Introduction

Down’s syndrome (DS) or trisomy 21 is the most common form of intellectual disability with an overall prevalence of 1.42/1000 that increases significantly with maternal age (Bray et al., 1998). Immunological abnormalities like functional impairments in fagocytes with low chemotactic ability as well as the depressed production of cytokines are intrinsic to the chromosomal disorder and result in an increased morbidity and mortality from infectious diseases (Ugazio et al., 1990). In addition, DS is associated with hypothyroidism in 20–40% of the patients and with neuropsychiatric disorders of which depression and Alzheimer-type dementia are the most frequent (Lovell and Reiss, 1993; Prasher, 1999; Bush and Beail, 2004).

Since the activity of the enzyme cystathionine β-synthase (CBS), which is encoded on chromosome 21 (21q22.3), is associated with the level of intelligence and with neuropsychiatric disorders (Abbott et al., 1987; Barbaux et al., 2000), the concentration of CBS was recently investigated in postmortem brains of DS persons (Ichinohe et al., 2005). It was found that the concentration of CBS was three times higher as compared to brain levels of normal individuals. CBS catalyzes the conversion of homocysteine and serine into cystathionine. The latter dipeptide is further metabolized into the amino acids cysteine and eventually to taurine. In humans, the sole source of homocysteine is through dietary intake of the essential amino acid methionine (Selhub et al., 1999; Ward et al., 2000).

Thus, the over-expression of CBS in persons with DS may alter homocysteine metabolism resulting in a metabolic imbalance such that folate-dependent resynthesis of methionine is compromised, and may be in favour of the transsulfuration pathway, leading eventually to increased levels of taurine (Huxtable, 1992). Due to the dosage effect of CBS, a decrease in plasma concentration of serine was found in DS persons. In addition, an increase in plasma lysine, which was explained by generalized premature aging in DS, was observed (Mircher et al., 1997).

Another amino acid abnormality is a decrease of systemic glutamate uptake as assessed by measuring glutamate uptake in platelets and fibroblasts from DS persons (Begni et al., 2003). The authors suggest the use of this peripheral model as a biochemical ex vivo marker of a central glutamatergic dysfunction. Excessive glutamatergic stimulation as a consequence of glutamate uptake deficits could be responsible for neuronal suffering, excitoxicity and cell death, and may play a key role in the pathophysiology of neuropsychiatric disorders (review: Van der Heijden et al., 2004).

Apart from the abovementioned amino acid abnormalities, impaired immunological function in DS persons results in increased serum neopterin levels (Mehta et al., 2005). Neopterin is not only an immune activator for the cell-mediated immune response, but also induces or enhances cytotoxicity and exhibits antioxidant properties (Hamerlinck, 1999). This observation is consistent with the increased susceptibility of DS persons to bacterial and viral infections (Cossarizza et al., 1990).

No studies have been reported on the measurement of peripheral precursors or metabolites of the neurotransmitters serotonin and dopamine in DS. Since serotonin is involved in brain growth and maturation, and experimentally induced high dopamine turnover may give rise to toxic metabolites and neurodegeneration (Ogawa et al., 1993; Alexander et al., 1997; Kupsch et al., 2001), measurement of peripheral correlates of central serotonin and dopamine metabolism is also warranted. Since at least 40% of circulating homovanillic acid (HVA) originates form the brain, the plasma HVA concentration is thought to be a fairly good indicator of the changes in dopamine metabolism in the brain (Kendler et al., 1982). In addition, plasma 5-hydroxyindoleacetic acid (5-HIAA) is suggested to be an indicator of brain serotonergic activity (Meltzer, 1989).

The present study was designed to investigate peripheral parameters that reflect changes in homocysteine metabolism, immune function and monoaminergic neurotransmission in a group older non-demented healthy persons with DS. To this end, especially the sulphur containing amino acids methionine and taurine as well as serine, glycine, glutamate, lysine, tryptophan and the other large neutral amino acids (LNAA) were measured. The ratio of tryptophan to LNAA is a fairly good indicator of central serotonin synthesis. Concentrations of the dopamine and serotonin metabolites HVA and 5-HIAA were determined as well as the levels of the immune activation marker neopterin.

Materials and methods

Subjects

Over a period of 4 years a community based sample of 505 DS persons aged 45 year and older from the Southern and the South-Western parts of the Netherlands was collected. The study protocol was approved by the Medical Ethetical Committee of the Erasmus University Medical Centre in Rotterdam, The Netherlands (protocol number: MEC 185.974/1999/202). In addition, the ethical committee of the local institutions provided approval. Written informed consent was obtained from the legal representatives. Details of the study population and the used screening instruments are presented elsewhere (Coppus et al., 2006).

For the present study persons were excluded in case of any relevant somatic comorbidity, including hypothyroidism and epilepsy, or the presence of a depressive disorder and signs of dementia. All persons were free of medication, including the use of folic acid, and did not smoke. The study group comprised 48 persons with DS (female: 11; male: 37; mean age: 50 ± 4.8 years) of whom 2 had to be excluded because of laboratory problems, and was compared to a group of 48 age and sex matched healthy controls (female: 16; male: 32; mean age: 50.2 ± 9.1 years).

