Genetics of susceptibility to 6-aminonicotinamide-induced cleft palate in the mouse: studies in congenic and recombinant inbred strains

Effects of phenytoin on Satb2 and Hoxa2 gene expressions in mouse embryonic craniofacial tissue

Cleft lip and cleft palate are common congenital craniofacial birth defects in humans. Phenytoin (PHT) is a risk factor of cleft palate formation; however, the molecular mechanisms by which phenytoin exerts its teratogenic effects resulting in cleft palate remain unknown. The Satb2 gene mutation is associated with cleft palate. Satb2-deficient mice exhibit cleft palate deformity and an up-regulation of Hoxa2 in the fronto-nasal region. In this study, phenytoin was administered intraperitoneally to pregnant C57BL/6 mice on the 10th day of gestation. Real-time PCR results showed that the expressions of Satb2 and Hoxa2 in craniofacial tissues of mouse embryos were obviously different at different time points. The Satb2 gene was down-regulated and the Hoxa2 gene was up-regulated in phenytoin-treated mouse embryonic craniofacial tissue. We conclude that phenytoin may regulate the expression of these two genes in C57BL/6 mice and it may also be involved in the formation of cleft palate.

ArylamineN-Acetyltransferases: What We Learn from Genes and Genomes

Arylamine N-acetyltransferases (NATs) are phase II xenobiotic metabolizing enzymes, catalyzing acetyl-CoA-dependent N- and O-acetylation reactions. All NATs have a conserved cysteine protease-like Cys-His-Asp catalytic triad inside their active site cleft. Other residues determine substrate specificity, while the C-terminus may control hydrolysis of acetyl-CoA during acetyltransfer. Prokaryotic NAT-like coding sequences are found in >30 bacterial genomes, including representatives of Actinobacteria, Firmicutes and Proteobacteria. Of special interest are the nat genes of TB-causing Mycobacteria, since their protein products inactivate the anti-tubercular drug isoniazid. Targeted inactivation of mycobacterial nat leads to impaired mycolic acid synthesis, cell wall damage and growth retardation. In eukaryotes, genes for NAT are found in the genomes of certain fungi and all examined vertebrates, with the exception of canids. Humans have two NAT isoenzymes, encoded by highly polymorphic genes on chromosome 8p22. Syntenic regions in rodent genomes harbour two Nat loci, which are functionally equivalent to the human NAT genes, as well as an adjacent third locus with no known function. Vertebrate genes for NAT invariably have a complex structure, with one or more non-coding exons located upstream of a single, intronless coding region. Ubiquitously expressed transcripts of human NAT1 and its orthologue, murine Nat2, are initiated from promoters with conserved Sp1 elements. However, in humans, additional tissue-specific NAT transcripts may be expressed from alternative promoters and subjected to differential splicing. Laboratory animals have been widely used as models to study the effects of NAT polymorphism. Recently generated knockout mice have normal phenotypes, suggesting no crucial endogenous role for NAT. However, these strains will be useful for understanding the involvement of NAT in carcinogenesis, an area extensively investigated by epidemiologists, often with ambiguous results.

Correlation of susceptibility to 6-aminonicotinamide and hydrocortisone-induced cleft palate

Our previous genome-wide Quantitative Trait Locus (QTL) mapping study using mouse A/J by C57BL/6J recombinant inbred (RI) lines suggested several chromosomal regions contain genes influencing susceptibility to phenytoin (PT)-induced cleft lip with or without cleft palate [CL(P)] and 6-aminonicotinamide (6-AN)-induced isolated cleft palate (CP). Importantly, the same chromosomal regions but different RI parental strain alleles were sometimes implicated in susceptibility to these different kinds of orofacial clefting. Here we report the susceptibility to hydrocortisone (HC)-induced CP in these RI lines. We treated pregnant females with HC and studied the incidence of CP in day 17 fetuses. RI lines showed highly correlated responses to HC and 6-AN. The A/J parental line and five RI lines showed very high levels of clefting in response to both of these teratogens. The C57BL/6J parental line and five other RI lines exhibited low incidence of CP for these teratogens. In contrast, there was no significant correlation between incidence of PT-induced CL(P) and HC-induced CP.

