40 Lesch-Nyhan syndrome

This disorder was first clearly defined by Lesch and Nyhan in 1964 when they reported two brothers with ‘hyperuricaemia, mental retardation, choreoathetosis and destructive biting’. Similar patients had been reported earlier by Catel and Schmidt (1959) and Riley (1960), but they had not investigated the biochemical abnormality in detail. A number of further reports followed and in 1967 Seegmiller et al. showed that the underlying biochemical defect was due to impaired activity of the enzyme hypoxanthine-guanine phosphoribosyl transferase (HPRT or HGPRT). Subsequently it was shown that HPRT was encoded by a gene on the X chromosome at position Xq26-27 (Pai et al. 1980) and the nucleotide sequence of this region was reported (Jolly et al. 1983). The observations made in these initial descriptions have been considerably expanded but not significantly altered.

Lesch-Nyhan syndrome occurs with an estimated prevalence of 1/380,000 live births in Canada, compared with about 1/235,000 live births in Spain (Crawhall et al. 1972, Torres and Puig 2007). The diagnosis of Lesch-Nyhan syndrome depends on clinical, biochemical, enzymatic, and molecular data:

Other abnormalities of purine metabolism and even HPRT activity have been found, but although they may be associated with hyperuricaemia and gout (Wyngaarden and Kelly 1983), they do not usually show the cerebral manifestations of the Lesch-Nyhan syndrome. Thus partial, as opposed to complete, defects in this enzyme do not usually produce neurological or behavioural symptoms (Kogut et al. 1970, Sweetman et al. 1978, Nyhan 1980) and it seems that only a small amount of residual HPRT activity is needed to prevent the neurological damage (Kelly et al. 1969). The precise genetic abnormality is not the same in all patients with Lesch-Nyhan syndrome. Considerable molecular heterogeneity has been demonstrated in HPRT (Kelley and Meade 1971, McDonald and Kelley 1971) and in the HPRT1 gene (see Sculley et al. 1992) from different Lesch-Nyhan patients. These structural variations in the enzyme, however, usually have the same functional result, namely complete absence of HPRT activity.

A few patients have been found who have lacked the tendency to self-mutilation and have shown variation in other neurological signs. These have included choreic and athetotic movements in the presence of normal intellect (Bakay et al. 1979, Gottlieb et al. 1982) and spastic tetraplegia with mental retardation (Schneider et al. 1976). Some of these cases may be due to variation in the structure of the defective HPRT and thus may be rare exceptions to the above general rule.

Purine ribonucleotides are involved in the formation of polynucleotides (DNA and RNA). In addition they take part in a number of normal cellular functions (McKeran and Watts 1978). Although purine ribonucleotides can be formed directly from small molecular precursors, the so-called purine salvage reactions constitute an alternative route (Fig. 40.1). This latter pathway utilizes a purine base and a purine phosphoribosyltransferase enzyme. These enzymes and reactions normally take place in all body tissues. There are two such enzymes. The first is HPRT and this changes hypoxanthine or guanine into their nucleotides, which are respectively inosinic and guanilic acids. This enzyme is absent in the Lesch-Nyhan syndrome, leading to a build-up of substrates and a reduction in these nucleotides. The second enzyme is adenine phosphoribosyltransferase (APRT) which converts adenine into its nuclotide adenylic acid. ARPT activity is actually increased in this syndrome (Nyhan 1977). The activity of these enzymes can be measured in most tissues but erythrocytes are usually used. These show no HPRT activity in the Lesch-Nyhan syndrome. Other tissues, including fibroblasts, may show what appears to be a small amount of activity of this enzyme. This may, however, be artifactual and be due to alternative metabolic pathways (Nyhan 1977).

