Is Sonic hedgehog (SHH) a candidate gene for spina bifida? A pilot study

Toward understanding the genetic basis of neural tube defects

Neural tube defects (NTDs) represent a common group of severe congenital malformations that result from failure of neural tube closure during early development. Their etiology is quite complex involving environmental and genetic factors and their underlying molecular and cellular pathogenic mechanisms remain poorly understood. Animal studies have recently demonstrated an essential role for the planar cell polarity pathway (PCP) in mediating a morphogenetic process called convergent extension during neural tube formation. Alterations in members of this pathway lead to NTDs in vertebrate models, representing novel and exciting candidates for human NTDs. Genetic studies in NTDs have focused mainly on folate-related genes based on the finding that perinatal folic acid supplementation reduces the risk of NTDs by 60-70%. A few variants in these genes have been found to be significantly associated with an increased risk for NTDs. The candidate gene approach investigating genes involved in neurulation has failed to identify major causative genes in the etiology of NTDs. Despite this history of generally negative findings, we are achieving a rapid and impressive progress in understanding the genetic basis of NTDs, based mainly on the powerful tool of animal models.

Current perspectives on the genetic causes of neural tube defects

Neural tube defects (NTDs) are a group of severe congenital abnormalities resulting from the failure of neurulation. The pattern of inheritance of these complex defects is multifactorial, making it difficult to identify the underlying causes. Scientific research has rapidly progressed in experimental embryology and molecular genetics to elucidate the basis of neurulation. Crucial mechanisms of neurulation include the planar cell polarity pathway, which is essential for the initiation of neural tube closure, and the sonic hedgehog signaling pathway, which regulates neural plate bending. Genes influencing neurulation have been investigated for their contribution to human neural tube defects, but only genes with well-established role in convergent extension provide an exciting new set of candidate genes. Biochemical factors such as folic acid appear to be the greatest modifiers of NTDs risk in the human population. Consequently, much research has focused on genes of folate-related metabolic pathways. Variants of several such genes have been found to be significantly associated with the risk of neural tube defects in more studies. In this manuscript, we reviewed the current perspectives on the causes of neural tube defects and highlighted that we are still a long way from understanding the etiology of these complex defects.

Candidate gene analysis in human neural tube defects

Biochemical and developmental pathways, mouse models, and positional evidence have provided numerous candidate genes for the study of human neural tube defects. In a survey of 80 studies on 38 candidate genes, few found significant results in human populations through case-control or family-based association studies. While the folate pathway has been explored extensively, only the MTHFR 677C > T polymorphism was significant, and only in an Irish population. Developmental pathways such as the Wnt signaling pathway and Hox genes have also been explored without positive results. More than 90 mouse candidates have been identified through spontaneous and knockout mutations, but only the T locus (mouse Brachyury gene) showed association in an initial study that was not confirmed on follow-up. Positional candidates have been derived from cytogenetic evidence, but preliminary genomic screens have limited power due to small sample sizes. Future studies would increase their power to detect association by using more samples. In addition a clarification of the phenotype would be beneficial as many studies used different inclusion criteria. Incorporating several types of data could highlight better candidates, as would looking beyond the traditional sources for candidate genes. Recent studies of an energy metabolism gene (UCP2) and vitamin B metabolism (Transcoalbumin) have produced promising results. Utilizing other model organisms may also be beneficial, as in a recent study from a chick model of NTDs in NCAM1. New approaches combined with traditional methods and increased sample sizes will help prioritize human NTD candidate genes and clarify the complex etiology of this condition.

Spina bifida

Spina bifida results from failure of fusion of the caudal neural tube, and is one of the most common malformations of human structure. The causes of this disorder are heterogeneous and include chromosome abnormalities, single gene disorders, and teratogenic exposures. However, the cause is not known in most cases. Up to 70% of spina bifida cases can be prevented by maternal, periconceptional folic acid supplementation. The mechanism underlying this protective effect is unknown, but it is likely to include genes that regulate folate transport and metabolism. Individuals with spina bifida need both surgical and medical management. Although surgical closure of the malformation is generally done in the neonatal period, a randomised clinical trial to assess in utero closure of spina bifida has been initiated in the USA. Medical management is a lifelong necessity for individuals with spina bifida, and should be provided by a multidisciplinary team.