Biochemical analyses

Plasma amino acids are analyzed by high-performance liquid chromatography (HPLC) using pre-column derivatization with o-phthaldialdehyde (Fekkes et al., 1995). The tryptophan-ratio (Trp-ratio) is calculated by dividing the total tryptophan level by the sum of the other large neutral amino acids (LNAA), i.e. valine, isoleucine, leucine, tyrosine and phenylalanine, which compete for the transport of tryptophan through the blood- brain barrier. The tyrosine-ratio (Tyr-ratio) is calculated in the same manner by substituting tryptophan for tyrosine. The concentrations of the monoamine metabolites 5-HIAA and HVA are analyzed by HPLC and electrochemical detection (Fekkes et al., 1997). The concentration of neopterin in plasma is determined as previously described (Hoekstra et al., 2001).

Statistical analyses

For statistical comparisons between persons with DS and controls, the unpaired t-test was used because all data sets showed a normal distribution. Normality of the distribution was tested with the Kolmogorov-Smirnov test. A value of p<0.05 was considered to be statistically significant. In case of finding statistically significant values for biochemical parameters, discriminant analysis was performed in order to assess the potential for further differentiating DS persons from controls, and to establish their relative contribution to the group difference.

Results

As can be inferred from Table 1, the concentrations of methionine, glutamate and all LNAA are significantly decreased in the DS group as compared to the control group. Of the LNAA, tryptophan shows the lowest significance. The concentrations of taurine and glycine are significantly increased in DS persons as compared to controls, whereas the level of glutamate is significantly decreased. In addition, the Trp-ratio but not the Tyr-ratio, is significantly higher in the DS group compared to the controls.

Table 1 Plasma levels (mean ± SD) of biochemical parameters in DS persons and controls
Full size table

With respect to neopterin, values in the DS group are significantly increased as compared to the control group. Furthermore, plasma HVA concentrations are significantly higher in the DS group, whereas no differences are found regarding 5-HIAA concentrations.

Finally, discriminant analysis (stepwise method) of the biochemical parameters with p-values <0.001 (valine, isoleucine, leucine, methionine, taurine and Trp-ratio) results in a discriminant function with valine, methionine and taurine as major predictors. Classification results are good: 84.8% of the DS persons were correctly classified as were 83.3% of the controls. As a whole 16% was wrongly classified.

Discussion

In the present study biochemical parameters related to neurotransmission and immunological function were investigated in a group of older persons with Down’s syndrome. It was found that the spectrum of amino acids shows widespread differences in the DS group as compared to controls. More specific, the amino acids valine, methionine and taurine were demonstrated to differentiate persons with DS from controls. Nearly all values of amino acids are lower in the DS group, which cannot be explained from dietary effects since all persons were provided with well balanced meals. Besides, Ciaccio et al. (2003) reported that dietary measures over one year have an only marginal influence on the plasma concentration of amino acids in children with DS.

With respect to the CBS-related parameters, plasma methionine was found to be significantly lower, while taurine was higher. In addition, plasma serine was slightly decreased in DS persons. These observations are in accordance with an over-expression of the CBS-gene in DS (Chadefaux et al., 1985; Progribna et al., 2001; review: Chango et al., 2002) and are also in line with results concerning serine and taurine plasma concentrations as reported by Mircher et al. (1997). Although homocysteine was unfortunately not measured in this primarily epidemiological study, these data corroborate an altered homocysteine metabolism in persons with DS and are as such in line with the lowered plasma concentration of homocysteine reported by Pogribna et al. (2001). Other investigators, however, reported increased concentrations of homocysteine (Brattstrom et al., 1989; Guéant et al., 2005). As reviewed by Townsend et al. (2004), the sulphur containing amino acids methionine, homocysteine and taurine are involved in the maintenance and integrity of cellular systems by influencing cellular redox state and cellular capacity to detoxify toxic compounds, free radicals and reactive oxygen species. An imbalance of these amino acids in DS persons may indicate neuropathological changes in this group.

The increased Trp-ratio in the DS group suggests an enhanced availability of tryptophan for the synthesis of serotonin in the central nervous system. Whether central serotonergic neurotransmission is increased in DS persons cannot be concluded from this study, especially since the plasma concentration of 5-HIAA is not changed. The latter parameter, however, is only a very weak and indirect measure of central serotonergic activity (Lambert et al., 1995).

On the other hand, plasma HVA concentrations are signifi- cantly increased, which is an indication of a higher turnover of dopamine that was reported previously also in cerebrospinal fluid studies in persons with DS (Kay et al., 1987; Schapiro et al., 1987).

Neopterin is increased in the DS group as compared to controls. This finding is in agreement with the observations by Metha et al. (2005) who additionally reported that increase of neopterin is not correlated with age and sex. High concentrations of neopterin as seen in DS is consistent with the impaired immune function that was reported in persons with DS (Nespoli et al., 1993; Park et al., 2000).

In conclusion, in the present study of persons with DS without any signs of dementia, somatic and/or neuropsychiatric comorbidity and free of any medication, extensive biochemical abnormalities are found suggesting a compromised homocysteine metabolism, an activated cellmediated immune response and an enhanced turnover of dopamine. Interestingly, a few peripheral biochemical parameters appeared to have a marked potential to differentiate between DS persons from controls.