The metabolic consequences of HPRT deficiency are two-fold. First, there is a deficiency of nucleotides. Some authors have considered this as being a possible cause of the neurological symptoms (Watts et al. 1982) while others have dismissed it (Kopin 1981). Second, there is an accumulation of hypoxanthine, xanthine, and guanine with shunting of metabolism into the formation of uric acid. Accumulation of these purine metabolites has also been regarded as a possible cause of the neurological features, although a particular substance has not been incriminated. As mentioned above, partial defects of HPRT lead to similar accumulations of purines and uric acid without the associated cerebral symptoms. Uric acid itself is not formed in the brain.

Patients with the Lesch-Nyhan syndrome usually have an elevated blood uric acid concentration and always show an increase in the total amount of uric acid excreted in the urine, often by as much as 300–400%. The urinary excretion of hypoxanthine is also markedly increased, although that of xanthine is usually normal.

At autopsy there are few, if any, morphological changes in the brain. Microcephaly is sometimes present but may reflect general failure of growth rather than specific cerebral maldevelopment. Occasional granules of probable uric acid and minor axonal swelling with demyelination have been reported (Sass et al. 1965), but these are not a regular feature (Crussi et al. 1969, Mizuno et al. 1976), and they are probably attributable to uraemia. In particular the basal ganglia are histologically normal (see Table 40.1A).

It seems likely that the neurological abnormality is at the biochemical rather than morphological level. Cerebrospinal fluid (CSF) shows reduced levels of the dopamine metabolite homovanillic acid and the noradrenergic metabolite 3-methoxy-4-hydroxyphenylethylene glycol (Silverstein et al. 1985, Jankovic et al. 1988). The concentration of the serotonin metabolite 5-hydroxy-indolacetic acid has been reported to be normal (Silverstein et al. 1985) and increased (Jankovic et al. 1988). The athetoid posturing and choreiform movements suggest involvement of the basal ganglia. The levels of HPRT in the basal ganglia are higher than elsewhere in normal brain and cerebral levels; in general, three or four times greater than in non-neurological tissues. In the Lesch-Nyhan syndrome, HPRT activity in the brain is barely detectable whereas ARPT is normal (Lloyd et al. 1981). The circumstantial evidence suggesting that the Lesch-Nyhan syndrome might have a particularly severe effect on the basal ganglia is supported by biochemical findings in autopsy material. The caudate nucleus, putamen, and nucleus accumbens show diminished levels of dopamine and its metabolite homovanillic acid, as well as reduced activity of the enzymes involved in dopamine formation (dopa decarboxylase and tyrosine hydroxylase). These levels are reduced by approximately 10–30%. The concentration of dopamine in the substantia nigra, however, is normal. Choline acetyltransferase, the enzyme involved in the formation of acetylcholine, is also reduced in the neostriatum, while glutamic acid decarboxylase activity, which reflects GABA function, is normal. The levels of serotonin and its major metabolite 5-hydroxyindoleacetic acid are not reduced and may even be slightly elevated. Noradrenaline levels have not been so extensively documented but have been normal where they have been studied in the nucleus accumbens and substantia nigra (Lloyd et al. 1981, Rassin et al. 1982). An hypothesis that has been proposed to explain the discrepancy between reduced concentration of dopamine in the neostriatum and normal levels in the substantia nigra is that patients have diminished arborization of dopamine nerve terminals without loss of their cells of origin.

The picture is further complicated as various purine derivatives may themselves act as neurotransmitters (Burnstock 1975), influence neurotransmitter release (Fredholm and Hedqvist 1980), or alter receptor sensitivity (Rodbell 1980). Derivatives of guanosine and adenosine may be particularly involved. In addition to the changes in neurotransmitters, the concentrations of a range of amino acids are reduced in the brain. The most substantial decreases have been in threonine, serine, valine, isoleucine, lysine, and arginine. Not all amino acids are decreased and a few, such as glutamine and urea, may be increased. It is uncertain if these changes are of aetiological significance in the production of the neurological features or if they are secondary to malnutrition or renal dysfunction (Rassin et al. 1982).

