Congenital defects, folic-acid, and homoeobox genes

Antisense modulation of the coding or regulatory sequence of the folate receptor (folate binding protein-1) in mouse embryos leads to neural tube defects

BACKGROUND

Although folic acid decreases the incidence of neural tube defects (NTDs) in humans, the mechanism for this protection is unknown. We have employed antisense technology to alter expression of the gene for the folate receptor (folate binding protein-1 [Folbp1]) in mouse embryos cultured in vitro.

METHODS

Embryos were explanted on day 8 of gestation and cultured for 44 hr. Several oligodeoxyribonucleotides designed to modulate the coding region or a regulatory sequence in the 5'-untranslated region of Folbp1 were microinjected into the amniotic sac of embryos at the beginning of the culture period.

RESULTS

Two different antisense sequences to the 5' and 3' coding region in Folbp1 produced concentration-dependent increases in the number of embryos with NTDs. Coinjection of 5-methyltetrahydrofolate with these sequences decreased the frequency of abnormal embryos. A semi-quantitative RT-PCR technique used to measure the amount of Folbp1 mRNA in treated and control embryos confirmed that the mRNA level was decreased by treatment with the antisense sequences. An antisense oligodeoxyribonucleotide to a 17 base cis regulatory element also generated a concentration-dependent increase in the frequency of embryos with NTDs, and a decrease in the level of Folbp1 mRNA.

CONCLUSIONS

These results demonstrate that alterations in expression of Folbp1 by perturbing either the coding sequence or a critical regulatory cis-element can play a role in NTDs.

Hypothesis: folate-responsive neural tube defects and neurocristopathies

BACKGROUND

What accounts for the wide spectrum of folate-responsive dysmorphogeneses? Both embryonic and fetal cells are entirely dependent on maternal folate to support their requirement for precisely timed proliferative bursts during gestation. Folate receptors (FRs) mediate transport into cells and are central to transplacental maternal-to-fetal folate transport. FRs are also critical for neural tube and neural crest development because recent murine "knock-out" and "knock-down" of FRs results in a high percentage of folate-responsive neural tube defects (NTDs) and neurocristopathies.

HYPOTHESIS

Central to our hypothesis is the fact that folate deficiency is accompanied by a reduction in the proliferative capacity of highly mitotic neural tube or neural crest cells. Therefore, depending on when in pregnancy various cohorts of highly proliferative cells are deprived of folate, and the origin of the affected cells will determine the type of developmental dysmorphogenesis. Thus, selective folate deficiency in early pregnancy of only highly proliferative neural tube or neural crest cells predisposes to NTDs or gross dysmorphogenesis, respectively. Folate deficiency that compromises placental development will predispose to small-for-date babies due to an overall nutrient deficiency, and the development of folate insufficiency later in pregnancy could predispose to more subtle midline birth defects involving atresia of neural crest cell-derived structures. Finally, a congenital folate transport defect would only be corrected by suprapharmacological doses of folate, which ensures passive diffusion.

CONCLUSION

This hypothesis can explain the results of several earlier and more recent clinical trials on folate supplementation in pregnancy, but it also raises the possibility that there may be several as yet undiscovered neurocristopathies that are folate responsive. Teratology 62:42-50, 2000. Published 2000 Wiley-Liss, Inc.

Maternal factors associated with severity of birth defects

OBJECTIVE

Due to inbreeding and nutritional factors, the Bedouin Arabs represent a high risk population for birth defects. The severity of birth defects is probably related to the time and extent of interference with embryogenesis. The present study was aimed at identifying factors associated with severity of birth defects, in pregnancies of Bedouin women examined at a third level ultrasound clinic.

METHODS

The study population consisted of 295 Bedouin women who attended an ultrasound clinic at the Soroka Medical Center between 1990 and 1996. The case group included 188 women carrying fetuses with severe birth defects, defined as incompatible with life or which significantly interfere with normal living. For those defects the option of pregnancy termination was discussed. The comparison group consisted of 107 women whose fetuses were diagnosed with mild defects.

RESULTS

Women carrying fetuses with severe birth defects had more pregnancies and more deliveries than women carrying fetuses with mild defects (P = 0.005, P = 0.04, respectively). The severity of defects was found to be unrelated to maternal age, consanguinity, residence, birth order, previously uncompleted pregnancies and birth defects in the family.

CONCLUSIONS

Higher birth order was associated with severity of birth defects detected at the second trimester.