Mechanisms of enhanced osteoclastogenesis in Alkaptonuria

Alkaptonuria (AKU) is a rare disorder characterized by the deficiency of the enzyme homogentisate 1,2-dioxygenase and consequent homogentisate accumulation, which leads to progressive and severe osteoarthopathy starting from the second decade of life. Thus, in AKU patients bone involvement represents an important clinical issue, which we investigated. Serum levels of RANKL, osteoprotegerin, sclerostin, DKK1, and bone remodeling markers were measured in nine AKU patients (two children and seven adults) and 22 controls, together with lumbar spine bone mineral density (LS-BMD) and femoral-BMD. In the two AKU children the average of LS-BMD and femoral-BMD Z-scores were within the normal range, but reduced with respect to the controls. Otherwise, in the adult AKU patients LS-BMD T-score were inside the normal range, but femoral-BMD T-score reached osteopenic levels. Consistently, in AKU adults higher RANKL and CTX and lower osteoprotegerin levels were observed than controls. Otherwise, spontaneous osteoclastogenesis was already evident in peripheral blood mononuclear cell cultures from AKU children together with a high percentage of circulating osteoclast precursors. Osteoclastogenesis was sustained by the high levels of TNFα, RANK, RANKL, and LIGHT. In conclusion, the altered osteoclastogenesis was observed already in AKU children despite the absence of evident injury. Thus, a preventive approach in young patients, targeting osteoclast activity, may prevent the macroscopic bone disease which appears in adult AKU.