Urate Oxidase, Pseudogene

Description

In most mammals, the activity of urate oxidase (EC 1.7.3.3) catalyzes the oxidation of uric acid to allantoin; humans and some primates lack this enzyme activity. The loss of urate oxidase in the human during primate evolution predisposes man to hyperuricemia, a metabolic disturbance that can lead to gouty arthritis and renal stones (summary by Wu et al., 1994).

Cloning and Expression

Lee et al. (1988) determined the amino-terminal amino acid sequence of porcine urate oxidase and used it in a novel procedure for generating cDNA probes. The procedure was PCR-based and used mixed oligonucleotide primers complementary to the reverse translation products of an amino acid sequence. Lee et al. (1988) suggested that this rapid and simple cDNA cloning procedure is generally applicable and requires only a partial amino acid sequence. They used a cDNA probe developed by this method to isolate a full-length porcine urate oxidase cDNA and to demonstrate the presence of homologous genomic sequences in humans by Southern blot analysis of human genomic DNA. (This is a situation similar to the enzymatic machinery for synthesis of ascorbic acid, which is also missing in man (see 240400); a nonfunctional gene homologous to the gene coding for the enzyme L-gulonolactone oxidase in other species may well be present in man.) Motojima and Goto (1988) cloned part of this homologous sequence from a human genomic library using a rat uricase cDNA probe and showed that the sequence is highly homologous to the 3-prime untranslated portion of the rat uricase mRNA.

Biochemical Features

In most mammals, urate oxidase (EC 1.7.3.3) is present in liver, with little or no detectable activity in other tissues. It is associated with the peroxisomes and exists as a tetramer with an apparent subunit size of 32,000 daltons. Humans and certain primates lack this enzyme activity. The identification of mice with complete HPRT deficiency but without any symptoms of the Lesch-Nyhan syndrome (300322) (Hooper et al., 1987; Kuehn et al., 1987) raised the possibility that the absence of urate oxidase activity in the purine metabolism pathway may contribute to the development of the neurologic symptoms observed in humans (see 308000). The enzyme catalyzes the oxidation of uric acid to allantoin (summary by Lee et al., 1988).

Molecular Genetics

Wu et al. (1989) identified 2 nonsense mutations in the human urate oxidase gene, consistent with its nonfunctional nature. Comparison of the sequences in man, mouse, and pig suggested that loss of urate oxidase function in man was due to a sudden mutational event.

Using the cDNA and selected genomic probes of rat urate oxidase, Yeldandi et al. (1990) screened the human genomic library and isolated 7 clones, one of which contained exonic regions corresponding to exons 5, 6, and 7 of the rat urate oxidase gene. The nucleotide sequences of these 3 exons and the exon/intron junctions were compared with those of the rat gene. They found a mutation resulting in a stop codon TGA in the fifth exon of the human urate oxidase gene. Sequence analysis of PCR-amplified DNA corresponding to the fifth exon of urate oxidase from DNA samples of 4 different individuals demonstrated the same TGA stop codon in all. They suggested that this mutation, with or without other mutations, may be responsible for the lack of urate oxidase activity in the human.

Kratzer et al. (2014) argued that the silencing of the uricase gene in the ape and human lineage resulted from multiple independent events rather than from a single ancestral mutation. When the authors synthesized and expressed predicted ancient primate uricases, they observed a gradual loss of activity culminating in complete inactivity in apes and man. Expression of ancient and modern uricases in human HepG2 liver cells showed localization to peroxisomes, as observed in other mammals. The cultured cells with active uricase showed both reduced uric acid and triglyceride accumulation. Kratzer et al. (2014) proposed that the reduced activity of uricase in primates and its loss in apes and humans may have been a consequence of evolutionary selection for the ability to more rapidly convert fructose into fat during a period of progressively cool climate change, in keeping with the 'thrifty genotype' hypothesis of Neel (1962).

Gene Structure

Ito et al. (1991) described the structure of the uricase gene in the rat, where it spans 40 kb and comprises 8 exons. All the exon-intron junctional sequences conform to the canonical GT/AG rule.

Mapping

Yeldandi et al. (1992) assigned the urate oxidase gene (UOX) to chromosome 1 by Southern analysis of human/hamster cell hybrids. Using fluorescence in situ hybridization, they refined the assignment to 1p22. Thus, the 'disease' urate oxidase deficiency, a universal aberration in man and hominoid primates, maps to chromosome 1. To determine whether urate oxidase is an essential enzyme in lower mammals and to develop a mouse model for hyperuricemia and gout, Wu et al. (1994) disrupted the urate oxidase gene in the mouse by homologous recombination in embryonic stem cells. Unlike the human situation, urate oxidase deficiency in mice caused pronounced hyperuricemia and urate neuropathy. More than half of the mutant mice died before 4 weeks of age, indicating that urate oxidase is essential in mice. Given its absence, urate oxidase is presumably a nonessential enzyme in humans. Although lack of this enzyme may contribute to the development of hyperuricemia and gout in adult life, most humans do not develop the disease except in conjunction with other factors.