Diabetic embryopathy in C57BL/6J mice. Altered fetal sex ratio and impact of the splotch allele.

Maternal diabetes (types 1 and 2) induces a broad array of congenital malformations, including neural tube defects (NTDs), in humans. One of the difficulties associated with studying diabetic embryopathy is the rarity of individual malformations. In an attempt to develop a sensitive animal model for maternal diabetes-induced NTDs, the present study uses chemically induced diabetes in an inbred mouse model with or without the splotch (Sp) mutation, a putatively nonfunctional allele of Pax3. Pax3 deficiency has been associated with an increase in NTDs. Female C57BL/6J mice, either with or without the Sp allele, were injected intravenously with alloxan (100 mg/kg), and plasma glucose was measured 3 days later. A wide range of hyperglycemia was induced, and these diabetic mice were bred to C57BL/6J males, some carrying the Sp allele. Gestational-day-18 fetuses were examined for developmental malformations. Fetuses from matings in which either parent carried the Sp allele were genotyped by polymerase chain reaction. Maternal diabetes significantly decreased fetal weight and increased the number of resorptions and malformations, including NTDs. A significant correlation was found between the level of maternal hyperglycemia and the malformation rate. The sex ratio for live fetuses in diabetic litters was significantly skewed toward male fetuses. Matings involving the Sp allele yielded litters with significantly higher percentages of maternal diabetes-induced spina bifida aperta but not exencephaly, and this increase was shown to be associated with the presence of a single copy of the Sp allele in affected fetuses. Thus, Pax3 haploinsufficiency in this murine model of diabetic embryopathy is associated with caudal but not cranial NTDs.

Identification of a murine locus conveying susceptibility to cadmium-induced forelimb malformations.

The heavy metal cadmium (Cd), an environmentally ubiquitous contaminant, is a potent teratogen in mice. When administered parenterally, it induces an array of malformations that vary in scope and severity with the route, dose, time of administration, and the strain of the animal. When administered intraperitoneally on day 9.0 of gestation, 4 mg/kg cadmium chloride produces forelimb defects (predominantly ectrodactyly) in over 80% of fetuses of the C57BL/6 mouse strain, while no limb defects are observed in the identically treated SWV strain. Like other examples of strain-specific teratogenic activity, the underlying nature of the differential susceptibility remains unknown. The present study investigates the segregation of sensitivity to Cd-induced forelimb defects in crosses between C57BL/6 and SWV mice and provides evidence for the involvement of both maternal and fetal factors in the determination of defect expression. In addition, quantitative trait loci (QTL) analysis of the fetal genetic component was performed among 198 backcross progeny, utilizing a genomic linkage map of 149 informative microsatellite markers. One QTL demonstrating significant linkage to expression of the defect, designated Cadfar (cadmium-induced forelimb autopod reduction), was mapped to the distal end of chromosome 6 with a lod score of 3.1.

Teratogenic response to arsenite during neurulation: relative sensitivities of C57BL/6J and SWV/Fnn mice and impact of the splotch allele.

Arsenic is an environmental contaminant that induces congenital malformations, primarily neural tube defects, in laboratory animals, and it may contribute to human birth defects. The acute doses of arsenicals required to elicit teratogenesis in outbred strains of mice, however, are orders of magnitude higher than those to which humans are exposed environmentally. In order to examine interactions between arsenite administration during neurulation and murine genotype, the present study compares two inbred mouse strains, establishes a teratogenic dose of arsenite, and evaluates the effect of the splotch mutation on arsenic-induced teratogenesis. SWV/Fnn or C57BL/6J females were injected intraperitoneally with sodium arsenite (10 mg/kg) on days 6.5, 7.0, 7.5, 8.0, 8.5, or 9.0 of gestation. A dose-response study was carried out in the C57BL/6J strain, and the effect of the splotch mutation, introduced via the male (C57BL/6J Sp/+), was assessed. Fetuses were examined for external, visceral, and skeletal malformations. Fetuses from crosses of C57BL/6J females with C57BL/6J Sp/+ males were genotyped by PCR. Ten-mg/kg sodium arsenite was teratogenic in nearly 50% of C57BL/6J fetuses, and the C57BL/6J strain was significantly more sensitive to arsenite-induced embryo-lethality and teratogenicity than the SWV/Fnn strain. The spectrum of malformations produced was dependent on the gestational time point of arsenite administration. Introduction of the splotch allele significantly increased neural tube defects and other specific malformations. This result demonstrates that a mutation in a single gene can increase sensitivity to arsenic-induced birth defects. This murine study examines the interaction between arsenite-induced teratogenicity and genotype.

Differential sensitivity of the SWV and C57BL/6 mouse strains to the teratogenic action of single administrations of cadmium given throughout the period of anterior neuropore closure.

When administered to mice during gestation, the heavy metal, cadmium, is known to induce malformations of the neural tube, craniofacial region, limbs, trunk, viscera, and axial skeleton that vary in scope and severity among inbred strains of mice. Two strains, C57BL/6 and SWV, were previously shown to differ in their susceptibility to exencephaly induced by many teratogenic treatments, including sodium 2-ethylhexanoate, hyperthermia, valproic acid, and carbon dioxide, with the SWV strain being consistently more sensitive than the C57BL/6 strain. These findings support the observation of Finnell et al. ([1988] Teratology 38:313-320) of shared hierarchies of relative susceptibility to exencephaly induced by biochemically distinct teratogens, and suggest that the SWV strain would also be more sensitive to exencephaly induced by cadmium. In the present study, pregnant mice from the two strains were exposed to single i.p. injections of cadmium chloride at 4 mg/kg-BW on day 6.5, 7.0, 7.5, 8.0, 8.5, or 9.0 of gestation. Fetuses were removed by cesarean section on day 18 of gestation and scored for malformations. The C57BL/6 strain was observed to be more sensitive than the SWV strain to the induction of exencephaly by cadmium on days 6.5, 7.0, and 8.0, with mean litter percentages of 3.6%, 88.3%, and 62.2%, respectively, compared to 0.0%, 4.1%, and 27.7% for the SWV strain. This finding provides evidence in contrast to the hypothesis of shared hierarchies of sensitivity to teratogen-induced exencephaly. Data on a number of other cadmium-induced malformations are also presented.