The MTHFR rs1801133 (c.677C>T) is the most intensively investigated variant in the homocysteine/folate pathway [11, 14, 65]. However, results of the MTHFR rs1801133 association in different CL/P populations are inconsistent ( Fig. 2), indicating the challenges of researching gene-disease associations [14]. Both fetal and maternal genetic susceptibilities may affect the intrauterine environment during palatogenesis. We found no association between maternal, as well as

embryonic, MTHFR rs1801133, and MTHFD1 (gene encoding trifunctional enzyme methylenetetrahydrofolate dehydrogenase 1) rs2236225 (c.1958G>A) INK-128 and CL/P risk [24, 32]. Maternal RFC1 (reduced folate carrier 1) rs1051266 (c80A>G) and embryonic MTR rs1805087 (c.2756A>G), MTRR (methionine synthase reductase) rs1801394, CBS 844ins68, TCN2 (transporter transcobalamin II) rs1801198 were not correlated with CL/P

Pirfenidone mouse susceptibility in the Polish population [23, 31]. Genetic processes that alter gene function without structural DNA alternation have become one of the chief focus areas of developmental medicine. Recently, there has been increased interest in epistasis and its influence on congenital anomalies in general. The nonparametric and genetic model-free Multifactor Dimensionality Reduction (MDR) analysis revealed a significant interactive effect of investigated SNPs in embryonic genes encoding enzymes involved in one carbon metabolism on clefting susceptibility (i.e. MTHFR rs1801133, MTR rs1805087, and PEMT/phosphatidylethanolamine N-methyltransferase/rs4646406 – a testing balance accuracy of 0.62 and a cross-validation consistency of 6/10, p=0.02) [31]. Even in the absence of an independent effect on CL/P risk in the Polish population,

the presence of the MTHFR rs1801133 may result in an increased CL/P risk. Studies using a variety of approaches have produced conflicting or inconclusive results on the MTHFR rs1801133 in clefting susceptibility, possibly because of the diversity of the investigated populations or the inadequate power of the studies. It is especially noteworthy that Polish mothers homozygous (GG) or heterozygous (AG) for 3-mercaptopyruvate sulfurtransferase the top-SNP of MTR, rs1805087, displayed a twofold increased risk of having a child with CL/P (ORAG+GGvsAA=2.19, 95%CI=1.19–4.05, p=0.01) [23]. Interestingly, maternal genotypes that include the G allele have also been associated with an increased risk of neural tube defects and conotruncal heart defects [66, 67]. Methionine synthase, encoded by MTR, is a vitamin B12-dependent enzyme that functions within the transmethylation cycle by catalyzing the 5-methyltetrahydrofolate-dependent remethylation of homocysteine to methionine.