Introduction
The Lesch-Nyhan syndrome is a rare, genetically inherited disorder first described by Michael Lesch and Willian Nyhan in 1964 (Nyhan et al., 2007). It affects males, and is present immediately after the child is born. The prevalence of the disease is of about 1/100,000 of live births worldwide, with around 1/380,000 in Canada and in the United States, and 1/235,000 in Spain (Torres and Puig, 2007). It is caused by the enzyme hypoxanthine-guanine phosphoribosyl transferase going wrong and unable to function well. For this, other doctors call this as the ‘hypoxanthine-guanine phosphoribosyl transferase deficiency’ and it is usually characterized in humans by several mental, motor and neurological effects (Rasko and Downes, 1995).
Inheritance
The Lesch-Nyhan syndrome (LNS) is an x-linked recessive disease. Sex linked disorders usually show a bias on their occurrence depending on the gender of the individual. For an x-linked recessive disease to be expressed in a human, it must not be masked by any other dominant gene. For females, both of their X chromosomes must be of the recessive trait for them to be affected. Males, however, only have one X chromosome. One recessive trait is then sufficient for them to express the trait (Campbell, et al., 1999). X-linked recessive traits are then frequently expressed by males, which are passed on to them by their mothers (National Institutes of Health, 2007).
Females are then the carriers since, although they do not express the disease, one of their chromosomes carry the trait.By using a Punnet square, the chances of a child inheriting the disease can be predicted. Assume that a female carrier would bear child with a normal male.Female: XX’ (X’ being the recessive trait)Male: The child having the X’Y chromosomes would be a male expressing the LNS, while the one with the XX’ chromosomes would be a female carrying the disease. A son would then have a 50% chance of being affected by LNS and a daughter would also have a 50% chance of being a carrier. There is always an almost zero percent chance of a woman externally manifesting the disease since the daughter would always get one of her X chromosome from her normal father.CausesThe malfunction of the hypoxanthine-guanine phosphoribosyl transferase is the sole cause of the Lesch-Nyhan syndrome. This enzyme is responsible for the recycling of purines, a component of nucleic acids.
Three purine bases are involved in the synthesis of nucleic acids: adenine guanine and hypoxanthine. For each, there are different enzymes used for the salvaging of the bases. The enzyme adenine phosphoribosyl transferase is used to resynthesize adenines while the hypoxanthine-guanine phosphoribosyl transferase (HPRT) is for the hypoxanthines and guanines. This enzyme converts these bases into inosine monophosphate or guanine monophosphate, which are then both converted by other enzymes into triphosphates (ATP or GTP).
These would be used in the production of DNA and RNA (Rasko and Downes, 1995).Some studies found out that cells produce the HPRT enzyme at high concentrations but with very specific activity. Also, the recreation alone of purines is very low, suggesting that they are highly dependent on the work of the HGPT to recycle them (Fisch, 2003). Therefore, a lack of the enzyme would immediately cause accumulations of the purines, which would lead to several symptoms. Some biochemical changes were also found in patient, such as the decreased in the basal ganglia’s dopamine function which can be related to the disorder’s symptom of self-mutilation (Boulton et al., 1989).The dysfunctional enzyme is caused by a mutation in the HPRT1, the gene coding for the particular enzyme. This mutation happens on the long arm of the X chromosome, at the location Xq26 (Torres and Puig, 2007).
The mutation is of a point type, meaning small deletions or insertions of single nucleotides happened in this specific area of the gene (Rasko and Downes, 1995). One example of this mutation is the HPRT1-Kinston, which substitutes the amino acid asparagine for aspartic acid. The resulting mutant enzyme, although similar in concentration of a normal HPRT, is not efficient in the process of recycling the purines (Bhagavan, 2002).The disease is then said to be directly related to genetic disabilities, although there are small chances that the environment can cause such mutations.
However, these mutations would take many years to occur, so a LNS caused solely by the surroundings is not yet likely to happen.Diagnosis and SymptomsOne effect of this syndrome is the overproduction of uric acid, a disorder called hyperuricemia. This can be detected in young children by measuring the urate-to-creatinine ratio. A ratio greater than 2 suggests overproduction (Nyhan et al., 2007). The accumulation of this waste product (of chemical processes happening in the urine and blood) can cause gouty arthritis, kidney and bladder stones (U.S. National Library of Medicine, 2008).
This arthritis targets the joints, making the affected unable to walk and have a hard time in sitting down. They usually become dependent on wheel-chairs for life (U.S. National Library of Medicine, 2008). After a few years, affected children show signs of spasticity and several involuntary actions such as flexing and extensor twitching (Nyhan et al., 2007).Cognitive impairment and behavioral disturbances are also seen in patients of two to three years of age. Several behaviors such as lip and finger biting, banging of heads and limbs, and check scratching are common signs of self-mutilations caused by the disorder (Nyhan et al., 2007). Severe symptoms of mutilation would result to possible isolation and even teeth extraction.PrognosisIndividuals suffering from the disorder usually die at an early age, around the first 10 to 20 years due to renal and kidney failure (National Institutes of Health, 2007). Few patients are able to live longer, at around ages of 40.
However, respiratory attacks are common in these patients (Health A to Z, 2006).Treatment and ManagementThere are still no possible treatments that would totally eliminate the effects of the disorder. However, some techniques and medications are used to help alleviate and extend the life span of the patient. Control of uric production is one, and this is done by using the chemical allopurinol. This has shown effects on prevention of gouty arthritis, but none on the behavioral and neurological aspects of the disease (Nyhan et al., 2007). Renal stones, which causes the early death of patients, can be removed through surgery or lithotripsy while spasticity can be treated with baclofen or benzodiazepines (Nyhan et al., 2007).
Some couples undergo pregnancy testing that would determine the activity of HPRT in cultured amniotic cells. At some times when the fetus is found to be male, and with no record of HPRT activity, the pregnancy is therapeutically terminated (Queenan, 1999).Aside from medicines and procedures, genetic counseling also plays an important role in the managing of the disease. A woman can undergo tests to be certain if she is a carrier of the disease and learn about the chances of giving birth to an affected individual and passing on the trait.
References
- Bhagavan, N. V. (2002). Medical biochemistry.
- Florida: Harcourt/Academic Press.Boulton, A. A., Baker, G. B. & Juorio, A. V. (1989).
- Drugs as tools in neurotransmitter research. New Jersey: Humana PressCampbell, N. A., Reece, J. B. & Mitchell, L. G. (1999).
- Biology. California: Benjamin/Cummings.Fisch, G. S. (2003)
- Genetics and genomics of neurobehavioral disorders. New Jersey: Humana PressHealth A to Z. (2006).
