Lesch-Nyhan syndrome

Lesch-Nyhan syndrome
Name	Lesch-Nyhan syndrome
Synonyms	Lesch–Nyhan disease, Nyhan's syndrome, Juvenile gout
Inheritance	X-linked recessive
Onset	Early infancy
Symptoms	Hyperuricemia, dystonia, choreoathetosis, self-injurious behavior, gout
Complications	Kidney failure, joint damage, severe neurological disability
Duration	Lifelong
Causes	Mutation in the HPRT1 gene
Risks	Family history
Diagnosis	Clinical evaluation, enzyme assay, genetic testing
Differential	Cerebral palsy, other purine metabolism disorders
Treatment	Allopurinol, supportive care, behavioral management
Medication	Allopurinol, benzodiazepines
Prognosis	Reduced life expectancy, significant disability
Frequency	~1 in 380,000 live births

Lesch-Nyhan syndrome. It is a rare, inherited metabolic disorder caused by a complete deficiency of the enzyme hypoxanthine-guanine phosphoribosyltransferase (HPRT). First described in 1964 by American medical student Michael Lesch and his mentor, pediatrician William Nyhan, the condition is characterized by severe neurological dysfunction and behavioral abnormalities. The syndrome follows an X-linked recessive pattern of inheritance, almost exclusively affecting males, and presents a profound challenge for clinical management due to its complex multisystem involvement.

Signs and symptoms

The clinical presentation is marked by a triad of symptoms: severe hyperuricemia, devastating neurological impairment, and compulsive self-injurious behavior. Neurological signs typically emerge between three to six months of age and include severe hypotonia and developmental delay. This progresses to pronounced extrapyramidal symptoms such as dystonia and choreoathetosis, as well as pyramidal tract signs like hyperreflexia and Babinski signs. The most distinctive and distressing feature is the onset of self-mutilation, often involving biting of the lips, fingers, and hands. Hyperuricemia leads to complications reminiscent of gout, including kidney stones, nephropathy, and tophaceous deposits in joints, which can be observed even in early childhood. Many patients also exhibit dysarthria, dysphagia, and vomiting.

Genetics

The disorder is inherited in an X-linked recessive manner, with the causative gene, HPRT1, located on the X chromosome at locus Xq26. As such, the full syndrome is almost exclusively seen in males, while female carriers are typically asymptomatic due to X-inactivation. The vast majority of cases arise from de novo mutations, with no prior family history. Over 1,000 different mutations in the HPRT1 gene have been identified, including missense, nonsense, and frameshift mutations, as well as larger deletions. Genetic heterogeneity is significant, and the specific mutation can sometimes correlate with the severity of the neurological phenotype.

Pathophysiology

The primary biochemical defect is a virtually complete absence of functional HPRT enzyme, a key component of the purine salvage pathway. This enzyme normally recycles purine bases, hypoxanthine and guanine, into inosine monophosphate (IMP) and guanosine monophosphate (GMP). Its deficiency causes a shift toward de novo purine synthesis, leading to overproduction of uric acid and the characteristic hyperuricemia. The precise mechanism linking the purine metabolism disorder to the severe neurological and behavioral symptoms remains incompletely understood. Hypotheses center on dopamine depletion in the basal ganglia, particularly the striatum, as studies in patients and animal models, such as the HPRT knockout mouse, show profound alterations in dopaminergic pathways. Abnormalities in other neurotransmitter systems, including serotonin and glutamate, may also contribute.

Diagnosis

Diagnosis is suspected based on the classic clinical features, particularly the combination of developmental delay, movement disorders, and self-injury in a male infant. Biochemical confirmation is achieved by demonstrating elevated uric acid levels in blood and urine, and a definitive diagnosis is made by measuring HPRT enzyme activity in cells such as erythrocytes or cultured skin fibroblasts. Molecular genetic testing for mutations in the HPRT1 gene provides conclusive evidence and is useful for carrier testing and prenatal diagnosis. Differential diagnoses include other causes of dystonia like cerebral palsy, and disorders of purine metabolism such as Kelley–Seegmiller syndrome (a partial HPRT deficiency).

Treatment

Management is multidisciplinary and primarily supportive, as there is no cure. The hyperuricemia and its renal complications are treated effectively with high-dose allopurinol, a xanthine oxidase inhibitor, which reduces uric acid production. Febuxostat is an alternative. Neurological and behavioral symptoms are extremely difficult to manage. Physical restraints, dental extractions, and behavioral therapy are often necessary to prevent self-injury. Pharmacological interventions for movement disorders and behavior, such as benzodiazepines, baclofen, or dopamine agonists, have limited and variable efficacy. Supportive care includes nutritional support, often via gastrostomy, and management of complications like scoliosis and contractures.

Prognosis

The prognosis is poor, with life expectancy often reduced due to medical complications. While allopurinol treatment can prevent death from renal failure in childhood, the neurological disease is progressive and unremitting. Most individuals have profound intellectual disability and never achieve independent walking or speech. Death in early adulthood is common, frequently resulting from aspiration pneumonia, complications of immobility, or sudden unexpected death. Ongoing research into gene therapy and stem cell transplantation offers hope for future therapeutic strategies, but these remain experimental.

Category:Genetic disorders Category:Neurological disorders Category:Inborn errors of purine metabolism