Etiology and Genetic Features
Lesch-Nyhan syndrome has a strictly defined genetic etiology. The disease is inherited in an X-linked recessive manner. Due to this inheritance pattern, the pathology clinically manifests exclusively in males.
The primary underlying cause of the entire cascade of metabolic disturbances is the complete loss of activity of a specific enzyme — hypoxanthine-guanine phosphoribosyltransferase (abbreviated as HGPRT). It is important to emphasize that this absolute absence of enzyme activity determines the severity of the disease, distinguishing it from other forms of hyperuricemia.
Biochemical Pathogenesis
In a healthy organism, the enzyme HGPRT acts as an "economizer," providing the salvage pathway for purine bases. It captures free purines and returns them to the nucleotide pool so the cell does not waste resources synthesizing them de novo. HGPRT catalyzes two main reactions:
- Combining hypoxanthine with 5-phosphoribosyl-1-pyrophosphate (PRPP) to form inosine monophosphate (IMP) and pyrophosphate.
- Combining guanine with PRPP to form guanosine monophosphate (GMP) and pyrophosphate.
In Lesch-Nyhan syndrome, this salvage pathway is completely blocked. A pathological process develops that strikes purine metabolism from two directions:
- Substrate accumulation (PRPP). Because PRPP is not consumed in salvage reactions, it accumulates within cells. Excess PRPP acts as a potent stimulator of de novo purine synthesis. As a result, purine production increases multifold.
- Oxidation of unused bases. Free hypoxanthine and guanine, which cannot be converted into nucleotides, are oxidized by the enzyme xanthine oxidase. The end product of this oxidation is uric acid.
Clinical Consequences
The double metabolic hit (activation of de novo synthesis and the inability to salvage free bases) results in uric acid being synthesized in huge, toxic quantities. A severe form of hyperuricemia develops.
Main manifestations of the syndrome:
- Systemic complications of hyperuricemia: development of gout and formation of urate kidney stones.
- Severe neurotoxicity: accumulation of metabolites triggers profound neurological and psychiatric abnormalities.
Early diagnosis: primulest clinical sign of the syndrome can be noticed in the first months of life, even before frightening neurological disorders appear. Characteristic pink and orange spots appear on the infant's diapers. This phenomenon is caused by massive uricosuria: uric acid crystals are excreted in the urine and physically damage the epithelium of the urinary tract, leaving colored stains.