Introduction

Overview of lesch-nyhan syndrome (LNS)

Lesch-Nyhan Syndrome (LNS) is a rare disorder caused by a lack of the hypoxanthine-guanine phosphoribosyltransferase (HPRT) enzyme.¹ HPRT is important for recycling purines so they can be utilised by the body.¹ A severe deficiency in HRPT causes several symptoms, including hyperuricemia, kidney stones, self-injurious behaviour, and dystonia.¹ Self-injurious behaviour is considered one of the hallmark symptoms of LNS, with patients displaying pain and remorse after an episode. 

LNS is an X-linked recessive disorder, meaning that it is caused by a genetic mutation in the X chromosome. Specifically, the mutation occurs in the HPRT1 gene in the X chromosome,² which codes for the HPRT enzyme. This mutation results in a reduced level of the enzyme. Males have one X chromosome and one Y chromosome, whereas females have two X chromosomes. When a male inherits the mutated X chromosome from his mother, he develops the disease. Meanwhile, females have two X chromosomes and would need both to contain the mutation to develop the disease. Therefore, it is rarer for a female to develop LNS.

Current challenges in treatment 

Treatment options currently focus on symptom management. Hyperuricemia is treated with allopurinol, which reduces the overproduction of uric acid and therefore reduces the likelihood of long-term complications of hyperuricemia, such as gout (a type of arthritis causing sudden, severe joint pain) or urolithiasis (stones found in the urinary system).¹

Regarding neurological symptoms, since the pathophysiology is not fully understood, treatment options are limited and require further research.¹ Behavioural symptoms are treated with a combination of medical, physical, and psychiatric interventions, which are often tailored to a patient’s symptoms.¹

Importance of therapies

This article will explore novel and emerging therapies, such as dopamine modulation and deep brain stimulation, as well as their potential impact on the quality of life of patients with LNS.

Due to the rarity of the disease, treatment options are limited, and the symptoms experienced by the patients can be distressing for both the patients and their caregivers. It is essential to explore other avenues to improve the quality of life for patients with LNS.

Pathophysiology of lesch-nyhan syndrome

LNS is caused by a deficiency in the HRPT enzyme.¹ HPRT is an enzyme that recycles purines, from nitrogenous bases to purine nucleotides.³ Purine bases, which consist of adenine and guanine, form one of the two main types of nitrogenous bases found in DNA and RNA.⁴ Purine nucleotides are comprised of a purine base, a sugar, and one or more phosphate groups. They are involved in brain function and cell signalling.⁵

In a healthy individual, purine bases are recycled.¹ Hypoxanthine, a purine base, is recycled into inosine monophosphate (IMP).³ Guanine, also a purine base, is recycled into guanine monophosphate (GMP)³. IMP and GMP then form adenosine triphosphate (ATP) and guanine triphosphate (GTP), which are utilised for energy and cell function, respectively.⁶

In a patient with LNS, the HRPT does not recycle the purine bases, so they can be reused. This means that hypoxanthine and guanine are not recycled and are instead converted to uric acid.² This is why one of the key symptoms of LNS is hyperuricemia, the overproduction of uric acid.² Moreover, there are fewer purine nucleotides formed, which are required for brain function, and thus, this impairs the dopamine system.⁵ This impairment results in neurological and behavioural symptoms.¹

Dopamine modulation as a therapeutic strategy

Rationale for targeting dopamine

The deficiency in the HRPT enzyme, which leads to impaired purine metabolism, has a profound impact on dopamine dysfunction. Dopamine dysfunction contributes to the manifestation of motor, neurological, and behavioural symptoms.¹ Dysfunction affects the basal ganglia, a part of the brain responsible for both emotional regulation and motor control.²

Since HRPT enzyme deficiency results in both a loss of basal ganglion dopamine and its associated enzymes, pharmacological therapies could play a role in restoring the balance of dopamine and thereby reducing symptoms.²

Dopamine agonists, such as ropinirole, work by stimulating receptors in the brain to increase dopamine activity.⁸ This is mainly used in Parkinson’s disease, but could potentially play a role in LNS regarding the deficiency of dopamine.⁸ However, further research on the use of dopamine agonists to treat LNS is yet to be done.⁸

Deep brain stimulation (DBS) in LNS

Overview of DBS technology

Deep brain stimulation (DBS) is a procedure that involves using electrodes in different parts of the brain and recording its activity.⁹ The electrodes generate an electrical current, controlled by a pulse generator, which can be fitted onto the chest.⁹ DBS is used to treat neurological symptoms when other treatments have failed.⁹ Treatment is tailored to the individual, including the strength and duration of the pulse and the number of times DBS is repeated to provide relief from the patient’s symptoms.⁹ 

The target area of DBS in LNS could be the basal ganglia, as it is now known that dopamine dysfunction is found in that particular region.⁹ This could stimulate activity and potentially reduce symptoms of LNS, including self-injurious behaviour and motor symptoms.⁹ This would result in a profound improvement in quality of life, as well as enable other avenues of treatment to be considered.⁹

Summary

In conclusion, LNS is a rare disorder characterised by several behavioural, neurological, and physical symptoms. Since LNS manifests in infants, it can have a profound impact on parents and loved ones of patients. Since a single treatment cannot manage the symptoms of LNS alone, a combination of treatments and a tailored treatment plan is needed, specifically for each patient. Living with LNS can be challenging, and other treatment avenues are required, specifically those targeting neurological symptoms. Emerging treatments such as dopamine modulation and deep brain stimulation are promising avenues. However, further research is needed to consider how these treatments can be utilised for patients with LNS and enable them to live with a better quality of life.

FAQs

What is lesch-nyhan syndrome?

Lesch-Nyhan Syndrome (LNS) is an X-linked recessive disorder that is caused by a genetic mutation in the HRPT1 gene. This causes a deficiency in the HRPT enzyme, resulting in purines (the building blocks of DNA and RNA) not being able to be recycled and reused. This leads to a build-up of uric acid, leading to gout or kidney stones. Purines are also involved in brain function, so a limited formation of purines also affects the dopamine system, which results in behavioural symptoms and neurological symptoms.

Who is most affected by lesch-nyhan syndrome?

Males are most affected by LNS, as they only require one mutated copy of the X chromosome for the disease to manifest. They receive the mutated X chromosome from their mother and a Y chromosome from their father. Although it primarily affects males, there have been reports of females developing LNS.¹ The prevalence of the disorder is between 1:235,000 and 1:380,000.¹

What treatment options are available?

Treatment options are limited and mainly focus on reducing symptoms. Hyperuricemia, the buildup of uric acid, can be treated with allopurinol to prevent chronic complications. Neurological symptoms are more difficult to treat, but emerging treatments such as dopamine modulation and deep brain stimulation are promising and may reduce these symptoms. 

How is lesch-nyhan syndrome diagnosed?

Diagnosis is based on a number of factors. LNS often presents early in infants, with hyperuricemia being one of the first symptoms.¹ However, as a child grows older, cognitive delays can be observed alongside movement disorders.¹ Self-injurious behaviour is also a prevalent hallmark symptom of LNS.¹ Diagnosis is usually done by observing the presence of hallmark symptoms and testing for the presence of the HRPT enzyme.⁷ Molecular testing is also utilised to check for mutations in the HRPT1 gene.⁷