Autopsy findings outside the nervous system are largely confined to changes produced by the overproduction and accumulation of uric acid. There is renal damage with deposition of urate, renal stones, and secondary urinary infection. The systemic effects of renal failure and secondary hypertension may be apparent. Microscopic accumulations of uric acid can be found in many tissues and macroscopic tophi accumulate in the usual sites. There may be gouty arthritic changes. Retarded development with emaciation is often present.

As mentioned above, the disorder is passed through a heterozygous female carrier and is X-linked. Fibroblasts grown from such heterozygous carriers show a mosaic pattern in keeping with random X-inactivation of the HPRT1 locus, but this is not the case with their erythrocytes in which inactivation is non-random and selectively involves the defective gene. Such female carriers thus have normal HPRT activity in red cells.

Initial attempts at pre-natal diagnosis utilized the absence of HPRT activity in chorionic villi (Gibbs et al. 1984), but subsequently direct analysis of fetal genomic DNA has been utilized in families in which the mutation has been defined (Gibbs et al. 1990).

The HPRT-encoding gene is located in the region of Xq26-27 and consists of nine exons and eight introns totally 44 kb (Patel et al. 1986). The mRNA which it transcribes contains a protein-encoding region of 654 nucleotides (Jolly et al. 1983). Abnormalities of this gene in Lesch-Nyhan syndrome are very heterogeneous and over 70 different mutations have been described (see Sculley et al. 1992). They fall into two categories, namely a change in the gene

that affects the size of the translated protein and a single base mutation that results in a single amino acid substitution without altering the size of the protein. The three dimensional structure of the HPRT protein is uncertain and it is thus not known exactly how these mutations affect its function.

The Lesch-Nyhan syndrome is inherited as an X-linked recessive disorder and thus occurs only in boys and is transmitted by asymptomatic heterozygote females. There have been rare instances in which females have been clinically affected (Hara et al. 1982, Ogasawara et al. 1989, Yukawa et al. 1992) and this has been attributed to failure of the Lyon effect in which the normal X-chromosome suppresses the defective one.

There does not appear to be any racial or ethnic predilection for the disease. The exact incidence is uncertain and estimates have varied from between 1:100,000 to 1:380,000 births (Crawhall et al. 1972, Nyhan 1981, Torres and Puig 2007).

Although hypotonia may be noticed from birth (Christie 1982, Watts et al. 1982) this is exceptional and most patients appear normal until appoximately 6–8 months of age (Table 40.1B). The presence of uric acid crystals as ‘orange sand’ may be noticed in the nappies from earliest times but is liable to be overlooked or misinterpreted. These infants tend to be irritable and ‘colicy’. It has been speculated that this may be due to urate accumulations in the renal drainage system, with resultant colic. The clinical picture, however, soon becomes dominated by neurological features and it is usually only at this stage that definite abnormality is noted (Table 40.2). Thus, if head control and sitting have developed these are lost and the child requires support. Hypotonia, even if absent or missed earlier, may be apparent at this stage. Conversely there may be rigidity of the ‘lead pipe’ type and this can develop over a few months. The most striking clinical manifestation is the appearance of involuntary movements (Fig. 40.2). These take the form of athetosis or dystonic posturing and episodes of torticollis, retrocollis, or opisthotonus may become apparent. The upper limbs are characteristically extended at the elbow with hyperpronation of the forearm, flexion of the wrist, and extension of the fingers. Inversion of the lower limbs with extension at the hips and knees plus plantar flexion of

the feet is typical. Adduction at the hips with scissoring may develop and eventually dislocation of the hips may occur. Choreic or ballistic movements may be superimposed, but generally dystonia tends to dominate and it is not infrequent for these children to be diagnosed intially as having ‘athetoid cerebral palsy’ (Christie et al. 1982). The involuntary movements are exaggerated by excitement or frustration. These children do not learn to walk or stand unsupported.

