Metabolismo do folato e síndrome de Down: análise de polimorfismos genéticos e homocisteína plasmática

Abstract

Down syndrome (DS) is, in the most cases, resulting from chromosomal nondisjunction during maternal meiosis. It is believed that the abnormal folate metabolism as result of genetic polymorphisms may lead to DNA hypomethylation and consequent chromosomal nondisjunction. Objective To establish the chromosomal anomalies frequencies of DS cases consulted by Genetics Outpatient Service of Hospital de Base (HB) in São José do Rio Preto to subsequent selection of patients with free trisomy 21; to evaluate the influence of the Methylenetetrahydrofolate reductase (MTHFR) C677T and A1298C, Methionine sinthase (MTR) A2756G and Reduced folate carrier 1 (RFC1) A80G polymorphisms and of plasma homocysteine (Hcy) concentrations as maternal risk factors for DS; to investigate the impact of the MTHFR C677T and A1298C, MTR A2756G and RFC1 A80G polymorphisms on Hcy concentrations in DS individuals. Subjects and Methods To molecular investigation and plasma Hcy quantification were included in the study 56 DS individuals with karyotypic result 47,X_,+21, 72 mothers of DS individuals with free trisomy 21 (DS mothers) and 194 mothers who had no children with DS (control mothers). The Hcy quantification was performed by liquid chromatography tandem mass spectrometry. DNA was extracted from leukocytes of peripheral blood to the investigation of the MTHFR C677T, MTR A2756G and RFC1 A80G polymorphisms by polymerase chain reaction (PCR) and enzyme digestion, and the MTHFR A1298C polymorphism by allele-specific PCR. Results The frequencies of chromosomal alterations in DS patients were 92.2% (n = 357) for free trisomy 21, 6.2% (n = 24) for translocation and 1.5% (n=6) for mosaicism. The molecular analysis in DS mothers and control group showed that the median of the number of polymorphic alleles for the four loci tested was higher Abstract x iii in DS mothers as compared to the control group (P = 0.02), and the presence of three or more polymorphic alleles increase the risk for having a child with DS in 1.74 times (P = 0.048). Elevated maternal risk for DS was also observed in the presence of Hcy concentration higher than 4.99 μmol/L (P = 0.003). The allele frequencies for the polymorphisms in DS group were 0.37 for MTHFR 677T, 0.21 for MTHFR 1298C, 0.18 for MTR 2756G and 0.47 for RFC1 80G. The Hcy mean concentration in this group was 5.2 ± 3.3 μmol/L. The Hcy concentrations were significantly increased in the presence of MTR 2756AG heterozygous genotype as compared to the MTR 2756AA wild-type genotype (P = 0.025). Conclusions The presence of three or more polymorphic alleles for MTHFR C677T, MTHFR A1298C, MTR A2756G and RFC1 A80G and plasma Hcy concentrations higher than 4.99 μmol/L are maternal risk factors for DS. The MTR 2756AG heterozygous genotype is associated with increased Hcy concentrations in DS individuals. This study confirms yet that chromosomal nondisjunction, represented by free trisomy 21, is the most frequent cause of DS.