp score >/=0.5 units/year), with an odds ratio (OR) of 5.9 (95% confidence interval [95% CI] 2.0-17.2) after adjustment for serum C-reactive protein (CRP) levels and baseline joint damage. Patients with increased levels of both urinary HELIX-II and CTX-II had the highest risk of progression (OR 17.5 [95% CI 3.1-99]).
The HELIX-II ELISA is specific for type II collagen degradation, has adequate technical performance, and can distinguish patients with knee OA or RA from healthy controls. Elevated HELIX-II levels are associated with increased risk of radiographic progression in RA independently of CRP levels, baseline joint damage, and urinary CTX-II levels. The HELIX-II ELISA should be useful for the clinical investigation of patients with arthritis and for identifying RA patients at higher risk of progression.