The methylenetetrahydrofolate reductase (MTHFR) gene encodes a critical enzyme involved in the one-carbon metabolism pathway, which includes methionine and folate cycles that are integral for cellular metabolic process like protein methylation and DNA synthesis (Raghubeer et al., 2021; Liew & Gupta, 2015). Specifically, the MTHFR enzyme catalyzes the conversion of 5,10-methylenetetrahydrofolate (5,10-CH-MTHF) to 5-methyltetrahydrofolate (5-MTHF), which is the biologically active form of folate required for homocysteine re-methylation (Leclerc et al., 2013).
Roughly 60-70% of the global population is suspected to carry at least one functional allele variant of this gene, many of which alter enzyme activity (Long & Goldblatt, 2016). Despite its prevalence, this complexity has made MTHFR a difficult target for systemic and multi-pathway investigation.
Adult humans obtain folate through dietary intake of both natural and synthetic forms, like folic acid, which is found in supplements and fortified foods (Courseault et al., 2023). However, folic acid is not bioactive, and therefore cannot be accessed by cells directly and must first be converted to a usable form (Leclerc et al., 2013). Dihydrofolate reductase (DHFR) converts folic acid to tetrahydrofolate (THF), which is the raw material other folate forms are formed from (Leclerc et al., 2013). THF is first converted in a process involving vitamin B6, serine, and the enzyme serine hydroxymethyltransferase (SHMT) to 5,10-methylenetetrahydrofolate. 5,10-methylenetetrahydrofolate is then further reduced by the MTHFR enzyme to produce 5-MTHF, which is the usable form of folate that circulates in blood (Leclerc et al., 2013; Hustad et al., 2007). This reduction is critical, since 5-MTHF is the active form of folate and when paired with Vitamin B12 as a cofactor, it is the primary folate form capable of recycling homocysteine (Hustad et al., 2007).
Homocysteine is a normal intermediate metabolite produced during the methionine cycle, when methionine, an amino acid essential for all methylation reactions, donates a methyl group (Zhang et al., 2019). This happens in a variety of processes, including but not limited to: DNA methylation, detoxification, and neurotransmitter pathways (Long & Goldblatt, 2016). Homocysteine levels within normal baseline ranges are not independently predictive of poor health outcomes, but chronically elevated levels are well-documented as markers of inflammation, cardiovascular risk, and oxidative stress (Dedoussis et al., 2005; Raghubeer et al., 2021). Therefore, to maintain metabolic balance, homocysteine is either recycled or eliminated through interconnected pathways (Hustad et al., 2007). One of the main cycles used to do this is the methionine cycle, which re-methylates homocysteine into methionine, using 5-MTHF and Vitamin B12 to re-attach free methyl groups to homocysteine molecules, lowering homocysteine levels and keeping the liver and metabolic status stable and operational (Christensen et al., 2025; Matté et al., 2009).
There are currently 34 rare but deleterious known MTHFR polymorphisms, with varying location and impact among them (Raghubeer et al., 2021). However, the C677T polymorphism (RSID: rs1801133), has consistently shown to have one of the largest enzyme deficiencies, with enzyme activity appearing at 34% reduction in heterozygous and 75% reduction in homozygous, highlighting the mechanistic impact on afflicted individuals (Wan et al. 2018). This polymorphism is located on the chromosome 1 exon 4, and results in the conversion of alanine to valine at codon 222 (Liew & Gupta, 2015). Research into C677T has revealed it to be connected to several disease states, including but not limited to: kidney disease, neural tube defects, cancer, and liver disease (Raghubeer et al., 2021). Specifically, this polymorphism has been known to reduce the functional capability of 5-MTHF, which leads to downstream effects that include elevated homocysteine levels (Hustad et al., 2007; Liew & Gupta, 2015).
Since this investigation explores the impact of the C677T MTHFR mutation, it was important to include measures of modal biochemical expression, like biomarkers, in addition to genotype. As explored further down below, downstream effects of polymorphisms can vary substantially across affected individuals, which indicates that genotype alone cannot fully capture functional metabolic status (Hustad et al., 2007; Tsang et al., 2015). Therefore, in order to attempt to capture the dynamic nature of one-carbon metabolism, circulating biomarkers such as homocysteine, folate, vitamin B6, and vitamin B12, were included in order to provide direct measures of metabolic activity in an approach that accounts for both genetic variation and environmental factors (Hustad et al., 2007; Rooney et al., 2020).
While the methionine cycle is the primary mechanism for regulating homocysteine concentrations, there is another independent re-methylation pathway that uses the enzyme betaine-homocysteine methyltransferase (BHMT); in this pathway, betaine, which is a methyl-rich compound derived from choline, donates a methyl group to homocysteine in order to recycle it into methionine (Raghubeer et al., 2021). This pathway is parallel and folate independent, which means it can potentially compensate to some degree in individuals with reduced folate-dependent pathway efficiency, like C677T carriers (Holm et al., 2007). Because the betaine/BHMT represents a mechanistically relevant compensatory system, betaine is included in this analysis as a possible MTHFR C677T biomarker profile component.
The primary aim of this analysis is to systematically characterize relevant biomarker patterns across MTHFR C677T genotypes. This will allow for analysis of factors that may be systemically relevant but mechanistically distant from related biochemical cycles, and therefore may not have been targeted as primary investigative factors in current and ongoing literature.