Dysarthria is usually prominent and largely due to the athetosis and chorea. This, combined with a limited vocabulary, makes communication difficult and frustrating. Speech is limited to single words in approximately half of patients (Christie et al. 1982). Dysphagia probably has a similar cause and is of such a degree as to produce major feeding difficulties. Vomiting is not infrequent. In the terminal stages, aspiration and pneumonia are common.

The frequency of cortico-spinal tract involvement has been debated. Spasticity and brisk tendon reflexes have been reported in the majority of patients in some series, while extensor plantar responses have been present in approximately a half (Christie et al. 1982). Watts et al. (1982), however, claimed that misinterpretation of the physical signs is common due to involuntary movements and that definite cortico-spinal involvement is relatively uncommon, occuring only in the presence of a myelopathy. They described two patients with recurrent attacks of cervical myelopathy producing tetraparesis, cortico-spinal tract signs, variable sensory loss below the level of the lesion, and incontinence. These episodes lasted a day or more and in one case became permanent. Bulbar involvement also occurred in one patient. Autopsy in a single case did not reveal any spinal cord abnormality and the attacks were attributed to spinal and medullary ischaemia, possibly secondary to cervical hypermobility caused by the involuntary movements.

One of the most striking aspects of the disease is aggressive behaviour. This is usually self-directed and will regularly result in destruction of tissue unless protective measures are taken. Biting of the lips (Fig. 40.3) or fingers is common and may result in partial amputation. Scratching or banging the face with the hands is also common and crashing the head against neighbouring hard surfaces may occur. Such actions are purposeful and not the result of involuntary movements. Aggression may also be directed at others with biting, punching, scratching, or kicking if the opportunity arises. Verbal abuse is common and some patients seem to develop purposeful vomiting as a means of aggression. Although self-destructive behaviour can occur in a number of other conditions associated with mental retardation, this is not as regular and severe as in the Lesch-Nyhan syndrome. Perhaps the most curious aspect is that these patients are distressed and terrified by their behaviour. They will scream with pain and be frightened if put in a situation where they can injure themselves. The tendency to self-injury tends to decline over the age of 10 years (Mizuno 1986) and older children may learn to partially control the temptation to injure themselves by such manoeuvres as sitting on their hands or placing them behind their backs. They only seem content when physically restrained so they cannot injure themselves and become terrified when such protection is removed.

Dysarthria, difficulty with using the limbs, and aggression make psychometric assessment difficult. None-the-less, most patients are mentally retarded. The mean intelligence quotient in nine children was 58, with a range from 25 to 101. Only about half can construct sentences and only occasional patients can read or become toilet trained (Christie et al. 1982).

Other neurological features include autonomic overactivity with abnormally dilated pupils, excessive sweating, tachycardia, and high skin conductance (Watts et al. 1982). Whether this represents specific autonomic dysfunction or a state of increased arousal and anxiety is uncertain. Although convulsions are not common they have been reported in a small number of patients (Christie et al. 1982). Cerebellar signs are not a feature (Watts et al. 1982). Table 40.1C summarizes the neurological features observed in 220 patients as presented by Jinnar et al. (2006).

Underdevelopment is usual and most patients are below the third percentile for height and weight, with the latter being affected to a disproportionately greater degree. Weight loss may result from increased activity due to the involuntary movements and loss of purines in the urine as well as from inadequate caloric intake. Bone age is also retarded and head circumference is reduced (Christie et al. 1982, Watts et al. 1982). Renal stones, with episodes of renal colic and haematuria, are a consequence of the overproduction of uric acid. Impaired tubular function with diminished concentrating ability is the initial renal lesion, but this is subsequently followed by glomerular involvement and eventual renal failure. Dehydration with or without accompanying systemic infection may precipitate acute anuria due to obstruction of the urinary tract by uric acid crystals. Tophi may appear in the usual cutaneous sites, particularly on the ears. There may be attacks of acute gouty arthritis. Congenital abnormalities have been described in a small number of cases, including testicular absence, undescended testes, deformation of the pinna, and imperforate anus (Christie et al. 1982, Watts et al. 1982). The fully developed clinical picture is so characteristic as to allow diagnosis almost at a glance. A small, emaciated child is strapped into a wheel chair and the limbs are restrained to prevent injury. There is partial destruction of the lips or digits and athetotic posturing.

