Development of an in vitro model to investigate joint ochronosis in alkaptonuria

Alkaptonuria (AKU) is a genetic disorder caused by lack of the enzyme responsible for breaking down homogentisic acid (HGA), an intermediate in tyrosine metabolism. HGA is deposited as a polymer, termed ochronotic pigment, in collagenous tissues. Pigmentation is progressive over many years, leading to CTDs including severe arthropathies. To investigate the mechanism of pigmentation and to determine how it leads to arthropathy, we aimed to develop an in vitro model of ochronosis. Osteosarcoma cell lines MG63, SaOS-2 and TE85 were cultured in medium containing HGA from 0.1 μM to 1 mM. Cultures were examined by light microscopy and transmission electron microscopy, and Schmorl's stain was used to detect pigment deposits in vitro, following the observation that this stain identifies ochronotic pigment in AKU tissues. The effects of HGA on cell growth and collagen synthesis were also determined. There was a dose-related deposition of pigment in cells and associated matrix from 33 μM to 0.33 mM HGA. Pigmentation in vitro was much more rapid than in vivo, indicating that protective mechanisms exist in tissues in situ. Pigment deposition was dependent on the presence of cells and was observed at HGA concentrations that were not toxic. There was an inhibition of cell growth and a stimulation of type I collagen synthesis up to 0.33 mM HGA, but severe cell toxicity at 1 mM HGA. We have developed an in vitro model of ochronosis that should contribute to understanding joint destruction in AKU and to the aetiology of OA.