The AKU mouse was established as a model of both the plasma biochemistry of AKU and its associated arthropathy. Early-stage treatment of AKU patients with nitisinone could prevent the development of associated joint arthropathies. The cellular pathology of ochronosis in AKU mice is identical to that observed in early human ochronosis and thus is a model in which the early stages of joint pathology can be studied and novel interventions evaluated.

Alkaptonuria (AKU) is an ultrarare autosomal recessive disorder resulting from a deficiency of homogentisate 1,2 dioxygenase (HGD), an enzyme involved in the catabolism of phenylalanine and tyrosine. Loss of HGD function prevents metabolism of homogentisic acid (HGA), leading to increased levels of plasma HGA and urinary excretion. Excess HGA becomes deposited in collagenous tissues and subsequently undergoes polymerisation, principally in the cartilages of loaded joints, in a process known as ochronosis. This results in an early-onset, devastating osteoarthropathy for which there is currently no effective treatment. We recently described the natural history of ochronosis in a murine model of AKU, demonstrating that deposition of ochronotic pigment begins very early in life and accumulates with age. Using this model, we were able to show that lifetime treatment with nitisinone, a potential therapy for AKU, was able to completely prevent deposition of ochronotic pigment. However, although nitisinone has been shown to inhibit ochronotic deposition, whether it can also facilitate removal of existing pigment has not yet been examined. We describe here that midlife administration of nitisinone to AKU mice arrests further deposition of ochronotic pigment in the tibiofemoral joint, but does not result in the clearance of existing pigment. We also demonstrate the dose-dependent response of plasma HGA to nitisinone, highlighting its efficacy for personalised medicine, where dosage can be tailored to the individual AKU patient.

Objective Alkaptonuria (AKU) is a rare genetic disease which results in severe early onset osteoarthropathy. It has recently been shown that the subchondral interface is of key significance in disease pathogenesis. Human surgical tissues are often beyond this initial stage and there is no published murine model of pathogenesis, to study the natural history of the disease. The murine genotype exists but it has been reported not to demonstrate ochronotic osteoarthropathy consistent with the human disease. Recent anecdotal evidence of macroscopic renal ochronosis in a mouse model of tyrosinaemia led us to perform histological analysis of tissues of these mice that are known to be affected in human AKU. Design The homogentisate 1,2-dioxygenase Hgd+/−Fah−/− mouse can model either hereditary tyrosinaemia type I (HT1) or AKU depending on selection conditions. Mice having undergone Hgd reversion were sacrificed at various time points, and their tissues taken for histological analysis. Sections were stained with haematoxylin eosin (H&E) and Schmorl’s reagent. Results Early time point observations at 8 months showed no sign of macroscopic ochronosis of tissues. Macroscopic examination at 13 months revealed ochronosis of the kidneys. Microscopic analysis of the kidneys revealed large pigmented nodules displaying distinct ochre colouration. Close microscopic examination of the distal femur and proximal fibula at the subchondral junctions revealed the presence of numerous pigmented chondrocytes. Conclusions Here we present the first data showing ochronosis of tissues in a murine model of AKU. These preliminary histological observations provide a stimulus for further studies into the natural history of the disease to provide a greater understanding of this class of arthropathy.

No abstract

No abstract

models: 1) intra-articular injection of monoiodoacetate (MIA) at the dose of 1 mg/kg, 2) medial meniscal tear (MMT) combined with medial collateral ligament transection (MCLT), 3) anterior cruciate ligament transection (ACLT) combined with partial medial meniscectomy (pMMx), 4) ACLT. Body weight, static weight bearing determined as hind paw weight distribution, and static mechanical allodynia determined as paw withdrawal threshold were followed in each model during the study. Knee joints were harvested and digital radiographs were obtained from them at two different time points in each model as follows: in the MIA model at 2 and 4 weeks, in the MMT + MCLT model at 3 and 6 weeks, in the ACLT + pMMx model at 4 and 8 weeks, and in the ACLT model at 5 and 10 weeks. Microscopic assessment of degenerative changes was performed in knee joints as recommended by the OARSI histopathology initiative. Results: Static weight bearing was decreased in operated knee joints during the first week of the study, demonstrating an operation-related joint discomfort. Paw withdrawal threshold was decreased in operated and MIA-injected knee joints during the first week and at the end of the study, indicating operation and OA-related joint pain. Microscopic assessment of OA demonstrated progressive degenerative changes in knee joints in all animal models. Mild to moderate changes were observed in the MIA and ACLT models. In the MIA model, mild degenerative changes included the loss of chondrocytes, proteoglycans (PG) and collagen matrix in superficial layer of articular cartilage in medial tibial plateau at 2 weeks, and the chondrocyte and PG loss exacerbated down to intermediate layer at 4 weeks. In the ACLT model, chondrocytes, PG and collagen matrix were lost mainly in superficial layer at 5 weeks, their loss exacerbated down to tidemark at 10 weeks, and these changes were associated with the presence of moderate to large osteophytes and minimal synovial inflammation. The MMT + MCLT and ACLT + pMMx models exhibited moderate to severe degenerative changes. In the articular cartilage of medial tibial plateau, chondrocytes, PG and collagen matrix were lost from superficial layer down to tidemark in the MMT + MCLT model at 3 and 6 weeks, and in the ACLT + pMMx model at 4 and 8 weeks. These changes were associated with the presence of large osteophytes and mild synovial inflammation in both models. Conclusions: This study characterized progressive degenerative changes as recommended by the OARSI histopathology initiative in knee joints of four rat OA models used frequently in the preclinical in vivo efficacy studies of DMOADs. Mild to moderate degenerative changes were observed in the MIA and ACLT models and moderate to severe changes in the MMT + MCLT and ACLT + pMMx models. In all OA models, the development of degenerative changes was associated with decreased paw withdrawal threshold, indicating OA-related joint pain.

No abstract