One-carbon pathway metabolites are altered in the plasma of subjects with Down syndrome: Relation to chromosomal dosage

Abstract

Introduction: Down syndrome (DS) is the most common chromosomal disorder and it is caused by trisomy of chromosome 21 (Hsa21). Subjects with DS show a large heterogeneity of phenotypes and the most constant clinical features present are typical facies and intellectual disability (ID). Several studies demonstrated that trisomy 21 causes an alteration in the metabolic profile, involving among all the one-carbon cycle.

Methods: We performed enzyme-linked immunosorbent assays (ELISAs) to identify the concentration of 5 different intermediates of the one-carbon cycle in plasma samples obtained from a total of 164 subjects with DS compared to 54 euploid subjects. We investigated: tetrahydrofolate (THF; DS n = 108, control n = 41), 5-methyltetrahydrofolate (5-methyl-THF; DS n = 140, control n = 34), 5-formyltetrahydrofolate (5-formyl-THF; DS n = 80, control n = 21), S-adenosyl-homocysteine (SAH; DS n = 94, control n = 20) and S-adenosyl-methionine (SAM; DS n = 24, control n = 15).

Results: Results highlight specific alterations of THF with a median concentration ratio DS/control of 2:3, a decrease of a necessary molecule perfectly consistent with a chromosomal dosage effect. Moreover, SAM and SAH show a ratio DS/control of 1.82:1 and 3.6:1, respectively.

Discussion: The relevance of these results for the biology of intelligence and its impairment in trisomy 21 is discussed, leading to the final proposal of 5-methyl-THF as the best candidate for a clinical trial aimed at restoring the dysregulation of one-carbon cycle in trisomy 21, possibly improving cognitive skills of subjects with DS.

Keywords: Down syndrome; chromosomal dosage; folates; one-carbon pathway; trisomy 21.

FIGURE 1

One-carbon pathway. The figure shows a schematic representation of folate pathway (in red), homocysteine-methionine cycle (in orange) and trans-sulfulration pathway (in yellow), that are part of one-carbon pathway. The metabolites analyzed in our study are reported in green and the enzymes directly involved in their production or transformation are indicated with the official symbol reported in Gene NCBI (https://www.ncbi.nlm.nih.gov/gene). Alternative forms of enzyme are shown together separated by “/”. The schematic representation was realized starting from “One carbon pool by folate - Homo sapiens” of KEGG pathway database (https://www.genome.jp/kegg/pathway.html).

FIGURE 2

Metabolite concentrations in subjects with DS and normal control subjects. The graphs report metabolite plasma levels of each subject in the study. On the x-axis there is the subdivision of subjects in DS and control groups. Subjects with DS are represented like red dots and normal control subjects are represented like blue dots. On the y-axis the concentration of the metabolite in μg/ml, ng/ml or pg/ml is reported. The asterisks above the graph indicate the level of statistical significance (*p < 0.05; ^**p < 0.005; ^***p < 0.0005). The middle black lines indicate the mean concentration values for each group and the external black lines indicate standard deviation (SD) values. The mean concentration, SD values and the number of subjects (n) are reported below each graph for DS and control groups. (A) shows THF concentrations; (B) shows 5-methyl-THF concentrations; (C) shows 5-formyl-THF concentrations excluding strong outliers; (D) shows SAH concentrations excluding strong outliers; (E) shows SAM concentrations. The graphs were created with GraphPad Prism software v.6.0 (San Diego, CA).

FIGURE 3

Heat Map figure of bivariate correlation between levels of each metabolite and levels of all the other metabolites excluding strong outliers. (A) presents bivariate correlations in DS group and (B) presents bivariate correlation in the control group (for complete data see Supplementary Table 11). At the top left of the figures the color code bar for Pearson correlation coefficient (r). Statistically significant correlations were marked with an “*”.