Congenital defects, folic-acid, and homoeobox genes
Authors: Aubrey Milunsky
Journal: The Lancet (1996)

Precise determination of the cause of a non-monogenic congenital defect is difficult, and evaluation of any measure taken to avoid such a defect is an even greater challenge. At the root of such difficulties lie largely unknown genes and environmental agents (single, multiple, interacting, or predisposing). Despite multivariate and other analyses adjusting for variables, confounding factors complicate epidemiological studies in the face of genetic heterogeneity and pleiotropism, and of known or unrecognised exogenous factors. To study the multifactorial aetiology of congenital defects requires a host of considerations, including family history, maternal age, weight and stature, race, maternal disorders (eg, diabetes, epilepsy, lupus erythematosus) drugs, dietary supplements, diet, smoking, heat exposure, and a change in partner. Failure to consider these factors, among others, may confound causal analysis. Indeed, the vast majority of epidemiological studies on congenital defects have not taken all known causal factors into account, but rather have rested on statistical power and methodology. Hence, it is no surprise that it took over a quarter of a century to confirm original claims that periconceptional folic acid supplementation reduces the recurrence rate of neural tube defects (NTDs). Most recently, and with appropriate caution, Czeizel1Czeizel AE Reduction of urinary tract and cardiovascular defects by periconceptional multivitamin supplementation.Am J Med Genet. 1996; 62: 179-183Crossref PubMed Scopus (241) Google Scholar has reported significantly reduced rates of occurrence of urinary tract defects (mainly obstructive) and cardiovascular malformations (mainly ventricular septal defects) in the offspring of pregnant women taking daily periconceptional multivitamin supplements containing 0·8 mg folic acid. Czeizel cautioned that (1) his observations could be explained by chance alone; (2) that ascertainment could have been unwittingly biased (by more sensitive diagnostic procedures in the control group); and (3) heterogeneous causes of urinary tract and cardiovascular anomalies are recognised. The fidelity of the message from this study is unclear, in view of conclusions in another report on the same cohort that “the rate of cleft lip ± cleft palate in the vitamin-supplemented group was nearly twice the rate of the Hungarian population” and there was “no difference in the rate of cleft lip ± cleft palate in the supplemented vs control groups”.2Czeizel AE Dudas I Prevention of the first occurrence of neural-tube defects by periconceptional vitamin supplementation.N Engl J Med. 1992; 327: 1832-1835Crossref PubMed Scopus (2618) Google Scholar Clearly, a worthy explanation is needed to mollify concern that some selection bias or other factor (s) is influencing the reported conclusions. A similar protective effect against congenital urinary tract defects has also been noted in a case-control study.3Li De-K Daling JR Mueller BA Hickok DE Fantel AG Weiss NS Periconceptional multivitamin use in relation to the risk of congenital urinary tract anomalies.Epidemiology. 1995; 6: 212-218Crossref PubMed Scopus (134) Google Scholar However, that study has serious limitations, such as ascertainment and selection bias, inefficient registry-based case-finding, non-blinding of cases and controls, over 37% of case mothers not interviewed, and differential recall bias between cases and controls. The place of folic acid and zinc4Milunsky A Morris JS Jick H et al.Maternal zinc and fetal neural tube defects.Teratology. 1992; 46: 341-348Crossref PubMed Scopus (40) Google Scholar in the pathogenesis of NTDs include their respective involvement in DNA and RNA biosynthesis. Precisely how these dietary or metabolic elements affect the rapidly proliferating, migrating, and organising cells of the embryo remains to be clarified. Cellular and molecular mechanisms are influenced by factors intrinsic and extrinsic to specific tissues being generated in organ formation and closure. A teratogenic effect in human beings of vitamin A excess5Rothman KJ Moore LL Singer MR Nguyen U-SaDT Mannino S Milunsky A Teratogenicity of high vitamin A intake.N Engl J Med. 1995; 333: 1369-1373Crossref PubMed Scopus (550) Google Scholar may well derive from an effect on the expression of the homoeobox gene Hoxb-1 (among others) that regulate axial patterning in the embryo.6Marshall H Studer M Popperl H et al.A conserved retinoic acid response element required for early expression of the homeobox gene Hoxb-1.Nature. 1994; 370: 567-571Crossref PubMed Scopus (388) Google Scholar Mutations in another murine gene (AP-2) invariably cause NTDs.7Zhang J Hagopian-Donaldson S Serbedzija G et al.Neural tube, skeletal and body wall defects in mice lacking transcription factor AP-2.Nature. 1996; 381: 238-241Crossref PubMed Scopus (527) Google Scholar Vitamin A excess may alter the expression of this gene (which resides on human chromosome 6) in human neural crest cells. New insights into mammalian embryology have been gained through study of mice with targeted gene deletions.7Zhang J Hagopian-Donaldson S Serbedzija G et al.Neural tube, skeletal and body wall defects in mice lacking transcription factor AP-2.Nature. 1996; 381: 238-241Crossref PubMed Scopus (527) Google Scholar, 8Zhao Q Behringer RR deCrombrugghe B Prenatal folic acid treatment suppresses acrania and meroanencephaly in mice mutant for the Cart1 homeobox gene.Nat Genet. 1996; 13: 275-283Crossref PubMed Scopus (207) Google Scholar These elegant studies reveal developmental genes that influence or control cell growth, migration, apoptosis, and organisation. Studies of knockout mice mutants made homozygous for the Cart1 paired-class homoeobox-containing gene (a transcription factor that regulates downstream target genes) suggest that at a critical embryonic stage a region-specific lesion in head mesenchyme may induce NTDs as well as craniofacial abnormalities.8Zhao Q Behringer RR deCrombrugghe B Prenatal folic acid treatment suppresses acrania and meroanencephaly in mice mutant for the Cart1 homeobox gene.Nat Genet. 1996; 13: 275-283Crossref PubMed Scopus (207) Google Scholar It is especially noteworthy that prenatal treatment with folic acid of these knockout mouse embryos suppresses the development of NTDs.8Zhao Q Behringer RR deCrombrugghe B Prenatal folic acid treatment suppresses acrania and meroanencephaly in mice mutant for the Cart1 homeobox gene.Nat Genet. 1996; 13: 275-283Crossref PubMed Scopus (207) Google Scholar The clear and apparent lesson from these mouse-mutants is that cellular defects in a subregion of the cranial region may influence the embryological process in adjacent tissues, ultimately affecting the entire cascade of craniofacial development. It is therefore conceivable that apparently disparate defects (cardiovascular, urinary tract, neural tube) may have common origins as neurocristopathies or maldevelop as a consequence of a disturbed embryological process. The blunt though valuable instrument that is epidemiology has introduced new hypotheses and clues about the aetiology of various congenital defects that now enable more precise molecular definition. Progress made through epidemiologically based claims is necessarily slow, requiring more detailed studies and confirmation. Such is the case for the suggestion that periconceptional multivitamin supplementation might reduce the occurrence of congenital urinary tract and cardiovascular anomalies.