The disorder is gradually progressive. Untreated patients may die within the first few years. Even with treatment, survival into adult life is unusual. Death is usually due to pulmonary infection or renal failure with superadded emaciation. Sudden unexplained death has been reported (Mizuno 1986).

Mild anaemia is not uncommon. This may be macrocytic and megaloblastic or microcytic and hypochromic. Impaired nutrition with or without haematuria may be involved (Manzke 1967, Van der Zee et al. 1970, Watts et al. 1982).

Blood uric acid levels are usually about double the upper limit of normal, although occasionally they may be normal. In these latter cases uric acid excretion is sufficiently high to prevent accumulation in the blood. The urinary excretion of uric acid is always elevated and may be up to three or four times greater than the upper limit of normal. In order to relate urinary excretion to body mass, values are often expressed as the amount of uric acid excreted per kilogram of body weight in 24 hours. The rate of urinary uric acid to creatinine enables comparison with muscle bulk and is applicable to random urine samples. Generally patients excrete between 40 and 70 mg of uric acid/kg/24 hours or between 3 and 4 mg/mg of creatinine (Nyhan 1977). Routine plasma biochemistry is otherwise usually normal unless it shows changes of renal impairment.

Although the clinical picture combined with evidence of uric acid overproduction is highly suggestive of the diagnosis, absolute confirmation depends on demonstration of absent HPRT activity. This is usually performed on erythrocytes. As mentioned above, it can be performed on other tissues and in these there may be evidence of slight residual enzyme activity although this may really be due to alternative metabolic paths.

Radiology has little to add although it may reveal reduced bone age (Christie et al. 1982). CT brain scans and electroencephalograms have been reported as normal (Watts et al. 1982). Jinnah et al. (2006) presented a work-up of 44 patients and 122 prior reports that included a total of 254 patients. Among these, imaging data were available as CT for 22 and as MRI for 25 patients. They found that most scans were normal and ‘atrophy’ was present in up to a third. They, however, concluded that although the term ‘atrophy’ was often used to describe cerebral volume loss, concurrent enlargement of the cranial sinuses in one imaging study suggested that the volume loss reflected poor brain development rather than a degenerative process (Harris et al. 1998). Therefore, they felt the term ‘dystrophy’ might be more appropriate.

CSF examination is not usually carried out. The CSF concentration of hypoxanthine is increased, while that of xanthine and uric acid is normal (Sweetman 1968). The neurotransmitter changes are mentioned under ‘Chemical and pathological changes’.

Muscle biopsy has shown a reduction in diameter of type I and type II fibres, with coarse mitochondrial clumping in the former (Watts et al. 1982). These findings are minor and may even be related to cachexia. There is little evidence to support a specific myopathy.

Different tests have to be used to detect female heterozygote carriers of the disease. The haemopoetic system of such carriers is derived from a single clone of cells containing normal amounts of HPRT. Thus they show normal plasma uric acid levels and normal levels of HPRT in erythrocytes and leucocytes. Fibroblast cultures, however, will demonstrate the mosaic nature of these carriers with two populations of cells, one positive and the other negative for HPRT activity. There are various methods of detecting these abnormal cells including radioautography (Rosenbloom et al. 1967) and pharmocological cell selection. The latter test depends on the fact that HPRT converts 6-mercaptopurine and related analogues into cytotoxic compounds. Thus, in mixed cell populations, cells containing this enzyme die while those without it survive (Felix and DeMars 1971). The usual test, however, does not involve cell culture and consists of analyzing the cells from individual hair follicles. The cells from a single follicle are all of the one type and contain either normal amounts or no HPRT. By collecting and analyzing sufficient follicles, the presence of a mosaic pattern can be reliably detected or excluded. Culture techniques need only be resorted to if brittle hair prevents plucking sufficient follicles (Francke et al. 1973).

