The contribution of mouse models in the rare disease alkaptonuria

Alkaptonuria is an ultra-rare autosomal recessive disorder of tyrosine metabolism, whereby the homogentisate 1,2-dioxygenase (HGD) enzyme is deficient, causing an elevation of its substrate homogentisic acid (HGA). Overtime, elevated HGA causes connective tissue ochronosis, leading to a severe and early onset osteoarthropathy. The use of HGD deficient mouse models in this metabolic bone disease have provided the opportunity to investigate AKU pathophysiology and potential treatments. An ENU mutagenesis AKU mouse model (BALB/c Hgd^−/−) provided the means to explore the onset of pigmentation in cartilage and treatment of AKU with nitisinone, an inhibitor of the upstream enzyme forming HGA. This work provided evidence that nitisinone could not only lower circulating HGA, but could also prevent ochronosis and halt disease progression, leading to its off-label use at the National Alkaptonuria Centre (Liverpool, UK) and its subsequent testing in human clinical trials (DevelopAKUre). Recently, a new targeted AKU mouse model (Hgd tm1a^−/−, C57BL/6) has been established, offering a LacZ reporter gene for localising gene expression and LoxP and FRT sites that enabled generation of an inducible and liver-specific HGD knockout model (Hgd tm1d MxCre^+/−). This conditional model determined the importance of the liver as a target organ for future gene/enzyme replacement therapies in AKU. The contribution of AKU mouse models has clearly accelerated the treatment and knowledge of this rare disease, and will continue to be used.