Methylation is a metabolic process that occurs in every cell and organ in our body. Life would not exist without it. Methylation occurs when one molecule passes a methyl group (a carbon atom with three hydrogens attached to it) to another molecule. This takes place around a billion times per second in our body. Methylation is critical for detoxification, immune function, DNA production, energy, mood balancing and controlling inflammation. Poor methylation function leads to many chronic conditions.
METHYLATION & DOWN SYNDROME by Gabi Giacomin
The folate cycle
Folate metabolism occurs due to several key enzymes including MTHFR which converts 5,10 mthf – 5 mthf; MTR which is involved in remethylation of homocysteine to methionine; MTRR which regenerates methionine synthase; CBS which catalyses the transulfuration of homocysteine to cystathionine and RFC-1 which transports 5-mthf into cells.[1,2,3,4,5]
Various clinical studies have found an association between MTHFR 677TT, elevated homocysteine and reduced methionine in mothers of DS children [6, 7] These results indicate that reduced MTHFR enzyme activity results in DNA hypomethylation which may lead to abnormal cell division and trisomy 21 [6]
MTHFR
At present we know a) the biochemical structure and function of the MTHFR enzyme b) the effect of both 677T and 1298C alleles of MTHFR on Homocysteine levels c) folate is not synthesised efficiently from the diet d) TT homozygotes are at risk when their folate status is low as the mutation requires much higher levels of folate to stabilise the binging of FAD e) FAD is held onto as folate levels increase [8]. Maternal nutritional status and lifestyle together with genetic mutations influence the stability of the homocysteine cycle and the production of SAMe.[8]
MTRR/ MTR & Vitamin B12
The MTRR enzyme converts cobalamin to methylcobalamin, (the active form of vitamin B12), using SAMe and B vitamins as cofactors. Mutations of MTRR, such as C524T and A66G, increase the likelihood that a person with Down Syndrome will develop Congenital Heart Defects (CHD) [9]. These polymorphisms could be used to evaluate CHD risk in DS [9] MTR influences the progression of Alzheimer’s disease [10].
Mitochondria
In DS, over expression of the CBS enzyme results in imbalances within the methylation cycle and methyl availability. Mitochondrial function is affected by methylation status, since mitochondria rely on the availability of SAMe as a methyl donor. Mitochondrial levels of SAMe are reduced in DS compared to controls indicating the effects of methylation imbalances on mitochondria [11]
Hypermethylation
Ironically, despite hypomethylation leading to low SAMe, DNA lymphocyte hypermethylation simultaneously exists in DS [12, 13] and is a potential cause of neurodegeneration [14]. Alterations in DNA methylation are associated with neurodevelopment deficits and premature ageing in DS [15, 16, 17]
Altered methylation was observed across the whole genome, and wasn’t enhanced on Chromosome 21 (HSA21) [17]. The gene DNMT3L sits on HSA21 and influences enzymes DNMT3a and 3b which are DNA methylators, and may be the cause of hypermethylation and neurological deficits in DS [17]. DNA methylation is linked to cognition in DS as measured by the Dalton Brief Praxis Test [18, 19].
CBS Enzyme
The gene for cystathionine beta-synthase (CBS) is located on chromosome 21 and is overexpressed in children with Down syndrome (DS), or trisomy 21. As a result, plasma levels of homocysteine, methionine, S-adenosylhomocysteine (SAH), and S-adenosylmethionine (SAMe) were all decreased in children with DS [20]. Levels of Cysteine and cystathionine were increased, relative to an increase in CBS activity [20].
An increase in CBS activity prevents the resynthesis of methionine from homocysteine, dependent on folate. A reduction in the availability of homocysteine creates a ‘folate trap’, resulting in folate deficiency which contributes to the pathology of DS [20].
A decrease in the activity of CBS leads to homocysteinuria, and issues with sulfur metabolism resulting in mental retardation and vascular disease [21]. Levels of CBS in DS brains are three times greater than controls and are found in astrocytes associated with plaque in the brains of DS people with AD [21]. CBS over expression may lead to cognitive decline and Alzheimer’s Disease pathology in DS [21].
Mice studies reveal CBS is over expressed in the cerebellum and hippocampus, and this over expression affects neurones in the hippocampus, facilitating the strengthening of synapses [22]. Many animal studies report that this is associated with improved spatial learning. This raises the possibility that CBS over expression might have a positive effect on cognitive function in DS [22].
SOD1
Plasma glutathione levels were reduced in children with DS possibly as a result of the over expression of the SOD gene located on chromosome 21 and the resulting increase in oxidative stress [20].
Nutrients associated with Methylation
Addition of folinic acid, methionine, methyl B12, thymidine and DMG to T21 lymphocyte cells in vitro improved the metabolic profile [20].
Homocysteine
Overexpression of CBS activity leads to low Homocysteine levels in people with DS. High homocysteine levels were found in a subgroup of people with DS who had low IQ [23]. Vitamin B12, folate and Body Mass Index weren’t related to Homocysteine levels [23]. Ageing is related to Homocysteine and IQ, although this association could not be related to DS [23]. Intelligence was associated with Homocysteine levels independent of age. The MTHFR 677T allele was associated with lower IQ due to its effect on Homocysteine levels [23]. Homocysteine and folate deficiencies increased the risk of neurodegeneration [23].
The Folate Trap
The Folate Trap in DS occurs due to the over expression of the CBS enzyme, increasing activity of the trans-sulfuration pathway. Homocysteine levels are reduced and its remethylation, dependent on 5-mthf and vitamin B12, cannot occur and neither resynthesis of 5-mthf to thf [20]. Supplementation of active folate and B12 is very effective at increasing methionine and SAMe. The MTHFR 677TT mutation may aggravate the folate trap further by reducing the remethylation of homocysteine and production of tetrahydrofolate [20]. A Mutated MTHFR also reduces production of SAMe, needed for DNA and protein methylation and synthesis of choline [20].
Maternal Risk Factors
Abnormal cell division of chromosome 21 occurs due to reduced methylation as a result of epigenetic changes [24]. Mutations of DNMT3B, a gene associated with folate metabolism, affects the activity of its enzyme and increases the chance of abnormal cell division in mothers of children with DS [24].
MTR gene mutations and homocysteine are risk factors for [Sicilian] mothers to have a child with DS [25].
Women with both MTHFR and MTRR mutations have a higher chance of producing a child with DS [26]. MTRR A66G is more common in mothers of children with DS, but isn’t associated with increased homocysteine [26].
Homocysteine levels were higher among mothers of DS children compared to controls and the homozygous MTHFR 677T allele was associated with altered levels of Homocysteine. Mothers of children with DS tended to have higher allele’s i.e. 677T, 1298C, 2756 G, 66G than controls [27].
DNA hypomethylation can lead to chromosomal instability and increase the likelihood of abnormal cell division which causes trisomy 21 [28].
Three combined genotypes (CTCC, TTAC and TTCC) were identified in Indian mothers of children with DS, but not in controls. These genotypes are very uncommon in the human population [28].
References
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