Infant C677TMTHFR polymorphism and severe mental retardation

Synthesis of genetic association studies on autism spectrum disorders using a genetic model-free approach

BACKGROUND

Autism spectrum disorder (ASD) is a clinically and genetically heterogeneous group of neurodevelopmental disorders. Despite the extensive efforts of scientists, the etiology of ASD is far from completely elucidated. In an effort to enlighten the genetic architecture of ASDs, a meta-analysis of all available genetic association studies (GAS) was conducted.

METHODS

We searched in the Human Genome Epidemiology Navigator (HuGE Navigator) and PubMed for available case-control GAS of ASDs. The threshold for meta-analysis was two studies per genetic variant. The association between genotype distribution and ASDs was examined using the generalized linear odds ratio (ORG). For variants with available allele frequencies, the examined model was the allele contrast.

RESULTS

Overall, 57 candidate genes and 128 polymorphisms were investigated in 159 articles. In total 28 genetic polymorphisms have been shown to be associated with ASDs, that are harbored in 19 genes. Statistically significant results were revealed for the variants of the following genes adenosine deaminase (ADA), bone marrow stromal cell antigen-1 (CD157/BST1), Dopamine receptor D1 (DRD1), engrailed homolog 2 (EN2), met proto-oncogene (MET), methylenetetrahydrofolate reductase (MTHFR), solute carrier family 6 member 4 (SLC6A4), Synaptosomal-associated protein, 25kDa (SNAP-25) and vitamin D receptor (VDR). In the allele contrast model of cases versus healthy controls, significant associations were observed for Adrenoceptor Alpha 1B (ADRA1B), acetyl serotonin O - methyltransferase (ASMT), complement component 4B (C4B), dopamine receptor D3 (DRD3), met proto-oncogene (MET), neuroligin 4, X-linked (NLGN4), neurexin 1 (NRXN1), oxytocin receptor (OXTR), Serine/Threonine-Protein Kinase PFTAIRE-1 (PFTK1), Reelin (RELN) and Ras-like without CAAX 2 (RIT2).

CONCLUSION

These significant findings provide further evidence for genetic factors' implication in ASDs offering new perspectives in means of prevention and prognosis.

A systematic review of the biological, social, and environmental determinants of intellectual disability in children and adolescents

AIM

This systematic review aimed to identify the most important social, environmental, biological, and/or genetic risk factors for intellectual disability (ID).

METHODS

Eligible were published prospective or retrospective comparative studies investigating risk factors for ID in children 4-18 years. Exclusions were single group studies with no comparator without ID and a sample size <100. Electronic databases (Medline, Cochrane Library, EMBASE, PsycInfo, Campbell Collaboration, and CINAHL) were searched for eligible publications from 1980 to 2020. Joanna Briggs Institute critical appraisal instruments, appropriate for study type, were used to assess study quality and risk of bias. Descriptive characteristics and individual study results were presented followed by the synthesis for individual risk factors, also assessed using GRADE.

RESULTS

Fifty-eight individual eligible studies were grouped into six exposure topics: sociodemographic; antenatal and perinatal; maternal physical health; maternal mental health; environmental; genetic or biological studies. There were few eligible genetic studies. For half the topics, the certainty of evidence (GRADE) was moderate or high.

CONCLUSION

Multiple studies have examined individual potential determinants of ID, but few have investigated holistically to identify those populations most at risk. Our review would indicate that there are vulnerable groups where risk factors we identified, such as low socioeconomic status, minority ethnicity, teenage motherhood, maternal mental illness, and alcohol abuse, may cluster, highlighting a target for preventive strategies. At-risk populations need to be identified and monitored so that interventions can be implemented when appropriate, at preconception, during pregnancy, or after birth. This could reduce the likelihood of ID and provide optimal opportunities for vulnerable infants.

SYSTEMATIC REVIEW REGISTRATION

[https://www.crd.york.ac.uk/prospero/display_record.php?RecordID=120032], identifier [CRD42019120032].

Linking genetics to epigenetics: The role of folate and folate-related pathways in neurodevelopmental disorders

There is growing evidence that epigenetic dysregulation plays a role in neurodevelopmental disorders. In humans, folate is one of the main donors of the methyl group required for the synthesis of S-adenosylmethionine, which in turn is needed for DNA and histone methylation as key neurodevelopment processes. Folate deficiency during pregnancy has been correlated with neural tube defects and with a higher incidence of neurocognitive and/or neurobehavioral deficits. A similar outcome may be exerted by gene polymorphisms in folate or folate-related pathways. This has been documented by numerous case/control association studies performed on neurodevelopmental disorders such as autism spectrum disorder and attention deficit hyperactivity disorder. In this regard, the folate cycle represents a "perfect model" of how genetics influences epigenetics. Gene variants in folate and folate-related pathways can be considered risk factors for neurodevelopmental disorders and should therefore be assessed by genetic testing in pregnant women. High-risk women should be considered for folate supplementation during pregnancy. Here, we review all published case/control association studies on gene polymorphisms in folate and folate-related pathways performed on neurodevelopmental disorders, provide an overview of neurodevelopment and DNA methylation changes occurring at this time, and describe the biological basis of neurodevelopmental disorders and recent evidence of their epigenetic dysregulation.

B vitamin polymorphisms and behavior: evidence of associations with neurodevelopment, depression, schizophrenia, bipolar disorder and cognitive decline

The B vitamins folic acid, vitamin B12 and B6 are essential for neuronal function, and severe deficiencies have been linked to increased risk of neurodevelopmental disorders, psychiatric disease and dementia. Polymorphisms of genes involved in B vitamin absorption, metabolism and function, such as methylene tetrahydrofolate reductase (MTHFR), cystathionine β synthase (CβS), transcobalamin 2 receptor (TCN2) and methionine synthase reductase (MTRR), have also been linked to increased incidence of psychiatric and cognitive disorders. However, the effects of these polymorphisms are often quite small and many studies failed to show any meaningful or consistent associations. This review discusses previous findings from clinical studies and highlights gaps in knowledge. Future studies assessing B vitamin-associated polymorphisms must take into account not just traditional demographics, but subjects' overall diet, relevant biomarkers of nutritional status and also analyze related genetic factors that may exacerbate behavioral effects or nutritional status.

Screening for methylenetetrahydrofolate reductase C677T and A1298C polymorphisms in Indian patients with idiopathic mental retardation

Although genetic, nutritional, and environmental factors have been found to aggravate mental retardation in approximately 1% of individuals, no cause is known till date. In this study, two genetic polymorphisms in methylenetetrahydrofolate reductase (MTHFR), C677T (rs#1801133) and A1298C (rs#1801131), have been investigated in idiopathic mental retardation (IMR) subjects. Significantly higher frequency of the C677 allele was observed in IMR (n = 155; chi(2) = 5.5; P = 0.019) and moderate IMR (n = 67; chi(2) = 6.16; P = 0.013) groups as compared to controls (n = 126); for A1298C, no significant difference was noticed. TDT analysis revealed preferential transmission of C677 allele to a small group of mild IMR probands (chi(2) = 5.545; P = 0.018). Higher frequency of CA haplotype was also noticed in IMR cases as compared to controls (chi(2) = 6.28; P = 0.012). We infer from the present investigation that these polymorphisms are not contributing to the aetiology of IMR in this population since both case-control and family-based analysis revealed no significant transmission of the mutated allele.