This study was conducted to determine the gene expression profile associated with NTDs among infants attending MNH and MOI. Findings revealed a significant low expression of MTHFR gene among cases than controls.
NTDs result from failure of closure of the neural tube during fetal embryogenesis. Evidence from studies done in different population groups strongly suggest that MTHFR polymorphism (MTHFR C677T) is associated with an increased risk of NTDs(7,8). In this study, DNA sequencing was not done, which is required for detecting specific gene mutation. However, the significantly low expression of MTHFR gene in the cases group could suggest the possibility of having variants with low enzyme activity amongst the cases with NTDs. The MTHFR gene plays a key role in folate and methylation cycles which are required for essential processes including; DNA synthesis, DNA/protein methylation, cell division, and tissue growth; especially in the rapidly developing cells(5). Thus, a defective gene variant could result in an impaired DNA synthesis or DNA methylation involved in the neurulation process which is essential during neural tube closure.
Folate pathway genes (those involved the uptake, transport and folate metabolic enzymes) have been intensively studied as candidates for NTD causation, in view of the role of folate in primary prevention. The main outputs of folate one-carbon metabolism (FOCM) are the generation of purines and pyrimidines for DNA synthesis during cell replication and the donation of methyl groups for regulation of gene, protein, and lipid function. Hence, genetic variants that reduce the efficiency of FOCM might increase the risk of NTDs either by compromising cell proliferation or the regulation of gene expression, or both. Indeed, when cells are derived from NTD foetuses and maintained in culture for several passage generations, around 30% of cell lines exhibited apparent inborn errors of folate metabolism, as indicated by diminished thymidylate biosynthesis(17); suggesting that genetic factors play a role in significant numbers of NTDs.
Despite decades of investigation into the genetic basis of NTDs, our knowledge of the inherited causative factors that impart disease risk remains incomplete. This is despite a wealth of information on the genetic requirements for neural tube closure in mice and many insights into the molecular basis of neurulation in lower vertebrates. Genetic counseling for NTDs is still empirical, based on population recurrence risk values, and there are no genetic tests for prospective parents in routine clinical usage. Genotyping for the C677T polymorphism of MTHFR and a few other folate-related genetic variants is available, but it is unclear how to interpret a particular parental MTHFR genotype in terms of NTD risk in a future pregnancy. Hopefully, full genome-wide assessment of potential risk variants, including coding, regulatory, and epigenetic modifications, in individuals with NTDs and their parents, will shed more light into this. However, the extent to which this information will “explain” the occurrence of sporadic NTDs or predict future risk of NTD in families, remains to be determined. A further vital step in this work will be to link nutritional status, in terms of folate, vitamin B12 and other factors, to the putative genetic risk factors. It seems likely that only once all such factors are taken into account, can a reliable battery of tests be expected to emerge that has NTD predictive value.
The study found no association of NTDs and the expression levels of other genes such as VANGL1, VANGL2, Dvl2 and Dvl3. This is contrary to the studies done in China and Italy, which found a significant association between the incidence of NTDs and other genes in addition to MTHFR gene(10–15). This could be partly explained by the fact that none of these studies were done in Africa and also the other studies included aborted fetuses. Therefore, there is the likelihood that, gene expressions can be influenced by genetic diversity among different population groups and specific fetal developmental stages.
Strengths and limitations
This was a preliminary study, a first of its kind in Tanzania, whose objective was to screen for differences in the expression of implicated genes for NTDs between cases and controls, which has shown a genetic basis for NTDs; though the data is limited in functionally defining the genotypes and phenotypes which should be further investigated.
In this study, due to limited funding, further elucidation through DNA sequencing which is required for identifying specific genetic mutation/polymorphism could not be performed. Therefore, we cannot link the gene expression profile with specific gene variant/mutation(s) which might be a risk factor among the NTDs cases at the time of embryogenesis.