The Helix-II epitope: a cautionary tale from a cartilage biomarker based on an invalid collagen sequence

Objective: An apparent database error in the sequence underlying the Helix-II cartilage biomarker immunoassay was investigated at the protein level. Methods and results: Tandem mass spectrometry established the peptide sequence ERGETGPP*GPA in human type II collagen, not ERGETGPP*GTS used to generate the antibody for the Helix-II assay. Conclusions: Recent reports in which the Helix-II assay was applied to urine or serum as a marker of cartilage collagen degradation need to be re-evaluated since the epitope does not occur in cartilage type II collagen. Based on collagen sequences and Helix-II epitope properties, type III collagen is one of several candidate sources of the cross-reacting signal in body fluids, but not type 11 collagen. The findings highlight the need for more stringent scrutiny of the origins and validation of molecular markers in body fluid assays in general. (C) 2009 Published by Elsevier Ltd on behalf of Osteoarthritis Research Society International.