Inborn Metabolic Diseases

35.1 Inborn Errors of Purine (P. 435)

Metabolism

Inborn errors of purine metabolism comprise errors of: 4 purine nucleotide synthesis: phosphoribosylpyrophosphate (PRPP) synthetase superactivity, adenylosuccinase (ADSL) deficiency, AICA-ribosiduria caused by ATIC deficiency,

4 purine catabolism: the deficiencies of muscle AMP deaminase (AMP-DA, also termed myoadenylate deaminase), adenosine deaminase (ADA), purine nucleoside phosphorylase (PNP) and xanthine oxidase, 4 purine salvage: the deficiencies of hypoxanthine-guanine phosphoribosyltransferase (HGPRT) and adenine phosphoribosyltransferase (APRT).

The deficiency of deoxyguanosine kinase causes mitochondrial DNA depletion (7 also Chap. 15). With the exception of muscle AMP-DA deficiency, all these enzyme defects are very rare.

35.1.1 Phosphoribosyl Pyrophosphate Synthetase Superactivity

Clinical Presentation

The disorder is mostly manifested by the appearance, in young adult males, of gouty arthritis and/or uric acid lithiasis, potentially leading to renal insufficiency [1, 2]. Uricemia can be very high, reaching 10-15 mg/dl (0.60- 0.90 mmol/l) [normal adult values: 2.9-5.5 mg/dl (0.17- 0.32 mmol/l)]. The urinary excretion of uric acid is also increased, reaching up to 2400 mg (14 mmol)/24 h, or 2.5 mmol/mmol creatinine [normal adult values: 500- 800 mg (3-4.7 mmol)/24 h, or 02-0.3 mmol/mmol creatinine].

A few patients have been reported in which clinical signs of uric acid overproduction already appeared in infancy and were accompanied by neurologic abnormalities, mainly sensorineural deafness, particularly for high tones, but also hypotonia, locomotor delay, ataxia and autistic features [2].

Metabolic Derangement

The enzyme forms phosphoribosyl pyrophosphate (PRPP) from ribose-5-phosphate and ATP (. Fig. 35.1). PRPP is the first intermediate of the de novo synthesis of purine nucleotides (not shown in full detail in . Fig. 35.1), which leads to the formation of inosine monosphosphate (IMP), from which the other purine compounds are derived.

PRPP synthetase is highly regulated. Various genetic regulatory and catalytic defects [1, 2] lead to superactivity, resulting in increased generation of PRPP. Because PRPP amidotransferase, the rate-limiting enzyme of the de novo pathway, is physiologically not saturated by PRPP, the synthesis of purine nucleotides increases, and hence the production of uric acid.

PRPP synthetase superactivity is one of the few known examples of an hereditary anomaly of an enzyme which enhances its activity. The mechanism of the neurological symptoms is unresolved.

Genetics

The various forms of PRPP synthetase superactivity are inherited as X-linked traits. In the families in which the anomaly is associated with sensorineural deafness, heterozygous females have also been found with gout and/or hearing impairment [2]. Studies of the gene in six families revealed a different single base change in each of them [3].