3. Discussion
PRO-C1, liver damage, ochronosis
DISEASE PROGRESSION, ALKAPTONURIA, RARE DISEASE, INFLAMMATION, DISEASE, LIVER DAMAGE, EMA, OCHRONOSIS, PATHOGENESIS
This study presents evidence that the remodelling of collagen type I and collagen type II, the main ECM proteins in bone and cartilage, respectively, is altered following nitisinone treatment in patients with alkaptonuria.The pathogenesis of AKU is characterized by the accumulation of the ochronotic pigment in joints and other cartilaginous tissues. Nitisinone has been approved by the EMA for the treatment of adult patients with AKU, on the basis of the significant lowering of the circulating and urinary HGA, responsibility for the development of ochronosis, and the demonstration of slower disease progression [Type II collagen constitutes up to 90% of the collagen content in cartilage [When looking at trajectories of biomarker change over time, it is possible to observe a sharp increase in the levels of both CTX-I and PRO-C1 after one year of treatment, that then returned to baseline levels at later visits. A similar trend was observed for total levels of the enzyme ALP. The main sources for this enzyme are the liver and bone. There were no signs of any liver damage in nitisinone-treated patients, so a likely explanation for the increase is that it here reflects bone remodelling. Since the three biomarkers presented the same pattern, this phenomenon may reflect an increased remodelling of the bone tissue after starting nitisinone treatment. This may be reflected in the observed increased range of motion of the joints, especially distally, in nitisinone-treated patients, which allowed greater mobility [Type II collagen remodelling was also altered by nitisinone treatment. Especially C2M levels had decreased markedly already at Year 1, suggesting that the treatment influences inflammatory-mediated turnover of cartilage. It was also possible to observe changes in the CTX-II/Creat biomarker that reflect remodelling of articular and calcified cartilage. The difference between C2M and CTX-II is related to their molecular origin. C2M is from the helical domain of type II collagen and is generated by matrix metalloproteinases (MMPs), which are typically upregulated and activated during inflammation, while CTX-II is from the telopeptide end of type II collagen and can be generated by proteases such as cathepsin K expressed by osteoclasts [The effect of nitisinone on the biomarker of type II collagen remodelling CTX-II/Creat was more pronounced in younger and male patients, while the reduction of C2M at late time points was more pronounced in patients with lower cAKUSSI at baseline and in male patients. This suggests that the effect of nitisinone on joint health may be more pronounced if the treatment is started earlier when the disease is not yet overt. It is currently not possible to find an explanation for the difference in treatment effect on these biomarkers between males and females.This study is limited by a relatively low number of examined patients. However, AKU is a rare disease, and this study involves the largest cohorts for which biomarker samples have been collected and the effect of nitisinone has been studied. Moreover, the treatment effect was observed over the course of four years, which may be an insufficient time to see significant changes in remodelling of cartilage and bone tissue, as the drug affects the production of the damaging ochronotic pigment, and it was tested exclusively in patients with chronic pathological signs already present at baseline.The non-invasive biomarkers shown in this investigation are related to tissue remodelling in joints in patients with AKU. They reflect the turnover of type I collagen, the most abundant collagen in bones, and type II collagen, the most abundant collagen in cartilage, and can describe changes at a molecular level induced by the treatment of nitisinone, likely an indirect effect of blocking the production of the ochronotic pigment. These biomarkers can have clinical utility in the evaluation and management of patients with AKU as they could help find patients in which the disease has already affected the remodelling of joints and who could therefore benefit from an early initiation of nitisinone therapy. Moreover, they can be useful in the evaluation of the long-term effect of nitisinone treatment on joint health, without the need for complex clinical and imaging evaluation.