Trisomy 21, or Down syndrome, is a relatively common affliction, and the most widely known cause of intellectual disability, hence the investigation and determination of the molecular basis of this disease and the search for potential predisposing factors for its occurrence are justified. Simple trisomy, constituting 95% of cases, results from the phenomenon of nondisjunction which occurs during meiotic division; however, the mechanisms responsible for the abnormal segregation of chromosomes are still unknown [2].
In the search for the causes of faulty segregation of chromosomes, their instability, fragility, or aneuploidy, ever-greater attention is being drawn to the potential contribution of epigenetic phenomena, primarily DNA hypomethylation [7, 8]. Deficiencies in dietary folate and disorders of the folate cycle which directly result in, inter alia, hyperhomocysteinaemia and insufficient synthesis of S-adenosylmethionine ‒ a fundamental donor of methyl groups necessary for DNA methylation ‒ have been linked to the hypomethylation of nucleic acids. Disorders of the cyclic transformation of folate derivatives result primarily from a defect in the activity of the key enzyme in this cycle, i.e. methylenetetrahydrofolate reductase (MTHFR). Currently, more than 40 SNP polymorphisms have been described in the MTHFR gene; the direct influence of some of these on the reduction of the activity of the enzyme encoded by this gene, ultimately resulting in SAM deficiencies and DNA hypomethylation, has been proven [1, 5‒7, 11, 17]. On this basis, a conclusion was drawn regarding the potential for linking nondisjunction, the reason for the vast majority of DS cases, with DNA hypomethylation, which can be brought about by disturbances of the folate cycle resulting from insufficient MTHFR activity, caused in turn by the occurrence of polymorphisms of single nucleotides within the associated gene.
In this study, the two SNP polymorphisms most frequently associated with trisomy of chromosome 21 in MTHFR were taken into consideration, namely 677CT and 1298AC, as well as three others, not previously researched for the existence of correlations with DS, namely rs3737964, rs4846048, and rs1994798. Observed differences between the study and control groups in the frequency of genotypes in 677CT and 1298AC, as well as in all three remaining polymorphic sites, showed no statistical significance in the chi-square test
(p > 0.05). The distribution of the percentage of frequency of mutant and wild alleles in the studied SNPs also fell within the confidence interval in the chi-square test (p > 0.05); thus no statistically significant differences were observed in the studied groups in the Polish population.
The results presented in this paper are congruent with many other reports published in the relevant literature since the appearance of the first paper on this subject in 1999. This applies equally to studies of Caucasians and of other populations. Worth mentioning, inter alia, are studies on French (Chango et al., 2005) [18], Italian (Stuppia et al., 2002) [19], Danish (Kokotas et al., 2009) [20], and Croatian (Vraneković et al., 2010) [21] populations. In addition, analogous studies which also excluded the association of DS with the SNP polymorphisms 677CT and 1298AC were carried out in Jordan (Sadiq et al., 2011) [22], Romania (Bucerzan et al., 2017) [6], Brazil (Balarin et al., 2017) [5], Turkey (Boduroglu et al., 2004) [23], China (Jiajin et al., 2018) [1], and India (Kohli et al., 2008; Kaur et al., 2013; Mohanty et al., 2012) [24‒26]. Confirmation was also obtained in an extensive meta-analysis by Yang et al. (2013), encompassing 32 articles excluding the association of MTHFR polymorphisms of 677CT and 1298AC with trisomy of chromosome 21 [27].
On the other hand, an equal number of researchers have proven a relationship between the occurrence of the described polymorphisms in the MTHFR gene in women and the birth of children with Down syndrome. This is evidenced by the first study by James et al. (1999) on an American population [7]. Similar results were obtained by Cyrus et al. (2009, southern India) [28], Coppedè et al. (2009, Italy) [29], Meguid et al. (2008, Egypt) [30], and
da Silva et al. (2005, Brazil) [31]. Mention may also be made of meta-analyses conducted by Wu et al. (2012) [32], Rai et al. (2014) [33], and Victorino et al. (2014) [34].
The glaring discrepancy between these reports renders any inferences about single nucleotide polymorphisms 677CT and 1298AC as potential predisposing factors for the occurrence of DS in children doubtful and controversial, at the very least. The present paper also indicates the absence of any connection between the other three polymorphisms (rs3737964, rs4846048, and rs1994798) and the occurrence of DS. All of the above-mentioned authors emphasize that these discrepancies exclude SNPs in the MTHFR gene as an independent risk factor for trisomy 21. The polymorphisms in question may nevertheless constitute one of many factors, including additional genetic, epigenetic, environmental, and other random factors, whose simultaneous interaction may result in a predisposition to the birth of a child with trisomy 21[57].
The results of the present study of a Polish population do not contradict the reports published to date. It is not out of the question that larger sample size (in both the test and control groups) would result in statistically significant differences between the frequencies of individual genotypes of 677CT and 1298AC in the studied groups, given that the results of statistical analysis in some comparisons were close to decisive values. However, neither does the exclusion of a relationship between these polymorphisms in the MTHFR gene in mothers and the birth of children with trisomy 21 in this Polish population equate to the lack of participation of the described changes in the molecular basis of DS. The most appropriate direction of research appears to be an investigation of the simultaneous influence of several factors in the pathomechanism of DS; this would require additional, extremely detailed analyses.