Identification and functional characterization of novel polymorphisms associated with the genes for arylamine N-acetyltransferases in mice

Arylamine N-acetyltransferase (NAT) polymorphism in humans has been associated with variation in susceptibility to drug toxicity and cancer. In mice, three NAT isoenzymes are encoded by Nat1, Nat2 and Nat3 genes. Only Nat2 has been shown previously to be polymorphic, a single nucleotide substitution causing the slow acetylator phenotype in the A/J strain. We sequenced the Nat genes from inbred (CBA and 129/Ola), outbred (PO and TO) and wild-derived inbred (Mus spretus and Mus musculus castaneus) mouse strains and report polymorphism in all three Nat genes of M. spretus and in Nat2 and Nat3 genes of M. m. castaneus. Enzymatic activity assays using liver homogenates demonstrated that M. m. castaneus is a 'fast' and M. spretus a 'slow' acetylator. Western blot analysis indicated that hepatic NAT2 protein is less abundant in M. spretus than M. m. castaneus. The new allozymes were expressed in a mammalian cell line and NAT enzymatic activity was measured with a series of substrates. NAT1 and NAT2 isoenzymes of M. m. castaneus exhibited a higher rate of acetylation, compared with those of M. spretus. Activity of the NAT3 allozymes was hardly detectable, although the Nat3 gene does appear to be transcribed, since mRNA was detected by RT-PCR in the spleen. Additional polymorphisms, useful for Nat-related genetic studies, have been identified between BALB/c, C57Bl/6J, A/J, 129/Ola, CBA, PO, TO, M. m. castaneus and M. spretus strains in four microsatellite repeats located close to the Nat genes.

Comparison of the expression patterns of two LIM-homeodomain genes, Lhx6 and L3/Lhx8, in the developing palate

OBJECTIVES

To compare and contrast the gene expression of two LIM-homeobox type transcription factors, Lhx6 and L3/Lhx8, in secondary palate formation.

METHODS

In situ hybridization histochemistry with digoxygenin (DIG) labelled cRNA probes specific for Lhx6 and L3/Lhx8.

MATERIALS

Serial cryo-sections of embryonic day (E)13.5, 14.5, and 15.5 mice (C57BL/6).

OUTCOME MEASURE

Comparison of the signal intensities of NBT/BCIP precipitate by alkaline phosphatase conjugated anti-DIG antibody.

RESULTS

From E13.5 to E15.5, Lhx6 and L3/Lhx8 signals are detected in palatal mesenchyme, but the L3/Lhx8 signal is much more intense than the Lhx6 signal. In palatal epithelium, covering the mesenchyme, Lhx6 mRNA is transiently expressed at E14.5, while L3/Lhx8 mRNA expression is never detected throughout the development.

CONCLUSION

Lhx6 and L3/Lhx8 functions may be partially redundant in the mesenchyme of the secondary palate, but not in the palatal epithelium.

Immunochemical detection of arylamine N-acetyltransferase during mouse embryonic development and in adult mouse brain

