Alkaptonuria: Current Perspectives

Abstract

The last 15 years have been the most fruitful in the history of research on the metabolic disorder alkaptonuria (AKU). AKU is caused by a deficiency of homogentisate dioxygenase (HGD), the enzyme involved in metabolism of tyrosine, and is characterized by the presence of dark ochronotic pigment in the connective tissue that is formed, due to high levels of circulating homogentisic acid. Almost 120 years ago, Sir Archibald Garrod used AKU to illustrate the concept of Mendelian inheritance in man. In January 2019, the phase III clinical study SONIA 2 was completed, which tested the effectiveness and safety of nitisinone in the treatment of AKU. Results were positive, and they will serve as the basis for the application for registration of nitisinone for treatment of AKU at the European Medicines Agency. Therefore, AKU might become a rare disease for which a cure will be found by 2020. We understand the natural history of the disease and the process of ochronosis much more, but at the same time there are still unanswered questions. One of them is the issue of the factors influencing the varying severity of the disease, since our recent genotype-phenotype study did not show that differences in residual homogentisic acid activity caused by the different mutations was responsible. Although nitisinone has proved to arrest the process of ochronosis, it has some unwanted effects and does not cure the disease completely. As such, enzyme replacement or gene therapy might become a new focus of AKU research, for which a novel suitable mouse model of AKU is available already. We believe that the story of AKU is also a story of effective collaboration between scientists and patients that might serve as an example for other rare diseases.

Keywords: alkaptonuria; nitisinone; ochronosis; ochronotic pigment; rare disease.

Figure 1

Phenylalanine/tyrosine metabolism in brief and main AKU clinical symptoms. Notes: Metabolic block in alkaptonuria (AKU) affects homogentisic acid dioxygenase (HGD) and causes its accumulation, while nitisinone blocks 4-hydroxyphenylpyruvic acid dioxygenase and impedes formation of HGA. Ochronotic pigment is formed in connective tissue from benzoquinone acetic acid intermediate product. Main AKU clinical symptoms are homogentisic aciduria (urine gets dark upon standing, due to the presence of HGA) and ochronosis (dark pigmentation) of ear, eye, and joints.

Figure 2

Overview of the number of patients with alkaptonuria reported worldwide. Note: Courtesy of AKU Society (www.akusociety.org), Ciarán Scott.

Figure 3

Proportion of different mutation types of 178 HGD gene variants affecting HGD function identified in about 530 AKU patients. Note: Data from HGD Mutation Database (http://hgddatabase.cvtisr.sk).