Raman Spectroscopy identifies differences in ochronotic and non-ochronotic cartilage; a potential novel technique for monitoring ochronosis

Abstract

Alkaptonuria (AKU) is a rare, inherited disorder of tyrosine metabolism, where patients are unable to breakdown homogentisic acid (HGA), which increases systemically over time. It presents with a clinical triad of features; HGA in urine, ochronosis of collagenous tissues, and the subsequent ochronotic arthritis of these tissues. In recent years the advance in the understanding of the disease and the potential treatment of the disorder looks promising with the data on the efficacy of nitisinone. However, there are limited methods for the detection and monitoring of ochronosis invivo, or for treatment monitoring. The study aim was to test the hypothesis that Raman spectra would identify a distinct chemical fingerprint for the non-ochronotic, compared to ochronotic cartilage. Ochronotic and non-ochronotic cartilage from human hips and ears were analysed using Raman spectroscopy. Non-ochronotic cartilage spectra were similar and reproducible and typical of normal articular cartilage. Conversely, the ochronotic cartilage samples were highly fluorescent and displayed limited or no discernible Raman peaks in the spectra, in stark contrast to their non-ochronotic pairs. Interestingly, a novel peak was observed associated with the polymer of HGA in the ochronotic cartilage that was confirmed by analysis of pigment derived from synthetic HGA. This technique reveals novel data on the chemical differences in ochronotic compared with non-ochronotic cartilage, these differences are detectable by a technique that is already generating invivo data and demonstrates the first possible procedure to monitor the progression of ochronosis in tissues of patients with AKU.