Arylamine N-acetyltransferases (NATs) are important in susceptibility to xenobiotic-induced disorders (e.g., drug-induced autoimmune disease, bladder cancer), but their role in endogenous metabolism is yet to be elucidated. The discovery that human NAT1 acts upon p-aminobenzoylgluatamate (p-ABG) to generate p-acetamidobenzoylglutamate (p-AABG), a major urinary metabolite of folic acid, suggests that human NAT1 may play a role in folic acid metabolism and hence in the normal development of the neural tube. In this study we examined the distribution of NAT in neuronal tissue from adult mice and embryos. Immunohistochemical staining of the adult mouse cerebellum revealed NAT2 (the mouse homologue of human NAT1) expression in the cell bodies and dendrites of Purkinje cells and in the neuroglia of the molecular layer. In embryos, NAT2 was detected in developing neuronal tissue on days 9.5, 11.5, and 13.5. It was expressed intensely in the nerual tube around the time of closure. The level of expression subsequently declined in the neuroepithelium but increased in glial cells. In addition, NAT2 was detected in the developing heart and gut. These findings demonstrate that the embryo itself expresses an enzyme which is involved in the metabolism of folic acid, so that the role played by both mother and embryo must be considered when examining the role of folic acid in embryonic development. These findings imply that polymorphisms in NAT genes could play a role in determining susceptibility to neural tube defects (NTD) and orofacial clefting, developmental disorders which can be prevented by dietary administration of folic acid.

Genome scan for teratogen-induced clefting susceptibility loci in the mouse: evidence of both allelic and locus heterogeneity distinguishing cleft lip and cleft palate

Nonsyndromic clefting of the lip and palate in humans has a highly complex etiology, with both multiple genetic loci and exposure to teratogens influencing susceptibility. Previous studies using mouse models have examined only very small portions of the genome. Here we report the findings of a genome-wide search for susceptibility genes for teratogen-induced clefting in the AXB and BXA set of recombinant inbred mouse strains. We compare results obtained using phenytoin (which induces cleft lip) and 6-aminonicotinamide (which induces cleft palate). We use a new statistical approach based on logistic regression suitable for these categorical data to identify several chromosomal regions as possible locations of clefting susceptibility loci, and we review candidate genes located within each region. Because cleft lip and cleft palate do not frequently co-aggregate in human families and because these structures arise semi-independently during development, these disorders are usually considered to be distinct in etiology. Our data, however, implicate several of the same chromosomal regions for both forms of clefting when teratogen-induced. Furthermore, different parental strain alleles are usually associated with clefting of the lip versus that of the palate (i.e., allelic heterogeneity). Because several other chromosomal regions are associated with only one form of clefting, locus heterogeneity also appears to be involved. Our findings in this mouse model suggest several priority areas for evaluation in human epidemiological studies.

Effect of the dam strain on the spontaneous incidence of cleft lip and palate and intrauterine growth of CL/Fr mouse fetuses

PURPOSE

The effects of dam strain on the spontaneous incidence of the cleft lip and palate (CLP) and the intrauterine growth of transferred CLP-susceptible CL/Fr embryos were examined with an embryo transfer technique. The CL/Fr strain of embryos at the early blastocyst stage was transferred to the same dam strain and also to the CLP-resistant C57BL dam strain. A laparotomy was done on the 18th gestational day at which time the number of fetuses, the resorption sites and the fetus weight were recorded. Each fetus was checked for the presence of CLP. Five criteria to assess the reproduction and fertility as well as the fetus weight were then compared between both dam strains.

RESULTS

The dam pregnancy rate and the fetus survival rate in the CL/Fr dam strain were both significantly lower than those in the C57BL dam strain. The resorption rate in the CL/Fr dam strain was significantly higher than that in the C57BL dam strain. The spontaneous incidence rate of CLP in the CL/Fr dam strain was also significantly higher than that in the C57BL dam strain. The fetus weight of the CL/Fr fetuses developed in the CL/Fr dam strain was significantly lighter than that of the CL/Fr fetuses developed in the C57BL dam strain.

CONCLUSION

The results indicated that the CLP-susceptible CL/Fr dam strain provided a less favorable uterine environment for the implantation, survival and intrauterine growth of the transferred CL/Fr embryos and also caused a higher spontaneous incidence rate of CLP. Thus, it can be concluded that the effect of the dam strain appears to play an important role on the spontaneous incidence of CLP and the intrauterine growth of the CL/Fr strain embryos transferred to both CL/Fr and C57BL dam strains.