Spinal Neural Tube Defects in the Curly Tail Mouse Mutant: Involvement of Retinoic Acid and its Nucleic Acid Receptors

Abstract

Human neural tube defects (NTD) are among the commonest congenital malformations. The frequency of NTD is 1/1000 births in North America, Africa, Japan and Mongolia, and it rises to 7.6-8.6/1000 in south Wales and Northern Ireland. NTD also occur in 4.9% of all abortuses and so about 16/1000 of all conceptions are affected. The etiology of NTD is, however, still unknown. The mouse mutant curly tail (ct/ct) develops spinal NTD in 54% of cases, resulting from delayed closure of the posterior neuropore (PNP), and displays many similar features to those of NTD in humans. Thus, it provides a valid model for studying human spina bifida. Two previous observations suggest an involvement of retinoic acid (RA) and its mucleic receptors in closure of the PNP in mouse embryos. The effects of RA on development and growth of ct/ct embryos and on the expression pattern of nucleic retinoic acid receptor (RAR) were therefore studied. We found that maternal treatment with the low dose of 5 mg/kg RA decreases tlie incidence of spinal NTD by 50% in a stage-specific manner, without concomitant increase of tlie incidence of cranial NTD. The effect of RA is specific to tlie ct/ct phenotype: there is no other alteration in resorption rate, litter size, morphogenesis, growth or development. In situ hybridization using RAR-β and RAR-γ riboprobes and computerized image analysis of affected ct/ct embryos, which are destined to develop spinal NTD, reveals a down-regulation of RAR-β and RAR-γ expression in the hindgut endoderm and it tlie PNP region, respectively. The extent of reduction of RAR-γ transcripts abundance correlates well with the severity of delayed PNP closure. As early as 2 hours following RA treatment at 10 days 8 hours post coituni, RAR-β is upregulated specifically in the hindgut endoderm. The deficates expression of RAR-γ in the PNP region is also upregulated. By 6 hours after RA treatment, PNP length is significantly reduced. Three to four days later, the incidence of spinal NTD is decreased by 50%. This sequential event indicates that both the genesis and prevention of spinal NTD in the ct/ct mouse mutant involve retinoic acid and its mucleic acid receptors, RAR-β and RAR-γ.