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title: "1.1 Introduction & Background"
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## <u> 1.1.1: MTHFR and Its Role in One-Carbon Metabolism </u>
The methylenetetrahydrofolate reductase (MTHFR) gene encodes a critical enzyme involved in the one-carbon metabolism pathway, which includes methionine and folate cycles that are integral for cellular metabolic process like protein methylation and DNA synthesis (Raghubeer et al., 2021; Liew & Gupta, 2015). Specifically, the MTHFR enzyme catalyzes the conversion of 5,10-methylenetetrahydrofolate (5,10-CH-MTHF) to 5-methyltetrahydrofolate (5-MTHF), which is the biologically active form of folate required for homocysteine re-methylation (Leclerc et al., 2013).
Roughly 60-70% of the global population is suspected to carry at least one functional allele variant of this gene, many of which alter enzyme activity (Long & Goldblatt, 2016). Despite its prevalence, this complexity has made MTHFR a difficult target for systemic and multi-pathway investigation.
## <u>1.1.2: The Folate Cycle (DHFR → THF → 5-MTHF)</u>
Adult humans obtain folate through dietary intake of both natural and synthetic forms, like folic acid, which is found in supplements and fortified foods (Courseault et al., 2023). However, folic acid is not bioactive, and therefore cannot be accessed by cells directly and must first be converted to a usable form (Leclerc et al., 2013). Dihydrofolate reductase (DHFR) converts folic acid to tetrahydrofolate (THF), which is the raw material other folate forms are formed from (Leclerc et al., 2013). THF is first converted in a process involving vitamin B6, serine, and the enzyme serine hydroxymethyltransferase (SHMT) to 5,10-methylenetetrahydrofolate. 5,10-methylenetetrahydrofolate is then further reduced by the MTHFR enzyme to produce 5-MTHF, which is the usable form of folate that circulates in blood (Leclerc et al., 2013; Hustad et al., 2007). This reduction is critical, since 5-MTHF is the active form of folate and when paired with Vitamin B12 as a cofactor, it is the primary folate form capable of recycling homocysteine (Hustad et al., 2007).
## <u>1.1.3: The Methionine/Homocysteine Cycle</u>
Homocysteine is a normal intermediate metabolite produced during the methionine cycle, when methionine, an amino acid essential for all methylation reactions, donates a methyl group (Zhang et al., 2019). This happens in a variety of processes, including but not limited to: DNA methylation, detoxification, and neurotransmitter pathways (Long & Goldblatt, 2016). Homocysteine levels within normal baseline ranges are not independently predictive of poor health outcomes, but chronically elevated levels are well-documented as markers of inflammation, cardiovascular risk, and oxidative stress (Dedoussis et al., 2005; Raghubeer et al., 2021). Therefore, to maintain metabolic balance, homocysteine is either recycled or eliminated through interconnected pathways (Hustad et al., 2007). One of the main cycles used to do this is the methionine cycle, which re-methylates homocysteine into methionine, using 5-MTHF and Vitamin B12 to re-attach free methyl groups to homocysteine molecules, lowering homocysteine levels and keeping the liver and metabolic status stable and operational (Christensen et al., 2025; Matté et al., 2009).
## <u>1.1.4: What is C677T and Why It Matters</u>
There are currently 34 rare but deleterious known MTHFR polymorphisms, with varying location and impact among them (Raghubeer et al., 2021). However, the C677T polymorphism (RSID: rs1801133), has consistently shown to have one of the largest enzyme deficiencies, with enzyme activity appearing at 34% reduction in heterozygous and 75% reduction in homozygous, highlighting the mechanistic impact on afflicted individuals (Wan et al. 2018). This polymorphism is located on the chromosome 1 exon 4, and results in the conversion of alanine to valine at codon 222 (Liew & Gupta, 2015). Research into C677T has revealed it to be connected to several disease states, including but not limited to: kidney disease, neural tube defects, cancer, and liver disease (Raghubeer et al., 2021). Specifically, this polymorphism has been known to reduce the functional capability of 5-MTHF, which leads to downstream effects that include elevated homocysteine levels (Hustad et al., 2007; Liew & Gupta, 2015).
## <u>1.1.5: Biomarkers vs. Genotype Justification</u>
Since this investigation explores the impact of the C677T MTHFR mutation, it was important to include measures of modal biochemical expression, like biomarkers, in addition to genotype. As explored further down below, downstream effects of polymorphisms can vary substantially across affected individuals, which indicates that genotype alone cannot fully capture functional metabolic status (Hustad et al., 2007; Tsang et al., 2015). Therefore, in order to attempt to capture the dynamic nature of one-carbon metabolism, circulating biomarkers such as homocysteine, folate, vitamin B6, and vitamin B12, were included in order to provide direct measures of metabolic activity in an approach that accounts for both genetic variation and environmental factors (Hustad et al., 2007; Rooney et al., 2020).
## <u>1.1.6: Alternate Relevant Paths - The Betaine/BHMT Pathway</u>
While the methionine cycle is the primary mechanism for regulating homocysteine concentrations, there is another independent re-methylation pathway that uses the enzyme betaine-homocysteine methyltransferase (BHMT); in this pathway, betaine, which is a methyl-rich compound derived from choline, donates a methyl group to homocysteine in order to recycle it into methionine (Raghubeer et al., 2021). This pathway is parallel and folate independent, which means it can potentially compensate to some degree in individuals with reduced folate-dependent pathway efficiency, like C677T carriers (Holm et al., 2007). Because the betaine/BHMT represents a mechanistically relevant compensatory system, betaine is included in this analysis as a possible MTHFR C677T biomarker profile component.
## <u>1.1.7 Primary Aim Statement</u>
The primary aim of this analysis is to systematically characterize relevant biomarker patterns across MTHFR C677T genotypes. This will allow for analysis of factors that may be systemically relevant but mechanistically distant from related biochemical cycles, and therefore may not have been targeted as primary investigative factors in current and ongoing literature.