The techniques enabling detection of mutations to the HGPRT gene have largely replaced such methods of detecting carriers, but because of genetic heterogeneity a large amount of work may need to be done to establish the precise defect in any one pedigree.

The uric acid overproduction can be easily controlled by the administration of allopurinol, which inhibits xanthine oxidase, leading to accumulation of xanthine and its precursors. Although this prevents tophus formation, arthritis, renal impairment, and urinary calculi, it does not affect the neurological aspects of the disease. Usually 200–400 mg of allopurinol daily is adequate but occasionally up to 800 mg daily may be required (7–40 mg/kg/day) (Christie et al. 1982). In people with normal HPRT levels, who are treated with allopurinol, the excretion of xanthine is greater than that of hyopxanthine and occasionally xanthine stones are formed in the urinary tract. This is not a problem in the Lesch-Nyhan syndrome as the major excretion is as the more soluable hypoxanthine (Sweetman and Nyhan 1967). A good fluid intake should be maintained, particularly at times of intercurrent illness. Alkali administration is also effective in preventing renal complications from high levels of uric acid excretion, but this therapy has been supplanted by the more effective allopurinol. It is important to erradicate urinary infection, but this is less of a problem if uric acid stone formation is prevented.

A number of therapies have been tried in an attempt to improve the neurological and behavioural manifestations of this disease. These include the serotonin precursor 5-hydroxytyptophan, dopamine receptor blockers (e.g. haloperidol, fluphenazine, and pimozide), l-dopa, and benzodiazepines (Anderson et al. 1976, Christie et al. 1982, Watts et al. 1982). None of these agents has been useful in long-term management, although 5-hydroxytryptophan may produce transient diminution in self-destructive tendencies (Nyhan et al. 1980). l-dopa may increase dystonia (Watts et al. 1982). The same has been claimed of dopamine receptor blockers (Goldstein et al. 1985). Bone marrow transplantation has not been successful in reversing established neurological features (Nyhan et al. 1986) and results in early disease have been inconclusive (Endres et al. 1991).

Behavioural modification techniques have been reported to be helpful in preventing self-inflicted injuries (Anderson et al. 1976), but convincing long-term benefit has not been demonstrated and some authors have found difficulty in obtaining therapeutically useful results (Gilbert et al. 1979). Almost invariably physical restraint is required to prevent injury. This involves strapping the hands down or covering them with padding (‘boxing gloves’). Neighbouring hard surfaces have to be padded and frequently teeth have to be extracted. Mouth guards have been advocated to prevent lip damage (Sugahara et al. 1994). Botulinum toxin may be considered as treatment in cases of severe tongue protrusion (Schneider et al. 2006). As children become older they may develop some ability to control these destructive impulses and protective measures may be able to be cautiously reassessed.

Speech difficulties may require the assistance of a speech therapist. Usually alternative ways of communicating have been developed as speech may not be able to be improved significantly. Special attention to feeding is needed to prevent emaciation. Physiotherapy may help in preventing contractures. Special educational needs may have to be met in those few patients whose intelligence is relatively preserved.

Genetic advice should be given to all female heterozygotes. Each pregnancy carries a 1 in 4 risk of producing a child with the disease. With culture of cells from the amniotic fluid or chorionic biopsy, an estimation of HPRT makes an antenatal diagnosis possible. Combined with foetal sexing and therapeutic abortion these techniques have allowed heterozygote mothers to have unaffected children (Gibbs et al. 1984). Fetal genetic analysis can be used instead of the above if the precise abnormality is known (see under ‘Genetics’).