Clinical Description
Affected Males
Pathogenic variants in HPRT1 are almost always associated with clinical evidence for overproduction of uric acid (hyperuricemia, nephrolithiasis, and/or gouty arthritis) as well as a range of neurobehavioral phenotypes. Historically, three phenotypes recognized in the spectrum of HPRT1 disorders were Lesch-Nyhan disease (LND) at the most severe end, the intermediate phenotype HPRT1-related neurologic dysfunction (HND), and HPRT1-related hyperuricemia (HRH) at the mild end (see Table 1). In reality, these neurobehavioral phenotypes cluster along a continuum from severe to mild [Fu et al 2014].
Overproduction of uric acid is present from birth. The serum uric acid concentration is usually elevated. This finding is often missed because hyperuricemia can be mild; rarely, serum uric acid concentration can be normal.
If untreated, overproduction of uric acid leads to precipitation of uric acid crystals in the urinary system. Crystals appear as an orange sandy material in the diapers. Larger stones may appear as "gravel" in diapers. Larger stones may be difficult to pass. Stones may cause hematuria and increase the risk for urinary tract infections. While crystals or gravel in the diaper may be an early feature, their significance is often not appreciated for years.
Another potential consequence of untreated overproduction of uric acid is gouty arthritis caused by precipitation of uric acid in the joints. Complex precipitates mixed with proteins may form visible swellings known as tophi. Gout is uncommon in children and typically develops long after other manifestations are present.
If overproduction of uric acid is not treated, renal failure is common. Some individuals develop renal failure even with treatment.
Lesch-Nyhan Disease (LND)
LND is characterized by uric acid overproduction, motor dysfunction resembling severe cerebral palsy, intellectual disability, and self-injurious behavior.
Motor dysfunction. Boys with LND typically have normal prenatal and perinatal histories. The most common initial findings during the first year of life are hypotonia and delayed motor skills. Children with LND fail to reach normal milestones such as sitting, crawling, and walking [Jinnah et al 2006].
Within the first few years of life, abnormal movements emerge. The characteristic feature in all individuals is severe action dystonia [Jinnah et al 2006]. Children with LND may also develop opisthotonos, choreoathetosis, and sometimes ballismus. Approximately one third develop corticospinal signs such as spasticity, hyperreflexia, and clonus. The neurologic picture resembles dyskinetic or athetoid cerebral palsy.
The motor disability is sufficiently severe that children with LND do not walk and are usually confined to a wheelchair. Most need assistance with feeding and hygiene.
Cognitive disturbances. Most individuals with LND are cognitively impaired. The degree of impairment is difficult to assess because the neurobehavioral problems make interpretation of standardized tests difficult [Schretlen et al 2001].
In general, individuals with LND do not have severe intellectual disability. Formal psychological testing typically yields scores in the mild-to-moderate range of dysfunction.
Behavioral abnormalities. Almost all affected individuals develop the hallmark feature of recurrent self-injurious behaviors [Schretlen et al 2005]. This problem most often develops between ages two and four years. In some instances, it may begin as early as the first year of life, or it may be delayed until the late teenage years.
Self injury most often involves biting of the fingers, hands, lips, and cheeks [Robey et al 2003]. It may also involve banging the head or limbs against hard objects. The severity and patterns of this behavior evolve over time, but they generally occur daily.
In addition to self-injurious behavior, affected individuals often have other difficult behaviors including impulsiveness, aggressiveness, oppositional defiance, recurrent vomiting, spitting at others, and coprolalia.
Other manifestations can include the following:
Life expectancy. LND is not a progressive neurodegenerative disorder [Göttle et al 2014]. If management of manifestations is effective, individuals with LND may survive into the second to fourth decades of life.
The most common causes of death include pulmonary failure (from pneumonia or recurrent aspiration from dysphagia), renal failure, or sepsis from different causes. Sudden unexplained death in an otherwise well-tended individual may also occur [Neychev & Jinnah 2006].
Heterozygous Females
Heterozygous females are virtually always clinically normal without evidence for motor or cognitive deficits. Production of uric acid may be slightly elevated, and some heterozygous females may develop gout when they are older [Puig et al 1998].
Rarely the LND phenotype has been observed in heterozygous females as the result of skewed (nonrandom) X-chromosome inactivation of the chromosome bearing the normal HPRT1 allele. (Of note, discordant phenotypes were observed in monozygotic twin girls [1 normal; 1 with LND] due to skewed X-chromosome inactivation [De Gregorio et al 2005].)
Presence of biallelic HPRT1 pathogenic variants has also been reported in females [Fu et al 2014].
Genotype-Phenotype Correlations
Because the amount of residual HGprt enzyme activity correlates with the severity of the phenotype [Fu et al 2014, Fu et al 2015], some general genotype-phenotype correlations are seen.
LND is associated with complete or near-complete loss of enzyme activity from the following types of HPRT1 variants:
Loss-of-function variants such as nonsense variants or frameshift variants due to small or large insertions or deletions
Missense variants with absent or near-absent enzyme activity. These variants may reduce enzyme activity through different mechanisms including impaired kinetic properties, impaired dimerization of HGprt subunits required for functional activity, and poor protein stability.
Most (but not all) splice site variants that result in major changes in the protein coding region
HND is associated with HPRT1 variants that result in small amounts of residual enzyme activity (usually 2%-8% of normal):
Most HND-causing variants are missense variants.
Occasionally variants affect splice sites, leaving a small proportion of correctly spliced transcripts.
Rare variants are duplications, which may rarely revert to normal to varying degrees in different tissues, producing somatic mosaicism with varying residual HGprt enzyme activity [
Fu et al 2014].
HRH is associated with missense HPRT1 variants that retain the highest level of abnormal enzyme activity (usually >8% of normal):